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Clear sky direct radiative effects of aerosols over Southeast Asia based on satellite observations and radiative transfer calculations
... These particles are pumped into the troposphere by convection systems, and last from several days to several weeks (Haywood and Boucher, 2000;Yu et al., 2012). Aerosols have had significant impacts on ecosystem health and humans, land and sea, biogeochemical cycles, characteristics of clouds, as well as weather systems and climate, especially in recent decades (Feng and Christopher, 2014;Basha et al., 2015;Yu et al., 2015;Lee et al., 2016). Awareness of the uncertainties stemmed from aerosols as the most important sources in the studies of climate systems and climate change is significant (Banks and Brindley, 2013;Tomasi et al., 2015). ...
... There are several methods for studying aerosols, including groundbased measurements (Welton et al., 2000;Holben et al., 2001;Murayama et al., 2001;Chung et al., 2005;He and Yi, 2015;Han et al., 2015;Ningombam et al., 2014;Vijayakumar et al., 2016;Campbell et al., 2016), aerosols models (McMurry, 2000;Nowottnick et al., 2015;Geng et al., 2015;Vijayakumar et al., 2016;Prijith et al., 2016) and remote sensing (Winker et al., 2007;Bréon et al., 2011;Knippertz and Todd, 2012;Kokhanovsky, 2013;Feng and Christopher, 2014). Ground-Based Measurement is a common method for aerosols monitoring which is done discretely and pointwise on ground stations. ...
... A large number of studies have used CALIPSO data for a wide range of atmospheric studies. CALIPSO profiles were utilized for various fields including; testing dust transfer (Liu et al., 2008;Abdi Vishkaee et al., 2012;Cabello et al., 2012;Yu et al., 2015;Francis et al., 2017), aerosol distribution (Chen et al., 2012;Mishra and Shibata, 2012), aerosol radiation (Feng and Christopher, 2014;Basha et al., 2015) and validation of climate models (Koffi et al., 2012). NASA presents an online bibliography related to atmospheric researches using CALIPSO data (https:// www-calipso.larc.nasa.gov/resources/bibliographies.php). ...
This paper presents a new algorithm based on the support vector machine (SVM) for classifying the Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) data into classes of clean air, cloud, thin aerosol, dense aerosol, surface, subsurface and totally attenuated. The procedure is as follows: At first, the considered features based on CALIPSO data are prepared. Brightness Temperature Differences between 10 and 12 μm (BTD11-12) is then used to better discriminate dense aerosols from clouds. The particle density feature proposed in this research is another feature participating in the classification. Training samples are automatically extracted by applying strict thresholds on the features. A wrapper feature selection is performed to rank the features based on their performance. Four post-processing steps are implemented to correct some misclassified cells e.g. edges of clouds and high-level clouds. The proposed algorithm was implemented on 4 datasets in the Middle East and North Africa (MENA), and India with various types and densities of aerosol. An accuracy assessment based on the comparison between the obtained results and ground truth samples indicated 0.94, 0.96 4, 0.92 and 0.89 kappa coefficients for the datasets. A statistical hypothesis test demonstrated that our SVM classification overcame CALIPSO vertical feature mask (VFM) product. The experimental result indicates the high accuracy of the proposed algorithm for the atmosphere scene classification using CALIPSO data.
... For the model-simulated method, aerosol optical properties obtained from ground-based or remote-sensing observations have been used as input to the RTM to calculate ADRE at TOA and surface [11][12][13][14][15][16]. For the remote sensing method, one is to estimate ADRE by the linear relationship between AOD and concurrent upward SW flux at TOA obtained from satellite observations [17][18][19][20][21][22]; the other is to take Fclr as the minimum upward SW flux at TOA in clear sky during one month, but it is not suitable for the regions with high cloud coverage or aerosol loading such as the study region (YRB) [4]. In other words, the remote-sensing technique (linear fitting) is more suitable for estimating ADRE at regional and global scales. ...
... During the past decades, a series of studies on aerosol optical properties and associated direct radiative effects continue to emerge over Amazonia [3,17], Southeast Asia [18,19], Northeast India [11,20,[23][24][25][26][27][28][29], Central India [12], and global land [2,[30][31][32][33] and ocean [4,5,21]. The results showed that the averaged ADRE at TOA was negative at global scale [4][5][6], but it varied regionally probably due to the comprehensive effects of aerosol optical properties and underlying surface characteristics. ...
... In addition, we calculate the IADRE efficiency by regressions from median values of IADRE and AOD. It is conducive to further analyze the effects of other aerosol optical prosperities except AOD on IADRE [19]. The detailed flowchart of our satellite-retrieved method for calculating IADRE at TOA over YRB during 2001-2015 is illustrated in Figure 2. ...
The spatiotemporal variation of aerosol optical depth at 550 nm (AOD550), Ångström exponent at 470–660 nm (AE470–660), water vapor content (WVC), and shortwave (SW) instantaneous aerosol direct radiative effects (IADRE) at the top-of-atmosphere (TOA) in clear skies obtained from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Clouds and the Earth’s Radiant Energy System (CERES) are quantitatively analyzed over the Yangtze River Basin (YRB) in China during 2001–2015. The annual and seasonal frequency distributions of AE470–660 and AOD550 reveal the dominance of fine aerosol particles over YRB. The regional average AOD550 is 0.49 ± 0.31, with high value in spring (0.58 ± 0.35) and low value in winter (0.42 ± 0.29). The higher AOD550 (≥0.6) is observed in midstream and downstream regions of YRB and Sichuan Basin due to local anthropogenic emissions and long-distance transport of dust particles, while lower AOD550 (≤0.3) is in high mountains of upstream regions. The IADRE is estimated using a linear relationship between SW upward flux and coincident AOD550 from CERES and MODIS at the satellite passing time. The regional average IADRE is −35.60 ± 6.71 Wm−2, with high value (−40.71 ± 6.86 Wm−2) in summer and low value (−29.19 ± 7.04 Wm−2) in winter, suggesting a significant cooling effect at TOA. The IADRE at TOA is lower over Yangtze River Delta (YRD) (≤−30 Wm−2) and higher in midstream region of YRB, Sichuan Basin and the source area of YRB (≥−45 Wm−2). The correlation coefficient between the 15-year monthly IADRE and AOD550 values is 0.63, which confirms the consistent spatiotemporal variation patterns over most of the YRB. However, a good agreement between IADRE and AOD is not observed in YRD and the source area of YRB, which is probably due to the combined effects of aerosol and surface properties.
... The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC, 2013) stated that the variation (−0.85 to +0.15 Wm −2 ) of aerosol radiative forcing is relatively significant compared with its counterpart (2.54 to 3.12 Wm −2 ) of well-mixed greenhouse gases [1]. The large fluctuation in radiative forcing (RF) is primarily related to the poor characterization of the microphysical and optical properties of atmospheric aerosols [2,3]. These results also suggest that each type of aerosol does not weigh equally with regard to the total amount of RF. ...
... 3). The results of all dual-between f AOD NDAI and f AOD INP , as ...
Quantifying aerosol compositions (e.g., type, loading) from remotely sensed measurements by spaceborne, suborbital and ground-based platforms is a challenging task. In this study, the first and second-order spectral derivatives of aerosol optical depth (AOD) with respect to wavelength are explored to determine the partitions of the major components of aerosols based on the spectral dependence of their particle optical size and complex refractive index. With theoretical simulations from the Second Simulation of a Satellite Signal in the Solar Spectrum (6S) model, AOD spectral derivatives are characterized for collective models of aerosol types, such as mineral dust (DS) particles, biomass-burning (BB) aerosols and anthropogenic pollutants (AP), as well as stretching out to the mixtures among them. Based on the intrinsic values from normalized spectral derivatives, referenced as the Normalized Derivative Aerosol Index (NDAI), a unique pattern is clearly exhibited for bounding the major aerosol components; in turn, fractions of the total AOD (fAOD) for major aerosol components can be extracted. The subtlety of this NDAI method is examined by using measurements of typical aerosol cases identified carefully by the ground-based Aerosol Robotic Network (AERONET) sun–sky spectroradiometer. The results may be highly practicable for quantifying fAOD among mixed-type aerosols by means of the normalized AOD spectral derivatives.
... Comparing spatial pattern of clear-sky SSR in Figure 3 and ARE in Figure 4a, regions with low clear-sky SSR correspond to regions with strong attenuation effect of aerosol on solar radiation. High ARE (larger than −40 W/m 2 ) are found in the neighboring region of Shandong, Hebei and Tianjin, which are much higher than the national mean ARE in China (−15.70 W/m 2 ) [60]. However, low ARE (smaller than −15 W/m 2 ) are found in north Hebei and south-central Inner Mongolia. ...
... W/m 2 ) are found in the neighboring region of Shandong, Hebei and Tianjin, which are much higher than the national mean ARE in China (-15.70 W/m 2 ) [60]. However, low ARE (smaller than -15 W/m 2 ) are found in north Hebei and south-central Inner Mongolia. ...
The distribution and trend of clear-sky surface solar radiation (SSR) and the quantitative effects of aerosol and water vapor are investigated in northern China during 2001–2015 using radiation simulations and satellite observations. Clear-sky SSR in northern China is high in summer and low in winter, which is dominated by astronomical factors and strongly modulated by the seasonal variations of radiative effects of aerosol (ARE) and water vapor (WVRE). The larger variation of WVRE than ARE indicates that water vapor plays a more important role in moderating the seasonal variation of clear-sky SSR. Clear-sky SSR shows an overall decreasing trend of –0.12 W/m2 per year, with decrease more strongly than –0.60 W/m2 per year in west-central Shandong and increase (about 0.40 W/m2) in south-central Inner Mongolia. The consistency of spatial distribution and high correlation between clear-sky SSR and ARE trend indicate that the clear-sky SSR trend is mainly determined by aerosol variation. Dust mass concentration decreases about 16% in south-central Inner Mongolia from 2001 to 2015, resulting in the increase in clear-sky SSR. In contrast, sulfate aerosol increases about 92% in west-central Shandong, leading to the decreasing trend of clear-sky SSR.
... In light of the findings of Jeong and Wang (2010), we use seasonally varying emissions in our study. Beyond seasonal variability, Feng and Christopher (2014) and Sena and Artaxo (2015) showed that the direct radiative effects of fire aerosols also exhibit interannual variability, something that is not taken into account in most climate modelling studies. We complement the sub-monthly focus of Clark et al. (2015) and the seasonal focus of Jeong and Wang (2010) by investigating interannually varying emissions. ...
... Three of the above-mentioned studies (Jeong and Wang, 2010;Feng and Christopher, 2014;Sena and Artaxo, 2015) considered the direct radiative effects of fire aerosols but did not consider other radiative effects. However, as pointed out by Jacobson (2014), the radiative effects of fire aerosols are not limited to the direct effect. ...
Open-burning fires play an important role in the earth's climate system. In
addition to contributing a substantial fraction of global emissions of carbon
dioxide, they are a major source of atmospheric aerosols containing
organic carbon, black carbon, and sulfate. These “fire aerosols” can
influence the climate via direct and indirect radiative effects. In this
study, we investigate these radiative effects and the hydrological fast
response using the Community Atmosphere Model version 5 (CAM5). Emissions of
fire aerosols exert a global mean net radiative effect of
−1.0 W m−2, dominated by the cloud shortwave response to organic
carbon aerosol. The net radiative effect is particularly strong over boreal
regions. Conventionally, many climate modelling studies have used an
interannually invariant monthly climatology of emissions of fire aerosols.
However, by comparing simulations using interannually varying emissions vs.
interannually invariant emissions, we find that ignoring the interannual
variability of the emissions can lead to systematic overestimation of the
strength of the net radiative effect of the fire aerosols. Globally, the
overestimation is +23 % (−0.2 W m−2). Regionally, the
overestimation can be substantially larger. For example, over Australia and
New Zealand the overestimation is +58 % (−1.2 W m−2), while over
Boreal Asia the overestimation is +43 % (−1.9 W m−2). The
systematic overestimation of the net radiative effect of the fire aerosols is
likely due to the non-linear influence of aerosols on clouds. However,
ignoring interannual variability in the emissions does not appear to
significantly impact the hydrological fast response. In order to improve
understanding of the climate system, we need to take into account the
interannual variability of aerosol emissions.
... Sensibly planned addition to these measurements would possibly be able to resolve current scale-induced discrepancies between in situ and space-based observations. Endeavours at the pan Himalayan scale are further needed for a reliable climate mitigation and adaptation strategies that serve millions of people downstream (Feng and Christopher 2014). Recent studies from higher Himalayan regions have highlighted the role of meteorology and strong seasonal variations in eBC concentration (Babu et al. 2011;Marinoni et al. 2010;Mu and Liao 2014;Wang et al. 2014;Sarkar et al. 2015). ...
Sufficient site-level observations are needed to resolve the discrepancies between in situ and space-based observations for reliable climate mitigation and adaptation strategies. Resolving such discrepancies from climate-sensitive and understudied Himalaya is the priority for the Indian sub-continent. This study investigates characteristics and dynamics of equivalent black carbon aerosol (eBC) including sources over a glaciated valley at the transitional climate zone between central and western Himalaya. Thermo-topographic factors influencing valley-scale dynamics of eBC were observed by coupled measurements on instantaneous eBC (Aethalometer) and meteorological state parameters at a high altitude (4000 m asl) in the Indian Himalaya covering an annual cycle (October 2015 to August 2016). Results indicate a very large variation (38–5638 ng m⁻³) in mean daily BC concentration (308 ± 37 ng m⁻³). Seasonally, the eBC concentration was found highest during the pre-monsoon (1276 ± 115 ng m⁻³) and lowest during the monsoon season (308 ± 37 ng m⁻³). In contrast, the magnitude was comparable during winter and post-monsoon seasons (400–500 ng m⁻³). BC-induced mean annual radiative forcing at the atmosphere was + 10.1 ± 3.0 W m⁻². These results indicate a substantial eBC burden over the region even at this high tropospheric altitude in this pristine glacier environment. On the other hand, the diurnal-scale variability in eBC concentration is primarily governed by high-altitude meteorological processes. Further, source apportionment analyses underscore the seasonal scale influence of monsoon and westerly circulation systems.
... Recently, MODIS Collection (C6) aerosol products have been widely validated over land and ocean surfaces [21][22][23][24][25][26][27][28][29][30][31][32][33], including the Indian subcontinent [34][35][36], Southeast Asia [37], East Asia [38,39], Greece [40,41], and China [42][43][44][45]. Bilal et al. [46] showed the missing aerosol optical depth (AOD) pixels for the MODIS C6 Dark Target (DT) and Deep Blue (DB) AOD products during several incidental haze events which occurred in 2013 over the Beijing-Tianjin-Hebei region. ...
In this study, Aqua-Moderate Resolution Imaging Spectroradiometer (MODIS) Collection (C6) and C6.1 Dark Target aerosol optical depth (AOD) retrievals at 3 km (DT3K) and 10 km (DT10K), Deep Blue AOD retrievals at 10 km (DB10K), and combined DT and DB (DTB) AOD retrievals at 10 km resolutions were validated from 2002 to 2014 against ground-based sunphotometer AOD measurements obtained from the Chinese aerosol remote sensing network (CARSNET). The CARSNET AOD data were obtained for sites at Mt. Waliguan (MW), Lanzhou (LZ), Ulate (UL), and Zhengzhou (ZZ) located in the Yellow River basin (YERB) region, China. Errors and agreement between satellite and ground data were reported using Pearson’s correlation (R) and relative mean bias (RMB). Results showed that the DT3K C6.1 highest quality flag (QF = 3) AOD retrievals were well correlated with the sunphotometer AOD data, with an R of 0.82 and an RMB of 1.01. Overestimation and underestimation in DT AOD retrievals were observed for AOD > 1.1 and AOD < 1.1, respectively. A significant underestimation of 37% in DB10K AOD retrievals was observed across all the sites except ZZ, which was indicated by a low-value RMB (0.63). Spatial distribution maps showed high AOD values (>0.8) over the lower part of the YERB and low AOD values (<0.4) across the upstream part of the YERB. This might be due to a large number of aerosol emissions over the lower developed areas and a scarcity of aerosols over the upstream mountain areas. Overall, this study supports the use of DT10K C6.1 AOD retrievals over the western semi-arid and arid regions of the YERB and DTB10K AOD retrievals over the north-central water system and eastern plain regions of the YERB.
... This may be attributed to its unique basin terrain (Tao et al., 2014). On the contrary, AOD is significantly positively correlated with ADRE ATM (R > 0.8) over the middle and High correlation between AOD and ADRE was also reported over the Beijing , YRD , China (Xia et al., 2016), Southeast Asia (Feng and Christopher, 2014) and global ocean Overall, for the entire study period , there is a slight upward trend (0.0065 year −1 ) at 95% significant level (p = 0.00) for MERRA-2 deseasonalized AOD. Before 2000, the increase in AOD is not obvious with a trend of 0.0025 year −1 . ...
... Recently, these MODIS C6 and VIIRS_EDR aerosol products have been widely validated against ground observations in clear sky over the global land and ocean [8,9], India [10,11], Southeast Asia [12], East Asia [13,14], Greece [15,16] and China [17][18][19]. However, their performances over frequent haze-fog areas are still unclear [20]. ...
The visible infrared imaging radiometer suite (VIIRS) environmental data record aerosol product (VIIRS_EDR) and the aqua-moderate resolution imaging spectroradiometer (MYD04) collection 6 (C6) aerosol optical depth (AOD) products are validated against the Cimel sun–photometer (CE318) AOD measurements during different air quality conditions over the Yangtze river basin (YRB) from 2 May 2012 to 31 December 2016. For VIIRS_EDR, the AOD observations are obtained from the scientific data set (SDS) “aerosol optical depth at 550 nm” at 6 km resolution, and
for aqua-MODIS, the AOD observations are obtained from the SDS “image optical depth land and ocean” at 3 km (DT3K) and 10 km (DT10K) resolutions, “deep blue aerosol optical depth 550 land”
at 10 km resolution (DB10K), and “AOD 550 dark target deep blue combined” at 10 km resolution (DTB10K). Results show that the high-quality (QF = 3) DTB10K performs the best against the CE318
AOD observations, along with a higher R (0.85) and more retrievals within the expected error (EE) ± (0.05 + 15%) (55%). Besides, there is a 10% overestimation, but the positive bias does not exhibit obvious seasonal variations. Similarly, the DT3K and DT10K products overestimate AOD retrievals by 23% and 15%, respectively, all over the year, but the positive biases become larger in spring and summer. For the DB10K AOD retrievals, there is an overestimation (underestimation)
in autumn and winter (spring and summer). Compared to the aqua-MODIS AOD products, the VIIRS_EDR AOD retrievals are less correlated (R = 0.73) and only 44% of the retrievals fall within EE. Meanwhile, the VIIRS_EDR shows larger bias than the aqua-MODIS C6 retrievals, and tends to overestimate AOD retrievals in summer and underestimate in winter. Additionally, there is an underestimation for the VIIRS_EDR AOD retrievals over the regions during high aerosol loadings. These indicate that the VIIRS_EDR retrieval algorithm needs to be improved in further applications over the YRB.
... Global monitoring of aerosols from space by using satellites provides a distinctive chance to study the effects of aerosols in different areas over the Earth's surface. A large number of studies on atmospheric aerosol have been performed by using Clouds and the Earth's Radiant Energy System (CERES) data over different places in the world ( Satheesh and Ramanathan, 2000;Li et al., 2000;Rajeev and Ramanathan, 2001;Zhang, 2002a, 2002b;Loeb and Kato, 2002;Zhang and Christopher, 2003;Zhang et al., 2005;Christopher et al., 2006;Gupta et al., 2008;Patadia et al., 2008b;Yan et al., 2011;Corbett et al., 2012;Sena et al., 2013;Feng and Christopher, 2013;Feng and Christopher, 2014;Sena and Artaxo, 2015;Sundström et al., 2015 etc.). The majority of the studies were conducted generally over either oceanic, deserts or land region. ...
... Global monitoring of aerosols from space by using satellites provides a distinctive chance to study the effects of aerosols in different areas over the Earth's surface. A large number of studies on atmospheric aerosol have been performed by using Clouds and the Earth's Radiant Energy System (CERES) data over different places in the world (Satheesh and Ramanathan, 2000; Li et al., 2000; Rajeev and Ramanathan, 2001; Zhang, 2002a, 2002b; Loeb and Kato, 2002; Zhang and Christopher, 2003; Zhang et al., 2005; Christopher et al., 2006; Gupta et al., 2008; Patadia et al., 2008b; Yan et al., 2011; Corbett et al., 2012; Sena et al., 2013; Feng and Christopher, 2013; Feng and Christopher, 2014; Sena and Artaxo, 2015; Sundström et al., 2015 etc. ). The majority of the studies were conducted generally over either oceanic, deserts or land region. ...
In order to understand the climatic implications of atmospheric aerosols, top of atmosphere (TOA) shortwave (SW, 0.3–5 µm) fluxes and aerosol optical depth (AOD) at 550 nm retrieved simultaneously by clouds and the earth's radiant energy system (CERES) and moderate resolution imaging spectroradiometer (MODIS) instruments, respectively, are analysed over North-East India and its adjoining areas for the period July 2002–December 2013. The aerosol-free TOA flux obtained by establishing the linear regression between CERES SW TOA fluxes and MODIS AODs exhibits strong seasonality with peak values in monsoon and minimum in winter. Same seasonality is captured by the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model, but with difference in absolute values. SBDART code is used to extend instantaneous radiative forcing estimates into 24-h averages. AOD over the North East India region with complex terrain shows altitudinal variation with maximum value at the lowest elevation site Dhaka and minimum value at the high-altitude locations Shillong and Aizwal. In general, strong seasonality in AOD is observed with a peak in pre-monsoon (March–May) and dip in post-monsoon (October–November) at all the locations. The direct instantaneous TOA shortwave aerosol radiative forcing (SWARF) shows maximum values in pre-monsoon over all the locations except at Guwahati, Banmauk, Aizawl, and Shillong. The lowest value of instantaneous SWARF is observed in post-monsoon except at Banmauk and Shillong. Climatologically TOA diurnally averaged SWARF varies between −6.95 W m−2 in Aizawl to −20.39 W m−2 in Shillong. In general, the TOA SW forcing efficiency is highest in monsoon at all the locations. The radiative forcing efficiency is found to be less negative when surface reflectance increases.
... Solar radiation is the primary energy source for life on our planet, and greatly affects energy balance, plant growth and climate change on earth [1e3]. Due to the presence of water vapor, dust and aerosols, the solar radiation is attenuated through absorption and scattering process [4e9], for example, aerosols affects the radiative budget directly by scatting and absorbing process and indirectly by modifying cloud properties [5,10,11]. This attenuation process is described by an index of the atmospheric turbidity, which is an important parameter in predicting the availability of solar radiation and daylight illuminance under cloudless skies [4,12]. ...
Accurate measurement and determination of the atmospheric turbidity is of great importance for solar radiation modeling and climate change studies. Daily values of global, direct and diffuse solar irradiation and meteorological variables (air temperature, relative humidity, sunshine duration and wind speed) during 1960e2013 are used to investigate the monthly variations of Ångstr€ om turbidity coefficient (b) at Zhengzhou, China. An improved method (IYHM-ZZ) is proposed by combining the format of the Yang hybrid model (YHM) with corrected spectral terms. The b value is obtained by adjusting the estimated direct radiation until it matches the measured values. Statistical indicators (RMSE, MBE and t-test) are used to evaluate the performance of YHM and IYHM-ZZ models, and the IYHM-ZZ model produces more accurate estimates than the YHM model. The results indicate that the b values are generally higher in winter and spring, lower in summer and autumn. An increasing trend of b is observed during 1960e2010 at Zhengzhou, and the annual mean b are 0.07, 0.09, 0.11, 0.12, 0.12 and 0.13 for 1960s, 1970s, 1980s, 1990s, 2000s and 2010-, respectively.
... Solar radiation is the primary energy source for life on our planet, and greatly affects energy balance, plant growth and climate change on earth [1e3]. Due to the presence of water vapor, dust and aerosols, the solar radiation is attenuated through absorption and scattering process [4e9], for example, aerosols affects the radiative budget directly by scatting and absorbing process and indirectly by modifying cloud properties [5,10,11]. This attenuation process is described by an index of the atmospheric turbidity, which is an important parameter in predicting the availability of solar radiation and daylight illuminance under cloudless skies [4,12]. ...
... This technique (CERES + MODIS) has also been widely applied to evaluate the mean DARF over a time period (usually 2-3 months) in several other regions (e.g. Zhang et al., 2005;Christopher, 2011;Feng and Christopher, 2014;Sundström et al., 2015). Although these studies focused on averages are useful, they lack the high temporal resolution needed to observe important details on the changes of the radiative balance due to the short residence time of aerosols in the atmosphere. ...
... In this product, the higher-resolution MODIS data such as scene identifications and cloud and aerosol properties are averaged over the larger CERES footprint using point spread functions [Smith, 1994]. Our group has experience in such measurement-based analysis for clear sky ARE of aerosols estimations over oceans [Zhang et al., 2005a and over land [Patadia et al., 2008;Feng and Christopher, 2014]. ...
The elevated layers of absorbing smoke aerosols from western African (e.g. Gabon, and Congo) biomass burning activities have been frequently observed above low level stratocumulus clouds off the African coast, which presents an excellent natural laboratory for studying the effects of aerosols above clouds (AAC) on regional energy balance in tropical and sub-tropical environments. Using spatially and temporally collocated Moderate Resolution Imaging Spectroradiometer (MODIS), Ozone Monitoring Instrument (OMI), and Clouds and the Earth's Radiant Energy System (CERES) data sets, the top-of-atmosphere (TOA) shortwave Aerosol Direct Shortwave Radiative Effects (ARE) of absorbing aerosols above low-level water clouds in the Southeast Atlantic Ocean was examined in this study. The regional averaged instantaneous ARE has been estimated to be 36.7±20.5 Wm−2 (regional mean ± standard deviation) along with a mean positive OMI Aerosol Index (AI) at 1.3 in August 2006 based on multi-sensors measurements. The highest magnitude of instantaneous ARE can even reach 138.2 Wm−2. We assess that the 660nm Cloud Optical Depth (COD) values of 8–12 is the critical value above (below) which aerosol absorption(scattering) effect dominates and further produces positive (negative) ARE values. The results further show that ARE values are more sensitive to aerosols above lower COD values than cases for higher COD values. This is among the first studies to provide quantitative estimates of shortwave ARE due to AAC events from an observational perspective.
... This technique (CERES + MODIS) has also been widely applied to evaluate the mean DARF over a time period (usually 2-3 months) in several other regions (e.g. Zhang et al., 2005;Christopher, 2011;Feng and Christopher, 2014;Sundström et al., 2015). Although these studies focused on averages are useful, they lack the high temporal resolution needed to observe important details on the changes of the radiative balance due to the short residence time of aerosols in the atmosphere. ...
A new methodology was developed for obtaining daily retrievals of the direct radiative forcing of aerosols (24h-DARF) at the top of the atmosphere (TOA) using satellite remote sensing. Simultaneous CERES (Clouds and Earth's Radiant Energy System) shortwave flux at the top of the atmosphere and MODIS (Moderate Resolution Spectroradiometer) aerosol optical depth (AOD) retrievals were used. To analyse the impact of forest smoke on the radiation balance, this methodology was applied over the Amazonia during the peak of the biomass burning season from 2000 to 2009. To assess the spatial distribution of the DARF, background smoke-free scenes were selected. The fluxes at the TOA under clean conditions (Fcl) were estimated as a function of the illumination geometry (θ0) for each 0.5° × 0.5° grid cell. The instantaneous DARF was obtained as the difference between the clean (Fcl (θ0)) and the polluted flux at the TOA measured by CERES in each cell (Fpol (θ0)). The radiative transfer code SBDART (Santa Barbara DISORT Radiative Transfer model) was used to expand instantaneous DARFs to 24 h averages. This new methodology was applied to assess the DARF both at high temporal resolution and over a large area in Amazonia. The spatial distribution shows that the mean 24h-DARF can be as high as −30 W m−2 over some regions. The temporal variability of the 24h-DARF along the biomass burning season was also studied and showed large intraseasonal and interannual variability. We showed that our methodology considerably reduces statistical sources of uncertainties in the estimate of the DARF, when compared to previous approaches. DARF assessments using the new methodology agree well with ground-based measurements and radiative transfer models. This demonstrates the robustness of the new proposed methodology for assessing the radiative forcing for biomass burning aerosols. To our knowledge, this is the first time that satellite remote sensing assessments of the DARF have been compared with ground-based DARF estimates.
The visible infrared imaging radiometer suite (VIIRS) environmental data record aerosol product (VIIRS_EDR) and the Aqua-moderate resolution imaging spectroradiometer (MYD04) collection 6 (C6) aerosol optical depth (AOD) products are validated against the Cimel sun-photometer (CE318) AOD measurements during different air quality conditions over the Yangtze river basin (YRB) from 2 May 2012 to 31 December 2016. For VIIRS_EDR, the AOD observations are obtained from the scientific data set (SDS) "aerosol optical depth at 550 nm" at 6 km resolution, and for Aqua-MODIS, the AOD observations are obtained from the SDS "image optical depth land and ocean" at 3 km (DT3K) and 10 km (DT10K) resolutions, "deep blue aerosol optical depth 550 land" at 10 km resolution (DB10K), and "AOD 550 dark target deep blue combined" at 10 km resolution (DTB10K). Results show that the high-quality (QF = 3) DTB10K performs the best against the CE318 AOD observations, along with a higher R (0.85) and more retrievals within the expected error (EE) ± (0.05 + 15%) (55%). Besides, there is a 10% overestimation, but the positive bias does not exhibit obvious seasonal variations. Similarly, the DT3K and DT10K products overestimate AOD retrievals by 23% and 15%, respectively, all over the year, but the positive biases become larger in spring and summer. For the DB10K AOD retrievals, there is an overestimation (underestimation) in autumn and winter (spring and summer). Compared to the Aqua-MODIS AOD products, the VIIRS_EDR AOD retrievals are less correlated (R = 0.73) and only 44% of the retrievals fall within EE. Meanwhile, the VIIRS_EDR shows larger bias than the Aqua-MODIS C6 retrievals, and tends to overestimate AOD retrievals in summer and underestimate in winter. Additionally, there is an underestimation for the VIIRS_EDR AOD retrievals over the regions during high aerosol loadings. These indicate that the VIIRS_EDR retrieval algorithm needs to be improved in further applications over the YRB.
In this study, a quantitative assessment of clear‐sky aerosol direct radiative effects (ADREs) at the surface over the Yangtze River Basin (YRB), China was conducted based on Mesoscale Atmospheric Global Irradiance Code (MAGIC) radiation code (direct method) and ground‐based measurements (indirect method). For the direct method, The ADRE at the surface was calculated using MAGIC radiation code with Moderate Resolution Imaging Spectroradiometer (MODIS) and interim ECMWF Re‐Analysis (ERA‐INTERIM) integrated water vapour as input. Results showed that the multi‐year average ADRE ranged from −39.29 to −1.95 W/m² with mean values of −18.19 ± 1.44 W/m². High ADRE (larger than −34 W/m²) occurred in Sichuan Basin, Central Hubei Province and Yangtze River Delta. The seasonal ADRE was high in spring (−23.05 ± 13.48 W/m²) and low in winter (−14.24 ± 8.51 W/m²). Generally, an increasing trend (0.27 W/m² per year) of ADRE was found from 2001 to 2010 and a decreasing trend (−0.72 W/m² per year) from 2011 to 2015. The water vapour radiative effect (WVRE) was calculated using MAGIC radiation code and the multi‐year average value was −52.59 ± 1.45 W/m², which indicated the attenuation of clear‐sky radiation in YRB due to water vapour was stronger than the aerosols. However, the trends of water vapour effects were 0.03 W/m² per year during 2001–2010 and −0.37 W/m² per year during 2011–2015, which were much smaller than the trends of ADRE. For the indirect method, the clear‐sky ADRE at sites was calculated using the solar radiation observations. The variations of annual average ADRE obtained by above methods gave good match. An obvious increasing trend (−0.71 W/m² per year) of aerosol radiative cooling effect was found in Central Hubei Province from 2001 to 2010 and an obvious decreasing trend (3.23 W/m² per year) in Sichuan Basin from 2011 to 2015. The determination coefficient (R²) between ADRE trends and clear‐sky solar radiation trends is 0.874, indicating that the trends of clear‐sky solar radiation were mainly determined by the trends of ADRE.
The spatio-temporal characteristics of aerosol loading over South China from 2001 to 2016 were investigated using aerosol optical depth (AOD) from the Moderate Resolution Imaging Spectroradiometer (MODIS) and NO2 from the Ozone Monitoring Instrument (OMI). AOD values were high in the central part and low in the southeast and northwest parts of South China. High AOD (larger than 0.7) were found in the Pearl River Delta, Nanning, and Hanoi (Vietnam). The seasonal average AOD was high in spring (approximately 0.7) and low in winter (approximately 0.4). Generally, an increasing trend of AOD was found from 2001 to 2004 and a decreasing trend from 2004 to 2016 in the continent due to the change in pollutant discharging, which was verified by annual NO2 data. Furthermore, the aerosol radiative effect (ARE) was calculated using the Mesoscale Atmospheric Global Irradiance Code (MAGIC) and MODIS AOD time series. The spatial distribution and temporal variation of ARE at surface showed similar patterns to AOD, with high values occurring in the Pearl River Delta (−39 W/m²), Hanoi (−36 W/m²), and Nanning (−30 W/m²). From 2001 to 2016, ARE at surface in South China decreased by approximately 4 W/m² with the highest value (−24.75 W/m²) occurring in 2007.
Open-burning fires play an important role in the Earth's climate system. In addition to contributing a substantial fraction of global emissions of carbon dioxide, they are also a major source of atmospheric aerosols such as organic carbon, black carbon, and sulphate. These "fire aerosols" can influence the climate via both direct and indirect radiative effects. In this study, we investigate these radiative effects and the hydrological fast response using the Community Atmosphere Model version 5 (CAM5). Emissions of fire aerosols exert a global mean net radiative effect of −1.0 W m−2, dominated by the cloud shortwave response to organic carbon aerosol. The net radiative effect is particularly strong over boreal regions. By comparing simulations using inter-annually varying versus inter-annually invariant emissions, we find that ignoring the inter-annual variability of the emissions can lead to systematic overestimation of the strength of the net radiative effect of the fire aerosols. Globally, the overestimation is +23 % (−0.2 W m−2). Regionally, the overestimation can be substantially larger. For example, over Australia and New Zealand the overestimation is +58 % (−1.2 W m−2), while over Boreal Asia the overestimation is +43 % (−1.9 W m−2). The systematic overestimation of the net radiative effect of the fire aerosols is likely due to the non-linear influence of aerosols on clouds. However, ignoring inter-annual variability in the emissions does not appear to significantly impact the hydrological fast response. In order to improve understanding of the climate system, we need to more accurately quantify the effects of aerosols, taking into account important characteristics such as inter-annual variability.
atmos-chem-phys-discuss.net/14/31515/2014/ doi:10.5194/acpd-14-31515-2014 © Author(s) 2014. CC Attribution 3.0 License. This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract A new methodology was developed for obtaining daily retrievals of the direct radia-tive forcing of aerosols (24h-DARF) at the top of the atmosphere (TOA) using satellite remote sensing. For that, simultaneous CERES (Clouds and Earth's Radiant Energy System) shortwave flux at the top of the atmosphere (TOA) and MODIS (Moderate
The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard both NASA's Terra and Aqua satellites is making near-global daily observations of the earth in a wide spectral range (0.41-15 m). These measurements are used to derive spectral aerosol optical thickness and aerosol size parameters over both land and ocean. The aerosol products available over land include aerosol optical thickness at three visible wavelengths, a measure of the fraction of aerosol optical thickness attributed to the fine mode, and several derived parameters including reflected spectral solar flux at the top of the atmosphere. Over the ocean, the aerosol optical thickness is provided in seven wavelengths from 0.47 to 2.13 m. In addition, quantitative aerosol size information includes effective radius of the aerosol and quantitative fraction of optical thickness attributed to the fine mode. Spectral irradiance contributed by the aerosol, mass concentration, and number of cloud condensation nuclei round out the list of available aerosol products over the ocean. The spectral optical thickness and effective radius of the aerosol over the ocean are validated by comparison with two years of Aerosol Robotic Network (AERONET) data gleaned from 132 AERONET stations. Eight thousand MODIS aerosol retrievals collocated with AERONET measurements confirm that one standard deviation of MODIS optical thickness retrievals fall within the predicted uncertainty of 0.03 0.05 over ocean and 0.05 0.15 over land. Two hundred and seventy-one MODIS aerosol retrievals collocated with AERONET inversions at island and coastal sites suggest that one standard deviation of MODIS effective radius retrievals falls within r eff 0.11 m. The accuracy of the MODIS retrievals suggests that the product can be used to help narrow the uncertainties associated with aerosol radiative forcing of global climate.
Southern Asia is one of the most heavily populated regions of the world. Biofuel and biomass burning play a disproportionately large role in the emissions of most key pollutant gases and aerosols there, in contrast to much of the rest of the Northern Hemisphere, where fossil fuel burning and industrial processes tend to dominate. This results in polluted air masses which are enriched in carbon-containing aerosols, carbon monoxide, and hydrocarbons. The outflow and long-distance transport of these polluted air masses is characterized by three distinct seasonal circulation patterns: the winter monsoon, the summer monsoon, and the monsoon transition periods. During winter, the near-surface flow is mostly northeasterly, and the regional pollution forms a thick haze layer in the lower troposphere which spreads out over millions of square km between southern Asia and the Intertropical Convergence Zone (ITCZ), located several degrees south of the equator over the Indian Ocean during this period. During summer, the heavy monsoon rains effectively remove soluble gases and aerosols. Less soluble species, on the other hand, are lifted to the upper troposphere in deep convective clouds, and are then transported away from the region by strong upper tropospheric winds, particularly towards northern Africa and the Mediterranean in the tropical easterly jet. Part of the pollution can reach the tropical tropopause layer, the gateway to the stratosphere. During the monsoon transition periods, the flow across the Indian Ocean is primarily zonal with the trade winds, and strong pollution plumes originating from both southeastern Asia and from Africa spread across the central Indian Ocean. This paper provides a review of the current state of knowledge based on the many observational and modeling studies over the last decades that have examined the southern Asian atmospheric pollutant outflow and its large scale effects.
Aerosols affect the Earth's energy budget directly by scattering and absorbing radiation and indirectly by acting as cloud condensation nuclei and, thereby, affecting cloud properties. However, large uncertainties exist in current estimates of aerosol forcing because of incomplete knowledge concerning the distribution and the physical and chemical properties of aerosols as well as aerosol-cloud interactions. In recent years, a great deal of effort has gone into improving measurements and datasets. It is thus feasible to shift the estimates of aerosol forcing from largely model-based to increasingly measurement-based. Our goal is to assess current observational capabilities and identify uncertainties in the aerosol direct forcing through comparisons of different methods with independent sources of uncertainties. Here we assess the aerosol optical depth (tau), direct radiative effect (DRE) by natural and anthropogenic aerosols, and direct climate forcing (DCF) by anthropogenic aerosols, focusing on satellite and ground-based measurements supplemented by global chemical transport model CTM) simulations. The multi-spectral MODIS measures global distributions of aerosol optical depth (tau) on a daily scale, with a high accuracy of +/- 0.03 +/- 0.05 tau over ocean. The annual average tau is about 0.14 over global ocean, of which about 21% +/- 7% is contributed by human activities, as estimated by MODIS fine-mode fraction. The multi-angle MISR derives an annual average AOD of 0.23 over global land with an uncertainty of similar to 20% or +/- 0.05. These high-accuracy aerosol products and broadband flux measurements from CERES make it feasible to obtain observational constraints for the aerosol direct effect, especially over global the ocean. A number of measurement-based approaches estimate the clear-sky DRE ( on solar radiation) at the top-of-atmosphere (TOA) to be about - 5.5 +/- 0.2 W m(-2) ( median +/- standard error from various methods) over the global ocean. Accounting for thin cirrus contamination of the satellite derived aerosol field will reduce the TOA DRE to -5.0 W m(-2). Because of a lack of measurements of aerosol absorption and difficulty in characterizing land surface reflection, estimates of DRE over land and at the ocean surface are currently realized through a combination of satellite retrievals, surface measurements, and model simulations, and are less constrained. Over the oceans the surface DRE is estimated to be - 8.8 +/- 0.7W m(-2). Over land, an integration of satellite retrievals and model simulations derives a DRE of - 4.9 +/- 0.7W m(-2) and - 11.8 +/- 1.9W m(-2) at the TOA and surface, respectively. CTM simulations derive a wide range of DRE estimates that on average are smaller than the measurement-based DRE by about 30 - 40%, even after accounting for thin cirrus and cloud contamination. A number of issues remain. Current estimates of the aerosol direct effect over land are poorly constrained. Uncertainties of DRE estimates are also larger on regional scales than on a global scale and large discrepancies exist between different approaches. The characterization of aerosol absorption and vertical distribution remains challenging. The aerosol direct effect in the thermal infrared range and in cloudy conditions remains relatively unexplored and quite uncertain, because of a lack of global systematic aerosol vertical profile measurements. A coordinated research strategy needs to be developed for integration and assimilation of satellite measurements into models to constrain model simulations. Enhanced measurement capabilities in the next few years and high-level scientific cooperation will further advance our knowledge.
This paper addresses the Amazonian shortwave radiative budget over cloud-free conditions after considering three aspects of deforestation: (i) the emission of aerosols from biomass burning due to forest fires; (ii) changes in surface albedo after deforestation; and (iii) modifications in the column water vapour amount over deforested areas. Simultaneous Clouds and the Earth's Radiant Energy System (CERES) shortwave fluxes and aerosol optical depth (AOD) retrievals from the Moderate Resolution Imaging SpectroRadiometer (MODIS) were analysed during the peak of the biomass burning seasons (August and September) from 2000 to 2009. A discrete-ordinate radiative transfer (DISORT) code was used to extend instantaneous remote sensing radiative forcing assessments into 24-h averages.
The mean direct radiative forcing of aerosols at the top of the atmosphere (TOA) during the biomass burning season for the 10-yr studied period was −5.6 ± 1.7 W m<sup>−2</sup>. Furthermore, the spatial distribution of the direct radiative forcing of aerosols over Amazonia was obtained for the biomass burning season of each year. It was observed that for high AOD (larger than 1 at 550 nm) the maximum daily direct aerosol radiative forcing at the TOA may be as high as −20 W m<sup>−2</sup> locally. The surface reflectance plays a major role in the aerosol direct radiative effect. The study of the effects of biomass burning aerosols over different surface types shows that the direct radiative forcing is systematically more negative over forest than over savannah-like covered areas. Values of −15.7 ± 2.4 W m<sup>−2</sup>τ<sub>550 nm</sub> and −9.3 ± 1.7 W m<sup>−2</sup>τ<sub>550 nm</sub> were calculated for the mean daily aerosol forcing efficiencies over forest and savannah-like vegetation respectively. The overall mean annual land use change radiative forcing due to deforestation over the state of Rondônia, Brazil, was determined as −7.3 ± 0.9 W m<sup>−2</sup>. Biomass burning aerosols impact the radiative budget for approximately two months per year, whereas the surface albedo impact is observed throughout the year. Because of this difference, the estimated impact in the Amazonian annual radiative budget due to surface albedo-change is approximately 6 times higher than the impact due to aerosol emissions. The influence of atmospheric water vapour content in the radiative budget was also studied using AERONET column water vapour. It was observed that column water vapour is on average smaller by about 0.35 cm (around 10% of the total column water vapour) over deforested areas compared to forested areas. Our results indicate that this drying contributes to an increase in the shortwave radiative forcing, which varies from 0.4 W m<sup>−2</sup> to 1.2 W m<sup>−2</sup> depending on the column water vapour content before deforestation.
The large radiative forcing values presented in this study point out that deforestation could have strong implications in convection, cloud development and the ratio of direct to diffuse radiation, which impacts carbon uptake by the forest.
Nine months of the Clouds and the Earth's Radiant Energy System (CERES)/Tropical Rainfall Measuring Mission (TRMM) broadband fluxes combined with the TRMM visible infrared scanner (VIRS) high-resolution imager measurements are used to estimate the daily average direct radiative effect of aerosols for clear-sky conditions over the tropical oceans. On average, aerosols have a cooling effect over the Tropics of 4.6 ± 1 W m-2. The magnitude is 2 W m-2 smaller over the southern tropical oceans than it is over northern tropical oceans. The direct effect derived from CERES is highly correlated with coincident aerosol optical depth () retrievals inferred from 0.63-m VIRS radiances (correlation coefficient of 0.96). The slope of the regression line is 32 W m-2 -1 over the equatorial Pacific Ocean, but changes both regionally and seasonally, depending on the aerosol characteristics. Near sources of biomass burning and desert dust, the aerosol direct effect reaches 25 to 30 W m-2. The direct effect from CERES also shows a dependence on wind speed. The reason for this dependence is unclear-it may be due to increased aerosol (e.g., sea-salt or aerosol transport) or increased surface reflection (e.g., due to whitecaps). The uncertainty in the tropical average direct effect from CERES is 1 W m-2 (20%) due mainly to cloud contamination, the radiance-to-flux conversion, and instrument calibration. By comparison, uncertainties in the direct effect from the Earth Radiation Budget Experiment (ERBE) and CERES `ERBE-like' products are a factor of 3-5 times larger.
Seven years analysis of clear-sky aerosol direct radiative forcing is presented for two locations in the Amazon Region heavily impacted by biomass burning emissions. During the dry season, the monthly average direct forcing at the top of the atmosphere, deduced from measurements, varied from -5 to -12 W/m2 and at the surface from -21 to -74 W/m2, with the difference associated with absorption of sunlight by the smoke aerosol layer. The spatial distribution of the forcings over Amazonia showed that they affect an area over 1.2-2.6 million square kilometers.
The smoke aerosol plume produced by intense forest fires over Indonesia during September–November 1997 provided a unique opportunity to investigate the radiative impact of aerosols on sea surface temperature (SST), primarily due to its episodic nature and occurrence of high aerosol optical depth (>0.8) for ∼2 months over the east equatorial Indian Ocean (EIO). The aerosol radiative effect was well discernible in SST because the month-to-month variation in shortwave aerosol direct cooling (ADC) was so large while the corresponding variations in the other governing factors such as surface wind and sea surface upwelling were less significant during this period, when the net cloud radiative forcing had a positive anomaly at the surface. The present study clearly shows that from September to October 1997, ADC at surface has increased by more than −46 Wm−2 over east EIO resulting a decrease in SST by >1°C. This might have provided a positive feedback to the Indian Ocean dipole influencing the meteorology of the south Asian region.
Using broadband shortwave radiance measurements from the Clouds and Earth Radiant Energy System (CERES) sensors onboard the Terra and Aqua satellites, empirical angular distribution models (EADM) are constructed for smoke aerosols. These EADMs are constructed for smoke aerosols emitted during the biomass burning season (August-October), in South America. All available years (2000-2008) of both rotating azimuth plane and cross-track radiance data from CERES have been utilized. Aerosol scenes are identified using coincident aerosol optical thickness retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS). The new EADMs are used to estimate top-of-atmosphere (TOA) shortwave flux for cloud free scenes. The CERES clearly shows the angular dependence of shortwave radiance on smoke aerosol optical thickness. A comparison of TOA shortwave fluxes estimated from the new smoke EADM with the existing CERES TOA shortwave fluxes shows that the CERES fluxes are higher (lower) for aerosol optical thickness less (greater) than 0.3, largely due to the use of aerosol optical thickness in characterizing the new EADMs developed in this study. Empirical ADMs for smoke aerosols over land are developed for the first time in this study, and our results demonstrate that large amounts of aerosols emitted during biomass burning activities contribute to the anisotropy of the radiance distribution at the TOA. Therefore, it is imperative to include aerosol information in the angular distribution models, especially now that more than a 10 year record of Terra data is available.
We analyze aircraft observations obtained during INTEX-A (1 July to 14 August 2004) to examine the summertime influence of Asian pollution in the free troposphere over North America. By applying correlation analysis and principal component analysis (PCA) to the observations between 6 and 12 km, we find dominant influences from recent convection and lightning (13% of observations), Asia (7%), the lower stratosphere (7%), and boreal forest fires (2%), with the remaining 71% assigned to background. Asian air masses are marked by high levels of CO, O3, HCN, PAN, C2H2, C6H6, methanol, and SO42–. The partitioning of NOy species in the Asian plumes is dominated by PAN (∼600 pptv), with varying NOx/HNO3 ratios in individual plumes, consistent with individual transit times of 3–9 days. Export of Asian pollution occurred in warm conveyor belts of midlatitude cyclones, deep convection, and in typhoons. Compared to Asian outflow measurements during spring, INTEX-A observations display lower levels of anthropogenic pollutants (CO, C3H8, C2H6, C6H6) due to shorter summer lifetimes; higher levels of biogenic tracers (methanol and acetone) because of a more active biosphere; and higher levels of PAN, NOx, HNO3, and O3 reflecting active photochemistry, possibly enhanced by efficient NOy export and lightning. The high ΔO3/ΔCO ratio (0.76 mol/mol) in Asian plumes during INTEX-A is due to strong photochemical production and, in some cases, mixing with stratospheric air along isentropic surfaces. The GEOS-Chem global model captures the timing and location of the Asian plumes. However, it significantly underestimates the magnitude of observed enhancements in CO, O3, PAN and NOx.
During the ACE-Asia campaign in March-May 2001, in situ measurements of
aerosol optical properties were made from multiple airborne and land- or
ship-based platforms. Using a suite of direct interplatform comparisons
and a campaign-wide statistical comparison, we test the precision of
these measurements, and we determine whether the platforms sampled
similar aerosol. Data included in the study are from the National Center
for Atmospheric Research C-130 aircraft; the CIRPAS Twin Otter aircraft;
the National Oceanographic and Atmospheric Administration (NOAA) ship
R.V. Ronald H. Brown; and the Gosan surface station on Jeju Island,
located off the southern tip of South Korea. Comparisons were made of
total and submicron light scattering at 450, 550, and 700 nm; total and
submicron absorption at 550 nm; the Ångström exponent; single
scatter albedo of the total aerosol, submicron and supermicron aerosol
at 550 nm; hemispheric backscatter fraction at 550 nm; and light
scattering hygroscopic growth at 550 nm. For the campaign-wide
comparison, the data are broken down by light scattering fine mode
fraction since the aerosol in the ACE-Asia study region were a variable
mix of pollution, dust, and sea salt. Finally, we calculate how the
observed uncertainties in the aerosol optical properties propagate to
uncertainties in top-of-atmosphere radiative forcing. Single scatter
albedo showed excellent agreement among all platforms other than the
Twin Otter, with discrepancies generally <0.02. These data sets
combine to give campaign-wide values of single scatter albedo of 0.885
± 0.023 for the submicron aerosol (i.e. pollution) and 0.957
± 0.031 for the supermicron aerosol (which, for these data, was
predominantly dust). The data also indicated that, as expected, the Low
Turbulent Inlet on the C-130 produced enhanced concentrations of coarse
mode aerosol. There also may have been significant coarse mode particle
losses on the other platforms. These effects combined to produce
generally lower fine mode fractions and Ångström exponents on
the C-130 than on the other platforms. Large discrepancies in
hemispheric backscatter fraction and light scattering hygroscopic growth
were observed in both the side-by-side and statistical comparisons. We
are not able to explain these differences, though possible causes are
discussed. Studies of the TSI, Inc. nephelometer backscatter measurement
and of the two methods used here to measure hygroscopic growth are
needed to clarify the source of these observed discrepancies. A better
understanding of the effects of nonsphericity on hemispheric backscatter
fraction is also needed.
The radiative parameters of mineral aerosols are strongly dependent on particle size. Therefore explicit modeling of particle size distribution is needed to calculate the radiative effects and the climate impact of mineral dust. We describe a parameterization of the global mineral aerosol size distribution in a transport model using eight size classes between 0.1 and 10 mum. The model prescribes the initial size distribution using soil texture data and aerosol size measurements close to the ground. During transport, the size distribution changes as larger particles settle out faster than smaller particles. Results of Mie scattering calculations of radiative parameters (extinction efficiency, single scattering albedo, asymmetry parameter) of mineral dust are shown at wavelengths between 0.3 and 30 mum for effective particle radii between 0.1 and 10 mum. Also included are radiative properties (reflection, absorption, transmission) calculated for a dust optical thickness of 0.1. Preliminary studies with the Goddard Institute for Space Studies (GISS) general circulation model (GCM), using two particle size modes, show regional changes in radiative flux at the top of the atmosphere as large as +15 Wm-2 at solar and +5 Wm-2 at thermal wavelengths in the annual mean, indicating that dust forcing is an important factor in the global radiation budget.
Using in situ measurements of aerosol optical properties and ground-based measurements of aerosol optical thickness (tau (s)) during the Smoke, Clouds and Radiation-Brazil (SCAR-B) experiment, a four-stream broadband radiative transfer model is used to estimate the downward shortwave irradiance (DSWI) and top-of-atmosphere (TOA) shortwave aerosol radiative forcing (SWARF) in cloud-free regions dominated by smoke from biomass burning in Brazil. The calculated DSWI values are compared with broadband pyranometer measurements made at the surface. The results show that, for two days when near-coincident measurements of single-scattering albedo omega (0) and tau (s) are available, the root-mean-square errors between the measured and calculated DSWI for daytime data are within 30 W m(-2). For five days during SCAR-B, however, when assumptions about omega (0) have to be made and also when tau (s) was significantly higher, the differences can be as large as 100 W m(-2). At TOA, the SWARF per unit optical thickness ranges from -20 to -60 W m(-2) over four major ecosystems in South America. The results show that tau (s) and omega (0) are the two most important parameters that affect DSWI calculations. For SWARF values, surface albedos also play an important role. It is shown that omega (0) must be known within 0.05 and tau (s) at 0.55 mum must be known to within 0.1 to estimate DSWI to within 20 W m(-2). The methodology described in this paper could serve as a potential strategy for determining DSWI values in the presence of aerosols. The wavelength dependence of tau (s) and omega (0) over the entire shortwave spectrum is needed to improve radiative transfer calculations. If global retrievals of DSWI and SWARF from satellite measurements are to be performed in the presence of biomass-burning aerosols on a routine basis, a concerted effort should be made to develop methodologies for estimating omega (0) and tau (s) from satellite and ground-based measurements.
Using 30 days of half-hourly, high temporal resolution GOES 8 imager data and radiative transfer calculations, dust aerosol optical thickness (AOT) was retrieved over the Atlantic Ocean (14degreesN similar to 26degreesN, 73degreesW-63degreesW) during the Puerto Rico Dust Experiment ( PRIDE). Dust aerosol size distributions and complex index of refraction inferred from ground-based measurements (1.53-0.0015i at 0.55 mum), which were used in Mie calculations and a plane-parallel discrete ordinate radiative transfer model (DISORT) to compute look up tables for AOT retrievals. Using a combination of spectral, spatial, and temporal tests, a dust detection algorithm was developed from the GOES 8 imager data. The degradation of the signal response relative to the prelaunched calibration of the GOES 8 visible channel was 39% in July 2000 and the GOES 8 AOT detection limit was estimated to be 0.04 in AOT (0.67 mum). The satellite-retrieved AOT were then compared with AOT values derived from ground-based Sun photometer (SP) sites. The comparison showed that GOES 8 retrieved AOT are in good agreement with the SP derived values, with linear correlation coefficient of 0.91 and 0.80 for the two sites. The GOES 8 monthly mean 0.67 mm AOT (0.19 +/- 0.13, 0.22 +/- 0.12) over the two SP sites matched the monthly mean SPAOT values (0.23 +/- 0.13, 0.22 +/- 0.10). The linear correlation between the GOES 8 retrieved AOT and the aircraft derived values from particle probe data and airborne Sun photometer AATS-6 measurements were 0.88 and 0.83, respectively. Besides the uncertainties from the nonspherical effect of dust aerosols, sensitivity studies showed that the uncertainties (Deltatau) of the GOES 8 retrieved AOT values were mainly from the uncertainties due to the imaginary part of refractive index (Deltatau = +/-0.05) and surface reflectance [Deltatau = +/-(0.02 similar to 0.04)]. This paper demonstrates the application of geostationary satellites to detect and retrieve dust AOT even at low to moderate AOTs. The GOES 8 imager with high temporal resolutions also captures aerosol diurnal variation in this study that can further reduce the uncertainties in the current aerosol forcing estimations caused by the high temporal variations of AOT, thereby playing a complementary role with global AOT retrievals from polar orbiting satellites.
Southeast Asia (SEA) hosts one of the most complex aerosol systems in the world, with convoluted meteorological scales, sharp geographic and socioeconomic features, high biological productivity, mixtures of a wide range of atmospheric pollutants, and likely a significant susceptibility to global climate change. This physical complexity of SEA is coupled with one of the world's most challenging environments for both in situ and remote sensing observation. The 7-Southeast Asian Studies (7SEAS) program was formed to facilitate interdisciplinary research into the integrated SEA aerosol environment via grass roots style collaboration. In support of the early 7SEAS program and the affiliated Southeast Asia Composition, Cloud, Climate Coupling Regional Study (SEAC(4)RS), this review was created to outline the network of connections linking aerosol particles in SEA with meteorology, climate and the total earth system. In this review, we focus on and repeatedly link back to our primary data source: satellite aerosol remote sensing and associated observability issues. We begin with a brief rationale for the program, outlining key aerosol impacts and, comparing their magnitudes to the relative uncertainty of observations. We then discuss aspects of SEA's physical, socio-economic and biological geography relevant to meteorology and observability issues associated with clouds and precipitation. We show that not only does SEA pose significant observability challenges for aerosol particles, but for clouds and precipitation as well. With the fundamentals of the environment outlined, we explore SEA's most studied aerosol issue: biomass burning. We summarize research on bulk aerosol properties for SEA, including a short synopsis of recent AERONET observations. We describe long range transport patterns. Finally, considerable attention is paid to satellite aerosol observability issues, with a face value comparison of common aerosol products in the region including passive and active aerosol products as well as fluxes. We show that satellite data products diverge greatly due to a host of known artifacts. These artifacts have important implications for how research is conducted, and care must be taken when using satellite products to study aerosol problems. The paper ends with a discussion of how the community can approach this complex and important environment. Published by Elsevier B.V.
A series of tables and charts is presented from which the atmospheric transmittance between any two points in the terrestrial atmosphere can be determined. This material is based on a set of five atmospheric models ranging from tropical to arctic and two aerosol models. A selected set of laser frequencies has been defined for which monochromatic transmittance values have been given. For low resolution transmittance predcition, a series of charts has been drawn providing the capability for predicting transmittance at a resolution of 20 wave-numbers. Separate sections are included on scattered solar radiation, infrared emission, refractive effects, and attenuation by cloud and fog. This third edition differs from the others in that the low resolution spectral curves for the uniformly mixed gases and in the short wavelength region for water vapor have been revised, providing some overall improvement in accuracy; and more importantly, an appendix has been added providing model data and equivalent sea level path data for the U.S. Standard Atmosphere, 1962.
A series of tables and charts is presented from which the atmospheric transmittance between any two points in the terrestrial atmosphere can be determined. This material is based on a set of five atmospheric models ranging from tropical to arctic and two aerosol models. A selected set of laser frequencies has been defined for which monochromatic transmittance values have been given. For low resolution transmittance prediction, a series of charts has been drawn providing the capability for predicting transmittance at a resolution of 20 wavenumbers. Separate sections are included on scattered solar radiation, infrared emission, refractive effects, and attenuation by cloud and fog.
Direct aerosol radiative forcing (DARF) remains a leading contributor to climate prediction uncertainty. To monitor the spatially and temporally varying global atmospheric aerosol load, satellite remote sensing is required. Despite major advances in observing aerosol amount, type, and distribution from space, satellite data alone cannot provide enough quantitative detail, especially about aerosol microphysical properties, to effect the required improvement in estimates of DARF and the anthropogenic component of DARF. However, the combination of space-based and targeted suborbital measurements, when used to constrain climate models, represents an achievable next step likely to provide the needed advancement.
Long-term measurements by the AERONET program of spectral aerosol optical depth, precipitable water, and derived Angstrom exponent were analyzed and compiled into an aerosol optical properties climatology. Quality assured monthly means are presented and described for 9 primary sites and 21 additional multiyear sites with distinct aerosol regimes representing tropical biomass burning, boreal forests, midlatitude humid climates, midlatitude dry climates, oceanic sites, desert sites, and background sites. Seasonal trends for each of these nine sites are discussed and climatic averages presented.
1] Direct and semidirect radiative effects of biomass burning aerosols from southern African fires during July–October are investigated using 20 year runs of the Community Atmospheric Model (CAM) coupled to a slab ocean model. Aerosol optical depth is constrained using observations in clear skies from Moderate Resolution Imaging Spectroradiometer (MODIS) and for aerosol layers above clouds from Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO). Over the ocean, where the aerosol layers are primarily located above cloud, negative top of atmosphere (TOA) semidirect radiative effects associated with increased low cloud cover dominate over a weaker positive all‐sky direct radiative effect (DRE). In contrast, over the land where the aerosols are often below or within cloud layers, reductions in cloud liquid water path (LWP) lead to a positive semidirect radiative effect that dominates over a near‐zero DRE. Over the ocean, the cloud response can be understood as a response to increased lower tropospheric stability (LTS) which is caused both by radiative heating in overlying layers and surface cooling in response to direct aerosol forcing. The marine cloud changes are robust to changes in the cloud parameterization (removal of the hard‐wired dependence of clouds on LTS), suggesting that they are physically realistic. Over land, decreased LWP is consistent with weaker convection driven by increased static stability. Over the entire region the overall TOA radiative effect from the biomass burning aerosols is almost zero due to opposing effects over the land and ocean. However, the surface forcing is strongly negative, which leads to a reduction in precipitation and also a reduction in sensible heat flux. The former is primarily realized through reductions in convective precipitation on both the southern and northern flanks of the convective precipitation region spanning the equatorial rain forest and the Intertropical Convergence Zone (ITCZ) in the southern Sahel. The changes are consistent with the low‐level aerosol‐forced cooling pattern. The results highlight the importance of semidirect radiative effects and precipitation responses for determining the climatic effects of aerosols in the African region.
1] Since first light in early 2000, operational global quantitative retrievals of aerosol properties over land have been made from Moderate Resolution Imaging Spectroradiometer (MODIS) observed spectral reflectance. These products have been continuously evaluated and validated, and opportunities for improvements have been noted. We have replaced the surface reflectance assumptions, the set of aerosol model optical properties, and the aerosol lookup table (LUT). This second-generation operational algorithm performs a simultaneous inversion of two visible (0.47 and 0.66 mm) and one shortwave-IR (2.12 mm) channel, making use of the coarse aerosol information content contained in the 2.12 mm channel. Inversion of the three channels yields three nearly independent parameters, the aerosol optical depth (t) at 0.55 mm, the nondust or fine weighting (h), and the surface reflectance at 2.12 mm. Retrievals of small-magnitude negative t values (down to À0.05) are considered valid, thus balancing the statistics of t in near zero t conditions. Preliminary validation of this algorithm shows much improved retrievals of t, where the MODIS/Aerosol Robotic Network t (at 0.55 mm) regression has an equation of: y = 1.01x + 0.03, R = 0.90. Global mean t for the test bed is reduced from $0.28 to $0.21.
1] Using spatially and temporally collocated multispectral, multiangle and broadband data sets from the Terra satellite, the role of biomass burning (BB) smoke particles on cloud-free top of atmosphere (TOA) direct shortwave aerosol radiative forcing (SWARF) is examined. A 5-year analysis during the peak biomass burning months of August and September is presented over South America (0°–20°S and 45°W–65°W). Our results indicate that over 5 years, the TOA diurnally averaged SWARF (DSWARF) from the Clouds and the Earth's Radiant Energy System (CERES) scanner ranges between À5.2 Wm À2 and À9.4 Wm À2 with a mean value of À7.6 Wm À2 and an estimated uncertainty of ±1.4 Wm À2 . The corresponding Multi Angle Spectroradiometer (MISR) aerosol optical thickness (AOT at 0.558 mm) ranged from 0.15 to 0.36 with a mean value of 0.24. The estimated mean TOA aerosol radiative forcing efficiency (E t) is À44.2 Wm À2 t À1 and is in good agreement with previous studies. We also examined the beta versions of the MISR data products such as the angstrom exponent (AE) and fraction of AOT in different size bins to assess the role of BB aerosol particle properties on SWARF. Our analysis indicates that the MISR retrieved 5 year mean AE is 1.54. Contribution to total AOT from small, medium and large particles is 66%, 16% and 18% respectively. This is the first multiyear assessment of SWARF for biomass burning aerosol particles using satellite observations alone and should serve as a useful constraint for numerical modeling simulations that estimate SWARF. (2008), A Multisensor satellite-based assessment of biomass burning aerosol radiative impact over Amazonia, J. Geophys. Res., 113, D12214, doi:10.1029/2007JD009486.
1] The direct radiative forcing by aerosols over the Indian Ocean region is simulated for the Indian Ocean Experiment (INDOEX) Intensive Field Phase during Spring 1999. The forcing is calculated for the top-of-atmosphere (TOA), surface, and atmosphere by differencing shortwave fluxes computed with and without aerosols. The calculation includes the effects of sea-salt, sulfate, carbonaceous, and soil-dust aerosols. The aerosol distributions are obtained from a global aerosol simulation including assimilation of satellite retrievals of aerosol optical thickness (AOT). The time-dependent, three-dimensional aerosol distributions are derived with a chemical transport model driven with meteorological analyses for this period. The surface albedos are obtained from a land-surface model forced with an identical meteorological analysis and satellite-derived rainfall and insolation. These calculations are consistent with in situ observations of the surface insolation over the central Indian Ocean and with satellite measurements of the reflected shortwave radiation. The calculations show that the surface insolation under clear skies is reduced by as much as 40 W/m 2 over the Indian subcontinent by natural and anthropogenic aerosols. This reduction in insolation is accompanied by an increase in shortwave flux absorbed in the atmosphere by 25 W/m 2 . The inclusion of clouds in the calculations changes the direct effect by less than 2 W/m 2 over the Indian subcontinent, although the reduction is much larger over China. The magnitude of the difference between all-sky and clear-sky forcing is quite sensitive to the three-dimensional spatial relationship between the aerosol and cloud fields, and other estimates of the difference for the INDOEX Intensive Field Phase are as large as 5 W/m 2 .
1] Using 10 months of collocated Clouds and the Earth's Radiant Energy System (CERES) scanner and Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol and cloud data from Terra, we provide estimates of the shortwave aerosol direct radiative forcing (SWARF) and its uncertainties over the cloud-free global oceans. Newly developed aerosol angular distribution models (ADMs) (Zhang et al., 2005), specifically for different sea surface conditions and aerosol types, are used for inverting the CERES observed radiances to shortwave fluxes while accounting for the effect of aerosol optical properties on the anisotropy of the top of atmosphere (TOA) shortwave radiation fields. The spatial and seasonal distributions of SWARF are presented, and the MODIS retrieved aerosol optical depth (t 0.55) and the independently derived SWARF show a high degree of correlation and can be estimated using the equation SWARF = 0.05 À 74.6 t 0.55 + 18.2 t 0.55 2 W m À2 (t 0.55 < 0.8). The instantaneous TOA SWARF from Terra overpass time is À6.4 ± 2.6 W m À2 for cloud-free oceans. Accounting for sample biases and diurnal averaging, we estimate the SWARF over cloud-free oceans to be À5.3 ± 1.7 W m À2 , consistent with previous studies. Our study is an independent measurement-based assessment of cloud-free aerosol radiative forcing that could be used as a validation tool for numerical modeling studies. (2005), Shortwave aerosol radiative forcing over cloud-free oceans from Terra: 2. Seasonal and global distributions, J. Geophys. Res., 110, D10S24, doi:10.1029/2004JD005009.
Using multiple satellite instruments, we demonstrate a new empirical method for obtaining shortwave (SW) aerosol angular distribution models (ADMs) over cloud-free oceans. We use nearly a year's worth of multispectral Moderate Resolution Imaging Spectroradiometer (MODIS) data to obtain aerosol properties within a Clouds and Earth Radiant Energy System (CERES) footprint and Special Sensor Microwave Imager (SSM/I) data to obtain near surface wind speed. The new aerosol ADMs are built as functions of near-surface ocean wind speed and MODIS aerosol optical depth at 0.55 μm (τ0.55). Among the new features are ADMs as a function of the ratio of fine mode to total aerosol optical depth (η), which contains infortnation on aerosol type, and the combination of the CERES rotation azimuth plane scan mode CERES data and MODIS aerosol products to characterize aerosol properties over glint regions. The instantaneous aerosol forcing efficiencies (SW flux per unit optical depth at τ0.55) are 80.5, 63.1, and 73.0 Wm-2, derived using the Earth Radiation Budget Experiment (ERBE), Tropical Rainfall Measuring Mission (TRMM), and the current Terra ADMs, respectively. This study highlights the necessity for building empirical aerosol ADMs as a function of wind speed, τ 0.55 and η, and gives examples of newly constructed aerosol ADMs over cloud-free oceans. We conclude that an overall uncertainty of 10% will be introduced in the derived SW aerosol direct forcing over cloud-free oceans if the ADMs are constructed without considering aerosol darkening effect over glint regions and aerosol brightening over nonglint regions (like ERBE ADMs) or the variations in aerosol properties (like TRMM ADMs). In a companion paper (Zhang et al., 2005), these new ADMs are used to calculate the shortwave aerosol radiative forcing over the global oceans.
1] Performance of the Multiangle Imaging Spectroradiometer (MISR) early postlaunch aerosol optical thickness (AOT) retrieval algorithm is assessed quantitatively over land and ocean by comparison with a 2-year measurement record of globally distributed AERONET Sun photometers. There are sufficient coincident observations to stratify the data set by season and expected aerosol type. In addition to reporting uncertainty envelopes, we identify trends and outliers, and investigate their likely causes, with the aim of refining algorithm performance. Overall, about 2/3 of the MISR-retrieved AOT values fall within [0.05 or 20% Â AOT] of Aerosol Robotic Network (AERONET). More than a third are within [0.03 or 10% Â AOT]. Correlation coefficients are highest for maritime stations ($0.9), and lowest for dusty sites (more than $0.7). Retrieved spectral slopes closely match Sun photometer values for Biomass burning and continental aerosol types. Detailed comparisons suggest that adding to the algorithm climatology more absorbing spherical particles, more realistic dust analogs, and a richer selection of multimodal aerosol mixtures would reduce the remaining discrepancies for MISR retrievals over land; in addition, refining instrument low-light-level calibration could reduce or eliminate a small but systematic offset in maritime AOT values. On the basis of cases for which current particle models are representative, a second-generation MISR aerosol retrieval algorithm incorporating these improvements could provide AOT accuracy unprecedented for a spaceborne technique. (2005), Multiangle Imaging Spectroradiometer (MISR) global aerosol optical depth validation based on 2 years of coincident Aerosol Robotic Network (AERONET) observations, J. Geophys. Res., 110, D10S04, doi:10.1029/2004JD004706.
Recently, global biomass-burning research has grown from what was primarily a climate field to include a vibrant air quality observation and forecasting community. While new fire monitoring systems are based on fundamental Earth Systems Science (ESS) research, adaptation to the forecasting problem requires special procedures and simplifications. In a reciprocal manner, results from the air quality research community have contributed scientifically to basic ESS. To help exploit research and data products in climate, ESS, meteorology and air quality biomass burning communities, the joint Navy, NASA, NOAA, and University Fire Locating and Modeling of Burning Emissions (FLAMBE) program was formed in 1999. Based upon the operational NOAA/NESDIS Wild-Fire Automated Biomass Burning Algorithm (WF_ABBA) and the near real time University of Maryland/NASA MODIS fire products coupled to the operational Navy Aerosol Analysis and Prediction System (NAAPS) transport model, FLAMBE is a combined ESS and operational system to study the nature of smoke particle emissions and transport at the synoptic to continental scales. In this paper, we give an overview of the FLAMBE system and present fundamental metrics on emission and transport patterns of smoke. We also provide examples on regional smoke transport mechanisms and demonstrate that MODIS optical depth data assimilation provides significant variance reduction against observations. Using FLAMBE as a context, throughout the paper we discuss observability issues surrounding the biomass burning system and the subsequent propagation of error. Current indications are that regional particle emissions estimates still have integer factors of uncertainty.
With the launch of NASA's Terra satellite and the MODerate Resolution Imaging Spectroradiometer (MODIS), operational Bidirectional Reflectance Distribution Function (BRDF) and albedo products are now being made available to the scientific community. The MODIS BRDF/Albedo algorithm makes use of a semiempirical kernel-driven bidirectional reflectance model and multidate, multispectral data to provide global 1-km gridded and tiled products of the land surface every 16 days. These products include directional hemispherical albedo (black-sky albedo), bihemispherical albedo (white-sky albedo), Nadir BRDF-Adjusted surface Reflectances (NBAR), model parameters describing the BRDF, and extensive quality assurance information. The algorithm has been consistently producing albedo and NBAR for the public since July 2000. Initial evaluations indicate a stable BRDF/Albedo Product, where, for example, the spatial and temporal progression of phenological characteristics is easily detected in the NBAR and albedo results. These early beta and provisional products auger well for the routine production of stable MODIS-derived BRDF parameters, nadir reflectances, and albedos for use by the global observation and modeling communities.
During 20 April to 21 May 2003, large amounts of smoke aerosols from
Central American Biomass Burning (CABB) fires were transported to
southeastern United States. Using a coupled aerosol, radiation, and
meteorology model built upon the heritage of the Regional Atmospheric
Modeling System (RAMS) with new capabilities called the Assimilation and
Radiation Online Modeling of Aerosols (AROMA), this paper, the second of
a two-part series, investigates smoke radiative impact on the regional
surface energy budget, temperature and relevant boundary layer
processes. Comparisons with limited ground-based observations and MODIS
aerosol optical thickness (AOT) showed that model consistently simulated
the smoke AOT and smoke radiative impacts on the 2 m air temperature
(2mT) and downward shortwave irradiance (DSWI). Over 30 days the 24-hour
mean smoke AOT was 0.18 (at 0.55 μm) near the smoke source region
(Yucatan Peninsula and southern Mexico), and 0.09 in downwind region
(e.g., southern Texas), both showing a diurnal variation of 24%. Maximum
AOT occurred during late afternoon and minimum during early morning in
smoke source region. The smoke radiative effects were dominant mostly
during the daytime and resulted in the decrease of DSWI, sensible heat
and latent heat by 22.5 Wm-2, 6.2 Wm-2, and 6.2
Wm-2, respectively, near the source region, in contrast to
15.8 Wm-2, 4.7 Wm-2, and 7.9 Wm-2,
respectively, in downwind regions. Both maximum and minimum 2mT were
decreased, and the overall diurnal temperature range (DTR) was reduced
by 0.31°C and 0.26°C in the smoke source and downwind regions,
respectively. The smoke absorption of solar radiation increased the
lapse rate by 0.1-0.5 K/day in the planetary boundary layer (PBL), thus
warming the air over the ocean surface. However, over the land surface
where the coupling between the lower PBL and the cooler land surface is
strong, such warming only occurred in the upper PBL and is amendable to
the diurnal variation of smoke emission. The simulation numerically
verifies the smoke self-trapping feedback mechanism proposed by Robock
(1988), where the increase of the atmospheric stability in the PBL
caused by the smoke radiative effects further traps more smoke aerosols
in the lower PBL. Such feedbacks, when coupled with favorable synoptic
systems, may have important implications for air quality modeling and
hydrological processes.
Using one year (December 2006–November 2007) of the Moderate Resolution Imaging SpectroRadiometer (MODIS), Multi-Angle Imaging SpectroRadiometer (MISR), and Clouds and the Earth's Radiant Energy System (CERES) data sets from NASA's Terra satellite, we assess the spatial and temporal distributions of aerosol properties (Aerosol Optical Depth, Fine Mode Fraction, and Single Scattering albedo) in the Southeast Asian region (SEA, 10°S–25°N, 90°E–150°E). We also provide a quantitative evaluation of regional cloud-free diurnally averaged shortwave aerosol radiative effects (SWARE) at the top of atmosphere (TOA) over both land and ocean. Our results indicate that the diurnally averaged shortwave radiative effects at the TOA over land and ocean are (− 6.4 ± 1.2 W m− 2) and (− 5.9 ± 1.3 W m− 2) with corresponding 550 nm aerosol optical depths of 0.27 ± 0.24 and 0.12 ± 0.10. Fine aerosol particles (< 0.6 μm) dominate the continental areas during the whole study period, which represents large fractions of biomass burning aerosols and anthropogenic pollutant aerosols. Our results also indicate that the monthly averaged cloud cover fractions over this region are above 60%. Therefore, further sampling of aerosols underneath these cloud layers is needed in future field campaigns.
A heavy dust storm that occurred in Northwestern China during April 24–30 2010 was studied using observational data along with the Fu–Liou radiative transfer model. The dust storm was originated from Mongolia and affected more than 10 provinces of China. Our results showed that dust aerosols have a significant impact on the radiative energy budget. At Minqin (102.959°E, 38.607°N) and Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL, 104.13°E, 35.95°N) sites, the net radiative forcing (RF) ranged from 5.93 to 35.7 W m−2 at the top of the atmosphere (TOA), −6.3 to −30.94 W m−2 at surface, and 16.77 to 56.32 W m−2 in the atmosphere. The maximum net radiative heating rate reached 5.89 K at 1.5 km on 24 April at the Minqin station and 4.46 K at 2.2 km on 29 April at the SACOL station. Our results also indicated that the radiative effect of dust aerosols is affected by aerosol optical depth (AOD), single-scattering albedo (SSA) and surface albedo. Modifications of the radiative energy budget by dust aerosols may have important implications for atmospheric circulation and regional climate.
During the last decade, the feedback between El Niño and biomass burning caused the Indonesia's forest fire aerosols to be the second most significant source of anthropogenic aerosol over the tropical Indian Ocean after the South Asian Haze. In this paper, the estimates of the radiative forcing during the 1997 Indonesia's forest fire have been obtained by integrating satellite derived aerosol optical depths and cloud cover with in-situ observations of single scattering albedo and a Monte-Carlo Aerosol-Cloud radiation model. The haze reduced the seasonal average solar radiation absorbed by the equatorial Indian ocean by as much as 30 to 60 W m-2 during September to November 1997, and increased the atmospheric solar heating by as much as 50% to 100% within the first 3 kilometers. The radiative forcing at the top of the atmosphere (TOA) was in the range of 5 to 15 W m-2 under cloudy skies. The significance of such large radiative flux changes to the tropical ocean-atmosphere heat budget and climate needs to be examined with climate models.
Hourly Geostationary Operational Environmental Satellite-8 (GOES-8)imager data (1344-1944 UTC) from 20 July-31 August 1998 were used to study the daytime variation of shortwave direct radiative forcing (SWARF) of smoke aerosols over biomass burning regions in South America (48-168S, 518-658W). Vicarious calibration procedures were used to adjust the GOES visible channel reflectance values for the degradation in signal response. Using Mie theory and discrete ordinate radiative transfer (DISORT) calculations, smoke aerosol optical thickness (AOT) was estimated at 0.67 mm. The GOES-retrieved AOT was then compared against ground-based AOT retrieved values. Using the retrieved GOES-8 AOT, a four-stream broadband radiative transfer model was used to compute shortwave fluxes for smoke aerosols at the top of the atmosphere (TOA). The daytime variation of smoke AOT and SWARF was examined for the study area. For selected days, the Clouds and the Earth's Radiant Energy System (CERES) TOA shortwave (SW) fluxes are compared against the model-derived SW fluxes. Results of this study show that the GOES-derived AOT is in excellent agreement with Aerosol Robotic Network (AERONET)-derived AOT values with linear correlation coefficient of 0.97. The TOA CERES-estimated SW fluxes compare well with the model-calculated SW fluxes with linear correlation coefficient of 0.94. For August 1998 the daytime diurnally averaged AOT and SWARF for the study area is 0.63 6 0.39 and 245.8 6 18.8 W m22, respectively. This is among the first studies to estimate the daytime diurnal variation of SWARF of smoke aerosols using satellite data.
Most assessments of the direct climate forcing (DCF) of anthropogenic aerosols are from numerical simulations. However, recent advances in remote sensing techniques allow the separation of fine mode aerosols (anthropogenic aerosol is mostly fine aerosol) from coarse mode aerosols (largely marine and dust, which are mostly natural) from satellite data such as the Moderate Resolution Imaging SpectroRadiometer (MODIS). Here, by combining MODIS narrowband measurements with broadband radiative flux data sets from the Clouds and the Earth's Radiant Energy System (CERES), we provide a measurement-based assessment of the global direct climate forcing (DCF) of anthropogenic aerosols at the top of atmosphere (TOA) only for cloud free oceans. The mean TOA DCF of anthropogenic aerosols over cloud-free oceans [60N-60S] is -1.4 +/- 0.9 Wm-2, which is in excellent agreement (mean value of -1.4 Wm-2) with a recent observational study by Kaufman et al. [2005].
The El Niño event of 1997-1998 caused a severe reduction of rainfall in Indonesia that promoted the spread of forest fires, leading to a pervasive haze in the region. Here we use fire coverage data from the 1997 World Fire Atlas with a review of other available data and literature to estimate the distribution of particulate emissions from August to November 1997 and the particle size and radiative properties. Our preferred estimate of the total particulate emissions is approximately 41 Tg. The emissions have been used to drive an atmospheric model to simulate the distribution of the haze and its direct radiative effect, with and without allowing for the effects of the smoke on the atmospheric evolution. Model diagnostics of the aerosol and its radiative impact are compared with measurements and output from other models. Large decreases in the incident solar flux at the surface are obtained in the region. The simulated global mean shortwave radiative forcing at the top of the atmosphere, averaged over the 4 months, is -0.32 Wm-2. The accuracy of this calculation is discussed, and the importance of the Indonesian fires in particular and of biomass burning in general is assessed.
The difference between the top of atmosphere shortwave clear sky (cloud and aerosol free, SWCLR) and aerosol sky radiative fluxes is known as direct radiative effect (DRE) for all aerosols or Direct Climate Forcing (DCF) for anthropogenic aerosols. There are several methods for calculating SWCLR including satellite-based methods and radiative transfer approaches. Since uncertainties in SWCLR can propagate into errors in DRE or DCF, we assess the SWCLR estimates over the global oceans using three approaches and quantify the differences among these methods both as a function of space and season. Our results indicate that the more commonly used intercept (73.4±3.6) and radiative transfer methods (74.7±4.0 Wm−2) are in close agreement to within±1.3 Wm−2. Values of SWCLR are provided as a function of space and season that can be used by other studies that require such values or as a source of validation. We further recommend that research studies report the methods and assumptions used to estimate SWCLR to facilitate easier intercomparisons among methods.
Daily aerosol optical thickness (AOT) at 0.55 μm over the desert regions is needed as a source of validation for numerical models such as the United Kingdom's Numerical Weather Prediction Unified Model. We examined the relationship between monthly mean ultraviolet (UV) absorbing aerosol index (AI) from the Ozone Monitoring Instrument (OMI) that is available on a daily basis with the Multiangle Imaging Spectroradiometer (MISR) AOT that is available once every nine days over North Africa. We then developed spatiotemporal AI-AOT relationships on a monthly mean basis that can be used to convert the daily AI to AOT during months when dust concentrations are high (June–August) to compare against months when a mixture of dust and biomass burning aerosols are present (January–March). We further examined the AOT data from the ground to validate our methods and results. While previous studies have examined the Total Ozone Mapping Spectrometer AI with limited ground-based Sun photometer data, our study extends this to the OMI over 2 years (2005–2006) and for the entire north African region (20°W–40°E and 0–30°N). Our results confirm that the MISR is an excellent sensor for retrieving AOT over desert regions. Comparisons between MISR and Aerosol Robotic Network (AERONET) data over multiple locations indicate that the linear correlation coefficient is 0.89. The AI-AOT relationship is region specific and is robust over locations where AI and AOT are high during June–August especially when the predominant aerosol is dust. This relationship breaks down closer to the equator when aerosol loading is small especially when biomass-burning aerosols are prevalent during January–March. Our analysis indicates that the estimated AOT (EAOT) from the AI-AOT relationship is within 28% of the MISR AOT for optical depths between 0.2 and 2.0 with large uncertainties (75%) for smaller optical depths (
Rainwater samples are collected from islands and seagoing cruises in the Yellow Sea and the East China Sea to determine concentration and to estimate deposition flux for dissolved silicate (DSi), together with other nutrients (e.g., nitrogen) by spectrophotometry. Concentrations of dissolved silicate in rainwater show considerable variation in time and space, with 0.5–15 μM at the Yellow Sea and the East China Sea. The (NO3− + NH4+)/DSi ratio in rainwater changes over up to 2 orders of magnitude, with high levels in winter and low in autumn. Levels of DSi falls with higher rainfall, and a positive relationship can be established with amount of total particles in rainwater samples. The deposition of DSi via rainfall is 0.97 × 109 mol yr−1 for the Yellow Sea and 2.0 × 109 mol yr−1 for the East China Sea, respectively, 20–40% higher than the atmospheric dry deposition estimated by previous work but shows critical importance to the marine ecosystems at low trophic level in terms of comparison of chemical budget and new production. The extrapolation to the global ocean indicates that aeolian inputs (i.e., wet and dry depositions) of dissolved silicate can be on the order of 0.9 × 1012 mol yr−1, with ∼20% of this amount is distributed in the continental margin.
Many Southeast Asian countries have been constantly plagued by recurring smoke haze episodes as a result of traditional slash-and-burn practices in agricultural areas to clear crop lands or uncontrolled forest fires. However, our current knowledge on the physiochemical and optical properties of ambient aerosols associated with regional haze phenomenon is still fairly limited. Therefore a comprehensive field study was carried out in Singapore from March 2001 to March 2002 under varying weather conditions to gain a better understanding of the characteristics. The physical (size distribution of mass and number concentrations), chemical (mass concentrations of chemical components: 14 ions, 24 metals, elemental carbon (EC) and organic carbon (OC)), and optical (light absorption (bap) and scattering (bsp) by particles) characteristics of ambient aerosol particles were investigated. The results are reported separately for clear and hazy days by categorizing the days as clear or hazy on the basis of visibility data. It was observed that the average concentrations of PM2.5 and most chemical components increased approximately by a factor of 2 on hazy days. Backward air trajectories together with the hot spot distributions in the region indicated that the degradation in Singapore's air quality on hazy days was attributable to large-scale forest fires in Sumatra. This visibility degradation was quantitatively measured on the basis of the light absorption and scattering by particles. As expected, scattering rather than absorption controlled atmospheric visibility, and PM2.5 particles present on hazy days were more efficient at scattering light than those found on clear days.
Using spatially and temporally collocated data sets from the Clouds and
Earth's Radiant Energy System (CERES) and Moderate-Resolution Imaging
Spectroradiometer (MODIS) instruments on the Terra satellite, a new
strategy is presented for studying the Shortwave Aerosol Radiative
Forcing (SWARF) over the global oceans. Using collocated data, for
September 2000, the global averaged optical thickness
(τ0.55) for cloud-free CERES pixels is 0.07 with a SWARF
of -6 Wm-2. The τ0.55 and SWARF values derived
from two independent instruments are in excellent agreement with the
following relationship: SWARF = 0.35 -105.34τ0.55
+61.47τ0.552 (0 <= τ0.55
<= 0.7) Wm-2. The synergistic use of the MODIS and CERES
data sets can be used to provide independent estimates of SWARF, and can
be used as a validation tool for studies that attempt to model the role
of aerosols on climate.
We examine diurnally averaged radiative forcing by mineral dust aerosols in shortwave and longwave spectral regions using a one-dimensional column radiation model. At the top of the atmosphere (TOA), net (shortwave plus longwave) dust radiative forcing can be positive (heating) or negative (cooling) depending on values of key variables. We derive an analytical expression for the critical single-scattering albedo at which forcing changes sign for an atmosphere containing both cloud and aerosol layers. At the surface, net dust forcing can be positive or negative under clear-sky conditions, whereas it is always cooling in the presence of a low-level stratus cloud. Longwave radiative forcing is essentially zero when clouds are present. We also study the sensitivity of dust diurnally averaged forcing to the imaginary part of refractive index (k), height of the dust layer, dust particle size, and dust optical depth. These variables play different roles as follows: (1) under both clear- and cloudy sky conditions, net TOA forcing is more sensitive to k than net surface forcing; (2) clear-sky longwave forcing and cloudy-sky TOA shortwave forcing are very sensitive to the altitude of the dust layer; although clear-sky shortwave forcing is not sensitive to it; (3) clear-sky shortwave forcing is much more sensitive to particle size than cloudy-sky shortwave forcing; longwave forcing is not sensitive to particle size; and (4) all forcings are sensitive to optical depth except cloudy-sky longwave forcing.
To understand climatic implications of aerosols over global oceans, the aerosol optical properties retrieved from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) are analyzed, and the effects of the aerosols on the earth's radiation budgets [aerosol radiative forcing (ARF)] are computed using a radiative transfer model. It is found that the distribution of the SeaWiFS-retrieved aerosol optical thickness is distinctively zonal. The maximum in the equatorial region coincides with the intertropical convergence zone, and the maximum in the Southern Hemispheric high latitudes coincides with the region of prevailing westerlies. The minimum aerosol optical thickness is found in the subtropical high pressure regions, especially in the Southern Hemisphere. These zonal patterns clearly demonstrate the influence of atmospheric circulation on the oceanic aerosol distribution.Over global oceans, aerosols reduce the annual-mean net downward solar flux by 5.4 W m2 at the top of the atmosphere, and by 5.9 W m2 at the surface. The largest ARF is found in the tropical Atlantic, Arabian Sea, Bay of Bengal, the coastal regions of Southeast and East Asia, and the Southern Hemispheric high latitudes. During the period of the big Indonesian fires (September-December 1997), the cooling due to aerosols is more than 10 W m2 at the top of the atmosphere, and more than 25 W m2 at the surface in the vicinity of Indonesia. The atmosphere receives extra solar radiation by more than 15 W m2 over a large area. These large changes in radiative fluxes are expected to have enhanced the atmospheric stability, weakened the atmospheric circulation, and augmented the drought condition during that period. It would be very instructive to simulate the regional climatic impact of the big Indonesian fires during the 1987-88 El Niño event using a general circulation model.
Results from a temporally intensive, limited area, radiative transfer model experiment are on-line for investigating the vertical profile of shortwave and longwave radiative fluxes from the surface to the top of the atmosphere (TOA). The CERES/ARM/GEWEX Experiment (CAGEX) Version 1 provides a record of fluxes that have been computed with a radiative transfer code; the atmospheric sounding, aerosol, and satellite-retrieved cloud data on which the computations have been based; and surface-based measurements of radiative fluxes and cloud properties from ARM for comparison. The computed broadband fluxes at TOA show considerable scatter when compared with fluxes that are inferred empirically from narrowband operational satellite data. At the surface, LW fluxes computed with an alternate sounding dataset compare well with pyrgeometer measurements. In agreement with earlier work, the authors find that the calculated SW surface insolation is larger than the measurements for clear-sky and total-sky conditions. This experiment has been developed to test retrievals of radiative fluxes and the associated forcings by clouds, aerosols, surface properties, and water vapor. Collaboration is sought; the goal is to extend the domain of meteorological conditions for which such retrievals can be done accurately. CAGEX Version 1 covers April 1994. Subsequent versions will (a) at first span the same limited geographical area with data from October 1995, (b) then expand to cover a significant fraction of the GEWEX Continental-Scale International Project region for April 1996 through September 1996, and (c) eventually be used in a more advanced form to validate CERES.
Smoke plumes originating from vegetation fires engulf Southeast Asia and East Equatorial Indian Ocean (EEIO) during October-November period in almost all the El Niño years. For the first time, observations of the vertical profiles of aerosol extinction coefficient using the Cloud Aerosol Lidar Pathfinder Satellite Observation (CALIPSO), along with the spatial distribution of aerosol optical depth (AOD) derived from NOAA-18-AVHRR provided an opportunity to study the 3-dimensional structure of the plume that spread over an area of ∼1 million km2 (0.5
1] Using measured and derived aerosol properties from the Puerto Rico Dust Experiment (PRIDE), a four-stream broadband radiative transfer model is used to calculate the downward shortwave irradiance (DSWI) at the surface and the shortwave irradiance at the top of atmosphere (TOA). The results of the calculated DSWI are compared against pyranometer measurements from the Surface Measurements For Atmospheric Radiative Transfer (SMART) instrument suite at Roosevelt Road (18.20°N, 65.60°W). Using aerosol optical thickness retrievals from half-hourly geostationary satellite data (GOES 8 imager), the diurnal short wave aerosol forcing (SWARF) of dust aerosols both at the surface and TOA are calculated for the entire study area (14°N $ 26°N, 61°W $ 73°W). For selected days, the Clouds and the Earth Radiant Energy System (CERES) TOA shortwave irradiance values from Terra are compared with radiative transfer calculations. Wang et al. [2003] show that the satellite derived aerosol optical thickness is in excellent agreement with Aerosol Robotic Network (AERONET) values. Results of this study show that the calculated direct, diffuse and total DSWI are in excellent agreement with the corresponding SMART values with biases of 1.8%, À3.3% and 0.5% respectively, indicating that dust aerosols are well characterized in the radiative transfer model. This is well within the measured uncertainties (1.3%) and the model uncertainties (5%). The monthly mean value and standard deviation of aerosol optical thickness at 670 nm (AOT670) during PRIDE are 0.26 ± 0.13, and the corresponding monthly mean daytime SWARF values are À12.34 ± 9.62 W m À2 at TOA and À18.13 ± 15.81 W m À2 at the surface, respectively. Our results also show that if diurnal changes in aerosol optical thickness are not considered, it leads to uncertainties in SWARF of 4 W m À2 at the surface and 2 W m À2 at the TOA. The CERES TOA short wave irradiance underestimates calculated values by about 10 W m À2 mainly due problems in misclassification of aerosols and lack of aerosol angular dependence models (ADMs) in the current CERES algorithms. This study is among the first to demonstrate the potential of the GOES 8 imagers in retrieving aerosol optical thickness and estimating the daytime diurnal SWARF of dust, both at the TOA and surface, in low to moderate dust loading regions over the oceans.
a b s t r a c t Aerosol radiative forcing at the Earth's surface is estimated by simultaneous measurements of broad-band global fluxes and aerosol optical depths (AODs) over an urban location in western India during 2008. AODs at 0.5 mm show large seasonal variability with higher values (0.52) during monsoon. Higher AOD during monsoon is mainly due to increase in relative humidity which overwhelms the effects of wet removal of aerosols and addition of sea salt. Forcing efficiency for monsoon season is found to be lower as compared to other seasons. Surface aerosol radiative forcing has the highest value of À44 Wm À2 during monsoon. The forcing values are similar for model independent and semi model dependent methods. Single scattering albedo (SSA) is higher in monsoon followed by pre-monsoon, winter and lowest in post-monsoon. SSA derived from ground-based measurements (aethalometer and nephelometer) is lower than columnar SSA estimated from Goddard Chemistry Aerosol Radiation and Transport (GOCART) model and derived from Ozone Monitoring Instrument (OMI). Lower radiative forcing at the surface is attributed to higher SSA during pre-monsoon. Model estimated surface forcing using ground-based SSA is about two times higher than observed forcing for different seasons except for monsoon. However, model estimated forcing using columnar SSA agrees well with observations except in monsoon. The differences during monsoon are probably caused by overestimation of SSA from GOCART and OMI. The study reveals that a small change in SSA can lead to significant change in aerosol forcing.
Spatiotemporal heterogeneity in aerosol radiative forcing and heating rate have been studied over Bay of Bengal and Arabian Sea during premonsoon (March–May 2006) using aerosol optical depth (AOD), total mass, aerosol chemical composition, and radiative transfer model. Mean 0.5 μm AOD over Bay of Bengal and Arabian Sea is 0.36 and 0.25, respectively. Water-soluble aerosols, sea salt, and mineral dust constitute ∼98% of total aerosol mass while black carbon aerosols contribute ≤2% over the two oceanic regions. Sensitivity tests reveal that (1) curvature effect in AOD spectra has insignificant impact in modifying the aerosol radiative forcing and heating rate and (2) the net Earth-atmosphere energy content shows minor differences when aerosol vertical profiles are used. Over Bay of Bengal the average aerosol forcing is estimated to be −12.0, −22.4, and 10.4 W m−2 at the top of the atmosphere (TOA), at the surface (SFC), and in atmosphere (ATM), respectively. The average aerosol radiative forcing is less negative over Arabian Sea and is −10.5, −15.8, and 5.3 W m−2 at TOA, SFC, and ATM, respectively. Aerosol radiative forcing decreases in magnitude from north to south over Bay of Bengal whereas an opposite trend is noteworthy over Arabian Sea. The average atmospheric heating rate over Bay of Bengal is ∼0.3 K/d, a factor of 2 higher than that over Arabian Sea. Furthermore, ATM warming and associated heating rate are the lowest compared to earlier results as scattering aerosols are dominant during premonsoon (March–May). These results have implications to the assessment of regional and seasonal climate impacts.