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Retrievals of column PWV for the year 2000 data from the two MFRSRs at SGPs Central Facility (C1 and E13).
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The Multi-Filter Rotating Shadowband Radiometer (MFRSR) measures direct and diffuse irradiances in the visible and near IR spectral range. In addition to characteristics of atmospheric aerosols, MFRSR data also allow retrieval of precipitable water vapor (PWV) column amounts, which are determined from the direct normal irradiances in the 940 nm spe...
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... SGP's CF m at noon varies from 0.5 in winter to 0.97 % 1 in summer. As it is seen from Figure 4, the typical PWV column at this site also has a seasonal variation with a winter minimum of 0.5 cm and a summer maximum of around 4 cm. Using equation (19) we can estimate that for the instrument characteristics listed above and a calibration error with c = 0.03, the error induced in the PWV column will be 0.05 cm (10%) in winter and 0.18 cm (4.5%) in summer. ...
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... theory, some error in PWV retrievals by MFRSR may occur at large solar zenith angles because of the angular dependence of instrument's filters spectral responsivity coupled with imperfect performance of the diffusor, which allows change in the angle of incidence of light on the filter surface during the day. However, comparison with normal incidence Sun photometers (CIMEL, AATS-6) shown in Figures 4 and 9 do not show specific deviations in retrieved PWV column amount from MFRSRs at low Sun angles (at least for air masses smaller than 5 used for retrievals), thus, we consider this effect to be negligible. ...
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... in this study we focus on the Central Facility MFRSRs (C1, E13), for which the technical information was available, while using retriev- als from other SGP's EFs in a qualitative manner. Figure 4 shows the PWV retrievals (daily means) from C1 and E13 MFRSRs for the year 2000. A strong summer maximum in PWV column amount is clearly seen. ...
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... To illustrate the feasibility of using MFRSR net- work data for creation of 2D data sets comparable with the MODIS satellite water vapor product, we constructed a spatial distribution of columnar PWV from the MFRSR data obtained on 14 September 2000 at local noon (overpass time of Terra satellite) shown in Figure 14 (right). This distribution is in agreement with the corresponding map of the Terra MODIS PWV product (from NIR channels [Gao and Kaufman, 2003]) shown in Figure 14 (left). ...
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... To illustrate the feasibility of using MFRSR net- work data for creation of 2D data sets comparable with the MODIS satellite water vapor product, we constructed a spatial distribution of columnar PWV from the MFRSR data obtained on 14 September 2000 at local noon (overpass time of Terra satellite) shown in Figure 14 (right). This distribution is in agreement with the corresponding map of the Terra MODIS PWV product (from NIR channels [Gao and Kaufman, 2003]) shown in Figure 14 (left). While lacking small-scale details (some of which are due to cloud contamination, especially in the southern part of the MODIS image), the MFRSR network provides an accurate spatial trend of PWV: increasing column amount from north-west to south-east of the site. ...
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... lacking small-scale details (some of which are due to cloud contamination, especially in the southern part of the MODIS image), the MFRSR network provides an accurate spatial trend of PWV: increasing column amount from north-west to south-east of the site. This agreement is confirmed by the quantitative compar- ison shown in Figure 15 both for the exact MFRSR locations (left) and for all points in Figure 14 plots (right). The former expectedly demonstrates better agreement with MODIS, which values are smaller on average then the MFRSRs' by 0.2 cm with 0.2 cm standard deviation of the differences. ...
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... SGP site, in particular, provides a unique example of a dense regional MFRSR network capable of producing aerosol and PWV data sets character- izing both temporal and spatial variability of these atmo- spheric species. 2D slices of these data sets (for a specific moment of time) can be compared to satellite products (e.g., from MODIS, Figures 14 and 15) with help of a spatial interpolation technique, while evolution of these atmo- spheric fields can be tracked by MFRSR network beyond the time of satellite overpass. This ability is particularly significant for characterization of highly variable WV fields. ...
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... To make a practical estimate of the retrieval accuracy, the PWV column amounts derived from measurements by two SGP's CF MFRSRs (C1 and E13) were compared with each other and with a number of correlative measure- ments both by other solar radiometers (AERONET's CIMEL, AATS-6) and nonsolar instruments (microwave radiometers, GPS receiver). Data from the year 2000 ( Figure 4) were chosen for intercomparisons. Some of these instruments (C1 MWR, AERONET, NPN GPS) routinely provide data throughout the year, while the other (ETL CSR, JPL WVR, and AATS-6) were deployed at SGP's CF during the Water Vapor IOP (WVIOP2000) from 18 September to 8 October 2000. ...
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... It is well known that atmospheric WV is highly variable both temporary and spatially. High temporal vari- ability of PWV column amount is seen in Figure 4, while Figure 14 (right) shows up to 50% differences in value between neighboring MFRSR locations (the average spacing of SGP network is 80 km). Our structure function analysis [cf. ...
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... It is well known that atmospheric WV is highly variable both temporary and spatially. High temporal vari- ability of PWV column amount is seen in Figure 4, while Figure 14 (right) shows up to 50% differences in value between neighboring MFRSR locations (the average spacing of SGP network is 80 km). Our structure function analysis [cf. ...
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... Over the past few decades, various techniques have been developed to measure the PWV content, including radiosondes (RSs), ground-based sun photometers, microwave radiometers, radar observations, and satellite remote sensing (Alexandrov et al., 2009;Gui, Che, Chen, Zeng, Liu, et al., 2017;Niell et al., 2001;Zeng et al., 2019). Among them, the RS is the primary in situ measurement to detect PWV due to its high accuracy. ...
Precipitable water vapor (PWV), despite being the key factor for assessing climate change over the Tibetan Plateau (TP), is undersampled in this region due to the complex terrain and climate conditions therein. The fifth‐generation European Center for Medium‐Range Weather Forecasts Reanalysis (ERA5) and Second Modern‐Era Retrospective Analysis for Research and Applications (MERRA‐2) data sets are the two newly released global reanalyzes that provide hourly PWV products with a high spatiotemporal resolution for water vapor research. The accuracies of these two PWV products on the TP require a comprehensive and cautious assessment. In this study, the hourly PWV values derived from ERA5 and MERRA‐2 are systematically assessed by a comparison with 33 Global Navigation Satellite System (GNSS) stations from 2017 to 2018 over the TP. To improve the reliability of this evaluation, an enhanced Tm model and a PWV vertical correction model, namely, TP‐Tm and TP‐PWVC, respectively, are developed, and both models show better performance when compared against other widely used models. The accuracies of the ERA5‐ and MERRA‐2‐derived PWV at the 33 GNSS stations are very high; ERA5 has the smallest mean bias and root mean square (RMS) error of −0.08 and 1.77 mm, respectively, compared to those of 0.53 and 2.12 mm for MERRA‐2. In addition, the biases and RMS errors show significant regional and seasonal differences. The performance of ERA5 is slightly better than MERRA‐2, mainly due to its higher spatial resolution. The spatial distributions of the PWV derived from the two reanalyzes are consistent with the GNSS‐derived PWV distribution. The diurnal PWV variations derived from ERA5 are in good agreement with the diurnal variation of the GNSS‐derived PWV, whereas significant differences are observed for MERRA‐2 at several stations. These results indicate that ERA5 can provide more reliable hourly PWV estimates over the TP, which can help users further understand the strengths and weaknesses of these two PWV products and promote climate change research.
... The columnar precipitable water content is determined based on the measurements at 940 nm (H 2 O absorption peak) and at 1020 nm (no absorption by water content). The details of the retrieval of water content and the types of errors involved in it can be found in the works of Schmid et al. (2001), Devara et al. (2001), Smirnov et al. (2004 and Alexandrov et al. (2009). The "Almucantar" scenario denotes a series of measurements taken at an elevation angle of the Sun for specific azimuth angles relative to the position of the Sun. ...
The AErosol RObotic NETwork (AERONET) is the most developed
ground-based network for aerosol remote sensing and has been playing a
significant role not only in monitoring air quality for protecting human
health but also in assessing the radiative budget of our planet Earth. In
this paper, we report the direct-Sun and inversion products, comprising of
spectral variation of aerosol optical depth (AOD), associated
Ångström exponent (AE), fine- and coarse-mode aerosol fractions,
aerosol size distribution (ASD), refractive index (RI), asymmetry parameter
(AP), single scattering albedo (SSA), aerosol radiative forcing (ARF) and
columnar concentration of gas constituents such as water vapor (H2O),
obtained from a Cimel Sun–sky radiometer, functioning in Pune, India, under
the AERONET program since October 2004. These long-term measurements carried
out from 2005 to 2015 could serve as an urban aerosol optical long-term
average or climatology. The AOD long-term variations at all wavelengths,
considered in the study, exhibited an increasing trend, implying year-to-year
enhancement in aerosol loading. The mean seasonal variations in AOD from
cloud-free days indicated greater values during the monsoon season, revealing dominance
of hygroscopic aerosol particles over the station. Contribution by different
aerosol types to AOD has also been deduced and discussed, and dominance of
a mixed type of aerosols (44.85 %) found, followed by combination of biomass
burning and urban industrial aerosols (22.57 %) compared to other types
of aerosols during the study period. The long-term datasets, derived aerosol
and trace gas products play a significant role in understanding aerosol climate
forcing, trends and evaluation of regional air pollution and validation of
aerosol transport models over the study region.
