Figure 5 - uploaded by Stuart Ashleigh Young
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A plot of integrated backscatter against infrared emittance for the temperature range shown. Bars represent variability in observed backscatter, -35 to -75 C.
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... More recently, Nee et al. [1998] found subvisual cirrus occurring approximately 50% of the time using a lidar operating at Chang-Li, Taiwan. Using lidar at 3°S in 1993, Platt et al. [1998] also detected subvisual cirrus with visible and infrared optical depths as low as 0.01 in Kavieng, Papua, New Guinea. From near-global observations of optically thin cirrus during the Lidar In-space Technology Experiment, Winker and Trepte [1998] found layers of cirrus occurring in thin sheets near the tropical tropopause with thicknesses between a few hundred meters and one kilometer that were unusually horizontally homogeneous. ...
Subvisual cirrus clouds that are defined as those whose optical
thickness is less than ˜0.3 are found in ˜50% of global
observations. Passive remote-sensing instruments, such as the Moderate
Resolution Imaging Spectroradiometer (MODIS), generally fail to detect
these optically thin clouds. The launch of NASA's Cloud-Aerosol Lidar
and Infrared Pathfinder Satellite Observation (CALIPSO) satellite
provides an unprecedented ability to detect thin cloud layers globally.
Also, the Clouds and the Earth's Radiant Energy System (CERES) provides
accurate measurements of top-of-atmosphere radiation. By using CERES,
MODIS, and CALIPSO measurements in a synergistic manner, a quantitative
assessment of the influence of subvisual clouds on the Earth's shortwave
(SW) radiation is accomplished. The difference between clear-sky
radiation flux and the flux obtained with the presence of subvisual
clouds clearly shows the cooling effect of subvisual clouds in the SW.
The subvisual clouds increase the diurnal mean reflected SW flux by
˜2.5 W m-2. The subvisual clouds' effect on outgoing
longwave radiation is also studied using a radiative-transfer model. The
model results show that a layer of subvisual clouds having optical
thickness of 0.1 can have a warming effect of ˜15 W
m-2. These clouds can also affect the polarization of the
reflected SW radiation and the accuracy of aerosol retrieval with
satellite measurements. This work demonstrates that the study of
subvisual clouds is necessary for an accurate and detailed understanding
of Earth-atmosphere radiation.
... Convective clouds do this directly through detrainment and mixing with ambient air, and may also aid in cirrus formation in the tropopause transition layer (TTL) through convective cloud top radiative cooling or causing the upward movement and expansion of air forced by wind over cumulus towers [Garrett et al., 2006]. The apparently extensive nature of thin tropical cirrus layers has been pointed out using satellites [Prabhakara et al., 1993;Wang et al., 1996;Bourassa et al., 2005], previous spaceborne lidars [Winker and Trepte, 1998;Dessler et al., 2006], and ground-based remote sensing [Platt et al., 1998[Platt et al., , 2002Comstock et al., 2002;Cadet et al., 2003;Iwasaki et al., 2004]. Limited aircraft studies of high tropical cirrus have also been made [Heymsfield, 1986;McFarquhar et al., 2000]. ...
... [6] Much of what we know regarding the importance of deep convection on the formation of tropical cirrus (both directly anvil generated and TTL level) has come, for example, from ground-based or airborne lidar case studies [Platt et al., 1998[Platt et al., , 2002Sassen et al., 2000] and passive satellite measurements combined with isentropic back trajectories to locate cirrus sources [Mace et al., 2006]. Drawbacks with these attempts, however, have to do with the quite limited field experiment locations and campaign periods, and the well-known limitations in inferring threedimensional cloud properties using only passive remote sensors on satellites. ...
... [11] To estimate the cirrus cloud visible optical depth t from the CALIPO data, we use the backward Fernald method with a mean effective lidar (extinction-to-backscatter) ratio of 20 sr. According to ground-based lidar studies, the lidar ratio is dependent on cirrus cloud optical depth as well as cloud temperature [Platt et al., 1998;Chen et al., 2002], but an effective lidar ratio (accounting for the contribution from multiple scattering) of 20 sr represents an acceptable mean value for our purposes. On an individual case basis, errors in t arising from using this simple approach could be as high as a factor of 2À3, but note that we calculate t only to put the cirrus into the broad categories of subvisual (t < $0.03), thin ($0.03 < t < $0.3), and opaque ($0.3 < t < $3.0) cirrus clouds [Sassen and Cho, 1992]. ...
Using a 2-year data set of combined lidar and cloud radar measurements from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat satellites, the occurrence of tropical cirrus and deep convective clouds is studied. The cloud identification algorithm takes advantage of the ability of the radar to probe deep precipitating clouds and of the lidar to sample even subvisual cirrus clouds. Examined are the frequency of occurrence and the geographical distribution of these clouds, and their apparent interconnections. There is a strong apparent diurnal variability in tropical cirrus mainly over land, with significantly more cirrus detected at night compared to day, but no clear diurnal pattern in deep convective activity. CALIPSO daylight signal noise effects do not appear to be not responsible for the diurnal cirrus pattern, because high, thin tropopause transitional layer (TTL) cirrus do not show a clear diurnal effect. Stratifying the global results by estimated visible cloud optical depth τ, we find that most of the planet's subvisual (τ < ∼0.03) cirrus clouds occur in the tropics and are more frequent at night and over ocean; thin (∼0.03 < τ < ∼0.3) cirrus have their highest global frequencies over equatorial landmasses and in the west Pacific region, and are also more frequent at night but occur mainly over land; and opaque (∼0.3 < τ < ∼3.0) cirrus are spread globally and tend to occur during the day over ocean. Although it is unknown which of the several proposed cirrus cloud formation mechanisms are key in the tropics, the close association of cirrus with convective clouds implies that tropical cirrus are linked to deep convective activity, with the likely exception of TTL cirrus clouds.
... Large number of experiments were conducted to investigate and improve our knowledge in different regions of the globe using various techniques employing lidars: Uthe and Russell, 1977;Liou,1986;Sassen et al.,1989;Ansmann et al.,1992;Wang et al., 1996;Platt et al., 1998Platt et al., , 2002Heymsfield et al., 1998;Sassen et al., 2000;Goldfarb et al., 2001;Comstock et al., 2002;Keckhut et al., 2005;Sunilkumar and Parameswaran, 2005;Seifert et al., 2007;Immler and Schrems, 2002;Immler et al., 2008, among others. However, over southern mid-latitudes the vertically and temporally resolved measurements of cirrus clouds properties are still scarce. ...
Cirrus clouds in the upper troposphere and the lower stratosphere have recently drawn much attention due to their important role and impact on the atmospheric radiative balance. Because they are located in the upper troposphere their study requires a high resolution technique not only to detect them but also to characterize their behaviour and evolution. A good dynamic range in lidar backscattering signals is necessary to observe and improve our knowledge of cirrus clouds, and thereof, atmospheric parameters in the troposphere and UT/LS due to their vicinity to the tropopause layer. The lidar system measures, in real time, the evolution of the atmospheric boundary layer, stratospheric aerosols, tropopause height and cirrus clouds evolution.The aim of the work is to present the main properties of cirrus clouds over central Argentina and to monitor tropopause height together with their temporal evolution using a backscatter lidar system located in Buenos Aires (34.6 °S, 58.5 °W). A cirrus clouds detection method was used to analyze a set of 60 diurnal events, during 2001–2005, in order to estimate tropopause height and its temporal evolution, using the top of cirrus clouds present on the upper troposphere as a tropopause tracer. The results derived from lidar show a remarkable good agreement when compared with rawinsonde data, considering values of tropopause height with differences less than or equal to 500 m, depending on the signal to noise ratio of the measurements. Clouds properties analysis reveals the presence of thick cirrus clouds with thickness between 0.5 and 4.2 km, with the top cloud located at the tropopause height.
... We use the GLAS prototype algorithms to separate clouds and aerosols in the LITE data processing between the ground level and the altitude of 8km. The lidar signal is explained in term of the attenuated volume backscatter coefficient β (r) (Platt et al., 1998), i.e., to the calibrated, range-corrected lidar signals within each layer. The discriminator used here is based on the threshold relation given by P =β max β / z max > X. β max is the maximum attenuated backscatter of the layer and β / z max is the maximum vertical gradient magnitude within the layer. ...
The distribution of clouds in a vertical column is assessed on the global scale through analysis of lidar measurements obtained from three spaceborne lidar systems: LITE (Lidar In-space Technology Experiment, NASA), GLAS (Geoscience Laser Altimeter System, NASA), and CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization). Cloud top height (CTH) is obtained from the LITE profiles based on a simple algorithm that accounts for multilayer cloud structures. The resulting CTH results are compared to those obtained by the operational algorithms of the GLAS and CALIOP instruments. Based on our method, spaceborne lidar data are analyzed to establish statistics on the cloud top height. The resulting columnar results are used to investigate the inter-annual variability in the lidar cloud top heights. Statistical analyses are performed for a range of CTH (high, middle, low) and latitudes (polar, middle latitude and tropical). Probability density functions of CTH are developed. Comparisons of CTH developed from LITE, for 2 weeks of data in 1994, with ISCCP (International Satellite Cloud Climatology Project) cloud products show that the cloud fraction observed from spaceborne lidar is much higher than that from ISCCP. Another key result is that ISCCP products tend to underestimate the CTH of optically thin cirrus clouds. Significant differences are observed between LITE-derived cirrus CTH and both GLAS and CALIOP-derived cirrus CTH. Such a difference is due primarily to the lidar signal-to-noise ratio that is approximately a factor of 3 larger for the LITE system than for the other lidars. A statistical analysis for a full year of data highlights the influence of both the Inter-Tropical Convergence Zone and polar stratospheric clouds.
... However, especially in the tropics vertically and temporally resolved measurements of cirrus cloud properties are still scarce. Recent studies have focused on the western Pacific warm pool [Platt et al., 1998[Platt et al., , 2002Heymsfield et al., 1998;Sassen et al., 2000;Comstock et al., 2002]. Other lidar statistics of cirrus cloud properties in the tropics are available only for the Seychelles [Pace et al., 2003] and eastern India [Sunilkumar and Parameswaran, 2005]. ...
Cirrus formation and geometrical and optical properties of tropical cirrus as a function of height and temperature are studied on the basis of INDOEX (Indian Ocean Experiment) lidar and radiosonde measurements and satellite observations of deep convection causing the generation of anvil cirrus. Lidar and radiosonde measurements were conducted at Hulule (4.1°N, 73.3°E), Maldives, during four field campaigns carried out in February–March 1999 and March 2000 (northeast (NE) monsoon season, characterized by increased concentrations of anthropogenic aerosols over the Indian Ocean) and in July and October 1999 (southwest (SW) monsoon season, characterized by clean maritime conditions). As a result of a stronger impact of deep convection on cirrus formation during the SW monsoon season, cirrus clouds covered the sky over the lidar site in only 35% (NE), but 64% (SW) of the measurement time. Subvisible cirrus (optical depth ≤0.03), thin (optical depth from 0.03 to 0.3), and opaque cirrus (optical depth ≥0.3) were observed in 18%, 48%, and 34% (NE) and in 8%, 52%, and 40% (SW) out of all cirrus cases, respectively. Mean midcloud heights were rather similar with values of 12.9 ± 1.5 km (NE) and 12.7 ± 1.3 km (SW). In 25% of the cases the cirrus top height was found close to the tropopause. Mean values of the multiple-scattering-corrected cirrus optical depth, cirrus layer mean extinction coefficient, and extinction-to-backscatter ratio were 0.25 ± 0.26 (NE) and 0.34 ± 0.29 (SW), 0.12 ± 0.09 km−1 (NE) and 0.12 ± 0.10 km−1 (SW), and 33 ± 9 sr (NE) and 29 ± 11 sr (SW), respectively. A functional dependency of the extinction coefficient of the tropical cirrus on temperature is presented. All findings are compared with several other cirrus lidar observations in the tropics, subtropics, and at midlatitudes. By contrasting the cirrus optical properties of the different seasons, a potential impact of anthropogenic particles on anvil cirrus optical properties was examined. Differences in the cirrus extinction-to-backscatter ratio suggest that NE monsoon anvil cirrus originating from deep-convection cumulus clouds had more irregularly shaped and thus slightly larger ice crystals than respective SW monsoon anvil cirrus. Because the meteorological conditions were found to vary significantly between the seasons, an unambiguous identification of the influence of Asian haze on cirrus optical properties is not possible.
... The Lidar/Radiometer (LIRAD) method was originally developed at the CSIRO Division of Atmospheric Research in order to retrieve detailed properties of cirrus clouds from ground-based observations. The LIRAD method has been described fully in Platt et al. (1987) and earlier papers. It has also been described in recent ARM Science Team proceedings and in Platt et al. (submitted). ...
... The original LIRAD results were 0.47and 0.16, respectively. Applying Equation 2 yields a published in Platt et al. (1987), and other data have been k value of 0.68 at about 0EC and 0.08 at -75E to -45EC. The submitted for publication (Platt et al., submitted). ...
The retrieval of cloud properties from Lidar/Radiometer dat a by the LIRAD method is described. Several data sets taken by the CSIRO Division of Atmospheric Research are now available for such retrievals as a result of several major fiel d experiments during the past six years. The LIRAD method is reviewed briefly and its application t o the various data sets is described. The data have bee n obtained at temperatures ranging from +5°C down to -80°C, representing a large fraction of global atmospheric tempera - tures. Results include information on cloud height and depth , infrared emittance and linear depolarization ratio, togethe r with functions that relate the lidar to the IR data and that lea d to information on both particle size and habit. The methods developed have been demonstrated to be suitable for processing large amounts of cloud data in what could b e close to real time. Thus, the methods should be suitable fo r retrieving detailed cloud optical and microphysical propertie s from cloud lidar and infrared data now being obtained at th e ARM Southern Great Plains (SGP) Cloud and Radiation Testbed (CART) site.
The existence of clouds significantly increases or decreases the net radiation of the Earth. The influence of cloud type and cloud phase on radiation is as important as cloud amount, and the physical processes of different types of clouds are obviously different. In this study, the occurrence frequency of different cloud types (low transparent, low opaque, stratocumulus, broken cumulus, altocumulus transparent, altostratus opaque, cirrus, and deep convective) and cloud phases (ice and water) over China and its surrounding areas (0–55°N, 70–140°E) are calculated based on cloud vertical feature mask products from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). The results show significant spatial differences and seasonal variations in the distribution of different cloud types and cloud phases. There are four prevailing cloud types over the whole year, among which cirrus and altocumulus transparent are the most widely distributed and have the highest occurrence frequency. Cirrus clouds are mainly distributed at altitudes above 6 km north of 30°N and south of 20°N. Altocumulus transparent clouds are mainly distributed over the Qinghai–Tibet Plateau and at an altitude of 3–6 km to the north of 40°N, and they are more widely distributed in winter than in summer. Water clouds are mainly distributed in the latitude range of 20°N–40°N and are obviously influenced by the Qinghai–Tibet Plateau. Water clouds are widely distributed in autumn and winter. Ice clouds are mainly distributed in the areas south of 20°N and north of 40°N. Regardless of the choice of altitude between 3 km and 7 km, the boundary between ice cloud and water cloud is always near the −14 °C isotherm, and when the −14 °C isotherm moves southward, the ice-cloud distribution range expands southward. The probability density functions of the temperature in the cloud always show the distribution characteristics of two peaks and one valley, which is particularly obvious in the middle and high clouds, and the peak temperature is warmer than the sub-peak temperature. The valley temperature and its corresponding latitude of all cloud types are different: the cirrus valley temperature is not significantly affected by the Qinghai–Tibet Plateau, but the plateau has an effect on the latitude of the valley temperature distribution of other types of cloud. The above conclusions lay the foundation for further research on the radiation effects of different clouds on China and its surrounding areas and also have certain indicating significance for weather effects caused by various cloud physical processes.
