Effects of cirrus heterogeneity on lidar CALIOP/CALIPSO data (original) (raw)
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Journal of Geophysical Research, 2010
Macrophysical and optical characteristics of cirrus clouds were investigated at the Semi-Arid Climate Observatory and Laboratory (SACOL; 35.951N, 104.141E) of Lanzhou University in northwest China during April to December 2007 using micro-pulse lidar data and profiling radiometer measurements. Analysis of the measurements allowed the determination of macrophysical properties such as cirrus cloud height, ambient temperature, and geometrical depth, and optical characteristics were determined in terms of optical depth, extinction coefficient, and lidar ratio. Cirrus clouds were generally observed at heights ranging from 5.8 to 12.7 km, with a mean of 9.071.0 km. The mean cloud geometrical depth and optical depth were found to be 2.070.6 km and 0.35070.311, respectively. Optical depth increased linearly with increasing geometrical depth. The results derived from lidar signals showed that cirrus over SACOL consisted of thin cirrus and opaque cirrus which occurred frequently in the height of 8-10 km. The lidar ratio varied from 5 to 70 sr, with a mean value of 26716 sr, after taking into account multiple scattering effects. The mean lidar ratio of thin cirrus was greater than that of opaque cirrus. The maximum lidar ratio appeared between 0.058 and 0.3 when plotted against optical depth. The lidar ratio increased exponentially as the optical depth increased. The maximum lidar ratio fell between 11 and 12 km when plotted against cloud mid-height. The lidar ratio first increased and then decreased with increasing mid-height.
Distinguishing cirrus cloud presence in autonomous lidar measurements
Atmospheric Measurement Techniques, 2015
2012 Level-2 Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite-based cloud data sets are investigated for thresholds that distinguish the presence of cirrus clouds in autonomous lidar measurements, based on temperatures, heights, optical depth and phase. A thermal threshold, proposed by Sassen and Campbell (2001) for cloud top temperature T top ≤ −37 • C, is evaluated versus CALIOP algorithms that identify ice-phase cloud layers using polarized backscatter measurements. Derived global mean cloud top heights (11.15 vs. 10.07 km above mean sea level; a.m.s.l.), base heights (8.76 km a.m.s.l. vs. 7.95 km a.m.s.l.), temperatures (−58.48 • C vs. −52.18 • C and −42.40 • C vs. −38.13 • C, respectively, for tops and bases) and optical depths (1.18 vs. 1.23) reflect the sensitivity to this constraint. Over 99 % of all T top ≤ −37 • C clouds are classified as ice by CALIOP Level-2 algorithms. Over 81 % of all ice clouds correspond with T top ≤ −37 • C. For instruments lacking polarized measurements, and thus practical estimates of phase, T top ≤ −37 • C provides sufficient justification for distinguishing cirrus, as opposed to the risks of glaciated liquid-water cloud contamination occurring in a given sample from clouds identified at relatively "warm" (T top > −37 • C) temperatures. Although accounting for uncertainties in temperatures collocated with lidar data (i.e., model reanalyses/sondes) may justifiably relax the threshold to include warmer cases, the ambiguity of "warm" ice clouds cannot be fully reconciled with available measurements, conspicuously including phase. Cloud top heights and optical depths are investigated, and global distributions and frequencies derived, as functions of CALIOP-retrieved phase. These data provide little additional information, compared with temperature alone, and may exacerbate classification uncertainties overall. 1 Motivation Cirrus clouds are recognized by sky gazers for their translucent and fibrous appearance, cast frequently as delicate white filaments across otherwise clear blue skies at relatively high tropospheric altitudes. To climate scientists however, cirrus clouds, which are composed almost exclusively of ice crystals, are distinct for their physical and radiative properties (e.g., Liou, 1986). As cold and optically thin counterparts to most liquid-water and mixed-phase clouds (e.g., Sassen and Cho, 1992), the net column-integrated radiative impact of cirrus cloud presence during sunlit hours varies between positive and negative, depending on the relative magnitudes of their simultaneous and offsetting contributions diurnally to tropospheric warming (infrared absorption and reemission) and cooling (solar albedo effects; Stephens et al., 1990). This attribute makes cirrus relatively unique among cloud genera. Combined with their relatively high occurrence frequencies globally (e.g., Holz et al., 2008), cirrus are significant and distinct contributors to climate overall (Sassen, 2002). Lidars are primary remote-sensing tools used for monitoring cirrus clouds (e.g., Sassen, 1991). Two complementary NASA lidar projects are presently tasked with compiling Published by Copernicus Publications on behalf of the European Geosciences Union.
Cirrus climatological results from lidar measurements at OHP (44°N, 6°E)
Geophysical Research Letters, 2001
A climatology of cirrus clouds over the Observatoire de Haute Provence in France has been constructed from the analysis of ground-based lidar measurements taken from 1997 to 1999. During this period the high-resolution Rayleigh/Mie lidar collected 384 nights of measurements and cirrus profiles are observed in about half of these cases. We find subvisible cirrus (τ < 0.03) constitute ∼ 20 % of cirrus cloud occurrences and that the mean thickness of a subvisible cirrus cloud layer is less than 1 km. A discussion of the error associated with these determinations is also presented.
Cirrus Clouds Optical Properties Measured With Lidar At Camagüey, Cuba
2006
Cirrus clouds play a key role in the earth's radiative budget. To understand and quantify its impact on earth's atmosphere radiative transfer it is necessary to have information about its optical properties. Such a knowledge is necessary both for comparison with other datasets of cirrus measurements as well as for modeling its role on radiative transfer. Cirrus clouds were measured by lidar at Camagüey (21.4º N, 77.9º W) between 1993 and 1998. We derived the extinction coefficient profile using a set of eight constant Extinction to Backscattering ratio conversion coefficients. Our lidar measurements are biased because we normally conduct measurements on clear nights to the naked eye. As result of the bias our cirrus lidar dataset is mostly representative of thin and subvisible cirrus. Making use of such fact we selected the most appropriated conversion coefficient considering the frequency of occurrence of the calculated optical depth values among the ranges attributed to thin and subvisible cirrus clouds. The 131 measurements were classified in three groups according to their optical depth (τ): opaque (τ > 0.3), thin (0.3> τ >0.03) and subvisual (τ > 0.03). The mean values for each one of the three groups were 0.498 ± 0.268, 0.074 ± 0.047 and 0.016 ± 0.010 respectively. The mean values of extinction along the complete cirrus vertical depth were derived too. The comparison with results reported at midlatitudes was conducted, the results are discussed.
EPJ Web of Conferences, 2016
Cirrus clouds are product of weather processes, and then their occurrence and macrophysical/optical properties can vary significantly over different regions of the world. Lidars can provide height-resolved measurements with a relatively good both vertical and temporal resolutions, making them the most suitable instrumentation for high-cloud observations. The aim of this work is to show the potential of lidar observations on Cirrus clouds detection in combination with a recently proposed methodology to retrieve the Cirrus clouds macrophysical and optical features. In this sense, a few case studies of cirrus clouds observed at both subtropical and polar latitudes are examined and compared to CALIPSO/CALIOP observations. Lidar measurements are carried out in two stations: the Metropolitan city of Sao Paulo (MSP, Brazil, 23.3°S 46.4°W), located at subtropical latitudes, and the Belgrano II base (BEL, Argentina, 78ºS 35ºW) in the Antarctic continent. Optical (COD-cloud optical depth and LR-Lidar Ratio) and macrophysical (top/base heights and thickness) properties of both the subtropical and polar cirrus clouds are reported. In general, subtropical Cirrus clouds present lower LR values and are found at higher altitudes than those detected at polar latitudes. In general, Cirrus clouds are detected at similar altitudes by CALIOP. However, a poor agreement is achieved in the LR retrieved between ground-based lidars and space-borne CALIOP measurements, likely due to the use of a fixed (or low-variable) LR value in CALIOP inversion procedures.
Journal of Applied Remote Sensing, 2013
The temporal variability of the 532-nm optical depth of cirrus clouds observed with a lidar at Observatory of Haute-Provence (43.9°N, 5.7°E, and 683-m altitude), has been analyzed. While advection dominates at the first order, variability of the optical depth on timescales of minutes can be related to spatial fluctuations of cloud properties on typical scales of a few kilometers. Log-normal distributions of the optical depth have been used to model the variability of the cirrus optical depth as observed by lidars. These investigations have been performed for three independent classes of cirrus. The log-normal distribution of the optical depth is applicable to the classes of thin clouds; however, for thick clouds, likely due to successive freezing/defreezing effects, the distribution is rather bimodal. This work compares the effects of visible solar light scattered by inhomogeneous cirrus to effects generated by homogeneous clouds having a constant geometrical thickness using the short-scale lidar observations of optical depth distribution and an analytical approach. In the case of thin cirrus, the scattering of solar light reaching the ground is stronger for inhomogeneous than homogeneous cirrus. In case of thick cirrus, multiplescattering processes need to be considered. The conclusion is that log-normal distribution of the cirrus optical depth should be considered in any radiative calculation in case of model grids larger than a few kilometers whatever the cirrus type is.
Atmospheric Research, 2017
Cirrus (Ci) cloud properties can change significantly from place to place over the globe as a result of weather processes, reflecting their likely different radiative and climate implications. In this work Cirrus clouds (Ci) features observed in late autumn/early winter season at both subtropical and polar latitudes are examined and compared to CALIPSO/CALIOP observations. Lidar measurements were carried out in three stations: São Paulo (MSP, Brazil) and Tenerife (SCO, Canary Islands, Spain), as subtropical sites, and the polar Belgrano II base (BEL, Argentina) in the Antarctic continent. The backscattering ratio (BSR) profiles and the top and base heights of the Ci layers together to their Cirrus Cloud Optical Depth (CCOD) and Lidar Ratio (LR) for Ci clouds were derived. In addition, temperatures at the top and base boundaries of the Ci clouds were also obtained from local radiosoundings to verify pure ice Ci clouds occurrence using a given temperature top threshold (b− 38°C). Ci clouds observed along the day were assembled in groups based on their predominant CCOD, and classified according to four CCOD-based categories. Ci clouds were found to be vertically-distributed in relation with the temperature, forming subvisual Ci clouds at lower temperatures and higher altitudes than other Ci categories at both latitudes. Discrepancies shown on LR values for the three stations, but mainly remarked between subtropical and polar cases, can be associated to different temperature regimes for Ci formation, influencing the internal ice habits of the Ci clouds, and hence likely affecting the LR derived for the Ci layer. In comparison with literature values, daily mean CCOD/LR for SCO (0.4 ± 0.4/21 ± 10 sr), MSP (0.5 ± 0.5/27 ± 5 sr) and BEL (0.2 ± 0.3/28 ± 9 sr) are in good agreement; however, the variability of the Ci optical features along the day present large discrepancies. In comparison with CALIOP data, Ci clouds are observed at similar altitudes (around 10-13 km height); however, differences are found mostly in CCOD values for subtropical Ci clouds, whereas LR values are in a closer agreement. These differences are carefully examined in relation with the closest CALIPSO overpass time and distance from the station (N 70 km far), inferring the irregular extension and inhomogeneity of the Ci clouds over each study area. These considerations can be useful for assimilation of the Ci features into climate models and evaluation of future space-borne lidar observations of Ci clouds, especially for the future ESA/Copernicus-Sentinel and ESA/EarthCARE missions.
Mid-latitude cirrus investigations at high-resolution through ground-based lidar measurements
Although cirrus vertical distributions determine their local cooling or warming effects, one of the main missing information in Global Climate Models (GCMs) is the characterization of their vertical location and stratification. Lidar technique, in contrast, can detect cirrus with high spatial and temporal resolution, providing accurate information on their vertical distribution. In this work, the recent and on going studies about the the characterization of mid-latitude cirrus through lidar systems located at the Observatory of Haute Provence (OHP, 43.9 ° N, 5.7 ° E) in France and at Rome Tor Vergata (RTV, 41.8 ° N, 12.6 ° E) in Italy are presented. Cirrus have been firstly studied in terms of quasi-stationary periods regarding statistical variability. A clustering approach has been then adopted to derive cirrus classification (and climatology) over the period 1996-2007 for OHP lidar measurements and over 2007-2010 for RTV dataset. Three independent cirrus classes have been identifi...