Macrophysical properties of tropical cirrus clouds from the CALIPSO satellite and from ground-based micropulse and Raman lidars

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Description

Lidar observations of cirrus cloud properties over the ARM Climate Research Facility’s Darwin site are compared from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, the ARM micropulse lidar (MPL), and the ARM Raman lidar (RL). Comparisons are made using the subset of profiles transparent to the lidar beam. RL and CALIPSO cloud fraction profiles show excellent agreement, while the MPL detects significantly less cirrus, particularly during the daytime. For cirrus-layer geometrical thickness, better agreement is found at night, while during the day both the MPL and CALIPSO cirrus layers are thinner than the RL. We find similar diurnal cycles in MPL and CALIPSO observations which could be due to increased daytime noise—during the daytime, cirrus clouds occur less frequently (noise prevents the detection of optically thin clouds) and are geometrically thinner (the tenuous cloud top and base are harder to detect, resulting in a thinner cloud). Daytime measurements using the RL are shown to be relatively unaffected by the solar background and are therefore used to check the validity of these diurnal cycles. The RL observations also show a similar decrease in daytime cirrus at the altitudes of maximum cirrus occurrence (~15–16 km). Below 15 km the RL diurnal cycle is of smaller magnitude and is of opposite sign below 11 km (thinner cirrus at night), although not all differences are statistically significant. Through the use of hourly MPL and RL cirrus cloud thickness and by the application of daytime detection limits to all CALIPSO data we find that the decreased MPL and CALIPSO cloud thickness during the daytime is likely due to noise. Also examined is the differences in cirrus optical depth due to method used to retrieve extinction,i.e., (1) a fixed lidar ratio, (2) the retrieval of the lidar ratio using layer transmission, and (3) the RL's ability to directly measure extinction using the nitrogen channel. Overall, the results of this study highlight the vast improvement the RL provides (compared to the MPL) in ARM’s ability to observe tropical cirrus clouds and consequently a ground-based data set better suited for CALIPSO comparisons. With continued operation the accumulation of a long-term RL data set of tropical cirrus, with independently retrieved extinction/backscatter and relatively higher-quality daytime measurements, will be a useful means for CALIPSO validation.