ABSTRACT
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 CTH. The resulting columnar results are used to investigate the interannual variability in the lidar CTHs. Statistical analyses are performed for a range of CTH (high, middle, low) and latitudes (polar, middle latitude, and tropical). The probability density function (PDF) of CTH are developed. Comparisons of CTH developed from LITE, for 2 weeks of data in 1994, with International Satellite Cloud Climatology Project (ISCCP) 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 signalto-noise ratio that is approximately a factor of three 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 (PSCs).
