Lidar observed characteristics of the tropical cirrus clouds

Abstract

Using the polarization diversity lidar data collected during March 1998 to April 2001, the characteristics of the tropical cirrus clouds observed over Gadanki (13.5°N; 79.2°E) are presented in this paper. Out of 210 nights of observations during this period, cirrus clouds were observed over the lidar site on 170 nights. The cloud mean height is found to be in the range of 8-17 km with peak occurrence at 13-14 km, just below the tropopause. The cloud thickness has values ranging 0.6-4.2 km with maximum occurrence at 0.9-1.2 km. The scattering ratio and the linear depolarization ratio are in the ranges of 1.14-36 and 0.01-0.75 with peak occurrences seen at 1.14-2 and 0.01-0.05, respectively. The optical depth can be as high as 2, but values less than 0.1 account for most (>80%) of the clouds. The clouds with low optical depth occur most frequently during fall equinox and high optical depth during summer. The formations of thin and thick cirrus clouds are seen to be closely related to the minimum tropospheric temperature and cumulonimbus outflows, respectively.