Multiyear lidar observations of the descending nature of tropical cirrus clouds

Sedimentation of ice particles from cirrus clouds is one of the main processes responsible for dehydration of the upper troposphere. Multiyear-long-duration lidar observations of tropical cirrus over Thiruvananthapuram (8.5 degrees N, 77 degrees E) show the descending nature of these clouds throughout the year, with a mean frequency of occurrence of similar to 20%. Majority of these descending cirrus occur near the base of tropical tropopause layer (TTL), while their frequency of occurrence and vertical extent of descent near the cold point tropopause are considerably less. On average, vertical displacement of the top and base of cirrus is almost similar. The most probable vertical displacement of cirrus is 1.5 - 2.5 km with descent duration of 1-2 h. However, in >20% of the cases, the vertical displacement is >3 km and similar to 40% of the descending cirrus layers last for 2-5 h. Around 95% of descending cirrus clouds have a settling speed <0.8 ms(-1). On average, the percentage of clouds that are thickening or thinning by >300 m during descent are almost equal (similar to 23% each), while the thickening/thinning of the remaining 54% is <300 m. About 79% of these descending cirrus layers have cloud optical depth (COD) between 0.1 to 0.5. None of these clouds are of sub-visual type (COD <0.03). For the typical range of vertical displacements and CODs observed in the present study, the descending of cirrus clouds are estimated to decrease the longwave cloud radiative forcing at top of the atmosphere by 1 to 12.9 Wm(-2).