Lidar observations of cirrus cloud near the tropical tropopause: temporal variations and association with tropospheric turbulence

Cirrus clouds forming just below the tropical tropopause and associated atmospheric turbulence are studied using a monostatic lidar and Mesosphere–Stratosphere–Troposphere (MST) radar. Lidar data show that the vertical extent of these clouds is confined to the region 14–16 km. Based on their visible optical depth, they can be classified as sub-visual (SVC), thin (TC) and dense cirrus (DC). Physical and optical properties of the cirrus clouds show significant temporal and spatial variability. While on some days, the clouds are strong and persistent throughout the night, they are weak and intermittent on some other days. Lidar backscattered signal from the clouds shows significant depolarisation indicating presence of non-spherical particles in abundance. The mean altitude of the cloud increases with decrease in tropopause temperature. The MST radar observations of vertical wind revealed that the region where these clouds are observed is highly turbulent. The vertical gradient of eddy diffusion coefficient shows a sharp discontinuity in the altitude region where the cirrus is strong and persistent, indicating that a region of divergence followed by a convergence above is conducive for cirrus formation.