Optical properties of cirrus clouds observed below the tropical tropopause

Abstract

Characteristics of cirrus clouds forming just below the tropical tropopause and associated atmospheric turbulence in this region are studied using a monostatic lidar and Mesosphere Stratosphere Troposphere (MST) radar. The lidar data shows quite frequent occurrence of cirrus clouds near the tropical tropopause. The vertical extent is confined to the region just below the tropopause with a width of about 1-2 km. Based on cloud optical depth these cirrus clouds can be classified as; the subvisual cirrus with optical depth (tau) <SUB>c</SUB> &lt; 0.05 and thick or opaque cirrus with optical depth (tau) <SUB>c</SUB> &lt; 0.05. Frequency of occurrence of subvisual clouds is larger than that of the thick clouds. While the subvisual cirrus occurs more frequently at an altitude approximately 16 km, the optically thick clouds occur more frequently around approximately 14 km. The lidar signal scattered by these clouds shows significant depolarization, indicating the presence of abundant amount of non-spherical particles. The MST radar observations of vertical wind during the lidar observation revealed that the region between 10 km to 16 km, where these clouds are observed, is highly turbulent. A study of the Turbulent Kinetic Energy profile has shown high er values during the period of cloud occurrence. altitude profile of the vertical gradient eddy diffusion coefficient in the latitude region 10 to 16 km shows a sharp positive peak followed by a sharp negative peak above, when the cloud is strong and continuous. On other periods when the cloud is weak and discontinuous in this region is relatively low and the peaks are rather broad. This means that a region of divergence followed by a region of convergence above is favorable for the cirrus cloud formation.