Lidar studies of particulates in the UTLS region at a tropical station over the Indian subcontinent

Characteristics of particulate matter in the upper troposphere and lower stratosphere (UTLS) region during the volcanic quiescent period of 1998-2003 are studied using dual polarization lidar observations at the tropical station Gadanki (13.5 degrees N, 79.2 degrees E). This study revealed a pronounced summer-winter contrast of the particulate backscatter coefficient (beta(a)) in the LS region with relatively high values during winter and low values during summer. High values of beta(a) (> 10(-6) m(-1) sr(-1)) in the UT region during summer are closely associated with the manifestation of semitransparent cirrus clouds in this region. High values of volume depolarization ratio (VDR) exceeding 0.04 and increasing up to 0.6 usually encountered in the UT region during this period could be due to the presence of relatively large nonspherical ice particles. These values in the LS region, which are relatively small (< 0.04), indicate that the particles in this region are relatively small and more or less spherical in nature. The present study shows that convective activity prevailing in the troposphere significantly influences the microphysical properties of particulates in the UTLS region. Convective available potential energy and outgoing longwave radiation, which are good representative indices for tropospheric convection, show good correlation with VDR in the lower stratosphere during summer and weak correlation during winter. The role of vertical transport from troposphere to stratosphere is also investigated using the vertical wind obtained from mesosphere-stratosphere-troposphere (MST) radar data. This study shows that the seasonal variation of mean backscatter coefficient matches well with the corresponding variation in mean vertical mass flux. These results suggest that highly nonspherical particles associated with cirrus clouds forming in the upper troposphere and subsequent uplift across the tropopause aided by strong convection in the troposphere during the summer monsoon period are responsible for the increased VDR observed in the upper troposphere and lower stratosphere region, while the transport of air from troposphere to stratosphere, which is more pronounced during winter, is responsible for the increase in backscatter coefficient in the lower stratosphere.