Cirrus observations using ground-based LIDAR and terra MODIS instrument

A climatological study of cirrus occurrence has been carried out using the ground-based lidar observations over Gadanki (13.5 degrees N, 79.2 degrees E) during 1998-2004. The Moderate Resolution Imaging Spectroradiometer (MODIS) measurements on the Terra spacecraft are also used for remote sensing of high clouds cirrus from space during 2001-2004. The interannual study using LIDAR and MODIS shows an enhancement in the cirrus occurrence during 2001 and fewer amount during 2002. The interseasonal variation of cirrus occurrence frequencies shows much of the occurrences during the monsoon season. Further lidar observations shows that the cirrus cloud tops typically extended to near the 16.53 km, the average tropical tropopause height. The distribution of maximum cloud base height frequency is confined to 10-12km. Frequency of occurrence of cloud physical thickness with respect to cloud base height (Zb) gives a higher occurrence between 11-15 km and typically the thickness of 2-4 km. At the cloud base height Zb > 15km, which is in the vicinity of tropopause, the cirrus is found to have lesser thickness. A significant observation from this statistical study over this latitude shows appearance of cirrus at two different altitudes because of different formation mechanisms. We will also discuss the formation mechanisms for the occurrence of tropical cirrus at this latitude.