Factors Behind the Variability of Cirrus Clouds over the Asian Summer Monsoon Region

Clouds play a major role in maintaining the Earth's radiant energy balance through cooling the Earth system by reflecting the solar shortwave radiation and warming its atmosphere by trapping the Earth emitted long wave radiation. Optically thin clouds such as cirrus commonly seen over tropical tropopause regions, allow solar radiation to pass through them and emit less long wave radiation due to their lower temperatures. The net effect of such upper level cirrus clouds is inducing a warming effect in the Earth's atmosphere. Therefore, an in-depth study is necessary to understand the factors affecting the variability of cirrus clouds. In this work we use observations (2006-2017) from CALIPSO satellite to generate the climatology of cirrus clouds' distribution over the Asian summer monsoon region (ASMR). Cirrus clouds show a clear seasonal variation with maximum incidence observed during the northern hemispheric summer. Through a trajectory modelling analysis, it is found that the major convective sources of cirrus are located over central India and Bay of Bengal. The presence of tropical easterly jet during the monsoon season is found to be responsible for the large-scale distribution of the cirrus clouds over the Indian sub-continent away from the major convective sources. In addition, the aspects influencing the variability of cirrus clouds over ASMR are also explored.