The role of ASM on the formation and properties of cirrus clouds over the Tibetan Plateau

Cirrus clouds play a significant role in the Earth's energy balance and in the hydrological cycle of the atmosphere. Here, a high-performance Micro Pulse Lidar was continuously used to investigate cirrus cloud formation and characteristics at Ali (32.50 degrees N, 80.08 degrees E; 4279m), in the western Tibetan Plateau from 25 July to 23 September 2016, a time frame that spanned the prevalence and degeneration period of the Asian summer monsoon (ASM). The cirrus clouds frequently occurred with sharp fluctuations in the vertical distribution from 8 to 14km above ground level (AGL) during the ASM period. In contrast, cirrus clouds were remarkably reduced and consistently existed near 10km in September, when the ASM began subsiding due to the lack of a driving force that triggers ice formation. Approximately half of the cirrus clouds were caused by deep convective activity during the ASM period, which held one-third of total cirrus clouds during the whole measurement period. These anvil cirrus clouds have a liquid origin and are characterised by optically thicker clouds with Cloud Optical Depth values greater than 0.2, high depolarisation ratios and high lidar ratios. These observations indicate that, in agreement with other studies at mid-latitudes and in the Arctic, liquid origin cirrus could be associated with thicker, larger and more complex nonspherical ice crystals in comparison to in situ formed cirrus. Cold perturbations were responsible for the formation and evolution of the remaining two-thirds of cirrus clouds. These clouds were mostly associated with in situ formation of ice crystals, in the slow updrafts in the tropical transition layer over the Tibetan Plateau.