The seasonal variation of Tibetan convective systems observed by satellite

Based on International Satellite Cloud Climatology Project (ISCCP) Convection Tracking Database (CTD) dataset, a new definition for the Tibetan Convective System (TCS) is introduced, in which the effects of cirrus and cirrostratus are excluded from the TCS over this region. 2032 TCSs have been collected to study their seasonal variations in initiation, frequency, spatial distribution, life cycle, cloud physics, precipitation, dynamic and thermodynamic characteristics. It is found that the number of TCSs varies seasonally, with the maximum in July and minimum in December. 73%, 10%, 8% and 9% TCSs originates from Tibetan trough, shear, vortex and low, respectively. TCSs play an important role in precipitation over the Tibetan Plateau and its adjacent regions. Even in winter, the TCS rainfall contributes up to 70% of the total precipitation over the central-eastern Tibetan Plateau, and 30%-70% over Sichuan basin and upper-middle segment of Yangtze River basin. The seasonal changes on cloud properties of the TCSs indicate that the strongest TCSs happen in summer and the weakest in winter. These variations are in consistent with the seasonal variation of the mean rainfall and heavy rain of TCSs from the TRMM estimation. The dynamic and thermodynamic characteristics demonstrate that TCSs show an asymmetric distribution, and it happens more frequently and violently in summer than other seasons because of its lower T-Td, high special humidity and secondary circulation.