Comparative analysis of two heavy rain events in Sichuan Basin during summer 2009

Two heavy rain events in Sichuan Basin are compared based on NCEP reanalysis, weather station precipitation and sounding data. The first event is from 26 to 29 June and the second from 30 July to 2 August 2009. The conclusions can be drawn as follows. (1) "6.26" case is mainly affected by the low trough in central basin and southwest vortex, but the "7.30" case is attributed to the shear line between two high-pressure systems, inverted trough and southwest vortex. (2) Results based on water vapor flux stream functions demonstrate that the two cases have different water vapor transport channels, the vapor contributed to the "6.26" case is mainly from the Bay of Bengal, while during the“7.30”heavy rain case the water vapor from the Bay of Bengal passed the Indo-China peninsula and merged with water vapor in the south China Sea and then transported to Sichuan Basin. The water vapor flux potential function and divergent component are important predictors for future 6-hour accumulated precipitation area and center. (3) The dry cold air at high levels declined into boundary layers and entered high energy area. During the adjustment of the Ekman non-balance flow in the boundary layer, the dry cold air induced strong vertical ascending motion in boundary layer which extended to the high levels. Meanwhile, secondary circulation system at middle levels is triggered by the dry cold air as well. This could be the important physical mechanism leading to the two heavy rain events.