Generation processes of mesoscale convective systems following midlatitude troughs around the Sichuan Basin

Abstract

[1] The generation processes of mesoscale convective systems (MCSs) that occurred over the Sichuan Basin in China were revealed in the early 2008 monsoon season. MCS occurrences detected in METEOSAT geostational satellite images are associated with the traveling of midlatitude troughs after the onset of the Indian monsoon. Three episodes of large-scale MCSs were generated under synoptic conditions of merging southwesterly low-level monsoon flows with a northerly midlatitude air mass following the trough in the leeward of the Tibetan Plateau without direct migration of the vortex generated from the plateau surface. Numerical simulations using a weather research and forecast (WRF) model showed that the MCS was triggered in the evening by strengthening of the low-level wind convergences with horizontal shear between the southerly monsoon flow, with large convective available potential energy, and the northerly dry intrusion. A sudden increase in the northerly winds was confirmed by sonde observation data in the western basin, and the winds were simulated as intrusions passing over the Qinling Mountains when the daytime clouds over the mountain were diminished. Sensitivity experiments by a WRF simulation revealed that the topography of the Sichuan Basin was able to capture the dry intrusion at the bottom to prevent the propagation of disturbances from the plateau and to cause the sudden onset of MCSs apart from the plateau with a heavy precipitation zone.