Impacts of Diurnal Variation of Mountain-plain Solenoid Circulations on Precipitation and Vortices East of the Tibetan Plateau during the Mei-yu Season

Diurnal variations of two mountain-plain solenoid (MPS) circulations associated with first-step terrain Tibetan Plateau (TP) and second-step terrain (high mountains between the TP and east plains) in China and their influence on the southwest vortex (SWV) and the mei-yu front vortex (MYFV) were investigated via a semi-idealized mesoscale numerical model Weather Research and Forecasting (WRF) simulation integrated with ten-day average fields (mei-yu period of 110 July 2007). The simulations successfully reproduced two MPS circulations related to first- and second-step terrain, diurnal variations from the eastern edge of the TP to the Yangtze River-Huaihe River valleys (YHRV), and two precipitation maximum centers related to the SWV, MYFV. Analyses of the averaged final seven-day simulation showed the different diurnal peaks of precipitation at different regions: from the afternoon to early evening at the eastern edge of the TP; in the early evening to the next early morning in the Sichuan Basin (SCB); and in the late evening to the next early morning over the mei-yu front (MYF). Analyses of individual two-day cases confirmed that the upward branches of the nighttime MPS circulations enhanced the precipitation over the SWV and the MYFV and revealed that the eastward extension of the SWV and its convection were conducive to triggering the MYFVs. The eastward propagation of a rainfall streak from the eastern edge of the TP to the eastern coastal region was primarily due to a series of convective activities of several systems from west to east, including the MPS between the TP and SCB, the SWV, the MPS between second-step terrain and the east plains, and the MYFV.