The persistent severe rainfall over southern China in June 2010：The evolution of synoptic systems and the Tibetan Plateau’s heating effect

KeyWord:Persistent severe rainfall over southern China;Convection-permitting ensemble simulation;Circulation and weather systems;Tibetan Plateau's heating effect

Abstract:

      Persistent severe rainfall occurs over southern China during June 2010, with a magnitude being comparable to June 2008 and exceeding the other recent years. The June of 2010 is characterized by frequent occurrences of low-value systems in the middle-and low-troposphere from the Qinghai-Tibet Plateau to the lower reaches of the Yangtze River, and the significantly intensified West Pacific subtropical high (WPSH), leading to more northward located low-level anomalous wind convergence and rain bands compared to June of 2008. Analyzing the four successive persistent severe rainfall events over southern China during 14-24 June 2010, it is found that the South Asian high, the middle-latitude trough and ridge and the WPSH in the middle troposphere, as well as the shear lines and eastward-moving vortices in the lower troposphere are the main synoptic systems inducing the persistent severe rainfall. Moreover, an ensemble of convection-permitting simulations (CTL) is carried out using the WRF model for these rainfall events in June 2010. The ensemble mean successfully reproduces the observed evolutions of precipitation and weather systems. Another ensemble of simulations (SEN) with the surface albedo over the Tibetan Plateau and its southern slope being changed artificially to one, otherwise it is identical to CTL, is carried out. Comparison between CTL and SEN suggests that the surface sensible heating of the Tibetan Plateau in CTL significantly effects the temperature distributions over the plateau and its surroundings and the thermal wind adjustment consequently changes atmospheric circulations and properties of synoptic systems from the lower to upper troposphere, which significantly intensifies the persistent severe precipitation over southern China. Specifically, at 200 hPa, anticyclonic and cyclonic anomalies are formed over the western and eastern plateau, respectively, which enhances the southward cold air intrusion along the eastern plateau and intensifies the divergence over southern China; at 500 hPa, the ridge over the northern plateau and the trough over eastern China are strengthened, the southwesterly flows along the northwestern side of the subtropical high are intensified, and the positive vorticity propagation from the plateau to the downstream is also enhanced significantly; and at 850 hPa, the low-pressure vortices strongly develop and move eastward while the southwesterly low-level jet over the South China coasts is intensified as shown in CTL, leading to stronger water vapor convergence and upward motion over the precipitation region.