The evolution of the East Asia summer monsoon in June 1994: Numerical simulations

The evolution of the summer monsoon over Asia during June 1994 is simulated using a regional climate model based on the Penn State-NCAR mesoscale mode l MM5. The National Center for Environment Prediction (NCEP) analysis are u sed as the initial and lateral boundary conditions, and the integration ext ends up to one month. The model is capable of reproducing the monthly mean features of the monsoo n circulation and total rainfall in June 1994. The changes of the large sca le circulation during the evolution of summer monsoon are also well simulat ed, which include: (1) the northward shift of the upper westerlies over the Tibetan Plateau and the development of upper easterlies south of the Plate au; (2) the extension of low level southwesterly monsoon flow from South Ch ina Sea to eastern China and the sequential jump of the rain belt from sout h to north; (3) the northward shift of the mid-tropospheric western Pacific subtropical high (WPSH) at 500 hPa, and the establishment of the upper tro pospheric high pressure over the Tibetan Plateau at 200 hPa. In order to examine the impact of convective heating on the evolution of th e monsoon, an experiment without convective heating is conducted. The resul ts show that the convective heating is crucial for: (a) the development of the low-level southwesterly monsoon; (b) the northward shift of the upper-l evel westerlies over the Tibetan Plateau; and, (c) the position and intensi ty of the western Pacific subtropical high.