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 model MM5. The National Center for Environment Prediction (NCEP) analysis are used as the initial and lateral boundary conditions, and the integration extends up to one month. The model is capable of reproducing the monthly mean features of the monsoon circulation and total rainfall in June 1994. The changes of the large scale circulation during the evolution of summer monsoon are also well simulated, which include: (1) the northward shift of the upper westerlies over the Tibetan Plateau and the development of upper easterlies south of the Plateau; (2) the extension of low level southwesterly monsoon flow from South China Sea to eastern China and the sequential jump of the rain belt from south to north; (3) the northward shift of the mid-tropospheric western Pacific subtropical high (WPSH) at 500 hPa, and the establishment of the upper tropospheric high pressure over the Tibetan Plateau at 200 hPa. In order to examine the impact of convective heating on the evolution of the monsoon, an experiment without convective heating is conducted. The results show that the convective heating is crucial for: (a) the development of the low-level southwesterly monsoon; (b) the northward shift of the upper-level westerlies over the Tibetan Plateau: and, (c) the position and intensity of the western Pacific subtropical high.