Simulation of Synoptic- and Subsynoptic-Scale Phenomena Associated with the East Asian Summer Monsoon Using a High-Resolution GCM

Abstract

A 20-yr simulation using a global atmospheric general circulation model with a resolution of 0.5° latitude ? 0.625° longitude is compared with observational findings. The primary goal of this survey is to assess the model performance in reproducing various summertime phenomena related to the continental-scale Asian monsoon in general, and the regional-scale East Asian monsoon in particular. In both model and observed atmospheres, the seasonal march of the precipitation centers associated with the Asian summer monsoon is characterized by onsets occurring earliest over the southeastern Bay of Bengal, followed by rapid northeastward advances over Indochina, the South China Sea?Philippine Sea and the western Pacific, northward evolution in the East Asian sector, as well as northwestward development over the Bay of Bengal, the Indian subcontinent, and the Arabian Sea. This onset sequence is accompanied by southwesterly low-level flows over the rainy regions, as well as northwestward migration of the 200-mb Tibetan anticyclone. Analysis of the heat sources and sinks in various regions illustrates the prominent role of condensational heating in the local energy budget during the mature phases of monsoon development. In accord with observations, the simulated monsoon rains in the East Asian sector are organized about zonally elongated ?mei-yu?baiu? (plum rain) systems. These precipitation features advance to higher latitudes during the June?July period, in conjunction with displacements of the axis of the low-level anticyclone over the subtropical western Pacific. A detailed case study is performed on a prominent rainy episode in the simulation. The model is capable of reproducing the observed intense gradients in temperature, humidity, and moist static stability in the vicinity of the mei-yu?baiu front, as well as the spatial relationships between the rainband and the three-dimensional flow field. The axis of the mei-yu?baiu rainband in this event is aligned with the trajectory of a succession of mesoscale cyclonic vortices, which originate from southwestern China and travel northeastward over the Yangtze River basin.