In this study, PSU/NCAR MM5 is used to simulate the evolution of the 1998 East Asian summer monsoon onset over the South China Sea (SCS). Simulation outputs are further utilized to study the development of mesoscale convective systems (MCSs) and their interactions with the synoptic/large-scale environment in the SCS region. During the monsoon onset, over southern China and the SCS, high equivalent potential temperature, convective instability, and the southwesterly confluence in the lower troposphere provided favorable large-calce thermodynamic and dynamic conditions for MCS development.

Model simulated results reveal that, in a warm-moist and unstable atmosphere, the downdraft outflows as a result of the MCS development could become a possible mesoscale triggering mechanism for the formation of a new MCS. Sensitivity tests depict the effect of latent heat release play­ing a significant role in the establishment of monsoon circulation. The ter­rain effect is significant for local cloud formation over land. For well organized MCSs occurring over the SCS, the influence of the mesoscale wind field is more important. The latent heat release in MCSs heats the atmosphere, reduces the surface pressure, and therefore enhances the low­level wind convergence and cyclonic circulation. The mesoscale adjustment of wind field and mass field, on one hand, enhances the MCS development or initiates the formation of a new MCS, and the organization of MCSs, on the other hand, integrates the mesoscale disturbances into a synoptic struc­ture and forms the local monsoon trough. Without the latent heat release, monsoon circulation cannot establish itself even if the model provides fa­vorable large-scale boundary forcing.