Cold air breaks out very often to the ocean off the east coast of China during winters. At the same time the surface water in the region can be very warm due to Kuroshio Current. Thus, a convective boundary layer develops over the East China Sea and the ocean surrounding Taiwan. The CBL can be as thick as 1 to 2 km and it is capped by a stratus cloud deck due to very large moisture flux from the ocean surface. The cloud deck is not uniform and the air-sea interaction process can be very intense due to the presence of mountains of Taiwan. As the northeasterly circulates around Taiwan, the airflow accelerates just off the east coast of Taiwan and induces extremely large surface heat and moisture fluxes. Thus, this region is known for active cyclogenesis during winters.

In this study, we simulate the evolution of the CBL and mesoscale circulations in the vicinity of Taiwan during winters with a newly-developed nonhydrostatic numerical model. The model is based on a fully compressible and three-dimensional system of equations with a terrain-following vertical coordinate. A major advantage of the model is the use of a forward-backward integration scheme to treat both high-frequency sound waves and internal gravity waves explicitly. The numerical procedure has been shown to be quite accurate and also is free from unstable computational modes. In addition, we use a forward semi-Lagrangian advection scheme which performs much better than a second order finite difference advection scheme. The results show that the air-sea interaction process is intensified by the mountains of Taiwan and the model is able to capture the major characteristics of the CBL and the mesoscale circulation system just to the east of Taiwan. The simulation precipitation field is also quite consistent with the observed climatological MSU precipitation rate in winters.