The Role of Offshore Convergence on Coastal Rainfall during TAMEX IOP 3

Abstract

The role of offshore convergence on the coastal rainfall maximum along the northwestern coast of Taiwan is analyzed based on TAMEX (Taiwan Area Mesoscale Experiment) data and numerical experiments using the fifth-generation Pennsylvania State University?National Center for Atmospheric Research (PSU?NCAR) Mesoscale Model (MM5). From a case study during TAMEX IOP 3, moderate rainfalls (?40 mm h?1) are observed along the northwestern coast of Taiwan associated with the arrival of three rainbands in succession during 1000?1600 LST. These rainbands form over the Taiwan Strait with northeast?southwest or north?south orientation. They intensify off the northwestern coast. There is a tendency for the most intense echoes to align in a northeast?southwest orientation off the northwestern coast. An orographically enhanced convergence zone with a northeast?southwest orientation occurs in the area where the deflected southerly flow converges with the prevailing southwesterly flow that is modified by the storm-induced westerlies immediately behind the convective line. From the numerical experiment with synoptic-scale forcing but without the island topography over Taiwan, a large-scale cloud band is simulated within the Taiwan Strait without the localized rainfall maximum along the northwestern coast. The coastal rainfall maximum is simulated when the influences of the orographic effects are included. The results from the radar analyses and numerical simulations indicate that the convergence off the northwestern coast is a combination of the synoptic-scale forcing, orographic effects, and the feedback of the convection. The offshore convergence is important for the production of the rainfall maximum along the northwestern coast of Taiwan.