A torrential rain episode (≥ 200 mm day^-1) was observed over coastal regions of southern Taiwan (south of 23.0°N) on 5 June 2008 under the influence of a meso-scale frontal cyclone (MFC) formed along the western flank of a Mei-yu frontal zone. Through the incorporation of additional observations over the ocean collected during Southwest Monsoon Experiment (SoWMEX)/Terrain-influenced Monsoon Rainfall Experiment (TiMREX), the location of a Mei-yu frontal boundary, defined by the wind shift, is better identified over the open ocean by the data assimilation of dropsonde observations. Meanwhile, the low-level cyclonic vorticity and horizontal convergence are increased along the Mei-yu frontal zone, and an increment maximum center of cyclonic vorticity locates over the north of South China Sea (SCS), where the MFC subsequently forms.

For the development and maintenance of a MFC over the northern SCS, the additional data assimilation, using the combination of the dropsonde and the radar radial velocity, has significantly improved the prediction of an MFC with a best track over the open ocean. Therefore, the daily rainfall distribution and amount over southern Taiwan are reproduced and significantly improved, particularly using dropsonde observations combined with the radar radial velocity.