A torrential rain episode (F 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.

AB STRACTA se ries of nu mer i cal sim u la tions are con ducted us ing the Weather Re search and Fore cast ing (WRF-ARW) model with a 9-km mesh to ex am ine the phys i cal pro cesses re spon si ble for the tor ren tial rain fall as so ci ated with a mesoscale con vec tive sys tem (MCS) along the Mei-Yu front that caused se vere dam age over south west ern Tai wan on 12 June 2005. In the con trol ex per i ment (with full Tai wan ter rain), the MCS tends to prop a gate north ward along the wind ward (west ern) slope, rather than be ing advected down stream, as it en coun ters the south ern Cen tral Moun tain Range (CMR). The low-level con ver gence be tween the dy nam i cally-driven re turn flow and the in com ing south west erly flow is an im por tant fac tor for the north ward prop a ga tion of the MCS. The val ues of the un sat u rated moist Froude num ber (F w ) and the con vec tive avail able po ten tial en ergy (CAPE) are about 0.333 and 2858 J kg -1 for the con vec tively un sta ble ba sic flow. This air flow be longs to the flow-around re gime as sug gested by pre vi ous stud ies. In this flow re gime, the tor ren tial rain fall as so ci ated with the sim u lated MCS oc curs on both the upslope side and adjacent plains over southwestern Taiwan, which agrees well with the observed rainfall distribution.A sen si tiv ity study of low er ing the Tai wan ter rain el e va tions to 80% (F w ~0.416) or 60% (F w ~0.555) of that in the con trol ex per i ment re veals that the air flow is char ac ter ized by a long-last ing orographic con vec tive sys tem near the moun tain peaks. The sim u lated tor ren tial rain fall mainly oc curs along the moun tain peaks of the south ern CMR. The ex per i ment with a re duced el e va tion to 40% (F w ~0.833) or 20% (F w ~1.666) in the con trol ex per i ment shows a down stream-prop a gat ing MCS, which falls into the flow-over re gime. This study dem on strates that the north ward prop a gat ing MCS, which is mainly caused by the flow de flec tion by south ern CMR, played a key role in pro duc ing tor ren tial rain fall over southwestern Taiwan during 0000 -1200 UTC 12 June 2005.