Precipitation forecast of MM5 in the Taiwan area during the 1998 Mei-yu season

This study presents precipitation verification, in the Taiwan area, for a real-time Pennsylvania State University-National Center for Atmospheric Research Fifth-Generation Mesoscale Model (MM5) system during the 1998 Mei-yu season. The highest equitable threat score (ETS) of precipitation forecasts, verified against observed precipitation, was about 0.2 at the 2.5-mm threshold for this nearly 2-month period. The complex and steep terrain in this region presented great challenges to the 15-km model in predicting realistic rainfall because the precipitation was driven by local forcings such as thermal effects and orographic lifting. In addition, the lack of observational data over the surrounding ocean greatly limited the quality of the model's initial data. It was found that the model system more accurately simulated nighttime rainfall than daytime precipitation. This was caused by the model underforecasting the rainfall events that resulted from solar heating and orographic lifting over the mountain slopes during the daytime hours. Precipitation, however, was overforecast over the high mountain regions (> 1200 m). Further, the analysis of ETS with regard to the terrain height indicated that the model performed better over the lowlands than over the mountainous areas (slopes and highlands). It was discovered that the ETSs were much higher for precipitation forecasts after the onset of the east Asia summer monsoon than prior to the onset. Overall, the model more accurately predicted precipitation for the rainfall events associated with the Mei-yu front and the accompanying mesoscale convective systems than it predicted precipitation associated with the local forcings.