On the average, Taiwan was affected by four typhoons with two of them made landfall every year. When a typhoon was affecting Taiwan, the heavy rainfall associated with typhoon often caused debris flow and flooding resulting in serious life losses and property damage. Since most of typhoon-related disasters in Taiwan were caused by the continuous heavy rainfall, the technique for forecasting rainfall amount during typhoon period has been highly desired. In this study, we analyzed the hourly rainfall data collected at 371 (automated) rain gauge stations around the whole island for the time period of 1989-2001. The average rainfall at different radial domains from typhoon center was computed and compared to those of other researches. The variations of rainfall amount with station elevation were also analyzed. Finally, a climatological model for estimating rainfall amount at each rain gauge station during typhoon period was developed. Currently, this climatological model is used at the National S&T Center for Disaster Reduction, Taiwan to forecast rainfall amount at each river basin during typhoon period using the predicted typhoon track issued by the Central Weather Bureau. The potential area of flooding is then estimated using the forecasted rainfall amount.

Analyses have been done to understand the capability of this climatological model. Results showed that the model could give reasonable estimates of rainfall pattern around the whole island for most typhoons when the best track of JTWC was used. Results also showed that the model could give reasonable estimate for the accumulated rainfall during the whole typhoon period for four selected major river basins, Dan-shui, Cho-shui, Kao-ping and Lan-yang river basins (located at northern, central, southern and eastern Taiwan, respectively). For the Dan-shui river basin, the correlation coefficient between the model estimate and the observation reached 0.77. However, if a shorter time period was considered, the accuracy of the model estimate decreased significantly. The correlation coefficient between the estimated 3-hourly rainfall and the observation for Dan-shui river basin was only 0.55. Results also showed that the model seriously underestimated the heavy rainfall at the northern Taiwan that was due to the interactions of typhoon circulation and the northeast monsoon. For example, the daily-accumulated rainfall reached 1136mm at Yang-Ming Mountain when the center of Typhoon Lynn (1987) was located at Bashi Channel (or about 500~600 km away). Therefore, a supplement model was developed for such special condition. In addition, the MM5 was used to simulate typhoon Lynn (1987) when it was affecting Taiwan and the mechanisms leading to heavy rainfall at northern Taiwan was analyzed. Results showed that significant low-level confluence and frontogenesis process occurred around northern Taiwan during the heavy rainfall period. Analyses of the frontogenesis function showed that although the confluence and deformation terms had large positive values at the confluence zone, their contribution was secondary. Major contribution came from the diabatic heating term especially at where heavy precipitation occurred. In summary, the forcing mechanisms leading to the heavy rainfall at northern Taiwan included the topographical lifting, the low-level confluence of typhoon surrounding circulation and the northeast monsoon, and the lifting of the warm and moist air associated with typhoon by the low-level cold air. The significant warm advection and diabatic effect associated with the slantwise ascending warm and moist air tended to maintain and intensify the baroclinic zone. This process appeared to be crucial to the continuous heavy rainfall during the interactions of typhoon and northeast monsoon.