The effect of estimating probable maximum precipitation by considering interaction typhoon and southwesterly flow

The purpose of this study is to make a description of what effect the southwesterly flow has on Probable Maximum Precipitation (PMP) by proposing Amplification Index of Southwesterly Flow (AISF). The catchment of Tseng-Wen Reservoir is selected as the study area. NCEP CFSR provides the daily data including wind velocity and precipitable water. The Alishan raingauge, near the study area, provides local precipitation daily data. All data has selected from May to August, 1979 to 2009. The study finds that precipitation has significant relationship with average value of both wind velocity and precipitable water from NCEP CFSR (The National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR)) over the southwest ocean of Taiwan. Regression equation for predicting the southwesterly precipitation is deduced by applying linear regression method and cross validation method which using ten southwesterly flow events data. AISF is mainly proposed for researches on typhoon events among which the typhoons are formed along three typhoon routes which have a greater effect on the study area, and is defined as the ratio of historical maximum southwesterly flow precipitation (PHMS) to southwesterly flow precipitation (P_ST). The study chooses 17 typhoon events to estimate their respective AISF. The result shows that under the effect of southwesterly flow, there occurred strong typhoons and heavy rainfalls on a larger scale. The AISFs of all typhoons are all larger than 1 and also matches the usual occurrences for a convergence region of typhoon and southwesterly flow. Finally, AISF was applied on storm transposition method to compare with the design PMP of Tseng-Wen Reservoir. Finally, AISF was applied on storm transposition method to compare with the design PMP of Tseng-Wen Reservoir. The result shows it is necessary to consider the interaction of two weather systems (in this study: typhoon and southwesterly flow) on PMP estimated.