IEstimating Probable Maximum Precipitation by Considering the Combined Effect of Typhoon and Southwesterly Flow

                                    Jheng-Cin Liou

This study proposes the independent approach (IA) to estimate probable maximum precipitation (PMP) by considering the combined effect of typhoon and southwesterly flow (SF). The concept of IA is that the PMP by considering the combined effect of typhoon and SF is equal to the sum of (1) the product of α and the PMP of SF and (2) the product of α and the PMP of typhoon, which is derived from the water vapor balance equation with the specific humidity and wind field. The parameter, α, represents the capability of the combined effect of typhoon and SF which could be triggered by the interaction of typhoon and SF wind fields. In the IA, the storm separation method (SSM) is adopted to estimate the PMP of typhoon and the PMP of SF is calculated by the linear regression equation established by the cross-validation method. The SSM is one of the approaches for calculating the rainfall of a storm, which decomposes the rainfall of a storm into circulation rainfall and orographic rainfall. The layers of circulation rainfall and orographic rainfall are from 1000 mb to 850 mb and from 1000 mb to 550 mb, respectively. Tsengwen Reservoir catchment in southern Taiwan is chosen as the study area. Eight historical typhoon events are collected as the data set. Among the eight typhoon event, the highest value of α is found to be 0.72. The 60-hour PMP by considering the combined effect of typhoon and SF at the Tsengwen Reservoir catchment is calculated to be 2,909 mm, which is larger than the highest 60-hour observed precipitation (1,697 mm). Further studies on this issue are essential and encouraged.

Keywords: The combined effect of typhoon and southwesterly flow, probable maximum precipitation, storm transposition method, storm separation method