北行颱風離開台灣後常帶來旺盛西南氣流，引進的西南氣流將帶來豐沛的雨量，其降雨主要受到風場輻合及受地形效應之影響，其中風場輻合的降雨甚至大於颱風本身所帶來的雨量，使人民生命財產蒙受巨大損失。而風場輻合效應與地形效應，何者為降雨的主要影響量尚無法判別，致使特定衛星所發展之特定地區的估算式來估算降雨時，結果往往不盡理想。因此發展適用於台灣的降雨估算技術是相當重要的，本研究將針對台灣地區受純西南氣流影響所導致的降雨量來進行探討。 本文首先利用衛星觀測南海海域其水氣及風速和地面降雨的相關性來估算西南氣流在台灣地區之降雨量，並建立西南氣流降雨指標(SFPI)作為西南氣流顯著與否之判斷依據，若值大於0.0表示有顯著的西南氣流影響，但由SFPI來篩選有無西南氣流時，會將受非純西南氣流之影響。所以需建立一篩選方法將非純西南氣流影響的案例濾除。先求得研究範圍內的平均風場，將平均風向加減20度且風速大於6(m/sec)定為篩選標準，若南海海域整體風向及風速比例達60%及70%時，判定為純西南氣流降雨案例。 由2009年個案驗證之結果指出，由西南氣流降雨指標所反演的降雨量，其平均誤差百分比為6.6%，顯示本研究建立之衛星估算西南氣流降雨量方法之可行性頗高。 In summer, southwesterly flow from South China Sea could enhance the precipitation in Taiwan, especially converged with typhoon peripheral circulation during the period of typhoon passed by. The rainfall is even intense than the typhoon itself which may cause the severe disaster in mountainous areas. Therefore, the investigation of the southwesterly flow effect on the precipitation around Taiwan areas is the main objective of this study. An index, southwesterly flow precipitation index (SFPI), composed of near sea surface wind field, water vapor content and cloud liquid water content within the study area (117˚-120˚E, 20˚-23˚N), was established for the precipitation potential in Taiwan with satellite data and ground-based observations. Nineteen cases of southwesterly flow precipitation from 2002 to 2008 were collected for constructing the precipitation potential, and showed a good consistency between SFPI and precipitation. However, the precipitation in some cases were underestimated when verified by the dataset of checking group (41 cases during 2009), which might be caused by the effect of convergence wind field and typhoon peripheral circulation. To eliminate the effect of convergence on precipitation, a screening method is proposed to select the case of southwesterly flow in the present study. Near sea surface wind field is averaged within the study area at first .If the wind direction and wind speed are quite uniform (more than 60%and 70%, respectively), the case will be identified southwesterly flow circumstance and then estimate the precipitation potential over Taiwan area. The results show that the average error of precipitation potential is less then 10% when comparing with ground-based observations, indicating that the SFPI is a valuable indicator in estimating the precipitation potential of southwesterly flow for Taiwan areas.
