本研究探討大氣-海洋交互作用對於一熱帶低壓登陸於臺灣西部所造成之臺灣地區劇烈降雨事件的影響，其發生於2018年8月23至24日。研究中使用兩個系集預報探討本事件，為使用COAWST（the Coupled Ocean-Atmosphere-Aave-Sediment Transport）耦合模式之海洋耦合(coupled)與非耦合(uncoupled)系集，其大氣初始條件來自WRF-LETKF（WRF-Local Ensemble Transform Kalman Filter）資料同化系統。 比較耦合與非耦合系集的預報結果顯示，耦合系集從臺灣西南部近海到沿岸比非耦合系集有更多雨，這個結果是由於海氣交互作用改變了天氣系統的特徵。由於只有耦合系集能夠模擬海面溫度的日夜變化，這個與非耦合系集的差異導致了兩者降雨量的不同。除此之外，熱通量是熱帶低壓發展的能量來源，因此熱通量在這個降雨事件中扮演了不可或缺的角色。總體來說，非耦合系集比耦合系集有更多熱通量，尤其是在臺灣西南部近海。濕度和溫度都藉由海氣交互作用被改變，導致在臺灣西南方的開放海域有更高的潛熱通量，造成耦合系集在此處有更多降雨。 依據系集成員的降雨表現與觀測雨量的差異，本研究將兩個系集皆分成三組：與觀測接近的相似組、降雨位置正確但雨量較觀測少的少雨組、以及位置與強度與觀測非常不同的差異組。每組的代表性成員則用來說明有無海氣耦合作用造成之降雨分佈差異。總體而言，非耦合系集傾向於在中國東南方產生另一個不真實的低壓系統，並造成8月24日的降雨。而在耦合系集中這個低壓系統因為海氣交互作用而被抑制。 ;This study investigates the impact of atmosphere-ocean interactions on a massive rainfall event of 23 and 24 August 2018 over Taiwan, which associates with the tropical depression making landfall in the western portion of Taiwan. There are two ensemble forecast experiments conducting in the present study, which uses the coupled ocean-atmosphere-wave-sediment transport modeling system (COAWST) in two modes (coupled and uncoupled) and the atmospheric initial condition provided by the WRF-Local Ensemble Transform Kalman Filter (WRF-LETKF) data assimilation system. Comparisons between the coupled and uncoupled ensemble forecasts show that the coupled run tends to have more rain than the uncoupled one from offshore to coastal region of southwestern Taiwan, a result from modifying the characteristics of weather systems by the air-sea interaction. The discrepancy in the magnitude of SST contributes to the difference in rainfall because the coupled run can simulate the SST′s diurnal cycle while the atmosphere-only uncoupled run does not. Besides, the heat fluxes play an essential role in the characteristic of this rainfall event because they are the primary sources of energy for TD′s development. In general, the uncoupled heat fluxes are stronger than that in the coupled run, especially over the offshore southeast of Taiwan. With air-sea coupling, both humidity and temperature are modified, leading to a higher latent heat flux over the open ocean southwest of Taiwan, causing the coupled run has more rainfall there. The ensemble members of both runs are grouped into three groups with rainfall behaviors similar to, less than, or very different from the observed rainfall. The representative ensemble member from each group is used to illustrate the distinctive features corresponding to the rainfall distribution of each group with and without air-sea coupling. Generally, the uncoupled run tends to generate another low-pressure system southeast of China, which is unrealistic and contributes to the rainfall on 24 August. The emerging of this system is discouraged after air-sea coupling.
