一個結合變分方法與系集卡曼方法的混合(Hybrid)資料同化方法將在本研究運用。在此，系集所計算的背景誤差協方差會透過變分資料同化求得最佳解，使所求得的最佳分析場同時具有傳統變分資料同化以及系集卡曼資料同化的雙重優點，以達成混合(Hybrid)的目的。 本研究中沒有使用任何虛擬渦旋方法並透過以GSI 3DVAR為基礎的混合資料同化系統，且在中央氣象局的全球預報模式來運行以了解其對於改善颱風預報準確度的影響。而熱帶氣旋主要的生命周期是以海上為主，在一般同化所需的觀測資料中，傳統觀測一直是模式初始分析最主要的資料來源，一般衛星觀測則為輔助來源，但在海上是缺乏傳統觀測資料的。近年發展的GPS掩星觀測主要是利用無線電波掩星折射觀測原理，得到「偏折角」及「折射率」資料，進而推求得知大氣層溫度、氣壓、乃至水汽等垂直剖面，由於其高垂直解析之優勢，更可涵蓋傳統觀測方式不易取得的海上資訊。 本研究所選取的個案為2015年中度颱風蘇迪勒(Soudelor)，並使用ERA Interim 的再分析資料來做為驗證場，實驗共分為兩個部分，第一部分比較運用混合資料同化系統相較於傳統三維變分資料同化方法對於預報準確度的關係，結果指出使用混合資料同化系統有效地改善全球環境場，進而使預報準確度提升。第二部分的實驗探討分別同化GPS掩星觀測資料裡的折射率以及偏折角對全球模擬結果以及颱風周圍環境場的改善和颱風模擬的影響，結果顯示在同化中加入GPSRO的觀測資訊會讓模擬更接近驗證場。 ;An ensemble -variational hybrid data assimilation (DA) system based on Ensemble Kalman filter (EnKF) and three-dimensional variational data assimilation (3DVar) system was applied in this study. In this hybrid system, flow-dependent ensemble covariances were estimated from an EnKF-generated ensemble and incorpotated into the variational minimization. In order to understand the improvement of the analysis for forecast of tropical cyclone (typhoon) in hybrid system , gridpoint statistical interpolation (GSI) 3DVar-based hybrid system were executed in CWB global forecast system model(CWB/GFS) without employing any bogus schemes . Conventional observation data are the main source for the model initialization in the DA method , but there is lack of conventional data over the ocean where the typhoon evolves. GPSRO data was developed recently , which makes use of radio signals transmitted by the global positioning system (GPS) satellites and then obtains the data of bending angle and refractivity to estimate temperature , pressure and vapor in the global atmosphere with high precision and vertical resolution over the entire ocean . In this study , the hybrid system is used to improve the forecast of Typhoon Soudelor(2015/08) and the ERA Interim data is regarded as the verification field. There are two main experiment in the research. One is the comparison of hybrid system and conventional 3DVar system on the impact of forecast accuracy. The result shows that using hybrid system would have a better performance than using 3DVar system in global domain. The other is the impact of assimilating GPSRO data (bending angle and refractivity) for the forecast accuracy. Sensitivity tests indicated that assimilation of GPSRO data would have a positive impact both on the prediction in global domain and typhoon domain.
