臺灣常受到西北太平洋及南中國海生成的颱風侵襲，如何準確地預報颱風的動向、強度、結構及降雨量相當重要。由於颱風的生成與發展常發生於廣大的洋面上，但海面上的觀測資料非常缺乏，沒有足夠的資訊和解析度來解釋颱風和附近的大氣環流，目前主要依賴遙測技術提供洋面上的觀測資料。許多研究證明全球導航衛星系統掩星技術之觀測資料具有便宜、準確及全球平均分佈等優勢，能彌補某些區域傳統觀測資料不足的問題。但影響颱風預報的因素很多，且無法完全掌握真實的大氣狀況，難以說明不同的掩星資料及使用不同的同化運算子對於颱風預報的影響。因此，本研究利用觀測系統模擬實驗（Observing System Simulation Experiments, OSSE），期望在假想的已知大氣情況下，利用自行設計的觀測位置，試圖了解同化不同的GPS掩星觀測資料─ ─折射率或偏折角─ ─對於颱風預報的影響。
本研究使用MM5四維資料同化系統（Four-Dimensional Variational Data Assimilation, 4DVAR），同化虛擬渦旋資料以及福爾摩沙衛星三號（FORMOSAT-3/COSMIC）衛星GPS RO折射率的觀測資料，期待找出一組最適當的結果作為Nature run ，提供二維射線追蹤模式（ray tracing model）作為真實的大氣狀態，並模擬COSMIC-2運行軌道以假想接收GPS衛星和GALELIO衛星之觀測點數，產生假想的掩星觀測資料，最後再以WRF三維資料同化系統（Three-Dimensional Variational Data Assimilation, 3DVAR），同化產生出來之折射率或偏折角GPS掩星觀測資料以進行觀測系統模擬實驗
研究中所選取的個案是2009年中度颱風莫拉克，在莫拉克模擬實驗中，同化虛擬渦旋資料能改善颱風的初始強度、結構以及中心定位；而同化真實的掩星觀測資料能調整初始資料的水汽場、溫度場與風場，降低模式初始場所存在的誤差。因此同時同化虛擬渦旋以及掩星觀測資料能使大氣環境和颱風本身的強度更接近真實，選其作為Nature run以進行OSSE實驗。在OSSE實驗中，不管是同化折射率或偏折角，結果皆顯示同化掩星技術之觀測資料筆數越多，改善程度越明顯。同化偏折角在第二、第三天雨量模擬結果，較同化折射率的結果更接近Nature run。且颱風路徑預報上，同化偏折角的結果又比同化折射率更接近Nature run。

Taiwan is often hit by the typhoons over the Northwest Pacific and the South China Sea. How to accurately forecast the typhoon's strength, structure and rainfall is very important. However, the generation and development of typhoons often occur in the ocean but lack of observations, without enough information to resolve the typhoon circulation. It’s more dependent on remote satellites to provide observations on ocean. Many studies have shown that the GNSS radio occultation (RO) observations possess the advantages of high resolution, high accuracy and global coverage, which can cover the shortage of the traditional observations. But, there are many factors that affect typhoon forecast; hence it is difficult to figure out how the model results are produced by using the different RO data or assimilation operators. Therefore, this study utilizes OSSE (Observing System Simulation Experiments), with simulated atmospheric circumstances and the pre-assumed RO positions, aiming to understand the relative impacts of different RO observations ─refractivity or bending angle, on typhoon prediction.
In this study, we use the MM5 4DVAR (Four-Dimensional Variational Data Assimilation) with bogus data assimilation (BDA) and the observed RO refractivity from the FORMOSAT-3/COSMIC to simulate a typhoon as the observed nature. The Nature run is then used to provide the atmospheric conditions required by the ray tracing model to simulate RO soundings for COSMIC-2 (GPS and GALELIO). Finally, we assimilate the simulated RO refractivity or bending-angle soundings using WRF 3DVAR (Three-Dimensional Variational Data Assimilation).
The case chosen for the impact study is the Morakot typhoon (2009) that affected Taiwan significantly. The initial strength, structure and position of Morakot were improved with reduced intensity error as bogus data assimilation was performed. The initial temperature and water vapor were adjusted by RO data assimilation. Therefore, the atmospheric environment and the strength of the typhoon are closer to the nature when assimilating both bogus vortex and refractivity. In the OSSE experiment, it was found that the improvement increases with more assimilated RO refractivity or bending-angle soundings from COSMIC-2. Furthermore, assimilation with bending angle appears to outperform that with refractivity in the simulated severe rainfall over Taiwan. The simulated track from assimilation with bending angle is also closer to the nature run than that from refractivity.

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