許多研究皆顯示全球定位系統(Global Positioning System)所提供的空載掩星觀測(Radio Occultation, RO)資料對改善劇烈天氣預報，如颱風路徑或是梅雨鋒面造成的強降水有相當正面的貢獻。由於掩星觀測能提供極高垂直解析度的溫濕度資訊，若能同化這些觀測資訊，預期可改善熱力場的初始條件，進而提升預報的準確度。雖然福衛三號任務(FORMOSAT-3/ COSMIC)已屆設計年限，導致實際獲得的觀測數量逐漸減少，但下一代的掩星觀測任務─福衛七號(FR-7/COSMIC-2)預計能適時地填補福衛三號的觀測缺口，加上更優秀的觀測儀器以及低傾角軌道設計，福衛七號任務將可加強對熱帶至副熱帶地區的觀測，提供更高水平密度的觀測資料。本計畫欲了解當掩星觀測數量與品質大幅提升後，同化福衛七號的掩星觀測對熱帶氣旋預報的影響。為能更仔細地分析福衛七號觀測的效益，我們將進行一系列的觀測系統模擬實驗(Observing System Simulation Experiment, OSSE)，探討2006年發生在大西洋的颶風Helene以及Gordon的發展過程。我們藉由真實場(Nature Run)、福衛三號的真實觀測位置以及模擬的福衛七號軌道位置製造出兩組虛擬的掩星折射率觀測。透過同化實驗，我們將分析福衛七號掩星觀測如何改善水氣分析場準確度，而此將如何影響後續預報過程的水氣傳輸與熱帶氣旋發展。 ;Many studies have shown that the space-borne GPS (Global Positioning System) RO (Radio Occultation) observation provide positive impact on severe weather prediction, such as typhoon and the heavy precipitation caused by Mei-Yu front. With the benefits of having atmospheric moisture and temperature profiles in high vertical resolution, assimilating the GPS RO observation is able to improve the thermodynamical initial condition and thus improve prediction. Although the FORMOSAT-3/ COSMIC mission reached the end of its design life and the real-time observing ability has been degrading, the next-generation mission, the FORMOSAT-7/COSMIC-2, is designed to be the replacement with a focus on tropic to sub-tropics. Given the higher horizontal observational density and better quality of FORMOSAT-7/COSMIC-2 data, we are motivated to understand to what extend this new generation GPS RO observation is capable to improve the accuracy of moisture analysis and to modify its transportation through the data assimilation. An Observing System Simulation Experiment (OSSE) was conducted with the WRF-LETKF data assimilation system for the period when Hurricanes Helene and Gordon (2006) developed on the Atlantic Ocean. The RO refractivity observations were generated from the nature run at the simulated FR-3 and FR-7 observation locations. Taking the experiment with FR3 RO data as the baseline, we plan to investigate the further impact from assimilation of FR-7/COSMIC-2 RO data on tropical cyclone prediction.
