| 摘要: | 同化福衛三號全球定位系統無線電掩星觀測對西北太平洋地區颱風路徑預報之影響
摘要
本文就颱風預報作業的觀點，系統性評估全球定位系統無線電掩星（Global Positioning System Radio Occultation, GPSRO）觀測對西北太平洋地區颱風路徑預報的效益。實驗以中央氣象局作業之45公里解析度TWRF（typhoon WRF, The Weather Research & Forecasting Model)模式分析預報系統，對西北太平洋地區2008至2010年間11個颱風個案（總共有327個分析預報時窗）分別對有無同化福衛三號（FORMOSAT-3/COSMIC）GPSRO折射率觀測作比較。平均而言，在每12小時的部分循環（partial cycle）同化期間，約有100筆GPSRO觀測被納入同化。實驗結果顯示，同化GPSRO觀測能有效改善36小時之後的路徑預報，至72小時預報則能減少路徑預報誤差約12km（5%），此預報誤差之改善程度雖不大，但具有統計意義上的顯著性。路徑預報誤差有所改進的原因是同化GPSRO對於模式溫度、水氣及風場的分析與預報均能有所改善。更進一步的分析顯示，颱風路徑預報誤差因西太平洋副熱帶高壓及其環流預報藉由同化GPSRO而獲得改善，而使得對環境駛流能有更佳的掌握。
此外，本文基於同化GPSRO折射率觀測能改善颱風路徑預報的結論，利用偏折角局地波譜寬（local spectral width, LSW），測試更進一步提升GPSRO觀測效益之可行性。LSW是在掩星觀測發生多重路徑傳播（multipath propagation）與繞射現象，為決定掩星觀測偏折角時所衍生之觀測產品。偏折角的LSW可用來表示該筆RO觀測之不確定性，透過此特性，本研究將LSW運用在同化福衛三號GPSRO觀測折射率時的觀測品質檢定（quality control, QC）及四維動態觀測誤差。透過颱風個案實驗顯示，利用LSW之QC與動態觀測誤差技術，對於改善該個案颱風路徑預報均有正面影響，建議LSW應用技術的發展有其可行性。
;The impacts of assimilation of FORMOSAT-3/COSMIC GPSRO data on the prediction of typhoons over the Northwestern Pacific
Abstract
In this research, we perform a systematic evaluation of the impact of Global Positioning System radio occultation (GPSRO) data on typhoon track prediction over the Northwestern Pacific with respect to the point of view of operational forecasts. Specifically, we perform data assimilation and forecast experiments using the Typhoon Weather Research and Forecasting (TWRF) system, the regional operational model, at 45-km resolution on eleven typhoons (with a total of 327 cases) in the period of 2008-2010 over the Northwestern Pacific, with or without the use of FORMOSAT-3/COSMIC (Constellation Observing System for Meteorology, Ionosphere and Climate) GPSRO refractivity observations. On average, about 100 GPSRO soundings are assimilated over a 12-h partially cycling assimilation period. The results indicate that the assimilation of GPSRO data reduces the typhoons’ track forecast errors after the 36-hour forecast, especially the reduction of the 72-hour track forecast errors by approximately 12 km (5%). Although this is only a modest improvement, it is statistically significant. The improvement of track forecast errors is due to improvement of the analysis and the forecast of temperature, water vapor, and wind fields by assimilating of GPSRO data. Further analysis shows that the reduction in typhoon track forecast errors can be attributed to the improved prediction of Western Pacific Subtropical High (WPSH) and its associated circulation, which leads to better forecasting of the environmental steering flow.
On the other hand, based on the conclusion of positive impacts of GPSRO data on forecast of typhoon movements, local spectral width (LSW) of bending angle is applied to further increase this impact. The LSW is a derivative obtained while retrieving the bending angle under multipath propagation and diffraction and can be used to as a measure of the uncertainty of RO observation. Hence, taking the advantage of this feature, this study applied the LSW on the quality control procedure of assimilating the FORMOSAT-3/COSMIC GPSRO refractivity data and its 4-D dynamic observational errors in TWRF. By the results from experiments for typhoon events, applications of the LSW show neutral impact on both track and field forecasts but have positive impact for some cases. According to the analyses of these cases, The LSW have the potential to increase the value of GPSRO data. |
