利用局地系集轉換卡爾曼濾波器雷達資料同化系統改善定量降水即時預報：莫拉克颱風（2009）

Improving QPN with an LETKF radar data assimilation system: Typhoon Morakot (2009)

蔡直謙(Chih-Chien Tsai)

指導教授 ： 廖宇慶 ； 楊舒芝

國立中央大學/地球科學學院/大氣物理研究所/博士(2014年)

摘要

本研究建置一套都卜勒雷達資料同化系統，結合局地系集轉換卡爾曼濾波器與天氣研究預報模式，並針對在台灣中、南部造成降雨紀錄和嚴重災害的莫拉克颱風（2009）個案，進行觀測系統模擬實驗與真實觀測實驗，以評估此雷達資料同化系統對於定量降水即時預報的助益。而研究的最大意義，為在台灣地形複雜且四周海域缺乏觀測等不利預報的條件下，提供以雷達資料同化改善颱風降雨即時預報的有效方案。

在觀測系統模擬實驗中，三維風場和雨水混合比為同化徑向風和回波之後改善最多的預報變數，可歸因於觀測算符內的直接關係。在降雨預報方面，雷達資料的正面影響可長達6小時。同化回波的改善主要在預報初期，同化徑向風的改善則較為延遲，而兩者皆同化的降雨預報表現最好。增加上游對流區域的觀測覆蓋量，亦可大幅提升降雨預報表現。另外，本研究針對颱風環流下所發展的對流雨帶，提出使用混合局地法進行雷達資料同化，以進一步改善降雨預報。

在真實觀測實驗中，此雷達資料同化系統仍能有效改善定量降水即時預報。同化回波時須使用變數局地化法，只用來更新雨水混合比。使用觀測空間的統計方法，能診斷預報偏差和理想系集離散度。混合局地化法在真實觀測實驗的效益更加明顯，尤其能提升觀測資料稀疏或破碎處的風場準確度，進而改善降雨預報。

關鍵字

局地系集轉換卡爾曼濾波器 ； 雷達 ； 資料同化 ； 定量降水即時預報 ； 莫拉克颱風

並列摘要

This study develops a Doppler radar data assimilation system, which couples the local ensemble transform Kalman filter with the Weather Research and Forecasting model. Its benefits to quantitative precipitation nowcasting (QPN) are evaluated with observing system simulation experiments (OSSEs) and real observation experiments on Typhoon Morakot (2009), which brought record-breaking rainfall and extensive damage to central and southern Taiwan. The purpose is to provide a useful plan of radar data assimilation for improving typhoon rainfall nowcasts in Taiwan, which are challenges due to complex terrain and the lack of in-situ observations over the surrounding sea.

In the OSSEs, the assimilation of radial velocity and reflectivity improves the three-dimensional winds and rain-mixing ratio most significantly because of the direct relations in the observation operator. For QPN, the positive impact of radar data lasts for 6 hours; the performance responds to reflectivity assimilation more quickly than radial velocity assimilation while assimilating both is most recommended. Increasing the observation coverage over upstream convection areas also largely enhances the QPN performance. For multi-scale interactions, we propose a mixed localization method, which yields further improvement.

Our system also improves QPN effectively with real observations. When real reflectivity data are assimilated, the variable localization method must be used to update only the rain mixing ratio. With observation-space statistics, the model bias and ideal ensemble spread can be diagnosed. The mixed localization method, which is more beneficial in the real case, enhances the accuracy of the wind field especially for the areas with sparse or discontinuous radar observations and also improves QPN.

並列關鍵字

local ensemble transform Kalman filter ； radar ； data assimilation ； quantitative precipitation nowcasting ； Typhoon Morakot

參考文獻

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