Title

利用局地系集轉換卡爾曼濾波器雷達資料同化系統改善短期定量降雨預報: SoWMEX IOP8 個案分析

Authors

邵彥銘

Key Words

WRF-LETKF雷達資料同化系統 ; SoWMEX IOP8 ; SoWMEX IOP8

PublicationName

中央大學大氣物理研究所學位論文

Volume or Term/Year and Month of Publication

2015年

Academic Degree Category

碩士

Advisor

廖宇慶;楊舒芝

Content Language

繁體中文

Chinese Abstract

本研究使用一套結合局地系集轉換卡爾曼濾波器(LETKF)與天氣研究預報模式(WRF),同化多座都卜勒雷達資料,針對2008年西南氣流密集觀測實驗(IOP8)的兩個個案做分析與模擬,這兩個個案(0614、0616)分別都對台灣南部帶來龐大降雨,而本研究的實驗目的為探討此雷達資料同化系統是否能夠改善梅雨個案的短期預報降雨能力以及改善的程度。 在0614的個案裡,首先比較在觀測使用0dbz與沒使用0dbz的同化實驗,而結果顯示,在沒有雷達回波的位置補上0dbz能夠有效的壓制模式中錯誤的回波生成,且不會降低主要雨帶的降雨強度。而同化實驗長度由一小時加長為兩小時後降雨預報在預報的最後幾個小時比其他實驗都好,但是在前期受到EnKF起轉問題而有低估降雨的現象,對此,若以隨機擾動採樣進行EnKF初始化較能展現出梅雨時期的大尺度不確定性,亦較有利於同化。 在0616的個案裡,由於在台灣東部非雷達觀測區域的初始擾動偏濕,而使得實驗結果在此區域有較大的濕偏差。若直接使用ECMWF再分析資料所做的單一預報此偏差情形並沒有那麼嚴重,比對再分析資料後發現,水相粒子在擾動後起轉的分布情形與再分析場的濕度有關,由於此個案的再分析場濕度較大,濕偏差的情形也較嚴重,而此現象也因無觀測而有誤差持續累積的情形。但台灣西南方的降雨一樣有較好的估計,因此綜合兩個個案的實驗結果,使用WRF-LETKF雷達資料同化系統能有效改善梅雨的定量降雨預報結果。

English Abstract

The Local Ensemble Transform Kalman Filter (LETKF) method, coupled with Weather Research and Forecast (WRF) model, is applied to assimilate data from five Doppler radars in Taiwan, with the purpose of investigating the improvement on short-term quantitative precipitation forecast (QPF) for rainfall events occurred during the Mei-Yu season. Two heavy precipitation cases from the 2008 SoWMEX IOP#8 field experiments are selected. The overall results demonstrate that by using WRF-LETKF to assimilate the radar data, the performance of model QPF for representing the Mei-Yu rainfall can be significantly improved. In the first case of June 14, 2008, it is found that by assimilating the 0 dBZ data, the spurious convection can be effectively suppressed. Extending the length of the radar data assimilation to two hours produces better rainfall forecast results. Generating initial perturbations from randomly selected, 6-hr apart data from the NCEP 1ox1o re-analysis data turns out to be a better way to capture the uncertainty related to the Mei-Yu frontal flow than the original NCEP NMC method does. The same model setup and assimilation method is applied to the second event on June 16, 2008. The pattern and amount of the forecasted rainfall pattern and over southwestern Taiwan indicates a very encouraging result. However, the rainfall prediction over eastern Taiwan becomes unrealistic strong, and this over-estimation cannot be mitigated due to the lack of radar data in this area. This indicates the importance of having a complete radar coverage over Taiwan and vicinity area.

Topic Category 基礎與應用科學 > 大氣科學
地球科學學院 > 大氣物理研究所
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