Improving Convective-Scale Quantitative Precipitation Nowcasting with the Mean Recentering scheme: A Case Study of Typhoon Morakot (2009)

• Main Authors: Pin-Ying Wu, 吳品穎
• Other Authors: Shu-Chih Yang
• Format: Others
• Language: zh-TW
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/86392b

碩士 === 國立中央大學 === 大氣科學學系 === 104 === In this study, the mean recentering (MRC) scheme was applied to the ensemble prediction system (EPS) and incorporated with the WRF-LETKF radar data assimilation system. The benefits of the MRC scheme to the convective-scale quantitative precipitation nowcasting (QPN) are investigated by a case study of Typhoon Morakot (2009). As a key step in the MRC scheme, the best member was selected by the spatial correlation coefficients (SCC) of the 3-hour rainfall accumulation and the performance of the precipitation nowcasting with the EPS was evaluated. In the deterministic QPN shown in probability-matched (PM) ensemble mean, the MRC scheme reduced the unrealistic excessive rainfall at the southern Central Mountain Range (CMR). Result suggests that the effect of the MRC scheme on QPN is sensitive to the choice of the best member. Selecting the best member base on the metrics related to precipitation helps improve the patterns of the QPN and the probability quantitative precipitation forecast (PQPF). In addition, taking the mean of a few good members as the best initial condition leads to more stable improvement. The MRC scheme was further incorporated with the WRF-LETKF radar data assimilation system. First, result shows that assimilating radar radial wind helped the model to capture the strong rainfall near Alishan (阿里山). However, the unrealistic excessive rainfall prediction at southern CMR still existed. After applying the MRC scheme, the amount of hydrometeor at southern Taiwan was reduced, and thus the unrealistic excessive rainfall could be alleviated. In order to make the MRC cycle effective with the metric of rainfall accumulation, it is suggested to assimilate both radar radial velocity and reflectivity. Finally, result from the sensitivity experiments suggests that the 1-hr interval is long enough for the MRC cycle to improve the 1-6 hr QPN.