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Data Assimilation Seminar Series

Dr. Chih-Chien Tsai (Aug. 4, 2017, 15:30-)

Affiliation Taiwan Typhoon and Flood Research Institute (TTFRI)
Title Preliminary Experimental Results of Polarimetric Radar Data Assimilation in the Case of Typhoon Soudelor (2015)
Abstract Authors: Chih-Chien Tsai (TTFRI) and Youngsun Jung (CAPS, University of Oklahoma)

Polarimetric radars provide polarimetric variables such as differential reflectivity (ZDR), specific differential phase (KDP) and co-polar correlation coefficient (ρHV) that probe more microphysical characteristics of hydrometeors in addition to radial velocity (Vr) and reflectivity (ZH). To investigate the effects of polarimetric radar data assimilation on simulating severe weathers in Taiwan, an observation operator for ZH, ZDR and KDP in accord with various bulk microphysical schemes is incorporated into a Weather Research and Forecasting (WRF)-local ensemble transform Kalman filter (LETKF) system. The case of Typhoon Soudelor (2015) is selected for its devastating wind and rainfall impinging on northern Taiwan, where Central Weather Bureau's RCWF S-band polarimetric radar is situated. Comparing five cold-start simulations using different double-moment microphysical schemes with RCWF observations, the one using the WRF double-moment six-class (WDM6) scheme is found to give the least over-estimation. Then, using WDM6, several assimilation experiments are carried out to assimilate various combinations of radar variables for nine 15-minute cycles after a 6-hour spin-up of the same perturbed initial ensemble. Preliminary results show that the assimilation of Vr and ZH greatly corrects the spatial distribution of very short-term rainfall prediction within three hours, and the additional assimilation of ZDR and/or KDP further enhances the accuracy. More investigations are currently in progress.
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