dc.description.abstract | Taiwan is located in the northeastern pacific region where tropical cyclone is very active and thus a major path for typhoons. Every typhoon season, approximately 30 typhoon forms in the northeastern pacific region. When the typhoon approaches Taiwan, the track and circulation of the typhoon changes dramatically due to the complex geographic features of the island, so it’s difficult to predict the typhoon’s track and rainfall.
Compared with conventional data, radar observations have an advantage of high spatial and temporal resolutions, and Doppler radars are capable of capturing detailed characteristics of flow fields, including typhoon circulation. The observation of radar network in Taiwan was used to investigate the impact of radar data assimilation for typhoon simulation. The case of Typhoon Nanmadol(2011)was chosen for this study, it struck Taiwan from 28 Aug 2011 to 29 Aug 2011. The purpose of this study was to adjust the initial field of the numerical model to improve the short-term typhoon predictions near Taiwan by using Doppler radar radial wind data assimilation. The typhoon track, structure, and precipitation were also inspected to clarify the effect of radar data assimilation simulated when typhoon approaching Taiwan.
In this study, Weather Research and Forecasting(WRF V3.2.1) model and 3D-VAR method of WRF Data Assimilation system(WRFDA V3.2.1)were used in doppler radar data assimilation.. The horizontal grid resolution of nested domains is 15 and 5 km, respectively, horizontal grid points were 301×253 and 241×241, model vertical layers extended from the surface up to 50 hPa with 35 levels. Initial conditions at 0000 UTC 28 Aug, and the WRF 3DVAR cycling from RCKT of CWB at 0600、0900、1200、1500 and 1800 UTC 28 Aug. The 3DVAR influence factor of horizontal scale for radar data assimilation was set R1 = 0.06 and R2 = 0.12.
After a series of experiments, we obtained some conclusions :(1) Compared with the one-time radar data assimilation, cycling of the multiple-time radar data assimilation has more positive impact on typhoon simulation, (2)Larger influence factor of horizontal scale (R2 = 0.12) also has more positive impact than smaller influence factor of horizontal scale(R1 = 0.06)for typhoon simulation in Taiwan. More assimilation cycle collocate with a larger influence factor of horizontal scale(R2 = 0.12), had improved the accuracy of the numerical simulation. Compared with the non-assimilate experiment, the precipitation pattern and typhoon track in the assimilation experiment were closer to the observations.
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