Assimilation of GBVTD-retrieved winds from single-Doppler radar for short-term forecasting of super typhoon Saotnai(0608) at landfall

摘要

A single-Doppler wind retrieval method called the Ground-Based Velocity-Track Display technique (GBVTD) has been developed in recent years to retrieve horizontal circulations of tropical cyclones. The technique is able to retrieve axisymmetric tangential and radial winds, asymmetric tangential winds for wave numbers 1-3, and along-beam mean winds in tropical cyclones. It has been successfully applied to tropical cyclone monitoring and warning. This study explores, for the first time, the assimilationof GBVTD-retrieved winds into a tropical cyclone prediction model, and examines its impact relative to that of directly assimilated radial velocity data, super typhoon Saomai (2006), the most intense landfalling typhoon ever recorded in China, is chosenas the test case, and data from the coastal operational radar at Wenzhou, China, are used. The ARPS 3DVAR system is used to assimilate either the radial velocity data directly or the GBVTD-retrieved winds, at 30 min intervals for 2 hours. The assimilation of the GBVTD-retrieved winds results in much improved structure and intensity analyses of Saomai compared to those in the Japan Meteorological Agency mesoscale reanalysis and compared to the analysis assimilating radial velocity (Vr) data directly. The ability of the GBVTD method in providing wind information covering the full circle of the inner-core circulation is the primary reason for its superior performance over direct assimilation of Vr data; for the latter, the azimuthal data coverage is often incomplete. With the improved initial conditions, the subsequent forecasts of typhoon intensity, track and precipitation are also improved. The improvements to both track and intensity predictions persist over a 12-hour forecast period, which is mostlyafter landfall. Subjective and quantitative evaluations of the precipitation and circulation patterns show consistent results. A further sensitivity experiment shows that the axisymmetric wind component in the GBVTD retrieval has the dominant impact onthe prediction.