Doppler Analysis And Simulation On Typhoon Bilis

The two-step variational method was used to analyse the 3D wind structure of typhoon Bilis by using two Doppler radars. WRF model and WRF 3D-Var were employed to estimate the effect of radar data assimilation on the forecasts of this typhoon process.Firstly, The 3D wind fields of this typhoon process was analyzed by using the two-step variational wind retrieval method. The results show that the method has ability to retrieve the wind characteristics of typhoon both in the horizontal and vertical sections. The asymmetrical structure can be obviously seen from the horizontal wind fields. The wind field is presented the funnel-shape characteristic with the increase of height. The downdraft occurred in the inner core and the updraft in the external part of the typhoon. By analyzing on the variables, it is found that there are a principal circulation in which the radius of the maximum wind speed dipped outward with the height and a secondary circulation with the inflow at the lower layers and outflow at the higher layers, then we utilize the simulation result by wrf model to test the retrieval result of this typhoon process.Subsequently, the retrieval wind fields were used to verify the common vortex models. The results show that the Rankine model fits well within the radius of the maximum tangential velocity, and the Chen 3 model fits well outside the radius of the maximum tangential velocity.Compared the retrieval wind from single radar with dual-doppler radar data, a constraint on this typhoon pattern was introduced into the cost function in the two-step variational method. It improves the retrieval ability of two-step variational method from single-Doppler radar data. The retrieved winds were consistent with that from two Doppler radar data.Finally, the radial velocities were assimilated into WRF 3D-Var system and the forecasts were performed by WRF model to test the effect of radial velocity assimilaton. Three experiments was designed: 1) No data was assimilated; 2) radial velocity was preprocessed to the size (about) model horizontal resolution and assimilated directly into 3D-Var system; 3) The retrieval winds by two-step variational method was assimilated into 3D-Var system. The results show that the wind is greatly adjusted and the horizontal wind is asymmetrical and eccentrical while assimilating retrieval wind. In the first nine-hour forecasts, radar data assimilation can improve the skill of the typhoon tracks forecasts. However, for all the experiments the forecasted central sea level pressures of this typhoon deepen than the observations. The forecasts of assimilating retrieval wind are better than assimilating radial velocity directly.