Owing to a lack of sufficient observational data to provide an accurate initial field in numerical modeling,the numerical prediction of severe convective weather systems is always difficult.With the rapid development of atmospheric detection technology,nonconventional observed data(e.g.,radar data)have been widely applied.Compared with conventional data,radar data-with their higher spatial and temporal resolution-can extract more useful information to improve the model's initial field,improve rainstorm mesoscale weather forecasting,and promote the simulation process capability.
In this paper,the impact of Doppler radar radial velocity on the prediction of a heavy rainfall event is examined.Based on the mesoscale nonhydrostatic Weather Research and Forecasting(WRF)model and its three dimensional variational assimilation system(WRF-3DVAR),a control experiment and three assimilation experiments were conducted for a heavy rainfall process that happened over the Huai-he River basin in July 2007.The model background fields were provided by NCEP data,and the Doppler radar radial velocity data were obtained from Fu-yang station,which houses a new generation of S-band Doppler Weather Radar(Model:CINRAD/SA).Because the effect of the radar data is related to the thinning method,this paper focuses on the influence of different thinning methods of radar data on assimilation results and rainstorm numerical simulations.To this end,a new radar data thinning method is introduced.Firstly,a horizontal polar coordinate plane with certain resolution is constructed;secondly,radar data on the elevation plane are projected onto the polar coordinate plane;then,the distances between the projected points and the polar coordinate plane grid points are calculated;and lastly,the nearest points are regarded as good points and the corresponding radar data are retained.In this method,the degree of radar data thinning depends on the density of the polar coordinate horizontal structured grid.
The numerical results showed:(1)By assimilating the Doppler radar radial velocity data,more sophisticated mesoscale information were retrieved in the initial wind fields,which was helpful in the simulation of the rainstorm process;(2)Through the assimilation of the Doppler weather radar radial velocity data,the circulation structure of the storm could be adjusted and the false precipitation in the control experiment could be reduced.Compared with the control experiment,both the rainfall intensity and the rainfall area in the assimilation experiments were improved greatly.Furthermore,it also made a positive contribution to further prediction of mesoscale heavy rainfall;(3)With more radar radial velocity data introduced into the model initial fields,the initial field and the simulation of precipitation could be improved more accurately.For different thinning methods,the assimilation of radar radial velocity data had different effects on the initial field,which also led to different results in the precipitation forecast.Compared with other tests,in the RV3 test(i.e.,radial polar coordinate grid resolution taken as 10 km),the pattern of precipitation and the storm center were the best.
It is important to note that the findings of this study are based on one heavy rain case;future research and more cases are needed to obtain more and deeper conclusions.