基于Hybrid EnSRF-En3DVar的雷达资料同化研究

Study on the assimilation of Doppler radar data using a hybrid EnSRF-En3DVar method

基于WRF模式构建了Hybrid En SRF-En3DVar同化系统,该系统使用En SRF方案直接更新集合扰动。利用构建的同化系统针对台风"桑美"分别进行集合协方差权重敏感性试验和同化雷达不同观测资料的敏感性试验。集合协方差权重敏感性试验发现:当集合协方差权重分别为0.25、0.5和0.75时,同化效果优于3DVar试验,其中0.75的集合协方差权重试验得到了分析场的最优估计;当集合协方差权重为1.0时,分析场最差。同化雷达不同观测资料的敏感性试验表明,联合同化雷达径向风及反射率能有效改善大气湿度场和风场,但对风场的改善效果不如仅同化雷达径向风好。将En SRF集合扰动更新方案与扰动观测方案综合分析发现,扰动观测方案集合离散度较小,计算代价大,En SRF方案优于扰动观测方案。

A hybrid ensemble square root filter and three-dimensional ensemble-variational （EnSRF-En3DVar） data assimilation system is developed based on WRF model.The EnSRF is used to update ensemble perturbations in this system.Aiming at Typhoon Saomai,two sets of sensitivity experiments are conducted to test this system.One is choosing different weight coefficients of ensemble covariance,the other is assimilating radar radial velocity（Vr） and/or reflectivity（Z）.The first set of experiments is to answer how the flow-dependent covariance estimated from the forecast ensemble works best in the hybrid EnSRF-En3DVar system.The results show that the assimilation effects from experiments with 30 ensemble members and ensemble covariance weight coefficients of 0.25,0.5 and 0.75 are better than those from 3DVar experiment.The best（worst） one is from the experiment when weight coefficient of ensemble covariance is 0.75 （1.0）.The second set of experiments is to examine the impact of assimilating radar Vr and/or Z on the analysis using EnSRF-En3DVar system.The results show that the combined assimilation of Vr and Z can improve the initial analysis of humidity and wind fields,but the only assimilation of Vr can improve the analysis of wind field most.Finally,the methods to generate the ensemble perturbations are compared.The results show that the ＂perturbed observation＂ method gives smaller ensemble spread and needs more expensive computational cost than the EnSRF method.