对流尺度数值预报中的云物理初始化方法改进及个例试验

Cloud physics initialization for convection-scale NWP:Scheme improvements and a case study

通过在云初始化方案中增加由地表感热和潜热通量确定的对流尺度速度作为对流判据,同时增加层云云冰、云水计算方案,改进云分析方法,并基于第2代华东快速更新循环同化模式预报系统,针对2015年4月28日华东强对流个例,进行对比试验,分析了改进的云初始化方案对云分析结果和模式预报效果的影响。试验表明:在云分析中增加对流判据,使得平均40%左右的云分析格点判定为非对流格点,对流格点分布与正的感热通量分布相似,在陆地上有显著日变化。在对流和层云格点判定之后,增加层云云冰、云水计算方案分析层云格点,显著地减小了模式初始场的云冰、云水混合比,有效地减弱了模式积分初始阶段云冰、云水含量的剧烈调整,尤其是在陆地区域。采用改进的云初始化方案进行预报,可以减少模式前1 h和前6 h的降水强度;尤其在个例的循环试验中,强降水中心强度和面积的预报比原方案显著减弱。

To improve the cloud initialization,a convective-scale velocity determined by surface sensible and latent heat fluxes is introduced into the scheme as a criterion of convection to estimate cloud ice and cloud water for stratiform and convective clouds separately.Using the second generation of Shanghai Meteorological Service-WRF ADAS Rapid Refresh System,the improved cloud analysis method was tested in a severe convective case occurred on 28 April 2015 in East China with a focus on the influence of the analysis method on cloud initialization and forecast results.The results show that about 40% of the total cloud analysis grids were determined to be non-convective grids based on the convection criterion.The distribution of convective grids was similar to that of upward sensible heat flux,which displayed a diurnal variation over the land.With separate stratiform and convective grids as well as different cloud water and ice schemes,the mixing ratios of cloud ice and cloud water were significantly reduced,which led to a weakening of radical adjustment of cloud particles at the initial stage of the model integration especially over the land area.The precipitation intensity in 1 h and 6 h model forecasts using the improved cloud initialization scheme were reduced.In the cycling experiments,the area with strong precipitation tended to be smaller and the intensity was significantly weakened.