Conventional meteorological data,Yinchuan CD Doppler radar and ECMWF,T639,WRF,NCEP/NCAR data are used to analyze the causes,predictability and significant forecast error margin of the rare severe rainstorm along the eastern Helan Mountain on 21 August 2016,which was missed by Ningxia Meteorological Observatory.The results show that the ECMWF,T639,WRF models predicted that there would be precipitation below 15 mm in the mid-north of Ningxia,but the magnitude was significantly smaller.Among the three models,the effect of ECMWF forecast was obviously superior to the other two.In the mid and late August 2016,the subtropical high was stronger continuously,located by north and getting to the strongest on the 21st.Under the control of the 588 dagpm line of the subtropical high,when the 592 dagpm line rapidly retreated eastward in southern Ningxia,the interaction of boundary layer shear line,low-level convergence and jet as well as the terrain of Helan Mountain triggered this severe rainstorm.Lack of experience,the local forecasters failed to capture the evolution of the circulation.The southeast and southerly low-level jet were formed when the stream in lower and middle layers were westbound,which was originated from the East China Sea,and established a warm convergence line in the eastern foot of Helan Mountain.The terrain friction convergence,the uplift of the wind and the movement of the mountain blocked the flow of Helan Mountain,and extended the rainfall time,contributing to the intensification of the rainstorm.Unfortunately,forecasters neglected the impact of Helan Mountain terrain on the rainfall.They didn't grasp the forecast indicators,such as K index≥38℃,LI≤-3,Q≥12 kg · kg-1.By the analysis,the predictability of the severe rainstorm on 21 August was put forward.%利用常规气象观测资料、银川CD多普勒雷达探测资料和ECMWF、T639、WRF、NCEP/NCAR等模式资料,分析了宁夏气象台漏报的2016年8月21日夜间贺兰山东麓历史罕见特大暴雨天气过程的成因,探讨了重大预报误差之缘由和可预报性,结果表明:(1) ECMWF、T639、WRF三种模式均预报宁夏中北部有15 mm以下的降雨,量级显著偏小是漏报诱因;而ECMWF降雨预报结果明显优于其他模式.(2)2016年8月中下旬西太平洋副热带高压持续偏强、位置偏北,21日达最强,在588 dagpm线控制下,592 dagpm线从宁夏南部东退,受低层切变线、辐合场和低空急流及贺兰山地形的共同影响,引发了宁夏极为罕见的特大暴雨.当地预报员对于极端暴雨事件预报经验匮乏,对副热带高压强盛、位置偏北的极强暴雨环流形势演变了解和认识欠缺.(3)源于东海的中低层偏东气流在西行中形成东南和南风低空急流,并在贺兰山东麓建立一暖性辐合线,由于贺兰山地形阻挡了气流的移动而产生的绕流、摩擦辐合及迎风波抬升,延长了降雨时间,对暴雨的增幅有促进和加强作用,预报中忽视了贺兰山地形对降雨影响.(4)对暴雨预报有效的物理量场θse500-θse850 ＜0 K、θse500≥337 K和θse850≥337 K、K指数≥38℃、LI≤-3、Q700≥12 kg· kg-1等指标掌握应用不熟练.分析结果表明:对于8月21日特大暴雨预报员可提前6h确定暴雨落区、订正降雨量级达到暴雨,可做到过程不漏报,但是经订正后的预报无法报出历史罕见的极端暴雨.
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