Analysis on ECMWF Model Medium-Term Forecast Transition of "8.22" Heavy Rainfall Process in Chongqing

Huan WANG; Cheng-zhi DENG; Jing LI; Chao LIU

doi:10.13718/j.cnki.xsxb.2020.05.018

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There was a significant deviation in the forecast of the heavy rainfall process in central Chongqing from August 22 to 23, 2018. The precipitation observation data, high-altitude detection data and the synoptic situation forecasts from the ECMWF model (hereinafter referred to as EC) have been used in this paper to analyse the cause and influencing weather system of this process. The effects of different reporting time from the EC model were also evaluated. The results showthat 1) This rainstorm process was under the joint influences of the high-altitude trough and themiddle-lower level shear line. 2) The precipitation forecasts from the EC model had a sudden change around July 19, which was caused by the transitional change of the 500hPa geopotential height field forecasts from the same model in Chongqing. 3) Under the condition that the 500hPa height field forecasts was basically consistent with the actual situation, the middle and lower layer shear line in the central part of Chongqing was the reason for the apparent deviation between the subjective precipitation forecast in August 20 and the actual rainfall distribution.

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Analysis on ECMWF Model Medium-Term Forecast Transition of "8.22" Heavy Rainfall Process in Chongqing

Received Date: 10/06/2019

Available Online: 20/05/2020

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Abstract: There was a significant deviation in the forecast of the heavy rainfall process in central Chongqing from August 22 to 23, 2018. The precipitation observation data, high-altitude detection data and the synoptic situation forecasts from the ECMWF model (hereinafter referred to as EC) have been used in this paper to analyse the cause and influencing weather system of this process. The effects of different reporting time from the EC model were also evaluated. The results showthat 1) This rainstorm process was under the joint influences of the high-altitude trough and themiddle-lower level shear line. 2) The precipitation forecasts from the EC model had a sudden change around July 19, which was caused by the transitional change of the 500hPa geopotential height field forecasts from the same model in Chongqing. 3) Under the condition that the 500hPa height field forecasts was basically consistent with the actual situation, the middle and lower layer shear line in the central part of Chongqing was the reason for the apparent deviation between the subjective precipitation forecast in August 20 and the actual rainfall distribution.

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暴雨是一种多发性的灾害性天气，常造成人民生命财产的巨大损失.陶新娥等^[1]对长江流域气象旱涝异常急转进行了识别和分析，王中等^[2-5]对西南涡引发的重庆暴雨过程进行了详细的分析，陈鹏等^[6-8]对切变线引发的重庆暴雨过程进行了数值模拟和诊断分析，李强^[9]对川渝盆地主汛期强降水特征进行了研究，但目前对重庆暴雨预报失误过程的研究较少.在中期预报中强降水的落区预报一直是主要难点之一，而在预报服务业务中，对中期预报的需求又十分迫切^[10-12].中期数值预报模式是中期降水预报的主要工具，初始误差、模式误差和天气与气候的可预报性都会造成数值天气预报的不确定性^[13-15]. 2018年8月21-23日重庆出现了区域暴雨天气过程，中东部地区大雨到暴雨，部分大暴雨.此次过程具有持续时间长、暴雨落区和雨强分布不均的特点.重庆市气象台8月20日发布《重要气象信息专报》中的落区预报与实况偏差较大，所以本研究利用降水实测资料、高空探测资料和EC模式资料对“8.22”重庆暴雨过程的中期预报效果进行分析，找出EC模式在暴雨中期预报中的转折和预报误差产生的原因.

5. 结论

(1) 此次暴雨过程是在高原槽东移配合中低层切变线的共同影响下产生的，700 hPa切变线和850切变线相交的位置与暴雨落区一致.

(2) EC模式降水预报在7月19日前后出现了突变，其原因是影响重庆地区的500 hPa高度场的预报发生了转折性的变化，且暴雨落区均预报在重庆中东部地区.

(3) 在500 hPa高度场预报与实况基本一致的情况下，通过对中低层风场的分析发现，重庆中部地区的中低层切变线是20日主观预报降水落区与实况出现明显误差的原因，中低层是否有切变线或辐合的存在是暴雨预报关键点，且风速辐合的区域与暴雨落区密切相关.

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Analysis on ECMWF Model Medium-Term Forecast Transition of "8.22" Heavy Rainfall Process in Chongqing

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