沂沭泗流域2020年一次致洪暴雨天气特征及其成因分析

Characteristics and Causes of a Flood-induced Rainstorm in 2020 in the Yishusi Basin

本文利用ERA5(European centre for medium-range weather forecasts re-analysis 5)逐小时资料、中国地面降水日值数据集(V2.0)和中国自动站与CMORPH降水产品融合的逐时降水量网格数据集(1.0版),对比分析沂沭泗流域2019年1909号台风“利奇马”和2020年8月13日特大暴雨2次致洪暴雨过程的时空特征。2次过程前期降水存在较大差异,“利奇马”过程降水持续时间长,影响区域广,过程雨量大,沂沭泗流域面雨量达1978年最强,但前期流域降水异常偏少五成以上。而8.13致洪暴雨过程前期沂沭泗出现多次强降水,降水异常偏多,降水较常年偏多八成,前30日累计降水总量为1978—2020年历史最大值。分析造成2020年前期降水异常偏差成因:7月20日~8月15日副热带高压强度异常偏强,脊线偏西,且贝加尔湖-蒙古地区冷涡不断有冷空气分裂南下,冷暖空气在江淮-黄淮地区对峙,沂沭泗流域有利于出现连续性强降水。此外,分析8.13致洪暴雨天气尺度和中小尺度系统可知,降水区处于东北冷涡底部和副高边缘,副高呈东北-西南走向,形成高压坝,有利于降水系统稳定少动;低层西南暖湿气流强盛、切变线维持。高、低空急流耦合作用使得低层辐合、高层辐散加剧,降水区垂直运动得以加强和维持。沂蒙山区地形不仅有利于流域坡面汇流,而且造成风场迎风坡辐合,对降水有一定的增强作用,并影响降水落区。最后,研究相对风暴螺旋度与强降水落区发现,两者具有较高的相关性,螺旋度对降水预报提前量超过4 h,且螺旋度中心值越大,雨强越大,螺旋度中心强度的维持预示着强降水的持续,因此相对风暴螺旋度在沂沭泗流域暴雨的预报中可作为重要的参考因子。

The temporal and spatial characteristics of two flooding rainstorm events in Yishusi Basin in 2019 and 2020 are compared and analyzed,based on the hourly data of ERA5(European centre for medium-range weather forecasts re-analysis 5),the China daily precipitation data set(V2.0)and the hourly precipitation of grid data set(version 1.0)which integrated by automatic station and CMORPH precipitation products.There was a significant difference in the pre-event precipitation between the two events.In the case of Typhoon Lekima,the rain lasted longer,affected a larger area,and resulted in a greater rainfall amount,with the basin experiencing the strongest rainfall since 1978.However,the pre-event precipitation anomaly was less than 50%of the normal amount.On the other hand,there were multiple heavy rainfall events in the basin before the August 13 event,with a significantly greater precipitation anomaly and a cumulative precipitation over the 30 days prior to the event reaching the historical maximum between 1978 and 2020,with a precipitation anomaly of 80%more than the annual average.The article analyzes the cause of the pre-event precipitation anomalies in 2020,which can be attributed to the exceptional strengthening of the subtropical high from July 20 to August 15,with its ridge line shifting westward,and the continuous southward intrusion of cold air masses split from the cold vortex in the Baikal-Mongolia region,causing a confrontation between warm and cold air in the Yangtze-Huaihe region,which was conducive to the occurrence of continuous heavy precipitation in the Yishusi Basin.Moreover,the analysis of weather scales and meso-and small-scale system during the August 13 flood event reveals that the precipitation zone was located at the northeast bottom of the cold vortex and the edge of the subtropical high,with the subtropical high extending in a northeast-southwest direction,forming a high-pressure dam that kept the precipitation system stable and less mobile.The low-level southwest warm and moist airflow was also strong,and the shear line was maintained.The coupling of high and low-level jet streams intensified the convergence in the lower troposphere and divergence in the upper troposphere,which enhanced and maintained the vertical motion in the precipitation area.The topography of Yimeng mountain area is not only conducive to the confluence of watershed slope,but also easy to cause the convergence of windward slope,which enhance the precipitation to a certain extent,and affect the falling area.Finally,by analyzing the relative storm helicity and heavy rainfall area,it is found that they have a good correlation.The helicity have an advance of precipitation forecast more than 4 hours.The larger the helicity center,the greater the rainfall intensity.The maintenance of helicity center intensity also indicates the last of heavy rainfall.Therefore,the relative storm helicity can be used as an important reference on the rainstorm prediction of Yishusi Basin.