2020年7月4-8日长江中游极端暴雨特征分析

Characteristics analysis of an extreme heavy rain event in the middle reaches of the Yangtse River from July 4 to 8 in 2020

利用多普勒天气雷达、气象卫星、自动气象站等监测数据及NCEP再分析资料,对2020年7月4-8日长江中游持续大暴雨过程的降水特征、中尺度系统演变以及梅雨锋上温度、湿度、涡度、垂直速度等物理量分布特征进行了分析。结果表明:这次暴雨过程具有持续时间长、覆盖范围广、暴雨落区集中、累积雨量大等特点。500 hPa中高纬度稳定的“Ω”型环流是持续大暴雨形成的有利环流背景。对流单体回波顶高、强度大、质心低以及“列车效应”是此次过程暴雨强度大的重要原因。梅雨锋上物理量分布具有如下特征:自下而上由假相当位温(θ_(se))构成的梅雨锋接近垂直状态,锋区内对流层低层特别是850 hPa以下大气层结不稳定,且低层锋区内风速小;35°N以北自对流层顶有干冷空气向南侵入至梅雨锋附近;暴雨中心上空经历了低层辐合、高层辐散和垂直速度由弱增强到再减弱的演变过程,有正涡度向梅雨锋前输送;梅雨锋对流层低层水平温度梯度非常小,且梅雨锋区低层存在一相对低温区;梅雨锋附近存在一定的水平比湿梯度,且其北侧比湿梯度较大。

Based on Doppler weather radar data,meteorological satellite cloud images,observations from automatic weather station(AWS)and NCEP reanalysis data,we have conducted the analysis of a sustained extra heavy rain event in the middle reaches of the Yangtse River from July 4 to 8 in 2020 including its precipitation characteristics,meso-scale convective systems(MCS)evolution and the distribution of temperature,humidity,vorticity,vertical velocity and other physical quantities on the Meiyu front.The results indicate that the event is characterized by long duration,wide coverage,concentrated rainstorm areas,large accumulated rainfall.This sustained extra heavy rain event is due to the ambient background of a stableΩ-shaped general atmospheric circulation in the mid-and high-latitudes at 500 hPa.The high echo top,great intensity and low centroid of convective cell and the"train effect"are the main reason that leads to the heavy rain event with high intensity.The following are the main characteristics of the physical quantities on the Meiyu front.The Meiyu front formed by pseudo-equivalent potential temperature(θ_(se))from down to up is close to vertical,then the atmospheric stratification in the lower troposphere especially below 850 hPa within front zone is unstable and the wind speed in the low-level front zone is small.The dry and cold air from tropopause in north of 35°N intrudes southerly to the Meiyu front.There is an evolution of convergence in the mid-and low-level and divergence in the upper level and the vertical velocity increasing and then decreasing over the heavy rain center,and positive vorticity transporting to front of the Meiyu front.The horizontal temperature gradient in the lower troposphere of the Meiyu front is very small,and there is a relative low temperature zone in the lower level of the meiyu front zone.There is a certain horizontal specific humidity gradient near the Meiyu front,and that on the north side of the meiyu front is greater.