川西高原冰雹时空分布特征及大冰雹环境条件分析

Characteristics of Hail and Environmental Conditions of Large Hail in Western Sichuan Plateau

选取2001—2016年川西高原31个县市气象站逐月冰雹日、3个探空站资料及ERA5逐小时再分析资料,分析川西高原冰雹时空分布特征及冰雹发生的天气背景和环境条件。结果表明:(1)近16 a川西高原冰雹日数总体呈减少趋势,高海拔降雹次数多于较低海拔地区。降雹频率分布曲线呈双峰偏态型,阿坝州的峰值出现在5月和9月,甘孜州出现在6月和9月。降雹主要发生时段为11—21时,占比95.5%,峰值出现在15时和17时。冰雹直径<5 mm冰雹占年降雹的72%以上。(2)连续冰雹天气的发生多伴随有高原低值系统的东移,黄淮低涡西侧的切变是影响夏季阿坝地区的重要天气系统之一。不同季节阿坝北部的大冰雹天气均是在高原低值系统的影响下发生,春、夏季大冰雹个例受明显的冷空气或冷平流的影响。(3)大冰雹个例中,-20℃层高度为7.2~8.6 km,0℃层高度距离地面4 km左右,对流有效位能CAPE≥980 J·kg^(-1)。无论春夏还是秋季,红原大冰雹个例的中低层温差(T600~400 hPa)≥21℃且600~400 hPa垂直风切变达到15~21 m·s^(-1),有利于形成高度组织化的对流风暴。

Based on the monthly hail days data,data of three sounding stations and ERA5 hourly reanalysis data of 31 meteorological stations in the western Sichuan Plateau from 2001 to 2016,the temporal and spatial distribution characteristics,the weather background and environmental conditions of hail were analyzed.The results showed that:(1)The hail frequency distribution curve in the west Sichuan Plateau from March to November showed a bimodal skew pattern,and the peak value of hail occurred in May and September in Aba,and in June and September in Ganzi.In recent 16 years,the number of hail days in western Sichuan Plateau showed a decreasing trend.Hail fell more frequently in high altitude areas than that in lower altitude areas.Hail occurred mainly from 11:00 to 21:00 in the west Sichuan Plateau,accounting for 95.5%.(2)The occurrence of continuous hail weather was usually accompanied by the eastward shift of plateau low value system and the shear effect on the west side of Huang-Huai low vortex,which was a weather system that could not be ignored in Aba area in summer.The large hail weather in the north of Aba in different seasons all occurred under the influence of the plateau low value system.The large hail in spring and summer were affected by obvious cold air or cold advection.(3)No matter in spring,summer or autumn,the temperature difference(T600~400 hPa)in the middle and low levels of the large hail case in Hongyuan was≥21℃and the vertical wind shear at 600~400 hPa reached 15~21 m·s^(-1),which was conducive to the formation of highly organized convective storms.The height of-20℃layer was 7.2~8.6 km,the height of 0℃layer was about 4 km from the ground,and the CAPE≥980 J·kg^(-1).