川藏地区雷暴大风活动特征和环境因子对比

Comparison of Characteristics and Environmental Factors of Thunderstorm Gales over the Sichuan-Tibet Region

利用2010-2017年中国气象局重要天气报、地面观测和探空资料以及欧洲中期天气预报中心ERA-Interim再分析资料,对川藏地区雷暴大风的活动特征、环境因子和环流形势进行统计分析,并对其中高原(海拔高度不低于1 km)和盆地(海拔高度低于1 km)区域雷暴大风活动进行对比。结果表明:川藏高原区域雷暴大风频次呈5-6月和9月双峰型分布,主要发生在午后;盆地区域主要发生在夏季,午后和夜间均较活跃。高原站雷暴大风年平均频次约为2次/站,在雷暴和大风中分别约占4.5%和8%。盆地站年平均频次仅为0.4次/站,雷暴中仅占1.5%,但在大风中约占60%。高原站雷暴大风的中低层环境温度递减率较大,一般呈上湿下干的逆湿垂直结构;而盆地站雷暴大风通常具有上干下湿的垂直结构。分别对5-6月和9月高原站雷暴大风两个峰值时段的环流形势进行合成分析,发现5-6月受高空西风槽影响,中层有弱冷平流侵入,高层位于高空急流入口区右侧,环境垂直风切变较大;而9月受副热带高压边缘影响,中高层较干,低层暖湿气流明显。这些均有利于雷暴大风发生。

Characteristics,environmental factors and synoptic situations of thunderstorm gales over the SichuanTibet Region from 2010 to 2017 are analyzed based on significant weather report,surface observations and sounding data from China Meteorological Administration and ERA-Interim reanalysis data from European Centre for Medium-Range Weather Forecasts(ECMWF).Distinct properties are revealed through comparison of characteristics and environmental parameters of thunderstorm gales over highland(1 km above sea level)and basin(1 km below sea level).Results show that thunderstorm gales occur over the highland during a full year except winter,with two peaks in May-June and September,respectively.Their diurnal variation shows a major peak at 2000 BT.However,thunderstorm gales over the basin are active both in the afternoon and in the evening mainly in summer.The annual station-averaged frequency of thunderstorm gales over the highland is about 2 times per station,proportions of which to thunderstorms and gales are about 4.5%and 8%,respectively.It is only 0.4 times per station for thunderstorm gales over the basin,which account for 1.5% of the thunderstorms but 60% of gales.The atmospheric water vapor content,convective available potential energy and downdraft convective potential energy over the highland are significantly lower than those over the basin.The mean vertical temperature lapse rate in the middle and lower troposphere over the highland is larger than that over the basin.Usually,there is a shallow moist layer in the middle troposphere overlaid on a drier air layer over the highland.However,there is usually significant dry air in the middle troposphere and a moist layer at low level over the basin.Synoptic situations of thunderstorm gales over the Sichuan-Tibet Region are composited during two peaks in May-June and September,respectively.During May and June,the vertical wind shear of the environment is strong,with the middle level affected by a westerly trough transporting weak cold advection at 500 hPa,and the upper level located on the right side of a jet entrance at 200 hPa.However,in September,the middle level over the Sichuan-Tibet Region is at the north edge of subtropical high pressure at 500 hPa,with significant dry air in the mid-upper troposphere and remarkable warm moist air flow at low level.Though synoptic situations are different in two seasons,both of them can provide favorable condition to the formation of thunderstorm gales.