一次干旱区极端暴雨天气的中尺度特征分析

Analysis on Mesoscale Characteristics of an Extreme Rainstorm in Arid Areas

干旱区内的暴雨天气发生频次很低,但影响重,深入分析其中尺度特征,对进一步理解干旱区暴雨的形成机理有重要意义。本文利用ECMWF的ERA5逐小时再分析资料、FY-4A卫星红外通道资料、嘉峪关雷达资料和实况观测等资料,分析了2019年6月20日引发河西走廊西部极端暴雨的β中尺度高空低涡的发展演变特征及暴雨局地性成因。结果表明:β中尺度高空低涡的雏形在青海海西州的西部生成,向甘肃肃北移动,从高层(400~250 hPa)向低层(700 hPa)发展,强度逐渐增强,移动过程中垂直涡度的空间结构从大范围倾斜涡柱逐渐收缩成近乎直立的深厚低涡,此阶段,200 hPa正涡度平流、正湿位涡下传等因子起主要作用,温度平流的作用较小。随后高空低涡缓慢东移,强度开始衰减,影响范围扩大,动力作用减弱,但中低层潜热反馈和低涡系统下山减缓高空低涡的衰减速度;高空低涡仍维持12 h以上,此时大气处于弱不稳定状态,水平尺度仅有40 km酒泉暴雨发生在此阶段。整体来看,湿位涡下传对高空低涡演变有较好的指示意义。酒泉站附近形成相较于周围更强的水汽辐合和上升运动,随着边界层偏强东风从鼎新-金塔垂直于祁连山脉灌入酒泉站附近,触发局地弱对流,雷达回波呈质心低,强度弱的特征,形成最大小时雨量为23.4 mm·h^(-1)的降水,大于10 mm·h^(-1)短时强降水持续3 h。

The frequency of rainstorms in the arid areas is very low,but its impact is serious.Analyzing the mesoscale characteristics of rainstorms in arid areas is of great significance to further understanding their formation mechanism.Based on hourly reanalysis data of ERA5 from ECMWF,FY-4A satellite infrared channel data,Doppler radar data at Jiayuguan,observations and etc,the evolution characteristics of mid-β scale high-altitude vortex and the local cause of the extreme rainstorm in the western Hexi Corridor on June 20,2019 were analyzed.The results showed that the mid-βscale high-altitude vortex was formed in the western Haixi of Qinghai Province and moved towards Subei of Gansu Province with developing from high(400~250 hPa)to low level(700 hPa)and increasing of the intensity.Meanwhile,the spatial structure of the vortex axis tilted in large scale at the beginning and then gradually evolved into a nearly vertical deep system.The positive advection of vorticity at 200 hPa,the moist potential vorticity and other factors played the major roles in the development of high-altitude vortex at this stage,while the effect of the temperature advection was limited.Then the high-altitude vortex slowly moved eastward with the weakening of intensity,but the influence scope expanding.The effect of dynamic on the evolution of high-altitude vortex was weakened,but the decay rate of the high-altitude vortex was slowed down by the medium-to-low level latent heat feedback and the low system downhill.The high-altitude vortex was maintained over 12 h with the atmosphere remained at weak unstable state and the rainstorm with the scale of only 40 km in Jiuquan was organized at this stage.Overall,the downward propagation of moist potential vorticity was a fairly good indicator in the evolution of high-altitude vortex.The stronger water vapor convergence and upward movement were formed near Jiuquan station than the surrounding areas.The radar echo was characterized by low-quality heart echo and weak intensity.The local weak convection was triggered as the strong easterly winds from Dingxin to Jinta in boundary layer that were perpendicular to Qilian Mountains poured into Jiuquan.The maximum hourly rainfall was 23.4 mm·h^(-1),and the short-time heavy rainfall greater than 10 mm·h^(-1) lasted for 3 h.