Abstract:

Abstract:The three hail processes in northern Shanxi Province in 2017 were analyzed comparatively using automatic meteorological station data, Doppler radar data，NCEP/NCAR FNL 1°×1° reanalysis data and disaster investigation data. The results were  as follows: (1) The three processes of hail  produced by pulse-storm occurred under the background of atmospheric circulation with the type of “two troughs and one ridge” and northwest airflow in the back of the trough at  500 hPa, the lower layer mesoscale shear lines or vortex were the main trigger system. Hails appeared in warming areas in the southeast of the mesoscale convergence line in the lower troposphere. The different lower layer mesoscale triggering systems and different atmospheric instability caused different strong convection dropping zones and intensity. (2) In the processes of hailstorm produced by pulse-storm, the wind direction from lower layer to upper layer rotated clock-wise with height, the wind speed increased with height from lower layer to 700 hPa level, and the vertical wind shear of 0-6 km ranged from 1×10-4 to 7×10-4 s-1, which was less than the hail weather threshold. (3) The radar echoes of the pulse storms were massive, and the strong center height was above the height on which the temperature was -10 ℃, its initial height was above 7 km, and the overshooting cloud top height was above 12 km. The formation, development and ending of pulse storms were closely linked to the overshooting, sinking and collapse of storm cloud top. The occurance and extinction of the storms could be determined by the shape of over-shooting cloud top, but it’s important to raise the elevation to 8 km above in order to find pulse storms ahead of time. From the radial velocity field, it could be seen that there were obvious differences among the three processes. Firstly, the duration of hailstorm was closely related to the thickness of the convergence layer. The thicker the convergence layer, the more intense the hailstorm, and the longer hailstorm lasted. Secondly, the velocity field in the form of single cell convergence in the lower layer and divergence in the upper layer developed more rapidly than that of multiple cells, and it caused more serious disasters. Before the occurrence of strong convection, VIL maximum was greater than 35 kg·m-2, and the VIL had a jump, the jumping increment was greater than 29 kg·m-2. (4) On the basic reflectance profile, the spatial structure of the side lobe echoes was shown as weak echo zones with an intensity less than 20 dBZ, perpendicular to the core area of the high reflectance factor. The development direction of the side lobe echo was from the central edge of the strong storm to the low azimuth. In the three hail processes, the process with the side lobe echo and three body scattering occurring lasted longer and was more disastrous.

Key words: hail, multi-scale, Doppler radar, pulse storm, comparative analysis

摘要：

利用加密自动气象站资料、多普勒雷达产品、NCEP/NCAR FNL 1°×1°再分析资料、灾情调查等资料，对2017年出现在山西北部的三次冰雹天气过程进行了对比分析。结果表明：（1）三次脉冲风暴冰雹过程均发生在500 hPa中高纬地区为“两槽一脊”型、槽后偏西北气流的环流背景下，低层切变线或涡旋是主要触发系统。冰雹出现在低层辐合线的东南侧暖区内。低层触发系统不同，大气不稳定度不同，造成的强对流落区和强度差异很大。（2）脉冲风暴冰雹过程中，从低层到高层风向随高度呈现一致的顺时针旋转，从低层到700 hPa附近风速随高度增加，0~6 km风垂直切变为1×10-4~ 7×10-4 s-1，均小于冰雹天气阈值。（3）脉冲风暴的雷达回波呈块状，强回波中心高度位于-10 ℃等温线所在高度以上，初始高度在7 km以上，上冲云顶高度大于12 km；脉冲风暴的形成、发展和结束与雷暴云顶的上冲、下降和崩溃紧密联系。可以利用上冲云顶的形态判断风暴的生消，但要抬高仰角到8 km左右高度观测，才有助于提前发现脉冲风暴。径向速度场上三次过程有明显差异，表现为降雹持续时间与辐合层厚度密切相关，辐合层越厚降雹越剧烈，持续时间越长；以单体形式在低层出现辐合、高层辐散的速度场发展迅猛程度要比多单体更剧烈，带来的灾害更严重；强对流发生前，VIL最大值大于35 kg·m-2，降雹前VIL出现跃增，跃增量大于29 kg·m-2。（4）基本反射率剖面图上，脉冲风暴产生的旁瓣回波的空间结构表现为与高反射率因子核区垂直，强度小于20 dBZ的弱回波带，旁瓣回波从风暴强中心边缘向低方位角方向伸展。三次过程中，出现旁瓣回波和三体散射的冰雹过程持续时间更长、灾害程度更重。

关键词: 冰雹, 多尺度, 多普勒雷达, 脉冲风暴, 对比分析