六盘山区一次典型暴雨过程的地形敏感性模拟试验

Topography sensitivity simulation test of a typical rainstorm process in Liupan Mountain region

基于WRF(weather research and forecasting)中尺度数值模式,对2018年7月10日六盘山区一次典型的暴雨天气过程进行模拟,分析此次过程的动力场、水汽场、云降水微物理结构的演变特征,通过改变模式初始场中六盘山地形高度进行敏感性试验,对六盘山地形影响该地区降水机制进行讨论。结果表明:蒙古冷涡底部冷空气和副热带高压西侧暖湿气流在六盘山区交汇配合低层700 hPa切变线辐合抬升导致此次暴雨过程;控制试验较好地模拟出雨带的分布范围、强降水中心位置及动力场结构特征,在降水发展和旺盛阶段,东南暖湿气流受地形强迫抬升和地形绕流共同影响,六盘山西坡和东坡均为上升气流,配合700 hPa切变线系统在六盘山山脊处上升气流汇聚加强,将云水带到负温层形成过冷水,云水、冰晶、雪和霰在0℃层至-40℃层之间共存,有利于冰相粒子碰冻增长和贝吉龙过程发生;地形敏感性试验发现改变地形对降水落区范围影响不大,而地形增高使六盘山区降水量级显著增大,尤其强降水更多集中在迎风坡一侧(山脉东侧),地形强迫抬升作用使得上升气流和水汽的垂直输送进一步加强,云中冰相过程发展充分,过冷云水为雪和霰的增长提供有利条件,因此使得地面降水增多。

The WRF(weather research and forecasting) mesoscale numerical model was used to simulate a typical rainstorm process in the Liupan Mountain region on July 10, 2018. The dynamic and moisture field, the evolution of cloud and precipitation micro-physical structure were analyzed in this paper. A sensitivity test was conducted by varying the height of the Liupan Mountain topography in initial field of the model, and mechanism of the Liupan Mountain topography affecting precipitation there was discussed. The results show that the rainstorm was caused by the cold air at the bottom of the Mongolian cold vortex and the warm-moist air at the west side of the WPSH(western Pacific subtropical high) intersecting at the Liupan Mountain region and the 700 hPa shear line at the lower level. The rainstorm zone, the center of heavy precipitation and the shear line at 700 hPa were well simulated by the WRF model in the control test. At the stage of development and prosperity of precipitation, the southeasterly warm and wet air was affected by topographic forced uplift and topographic circumfluence, and there were updrafts in western and eastern slopes of the Liupan Mountain. Cloud water was brought to negative temperature layer and formed supercooled water. Cloud water, ice crystals, snow and graupel coexisted between 0 ℃layer and-40 ℃ layer, which was conducive to collision growth of ice particles and the Bergeron process. The terrain sensitivity test shows that the change of terrain had little effect on rainfall area, and elevation of the terrain made rainfall level increase significantly,especially the heavy precipitation was more concentrated on the windward slope side. Forced uplift of terrain further strengthened vertical transport of water vapor and updraft, and the ice phase process in clouds developed fully. Supercooled cloud water provided favorable conditions for snow and graupel growth, thus increasing surface precipitation.