南支槽与孟加拉湾风暴结合对一次高原暴雪过程的影响

A Case Study on the Effect of Southern Branch Trough in the Subtropical Westerlies Combined with Storm over the Bay of Bengal on Plateau Snowstorm

利用NCEP/NcAR逐6 h 1°×1°。再分析资料与常规和非常规观测资料,对2007年11月云南德钦高原暴雪产生的原因进行了研究,探讨南支槽与孟加拉湾风暴结合对高原东南部强烈天气的影响过程。结果表明:(1)在南支槽和孟加拉湾风暴结合的天气尺度条件下,槽前偏南风低空急流受高原大地形阻挡产生的高原切变线是高原暴雪的直接影响系统;(2)由于地形和冷空气的作用,上升运动向北倾斜使高原对流层中上层首先出现上升运动,整层上升运动在高原切变线和次级环流上升支的共同作用下强烈发展。孟加拉湾风暴北上与南支槽结合、高原切变线北移和风暴低压临近使德钦上升运动出现三次增强;(3)南支槽前偏南风低空急流向北输送水汽,部分水汽被抬升到高空,部分水汽绕过高原东南角向下游输送。高空水汽经高原上空沿着高空西风急流向下游远距离输送。高、低空水汽通道不重合往往会影响高原及其下游强降水落区的预报。受高空水汽输送影响,高原东南部纵向岭谷区具有高层大气最先增湿的特征,近地层水汽通量长时间强烈辐合有利于高原暴雪的形成;(4)上游冷空气沿南支西风到达孟加拉湾,促使南支槽加深和维持有利于引导盂加拉湾风暴北上,南支槽前偏南风低空急流把暖湿空气输送上高原,同时横槽转竖冷空气从高原南下,冷暖空气在德钦交汇形成强锋区也是暴雪产生的一个有利条件。(5)高原暴雪的锋区结构具有中纬度锋面天气特征,在暴雪发生的锋区附近,满足倾斜位涡发展和条件性对称不稳定。

Using NCEP(1°× 1°) 6h reanalysis data and the conventional and unconventional observational data,a case study on the plateau snowstorm that occurred in November 2007 was performed to explore the generation process of the severe weather over the southeast part of Tibetan Plateau due to combination of Southern Branch Trough(SBT) in the subtropical westerlies and storm of Bay of Bengal(BOB).The results show that:(1) The plateau snowstorm synoptic scale condition is formed by the combination of SBT and storm of BOB.The plateau shear line produced by low-level jet is the direct influence system.(2) Because of the effect of the terrain and cold air,upward movement sloping to the north makes upward movement first appears in the mid-upper troposphere over the plateau.Both the plateau shear and the ascending branch of secondary circulation of the upper jet stream enhance the upward motion.Due to the combination of SBT and storm of BOB,the northward moving of plateau shear line and the nearing storm causes the ascending motion of Deqin to strengthen three times.(3) The southern lower-level jet occurs in front of the trough,which transports moisture to the north,and part of water vapor is lifted to the high level while other part of the water vapor passes by the southeast corner of plateau to the downstream.Meanwhile the high-level water vapor travels over the plateau along the upper westerly jet downstream advection over a long distance.High and low water vapor channels do not overlap,often affecting the plateau and its downstream forecast in heavy rainfalls.Due to the high-level water vapor transportion,the longitudinal rangegorge region of southeast plateau has the characteristics of upper atmosphere being moistured first,and surface layer moisture flux strong convergence for a long time is conducive to the formation of the plateau snowstorm.(4) The cold air upstream along the southern branch of the westerly to the BOB,deepens and maintains SBT,which is beneficial to the northward moving of the storm BOB.The cold air from the plateau and the warm air from BOB meet over Deqin,forming strong front,which is the main synoptic system of snowstorm.(5) The frontal structure of plateau snowstorm has the mid-latitude frontal synoptic characteristics.Snowstorm occurs near frontal zone,meeting the slantwise vorticity development and conditional symmetric instability.