西藏高原西南部地区一次暴雪天气过程诊断分析

Diagnostic analysis of a snowstorm event over the southwest region of the Tibetan Plateau

利用NCEP/NCAR的2.5°×2.5°逐6 h再分析资料、常规气象观测资料和卫星云图资料,对2013年1月17—19日西藏高原西南部地区的一次暴雪天气过程进行了综合分析。结果表明:此次西藏高原西南部地区暴雪天气过程中高纬地区为两槽两脊型,深厚的南支槽、西南急流和西太平洋副热带高压是此次暴雪过程的主要影响系统。此次暴雪过程气旋性涡度可达15.0×10^(-5)s^(-1),低层辐合和中高层辐散有利于产生上升运动,250 h Pa附近正散度为3.5×10^(-5)s^(-1),中高层的强抽吸效应和强上升运动对暴雪的发生具有重要作用;主要水汽来源为阿拉伯海,水汽通量增加和水汽通量散度中心向东北方向移动说明西南暖湿气流源源不断地向暴雪区输送水汽并辐合;同时,地形的抬升作用有利于水汽凝结,云系接近西藏高原时云顶亮温(Black Body Temperature,TBB)明显减小,到达暴雪区上空时TBB为-50℃以下,其中西藏高原西部的普兰地区上空TBB达-60℃以下。

The weather process of a snowstorm event in the southwest region of the Tibetan Plateau on January 17 to 19,2013 was comprehensively analyzed using the NCEP/NCAR（ National Centers for Environmental Prediction/National Center for Atmospheric Research） reanalysis data with resolution of 2. 5° × 2. 5°,conventional meteorological data and satellite cloud images in this study. The results show that the circulation situation during the snowstorm event in the southwest region of the Tibet Plateau is characterized by two troughs and two ridges at the middle and higher latitudes. The deep southern branch trough, southwesterly jet stream,and the Western Pacific subtropical high are the main influencing systems of the snowstorm event. The cyclonic vorticity reaches15. 0 × 10- 5 s- 1 during the snowstorm event,and the convergence at lower levels and divergence at middle and higher levels favor the ascending motion of the air. The positive divergence near 250 hPa is 3. 5 × 10- 5 s-1,and the powerful suction effect at middleand higher levels and the strong ascending motion play important roles in the snowstorm occurrence. Water vapor mainly comes from the Arabian Sea,and the increasing water vapor flux and its moving northeastwards divergence center indicate that water vapor is transported continually by warm and humid southwesterly flows and converges over the snowstorm area. Meanwhile, the orographic uplifting effect is favorable for the condensation of water vapor. The temperature of black body（ TBB） obviously decreases when the clouds reach over the Tibet Plateau. TBB drops below- 50 ℃ when clouds reach over the snowstorm area,and it is even lower than- 60 ℃ over the Burang area in the western Tibet Plateau.