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
Using NCEP (1°×1°) 6 h 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 range gorge 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.