2011年两次中亚低涡影响南疆西部降雪机制分析

Influence Mechanism Analysis of Snow Caused by Two Central Asian Vortexes in West of Southern Xinjiang in 2011

利用常规观测、T639、NCEP客观分析、FY-2D卫星等资料,对比分析2011年12月中亚低涡造成南疆西部5-9日暴雪过程(简称1205过程)和17-22日弱降雪过程(简称1217过程)。结果表明,中亚低涡是深厚且相当正压的天气尺度系统,造成两次降雪过程的低涡在对流层整层均为正涡度,低涡前部整层为上升运动,400 hPa以下为辐合区。水汽主要为来自500 hPa低涡前部的西南气流,且水汽收支变化均表现为西、南边界流入多,北边界流出多。虽然两次中亚低涡影响南疆西部降雪的气候背景、位置、强度、结构、维持时间及水汽输送相似,但形成低涡的大尺度环流背景、移动路径、高低空环流配置及水汽辐合机制有所不同:1205过程500 hPa为北脊南涡环流配置,低涡向东北方向移动,由于850 hPa低空偏东急流在南疆盆地的建立及维持,使得低层冷空气自东向西侵入,低涡前部暖湿气流与低层冷湿气流在南疆西部交汇造成南疆西部强降雪,且降雪前东边界700 hPa以下水汽流入增大。而1217过程500 hPa低涡脱离极锋锋区快速南压至中亚地区,为自北向南移动路径,虽然低涡前西南气流携带充分的水汽,但南疆盆地850 hPa无低空偏东急流,冷空气自帕米尔高原翻山下沉进入盆地,缺乏动力和水汽辐合条件,降雪时东边界500hPa以下水汽流入增大。

The snowstorm forecast of Southern West Xinjiang has always been a difficult point. The Central Asian Vortexes are the important systems producing the heavy snow or rainfall over Southem West Xinjiang. Dif- ferent vortexes traverse routes made different snow or rainfall scope and intensity. Taking the snowstorm during 5th to 9th （Remembered as 1205 process） and the weak snow with better advantages during 17th to 22nd （Re- membered as 1217 process） on December 2011 in S outhem West Xinjiang as examples, the reason of the differ- ences between the two was discussed to promote prediction capability. Based on the data of conventional observa- tion data, T639, NECP/NCAR reanalysis data and FY-2D satellite, the mechanisms differences between the two cases caused by Central Asian vortexes were analyzed comparatively. The results showed that Central Asian Vor- texes are scale weather system with regional characteristics, deep and fairly positive pressure. Two cases had in common were several points, both with positive vortices in the whole layer of the vortexes, upward motion in the front of that and convergence areas below 400 hPa. Also, the water vapor was both provided by the south- westerly airstream in the front of the vortexes at 500 hPa, with same water vapor budget of more input in the western and southern boundary and more output in the northern boundary. Although, the climatic background, location, strength, structure, holding time and water vapor transports of the vortexes in the two cases were simi- lar as mentioned above, there was something different as well, such as the large scale circulation backgrounds where the vortexes formed, the vortexes tracks, the configurations of high and low altitude and the mechanisms of moisture convergence. In the case 1205 process, under the circulation of northern ridge and southern vortex, the vortex moved toward northeast at 500 hPa while easterly jet established over the basin at 850 hPa, inserting cold air from the east at the surface. The warm moisture of the mid-layer had been butting up the cold moisture of the low-layer, the good cooperation of which made it snow heavy in Southern West Xinjiang. Nevertheless, in the case 1217 process, the vortex broke away from the polar front frontal zone and moved toward south to the Central Asian, moving in the north path. The water vapor, provided by the southwesterly airstream in the front of the vortexes, was abundant, but there＇s no easterly wind over the basin at 850 hPa and the cold air at the sur- face climbed over the hill to the basin from Southern West Xinjiang, with the mechanisms of moisture conver- gence lacked.