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

Abstract：The snowstorm forecast the west of Southern Xinjiang has always been a difficult point. The Central Asian Vortexes are the important systems producing the heavy snow or rainfall over the west of Southern Xinjiang. Different 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（Remembered as 1217 process）on December 2011 in the west of Southern Xinjiang as examples，the reason of the differences between the two was discussed to promote prediction capability. Based on the data of conventional observation 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 Vortexes 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 400hPa. Also，the water vapor was both provided by the southwesterly airstream in the front of the vortexes at 500hPa，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 similar 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 500hPa while easterly jet established over the basin at 850hPa，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 the west of Southern 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 surface climbed over the hill to the basin from the west of Southern Xinjiang，with the mechanisms of moisture convergence lacked.

Key words：Central Asia vortex；Jet stream；Spatial structure；Moisture convergence；Snow differences