The Research On Environmental Conditions And Formation Mechanism Of Two Typical Snowstorm In Beijing

From late October to early November in Beijing in 2009,there are several snowfall, rarely seen in history. In particular, Nov. 1 and Nov. 9 during the two reached the blizzard snow magnitude, the snowfall in Nov 1 is the largest amount of snow in winter snow over the past 60 years in Beijing, and snow occurs earlier than normal time 28 days, the nature of precipitation changes from rain turned into snow is the unexpected point of failure for the forecast forecaster; and the snowstorm in Nov 9 appears an explosion of snow accompanied by thunder and lightning phenomena. Using many high spatial-temporal resolution data of Beijing, such as auto-weather station (AWS) network data, Doppler radar data, wind profile data, and BJ-RUC Numerical simulation data ,a much in-depth comparative analysis and study are carried on two snowstorm cases in Beijing on 1th and 9th November 2009 are shown. It is revealed that:1, Both the 1101 snowstorm and the 1109 snowstorm have significant snowfall intensity,long duration,concentration in the central and southern regions and large snowfall in town. However, there are significant differences between the two processes: firstly, duing to the differences between the frontal systems, there are significantly different shapes of the snow area in the north of the snow line.Secondly,the 1101 snowstorm is relatively stable during the process of snow, having snowfall intensity of uniform , while the 1109 snowstorm has concentrated periods of snow, accompanied by thunder.2, These two snowfall that intervals of less than ten days,can not be exist without the better climatic background conditions:In early November 2009,the meridional circulation of 500hPa is significantly stronger than normal year,the position of trough in East Asia is significantly more west than climate average.Beijing is in the strong cold air region.Meanwhile, Whether in high or low,the cold air of early November 2009 is stronger than normal year,the activities of low-level cold air is , more frequent,contributing to the occurrence of snowfall.3, The same point on the large circulation of two snowfall is that they are both in the activities of East Asia trough and have strong front at low level.The differences about weather circulation and conditions include:(1) The positions and shapes of 500hPa trough are different. The 1101 snowstorm is affected by two troughs of 500hPa,while the major effect system of 1109 process in 500hPa is the trough of East Asian which is relatively stable,the north and the south China is always under the control of frontal zone before the trough of East Asian.(2) The differences between the two processes of 850hPa is not only the different configuration of low and high,but also the low-level wind circulation and the surface pressure field.(3) The significant convergence and positive vorticity in low level, and the significant divergence and negative vorticity in high level produce a continuous and strong upward motion in 1101 process before snow. Either the divergence or the vorticity in low and high level, the process in 1101 is more ideal than that in 1109. The Large-scale dynamic conditions in 1101 is superior. But using BJ-RUC data find that although the whole dynamic conditions of 1109 process is not good, but near the rainfall concentrated period ,the conditions is still more ideal, the intensity of convergence center and positive vorticity areas are obvious stronger than other times.(4) Differences in moisture sources.The water vapor of 1101 snowfall comes from two directions of same level,which is located below 850hPa,but the water vapor of 1109 snowfall comes from two directions of different levels, the water vapor carried by the westerly system of 700hPa combines with the water vapor transported from the Bohai of 850hPa together provide the necessary moisture conditions for the occurrence of 1109 snowstorm.4, The analysis of the mechanism of 1101 snowfall shows that the snowstorm is forced by the occluded front from the north. (1) With the formulation of the occluded front,water vapor transport From two different directions converge between 35-40N in Southwest of Beijing,inducing a narrow channel of moisture transportation directly to the occluded front in North China .(2) The Structure of front is shallow and the weakening of the occluded front is related to the east of cold advection by the way of invading the mature occluded front. Firstly the cold advection from layers below 500-800m cuts in the occluded front, the occluded front structure weaken from the bottom and finally die away .(3) The occluded front's precipitation area is decided by the east-dipping structure on this terrain, during its whole life stage (15h or so), there is a process of precipitation correspondingly in Beijing. The snowstorm of Beijing belongs to precipitation of occluded front and the snow is over with the disappearance of occluded front.(4) The process from rain to snow in Beijing is due to the rapidly decreasing and closing to the freezing point of air temperature near the surface. The decrease of air temperature before snow (from 0000 BT 1 to 0800 BT 1 November) is because of the rain by the means of evaporative cooling. While, the east of low-level cold air advection contributes to air temperature maintaining low near the surface in Beijing during the heavy snowfall (from 0800 BT 1 to 1400 BT 1 November).5, The analysis of the causes of 1109 snowfall shows that the heavy snow is forced by the typical cold front, associated with the mesoscale activities:(1) Using the AWS data, satellite images and radar data can reveal the mesoscale features on 1109 process. (2) according to AWS surface wind, there was a east-west directionβ-scale convergence line in the intersection between Daxing district and Beijing city before the lighting and explosion of snow about 2 hours ago.,the process lasts until about 22:40 pm.(3) According to the analysis of the stability conditions by BJ-RUC model data for "1109" process, it can be seen that the stable layer into neutral and pseudo-equivalent temperature changed shape above 700hPa.The process snows that contour slope of pseudo-equivalent temperature is greater than the contour slope of the absolute momentum and symmetric unstable conditions are met. Meanwhile, the wind climb along the front area to the symmetric instability area in order to getting the unstable energy. After that,a very strong upward motion emerges , so that the explosion of snow and thunder during snow becoming possible.