Analysis On Late Autumn And Early Winter Sleet To Snow Storm In Heilongjiang Province

Three processed of sleet to heavy snowstorm(‘041105',‘071020' and ‘121112')were analyzed by 1°×1°every 6h NCEP/NCAR reanalysis data,conventional surface data,sounding data,regional station data and radar data of Heilongjiang Province at early winter in this paper.Comprehensive comparative analysis from the circulation situation and influence system,and physical field of mesoscale system and precipitation type changes,occurred in November 2012 on 11-14,a rare snowstorm of extreme analysis,summed up the reason causes the winter sleet to Snowstorm Weather in late autumn and extreme weather may occur in Heilongjiang.The main conclusions are as follows:(1)Three sleet turn snow weather process and its atmospheric circulation have a common characteristic,which 500 h Pa have low vortex system development and eastward in the process of the three,resulting in a wide range of rain and snow in our province.(2)The three processes are established,which not only provide the water vapor which is conducive to the production of snow storm,but also the coupling of the high and low level jet provides a powerful driving force for precipitation.In the power structure of sleet to snow storm regions has a divergence in low level convergence,strengthen the ascending motion.But the three process due to the different position of upper air system,the water vapor sources have significant difference.(3)Analysis of the three process of the evolution of the state of the state can be obtained,0 degree Celsius and the ground temperature can be used as the standard of precipitation in Heilongjiang province.The phase transition of precipitation particles in the course of descent is determined by the height of the 0 degree Celsius layer from the atmospheric physical point of view.But because of the ground system such as different cold fronts and south to North cyclone influence,caused by small differences in 0 temperature level and temperature threshold due to the difference of advection may.From the analysis of the three processes,0 degrees Celsius in 925 h Pa layer and ground temperature near 1? can be used as the critical value of Heilongjiang province and conversion.When the temperature of 2m is greater than 1 and the temperature of 0 is higher than 925 h Pa,it is a process of rainfall.When the 2m temperature is about 1 degrees Celsius and 0 degrees Celsius layer is located between the 925-950 h Pa,it is a process of sleet weather;When the 2m temperature is less than 0 degrees and 0 degrees Celsius layer is located 950 h Pa to the ground,it is a process of snow weather.(4)In water vapor,dynamic,thermodynamic conditions,sleet to snow storm and heavy snow have obvious similarities and differences.Similarities: In water vapor condition,water vapor flux has high value,water vapor convergence;the lower layer has a high relative humidity value.The 925 h Pa and 850 h Pa have the high values of specific humidity.On the dynamic condition,the 925 h Pa and 850 h Pa all have ascending motion,there is convergence in the lower layer,in the upper divergence.In the thermal condition,the lower layer which is stronger and the warm air along the cold air is inclined to the upper layer,the warm air is permeated with.cold air.Differences: In the water vapor flux value,the sleet to snow storm is significantly greater than that of the snow.In the water vapor flux divergence,sleet to snow storm is stronger convergence than heavy snow.Contrast from the lower layer humidity,sleet to snow storm is 2g/kg more wetter than heavy snow,a process of sleet to snow storm has better water vapor condition.Sleet to snow storm divergence was significantly higher than that of heavy snow,the convergence is stronger in low layer.Comparison of convergence and divergence,sleet to snow storm has divergence on the vertical structure of convergence,convergence center of low layers have large gradient divergence.The heavy snow has a low level convergence and weak convergence area,at low level there is a ramp area to the upper extension and is a small divergence gradient.In thermal conditions,the structure of temperature layer is different.Sleet to snow storm has strong warm air in the low layer,and has a inversion region of temperature.The snow storm is easy to appear in the inversion layer of warm air side.The temperature structure of the heavy snow is a warm air in low level,a cold air is in high level.