江苏不同量级降雪过程的对比分析和预报指标研究

Comparative Analysis and Forecast Index of Snowfall Process of Different Magnitudes in Jiangsu Province

利用常规观测资料和NCEP 1°×1°再分析资料,通过对2008-2018年共11年间发生在江苏省的区域性中雪、区域性大雪、区域性暴雪天气过程的对比分析,发现影响江苏区域性降雪的主要天气系统是500 hPa西风槽、700 hPa西南急流和地面冷空气。决定降雪量级的因素主要是700 hPa西南急流强度和范围,降雪区上空水汽输送强度、水汽辐合强度、水汽辐合厚度也与降雪量级有一定的正相关关系。暴雪时700 hPa水汽通量≥14 g·cm^(-1)·hPa^(-1)·s^(-1),且水汽来源更为丰富,均来自于孟加拉湾和南海;大雪和中雪时,700 hPa水汽通量分别≥12 g·cm^(-1)·hPa^(-1)·s^(-1)和10 g·cm^(-1)·hPa^(-1)·s^(-1)。暴雪期间,水汽辐合区内水汽通量散度都≤^(-1)×10^(-7)g·s^(-1)·hPa^(-1)·cm^(-2),水汽辐合厚度达200~400 hPa,明显强于大雪和中雪。有利于江苏发生区域降雪过程的温度垂直分布条件为:地面≤2℃、t_(925)≤-1℃、t_(850)≤-2℃、t_(700)≤-1℃、t_(500)≤-14℃。随着降雪量级的增大,中低层温度阈值呈降低趋势。中低层逆温是产生区域性大雪及暴雪的必要条件,而中雪发生时不一定都有逆温层结,只要近地层温度条件合适,就能产生降雪。随着降雪量级的增大,逆温层强度明显增强、厚度明显增厚。暴雪、大雪和中雪时逆温强度阈值分别为3~8℃、2~8℃和1~3℃,其逆温层厚度分别为150~200 hPa、100~200 hPa和50~100 hPa。降雪过程中上升运动强中心位于600400 hPa。暴雪时,上升运动区相对大雪和中雪时的更为深厚,基本整层都为上升运动区,垂直运动发展旺盛。暴雪和大雪时上升运动中心值均≤-0.7 Pa·s^(-1),中雪时中心值≤-0.3 Pa·s^(-1)。

Based on the conventional observation data and NCEP 1°×1°reanalysis data,through the comparative analysis of regional moderate snow,regional heavy snow and regional blizzard weather processes in Jiangsu Province during the 11 years from 2008 to 2018,this research reveals that the main weather systems affecting the regional snowfall in Jiangsu Province are the 500 hPa westerly trough,the 700 hPa southwest jet and the surface cold air.The intensity and range of the southwest jet at 700 hPa are the major factor determining the snowfall magnitude.The intensity of water vapor transport,intensity of water vapor convergence,and thickness of water vapor convergence over the snowfall area also have a certain positive correlation with the magnitude of snowfall.During blizzard process,the water vapor flux at 700 hPa is≥14 g·cm^(-1)·hPa^(-1)·s^(-1),and water vapor sources are more abundant,all coming from the bay of Bengal and the south China sea.In heavy snow and moderate snow processes,the water vapor flux at 700 hPa is≥12·cm^(-1)·hPa^(-1)·s^(-1) and 10 g·cm^(-1)·hPa^(-1)·s^(-1) respectively.During blizzard processes,the divergence of the vapor flux in the vapor convergence area is≤-1×10^(-7)g·s^(-1)·hPa^(-1)·cm^(-2),and the thickness of the vapor convergence reaches 200—400 hPa,which is obviously stronger than in the precesses of heavy snow and moderate snow.The vertical temperature conditions that are conducive to the occurrence of regional snow in Jiangsu Province are that the surface temperature is≤2℃,t_(925)≤-1℃,t_(850)≤-2℃,t_(700)≤-1℃ and t_(500)≤-14℃.With the increase of the snowfall magnitude,the temperature threshold of the middle and lower layers shows a decreasing trend.The inversion is a necessary condition for regional heavy snow and snowstorm,while the inversion stratification is not always present when the moderate snow occurs.As long as the temperature conditions near the surface are suitable,snowfall can occur.With the increase of snowfall magnitude,the intensity of and thickness of inversion layer are obviously enhanced.The inversion intensity thresholds of blizzard,heavy snow and moderate snow are 3—8℃,2—8℃and 1—3℃,respectively,and the inversion layer thickness is 150—200 hPa,100—200 hPa and 50—100 hPa,respectively.The strong center of ascending motion lies in the range of 600—400 hPa when it snows.Actually,the ascending motion area is deeper in blizzard than in heavy snow and moderate snow,and the whole layer is basically the ascending motion area with vigorous development of vertical movement.The center value is≤-0.7 Pa·s^(-1) in blizzard and heavy snow processes,and≤-0.3 Pa·s^(-1) in moderate snow.