Snowstorm Feature Analysis And Forecasting Research In Jilin Province

Snowstorm is one of the most common severe weather phenomena in Jilin province in winter, which not only causes enormous economic losses, but also affects people’s life and property. By using the NCEP/NCAR reanalysis data, the daily precipitation data of the national observatory, and the weather phenomenon observation data, the temporal and spatial distribution characteristics of different types of snowstorms in Jilin province are analyzed. The snowstorm events can be grouped into two types: one with only pure snowfall(Type Ⅰ) and the other with sleet(Type II). And each type can be further classified into three groups: local, regional and large-scale according to the precipitation area. In this study, firstly, the statistical analysis were carried out for the synoptic systems that influenced the Type I and Type II snowstorm in Jilin province, and the conceptual models were set up respectively. Secondly, related physical quantities were calculated for the TypeⅠand Type II snowstorm, and the prediction indicators were summarized. Finally, the conceptual models and the prediction indicators were tested, and the main conclusions are drawn as follows:(1) Comparing the three groups of snowstorms in Jilin Province, local snowstorm occurs most frequently, followed by regional snowstorm, and the large-scale snowstorm happens least. Between the two types of snowstorms, the proportion of occurrences of the TypeⅠ snowstorm is higher than the Type II snowstorm.(2) Spatial distributions of snowstorms are quite different in Jilin Province, and the regional features are very obvious. Caused by the interaction of the synoptic systems and the Changbai Mountains, most of the snowstorms occur on the windward slopes of the Changbai Mountains, and significantly more than the ones on leeward slopes and plains of the Greater Higgnan Mountains.(3) According to the monthly distribution of the snowstorm occurrence in Jilin Province, snowstorm occurs most in November, and least in January. TypeⅠsnowstorm mainly occurs in November and March, and Type Ⅱ snowstorm concentrates in November and October. Regional and large-scale Type I snowstorms often happen in March, regional and large-scale Type Ⅱ snowstorms usually occur in October. And the interdecadal characteristics are also obvious.(4) The main synoptic systems on upper air which affect snowstorms in Jilin Province are upper level trough, cold vortex and wind shear. The main surface synoptic system are cyclones(Mongolia cyclone, North China cyclone and Changjiang-Huaihe cyclone),cyclone inverted trough and cold front. The upper level trough is the most important one of the upper air systems, followed by cold vortex and wind shear. When affected by upper level trough, the local snowstorms are mainly caused by cyclone inverted trough, and regional and large-scale snowstorms are mainly caused by North China cyclone and Changjiang-Huaihe cyclone. When affected by upper air cold vortex, local snowstorms are mainly caused by Mongolia cyclone, and regional and large-scale snowstorms are caused by cyclone inverted trough. When affected by the surface synoptic system, regional and large-scale Type I snowstorms are usually caused by North China cyclone, and Type Ⅱ snowstorms are mainly caused by Mongolia cyclone.(5) In terms of the upper air synoptic system, the circulation patterns associated with the snowstorms can be categorized into three types: upper-level trough, cold vortex, and wind shear. The features of these three types of circulation patterns are be summarized, and the synoptic system models for the two types of snowstorms are be established. The strength and pathway of the cold mass and the 0℃ isothermal line at the 850 hPa are all important factors to distinguish between the two types of snowstorm.(6) Water vapor flux, moisture flux divergence, specific humidity, vertical velocity,divergence,potential pseudo-equivalent temperature,K index and temperature advection are different between Type I and Type Ⅱ snowstorms, and they are important indicators to distinguish and forecast these two types of snowstorms in Jilin province.(7) According to the synoptic system models and the forecast indices for the two types of snowstorm, 20 snowstorms in winter(October to April in next year) during 2012-2014 are tested by using the NWP products from the T639 model, the results show the synoptic system model and physical indicators can indicate the process and snowfall region of the snowstorm. Compared with the public forecasts, the false rate and missing rate of forecasts are reduced by 26% and 23.7% respectively, and the accuracy are increased by 22.4%. These methods can efficiently reduce the false rate and missing rate of snowstorm forecasts and help to improve the accuracy.(8) Compared with the conventional weather forecast, the combination of synoptic system models, physical indicators and numerical model products can forecast the snowstorm more efficiently and more accurately, and can be used to accurately determine the location of the snowstorm.