Diagnosis of a rare snowstorm process in autumn over Shijiazhuang, Hebei province

Based on reanalysis data from NCEP, the circulation characteristics and physical fields of the rare snowstorm process on November 10-12, 2009 in Shijiazhuang of Hebei province were analyzed by synoptic diagnosis method. The results show that the snowstorm belongs to the typical northeast backflow type. The main influencing systems include the southward cold high pressure from Lake Baikal, the low pressure inverted trough in Hetao regions, the warm shear line in 700 hPa and the upper level trough in 500 hPa. The southwest and northeast low level jet is coupled each other, which not only affords water and vapor and energy transportation for the snowstorm but also strengthens uplift motion. Vertical allocation of water vapor leads to the increase of the local specific humidity. The deep humidity layer and strong water vapor convergence provide the abundant water vapor conditions for the snowstorm. The upper level divergence and low level convergence, the strong uplift motion are the dynamic conditions of this process. The period of large snowfall is in accord with the strong development stage of uplift motion. The temporal and spatial characteristics show that water vapor increases rapidly and ascends in initial stage of the snowstorm. At the same time, it indicates that forced uplift action of topography is also important. Temperature is higher in 850 hPa than in 700 hPa, which is favorable to water vapor desublimation. The inversion layer is ahead of 24 hours and the differences of temperature in the strongest stage of snowstorm is more than 5 ℃, which is indicative to snowstorm forecast.