Abstract:In this paper, an analysis is performed on the snow weather process taking place in Beijing on February 9 and 10, 2011. The NCEP final operational global analysis data as well as the observation data, both conventional and densely AWS data, are used in the analysis. It is shown in the analytical results that this snow process is a combined consequence of the eastward returning current and the surface inverted trough. The eastward returning current is characterized by its wedge structure of being shallow in northwest direction and deep in southeast direction, as well as being wet and cold. Although there is convergent ascending current and frontogenesis, the eastward returning current is not the primary dynamic mechanism of the snowfall. Instead, it plays the role as a cooling and wetting cushion. There are three primary dynamic mechanisms. First, a positive vorticity advection in front of 500 hPa short wave trough and a temperature advection at 850 hPa facilitate the development and eastward movement of surface inverted trough near Hetao. Second, the southwest current at east side of inverted trough ascends on the cooling cushion. Third, the overlay of convergent rising regions at the inverted trough and the front of east wind makes ascending motion strong. Finally, southwest warm and wet currents serve as the main water vapor source for the snow process.