Diagnostic analysis of the backflow snowstorm in early winter of 2011 in the southern part of North China

Data from conventional observations, automatic weather stations, intensive snowfall observations and GFS 6-hourly reanalysis with 0.5°×0.5° resolution are used to analyze the snowstorm occurred in the southern part of Hebei province on November 29, 2011. The results show that the upper trough and the surface cold front are dominant systems producing the snowstorm weather. Meanwhile, there are three major streams during the snowstorm event, i.e., cold airmass from northeast, and warm airmasses from southwest and southeast. Water vapor was mainly transported from the East China Sea at lower troposphere, while that of middle level comes from the Bay of Bengal and South China Sea. Warm moist air climbs over the cold and dry airmass and frontal zone is formed at about 800 hPa. Cold advection is the main factor for frontogenesis. While the contribution of each term in frontogenesis function is different, the tilting term is the largest contribution for frontogenesis.