Diagnostic Analysis of the “1106” Snowfall Process in North China in 2015

In this paper, the ambient circulation and thermal dynamics of a snowfall affecting Beijing and Hebei from 5 to 7 November 2015, were analyzed using the observed data of MICAPS (Meteorological Information Combine Analysis and Process System) and the reanalysis data, 0.25°×0.25° every 6 h, from the European Center for Medium-Range Weather Forecasts. The characteristics and causes of the snowfall were revealed. According to the analysis of the existing circulation, the snowfall was a "return-flow" snowfall under the superposition of two long wave troughs, one long wave ridge, and a series of short wave troughs. The development of a Siberian ridge at 500 hPa and a cyclonic vortex over Inner Mongolia and its weak trough forced the northerly cold air to meet the southwesterly warm air in Hebei. A low-pressure vortex developed at 700 hPa, which was the direct cause of the snowfall. At 500 hPa, several short wave troughs moved eastward, which made the rain and snow persist for an extended period. North China was affected by southerly warm and humid air flow at 700 hPa, easterly flow at 850 hPa, and the easterly wind at the bottom of ground high pressure combined with an inverted trough, which provided favorable conditions for vertical lift and water vapor transport. Upper and low-level jets formed, coupled with upper divergence and positive vorticity and vertical velocity fields, creating the dynamic conditions for snowfall. A moist environment and the convergence of water vapor in the lower layer provided abundant water vapor for snowfall.