摘要:利用中国气象局MICAPS地面、高空等常规观测资料及欧洲中心ERA-Interim的0.25°（纬度）×0.25°（经度）逐6 h再分析资料，对2015年11月5日至7日影响北京、河北的一次降雪过程的环流形势和动热力物理量进行了诊断分析，揭示了降雪特征及其形成原因。环流形势分析发现，此次降雪是在高空两槽一脊叠加短波槽活动天气背景下的“回流型”降雪。500 hPa有西伯利亚脊的发展和内蒙古地区气旋性涡旋及其向南发展出的弱槽，使得偏北冷空气与西南暖气流在河北地区相遇，伴随低层700 hPa的低涡发展，造成了此次降雪天气。500 hPa多小槽波动东移，使得雨雪天气维持较长时间；700 hPa受偏南暖湿气流影响，850 hPa为偏东风，地面高压底部偏东风配合倒槽，有较好的上升运动和水汽输送条件；高湿的大气环境条件和低层水汽辐合及抬升为降雪发生提供了充沛的水汽；高低空急流的形成，与散度场、涡度场和垂直速度场的高低空耦合配置，为降雪天气的发生创造了动力条件。
关键词:回流型降雪/
多小槽波动/
地面高压/
高低空急流
Abstract: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.
Key words:Return-flow snowfall/
Small troughs/
Ground high pressure/
Upper-level and low-level jet
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