利用雨滴谱仪、多普勒天气雷达、微波辐射计、地面加密自动站、EC再分析资料及气候整编资料等多源观测资料,分析了2018年4月4—5日,北京地区罕见暴雪过程的极端性及形成机制。结果表明:(1)此次过程是北京地区4月首次出现纯雪日,降雪量和积雪深度均突破历史同期记录,1000~850 hPa温度标准化异常SD值均小于-3,日降雪量排在整个冷季的前5%,是一次极端天气过程。(2)低层强冷空气入侵形成冷垫,700 hPa强西南低空急流输送充沛水汽,使北京地区上空800~500 hPa产生条件性对称不稳定,暖空气在锋区以上的强上升运动触发不稳定能量,产生高架对流,局地雷达回波具有夏季对流单体的倾斜结构特征,有利于暴雪增幅。(3)降雪过程先后受到两次冷空气叠加影响,前期强冷空气持续剧烈降温导致低层温度偏低,使得温度达到降雪阈值,是此次极端降雪过程产生的主要原因。(4)微波辐射计监测显示,降雪的起止时间与逆温具有良好的对应关系,降水相态主要取决于1 km以下的温度变化。
During 4-5 April 2018,an infrequent snowstorm stuck Beijing.This paper analyzes the extremes and formation mechanism of the blizzard process by using multi-source observation data such as raindrop disdrometer,Doppler weather radar,microwave radiometer,ground encryption automatic station,EC reanalysis data and climate reorganization data.The results showed that(1)the 5th of April 2018 was the first pure snow day in Beijing in April.The snowfall and snow depth both exceeded the historical record and the 1000-850 hPa temperature normalized SD value was less than-3.So,it was an extreme weather process.(2)The invasion of low-level cold air forced the warm and humid air to climb along the cold pad.Conditional symmetric instability from 800 hPa to 500 hPa in Beijing Area was caused by strongly warming and humidification from 700 hPa strong southwest low-level jet and triggered by the strong ri-sing motion of warm air above the front,resulting in elevated convection and radar echo like thunderstorm cells in summer,which enhanced the snowfall.(3)The snowfall process was affected by two cold air superimposition successively.The continuous violent cooling induced by a strong cold air before the snowfall caused the low-level temperature to be extremely low and reached the temperature threshold of snow in Beijing,which was the decisive factor for the extreme snowfall.(4)Microwave radiometer monitoring showed that the duration of snowfall had a good correspondence with the temperature inversion,and the precipitation phase mainly depended on the temperature change below 1 km.
strong cold air