华北一次冷空气回流路径对降水相态的影响分析

针对2018年4月3-5日东北冷空气回流到京津冀地区造成复杂相态降水过程典型华北回流天气个例，利用ERA5再分析资料和MICAPS地面资料，详细分析了冷空气路径、形态、对降水范围及相态的影响等。分析表明：回流冷空气对京津冀地区的影响，可分为4个阶段，即低层冷舌侵入、沿山堆积扩散、增强维持、变性消散阶段。在低层冷舌侵入阶段，冷空气以冷舌形式经东北、渤海侵入京津冀地区，冷舌在不同高度位置不同；冷舌在垂直方向位于干、湿过渡区，降水粒子经冷舌下方干区蒸发，造成阴天无降水天气。在沿山堆积扩散阶段，低层冷空气遇太行山堆积并向南北以扇形形式扩散，较高层次冷空气西边界扩散至太行山山区；燕山南部、太行山东部存在深厚湿层，且温度较低，出现降雪；距离山脉较近的平原地区出现雨夹雪或雪；距离山脉较远的平原地区无降水。在增强维持阶段，冷空气强度达到最强，范围达到最大；深厚湿层从太行山、燕山向平原扩展，降水范围扩大，降水相态主要取决于近地面温度。在冷空气变性消散阶段，较高层次回暖先于较低层次，冷空气变性消失，降水趋于结束。

Analysis on path of cold air in a returning flow weather in North China and its impact on precipitation type

Based on the typical North China returning flow weather case with multiple precipitation types caused by Northeast cold air returned to Beijing, Tianjin and Hebei region from April 3 to 5, 2018, ERA5 reanalysis data and MICAPS ground data were used to analyze the cold air path, structure, impact on precipitation area and precipitation type in detail. The analysis shows that the influence of returning flow can be divided into four stages: lower layer cold air tongue intrusion, cold air heaping and spreading along the mountain, strengthening and maintaining, weakening and disappearing. In the cold air tongue intrusion stage, cold air intrudes into Beijing, Tianjin and Hebei region via the Northeast Plain and Bohai Sea in the form of cold air tongue, with locations differ in different height. The cold air tongue is located in the dry-wet transition height vertically. Precipitation particles evaporate through the dry range under the cold air tongue, resulting in cloudy days without precipitation. During the cold air heaping and spreading along the mountain stage, the lower cold air spread in a fan-shaped form along the Taihang Mountains to the north and south, and the higher cold air west boundary spread to the inner Taihang Mountains. There are deep wet layers in the southern part of Yanshan and the eastern part of Taihang Mountains with low temperature which caused snowfall weather. Plain areas near the mountain ranges, there is snowfall or rainfall. There is no precipitation in plain areas far from the mountain ranges. During the strengthening and maintaining stage of cold air, the intensity and scope of cold air reach the maximum. The deep wet layer extends from Taihang Mountains and Yanshan Mountains to the plain, so the precipitation area is expanded. The phase type of precipitation mainly depends on the near surface temperature. In the weakening and disappearing stage of cold air, the warming of higher level is prior to that of lower level, the cold air denatures and disappears and precipitation tends to end.