冬季中尺度降雪系统暖输送带云微物理垂直特征的飞机观测研究

Airborne Observations of the Cloud Vertical Microphysical Characteristics of Warm Conveyor Belt within a Winter Mesoscale Snowstorm

本文利用飞机云物理探测系统观测数据,分析了2017年河北南部一次暖锋影响下的降雪云系微物理特征。受到西南涡暖锋暖湿输送带WCB(Warm Conveyor Belt)的影响,降雪前期云系可分为三层结构,上层为比较稀疏的冰云,冰雪晶单晶为主且粒径较小;中间层云厚与暖输送带厚度相当(0.9 km),其中液态水含量最大可达0.51 g m^(−3),冰晶非常稀少,云内液态含水量廓线与绝热含水量非常接近;下层云为纯冰云,冰雪晶粒子数浓度比云系上部高两个量级,粒子谱宽显著大于云系上部,冰雪晶粒子形状以柱状、柱状束和霰粒子为主,霰粒子是云系高层的单晶在过冷水丰富的云层中凇附增长下落形成的,并通过Hallett-Mossop繁生机制产生大量柱状冰晶。暖湿输送带中丰富的过冷水对降雪前期冰晶凇附、繁生等微物理过程有极为重要的影响。降雪后期,700 hPa急流轴移出观测区,云系垂直混合较为均匀,此时云中观测到的粒子以片状冰雪晶为主,在云系上部片状聚合体粒径较小,越向下片状聚合体粒径越大,体现了冰雪晶粒子在下落过程中的碰连增长。

The microphysical characteristics of the warm conveyor belt(WCB)within a winter mesoscale snowstorm in southern Hebei Province were analyzed using aircraft observation data.Influenced by the WCB associated with the Southwest vortex warm front,the cloud can be divided into three layers during the early period of snowfall.A few smallsized ice crystals existed in the upper part of the cloud.The thickness of the middle layer was equivalent to the thickness of the WCB(0.9 km).In this layer,the maximum liquid water content was 0.51 g m^(-3),with a low concentration of ice crystals,and the vertical profiles of the liquid water content and the adiabatic water content were similar.In the lower part of the cloud,the number concentration of ice crystals increased by two orders of magnitude compared with the upper part of the cloud,and the particle spectrum width increased significantly.The predominant ice crystal habits were columns,bundles of columns,and graupel.Graupel particles were formed through the process of riming,which occurred in the middle layer with abundant supercooled water,and numerous columnar ice crystals were produced by the Hallett–Mossop mechanism.The abundance of supercooled water in the WCB considerably influenced the microphysical processes during the early snowfall period.The vertical structure of the cloud was relatively uniform during the later period of snowfall,as the jet at 700 hPa had moved out of the observation area.At this time,the predominant ice crystal habits were plates,and the particle size increased as the height decreased,reflecting the growth of the ice crystals through the aggregation process.