回流与倒槽作用引发的内蒙古自治区两次暴雪天气过程分析

摘要

利用常规气象观测资料、NCEP再分析资料(1°×1°)、FY-2C卫星云图及呼和浩特多普勒天气雷达资料,从环流形势、物理量场及红外云图和雷达回波特征方面,对2015年11月5-6日和21-22日内蒙古自治区两次回流与倒槽作用引发的暴雪过程进行分析.结果表明:500 hPa 西风槽、700 hPa急流和切变线及地面倒槽是内蒙古自治区两次暴雪过程的主要影响系统,中尺度云团是造成两次暴雪过程的直接原因.700 hPa急流对暖湿空气的输送及强烈的水汽辐合为暴雪产生提供了充足的水汽,700 hPa急流越强且持续时间越长,降雪持续时间越长,降雪量也越大;系统性抬升为暴雪产生提供了动力条件,高低空急流耦合有利于上升运动加强,冷空气"楔入"构成低层冷垫,温度差异及其垂直切变导致动力锋生,锋面次级环流产生强烈的上升运动;暖湿气流在"冷空气楔"上爬升,"冷垫"与"暖盖"是暴雪产生的热力条件;最强降雪出现在锋区最强且开始减弱期间,暴雪区对应相当黑体温度低值中心(TBB≤220 K)、850 hPa水汽辐合中心及最大上升运动中心;基本反射率因子达30-35 dBz,可导致局部强降雪;径向速度图上零速度线呈"S"型结构,"牛眼"结构长时间存在可使降雪持续或加强.%Using the conventional meteorological data,NCEP (National Centers for Environmental Prediction) reanalysis data with resolution of 1°×1°,FY-2C satellite cloud images,and Doppler radar data in Hohhot,the circulation situations,physical quantity fields,characteristics of infrared cloud images and radar echoes during the two snowstorm events caused by reflow and inverted rough on November 5-6 and on November 21-22,2015 in Inner Mongolian Autonomous Region were analyzed.The results show that the westerly trough at 500 hPa,the jet and shear line at 700 hPa and the surface inverted rough are the main influencing systems and mesoscale clouds are the direct cause for the two snowstorm events.Warm and wet airflows transported by jets at 700 hPa and strong convergence of water vapor provide plenty of water vapor for snowstorms.The stronger and longer duration of jets at 700 hPa result in the longer duration and lager amount of snowfall.Systematic ascending motions provide the dynamic conditions for snowstorms,and the coupling of jets at the upper and lower levels favors the enhancement of ascending motion.Cold air forms a cold pad at the lower levels,temperature difference and vertical shears induce dynamical frontogenesis,and strong ascending motion is caused by the frontogenesis secondary circulation.Warm and wet airflows climb up along the cold pad,which provides the thermal conditions for snowstorms.The most severe snowfall occurs when the frontal zone is strongest and begins to weaken,and the snowstorm area occurred at the low-value center of equivalent blackbody brightness temperature (TBB≤220 K),the convergence center of water vapor at 850 hPa,and the ascending motion center.The composite reflectivity factor reaches 30-35 dBz,which means local strong snowfall.The zero velocity line in radial velocity field shows an "S" pattern,and the "bovine eye" exists for a long time,which makes snowfall continue or strengthen.