山东省线状中尺度对流系统的天气学特征

Synoptic Characteristics of Linear Mesoscale Convective System in Shandong Province

按照如下标准确定一个线状中尺度对流系统(linear mesoscale convective system,LMCS):40 dBz以上反射率因子连续或准连续回波带尺度≥100 km并持续至少1 h,镶嵌着40 dBz回波的35 dBz回波要求严格连续,线状或准线状的对流区域拥有一个共同的前边缘,最大回波强度≥50 dBz。从2012—2016年雷达资料中挑选出27个影响山东的LMCS,分析其天气学特征。结果表明:影响山东的LMCS 8月出现次数最多,形成时间集中在傍晚到前半夜,生命史一般为1~2 h,大多数具有后向传播特征;形成LMCS的初始对流单体绝大多数位于河北省,单体生成后一般向东偏南方向移动;LMCS大多数是东北—西南向,尺度一般介于100~200 km。文章提炼了形成LMCS的后倾槽、前倾槽和冷涡等三类天气学模型。850 hPa伴有暖温度脊或暖中心是形成LMCS的一个重要特征,冷涡和前倾槽类500 hPa中空急流以及后倾槽类700 hPa以下低空急流在形成LMCS中起着重要作用。当850 hPa比湿>8 g·kg^-1,沙氏指数和抬升指数均为负值时,可能出现LMCS。若对流有效位能>1000 J·kg^-1,对流抑制较小,且850 hPa与500 hPa气温差大于25℃,出现LMCS的概率达80%。LMCS出现时均伴有短时强降水,70.4%的LMCS造成雷暴大风、冰雹或强降水灾害。冰雹和大风比短时强降水需要大气层结的不稳定度更高,仅有短时强降水出现时,0℃层和-20℃层的高度明显比冰雹和大风出现时的高度高。

The synoptic characteristics of LMCS(linear mesoscale convective system)were analyzed based on 27 selected LMCSs affecting Shandong from the 2012-2016 radar data.The filter conditions are as below:the contiguous or quasi-contiguous echo band larger than 40 dBz is over 100 km and lasts for at least 1 h,the 35 dBz echo embedded with the 40 dBz echo is strictly contiguous,the linear or quasi-linear convection area shares a common leading edge,and the maximum echo intensity is over 50 dBz.The characteristics of the study are concluded as follows.The LMCS affected Shandong has a high frequency in August,and the formation time is concentrated in the dusk to the first half of the night,lifespan is generally 1-2 h,and most of them have characteristics of backward propagation.Most of the initial convective cells are generated in Hebei and generally move eastward by southward.Most of the LMCSs are in a northeast-southwest trend,and the scale is between 100 and 200 km.Three types of synoptic models of LMCS formation are constructed,including forward-tilting trough,backward-tilting trough and cold vortex.The warm temperature ridge or warm center at 850 hPa is the important feature of LMCS formation.The mid-level jet stream at 500 hPa in cold vortex and forward-tilting trough category,and the low-level jet stream below 700 hPa in backward-tilting trough category all play very important roles in the LMCS formation.LMCS constantly occurs when 850 hPa specific humidity is greater than 8 g·kg^-1 and the Lift Index(LI)and Showalter Index(SI)are both negative.The probability of LMCS occurrence is up to 80%when T 850-500>25℃and CAPE>1000 J·kg^-1 with small CIN.When LMCS appears,it is usually accompanied by short-duration heavy precipitation,70.4%LMCS contributes to disasters of thunderstorm gale,hail and severe precipitation.The hail and gale require higher stratification instability than short-duration severe precipitation.When only the short-duration heavy precipitation occurs,the heights of 0℃and-20℃layers are significantly higher than those when hail and gale occur.