金沙江下游峡谷一次飑线和下击暴流的机理分析

Analysis on a Squall Line and its Downburst over the Lower Reaches of Jinsha River

产生极端灾害性大风的飑线对水电站安全至关重要。本文以2016年6月4日金沙江下游白鹤滩水电站峡谷产生13级大风的飑线为例,从天气系统和中小尺度上讨论其形成机制。获得以下结论:(1)飑线到达前,受飑前低压影响水电站气压稳降了9 hPa,气温上升超过14℃。飑线到达时受雷暴高压作用1 h内气压陡升10 hPa,气温骤降超过10℃,相对湿度陡增到70%,伴随短时降水发生。飑线过境后以上要素恢复到原值。(2)天气尺度环流的分析发现,飑线当天水电站在高空急流入口右侧辐散和低空西南涡南侧切变辐合的耦合作用下,次级环流的上升运动为飑线深对流提供了环境背景。飑线前中高层干冷平流叠加在中低层暖湿平流上,加上低空强烈升温增加了气温直减率,积累了对流发展的静力和对流不稳定,为飑线长生命提供了有利环境条件。垂直风速显示峡谷的上空下沉运动增强了动量下传,促进飑线前侧的抬升,正反馈作用下促进风暴稳定发展。(3)昭通雷达探测表明,飑线中30 dBZ以上多单体紧密聚合成带状回波,镶嵌着超过10 km高度柱状发展的50 dBZ以上强回波核。径向速度场上,垂直于峡谷的中尺度辐合线与带状强回波吻合,是飑线的触发机制,对应深厚的中层径向辐合MARC(Mid-Altitude Radial Convergence)特征。在云体剖面结构上,飑线前强上升运动形成穹窿,飑线到达时高悬的回波核快速下降,形成下击暴流,并诱发顺着河谷的新单体生成,改变了飑线走向,形成顺着的峡谷的极端大风。

Squall lines often produce extremely disastrous gales and play a crucial role in the safety of hytropower stations.This article explores a squall line event over the lower reaches of Jinsha River on June 4,2016,which brought 13-level wind at the canyon of Baihetan Hytropower Station.This research detect the formation mechanism of this squall line from environmental conditions and medium-scale meteorology.The following conclusions are obtained:(1)Before the squall line arrived,the ground pressure dropped steadily by 9 hPa because of the low pressure,and the temperature of the canyon rises up above 14℃.As the squall line arrived,the pressure rises sharply with thunderstorm high pressure,and the temperature dropped more than 10℃.At the same time,the relative humidity reached as higher as 70%,accompanied by short-term precipitation.After the squall line left,the aforementioned meteorological elements restored to their original values.(2)Analysis on the synoptic scale circulation,we finds that Baihetan hytropower station located in the south of southwest vortex,which induced a wind convergence background at the low level.Meanwhile,the station lied in right side of the jet stream inlet at upper level,and strong vertical motion formed by the wind shear.As a result,the squall line to come into being.Furthermore,in the troposphere before the squall line,the atmospheric circulation displayed a dry-cold advection in the middle-upper level superimposed over a warm-wet advection and higher ground temperature in the lower levels,increasing the temperature lapse rate.t As a result,the strong static instability and convection accumulated,and the conditions were favorable for the squall line to live a long time.The analyses from vertical wind speed showed that the subsidence movement made the momentum spread down,and promoted the uplift in the front side of line,and a positive feedback effect developed,promoting a stable mechanism for the strong storm.(3)Radar of Zhaotong detection indicates that echo cell that greater than 30 dBZ aggregated costly into a band and embed with the strong echo cores of greater than 50 dBZ,as a cylindrical convective cloud of higher than 10 km.On the radical velocity field,the meso-scale convergence line coinciding with the banded strong echo,as well as MARC(Mid-Altitude Radial Convergence)characteristics of the mid-altitude radial convergence,were the trigger mechanism for the squall line.As the cloud structure of the storm,strong upward movement in front of the squall line leaded to the dome.As the squall line arrived,the echo core dropped rapidly,and the extreme gale of downburst formed,resulted the new convective monomers come in being along the canyon.