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

Abstract

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.

Keywords

• squall line
• downburst
• canyon
• mid-altitude radial convergence