The Characteristics Of A Pre-Tropical Cyclone Squall Line Under Weak Vertical Shear And Its Influence Mechanism

Pre-TC squall line is generated and developed in a environment which the vertical wind shear is significantly weaker than the mid-latitude squall line,therefore,the role of vertical wind shear on the pre-TC squall line in the occurrence,development and maintenance is worthy in-depth discussion.Take a pre-TC(Tropical Cyclone)squall line process occurred in Hunan and Jiangxi provinces,which effected by the super typhoon "Rammasun" in July 18,2014 for instance,discusses the mechanism of formation and development of this pre-TC squall line when the vertical wind shear was significantly weaker than that of midlatitude.The results showed that:(1)The vertical wind shear of the Pre-TC squall line in the middle and low level is significantly weaker than the mid-latitude squall line.Its main features are:The Typhoon moved northward and the subtropical anticyclone moves westward make the pressure gradient increases,which in turn leads to the enhancement of the low air flow.The lower troposphere is controlled by the warm and humid southeast airflow,so the wind direction and wind speed change in the vertical direction are not significant.(2)At the initial stage of the pre-TC squall line,the balance between weak vertical shear and weak cold pool lead to the erect updraft at the leading edge of the squall line.Unstable environmental conditions(cold and dry air overlap warm and moist air)on the front of the squall line is the important thermodynamic factors help to its development and enhancement.The horizontal wind shear on the right side of the typhoon inverted trough and the gust front in the front of squall line are the main dynamical factors of its development.During the mature stage,the cold pool range becomes larger and intensity becomes stronger on the rear side,resulting in reduced moisture and energy supply;at the same time,the middle level cold and dry air invasion strengthened,the updraft tilted toward the cold pool side and the vertical lifting weakened,this is not conducive to the development and maintained of squall line.These two features indicate the decay of the squall line.In the dissipation stage,the squall line move away from the typhoon and the influence of typhoon become weak.Insufficient water vapor supply and lifting lead to the demise of the pre-TC squall line.(3)Through qualitative analysis and quantitative comparison found that the influence of low level vertical wind shear on the Pre-TC squall line is more obvious than that of middle vertical wind shear;The stronger the vertical wind shear,the stronger intensity,the longer length,the longer maintenance time of the squall line,while the mid-level vertical wind shear have a weak impact to the squall line.The mechanism discussion shows when the low-level vertical wind shear is strong,the wind converges in front of the squall line is obviously;When increase the low-level vertical wind shear,it corresponds to the stronger horizontal vorticity and storm back-side environmental dry air inflow,enhance the rotation rises on the front side of the squall line and the descending airflow on the back side which is beneficial to the enhancement and maintenance of the squall line system.