The Characteristics Of Thunderstorm Gales In Tibet-sichuan Region And Preliminary Study On Its Formation Mechanisms

Thunderstorm gales?TG?occur locally and suddenly,and have the potential of causing devastating destruction and disasters.It is difficult to forecast their activities and defend their influences.The Tibet-Sichuan region is a key area for the development of western China,where convective weather such as TG occur frequently.Due to the coexisting terrains of highland,basin and deep-cut canyons,the complex atmospheric circulation s ystems,and the sparse observations,it is still a challenge to make an accurate prediction on TG over there.Therefore the activity characteristics and formation mechanisms of TG need further study.In this study,the station observational data is firstly used to analyze the characteristics of TG activity in Tibet-Sichuan region from 2010 to 2017 and the differences of TGs over highland and basin.Then Doppler weather radar data in Sichuan and surrounding provinces is employed to analyze the features of organizational modes of TG convective systems.And based on surface observational and sounding data,the water vapor,thermal and dynamic parameters of the atmosphere are calculated to reveal the environmental differences of TG.Finally,a rare TG event over the Tibet-Sichuan highland is investigated to represent the atmospheric thermodynamic environment,the structural evolution of convective system,and the formation mechanisms of TG.Comparison of the characteristics of TG over the highland?TGH?and basin?TGB?shows that:?1?TGH occur during a full year except winter,with two peaks in May-June and September.Their diurnal variation shows a major peak at 20:00 in the afternoon.However,TGB are active both in the afternoon and in the evening mainly in sum mer.?2?The annual station-averaged frequency of TGH is about 2 times,the proportions of which to thunderstorms and gales are about 4.5%and 8%respectively.It is only 0.4 times per station for TGB,which account for 1.5%of the thunderstorms but 60%of the gales.?3?The intensities of TGH and TGB are almost same,most of which are equal to or less than 20 m s^-1,but the extreme value of the former is greater.The convective systems that produce TG are divided into small-scale storms?isolated cell,cluster cells,broken lines?and MCS type?nonlinear-shaped MCS,squall lines,bow echoes?,which includes six sub-types according to their organizational modes.Results show that:?1?The convective systems mainly occur from April to September.The peak and minor-peak month are May and July.Small-scale storms mainly occur in the afternoon,with a peak around 20:00,and MCS are active both in the afternoon and at night.?2?TGH are mainly produced by small-scale storms with varying echo intensities.However,MCS with stronger echo intensity contribute to the most of TGB,the proportions of which are 45.4%and 27.5%for nonlinear MCS and squall line type.?3?MCS show more features with higher proportions than small-scale storms,and the drop of the reflectivity core and low-level strong wind region are better predictive indicators.Analysis of environmental parameters of TGH and TGB shows:?1?The atmospheric water vapor content,CAPE/DCAPE and vertical wind shear are smaller over the highland than that over the basin.?2?A shallow moist layer in the middle troposphere above a drier air layer commonly exists over the highland,with large mean vertical temperature lapse rate in the middle and lower troposphere.?3?There is usually significant midleve l dry air above a moist layer over the basin.?4?Synoptic situations over Tibet-Sichuan are composited during the two TGH peaks respectively.During May and June,TG occur on the right side of an upper jet entrance,and there is a westerly trough transporting weak cold advection at middle level,and strong environmental vertical wind shear.In September,however,TG occur on the north edge of subtropical high pressure,with midlevel drier air and low-level warm moister air flows.Both of the situations can provide favorable conditions for the formation of TG.A case study of rare TGH event with more than 10 mm hourly rainfall and severe hails of more than 18 mm in September 2016.Note that?1?The MCS occurred near low-level shear line with weak cold advection at 500 h Pa.Large environmental CAPE,vertical wind shear,and the thick moist atmospheric layer are conductive to the formation of the supercell.?2?The initial convection was generated on a surface convergence line,with multiple meso-?-scale cells embedded in stratiform cloud,which merged with the cluster cells in the south.When MCS entered the favorable environment,the cell on the south side evolved into a supercell.?3?The front inflow gap,hook echo and mesoscale cyclone were clear in a mature supercell at low levels.Mid-altitude radial convergence,weakening of updrafts and rapid drop of the reflectivity core,indicated the occurrence of d ownbursts.?4?The cooling effect due to the entrainment of midlevel dry air,drag effect related to the rapid drop of heavy raindrops and hails,and the narrow tube effect of the canyon terrain,contributed to the formation of TG near the ground.