Characteristic Analysis Of Plateau Shear Lines Related To Heavy Rainfall

Firstly,this article selected three transverse shear lines(TSLs)and perpendicular shear lines(PSLs)over Qinghai-Xizang Plateau at 500 h Pa respectively according to the Tibetan Plateau vortex and shear line yearbook from 2012 to 2014 combined the NCEP FNL 1°×1° reanalysis data,and comparative analysis had been made on the vertical structure characteristic of TSLs and PSLs.Then a warm TSL and a PSL under the saddle pattern background circulation from these cases had been selected to analyze the heavy rainfall(HR)mechanism.Finally,The evolution and dynamic mechanism of plateau vortex weakening and plateau shear line formation was further analyzed,studying on plateau shear line formation could affect plateau vortex development.The followings are the main conclusions that were obtained through above chapters:(1)Analysis on structure characteristics of TSLs and PSLs under different background circulations showed that: TSLs' structure characteristics are related to large scale background circulation.When TSLs are situated in the north edge of the 100 h Pa South Asia High(SAH)center,they are baroclinic configuration and slanted upward flow along the warm interface of baroclinic surface.When TSLs are situated in the center or nearby areas of SAH axis,they are warm vertical configuration and vertical upward flow is situated in the warmer and wetter areas.PSLs pass through the axis of 100 h Pa SAH,their southern extremity is warm vertical configuration and northern extremity is baroclinic configuration.(2)Comparative analysis on dynamical conditions of HR caused by a warm TSL and a PSL under the saddle pattern background circulation showed that: TSL and PSL both have positive vorticity band and convergence band at different stages.The configuration of vorticity and divergence in the vertical direction contribute to TSL'vertical upward movement and PSL' slanted circulation.TSL' non-geostrophic wet Q-vector convergence band is mainly located in the position of shear line,but PSL'non-geostrophic wet Q-vector convergence band is mainly located in the southern nearby position of shear line.TSL' wet Q-vector convergence center showed vertical distribution and vertical upward movement could reach tropopause.WetQ-vector convergence band climbed along the terrain on the east side of TSL,and its altitude was below 500 h Pa.PSL' wet Q-vector convergence band tilted towards the west,there are slanted upward flow between 500 hPa and 300 hPa.(3)Comparative analysis on moisture conditions of HR caused by warm TSL and PSL under the saddle pattern background circulation showed that: Moisture convergence band is situated in TSL and PSL basically at different stages.The banded structure of moisture convergence is more obvious on PSL than TSL.The distribution of wind field on shear line has a very important effect on moisture convergence.The moisture convergence levels are situated in the mid-low troposphere,and the moisture convergence intensity is stronger on TSL than PSL.(4)Comparative analysis on instability conditions of HR caused by warm TSL and PSL under the saddle pattern background circulation showed that: The rainfall of TSL is dominated by convective instability,but PSL is dominated by convectivesymmetric instability.The HR band on the southeast side of TSL is caused by the east flank of the Qinghai-Xizang Plateau.Or rather,the rainfall of TSL is caused by shear line and topograpy,but the rainfall of PSL is caused by shear line and slanted isentropic surface.(5)Evolution and Mechanism analysis on the weakening of an eastward moving plateau vortex and the formation of a plateau shear line showed that: The Okubo-Weiss(OW)parameter is frequently used in tropical cyclone to measure the relative amount of rotation and deformation in vortex and shear flow,and has a good potential for identifying the evolution of plateau vortex and location of plateau shear line formation and development.The weakening and disappearance of the plateau vortex were mainly affected by the divergence term in vorticity equation.The temporal evolution analysis indicates that this system transformed from plateau vortex with strong cyclonic vorticity to plateau shear line with strong convergence.The twisting term in the total deformation equation made the largest contribution to the formation of the shear line,followed by the horizontal pressure gradient term,and the divergence term made the minimum contribution.When the plateau shear line was dominated by stretching deformation,it restrained the development of plateau vortex.The shear line could be the background flow field that affected the vortex development.