Potential Vorticity Diagnosis On The Evolution Of The Transverse Shear Lines Over The Tibetan Plateau In Boreal Summer

Using the ERA-Interim reanalysis dataset provided by the European Centre for Medium-Range Weather Forecasts(ECMWF)for the period of June–August during 1981–2018,on the basis of objective identification with three criterions(meridional shear of zonal wind,relative vorticity and null point of zonal wind speed),13 cases of transverse shear line,which maintained for 5 days over the central Tibetan Plateau(33°N-35°N)and caused heavy precipitation,are selected and composited.Three episode of the evolution of composited shear line(CSL)are defined according to the relative vorticity and potential vorticity(PV)related to CSL.Initial episode for the first day of the CSL's evolutionary process,mature episode for the third day,and weakening episode for the fifth day.Distribution and evolution of PV at the initial,mature and weakening episode are analyzed.PV budget equation in the isobaric coordinate system was used to diagnose the evolution mechanism of CSL from PV perspective.The conclusions are as follows.The CSL orients in the east-west direction between 80°E-100°E,and extends horizontally to 2000 km and vertically to 2km.At 500 h Pa,the CSL lies between the 580 and584 dagpm contours,and on the north side of the warm center contoured by 272 K and the specific humidity center.At 200 h Pa,the CSL is located in the northern margin of the South Asian High and to the south of the jet stream.PV and relative vorticity at 500 h Pa near the CSL maintained positive during the evolution of the CSL,and is characterized by obvious diurnal variation.PV and relative vorticity increases during 18-00LT(local time,defined as LT,LT=UTC+6h)of each day,and relative vorticity reaches its peak at 00 LT,while PV reaches its peak 6hours(06LT)later.PV at lower-level increases and stretches upward during the development of CSL,and the PV center rises to 500 h Pa at mature episode,joints the upper-level PV to form a so-called PV tower.Upper-level PV stretches downward along 330 K isentropic surface to about 80°E near the CSL,characterized by 0.9PVU contour.Results of PV budget analyses shows that PV tendency term around the CSL is positive during initial and mature episode,and turns tonegative during weakening episode.The vertical component of the diabatic heating effect term contributes the most of the local variation of PV,and the PV advection contributes negatively to the CSL's development.The evolutionary model of the CSL influenced by diabatic heating is concluded as follows.On account of the remarkable diurnal variation of surface sensitive heating,vertical motion in near-surface layer of the Tibetan Plateau increases at 18 LT of each day.At 18 LT in the day before the mature episode,the water vapor near the CSL appears to be abundant,thus latent heat release center rises to about 400 h Pa,and reaches its peak at this time,leading to the enhancement of divergence at upper level.Meanwhile,enhanced upper-level jet also provides favorable divergence conditions.The enhancement of divergence at upper level encourages the convergence at lower level.The interaction of the motion between upper-and lower-level promotes the development of CSL.And at 18 LT in the day before the weakening episode,with the water vapor near the CSL decreases significantly,it is difficult to maintain the diabatic heating process mentioned above.The CSL finally dissipates with the decrease of the convergence.