A synthetic study of the structure and evolution characteristics of a meridionally-oriented shearline over the Tibetan Plateau based on objective identification

Using the 4 times daily ERA-Interim reanalysis dataset on global 1°×1° grids for June-August from 1981 to 2016 the automatic method to identify meridionally-oriented shear lines over the Tibetan Plateau (MOSLs) is applied to determine shear lines based on three parameters,i.e. In total 13 cases of MOSLs between 33°-35°N are selected. Using synthetic analysis technique,the structure and evolution characteristics of the MOSLs are studied. The results show that MOSLs are largely located at the main body of the Tibetan Plateau within the scope of 80°-100°E,extending along the east-west direction with a quasi-horizontal distribution at 500 hPa,Its horizontal scale near 2000 km. In the vertical direction,the MOSL over the Tibetan Plateau can extend up to 480 hPa with a thickness of neary 2 km. The circulation background for the MOSL is characterized two-trough and two-ridge pattern in the high-latitude region at 500 hPa,while the western Pacific subtropical high (hereafter WPSH) and the Iran High are distributed on both sides of the Qinghai-Xizang Plateau respectively. On the dynamic field,the extension of MOSL is consistent with the axis of positive vorticity zone at 500 hPa; there are zonally distributed positive vorticity and ascending motion near the MOSL,corresponding to the non-divergence zone. Divergence and convergence zones are located on the northern and/southern sides of MOSL,respectively. The positive vorticity zone near the MOSL can vertivally extend to 350 hPa,and the ascending motion can reach 200 hPa. However,the shear line can only extend to about 480 hPa,suggesting that it is a shallow weather system with a certain baroclinic property and a northward inclination with height. On the water vapor and thermal fields,the MOSL corresponds to the water vapor convergence zone,and there is a high pseudo-equivalent temperature center between 600-500 hPa on the southern side of MOSL. It obviously has a clear feature of high temperature and high humidity. The evolution process of MOSL from its initial generation to strong development and then weakening takes about 4 d. Along with the westward shift of the WPSH,the range and intensity of positive vorticity zone near the MOSL increase,and the MOSL develops. With the invasion of dry cold air,the intensity of the MOSL decreases. Under certain extreme conditions,the MOSL even disappears.