Climatology of Shear Line and Related Rainstorm over the Southern Yangtze River Valley Based on an Improved Intelligent Identification Method

Based on four reanalysis datasets including CMA-RA,ERA5,ERA-Interim,and FNL,this paper proposes an improved intelligent method for shear line identification by introducing a second-order zonal-wind shear.Climatic characteristics of shear lines and related rainstorms over the Southern Yangtze River Valley(SYRV)during the summers(June-August)from 2008 to 2018 are then analyzed by using two types of unsupervised machine learning algorithm,namely the t-distributed stochastic neighbor embedding method(t-SNE)and the k-means clustering method.The results are as follows:(1)The reproducibility of the 850 hPa wind fields over the SYRV using China’s reanalysis product CMARA is superior to that of European and American products including ERA5,ERA-Interim,and FNL.(2)Theory and observations indicate that the introduction of a second-order zonal-wind shear criterion can effectively eliminate the continuous cyclonic curvature of the wind field and identify shear lines with significant discontinuities.(3)The occurrence frequency of shear lines appearing in the daytime and nighttime is almost equal,but the intensity and the accompanying rainstorm have a clear diurnal variation:they are significantly stronger during daytime than those at nighttime.(4)Half(47%)of the shear lines can cause short-duration rainstorms(≥20 mm(3h)^(-1)),and shear line rainstorms account for one-sixth(16%)of the total summer short-duration rainstorms.Rainstorms caused by shear lines are significantly stronger than that caused by other synoptic forcing.(5)Under the influence of stronger water vapor transport and barotropic instability,shear lines and related rainstorms in the north and middle of the SYRV are stronger than those in the south.

Structural Characteristics of the Spring Transverse Shear Line over the Qinghai-Tibet Plateau and the Influence Mechanism of the Upper-level Jet on Its Evolution

Based on the four-times-daily ERA-Interim data with the resolution of 0.75°×0.75°,the structure and evolution characteristics of a transverse shear line(TSL)over the Qinghai-Tibet Plateau in April 2017 were analyzed,and the influence mechanism of the frontogenesis and frontolysis caused by the upper-level jet on its evolution was also investigated.The results show that the TSL was mainly located near the axis of the positive vorticity zone,which was a low-value area of the wind speed.It was a shallow baroclinic system with weak ascending motion.In the vertical direction,the TSL extended to the lowest height at 00:00 and the highest at 18:00.In the horizontal direction,the length of the TSL in the east-west direction was relatively shorter during 00:00-06:00 and relatively longer during 12:00-18:00.Besides,the position of the TSL was slightly northward at 06:00 and slightly southward at 18:00.The moving direction of the TSL was generally consistent with that of the upper-level jet.In addition,the vertical stretching height of the TSL and the near-surface wind speed were positively correlated with the intensity of the upper-level jet.The calculation by frontogenesis function indicates that the frontogenesis(frontolysis)was conducive to the formation(weakening)and strengthening(dissipation)of the TSL.The horizontal deformation-induced and diabatic heating-induced frontogenesis were favorable for the formation of the TSL,while the middle-level horizontal convergence-induced and diabatic heating-induced frontogenesis were beneficial to its maintenance.Besides,the moving direction and baroclinicity of the TSL over the Qinghai-Tibet Plateau were determined by the horizontal deformation-induced frontogenesis.In the frontogenesis function,the terms of horizontal deformation and horizontal convergence together determined the position of the TSL,and the diabatic heating term was conducive to the upward extension of the TSL.

Climatology of Transverse Shear Lines Related to Heavy Rainfall over the Tibetan Plateau during Boreal Summer

Based on ERA-Interim data and precipitation data of 2474 stations in China during May-October from 1981 to 2013, transverse shear lines （TSLs） were identified, and their climatic characteristics and association with torrential rainfall events over the Tibetan Plateau and the region to its east during boreal summer were analyzed statistically, based on three criteria： the meridional shear of zonal wind, the relative vorticity, and the zero contour line of zonal wind. It was found that TSLs are generally west east oriented over the Tibetan Plateau, with the highest occurrence frequency in June, and least occurrence in October. The high frequency axis of TSLs, parallel to the terrain of the Tibetan Plateau, shifts southward from May to August, and then slightly northward from September to October. The annual average TSL frequency is 65.3 days, and there are obvious interannual and interdecal variations of TSLs. The annual fluctuation of TSL frequency is most distinct in the 1980s, followed by the 2000s, with average frequency appearing during 1995 2000. It was found that the occurrence frequency of TSLs and that of heavy rainfall events over the Tibetan Plateau are stable during 1981-2013. However, the occurrence frequency of the heavy rainfall events resulting from TSLs is decreasing. More than 50% of the TSLs can lead to heavy rainfall, while 40% of the heavy rainfall events are caused by TSLs. TSLs are closely related to heavy rainfalls in the flooding season of June-August over the Tibetan Plateau.