Analysis On Structural Characteristics And Evolution Mechanism Of The Yangtze-Huaihe Cold Shear Line

Based on ERA-Interim data from June to July during 1981-2016 and daily meteorological dataset of China Surface Meteorological Stations(V3.0),10 typical Yangtze-Huaihe cold shear lines over eastern China in summer are selected,and the structural characteristics of the Yangtze-Huaihe cold shear line during the evolution process is investigated by the composite analysis.The results indicate that the Yangtze-Huaihe cold shear line is horizontally in a northeast-southwest direction and vertically inclines northward from lower layer to upper layer.The vertical extension of the Yangtze-Huaihe cold shear line can reach 750 hPa,and its life time is about 54 hours.The evolution process of the Yangtze-Huaihe cold shear line is affected by the comprehensive configuration of the high-level,medium-level and low-level weather systems.The southward advancement,strengthening and eastward movement of the north branch low-pressure trough over the Yangtze-Huaihe region at 850 hPa is a key factor for the evolution of the Yangtze-Huaihe cold shear line.Because the structure characteristics of the Yangtze-Huaihe cold shear line have obvious changes in the evolution process,so the evolution process can be divided into development stage,strong stage and weakening stage.In terms of dynamic structures,the Yangtze-Huaihe cold shear line corresponds well with the axis of the positive vorticity belt,whose center is located at 850 hPa,and reaches the maximum in the strong stage.The Yangtze-Huaihe cold shear line is located in the non-divergence zone,and there are strong convergence centers located on its south side.The Yangtze-Huaihe cold shear line also locates in the ascending motion zone between two secondary circulations on the north and south sides,with the maximum ascending velocity in the strong stage,and its large-value area presents an upright structure.In the development stage,there is an ascending motion along the Yangtze-Huaihe cold shear line,but in the strong and weakening stages,there are an ascending motion below 800 hPa and a descending motion above 800 hPa along the Yangtze-Huaihe cold shear line.In terms of thermal structures,the Yangtze-Huaihe cold shear line is located in the low temperature zone of the lower layer,and there is a high temperature zone around 500 hPa.Due to the dominant role of dry and cold airflow from the north,the Yangtze-Huaihe cold shear line locates in the dry and cold air during the development and strong stages,and then the warm and moist airflow from the south invades,resulting in the weakening of the Yangtze-Huaihe cold shear line.There is a convective unstable layer on the south side of the Yangtze-Huaihe cold shear line and a neutral layer on the north side.The water vapor gathers near the Yangtze-Huaihe cold shear line,and there are two water vapor convergence centers on the east and west sides of the Yangtze-Huaihe cold shear line respectively.The water vapor convergence zone is mainly below 600 hPa in the low troposphere and the convergence center locates at around 900 hPa.The atmospheric baroclinicity is one of the reasons for the northward inclination of the Yangtze-Huaihe cold shear line.The evolution of the Yangtze-Huaihe cold shear line is closely related to the diabatic heating.In the horizontal direction,the Yangtze-Huaihe cold shear line is located in the northeast-southwest diabatic heating zone.The intensity of the diabatic heating is consistent with the Yangtze-Huaihe cold shear line.Meanwhile,the diabatic heating belt moves from north to south with the Yangtze-Huaihe cold shear line.In the process of evolution,the diabatic heating of the atmosphere in the Yangtze-Huaihe region mainly comes from the condensation latent heat released by precipitation.The strong ascending motion conveys the latent heat of the lower layer upward and enhances the diabatic heating of the upper atmosphere.The strongest diabatic heating of the atmosphere is located at about 500 hPa in the middle troposphere.Using the complete form of vertical vorticity tendency equation,it is found that in the development stage,the vertical non-uniform heating term and the relative vorticity advection term play a major role in promoting the strength of the Yangtze-Huaihe cold shear line.In the weakening stage,the horizontal non-uniform heating term,the relative vorticity advection term,the beta effect term,the ascending motion term and the heat source change term play a major role,which contributed to the weakening of the Yangtze-Huaihe cold shear line.The vertical non-uniform heating term is a key factor affecting the vertical vorticity tendency.Its distribution and intensity have a major impact on the movement and strength of the Yangtze-Huaihe cold shear line.