Observational And Model Studies On The Initiation Conditions Of Severe Convections Over The Lower Area Of Yangtze-Huaihe River Basin

Severe convective weather happens frequently in China,especially in the lower area of Yangtze-Huaihe river basin,causing huge economic losses and casualties to our country.The forecast of severe convective weather is still difficult,and forecasting the specific location and time of convective initiation(CI)is one of the most challenging problems in forecasting severe weather.In this paper,the trigger mechanism of severe convective weather in the lower reaches of the Yangtze-Huaihe River is studied by observational and modeling analysis of several cases.Based on the detailed analysis of the severe weather on June 5,2009,it is found that a mesoscale disturbance convergence center existed in the southwest flow of the lower atmosphere in the northern part of Anhui Province.The disturbance triggers the first convective cloud in the northern part of Anhui Province.There was a convergence line formed by the confluence of the southwesterly flow and the westward flow in the northern part of Anhui Province,which provided a favorable area for CI.Convection occurred at the eastern end of the convergence line after a wind disturbance occurred,which proves that the CI is related to the disturbance of the surface wind field.The WRF model simulates the convergence line,the wind disturbance,and the convection initiated at the eastern end of the convergence line in the northern part of Anhui Province.Model results show that the updraft caused by the convergence line was confined below 3 km height.The wind disturbance was caused by a high-level gravity wave,which is an n = 1 mode gravity wave generated by coastal convective systems.When the updraft of the gravity wave intersected with the convergence line,the low-level updraft at the intersection strengthened and led to CI.The structural characteristics of n = 1 gravity waves and their effects in triggering new convection are analyzed by studying different cases.The n = 1 gravity wave consists of a downdraft-updraft couplet with a depth of about 12 km.The gravity wave is throughout the troposphere and the vertical velocity center of the downdraft-updraft couplet is located at 6 km.The updraft(downdraft)corresponds to divergence(convergence)in the upper half of the troposphere and convergence(divergence)in the lower half of the troposphere.The n =1 gravity wave forms a pressure dip on the ground,and the pressure drop(rising)stage of the pressure dip is caused by the subsidence warming(upward cooling)of upper-level atmosphere.The wave speed is about 30 m/s.The low pressure area before a squall line is partly caused by the subsidence of the n = 1 gravity wave.The n = 1 gravity wave has a positive contribution to CI when the updraft of the wave is stronger than the downdraft,which is characterized by an increase of the wind component towards the propagating direction of the wave to the surface wind field.The squall line on June 14th 2009 generated such n = 1 gravity wave and initiated severe convection when the updraft of the wave intersected with a band of shallow clouds.In this paper,typical surface convergence line types in the lower reaches of the Yangtze-Huaihe River include:1)convergence lines formed by the air flow bypassing the Dabie Mountain,Huangshan Mountain and Tianmu Mountain in the mountains' leeward side;2)convergence lines(including gust fronts)formed by outflow of previous convection clouds;3)convergence lines near the coastal region formed by the airflow from inland and the airflow from sea.These convergence lines provide a favorable location for convection and when they intersect with other convergence lines or systems(e.g.,shallow cloud bands,convective systems,gravity waves,etc.),deep convection is liable to occur in the intersection.