Dynamic And Thermodynamic Characteristics And Their Evolution Of Mesoscale Convection In Response To Wind Shear And Moisture

Environmental wind vertical shear is one of the important environmental factors affecting mesoscale convection.Its impact on the development and maintenance of convection is still controversial and needs further investigation.It has previously been suggested that vertical wind shear in the upper troposphere is a key control on supercell morphology through statistical observations.The present study represents the first attempt to test this hypothesis,using the Weather Research and Forecasting(WRF)idealized simulations to explore the effect of upper-level wind shear on mesoscale convection.The increase of the upper-level shear causes the increase of storm motion and low-level inflow.And further characteristics of the convective system are dramatically changed,including wider updrafts and stronger surface precipitation and outflow.To further explore the effect of different level wind shear on mesoscale convection,the sounding data from Shanghai Bao Shan meteorological observation station in China at 20:00on June 5th 2009(LTS)is used for generating the initial field of the idealized simulation.A series of test with different levels of wind shear are employed and the main results are as follows:(1)Low level wind shear is a necessary condition for the generation of convective system.Under the premise of the existence of low level wind shear,changing wind shear in the entire layer has the most significant impact on mesoscale convection,which is followed by the mid level shear.When increase(decrease)the entire layer vertical wind shear,the convective system has the strongest(weakest)strength and organization,life span of system becomes longer(shorter).(2)As the vertical wind shear increased,the horizontal vorticity increased,which is converted into vertical vorticity by the influence of vertical motion.The positive feedback process formed by the interaction between the vortex and the vertical motion is an important reason for the enhance of system strength and the maintenance of the linear structure for a long time.(3)As the wind shear decreased,the storm motion is much lower than the gust front.The gust front moves to the front of the system and blocks the necessary warm transportation of the updraft at the front of the system.The cold and stable environment after the gust front dissipates the system.In addition to wind shear,water vapor is also an important environmental factor that affects convection.Based on the control test of the actual sounding to generate the initialfield,further sensitivity test of water vapor at different levels are designed.It is found that the convective system is more sensitive to the low level water vapor.As the low level moisture increased,the convective available potential energy(CAPE)increased,the lifted condensation level(LCL)and level of free convection(LFC)decreased.The lifting capacity of the air parcel is enhanced which causes the increase of updraft and convection.In the environment with dryness of the middle layer and the wetness of the lower layer,the convective system is easier to maintain the linear structure,and its organization is enhanced.