DYNAMIC AND THERMODYNAMIC CHARACTERISTICS OF MESOSCALE CONVECTIVE SYSTEM AND THE IMPACT OF ITS EVOLUTION ON VERTICAL WIND SHEAR
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摘要: 为探究环境风切变在对流系统发生发展与维持过程中的重要性，利用2009年6月5日20时（北京时）上海宝山站的探空资料生成理想试验初始场，设计了包含改变整层、中层和低层风切变在内的多组试验，对比分析各试验系统的动热力结构特征及其演变发现：(1)整层环境风切变的改变对中尺度对流系统的影响最显著，其次是中层风切变。增大整层风切变时，对流系统强度及组织性最强，生命史增长。减小整层风切变时，系统强度最弱且组织结构易发散。(2)风切变增加，水平涡度增大，其受垂直运动影响转化为垂直涡度，涡旋对与垂直运动间相互作用形成的正反馈过程是系统强度增强并可以长时间维持线状结构的重要原因。(3)风切变减小，对流系统移动速度远小于阵风锋，阵风锋移至系统前方，阻断系统前沿上升气流必需的暖输送。阵风锋后冷而稳定的环境令系统逐渐消散。Abstract: To explore the importance of wind shear in the development and maintenance of convective system, the present study uses the sounding data from the Shanghai Baoshan meteorological observation station at 20:00 on June 5th 2009(LTS) to generate the initial field of idealized simulation, and designs a series of tests including the change of the entire level, mid-level, and low-level wind shear. The system's dynamic and thermodynamic characteristics and the evolution of the system are analyzed, and the following results are found. (1) The change of wind shear in the entire layer has the most significant impact on the mesoscale convective system, followed by the change of wind shear at mid-level. When the entire layer vertical wind shear is increased (decreased), the convective system has the strongest (weakest) strength and organization, and the life span of the system becomes longer (shorter). (2) As the vertical wind shear increases, the horizontal vorticity also increases, which will be converted into vertical vorticity under the influence of vertical motion. The positive feedback developed by the interaction between the vortex and vertical motion is an important reason for the enhancement of the strength of the system and the maintenance of the linear structure for a long time. (3) When the wind shear decreases, the movement speed of the convection system becomes much lower than that of the gust front, which 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.
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Key words:
- idealized simulation /
- wind vertical shear /
- vertical motion
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图 4 控制试验3 h（a）、5 h（b）和7 h（c）沿图 3直线AB所做剖面上的最大反射率（阴影，dBZ）、垂直速度(等值线，实线为正，虚线为负，m/s)、风场(箭头，m/s)
图 5 积分3 h时增加(a、c、e)及减小(b、d、f)低层（a、b）、中层（c、d）和整层（e、f）风切变试验的最大反射率（阴影，dBZ），地面风场（箭头，m/s）
黑框为图 7对流区位置。
图 7 积分3 h时US5试验（a1~c1）和US-5试验（a2~c2）对流区（图 5黑框位置内）在1.5 km（a）、2.5 km（b）和4 km（c）上的垂直速度（阴影，m/s）和水平风场（箭头，m/s）
图 9 积分3 h增加（a、c、e）及减小（b、d、f）低层（a、b）、中层（c、d）和整层（e、f）风切变试验在1.5 km高度的垂直涡度分布（阴影，单位：10-4s-1）
图 10 积分3 h时US5试验（a1~e1）和US-5试验（a2~e2）在1.5 km上的涡度局地变化项（a）、涡度倾侧项（b）、相对涡度平流项（c）、涡度垂直输送项（d）和散度项（e，单位：10-5s-2）
图 11 US5(a1~c1)和US-5(a2~c2)试验积分3 h在1.5 km高度垂直涡度（阴影，10-4 s-1），垂直速度（等值线，m/s）和水平风场（箭头，m/s）分布
红圈标示出气旋与反气旋中心位置，两条黑色实线A1A2、B1B2分别表示图b、c中垂直剖面位置。b1、c1和b2、c2分别对应US5和US-5试验为沿a中直线位置所做剖面上的垂直涡度（阴影），流线涡度（等值线，实线为正，虚线为负，10-4 s-1）和风场（箭头）。
图 12 US5（a、b）和US-5（c、d）试验在3 h(a、c)和5 h(b、c)的近地面扰动位温（阴影，K）和组合反射率（等值线，25 dBZ）直线为图 13剖面位置
表 1 试验方案名称及中、低层切变值
试验名称 CTRL US-5 US5 LUS-5 LUS5 MUS-5 MUS5 As/(m/s) 0 -5 5 -5 5 -5 5 改变风速的层 原始 整层 整层 0~3 km 0~3 km 3.5~6.5 km 3.5~6.5 km 0~3 km切变/(m/s) 17.0 14.5 19.8 14.5 19.8 17.0 17.0 3.5~6.5 km切变/(m/s) 5.5 4.9 6.1 5.5 5.5 4.9 6.1 
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