A MECHANISM FOR EFFECTS OF THE LARGE-SCALE MOTION AND HORIZONTAL INHOMOGENEITY OF THE MOIST STATIC INSTABILITY ON MCS AT MIDLATITUDES

Under the influence of the synoptic-scale environmental flows, the implicit nonthermal wind vorticity imbalance with ξT0 may arise when the change in the vertical shear vorticity of the flows,which is caused by the change of vorticity advection with altitude, is larger than that in the thermal wind vorticity of temperature fields, which is caused by the horizontal inhomogeneity of temperature advection. This kind of implicit imbalance will cause ascending motion to develop in the adjustment process. If the static stratification is horizontally inhomogeneous with the presence of its mesoscale high-value regions or the conditionally unstable region where moisture can be fully provided, the development of mesoscale disturbance with scale Lm may result from the coupling of the ascending motion on synoptic scale L1 with the parameter field of static stratification on mesoscale L2. The disturbance possesses such a vertical structure as ascending motion in the center, horizontal divergence in the upper and convergence in the lower atmosphere. This developmental process of instability may be one of the most important processes responsible for the growing of heavy rains in summer half year in China.