Simplification of potential vorcticity and mesoscale quasi-balanced dynamics model

The physical characteristics of mesoscale are analyzed, and results show that the unbalancedf'orced motion is the fundamental cause, which leads to the evolution of some important mesoscaleweather systems. In this paper, an alternative asymptotic expansion method, which is quite different from the conventional Rossby-number expansion, is used to simplify the potential vorticityequation. And the quasi balanced (QB) model based on nonlinear balance equation is derived.The QB model, which is in analogy with the quasi-geostrophic model, can describe the fundamental characteristics of the mesoscale accurately and may be used as the basis of theoretical studies onthe mesoscale atmospheric dynamics.