Abstract: A two-layer model both applicable to graphical integration and numerical computation is proposed. From this model the height of 500 mb surface and the thickness between a certain selected isobaric surface in the vicinity of the top of friction layer and 500 mb surface can be forecasted. An extended model involving three layers is also designed. In designing them, no prescription with regard to the distribution of vertical velocity is made.Finally, the steadiness of space mean velocity field and the selection of lateral boundary conditions are discussed.