A TWO-WAVE SCHEME FOR OROGRAPHIC GRAVITY WAVE DRAG PARAMETERIZATION

When the magnitude of sub-scale ographic forcing is comparable with explicitly ordinary dynamic forcing, the drag effect reduced by ographic gravity wave is to be significant for maintaining dynamic balance of atmospheric circulation, as well as the momentum and energy transport. Such sub scale ographic forcing shouldbe introduced into numerically atmospheric model by the means of drag being parameterized. Furthermore, the currently mature ographic gravity wave drag parameterization, whatever the first-generation (based on lineal singlewave theoretical framework) or the secondgeneration drag parameterization (an important extra forcing by the contribution of critical level absorption), they can not correctly and effetely describe the vertical profile of wave stress under the influence of ambient wind shearing. Based on aforementioned consideration, a two-wave scheme was proposed to parameterize the ographic gravity wave drag by freely propagating gravity waves. It starts with a second order WKB approximation, and treats the wave stress attenuations caused either by the selective critical level absorption or the classical critical level absorption explicitly; while in regions where critical levels are absent, it transports the wave stress vertically by two sinusoidal waves and deposits them according to the wave saturation criteria. This scheme is thus used to conduct some sample computations over the Dabie Mountain region. The results showed that the new two-wave scheme is able to model the vertical distribution of the wave stress more realistically.