An Orographic-drag Parametrization Scheme including Orographic Anisotropy for All Flow Directions

中国科学院大气物理研究所

The orographic drag is an essential process for weather predictions in the complex terrain regions, which depends on the inflow direction. In this study, we define the orographic asymmetry vector (OAV) for a coarse grid as the normalized vector between the grid’s center point and its center of mass, and the orographic asymmetry in a flow direction-which describes the inclination direction and extent of the sub-grid terrain-is then calculated as the projection of OAV on this direction. Calculation of the Effective orographic length (OL) and the model grid length λ are extended to all flow directions. A new orographic drag scheme, which considers the effects of orographic anisotropy in all directions is then developed based on the OAV projection and the extended OL and λ for a given direction. Sensitivity tests of the orographic drag under the new scheme were conducted using a 5 m/s vertically uniform wind along different directions for four coarse grid points in typical mountain regions. The new scheme is shown to provide a more continuous transition of the orographic parameters and the resulting drag as a function of flow direction than the piecewise transition of scheme with only eight directions. The predicted drag and momentum flux profile of the new scheme were compared with the mountain-wave simulations obtained from the integrated modeling system IAP-WRF over the Rocky Mountains. The new scheme was shown to predict an overall narrower drag scatter about the reference simulation than the old scheme.