Treatment and Effects of Orography in NWP Models

抄録

 This paper reviews the treatment of the orographic effects in NWP models. Although it is difficult to separate the dynamical, thermal and frictional (diffusive) effects of orography, to discuss them respectively is still convenient for the description of this topic. Through developing a proper vertical coordinate system and improving the numerical schemes, the dynamic effect of orography can be treated successfully at present. It is found that the orographic forcing of air flow in winter is significantly underestimated by smoothed grid-square mean orography. The use of enhanced orography, or the use of a parameterization scheme for orographically induced gravity wave drag decreases systematic forecasting errors. Many works have shown that the dynamic orographic effect is important in the Northern Hemisphere winter season, but it is not a major factor in summer season. On the other hand the relative contribution of the mean error variance to the total error variance is much higher in summer than in winter. This means that the treatment of thermal effect is generally inadequate. In order to improve the accuracy of summer forecasts more attention should be given to the better treatment of the thermal effect of orography. Better treatment of the effect of orography should be closely related to improving the method of parameterization of sub-grid scale physical processes which are believed to be important in the presence of orography, and also closely related to improving the method of analysis and initialization in the presence of orography.