A Simple Gravity Wave Drag Parametrization for Use in Medium-Range Weather Forecast Models

A simple gravity wave drag parametrization over mountainous terrain is tested for its ability to reduce the systematic errors of medium-range weather forecasts. Following Boer et al. (1984), this parametrization is a function of the low-level wind speed and stability, the local Froude number, and the variance of the subgrid-scale orographie features. A comparison study of ten 7-day forecasts obtained with envelope orography, wave drag or standard orography, shows that wave drag is as effective as envelope orography in reducing the systematic errors. A further comparison where the combined effects of the wave drag and that of a complementary enhanced orography (that is one that includes only the subgrid-scale elements not treated separately by wave drag) are taken into account shows this latter approach to be the most promising in reducing orographically-related systematic errors.