A Three-Parameter Model for Limited Area Forecasting

This report describes an operational quasi-geostrophic three-parameter model. The original model was developed by Dr. L. Bengtsson and has been used operationally for several years at the Swedish Meteorological and Hydrological Institute. The improved model described in this report incorporates an Ekman function and the effect of the flow over mountains as well as sensible and latent heat sources. Humidity and precipitation are also predicted by the model. An additional feature is the optional inclusion in the vorticity equation of the terms which are usually considered negligible: (1) the advection of vorticity by the divergent wind, (2) the product of relative vorticity and divergence, (3) the vertical advection of vorticity, and (4) the twisting term. The model can easily be adapted for different geographical areas with different grid lengths and also be integrated over different vertical layers. The lateral boundary values can be fixed or allowed to vary in time and thus it is possible to apply the so called nesting or telescoping technique. A special, relatively simple method to apply the nesting techniques is described.