High-Resolution Experiments with the ECMWF Model: A Case Study

Abstract

The development of mesoscale features in numerical weather forecasts and their evolution as a function of the resolution of the numerical model are studied for the case of Alpine cyclogenesis of 5 March 1982. The results suggest that the European Centre for Medium Range Weather Forecasts (ECMWF) model with a reduced grid of about 0.47° (37.0 km at latitude 45°) is capable of improving the forecast and in particular the detail in the surface wind and precipitation patterns. The inadequacy of a coarse mesh analysis for the representation of the mesoscale structure of the fields is discussed and the need for a fine-mesh analysis to initialize and verify fine mesh models is stressed. The use of an ?envelope? type orography to parameterize the effect of mountains on the atmospheric circulation is shown to improve the forecast of the ECMWF Model with a 1.875° grid. The height he, of the ?envelope? orography is computed from the United States Navy data ad (resolution 0.167°), using the expression he = h + eσ, where h and σ are the mean and standard deviation of the height in a model grid square and e is a constant. Experimentation with this envelope orographic shows that e = 1 provides a good parameterization for subgrid-scale orography forcing at resolution 1.875°, but e should be decreased with increasing resolution. The separate effect, as well as the combined effect, of resolution and orography is examined.