Incorporation of steep mountains into numerical forecasting models

Abstract

Numerical forecasting and general circulation models which include orographic effects use a smoothed topography, where the steepness of the mountains is greatly reduced. An incorporation of more realistic profiles seems desirable. To that effect a new condition at the lower boundary of the atmosphere is tested which treats steep mountains as impenetrable vertical barriers. Numerical experiments are carried out in a channel where this blocking condition is applied to an alpine profile as an example of subgrid orography and to the massiv of Greenland. Two forecasts are made for each mountain starting from an initial state which imitates a low approaching the mountain from the west in a baroclinic zonal current. The first one uses a smoothed topography and the second one applies the blocking condition to the obstacle. The latter yields patterns which agree well with synoptic experience. Cyclogenesis is predicted in northern Italy which is observed to occur typically with cold outbreaks from the north in the rear of a low. Just so, cyclogenesis is frequent near the southern tip of Greenland when a low crosses the ice cap and is predicted indeed. Both phenomena are not forecast in the experiments with smoothed topography. These and other results indicate that steep mountains are incorporated more realistically into numerical models by he new blocking condition than by a smoothed topography.