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IMPROVED TERRAIN EFFECTS IN BAROTROPIC FORECASTS

GEORGE P. CRESSMANJoint Numerical Weather Prediction Unit, U.S. Weather Bureau, Suitland, Md.

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Abstract

The persistent occurrence of systematic errors of barotropic forecasts over mountainous areas is strongly suggestive of significant effects of mountains and friction which have not been included in the previously used forecast models. This study reports on experiments with a new barotropic forecast model which contains an improved mountain effect and a surface friction effect. For computation of the surface stress depending on the wind and on the terrain, a hemispheric map of the drag coefficient is obtained.

The results of tests of the forecast model on an initially zonal flow, and on ten observed meteorological situations, indicate that the effects of terrain on the evolution of atmospheric flow patterns can be of large magnitude, and can account for significant errors in numerical prediction. Some success at accounting for these effects is attained with the barotropic representation of the atmosphere.

Abstract

The persistent occurrence of systematic errors of barotropic forecasts over mountainous areas is strongly suggestive of significant effects of mountains and friction which have not been included in the previously used forecast models. This study reports on experiments with a new barotropic forecast model which contains an improved mountain effect and a surface friction effect. For computation of the surface stress depending on the wind and on the terrain, a hemispheric map of the drag coefficient is obtained.

The results of tests of the forecast model on an initially zonal flow, and on ten observed meteorological situations, indicate that the effects of terrain on the evolution of atmospheric flow patterns can be of large magnitude, and can account for significant errors in numerical prediction. Some success at accounting for these effects is attained with the barotropic representation of the atmosphere.

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