The Hydrological Cycle as Simulated by an Atmospheric General Circulation Model

Abstract

The Meteorological Office five layer atmospheric general circulation model has been integrated through three complete annual cycles. The basic model has five layers in the vertical, with a quasi-uniform 330 km horizontal grid, giving 4626 points over the globe. Values of temperature, northward and eastward wind components and specific humidity were predicted at each grid point at each level; surface pressure was predicted at each point.

The model reproduces the main features of the hydrological cycle, including the major regional and seasonal variations in precipitation and evaporation. Over land, the hydrological cycle was more intense than observed, and the model’s deserts were less extensive than their real counterparts. The seasonal variation of snow cover was close to that observed, except over central North America where the model was too warm. Although the external forcing in the model was repeated exactly over each annual cycle, there were changes in the monthly mean circulation patterns from year to year, particularly along the model’s depression belts in winter. Precipitation varied most where it was heaviest. The fractional change in precipitation was largest over the drier regions in the subtropics.