A study of the earth's radiation budget using a general circulation model

Abstract

AbstractObservations by satellites have shown that the various components of the global mean radiation budget have a significant seasonal variation, the causes of which are not properly understood. Results from an annual cycle integration with the Meteorological Office 5‐level general circulation model have been used to clarify the roles played by clouds and the surface in determining the seasonal variations in the radiation budget. The model uses an interactive radiation scheme with seasonally varying climatological sea surface temperatures, incoming solar radiation and zonal mean climatological cloud amounts. Its simulation of the various components of the earth's radiation compares favourably with results from satellites and it has therefore been possible to use its results to explain some aspects of the observed budgets.Many of the features of the annual cycle of the global mean radiation can be associated with the asymmetry in the distribution of land and sea between the two hemispheres. Variations in the global mean planetary albedo can be attributed partly to the illumination of Antarctica near the solstice and partly to the southwards spread of winter snow cover over the northern hemisphere continents. For the globe as a whole variations in cloudiness have little effect. The main determinants of the global mean outgoing flux appear to be the surface and associated tropospheric temperatures. Since the seasonal change in the surface temperatures of the land masses of the northern hemisphere is greater than that of the extensive oceans of the southern hemisphere, the variation in the global mean outgoing flux is dominated by that in the northern hemisphere. Again the effect of seasonal variations in cloudiness is small.The model results have also been used to estimate the earth's annual mean total energy budget and, in particular, the poleward flux of heat by the oceans. The oceanic heat flux implied by the model compares quite favourably with estimates based on observations, the main differences being at the equator and near the North Pole. The model gives a cross‐equatorial oceanic flux from the southern to the northern hemisphere and a significant flux of heat by the oceans into the Arctic Basin.