Earth Radiation Budget data and climate research

An overview is presented of the uses of top of the atmosphere radiation budget measurements in studies of climate. The net radiative energy flux at the top of the atmosphere must be balanced by local heat storage in the earth-atmosphere column or by horizontal transport in the atmosphere and ocean. Regional variations in the components of the radiation balance are significant and place important constraints on regional and global climate. If suitable time averaging is applied, regional net radiation can be used to infer horizontal transport of energy in the atmosphere and ocean. Estimates of equator-to-pole transport in the atmosphere and ocean based upon currently available top-of-atmosphere radiation budget measurements contain unacceptably large uncertainties associated with uncertainties in the radiation budget measurements themselves. Diurnal and interannual variations in regional radiation balances are large and important but have not yet been properly sampled with broadband instruments. Both have the potential for providing important insights into climate. The role of cloudiness in climate sensitivity remains one of the major uncertainties in quantitative estimates of climate changes associated with particular perturbing influences. Radiation budget data can be used to estimate the effects of actual cloudiness on the top-of-atmosphere heat balance and the dependence of these effects on location and season. The observational studies of the effect of cloudiness on the radiation balance at the top of the atmosphere that have been completed to date all suffer from one or more of three fundamental problems. These are that the parameters of the cloudiness are not accurately known, the radiation budget data are not based on measurements whose frequency response is unbiased in relation to the emissions of the sun and the earth, and the diurnal sampling of the measurements is not complete. The uncertainties associated with each of these problems are expected to be reduced as a result of analyses based on data from the Earth Radiation Budget Experiment. In order that quantitative estimates of climate change be as accurate as possible the general circulation models (GCMs) which produce these forecasts must be critically evaluated with observed data. Many of the most important mechanisms for perturbing climate (e.g., CO2, volcanic aerosols, surface albedo changes) and many of the most important feedback processes (e.g., relative humidity feedback, cloud feedback, ice-albedo feedback) directly involve the radiation balance at the top of the atmosphere and its relation to surface climate. Since the radiation balance at the top of the atmosphere can, in principle, be measured very accurately from space, it is natural that the simulation of top-of-atmosphere energy fluxes by GCMs should be carefully validated against observations. Because of the complexities introduced by clouds, a complete validation of the radiation budget of a GCM is a long and difficult task. A strategy is suggested here in which the clear-sky fluxes of the model are first compared with a clear-sky radiation budget climatology derived from observations. The radiation budgets with clouds included can then also be compared. This procedure should isolate problems not associated with cloudiness and indicate whether, in the grossest sense, the model clouds are correctly influencing the radiation balance. Comparisons between instantaneous synoptic maps of the radiation budget components and numerical forecasts can also be used to diagnose the performance of models.