The role of earth radiation budget studies in climate and general circulation research

Abstract

Two decades of near‐continuous measurements of earth radiation budget data from satellites have made significant contributions to our understanding of the global mean climate, the greenhouse effect, the meridional radiative heating that drives the general circulation, the influence of radiative heating on regional climate, and climate feedback processes. The remaining outstanding problems largely concern the role of clouds in governing climate, in influencing the general circulation, and in determining the sensitivity of climate to external perturbations, i.e., the so‐called cloud feedback problem. In this paper a remarkably simple and effective approach is proposed to address these problems, with the aid of the comprehensive radiation budget data collected by the Earth Radiation Budget Experiment (ERBE). ERBE is a multisatellite experiment which began collecting data in November 1984. The simple approach calls for the estimation of clear‐sky fluxes from the high spatial resolution scanner measurements. A cloud‐radiative forcing (or simply cloud forcing) is defined which is the difference between clear‐sky and cloudy‐sky (clear plus overcast skies) fluxes. The global average of the sum of the solar and long‐wave cloud forcing yields directly the net radiative effect (i.e., cooling or warming) of clouds on climate. Furthermore, analyses of variations in clear‐sky fluxes and the cloud forcing in terms of temperature variations would yield the radiation‐temperature feedbacks, including the mysterious cloud feedback, that are needed to verify present theories of climate. Finally, general circulation model results are used to discuss the nature of the cloud radiative forcing. It is shown that the long‐wave effect of clouds is to enhance the meridional heating gradient in the troposphere, while the albedo or solar effect of clouds is largely to reduce the available solar energy at the surface. The long‐wave cloud‐induced drive for the circulation is particularly large in the monsoon regions. Thus it is concluded that analyses of ERBE data in terms of cloud forcing would add much needed insights into the role of clouds in the general circulation. With respect to the future, the scientific need is discussed for continuing broadband measurements of earth radiation budget data into the next century in order to understand the processes that govern interannual and decadal climate trends. Finally, the spectral variations in clear‐sky fluxes and cloud forcing and the need for broadband data to obtain the desired accuracies are described.