On the net radiative effectiveness of clouds

The net effect of clouds on the energy balance at the top of the atmosphere is investigated using data from the Earth Radiation Budget Experiment. A regional analysis is made of the relative magnitudes of the effects of clouds on outgoing longwave radiation and on reflected solar radiation. The ratio of these two effects is expressed in terms of a cloud factor angle defined in coordinates of reflected solar versus outgoing longwave radiation. This parameter is a measure of the net radiative effectiveness of clouds, which is insensitive to the average amount of cloud present. It is calculated in two independent ways. One method uses monthly averaged clear-sky and average values, and the other uses regression between daily values of solar and longwave exitance. Because the effect of cloud amount variations is minimized by the analysis procedure used here, the spatial pattern of cloud radiative effectiveness is simpler than that of cloud radiative forcing and corresponds to the pattern of circulation and cloud type regimes. The two methods of estimating the effectiveness give essentially the same spatial pattern. Both estimates of the cloud radiative effectiveness indicate that the global average net radiative effect of today's clouds is cooling, with the decrease in absorbed solar greater than the decrease in outgoing longwave radiation. The ratio of the solar to the longwave effect of cloud is about 1.85 based on the regression method and 1.55 based on the comparison of clear-sky and average radiation budget climatologies. The differences between the two methods arise primarily from regions of stratus cloud, tropical cumulonimbus cloud and clouds over snow. In each of these regions the regression method indicates a more strongly cooling effect. The difference between the global mean estimates is within their uncertainty. Investigation of the areas of disagreement between the two methods reveals remaining problems. Significant improvement in our understanding of the effect of clouds on the radiation balance will require that the analysis procedure incorporate simultaneous measurements of temperature, humidity, and cloud structure, with particular importance attached to humidity in the deep tropics and temperature over land surfaces. Additional spectral information will be necessary to discriminate the role of clouds over snow and ice.