Shortwave, longwave, and net cloud‐radiative forcing as determined from Nimbus 7 observations

Abstract

We studied the effect of clouds on the Earth's radiation budget at the top of the atmosphere. Both the northern hemisphere summer and winter are considered in terms of cloud‐radiative forcing, which describes the total effect of clouds on the Earth's radiation budget components. The cloud forcing is obtained by using statistically generated models correlating simultaneous observations of cloud properties and radiation budget parameters obtained from observations from Nimbus 7 satellite observations. The models estimate the broadband albedo and longwave flux throughout the range of variation, including periods of extensive to near‐total cloud cover where more simplistic representations fail. As such, the models are used to estimate period‐averaged clear‐sky radiation budget components. Knowledge of the cloudy‐sky and clear‐sky radiation budget components permits the computation of cloud‐radiative forcing. We show that the net effect of clouds is to cool the Earth. On the average, for an Earth observed to be 52.1% cloud covered, the net cloud forcing is −26.8 Wm−2. This net cooling is obtained from a longwave warming of 24.1 Wm−2 and a shortwave cooling of −50.9 Wm−2. Over much of the Earth there is a general compensation between the warming and cooling effects of clouds. However, over the middle‐ and high‐latitude oceans the albedo effect of clouds is dominant. This is particularly true in each summer hemisphere, where the effects of persistent cloudiness, low oceanic albedos, and high solar insolation combine to drive the net cloud forcing strongly negative. The net effect of cloud forcing cools the Earth, primarily due to the effect of clouds in the summer hemisphere oceans.