Cloud Types and the Tropical Earth Radiation Budget

Abstract

Nimbus-7 cloud and earth radiation budget data are compared in a study of the effects of clouds on the tropical radiation budget. The data consist of daily averages over fixed (500 km)2 target areas, and the months of July 1979 and January 1980 were chosen to show the effect of seasonal changes. Six climate regions, consisting of 14 to 24 target areas each, were picked for intensive analysis because they exemplify the range in the tropical cloud/net radiation interactions. The normal analysis was to consider net radiation as the independent variable and examine how cloud cover, cloud type, albedo and emitted radiation vary with the net radiation. Two recurring themes keep repeating on a local, regional, and zonal basis: the net radiation is strongly influenced by the average cloud type and amount present, but most net radiation values could be produced by several combinations of cloud types and amount. The regions of highest net radiation (greater than 125 W m?2) tend to have medium to heavy cloud cover. In these cases, thin medium altitude clouds predominate. Their cloud tops are normally too warm to be classified as cirrus by the Nimbus cloud algorithm. A common feature in the tropical oceans are large regions where the total regional cloud cover varies from 20% to 90%, but with little regional difference in the net radiation. The monsoon and rain areas are high net radiation regions. The deep convective storm centers tend to have low, often highly negative, net radiation, but these are surrounded by large areas of high net radiation covered by thin medium and high level clouds. large regional differences in the net radiation caused by varying cloud cover and type do, however, occur over the tropical oceans. But the most noticeable difference is between continental and ocean regions. The net radiation is considerably higher over the oceans. The largest longitudinal variations in net radiation in July and January occur in high solar insulation regions somewhat poleward of the subsolar point. Over the oceans, net radiation maxima are associated with an average cloud cover of low albedo clouds. Some or most of these clouds may actually be high attitude cirrus clouds with emissivity less than one, although in the Nimbus-7 cloud cover dataset, most are identified as midaltitude clouds. The ocean net radiation minima are associated with bright low-altitude clouds. The largest differences, over 100 W m?2, are between the ocean maxima and the deep minima over the continental deserts. Over the deserts during the summer, however, cloud variations appear less important than regional variations in the surface albedo.