The Effect of Changes in Cloud Amount on the Net Radiation at the Top of the Atmosphere

Abstract

Due to the opposing albedo and greenhouse effects of clouds, the possibility exists that the net radiation at the top of the earth-atmosphere system is, in the mean, insensitive to changes in cloud amount. If so, this would have important implications for climate studies. This question is examined with the use of data on the components of the radiation budget at the top of the atmosphere obtained from the processing of 45 months of scanning radiometer observations of the NOAA satellites. Year-to-year changes in monthly mean values of outgoing longwave radiation and albedo are analysed at a sample of geographic and climatic areas of the earth. By using the albedo changes as a measure of changes in cloud amount, it is possible to determined the sensitivity of the outgoing longwave radiation and the net radiation to changes in cloud amount. For each geographic/climatic area, the results indicate that the net radiation at the top of the atmosphere is sensitive to cloud amount changes and the sensitivity is such that the albedo effect of the clouds predominates over their greenhouse effect. Thus, for the earth as a whole the net radiation at the top of the atmosphere is sensitive to changes in cloud amount. Estimates of the numerical values of the global mean sensitivity of net radiation and outgoing longwave radiation to changes in cloud amount are presented and compared with previous findings.