~-~ ;.,'All ELSEVIER AtmosphericResearch37 (1995) 133-146 iii ATMOSPHERIC RESEARCH Diurnal variations of cloud from ISCCP data B. Cairns NASA GISS, 2880 Broadway, New York, NY 10025, USA Accepted 17 November1994 Abstract Information on the diurnal cyle of low, mid and high level cloud amount, for the period December 1984 to November 1990, compiled by the International Cloud Climatology Project (ISCCP), is analyzed using complex empirical orthogonal functions applied to the complex envelope of seasonal variations in the diurnal cycle. It is found that previous results on the diurnal variation of cloud amount, obtained from satellite and station data for more restricted periods and regions than that used here, are verified by the ISCCP data. The early afternoon maximum in low level cloud amount over all the world's continental land masses implies that diurnal variability enhances this cloud type's albedo effect. Conversely the nightime and early morning maxima in mid and high level cloud amount, over much of the globe, enhance their greenhouse effect. Any secular change in the diurnal variability of these cloud types could therefore have a significant effect on the net radiation at the Earth's surface. 1. Introduction Water vapor and clouds of liquid water and ice in the Earth's atmosphere are crucial elements in the determination of the radiative properties of the atmosphere and the sensitivity of the Earth's climate to forcings, both anthropogenic and natural. Clouds play a key role in determining the radiative balance of the Earth, in the hydrological cycle and in the determination of the water vapor distribution in the atmosphere. The increasing concentra- tion of carbon dioxide and other anthropogenic gases in the Earth's atmosphere and their predicted warming effect has led to considerable debate about the magnitude and sign of the cloud feedbacks. These may be expected to act in a warmer climate as a result of changes in the distribution, structure and optical properties of clouds and consequent modifications of the water vapor distribution. These feedbacks are important in the problem of climate change, since the radiative effects of clouds are large compared to virtually all climate forcing mechanisms that have been proposed. The uncertainties associated with cloud feedbacks have been noted in an intercomparison of global circulation models (Cess et al., 1989). 0169-8095/95/$09.50 © 1995Elsevier ScienceB.V. All rights reserved SSD10169-8095(94)00074-3