DIURNAL-VARIATIONS OF CLOUD FROM ISCCP DATA

Information on the diurnal cyle of low, mid and high level cloud amoun t, for the period December 1984 to November 1990, compiled by the Inte rnational Cloud Climatology Project (ISCCP), is analyzed using complex empirical orthogonal functions applied to the complex envelope of sea sonal variations in the diurnal cycle. It is found that previous resul ts on the diurnal variation of cloud amount, obtained from satellite a nd 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 im plies that diurnal variability enhances this cloud type's albedo effec t. Conversely the nightime and early morning maxima in mid and high le vel cloud amount, over much of the globe, enhance their greenhouse eff ect. Any secular change in the diurnal variability of these cloud type s could therefore have a significant effect on the net radiation at th e Earth's surface.