Description:
AbstractThe global and monthly distribution of the diurnal variation of outgoing longwave radiation (OLR) has been calculated by compositing estimates of the planetary OLR obtained from polar orbiting satellites at different equator crossing times. These data include NOAA and NASA polar orbiting satellites for the year 1974 through 1983, giving a total of ten different local observations per day throughout the whole data period. A Fourier analysis was then applied to calculate the phase and amplitude of the diurnal cycle on a 2.5° latitude/longitude grid for each month of the year. Assumption was implicitly made that the first harmonic alone explains most of the diurnal variation.The results are represented on maps extending from 55° south to 55° north for four different seasons from which a large seasonal variability of the diurnal harmonic primarily experienced by the mid‐latitude continents was clearly depicted. Over land areas the amplitude of the diurnal variation was large. In desert and semi‐arid areas it seem to be clearly related to surface solar heating i.e. phases of maximum OLR generally occurring near noon during summer and afternoon during winter months. Over the oceans, the amplitude is generally small even in the convectively active regions, such as the ITCZ, and the diurnal cycle appears to be modulated primarily by clouds.Finally, a ‘harmonic dial’ was constructed to examine the most dramatic features of the diurnal variation representative of such areas as the ITCZ, deserts, tropical convective clouds regions and highly variable mid‐latitude areas. Results of this study are consistent with other studies of the diurnal variability of clouds and radiation budget parameters.