The diurnal cycle of insolation at the surface of the Earth creates diurnal cycles of temperature of the surface and atmosphere, resulting in diurnal cycles of longwave radiation from the surface and from the atmosphere to the surface. Clouds are important in modulating the insolation at the surface and the downward longwave radiation from the atmosphere, thus it behooves us to understand quantitatively the effects of clouds on the surface radiation budget, as these heating effects govern the energy which is responsible for forcing the evolution of processes during the day. In particular, the development of convective clouds is driven by this energy flux, producing an important feedback mechanism.

The NASA/GEWEX Surface Radiation Budget Data Set 2.7 provides a global data set of upward, downward and net shortwave and longwave radiation fluxes at the surface of the Earth. These fluxes are computed using cloud conditions provided by ISCCP and temperature and humidity profiles from the Goddard Earth Observing System-4 (GEOS-4) reanalysis project. The radiation fluxes are computed on a one-degree grid for every three hours beginning at midnight Greenwich time and include the radiation as retrieved for existing conditions of cloudiness and also as it would be for clear sky conditions. The cloud forcing of surface radiation is computed by subtracting the clear-sky flux values from the values as computed for the observed cloud conditions. For this study, the average diurnal cycles for July over the years 1983 to 2004 are computed.

For the daily mean, the largest effects of clouds on total net radiation are over the oceans near 40 to 50 degrees north latitude and the dateline and the same latitude region south of Greenland, where clouds decrease the total net radiation by more than 100 Wm^-2, mainly by reducing the surface insolation. Over much of the Southern Hemisphere clouds have only a small effect on the surface insolation but increase the downward longwave flux significantly, so that they increase the total net radiation flux by up to 60 Wm^-2. The subsidence regions on both sides of the Equator associated with the Hadley circulation have less than 20 Wm^-2 change of total net flux.

A principal component analysis was performed for the diurnal cycles of the surface radiation components over the globe for land and ocean separately. For the downward and net shortwave flux over land, more than 95% of the diurnal cycle is described by the first principal component PC-1, which is fairly symmetric about noon and which has a peak that is 150 Wm^-2 above the nighttime level. The map describing the geographical distribution of the net shortwave PC-1 is very similar to that for downward shortwave flux for cloud forcing, indicating that the primary driver of this term is simply top-of-atmosphere insolation with the daily-mean cloud amount and transparency, and that the effects of cloud diurnal variation on surface net shortwave flux are small in a global context. For cloud forcing of downward longwave flux over land, 72% of the variance is described by PC-1. This term is approximately symmetric about 1300 hours at which time the maximum of 10 Wm^-2 occurs. The minimum of -6 Wm^-2 is just before the onset of insolation. PC-2 accounts for 13% of the variance of downward longwave flux and is approximately skew-symmetric about noon, with the maximum of 4 Wm^-2 at 0900 hours and the minimum of -3 Wm^-2 near 1900 hours. Geographically, PC-2 has a small coefficient over most areas but has values exceeding two around parts of the African coast, including the monsoon region of the southern coast of northwest Africa.