An understanding of the diurnal cycle of surface radiation is fundamental to modeling climate. For example, deep convection is initiated by the solar heating of the surface during the day, and the height to which the convection carries heat, water vapor and momentum greatly influences the resulting global circulation. The NASA/GEWEX surface radiation budget (SRB) data set is used to examine the relation of the diurnal cycle of SRB to regional climate for the average July.

Over land, the net shortwave flux (SWN) at the surface provides the driving energy for the diurnal cycle of the surface temperature, which creates the diurnal cycle of the upward longwave flux (LWU). The boundary layer heating results in the cycle of downward longwave flux (LWD). The diurnal cycles of SWN, LWU and LWD can each be described to first order as a principal component for the time variation and a geographical coefficient, which is the empirical orthogonal function. The map is partitioned into climate classes, and a two-dimensional histogram in the domain of LWU and SWN is constructed for each class. The points cluster in different parts of the plot according to the climate class. These clusters are explained by the circulation and cloud patterns of the climate classes. In a similar manner histograms are made for the LWD-LWU domain.

For ocean, the processes are different. The SRB data set is developed using Reynolds sea surface temperatures, and thus the surface skin temperature is constant during the day-night; therefore, the diurnal cycle of LWU over ocean is neglected in this study. The diurnal cycle of LWD over ocean is due to the absorption of shortwave flux within the atmosphere. Two-dimensional histograms of LWD and SWN are used to show the relations of SRB to climate class over the oceans.