Analysis and comparison of diurnal variations of cloud radiative forcing: Earth Radiation Budget Experiment and International Satellite Cloud Climatology Project results

Abstract

Cloud radiative forcing (CRF) is the radiative impact of clouds on the Earth's radiation budget. This study examines the diurnal variations of CRF using the Earth Radiation Budget Experiment (ERBE) monthly hourly flux data and the flux data derived from the International Satellite Cloud Climatology Project (ISCCP) using the Goddard Institute for Space Studies general circulation model radiation code. The results for the months of April, July, and October 1985 and January 1986 are analyzed. We found that, in general, two data sets agreed. For longwave (LW) CRF the diurnal range over land is generally greater than that observed over oceans. For the 4‐month averages the ERBE values are 15.8 W m−2 and 6.8 W m−2 for land and ocean, respectively, compared with the ISCCP calculated values of 18.4 W m−2 and 8.0 W m−2, respectively. The land/ocean contrast is largely associated with changes in cloud amount and the temperature difference between surface and cloud top. It would be more important to note that the clear‐sky flux (i.e., surface temperature) variabilities are shown to be a major contributor to the large variabilities over land. The maximum diurnal range is found to be in the summer hemisphere, and the minimum values in the winter hemisphere. It is also shown that the daytime maximum and the nighttime minimum are seen over large portions of land, whereas they occur at any local hour over most oceans. For shortwave (SW) CRF the daytime maximum values are about twice as large as monthly averages, and their highest frequency occurs at local noon, indicating that solar insolation is a primary factor for the diurnal variation of SW CRF. However, the comparison of the ERBE data with the ISCCP results demonstrated that the largest differences in the diurnal range and monthly mean of LW CRF were associated with tropical convergence zones, where clear‐sky fluxes could be easily biased by persistent cloudiness and the inadequate treatment of the atmospheric water vapor.