Clouds reduce the insolation at the surface, reducing evaporation of water and hence affecting hydrology and precipitation. The reduced insolation also reduces sensible heating of the surface, and consequently its temperature. Temperature and precipitation are the basic parameters of weather and climate. Thus, it is important to know how much clouds reduce the solar radiation at the surface. Clouds also emit longwave radiation to the surface, providing heat for raising the surface temperature and for evaporation. As with cloud radiative forcing at TOA, the shortwave and longwave forcing at the surface counteract each other.

The present paper investigates the seasonal cycles of cloud radiative forcing at the surface. The CERES SYN1deg data product is used. This data set includes monthly means of upward and downward shortwave and longwave fluxes at the surface on a grid with a 1° latitude by 1° longitude resolution. The CERES/Terra data set covers the period March 2000 through February 2010, for a ten-year period. All-sky and clear-sky fluxes are listed, so that the difference gives the cloud radiative forcing. In computing the seasonal cycle, the monthly means of the quantities are averaged for each calendar month of the data period in order to minimize interannual variations. Principal component analysis is used to obtain concise temporal and spatial descriptions of the seasonal cycles of the surface fluxes.

For the global average annual mean, clouds reduce the net shortwave flux at the surface by 49 W m^-2 and increase the downward longwave flux by 27 W m^-2, for a net cooling of 22 W m^-2. The cloud forcing is greater over ocean, where the net shortwave flux is reduced by 56 W m^-2 and the longwave flux is increased by 28 W m^-2. Over land clouds reduce the net shortwave flux by 35 W m^-2 and increase downward longwave flux by 25 W m^-2. The seasonal cycle may be measured by the root-mean-square of the variation about the annual mean. The RMS of the net shortwave seasonal cycle is 26 W m^-2 and of downward longwave is 6 W m^-2.