A goal of this presentation is to document the effects of clouds on the surface fluxes on the month-to-month time scale over the US. This goal will be achieved through the use of a quantity known as the surface cloud radiative forcing, which represents the radiative effects of clouds at the surface. The surface cloud radiative forcing is based on the downward and upward radiative fluxes under clear and cloudy conditions. The observed monthly total surface cloud forcing for a 10-year period (1985 to 1995) is determined from a combination of satellite observations, model analyses, and radiative transfer models. The variability of the surface cloud forcing on the month-to-month time scale will be discussed and related to several cloud properties and atmospheric parameters.

The month-to-month variability in surface cloud forcing is about 10-20 W m-2, with a swath of higher variability from the Gulf Coast to the Northern Plains. The lowest variability is found over the southwestern U.S., in a region of minimum cloud amounts in the time-mean. To determine the relative importance of cloud properties and atmospheric variables to the variability of surface cloud forcing, correlations of surface cloud forcing with cloud and atmospheric features at gridpoints over the US and for an average of land points only were calculated. The highest correlation occurs with surface cloud forcing and cloud optical depth, where months with high cloud optical depth lead to low (large negative) values of surface cloud forcing over the U.S. Optically thick clouds attenuate the insolation, and result in cooling at the surface. Correlations are highest in a swath that extends from the southern US to the Great Lakes region across the US, in the region of high variability of surface cloud forcing. The correlation between total cloud fraction and surface cloud forcing shows relatively high correlations over the southern portion of the US, although not as high as those with optical depth and cloud forcing. These correlations drop off significantly as one moves poleward, however, such that over the Northern Great Plains, there is very little relationship between total cloud fraction of surface cloud forcing. Over this region of the US, the optical thickness of the clouds is much more important than cloud amount alone to the surface cloud forcing.