Clouds modulate the gain and loss of energy at the surface, which in turn affects the surface fluxes of sensible and latent heat. A goal of this presentation is to assess the effects of clouds on the surface radiation budget over the US in the time-mean, seasonal, and interannual time scales. This goal will be achieved through a quantity known as the surface cloud radiative forcing, CRF(S), which is defined as the difference in net radiative fluxes at the surface in all-sky and clear-sky conditions. The shortwave (SW) and longwave (LW) CRF(S) is determined by satellite measurements in combination with radiative transfer models, and it effectively identifies the radiative effects of clouds at the surface.

In the time-mean, the SW CRF(S) is negative everywhere over the US, with values that range from –25 W m-2 over the southwestern US to –65 W m-2 over the northeastern US. This geographical distribution of the SW CRF(S) is a result of the distribution of cloud amount and cloud opacity over the US, with regions of greater cloud amount and optically thicker clouds having greater magnitudes of SW CRF(S). The time-mean LW CRF(S) is positive everywhere over the US, with a maximum over the northern portions of the US and a minimum over the southwestern US, with values ranging from 25 to 35 W m-2. The geographical distribution of the LW CRF(S) is primarily a result of the distribution of cloud amount. The total CRF(S), which is the sum of the SW CRF(S) and LW CRF(S), is negative almost everywhere over the US, indicating that clouds tend to cool the surface in the time-mean. This cooling is greater to the east of the Mississippi River valley than to the west.

Annually, the maximum magnitude of SW CRF(S) occurs in the warm season, and the annual cycle exhibits maximum amplitude across the northern portion of the US. This distribution is largely influenced by the annual cycle of available insolation. However, regions with maximum cloud amount in the summer, such as the summertime stratus in the Eastern Pacific and summer monsoon region of Mexico, exhibit local maximum in summertime SW CF(S). The annual cycle of LW CRF(S) indicates a maximum in LW CRF(S) in the cool season, and exhibits less amplitude than the SW CRF(S). The maximum amplitude of the annual cycle of LW CRF(S) occurs over the lower Mississippi River valley, and is largely a result of the large annual cycle of cloud amount and water vapor over this region. The interannual variability of CRF(S) will also be discussed.