Cloud radiative forcing 561 Definition and model results

Ramanathan et al. (1989) formalized the cloud longwave and shortwave radiative forcing as:

Longwave forcing = LWcloud - LWclear (5.4)

Shortwave forcing = SWcloud - SWclear (5.5)

where SW and LW are the net longwave and solar fluxes for given conditions of cloud cover and for clear skies. The values of cloud forcing are positive for warming and negative for cooling. The net cloud forcing is simply the sum of the longwave and shortwave forcings. The cloud forcing can be defined either for the top of the atmosphere or for the surface. Curry and Ebert (1992) examined the cloud radiative forcing at the TOA and surface for latitude 80° N using a single-column coupled model. Their results indicate that the total TOA cloud forcing is positive only from mid-September through mid-October, with values near zero during winter and strongly negative in midsummer. At the surface, values are positive except for two weeks in midsummer. The early analysis of Herman (1975) suggests that the net surface forcing is positive in all months but July. Based on observations from the SHEBA experiment, Intrieri et al. (2002) also show that clouds warm the surface except for a brief period in summer.

The annual cycle of the surface and TOA cloud radiative forcing from Curry and Ebert (1992) (Figure 5.7) illustrates that the competing effect of the shortwave and longwave forcing is most pronounced at the surface. The warming effect of clouds at the surface of the Arctic Ocean averaged over the year contrasts with lower latitudes, where clouds have a net cooling effect. The net surface warming in the Arctic Ocean is due to: (1) the absence of solar radiation during polar night; (2) the high albedo of the sea ice surface. The shortwave cloud forcing and thus the net forcing are strongly sensitive to variations in the surface albedo (Curry and Ebert, 1992). At the TOA, the net cloud

Figure 5.8 Mean monthly total cloud radiative forcing at the surface (W m 2) for the four mid-season months based on ISCCP-D data (courtesy of J. Key, NOAA, Madison, WI).

forcing is near zero in the winter months, but strongly negative in summer. This is primarily due to the high albedo of clouds.

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