Cloudradiation feedback

Numerous modeling studies suggest that changes in cloud fraction that would accompany global warming will modify the surface temperature. However, the sign of the feedback between different models ranges from negative (clouds cause cooling) to strongly positive (clouds cause warming). The issue is complicated in that there are feedbacks not only related to cloud fraction, but to cloud height, optical depth, and cloud microphysical properties. In turn, because of its unique thermodynamic and radiative environment, conclusions drawn from global studies may be inappropriate in the Arctic.

The cloud-radiation feedback mechanism in the Arctic is illustrated conceptually in Figure 5.15. A perturbation in the surface radiation balance may arise from increased greenhouse gas concentrations and/or increasing aerosol amounts. A perturbation in the surface radiation balance of the snow/ice changes the snow/ice characteristics (e.g., ice thickness and areal distribution, surface temperature and surface albedo). These changes, especially in the surface temperature and fraction of open water, modify fluxes of radiation and surface sensible and latent heat, modifying the atmospheric temperature, humidity and dynamics. This will in turn modify cloud properties (e.g., cloud fraction and optical depth), which will in turn modify the radiative fluxes. Resolving the interactions requires an understanding of changes in cloud fraction coverage and vertical distribution as the vertical temperature and humidity profile changes and adjustments in cloud water content, phase and particle size as atmospheric temperature and composition change.

As outlined earlier, for most of the year, the observed net cloud radiative forcing at the surface is positive in the Arctic (clouds cause warming). However, with respect to feedback processes, a major uncertainty is how cloud characteristics (e.g., optical

Figure 5.15 Same as in Figure 5.14 except for the cloud-radiation feedback mechanism (from Curry et al., 1996, by permission of AMS).

depth and microphysical properties) will be altered in a changing climate. Because of the impact of clouds on the surface radiation flux, cloud-radiation feedback processes in the Arctic are very much intertwined with the ice-albedo feedback. At present, the sign of the cloud-radiation feedback is still uncertain, but appears to be positive.

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