An alternative method to calculate cloud radiative forcing: Implications for quantifying cloud feedbacks

Abstract

A modification to the traditional cloud radiative forcing (CRF) formula is presented. This alternative approach uses incident rather than absorbed-solar radiation to calculate shortwave cloud radiative forcing at the surface. By removing the competing influence of surface albedo on CRF, the new formula more effectively isolates the impact of clouds and cloud changes under different climatic regimes. The removal of surface albedo effects makes the change in CRF a more useful measure of cloud feedback, although other factors (such as changes in clearsky solar absorption) can still muddle the interpretation. I present examples from GCM greenhouse simulations that demonstrate how this new formula can help to differentiate between actual cloud feedbacks versus apparent ones induced by snow and ice meltback using the traditional CRF equation. The results suggest that changes in surface albedo contribute approximately 20 to 40% of the high-latitude CRF anomaly using the standard formula applied to an equilibrium 2 × CO2 simulation and about 50% to the greater Arctic when used on 21st-century transient greenhouse experiments. Copyright 2006 by the American Geophyical Union.