EFFECTS OF CLOUD OPTICAL PROPERTY FEEDBACKS ON THE GREENHOUSE WARMING

Cloud optical properties, in particular the optical thickness tau, affect the earth-atmosphere radiation and their potential changes associated with climate changes may induce feedback effect. A one-dimensional radiative-convective model was used to illustrate that the difference in the vertical distribution of the radiative forcing between CO2 increase and changes of solar constant can result in a different tau feedback. Recently, Wang et al. carried out a general circulation model study of the climatic effect of atmospheric trace gases CH4, CFCs, and N2O, and the model results indicate that these trace gases provide an important radiative energy source for the present climate. Because the radiative-forcing behavior of CO2 is different from that of these other gases, the simulations also show that different radiative forcing can lead to quite different climatic effects. Consequently, increases in these trace gases may also induce different tau feedback than that due to CO2 increases. Since no study was attempted before to address this aspect, here a one-dimensional model is used to investigate the tau feedback associated with trace gases using an updated tau scheme that relates tau to cloud liquid water content through cloud layer latent heat flux. Because of the different changes in the tau vertical distributions, the tau feedback is calculated to be a small negative value for a CO2 increase, but much larger negative values for increase of trace gases. The strongest negative feedback is found for CFCs. Similar experiments were also conducted using a revised version of the Somerville and Remer tau scheme, which relates tau to cloud liquid water content through cloud temperature. The results indicate that the negative feedback for CO2 increases for a single cloud layer becomes much smaller when multiple-layer clouds are used, mainly due to the compensating effect of changes in tau values between high and low clouds. Because this scheme assumes a strong functional dependence of the local temperature, the tau feedback is also found to be sensitive to model dimensionality. In addition, the strength and sometimes even the sign of the tau feedback calculated from both schemes depend on the vertical distribution of cloud cover for the control climate, indicating the complexity of cloud-radiation interactions. Clearly, more observational and theoretical studies am needed to understand the cloud microphysics and their relation to large-scale climate variables.