Sensitivity of the outgoing long-wave radiation to surface temperature: modelling the opacity feedback and experiments with a variable cloud-top temperature provision

An efficient long-wave scheme for climate models originally suggested by Sasamori is modified to simulate the water vapor-temperature feedback mechanism correctly. It is found that the modified scheme with a fixed cloud-top altitude (FCA) correctly simulates the long-wave sensitivity to surface temperature, delta F super(1) / delta T sub(s) , over the clear portion of the sky, but that over the cloud portion of the sky, delta F super(1) / delta T sub(s) remains too high. The fixed cloud-top temperature (FCT) method is similarly reviewed and tested. Comparisons with observational Budyko-type correlations are shown to be indecisive over the question of FCA vs. FCT. A direct observational correlation between the effective cloud-top and surface temperatures, based upon annually averaged cloud statistics data, suggests a variable cloud-top temperature (VCT) model. In such a model, the temperature of the effective cloud-top layer is varied according to changes in the surface temperature at a rate that is intermediate between that of the FCA and FCT models. This model results in a reasonable delta F super(t) / delta T sub(s) over the cloud portion of the sky. The modified long-wave scheme is implemented into a zonally averaged dynamic climate model. It is shown that, when the VCT mechanism is invoked, climate sensitivity is doubled compared to that simulated with the FCA model. The importance of simulating not only the correct long-wave flux but also the correct long-wave sensitivity to temperature changes is, therefore, stressed for radiation schemes in studies involved with climate change.