Perturbed physics configurations of the HadAM3 atmospheric model driven with observed sea surface temperatures (SST) and sea ice were tuned to outgoing radiation observations. Four key parameters, that previous research found affected climate sensitivity, were adjusted to several different target values including observations. We used a Gauss-Newton line-search algorithm to adjust the model parameters. The observations used were the global average Reflected Shortwave Radiation (RSR) and Outgoing Longwave Radiation (OLR) from the Clouds and Earth's Radiant Energy System instruments combined with observations of ocean heat content. Using the same method, configurations were also generated that were consistent with the earlier Earth Radiation Budget Experiment results. Many, though not all, tuning experiments were successful with model failure almost as common as failure to converge. About 2500 configurations were generated and the changes in simulated outgoing radiation in these configurations were largely due to changes in clouds. Clear sky radiation changes were small largely due to a cancellation between changes in upper-tropospheric relative humidity and temperature. Changes in other climate variables are very strongly related to changes in OLR and RSR particularly on large scales. There appears to be some equifinality with different parameter configurations producing similar results though two clusters of models appear. One cluster is similar to the standard configuration while the other is warmer but drier in the tropics.
Submitted for publication