Evaluating the cloud response to climate change and current climate variability

The radiative feedback from clouds remains one of the largest uncertainties in determining the climate sensitivity to increasing greenhouse gas (GHG) concentrations. This study presents a diagnostic method of evaluating the simulated tropical cloud response to climate change. The method is illustrated by comparing the effect on the climate response of several new developments to the Meteorological Office's Hadley Centre atmosphere model coupled to a mixed-layer ocean. It is shown that the cloud response over tropical oceans to a doubling of CO, at individual locations appears to depend more upon the change in vertical velocity and whether the sea surface warms more or less than the tropical mean, rather than the absolute value of the surface temperature. By using compositing techniques, the model cloud response to doubling Col is related to the model cloud response to present-day spatio-temporal sea surface temperature anomalies. The use of special model diagnostics to take account of cloud overlap assumptions allows comparison of the modelled present-day variability with satellite data to evaluate the cloud response. Based upon this evaluation, it is possible to identify which of the new physical schemes are likely to provide a more reliable cloud response in the climate change simulation. It is found that overall, the model developments improve the simulation of present-day cloud processes and, through the relation with the climate change simulation, we may have more confidence in the cloud response to increasing GHGs occurring via these processes.