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Evaluation of the cloud response to climate change using cloud compositing techniques

PI: Keith Williams
Institution: Met Office Hadley Centre
Additional Investigators: Mark Ringer
The radiative feedback from clouds remains one of the largest uncertainties in determining climate sensitivity. Traditional methods of evaluating clouds in general circulation models (GCMs) have focused on comparing multi-annual mean maps of cloud variables with satellite observations. However, the ability to simulate the time-mean geographical distribution of present-day cloud may not offer a stringent constraint on the cloud radiative feedback under climate change. A possible alternative approach is to identify key processes associated with the cloud response to climate change, which are demonstrated to be important in a range of GCMs, and attempt to find analogues in present day climate variability. Compositing techniques to define ‘regimes’ (dynamical regimes, cloud regimes, physico-dynamical regimes, etc.) have been shown to be especially useful in separating particular processes, however there are a number of different compositing methods in the literature, many of which have not, to date, been demonstrated to constrain the cloud response to climate change.

The Cloud Feedback Model Intercomparison Project (CFMIP) has two sub-projects (sub-projects 1 and 3) which aim to test a range of these compositng methods in ±2K atmosphere-only and 2xCO2 atmosphere-mixed layer ocean experiments. It would be extremely valuable to extend these techniques to coupled atmosphere-ocean simulations, in order to test the generality of the results in a full coupled system. The CFMIP diagnostic list was designed with the requirements for cloud compositing as a focus. In particular, the project requests high temporal resolution (daily) data from the ISCCP simulator. ISCCP simulator diagnostics are marked as ‘lower priority output’ in the CMIP lists and only requested as monthly means, hence not all of the analysis to be carried out on the CFMIP experiments can be repeated on the CMIP models. However, some of the simpler cloud compositing techniques being utilised in CFMIP either do not use ISCCP simulator output, or use monthly mean ISCCP data. We intend to apply these techniques to the 1%/year simulations in CMIP in order to test whether there are significant differences in cloud processes occurring as part of transient climate change in a full coupled system compared with the equilibrium climate in more idealised atmosphere-mixed layer and atmosphere-only experiments. It will also form a valuable test of the generality of the conclusions from the CFMIP compositing sub-projects for the IPCC AR4.
  • Williams, K.D., M. A. Ringer, C. A. Senior, M. J. Webb, B. J. McAvaney, N. Andronova, S. Bony, J. -L. Dufresne, S. Emori, R. Gudgel, T. Knutson, B. Li, K. Lo, I. Musat, J. Wegner, A. Slingo and J. F. B. Mitchell, 2006: Evaluation of a component of the cloud response to climate change in an intercomparison of climate models. Clim. Dyn., 26, 145-165, 10.1007/s00382-005-0067-7. Abstract. Edit.

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