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Modelling Cloud Feedbacks in Today's General Circulation Models

Paper Reviewed
Lacagnina, C. and Selten, F. 2014. Evaluation of clouds and radiative fluxes in the EC-Earth general circulation model. Climate Dynamics 43: 2777-2796.

Facing the reality of the problem head-on, Lacagnina and Selten (2014) write that "despite the importance of clouds, their representation in general circulation models (GCMs) continues to account for much of the uncertainties in climate projections," citing Cess et al. (1996), Stocker et al. (2001) and Solomon et al. (2007), while noting that "the spread associated with intermodel differences is roughly three times larger than that associated with other main feedbacks," citing Dufresne and Bony (2008). And six years after the last of these assessments, they continue to find little that encourages them.

Working with the EC-Earth GCM version 2.3 that they coupled to an ocean GCM based on version 2 of the Nucleus for European Modeling of the Ocean (NEMO) model, the Dutch duo from the Royal Netherlands Meteorological Institute compared the married models' output to a wealth of real-world observational data obtained from numerous satellite and land-based sensors that they describe in detail - and to which they provide profuse references.

These efforts revealed, as they describe it, that (1) "EC-Earth exhibits the largest cloud biases in the tropics," where it (2) "underestimates the total cloud cover," but that it (3) "overestimates the optically thick clouds," with the net effect that (4) "clouds exert an overly strong cooling effect in the model," that (5) "the magnitude of the cooling due to the shortwave cloud radiative effect is underestimated for the stratiform low-clouds," because (6) "the model simulates too few of them," that (7) the "shortwave cloud radiative effect is overestimated for trade wind cumulus clouds," because (8) "in the model these are too thick," that (9) "the clouds in the deep convection regions also tend to overestimate the shortwave cloud radiative effect," because (10) "these clouds are generally too thick" and that (11) "there are too few mid and high thin clouds."

As for the ultimate take-home message of these several findings, Lacagnina and Selten conclude that "the model weaknesses discussed above indicate that more effort is needed to improve the physical parameterizations employed." And that may be an understatement, for after several generations of "improvements" costing multiple hundreds of millions of dollars these theoretical constucts prove yet again that they are still not ready for primetime!

References
Cess, R.D., Zhang, M.H., Ingram, W.J., Potter, G.L., Alekseev, V., Barker, H.W., Cohen?Solal, E., Colman, R.A., Dazlich, D.A., Del Genio, A.D., Dix, M.R., Dymnikov, V., Esch, M., Fowler, L.D., Fraser, J.R., Galin, V., Gates, W.L., Hack, J.J., Kiehl, J.T., Le Treut, H., Lo, K.K.-W., McAvaney, B.J., Meleshko, V.P., Morcrette, J.-J., Randall, D.A., Roeckner, E., Royer, J.-F., Schlesinger, M.E., Sporyshev, P.V., Timbal, B., Volodin, E.M., Taylor, K.E., Wang, W. and Wetherald, R.T. 1996. Cloud feedback in atmospheric general circulation models: an update. Journal of Geophysical Research 101: 12,791-12,794.

Dufresne, J.-L. and Bony, S. 2008. An assessment of the primary sources of spread of global warming estimates from coupled atmosphere-ocean models. Journal of Climate 21: 5135-5144.

Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and H.L. Miller, H.L. (Eds.) 2007. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, New York, New York, USA.

Stocker, T.F. et al. 2001. Physical climate processes and feedbacks. In: Houghton, J.T. et al. (Eds), Climate Change 2001: The Scientific Basis, Contributions of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom.

Posted 9 March 2015