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Problems with Global Climate Models: Cloud Representations
Reference
Siebesma, A.P., Jakob, C., Lenderink, G., Neggers, R.A.J., Teixeira, J., van Meijgaard, E., Calvo, J., Chlond, A., Grenier, H., Jones, C., Kohler, M., Kitagawa, H., Marquet, P., Lock, A.P., Muller, F., Olmeda, D. and Severijns, C.  2004.  Cloud representation in general-circulation models over the northern Pacific Ocean: A EUROCS intercomparison study.  Quarterly Journal of the Royal Meteorological Society 130: 3245-3267.

What was done
Quoting the authors, "simulations with nine large-scale models [were] carried out for June/July/August (JJA) 1998 and the quality of their results [were] assessed along a cross-section in the subtropical and tropical North Pacific ranging from (235E, 35N) to (187.5E, 1S)," in order to "document the performance quality of state-of-the-art GCMs (general-circulation models) in modeling the first-order characteristics of subtropical and tropical cloud systems."

What was learned
The main conclusions, according to Siebesma et al., were that "(1) almost all models strongly underpredicted both cloud cover and cloud amount in the stratocumulus regions while (2) the situation is opposite in the trade-wind region and the tropics where cloud cover and cloud amount are overpredicted by most models."  In fact, they report that "these deficiencies result in an overprediction of the downwelling surface short-wave radiation of typically 60 W m-2 in the stratocumulus regimes and a similar underprediction of 60 W m-2 in the trade-wind regions and in the intertropical convergence zone (ITCZ)," which discrepancies are to be compared, we note, with a radiative forcing of only 4 W m-2 for a 300-ppm increase in the atmosphere's CO2 concentration.  They also state that "similar biases for the short-wave radiation were found at the top of the atmosphere, while discrepancies in the outgoing long-wave radiation are most pronounced in the ITCZ."

What it means
The seventeen scientists from nine different countries state that "the representation of clouds in general-circulation models remains one of the most important as yet unresolved [our italics] issues in atmospheric modeling."  This is partially due, they continue, "to the overwhelming variety of clouds observed in the atmosphere, but even more so due to the large number of physical processes governing cloud formation and evolution as well as the great complexity of their interactions."  Nevertheless, they conclude that through repeated critical evaluations of the type they conducted, "the scientific community will be forced to develop further physically sound parameterizations that ultimately [our italics] result in models that are capable of simulating our climate system with increasing realism."  Until that time (indeed, until climate simulations can be done, not with increasing realism, but with true realism), we suggest it is not too wise to put much credence in what these admittedly inadequate state-of-the-art GCMs suggest about the future; and to actually mandate drastic reductions in fossil-fuel energy production on the basis of what they suggest currently is downright foolish.

Reviewed 18 May 2005