Séminaire T. Garrett
Jeudi 27 avril 2017, 14:30, amphithéâtre P. Glorieux, Université de Lille Sciences et Technologies
Are the shapes of clouds impossibly complex or fundamentally simple ?
Decades have passed with the climate science community remaining unable to narrow the range of plausible values for the climate sensitivity to a doubling of CO2 concentrations. The spread of values always seems to range from about 1.5 K to 4.5 K in each successive IPCC report. But why? Isn’t this a basic failure of the science community? The normal excuse is that clouds are complicated: even as we increase model sophistication new sensitivities are uncovered. Although perhaps we should consider, maybe, that complex deterministic calculations have simply been the wrong approach. In this talk I present an application of some simple ideas from equilibrium statistical mechanics to argue that tropical cloud geometric structures are fundamentally constrained by such basic atmospheric thermodynamic properties as atmospheric stability and sea surface temperatures, and that their size distributions follow power laws or exponentials found throughout nature. These theoretical concepts are tested with one of the largest simulations of clouds to date and with Oxygen A-band observations of cloud 3D structure. A prediction is made that tropical cloud cover will increase by 3% per 1 K increase in sea surface temperature, but with no radiative climate impact, suggesting climate sensitivities closer to 1.5 to 2 K per CO2 doubling.