Saturday, October 27, 2012

Whoops: paper finds supposed positive feedback from low clouds in models is exaggerated 50%

A mew paper published in Geophysical Research Letters finds computer models greatly exaggerate positive feedback from tropical low clouds by 50%. Meanwhile, ample direct observations indicate that both low and high cloud feedback is instead net negative.

Key Points
  • Correlation between low-cloud radiative effects in present and future climates
  • Due to a positive radiative feedback between low clouds and relative humidity
  • Observations help constrain the strength of climate change low-cloud feedbacks
F. Brient
Laboratoire de Météorologie Dynamique, Université Pierre et Marie Curie, CNRS, Paris, France
S. Bony
Laboratoire de Météorologie Dynamique, Université Pierre et Marie Curie, CNRS, Paris, France
The influence of cloud modelling uncertainties on the projection of the tropical low-cloud response to global warming is explored by perturbing model parameters of the IPSL-CM5A climate model in a range of configurations (realistic general circulation model, aqua-planet, single-column model). While the positive sign and the mechanism of the low-cloud response to climate warming predicted by the model are robust, the amplitude of the response can vary considerably depending on the model tuning parameters. Moreover, the strength of the low-cloud response to climate change exhibits a strong correlation with the strength of the low-cloud radiative effects simulated in the current climate. We show that this correlation primarily results from a local positive feedback (referred to as the “beta feedback”) between boundary-layer cloud radiative cooling, relative humidity and low-cloud cover. Based on this correlation and observational constraints, it is suggested that the strength of the tropical low-cloud feedback predicted by the IPSL-CM5A model in climate projections might be overestimated by about fifty percent.

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