Thursday, February 28, 2013

New paper finds IPCC models underestimate cooling effect of clouds

A paper published today in the Journal of Climate finds that IPCC climate models underestimate the cooling effect of clouds in the Arctic. The paper adds to many other peer-reviewed publications demonstrating that climate models exaggerate warming by underestimating the net cooling and negative feedback from clouds.

Sensitivity of CAM5 Simulated Arctic Clouds and Radiation to Ice Nucleation Parameterization

Shaocheng Xie,1 Xiaohong Liu,2 Chuanfeng Zhao,1 and Yuying Zhang1
1 Lawrence Livermore National Laboratory, Livermore, California, USA
2 Pacific Northwest National Laboratory, Richland, Washington, USA
Sensitivity of Arctic clouds and radiation in the Community Atmospheric Model version 5 to the ice nucleation process is examined by testing a new physically based ice nucleation scheme that links the variation of ice nuclei (IN) number concentration to aerosol properties. The default scheme parameterizes the IN concentration simply as a function of ice supersaturation. The new scheme leads to a significant reduction in simulated IN number concentrations at all latitudes while changes in cloud amount and cloud properties are mainly seen in high latitudes and middle latitude storm tracks. In the Arctic, there is a considerable increase in mid-level clouds and a decrease in low clouds, which result from the complex interaction among the cloud macrophysics, microphysics, and the large-scale environment. The smaller IN concentrations result in an increase in liquid water path and a decrease in ice water path due to the slow-down of the Bergeron-Findeisen process in mixed-phase clouds. Overall, there is an increase in the optical depth of Arctic clouds, which leads to a stronger cloud radiative forcing (net cooling) at the top of the atmosphere.

The comparison with satellite data shows that the new scheme slightly improves low cloud simulations over most of the Arctic, but produces too many mid-level clouds. Considerable improvements are seen in the simulated low clouds and their properties when compared to Arctic ground-based measurements. Issues with the observations and the model-observation comparison in the Arctic region are discussed.

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