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Tropical cirrus and water vapor: an effective Earth infrared iris feedback?

by Q Fu, M Baker, D L Hartmann
Atmos. Chem. Phys ()
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Abstract

We revisit a model of feedback processes pro-posed by Lindzen et al. (2001), in which an assumed 22% re-duction in the area of tropical high clouds per degree increase in sea surface temperature produces negative feedbacks as-sociated with upper tropospheric water vapor and cloud ra-diative effects. We argue that the water vapor feedback is overestimated in Lindzen et al. (2001) by at least 60%, and that the high cloud feedback is small. Although not men-tioned by Lindzen et al. (2001), tropical low clouds make a significant contribution to their negative feedback, which is also overestimated. Using more realistic parameters in the model of Lindzen et al. (2001), we obtain a feedback fac-tor in the range of −0.15 to −0.51, compared to their larger negative feedback factor of −0.45 to −1.03. It is noted that our feedback factor could still be overestimated due to the as-sumption of constant low cloud cover in the simple radiative-convective model.

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