Atmospheric Chemistry and Physics Discussions www.atmos-chem-phys-discuss.net 1680-7367 1680-7375 1 1 2001 10.5194/acpd-1-221-2001 http://www.atmos-chem-phys-discuss.net/1/221/2001/ http://www.atmos-chem-phys-discuss.net/1/221/2001/acpd-1-221-2001.html http://www.atmos-chem-phys-discuss.net/1/221/2001/acpd-1-221-2001.pdf 221 238 2001-09-03 Tropical cirrus and water vapor: an effective Earth infrared iris feedback? Q. Fu M. Baker D. L. Hartmann Department of Atmospheric Sciences, Box 351640, University of Washington, Seattle, Washington 98195, USA We revisit a model of feedback processes proposed by Lindzen et al. (2001), in which an assumed 22% reduction in the area of tropical high clouds per degree of sea surface temperature increase produces negative feedbacks associated with upper tropospheric water vapor and cloud radiative 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 should be small. Although not mentioned by Lindzen et al, 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., we obtain a feedback factor in the range of −0.15 to −0.51, compared to their larger negative feedback factor of −0.45 to −1.03.