Robust Responses of Tropical High Clouds in CGMs (original) (raw)

2009

Abstract

A positive longwave cloud feedback is a robust feature of all global climate models submitted to the IPCC AR4 archive. In this study we show that this is in large part due to the fact that tropical high clouds rise vertically as the planet warms, but do so in such a way as to remain at nearly the same temperature. Thus the longwave emission from tropical high clouds remains approximately constant rather than increasing in step with the warming planet, resulting in a positive longwave cloud feedback. We demonstrate that this robust feature of the models' tropical high clouds makes sense physically due to the fundamental constraint imposed by the clear-sky radiative cooling rate, as in the fixed anvil temperature (FAT) hypothesis of Hartmann and Larson (2002). Furthermore, the slight reduction in fractional coverage of high clouds that occurs over the course of the 21st Century simulations is also well-explained by considering the clear-sky balance between radiative cooling and subsidence: The simulated increase in static stability out-paces the increase in longwave cooling, thereby causing less clear-sky upper tropospheric convergence and - by mass conservation - a reduction in high cloud cover.

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