Radiative Destabilization in Clear-Sky CoolingCalculations

Abstract

JANUARY 2002 AMERICAN METEOROLOGICAL SOCIETY | indzen et al. (2001, hereafter LCH) recently discussed observations of clouds and water vapor over the tropical oceans and concluded that the earth may exhibit an “adaptive infrared iris” effect, in which tropical cloudiness and uppertropospheric water vapor respond to changes of sea surface temperature in a manner to produce a negative feedback mechanism in the climate system. LCH also discussed preliminary results that suggest the current generation of global climate models does not exhibit the irislike behavior observed in the satellite datasets they examined, a deficiency that needs to be addressed. This comment provides one possible source of climate-model bias that could explain some of the differences noted by LCH. This bias is analogous to that which results from ignoring the differences between cloudy and clear-air column radiative transfer. It arises due to the steep water vapor gradients discussed by LCH. Because steep water vapor gradients are also observed in midlatitude synoptic systems (e.g., see just about any day’s image of the water vapor channel on the National Oceanic and Atmospheric Administration Geostationary Operational Environmental Satellite Web site, www.goes.noaa.gov) this is not a problem confined to calculations in the Tropics.