92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Thursday, 26 January 2012
Climate Response to the Physiological Forcing of Carbon Dioxide in Radiative-Convective Equilibrium: An Idealized Study
Hall E (New Orleans Convention Center )
T.W. Cronin, MIT, Cambridge, MA; and K. A. Emanuel and R. G. Prinn

Elevated CO2 is likely to result in reduced stomatal conductance and transpiration over land surfaces a biophysical effect that has been termed physiological forcing (to distinguish it from the radiative forcing of CO2). Recent studies with GCMs have concluded that the surface air warming over some land regions due to physiological forcing could be as large as 20-30% of the total warming due to doubled CO2. As an idealization of the problem of physiological forcing, we explore the effect of the land surface evaporative fraction, β, on a near-equatorial atmosphere in radiative-convective equilibrium (RCE). Simple theory based on this framework leads to a diagnostic relationship between increases in sensible heat flux and surface air warming over land, which is in rough agreement with GCM results. To further build intuition about the effects of land surface moisture on the atmosphere in RCE, we perform many experiments with a radiative-convective (RC) model in single- and multiple- column modes, with β varying from 1 (saturated surface) to 0 (completely dry surface). RC model runs with a single column suggest that land surface drying would have significant dynamical consequences, and cannot generally be well-understood using a single-column model, motivating the use of a 2D, multi-column model. By performing simulations with small numbers of columns (part land, part ocean), we find that decreasing β generally increases the difference in surface air temperature between the land and ocean (ΔTA). We also develop a theoretical relationship between ΔTA, the sensible heat flux, and the width of the land, and compare it to the results of multiple-column simulations.

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