Vapor-Buoyancy Feedback in an Idealized GCM

Seth Seidel aNASA Goddard Space Flight Center, Greenbelt, Maryland
bDepartment of Land, Air, and Water Resources, University of California, Davis, Davis, California

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https://orcid.org/0000-0001-6002-9575
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Da Yang cDepartment of the Geophysical Sciences, University of Chicago, Chicago, Illinois

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Abstract

Humid air is lighter than dry air at a given temperature and pressure, as the molecular weight of water vapor is less than that of dry air. This effect is known as vapor buoyancy (VB). Although VB is a straightforward consequence of the ideal gas law, its influence on climate has been understudied. This study investigates VB’s influence on atmospheric temperature, radiation, and clouds. In mechanism-denial experiments, we remove VB from the dynamics and parameterizations of an idealized general circulation model (GCM). These experiments show that VB warms the tropical free troposphere by approximately 1 K in an Earth-like climate, and the magnitude of this effect increases with warming. This additional atmospheric warming causes greater outgoing longwave radiation (OLR) to be emitted as the climate warms. This constitutes a negative climate feedback which attains a magnitude of 0.1–0.2 W m−2 K−1 in the tropics. The simulations further show that, despite warming the atmosphere, VB does not introduce a countervailing positive water vapor feedback. Finally, we find that VB makes the net cloud feedback in the model more negative than it otherwise would be, reinforcing the clear-sky feedback.

© 2024 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Seth Seidel, sdseidel@ucdavis.edu

Abstract

Humid air is lighter than dry air at a given temperature and pressure, as the molecular weight of water vapor is less than that of dry air. This effect is known as vapor buoyancy (VB). Although VB is a straightforward consequence of the ideal gas law, its influence on climate has been understudied. This study investigates VB’s influence on atmospheric temperature, radiation, and clouds. In mechanism-denial experiments, we remove VB from the dynamics and parameterizations of an idealized general circulation model (GCM). These experiments show that VB warms the tropical free troposphere by approximately 1 K in an Earth-like climate, and the magnitude of this effect increases with warming. This additional atmospheric warming causes greater outgoing longwave radiation (OLR) to be emitted as the climate warms. This constitutes a negative climate feedback which attains a magnitude of 0.1–0.2 W m−2 K−1 in the tropics. The simulations further show that, despite warming the atmosphere, VB does not introduce a countervailing positive water vapor feedback. Finally, we find that VB makes the net cloud feedback in the model more negative than it otherwise would be, reinforcing the clear-sky feedback.

© 2024 American Meteorological Society. This published article is licensed under the terms of the default AMS reuse license. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses).

Corresponding author: Seth Seidel, sdseidel@ucdavis.edu

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