The Incredible Lightness of Water Vapor

Journal Article · Wed Apr 01 00:00:00 EDT 2020 · Journal of Climate

DOI:https://doi.org/10.1175/JCLI-D-19-0260.1· OSTI ID:1603216

Yang, Da ^[1]; Seidel, Seth D. ^[1]

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1. University of California, Davis, and Lawrence Berkeley National Laboratory, Berkeley, California

The molar mass of water vapor is much less than that of dry air. This makes a moist parcel lighter than a dry parcel of the same temperature and pressure. This effect is referred to as the vapor buoyancy effect and has often been overlooked in climate studies. We propose that the vapor buoyancy effect increases Earth’s outgoing longwave radiation (OLR) and that this negative radiative effect increases with warming, stabilizing Earth’s climate. We illustrate this mechanism in an idealized tropical atmosphere, where there is no horizontal buoyancy gradient in the free troposphere. Temperature increases toward dry atmosphere columns to compensate the reduced vapor buoyancy, increasing OLR by O(1 W m ⁻² ) at the reference climate. In warmer climates, the temperature difference between moist and dry columns would increase as a result of increasing atmospheric water vapor, leading to enhanced radiative effect and thereby stabilizing Earth’s climate. We estimate that this feedback strength is about O(0.2 W m ⁻² K ⁻¹ ) in the idealized atmosphere, which compares to cloud feedback and surface albedo feedback in the current climate. We further show evidence from observations and real-gas radiative transfer calculations for a significant radiative effect of vapor buoyancy in the tropical atmosphere.

• Sponsoring Organization: USDOE
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1603216
• Journal Information: Journal of Climate, Journal Name: Journal of Climate Journal Issue: 7 Vol. 33; ISSN 0894-8755
• Publisher: American Meteorological Society
• Country of Publication: United States
• Language: English