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Benchmark calculations of radiative forcing by greenhouse gases
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  • Robert Pincus,
  • Stefan Alexander Buehler,
  • Manfred Brath,
  • Omar Jamil,
  • Frank Evans,
  • James Manners,
  • Raymond Menzel,
  • Eli J. Mlawer,
  • David Paynter,
  • Rick Pernak,
  • Cyril Crevoisier,
  • Yoann Tellier
Robert Pincus
University of Colorado/NOAA Earth System Research Laboratory, University of Colorado/NOAA Earth System Research Laboratory

Corresponding Author:robert.pincus@colorado.edu

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Stefan Alexander Buehler
Universität Hamburg, Universität Hamburg
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Manfred Brath
University of Hamburg, University of Hamburg
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Omar Jamil
Met Office, Met Office
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Frank Evans
University of Colorado Boulder, University of Colorado Boulder
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James Manners
Met Office, Met Office
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Raymond Menzel
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Eli J. Mlawer
Atmospheric & Environmental Research, Atmospheric & Environmental Research
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David Paynter
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Rick Pernak
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Cyril Crevoisier
Laboratoire Météorologique Dynamique
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Yoann Tellier
Laboratoire Météorologique Dynamique
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Changes in the concentration of greenhouse gases within the atmosphere lead to changes in radiative fluxes throughout the atmosphere. The value of this change, called the instantaneous radiative forcing, varies across climate models, due partly to differences in the distribution of clouds, humidity, and temperature across models, and partly due to errors introduced by approximate treatments of radiative transfer. This paper describes an experiment within the Radiative Forcing Model Intercomparision Project that uses benchmark calculations made with line-by-line models to identify parameterization error in the representation of absorption and emission by greenhouse gases. The clear-sky instantaneous forcing by greenhouse gases to which the world has been subject is computed using a set of 100 profiles, selected from a re-analysis of present-day conditions, that represent the global annual mean forcing with sampling errors of less than 0.01 \si{\watt\per\square\meter}. Six contributing line-by-line models agree in their estimate of this forcing to within 0.025 \si{\watt\per\square\meter} while even recently-developed parameterizations have typical errors four or more times larger, suggesting both that the samples reveal true differences among line-by-line models and that parameterization error will be readily resolved. Agreement among line-by-line models is better in the longwave than in the shortwave where differing treatments of the water vapor vapor continuum affect estimates of forcing by carbon dioxide and methane. The impacts of clouds on instantaneous radiative forcing are roughly estimated, as are adjustments due to stratospheric temperature change. Adjustments are large only for ozone and for carbon dioxide, for which stratospheric cooling introduces modest non-linearity.
16 Dec 2020Published in Journal of Geophysical Research: Atmospheres volume 125 issue 23. 10.1029/2020JD033483