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Simple Spectral Models for Atmospheric Radiative Cooling

Author(s): Jeevanjee, Nadir; Fueglistaler, Stephan

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Abstract: Atmospheric radiative cooling is a fundamental aspect of Earth’s greenhouse effect, and is intrinsically connected to atmospheric motions. At the same time, basic aspects of longwave radiative cooling, such as its characteristic value of 2 K day−1, its sharp decline (or “kink”) in the upper troposphere, and the large values of CO2 cooling in the stratosphere, are difficult to understand intuitively or estimate with pencil and paper. Here we pursue such understanding by building simple spectral (rather than gray) models for clear-sky radiative cooling. We construct these models by combining the cooling-to-space approximation with simplified greenhouse gas spectroscopy and analytical expressions for optical depth, and we validate these simple models with line-by-line calculations. We find that cooling rates can be expressed as a product of the Planck function, a vertical emissivity gradient, and a characteristic spectral width derived from our simplified spectroscopy. This expression allows for a pencil-and-paper estimate of the 2 K day−1 tropospheric cooling rate, as well as an explanation of enhanced CO2 cooling rates in the stratosphere. We also link the upper-tropospheric kink in radiative cooling to the distribution of H2O absorption coefficients, and from this derive an analytical expression for the kink temperature Tkink ≈ 220 K. A further, ancillary result is that gray models fail to reproduce basic features of atmospheric radiative cooling.
Publication Date: Feb-2020
Electronic Publication Date: 22-Jan-2020
Citation: Jeevanjee, Nadir, and Stephan Fueglistaler. "Simple spectral models for atmospheric radiative cooling." Journal of the Atmospheric Sciences 77, no. 2 (2020): 479-497. DOI: 10.1175/JAS-D-18-0347.1.
DOI: doi:10.1175/JAS-D-18-0347.1
ISSN: 0022-4928
EISSN: 1520-0469
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Pages: 479 - 497
Type of Material: Journal Article
Journal/Proceeding Title: Journal of the Atmospheric Sciences
Version: Final published version. Article is made available in OAR by the publisher's permission or policy.
Notes: Related Item is Corrigendum, published in April 2020.

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