Preprints
https://doi.org/10.5194/gmd-2021-325
https://doi.org/10.5194/gmd-2021-325

Submitted as: model description paper 15 Nov 2021

Submitted as: model description paper | 15 Nov 2021

Review status: this preprint is currently under review for the journal GMD.

Computation of longwave radiative flux and vertical heating rate with 4A-Flux v1.0 as integral part of the radiative transfer code 4A/OP v1.5

Yoann Tellier1, Cyril Crevoisier1, Raymond Armante1, Jean-Louis Dufresne1, and Nicolas Meilhac2 Yoann Tellier et al.
  • 1LMD/IPSL, École Polytechnique, Institut Polytechnique de Paris, ENS, PSL Research University, Sorbonne Université, CNRS, Palaiseau France
  • 2FX CONSEIL, École Polytechnique, F 91128, Palaiseau Cedex, France

Abstract. Based on advanced spectroscopic databases, line-by-line and layer-by-layer radiative transfer codes numerically solve the radiative transfer equation with a very high accuracy. Taking advantage of its pre-calculated optical depth look-up table, the fast and accurate radiative transfer model Automatized Atmospheric Absorption Atlas OPerational (4A/OP) calculates the transmission and radiance spectra for a user defined layered atmospheric model. Here we present a module, called 4A-Flux, developed and implemented into 4A/OP in order to include the calculation of the clear-sky longwave radiative flux profiles and heating rate profiles at a very high spectral resolution. Calculations are performed under the assumption of local thermodynamic equilibrium, plane-parallel atmosphere and specular reflection on the surface. The computation takes advantage of pre-tabulated exponential integral functions that are used instead of a classic angular quadrature. Furthermore, the sublayer variation of the Planck function is implemented to better represent the emission of layers with a high optical depth. Thanks to the implementation of 4A-Flux, 4A/OP model have participated in the Radiative Forcing Model Intercomparison Project (RFMIP-IRF) along with other state-of-the-art radiative transfer models. 4A/OP hemispheric flux profiles are compared to other models over the 1800 representative atmospheric situations of RFMIP, yielding an Outgoing Longwave Radiation (OLR) mean difference between 4A/OP and other models of −0.148 W .m−2 and a mean standard deviation of 0.218 W .m−2, showing a good agreement between 4A/OP and other models. 4A/OP is applied to the Thermodynamic Initial Guess Retrieval (TIGR) atmospheric database to analyze the response of the OLR and vertical heating rate to several perturbations of temperature or gas concentration. This work shows that 4A/OP with 4A-Flux module can successfully be used to simulate accurate flux and heating rate profiles and provide useful sensitivity studies including sensitivities to minor trace gases such as HFC134a, HCFC22 and CFC113. We also highlight the interest for the modeling community to extend intercomparison between models to comparisons between spectroscopic databases and modelling to improve the confidence in model simulations.

Yoann Tellier et al.

Status: open (until 10 Jan 2022)

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Yoann Tellier et al.

Model code and software

4A-Flux v1.0: the radiative flux and heating rate module integrated into 4A/OP v1.5 radiative transfer code Tellier, Y., Crevoisier, C., Armante, R., Dufresne, J.-L., and Meilhac, N. https://doi.org/10.5281/zenodo.5667737

Yoann Tellier et al.

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Short summary
Accurate Radiative Transfer Models (RTM) are required to improve climate model simulations. We describe the module named 4A-Flux, implemented into 4A/OP RTM, aimed at calculating spectral longwave radiative fluxes given a description of the surface, the atmosphere and the spectroscopy. In Pincus et al. (2020), 4A-Flux has shown good agreement with state-of-the-art RTM. Here, it is applied to perform sensitivity studies and will be used to improve the understanding of radiative flux modeling.