Bridging Clarity and Accuracy: A Simple Spectral Longwave Radiation Scheme for Idealized Climate Modeling

Abstract

Parameterizing radiative transfer in means navigating trade-offs between physical accuracy and conceptual clarity. However, currently available schemes sit at the extremes of this spectrum: correlated-k schemes are fast and accurate but rely on lookup tables which obscure the underlying physics and make such schemes difficult to modify, while gray radiation schemes are conceptually straightforward but introduce significant biases in atmospheric circulation. Here we introduce a Simple Spectral Model (SSM) for clear-sky longwave radiative transfer which bridges this `clarity-accuracy' gap. The SSM accomplishes this by representing the spectral structure of H$_{2}$O and CO$_{2}$ absorption using analytic fits at reference conditions, then uses simple functional forms to extend these fits to different atmospheric conditions. This, coupled to a simple, two-stream solver, yields a system of six equations and ten physically-meaningful parameters which can solve for clear-sky longwave fluxes given atmospheric profiles of temperature and humidity. When implemented in an idealized aquaplanet GCM, the SSM produces zonal-mean climate states which accurately mimic the results using a benchmark correlated-k code. The SSM also alleviates the significant zonal-mean climate biases associated with using gray radiation, including an improved representation of radiative cooling profiles, tropopause structure, jet dynamics, and Hadley Cell characteristics both in control climates and in response to uniform warming. This work demonstrates that even a simple spectral representation of atmospheric absorption suffices to capture the essential physics of longwave radiative transfer, and the SSM promises to be a valuable tool both for idealized climate modeling, and for teaching radiative transfer in the classroom.