Representing 3D Radiative Effects in Weather and Climate Models
Representing 3D Radiative Effects in Weather and Climate Models
Previously Published Material: The background to this work is published in Hogan and Shonk (J. Atmos. Sci., 2013) but there have been numerous advances since then, e.g. incorporation of the longwave, cloud inhomogeneity, the solution in terms of matrix exponentials, and gas absorption.
Abstract ID#: 34822
English Abstract:
The flow of radiation through cloud sides is neglected in all current weather and climate models, yet can significantly change cloud radiative effects in both the shortwave and longwave. This talk presents a new approach for representing 3D effects efficiently in models by adding terms to the two-stream equations to represent the lateral exchange of radiation between clear and cloudy regions. Formulating these equations in terms of matrices leads to an elegant solution in terms of the matrix exponential. This new scheme we refer to as SPARTACUS (Speedy Algorithm for Radiative Transfer through Cloud Sides), and incorporates the RRTM gas absorption model and the Tripleclouds representation of cloud inhomogeneity. Comparison to Monte Carlo calculations for cumulus clouds reveal that the new scheme performs well, and is able to capture the change to cloud radiative effect of between -30% to +100% in the shortwave, depending on solar zenith angle. We also demonstrate that 3D effects can systematically increase the effect of cumulus clouds on surface longwave fluxes by of order 50%, an effect that has received very little attention in the literature. The potential impact of 3D radiative effects in large-scale models will be discussed.