Longwave 3D Benchmarks for Inhomogeneous Clouds and Comparisons with Approximate Methods

Abstract

he purpose of this study is twofold: to (i) establish three-dimensional (3D) longwave radiative transfer benchmarks for inhomogeneous cloud fields and (ii) compare the results with three approximate, 1D methods. The benchmark results are calculated using a correlated-k three-dimensional Monte Carlo (3DMC) algorithm that is validated via comparisons to line-by-line calculations for simple atmospheres. The approximate methods include an independent column approximation (ICA) and two cloud-overlap schemes: maximum/random (MRO) and random (RO). Six inhomogeneous cloudy-sky test cases are used and encompass a wide range of domain sizes used by general circulation models. Domain-averaged fluxes and heating rates from these atmospheres show that the ICA is consistently more accurate than the cloud-overlap models with respect to the 3D benchmarks. For example, comparisons of model results for the Atlantic Trade Wind Experiment (ATEX), a marine boundary layer cumulus field, yield a maximum cloud-layer heating rate error of 15.73 K day?1 from using cloud-overlap models, whereas the ICA error is only 2.17 K day?1. This paper presents results showing that these differences are attributed to the 3D effects of unresolved clouds and indicate that there is an inherent deficiency in the ability of 1D models to accurately calculate radiative quantities in these atmospheres.