Implementing the Delta-Four-Stream Approximation for Solar Radiation Computations in an Atmosphere General Circulation Model

Abstract

Proper quantification of the solar radiation budget and its transfer within the atmosphere is of utmost importance in climate modeling. The delta-four-stream (DFS) approximation has been demonstrated to offer a more accurate computational method of quantifying the budget than the simple two-stream approximations widely used in general circulation models (GCMs) for radiative-transfer computations. Based on this method, the relative improvement in the accuracy of solar flux computations is investigated in the simulations of the third-generation Canadian Climate Center atmosphere GCM. Relative to the computations of the DFS-modified radiation scheme, the GCM original-scheme whole-sky fluxes at the top of the atmosphere (TOA) show the largest underestimations at high latitudes of a winter hemisphere on the order of 4%?6% (monthly means), while the largest overestimations of the same order are found over equatorial regions. At the surface, even higher overestimations are found, exceeding 20% at subpolar regions of a winter hemisphere. Flux differences between original and DFS schemes are largest in the tropics and at high latitudes, where the monthly zonal means and their dispersions are within 5 W m?2 at the TOA and 10 W m?2 at the surface in whole sky, but differences may be as large as 20 and ?40 W m?2. In clear sky, monthly zonal means and their dispersions remain within 2 W m?2, but may be as large as 25 and ?12 W m?2. Such differences are found to be mostly determined by variations in cloud optical depth and solar zenith angle, and by aerosol loading in a clear sky.