Studies with a flexible new radiation code. I: Choosing a configuration for a large‐scale model

Abstract

A comprehensive new radiation code based on the two-stream equations in both the long-wave and short-wave spectral regions is described. The spectral resolution of the code is variable, enabling it to be used in a wide range of applications. Because of its flexibility, the code is well-suited to the investigation of the sensitivity of radiative calculations to changes in the way in which physical processes are parametrized. The gaseous transmission data are derived from a line-by-line model. Particular attention is directed towards the treatment of the water vapour continuum, the overlap between gases, and the sensitivity to changing the carbon dioxide concentrations.The performance of the code is examined both at high spectral resolution and in a lower-resolution configuration designed for the UK Meteorological Office Unified Forecast/Climate Model (UM). Particularly for use in the UM, the code must be shown to perform satisfactorily across the whole range of atmospheric conditions. Comparisons are therefore made with reference calculations in both the long-wave and the short-wave, in clear and cloudy skies, and the accuracy with which various processes may be represented is studied.For the cloudy calculations in the short-wave, a new method is presented for deriving the single-scattering properties in broad bands, based on the analytic expression for the reflectivity of an optically thick cloud. This minimizes the errors in calculating the short-wave radiative properties of water clouds when the spectral resolution is reduced to that designed for the UM. In contrast, for ice clouds the errors are minimized by deriving the singlescattering properties using linear averaging, as appropriate for optically thin clouds. In the long-wave, the vertical distribution of the radiative heating in cirrus clouds is examined at high spectral resolution. The effect of scattering of long-wave radiation, usually ignored in large-scale models, is examined in some detail and is explained using a simple model. Taking all these studies into account, it is concluded that the configuration designed for the UM retains the generality of the code, without significantly compromising the overall accuracy.

Aerosol climate feedback due to decadal increases in Southern Hemisphere wind speeds

“…We used monthly mean cloud fields and surface albedo from ISCCP [Rossow and Schiffer, 1999] together with a radiative transfer model [Edwards and Slingo, 1996]. For the baseline runs liquid water paths for low level clouds were computed from ISCCP optical depth measurements assuming an effective drop radius of 10 mm.…”

Accuracy tests of radiation schemes used in hot Jupiter global circulation models

“…In previous papers we have tested and confirmed the dynamical core's suitability to model hot Jupiter-like atmospheres (Mayne et al 2013(Mayne et al , 2014. This paper presents the adaptation of the radiation scheme, the EdwardsSlingo (ES) radiation scheme (Edwards & Slingo 1996), to conditions prevailing in hot Jupiter atmospheres. Opacities from high-temperature line lists have been calculated for the dominant absorbers in these atmospheres and the radiation scheme, using k-coefficients calculated from these opacities, has been tested against more accurate DO LbL calculations.…”

Two fast radiative transfer methods to improve the temporal sampling of clouds in numerical weather prediction and climate models

“…The Met Office Unified Model currently uses the 'Edwards-Slingo' radiation scheme (Edwards and Slingo, 1996). The spectral overlap of absorption by different gases is dealt with through the method of equivalent extinction (Edwards, 1996), where full calculations are only done for the k-terms of the 'major' gas in each band.…”

“…The optical properties for ice and liquid cloud condensate were recalculated for the band using the same parametrization as the HadGEM spectral file. (See Edwards and Slingo (1996) for the liquid condensate parametrization based on Mie scattering. The ice crystal parametrization, based on ice aggregates, is described in Edwards et al (2007).)…”

“…The number of k-terms can be adjusted to give the desired trade-off between accuracy and efficiency that is required for a given application. For example, the Met Office currently uses a scheme with a total of 115 k-terms for climate and global forecasting applications, and a scheme with 55 k-terms for short-term local area modelling (Edwards and Slingo, 1996;Cusack et al, 1999).…”