The Use of New Parameterizations for Gaseous Absorption in the CLIRAD-SW Solar Radiation Code for Models

Abstract

The new gaseous absorption parameterizations are incorporated in the CLIRAD-SW solar radiation code for models, openly distributed for the scientific community. In the new parameterizations, the magnitude of absorption coefficients in each homogeneous layer depends on both species concentrations in the layer and species amounts accumulated along the direct solar radiation path from the top of the atmosphere (TOA). The number of the k-distribution terms varies from 1 to 4 in each of the eight bands. The total number of pseudomonochromatic intervals in the new version of the code is 15, compared with 38 currently in CLIRAD-SW. This reduces computational time of the code by approximately 2 times and facilitates its using in the numerical models. The error of the new version of CLIRAD-SW is determined here as the difference between the flux or heating rate values calculated with the code and line-by-line model. The surface and top-of-the-atmosphere flux difference is less than 1.5 W m⁻² in calculations for the standard gaseous atmospheres and less than 7 W m⁻² in calculations for the standard gaseous atmosphere with aerosol or cloud scattering and absorption incorporated. The relative flux error is less than 1% for all cases of gaseous atmosphere with and without molecular scattering and aerosol extinction. This error is less than 1.5% for the cases with cloudiness. The errors of heating rate calculations in the clear-sky atmospheres are less than 6% up to the height of 70 km, while these errors in cloudy layers reach values of about 30%, which is typical for current broadband parameterizations. The method to reduce these errors is suggested.