Direct effect from GLOMAP-mode aerosol to the Edwards-Slingo radiation scheme

The UKCA model is configured into the HadGEM3 climate model with a double call to the radiation scheme to allow the direct radiative effect from the GLOMAP-mode aerosol to be calculated online in the model.

For the existing CLASSIC aerosol scheme used in HadGEM2-ES and previous HadGEM versions, the aerosol is considered as an external mixture of different aerosol types (sulphate, biomass, soot, biogenic and sea-salt). Then, for the direct aerosol forcing, the optical properties from each of these types was considered separately. The interface to the Edwards-Slingo radiation scheme used in HadGEM is based on look-up tables (from Mie calculations) for the optical properties of each particle type as a function of relative humidity.

In contrast to CLASSIC, GLOMAP-mode assumes each mode consists of an internal mixture of the different aerosol components (e.g. sulphate, black carbon and organic carbon). The water content is evaluated within this assumption using the ZSR algorithm (Stokes and Robinson, 1966), using water activity data from Jacobson et al. (1996).

Whereas the look-up tables for the CLASSIC aerosol scheme were based only on relative humidity and wavelength; for GLOMAP-mode, the information on mean particle radius and composition for each of the internally-mixed size modes is used. A volume-average mixing rule is used over the components present (including water) to calculate the real and imaginary parts of the refractive index. The particle size determines the Mie parameter in relation to the wavelength. The look-up tables have been calculated based on the integrals across each of the spectral bands for the short-wave and long-wave used by HadGEM.

For more details about the interface to the Edwards-Slingo radiation scheme and the calculations of the optical properties of the aerosol, please contact Nicolas Bellouin (UK Met Office) nicolas.bellouin@metoffice.gov.uk.