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


The first aerosol indirect effect involves increased cloud albedo from enhanced cloud droplet number concentrations (CDNC) during higher aerosol loadings.

The HadGEM2-ES model, which uses the CLASSIC aerosol scheme, already includes this effect using an empirical parameterization (Jones et al, 1994) that relates CDNC directly to the number concentration of those aerosol particles that act as cloud condensation nucleii (CCN).

When the CLASSIC aerosol scheme is used, the CCN concentration is derived from the mass of each aerosol type simulated, based on an assumed size distribution for each type which is globally uniform (see, e.g. Jones et al., 2007).

One of the key benefits of the new GLOMAP-mode aerosol scheme for UKCA, is that the evolution of the size distribution is simulated dynamically, with the number and mass in each size mode evolving according to the underlying aerosol microphysical processes. Consequently, the global CCN distribution, which is the critical driving property for the indirect forcing, will be simulated much more realistically by GLOMAP-mode than by CLASSIC.

There are two ways in which the indirect forcing from GLOMAP-mode (via modified cloud albedo) can be coupled to the Edwards-Slingo radiation scheme used by HadGEM.

The first is an incremental advance on the existing approach in HadGEM2-ES, whereby the Jones et al (1994) empirical relation for CDNC is still used, but the CCN concentration that drives it is diagnosed directly from GLOMAP-mode. This will give a more realistic perturbation to the cloud albedo, than the existing approach, but will not resolve the variability in CDNC that occurs from differences in size distribution and composition, for an equivalent CCN concentration (see e.g. Pringle et al, 2009).

The second approach (preferred) is to use a new mechanistic CDNC module that has been developed specifically for UKCA. The module employs the parameterization of Abdul-Razzak and Ghan (2000) to calculate CDNC combining the online size distribution and composition information from GLOMAP-mode, with a cloud updraft velocity derived from the UM-derived turbulent kinetic energy in the boundary layer.

For more details about the mechanistic cloud droplet number module please contact Rosalind West (Oxford University)


Abdul-Razzak, H. and Ghan, S. J.: A parameterization of aerosol activation 2: Multiple aerosol types. J. Geophys. Res., vol. 105, pp. 6837-6844, 2000.

Jones, A., Roberts, D. L. and Slingo, A. A climate model study of indirect radiative forcing by anthropogenic sulphate aerosols, Nature, vol. 370, pp. 450-453, 1994.

Jones, A., Haywood, J. M. and Boucher, O. Aerosol forcing, climate response and climate sensitivity in the Hadley Centre climate model J. Geophys. Res., doi:10.1029/2007JD008688 2007.

Pringle, K. J., Carslaw, K. S., Spracken, D. V., et al.: The relationship between aerosol and cloud drop number concentrations in a global aerosol microphysics model Atmos. Chem. Phys., 9, pp. 4131-4144, 2009.