View source for 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)
west@atm.ox.ac.uk.

References:
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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.