Uncertainty associated with convective wet removal of entrained aerosols in a global climate model 1Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada
16 Nov 2012
2Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA
3Karlsruhe Institute of Technology, Karlsruhe, Germany
4Institute of Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
Received: 03 Jan 2012 – Published in Atmos. Chem. Phys. Discuss.: 19 Jan 2012Abstract. The uncertainties associated with the wet removal of aerosols entrained above
convective cloud bases are investigated in a global aerosol-climate model
(ECHAM5-HAM) under a set of limiting assumptions for the wet removal of the
entrained aerosols. The limiting assumptions for the wet removal of entrained
aerosols are negligible scavenging and vigorous scavenging (either through
activation, with size-dependent impaction scavenging, or with the prescribed
fractions of the standard model). To facilitate this process-based study, an
explicit representation of cloud-droplet-borne and ice-crystal-borne aerosol
mass and number, for the purpose of wet removal, is introduced into the
ECHAM5-HAM model. This replaces and is compared with the prescribed
cloud-droplet-borne and ice-crystal-borne aerosol fraction scavenging scheme
of the standard model.
Revised: 31 Oct 2012 – Accepted: 06 Nov 2012 – Published: 16 Nov 2012
A 20% to 35% uncertainty in simulated global, annual mean aerosol mass
burdens and optical depth (AOD) is attributed to different assumptions for
the wet removal of aerosols entrained above convective cloud bases.
Assumptions about the removal of aerosols entrained above convective cloud
bases control modeled upper tropospheric aerosol concentrations by as much as
one order of magnitude.
Simulated aerosols entrained above convective cloud bases contribute 20%
to 50% of modeled global, annual mean aerosol mass convective wet
deposition (about 5% to 10% of the total dry and wet deposition),
depending on the aerosol species, when including wet scavenging of those
entrained aerosols (either by activation, size-dependent impaction, or with
the prescribed fraction scheme). Among the simulations, the prescribed
fraction and size-dependent impaction schemes yield the largest global,
annual mean aerosol mass convective wet deposition (by about two-fold).
However, the prescribed fraction scheme has more vigorous convective
mixed-phase wet removal (by two to five-fold relative to the size-dependent
impaction scheme) since nearly all entrained accumulation and coarse mode
aerosols are assumed to be cloud-droplet borne or ice-crystal borne, and
evaporation due to the Bergeron-Findeisen process is neglected.
The simulated convective wet scavenging of entrained accumulation and coarse
mode aerosols has feedbacks on new particle formation and the number of
Aitken mode aerosols, which control stratiform and convective cloud droplet
number concentrations and yield precipitation changes in the ECHAM5-HAM
model. However, the geographic distribution of aerosol annual mean convective wet
deposition change in the model is driven by changes to the assumptions
regarding the scavenging of aerosols entrained above cloud bases rather than
by precipitation changes, except for sea salt deposition in the tropics.
Uncertainty in the seasonal, regional cycles of AOD due to assumptions about
entrained aerosol wet scavenging is similar in magnitude to the estimated
error in the AOD retrievals.
The uncertainty in aerosol concentrations, burdens, and AOD attributed to
different assumptions for the wet scavenging of aerosols entrained above
convective cloud bases in a global model motivates the ongoing need to better
understand and model the activation and impaction processes that aerosols
undergo after entrainment into convective updrafts.
Citation: Croft, B., Pierce, J. R., Martin, R. V., Hoose, C., and Lohmann, U.: Uncertainty associated with convective wet removal of entrained aerosols in a global climate model, Atmos. Chem. Phys., 12, 10725-10748, doi:10.5194/acp-12-10725-2012, 2012.