Aerosol size-dependent below-cloud scavenging by rain and snow in the ECHAM5-HAM 1Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Canada
2Institute of Atmospheric and Climate Science, ETH Zurich, Zurich, Switzerland
3Harvard-Smithsonian Center for Astrophysics, Cambridge, USA
4Atmospheric, Oceanic, and Planetary Physics, University of Oxford, Oxford, UK
5Landesamt fur Natur, Umwelt und Verbraucherschutz NRW (LANUV), Recklinghausen, Germany
6Max Planck Institute for Meteorology, Hamburg, Germany
7Federal Office of Meteorology and Climatology MeteoSwiss, Zurich, Switzerland
Received: 15 January 2009 – Published in Atmos. Chem. Phys. Discuss.: 25 March 2009 Abstract. Wet deposition processes are highly efficient in the removal of aerosols from
the atmosphere, and thus strongly influence global aerosol concentrations,
and clouds, and their respective radiative forcings. In this study,
physically detailed size-dependent below-cloud scavenging parameterizations
for rain and snow are implemented in the ECHAM5-HAM global aerosol-climate
model. Previously, below-cloud scavenging by rain in the ECHAM5-HAM was
simply a function of the aerosol mode, and then scaled by the rainfall rate.
The below-cloud scavenging by snow was a function of the snowfall rate alone.
The global mean aerosol optical depth, and sea salt burden are sensitive to
the below-cloud scavenging coefficients, with reductions near to 15% when
the more vigorous size-dependent below-cloud scavenging by rain and snow is
implemented. The inclusion of a prognostic rain scheme significantly reduces
the fractional importance of below-cloud scavenging since there is higher
evaporation in the lower troposphere, increasing the global mean sea salt
burden by almost 15%. Thermophoretic effects are shown to produce increases
in the global and annual mean number removal of Aitken size particles of near
to 10%, but very small increases (near 1%) in the global mean
below-cloud mass scavenging of carbonaceous and sulfate aerosols.
Changes in the assumptions about the below-cloud scavenging by rain of
particles with radius smaller than 10 nm do not cause any significant changes to the global and annual mean
aerosol mass or number burdens, despite a change in the below-cloud number
removal rate for nucleation mode particles by near to five-fold. Annual and
zonal mean nucleation mode number concentrations are enhanced by up to 30%
in the lower troposphere with the more vigourous size-dependent below-cloud
scavenging. Closer agreement with different observations is found when the
more physically detailed below-cloud scavenging parameterization is
employed in the ECHAM5-HAM model.
Revised: 25 June 2009 – Accepted: 01 July 2009 – Published: 17 July 2009
Citation: Croft, B., Lohmann, U., Martin, R. V., Stier, P., Wurzler, S., Feichter, J., Posselt, R., and Ferrachat, S.: Aerosol size-dependent below-cloud scavenging by rain and snow in the ECHAM5-HAM, Atmos. Chem. Phys., 9, 4653-4675, doi:10.5194/acp-9-4653-2009, 2009.