1. Influences of Wet Scavenging on Aerosol Concentrations and Deposition in the ECHAM5-HAM Global Climate Model
Betty Croft1 Ulrike Lohmann2, Randall V. Martin1, Philip Stier3, Sabine Wurzler4, Johann Feichter5, and Corinna Hoose6
1Dalhousie University, Canada, 2ETH Zurch, Switzerland, 3University of Oxford, U. K., 4LANUV, Germany, 5Max Planck Institute for Meteorology, Germany, 6University of Oslo, Norway
Aerosols influence climate directly by scattering and absorbing radiation, and indirectly by modifying cloud properties
Wet scavenging strongly controls global aerosol three-dimensional distributions
Aerosol size-dependent impaction, and diagnostic nucleation scavenging has been implemented into the ECHAM5-HAM global climate model
Diagnostic, prognostic, and prescribed coefficient wet scavenging schemes are compared
The global and annual mean accumulation mode number burden is increased by 60% and 40% for the prognostic and diagnostic schemes, respectively, compared to a simulation assuming 100% of the in-cloud aerosol is cloud-borne.
The standard ECHAM5-HAM GCM (Stier et al. (2005)) uses prescribed coefficients for impaction scavenging by rain and snow (shown by the dark red and green steps to the right). Since the model predicts aerosol radius for each of seven lognormal modes, we have introduced size-dependent impaction scavenging.
Size-dependent impaction scavenging of aerosols by rain, snow, cloud droplets and ice crystals:
(Data sources described in detail in Croft et al. (2009))
New double-moment diagnostic nucleation scavenging:
assume each cloud droplet and ice crystal scavenge one aerosol by nucleation, apportion this number between the 4 internally mixed modes. Scavenge the aerosol number and mass distributions above this critical radius,
rcrit = rg(exp(20.5lnơgerf-1(1-((2CDNC+ICNC) x fracnj/N>35nm))
In-cloud nucleation and impaction scavenging prescribed coefficients for the seven modes of the standard ECHAM5-HAM are shown to left. Here we introduce double-moment diagnostic nucleation scavenging and size-dependent impaction scavenging in clouds.
NS KS AS CS KI AI CI
Annual Mean Aerosol Concentrations and Burdens:
Annual and zonal mean BC, POM and dust concentrations with and without in-cloud (IC) impaction:
Observed Annual Mean Wet Deposition and AOD:
% change sea salt for size-dependent rain and snow impaction vs. standard
Sea Salt Burden [mg m-2]
The prescribed coefficient method, the new diagnostic scheme, and the prognostic approach perform similarly in comparison to observed sulfate wet deposition. The zonal mean AOD over-prediction over the southern oceans is reduced by near to 30% with size-dependent impaction scavenging.
% change dust for size-dependent rain and snow impaction vs. standard
Dust Burden [mg m-2]
Zonal Mean Nucleation Mode (NS) Number Concentration [cm-2]
% change NS number for size-dependent rain and snow impaction vs. standard
The sea salt burden reduction, due to enhanced impaction scavenging by rain and snow, leads to greater new particle nucleation in the lower troposphere. Dust and carbonaceous aerosols are sensitive to in-cloud impaction in regions of mixed and ice clouds. Accumulation mode number burdens increase by 60% for the prognostic scavenging scheme of Hoose et al. (2008) versus the assumption that 100% of the in-cloud aerosol is cloud-borne.
90S
90N
90S
90N
Observations from Dentener et al. (2006), and Kinne (2009)
SO4 BC POM SS DU AS
References:
Croft, B., U. Lohmann, R. V. Martin, P. Stier, S. Wurzler, J. Feichter, R. Posselt, and S. Ferrachat (2009), Aerosol size-dependent below-cloud scavenging by rain and snow in ECHAM5-HAM, Atmos. Chem. Phys. Discuss., 9,
Dentener et al. (2006), Nitrogen and sulfur deposition on regional and global scales: A multi-modal evaluation, Global Biogeochem. Cycles, 20, GB4003, doi: /2005GB
Hoose C., U. Lohmann, R. Bennartz, B. Croft, G. Lesins (2008), Global simulations of aerosol processing in clouds, Atmos. Chem. Phys., 8,
Kinne, S., (2009), Remote sensing data combinations - superior global maps for aerosol optical depth, in: Satellite Aerosol Remote Sensing Over Land, edited by: Kokhanovsky, A. A. and De Leeuw, G., Springer
Stier et al. (2005), The aerosol-climate model ECHAM5-HAM, Atmos. Chem. Phys., 5,
Acknowledgments: