Global Simulations of Aerosol Impaction Scavenging with the ECHAM5-HAM GCM Betty Croft, and Randall V. Martin – Dalhousie University, Canada Ulrike Lohmann – ETH Zurich, Switzerland Philip Stier – Oxford University, U.K. Sabine Wurzler – Landesamt fur Umwelt, Natur, und Verbrauchershutz, Germany Johann Feichter – Max Planck Institute for Meteorology, Germany Corinna Hoose – University of Oslo, Norway ---------------- CMOS 2009 Congress ------------------- Radiation, Aerosols, and Cloud Session, June 3, 2009 ------------------------------------------------------------------

Outline: Introduction to Aerosol Scavenging Processes Impaction Scavenging by Rain and Snow Impaction Scavenging by Cloud Droplets and Ice Crystals Results of Global Simulations Summary and Outlook

Aerosol Scavenging Processes: Sedimentation and dry deposition (Figure from Hoose et al. (2008)) Uncertainty in aerosol scavenging contributes to the uncertainties in the estimates of the direct and indirect radiative forcing of aerosols on climate.

Impaction Scavenging by Rain and Snow: Stier et al. (2005) prescribed coefficients for rain of 4mm drops (red steps) Stier et al. (2005) prescribed coefficient for snow (green) (Figure from Croft et al. (2009))

Contribution of Impaction Scavenging By Rain and Snow to Global and Annual Mean Aerosol Deposition: Size-dependent impaction [%] Prescribed coefficients Sulfate BC POM Dust Sea Salt Red: Snow; Blue: Rain; Left: Prescribed coefficients; Right: Size-dependent impaction All results are for a 1-year T42 nudged simulation with ECHAM5-HAM GCM

Global and Annual Mean Sea Salt and Dust Burdens: Sea salt [mg m-2] Change relative to prescribed coefficients [%] Dust [mg m-2] Change relative to prescribed coefficients [%]

Percent Change of Global and Annual Mean Sea Salt and Dust Mass Burdens Relative to Prescribed Coefficient Simulation: Rain Drop spectra Fixed 0.4 mm Drops Fixed 4.0 mm Drops DU DU SS DU SS SS Dust and sea salt burdens are sensitive to the impaction scavenging parameterization for rain and snow.

Impaction Scavenging by Cloud Droplets: Example for CDNC 40cm-3, assuming a gamma distribution Prescribed coefficients of Hoose et al. (2008) are shown with red steps Number scavenging (solid lines); Mass scavenging (dashed lines) Data sources described in Croft et al. (2009)

Impaction Scavenging by Column and Plate Ice Crystals: Prescribed coefficients of Hoose et al. (2008) (red steps) Assume columns for T<238.15K Assume plates for 238.15<T<273.15 K (Data from Miller and Wang, (1991), and following Croft et al. (2009))

Mass mixing ratios: Black carbon, particulate organic matter, and dust are sensitive to the in-cloud (IC) impaction parameterization in regions of mixed and ice clouds.

Percent Change in Aerosol Mass and Number Burdens Relative to A Simulation With No In-Cloud Impaction: NS SU BC POM SS DU KS AS CS KI AI CI [%] The dust mass, and the number of insoluble accumulation and coarse mode aerosols are most sensitive to impaction scavenging.

Dust: Fractional Contribution of Stratiform Cloud Processes to the Annual and Global Mean Mass Deposition: [%] Warm Mixed Ice Warm Mixed Ice T>273K 238<T<273K T<238K T>273K 238<T<273K T<238K Nucleation Impaction

Comparison with observed zonal mean aerosol optical depth: Observations are a MODIS, MISR, AERONET composite from Kinne, (2009) AOD is more sensitive to impaction scavenging by rain and snow, as opposed to impaction with cloud droplets and ice crystals.

Summary and Outlook: Size-dependent impaction scavenging by rain and snow reduced global sea salt burdens by 15%. The assumption of a rain drop spectra as opposed to a fixed mean raindrop size changed the global sea salt burdens by near to 10%. Size-dependent impaction scavenging by cloud droplets and ice crystals reduced concentrations by up to 25% and 100% for carbonaceous aerosols and dust, respectively, in the regions of mixed phase and ice clouds.  prediction of climate change due to absorbing aerosols, particularly in the polar regions, requires consideration of in-cloud impaction scavenging. Impaction scavenging in convective clouds will be investigated in future work.

Acknowledgments: Thanks! Questions?

Percent Change in Aerosol Mass and Number Burdens Relative to A Simulation With No In-Cloud Impaction: Aerosol Burden Sensitivity to In-Cloud Scavenging Parameterizations [%] SU BC POM SS DU NS KS AS CS AI CI KI Blue: Size-dependent impaction (diagnostic nucleation) Red: Hoose et al. (2008) prescribed in-cloud impaction (diagnostic nucleation) Yellow: Hoose et al. (2008) prescribed in-cloud impaction (prognostic aerosol cloud processing)