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B. Croft 1 , J.R . Pierce 1 , R.V. Martin 1,2 , C. Hoose 3 , and U. Lohmann 4

Strong Sensitivity of Aerosol Concentrations to Convective Wet Scavenging Parameterizations in E CHAM5-HAM. B. Croft 1 , J.R . Pierce 1 , R.V. Martin 1,2 , C. Hoose 3 , and U. Lohmann 4 1 Dalhouse University, Canada 2 Harvard-Smithsonian Centre for Astrophysics, USA

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B. Croft 1 , J.R . Pierce 1 , R.V. Martin 1,2 , C. Hoose 3 , and U. Lohmann 4

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  1. Strong Sensitivity of Aerosol Concentrations to Convective Wet Scavenging Parameterizations in ECHAM5-HAM B. Croft1, J.R. Pierce1, R.V. Martin1,2, C. Hoose3, and U. Lohmann4 1Dalhouse University, Canada 2Harvard-Smithsonian Centre for Astrophysics, USA 3Karlsruhe Institute of Technology, Germany 4ETH Zurich, Switzerland HAMMOZ Workshop ETH Zurich March 27, 2012 Atmos. Chem. Phys. Discuss. 12, 1687-1732, 2012

  2. Outline • Motivation • Standard ECHAM5-HAM: convective wet scavenging (prescribed cloud-droplet-borne and ice crystal-borne aerosol fractions: R) • R calculated from convective cloud microphysics • Sensitivity studies for aerosol wet scavenging • Outlook and summary

  3. Motivation: Textor et al. (2006)

  4. Motivation: Koch et al. (2010)

  5. Convective Wet Scavenging of Aerosols in Standard ECHAM5-HAM: Prescribed cloud-droplet-borne, ice crystal-borne fractions: Aerosols are transported upwards; detrain at cloud top Cloud-droplet-borne/ice crystal-borne aerosol wet removal occurs as precipitation forms Aerosols entrain and detrain in updrafts R: Cloud-droplet-borne/Ice-crystal-borne aerosol fraction [%] Aerosols entrain at cloud base CS AS AI CI NS KS KI

  6. Coupling 2-Moment Convective Microphysics of Lohmann, 2008 and Wet Scavenging: CalculatedR withthe Physical Processes: Outflow at cloud top  StratiformCDNC/ICNC Standard model: no activation of aerosols entrained above cloud base layer Precipitation Formation (autoconversion, accretion and aggregation) Freezing and Bergeron-Findeisen process Transport upwards Cloud droplet activation at base ONLY

  7. Revised Convective Wet Scavenging in ECHAM5-HAM: New temporary variables: For each aerosol species and mode: Cloud-droplet-borne mass and number Ice-crystal-borne mass and number Interstitial mass and number + ∆ mj,k,ent- ∆ mj,k,det Note: 1) Cloud droplet-aerosol and ice crystal-aerosol collisions at all levels in updraft (applied collision kernels of Hoose et al. (2008)) 2) Nucleation scavenging occurs only at cloud base.

  8. Nucleation Scavenging Occurs Only at Convective Cloud Base: rscav Assume each cloud droplet scavenges one aerosol by nucleation processes Aerosol Number (N) Aerosol Mass (M) N> rscav = CDNC  cloud-borne aerosol number fraction (fn) = (N> rscav )/N cloud-borne aerosol mass fraction (fm) = (M> rscav )/M

  9. PF_init Revised convective wet scavenging: CF_init: Cloud droplet-borne and ice-crystal-borne aerosol fractions (R) are calculated based on microphysics PF_init: Prescribed R (CF_init – PF_init) Results: Global and annual mean internally mixed accumulation mode number burden increased by about 55% relative to standard model.

  10. Sensitivity Studies: Scavenging of Aerosols Entrained Above Cloud Base: n CF_ed CF_pipe Scavenge aerosols entering at cloud base only Allow nucleation scavengingof aerosols above cloud base

  11. Internally Mixed Accumulation Mode (AS) Mass Concentrations: CF_ed CF_ed – CF_pipe

  12. Convective Wet Deposition and Precipitation: [mm day-1 ] CF_ed (CF_ed – CF_pipe ) (CF_ed – CF_pipe ) CF_ed

  13. Contribution of Convective Wet Deposition to Total Wet Deposition in ECHAM5-HAM: PF_init CF_init CF_pipe CF_ed

  14. AOD Comparisons: R=0.50 Slope =0.87 Offset =0.04 CF_pipe R=0.56 Slope=0.77 Offset=0.04 R=0.65 Slope =0.62 Offset=0.03 PF_init CF_ed MODIS/MISR/AERONET AOD: van Donkelaar et al. (2010)

  15. Observations: Koch et al. (2010)

  16. Summary and Outlook: • Assumptions about convective clouds strongly influence aerosol scavenging, and resultant burdens and AOD. • Closest agreement with AOD observations was found for limiting case that allowed activation of aerosols entraining above cloud base. • Care must be used when applying prescribed fractions across broad temperature ranges (e.g. 238K-273K). Explicit representation of processes shows prescribed fraction of 0.75 was excessive for these clouds. • Need for ongoing observations, field studies, and modeling related to aerosol scavenging processes for convective clouds. • Assumptions about convective wet scavenging, particularly related to aerosols that entrain and detrain above cloud base strongly influence aerosol wet removal, resultant concentrations, burdens and AOD. Whether these aerosols enter the hydrometeors primarily by nucleation or impaction is an open question. • 2) Scavenging of aerosols entrained above cloud base decreases global and annual mean aerosol burdens by near to 50%. • 3) More vigorous convective wet scavenging parameterization  increased convective wet deposition by about a factor of 2, and reduced aerosol burdens even though modeled convective precipitation decreased. • 4) Need for ongoing observations, field studies, and modeling related to aerosol scavenging processes for convective clouds.

  17. Extra Slides

  18. Sulfate Wet Deposition (30°S to 30°N): Observed and Modeled PF_init CF_init CF_pipe CF_ed

  19. Annual, Zonal Mean Convective Cloud Droplet Number Concentration: ∆ Convective CDNC (CF_ed – CF_pipe) Convective CDNC (CF_ed) 400 cm-3 in polluted Northern Hemisphere 200 cm-3 in tropics 100 cm-3 increase in tropics

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