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, Canada2Harvard-Smithsonian Centre for Astrophysics, USA3Karlsruhe Institute of Technology, Germany4ETH Zurich, Switzerland

HAMMOZ Workshop ETH Zurich March 27, 2012

Atmos. Chem. Phys. Discuss. 12, 1687-1732, 2012

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

Motivation:

Textor et al. (2006)

Motivation:

Koch et al. (2010)

Convective Wet Scavenging of Aerosols in Standard ECHAM5-HAM:Prescribed cloud-droplet-borne, ice crystal-borne fractions:

Aerosols entrain at cloud base

Aerosols entrain and detrain in updrafts

Aerosols are transported upwards; detrain at cloud top

Cloud-droplet-borne/ice crystal-borne aerosol wet removal occurs as precipitation forms

R: Cloud-droplet-borne/Ice-crystal-borne aerosol fraction [%]

NS KS CSAS KI AI CI

Cloud droplet activation at base ONLY

Transport upwards

Freezing and Bergeron-Findeisen process

Precipitation Formation (autoconversion, accretion and aggregation)

Outflow at cloud top Stratiform CDNC/ICNC

Coupling 2-Moment Convective Microphysics of Lohmann, 2008 and Wet Scavenging: Calculated R with the Physical Processes:

Standard model: no activation of aerosols entrained above cloud base layer

Revised Convective Wet Scavenging in ECHAM5-HAM:

New temporary variables: For each aerosol species and mode:

1) Cloud-droplet-borne mass and number

2) Ice-crystal-borne mass and number

3) Interstitial mass and number

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.

+ ∆ mj,k,ent- ∆ mj,k,det

Nucleation Scavenging Occurs Only at Convective Cloud Base:

rscav

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

Assume each cloud droplet scavenges one aerosol by nucleation processes

Results:

Global and annual mean internally mixed accumulation mode number burden increased by about 55% relative to standard model.

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

PF_init

(CF_init – PF_init)

Sensitivity Studies: Scavenging of Aerosols Entrained Above Cloud Base:

n

CF_pipe CF_ed

Scavenge aerosols entering at cloud base only

Allow nucleation scavenging of aerosols above cloud base

CF_ed – CF_pipe

Internally Mixed Accumulation Mode (AS) Mass Concentrations:

CF_ed

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

CF_ed (CF_ed – CF_pipe )

PF_initCF_initCF_pipeCF_ed

Contribution of Convective Wet Deposition to Total Wet Deposition in ECHAM5-HAM:

PF_init CF_ed

CF_pipe

MODIS/MISR/AERONET AOD: van Donkelaar et al. (2010)

AOD Comparisons:

R=0.65Slope =0.62Offset=0.03

R=0.50Slope =0.87Offset =0.04

R=0.56Slope=0.77Offset=0.04

Observations: Koch et al. (2010)

Summary and Outlook:

1) Assumptions about convective clouds strongly influence aerosol scavenging, and resultant burdens and AOD.

2) Closest agreement with AOD observations was found for limiting case that allowed activation of aerosols entraining above cloud base.

3) 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.

4) Need for ongoing observations, field studies, and modeling related to aerosol scavenging processes for convective clouds.

1) 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.

Extra Slides

PF_init

CF_ed

CF_init

CF_pipe

Sulfate Wet Deposition (30°S to 30°N): Observed and Modeled

Convective CDNC (CF_ed)

∆ Convective CDNC (CF_ed – CF_pipe)

100 cm-3 increase in tropics200 cm-3 in tropics 400 cm-3 in polluted Northern Hemisphere

Annual, Zonal Mean Convective Cloud Droplet Number Concentration: