Aerosol Size-Dependent Impaction Scavenging in Warm, Mixed, and Ice Clouds in 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

-------------------------------------------------------------------------------------MOCA 09 - Clouds in Global Models Session, July 21, 2009 -------------------------------------------------------------------------------------

Aerosol Scavenging Processes:

(Figure from Hoose et al. (2008))Wet scavenging accounts for 50-95% of

aerosol deposition, and strongly controls aerosol 3-dimensional distributions, which influence climate both directly and indirectly.

Sedimentation and dry deposition

Modeling In-Cloud Impaction Scavenging:

Global climate model methodologies -

1) Prescribed coefficients (e.g., Stier et al. (2005))

2) Size-dependent impaction with diagnostic nucleation scavenging (e.g., this study)

3) Prognostic in-droplet and in-crystal aerosol modes with prescribed impaction coefficients (e.g., Hoose et al. (2008))

Questions we will address in this talk:

1)Are certain aerosol species more strongly influenced by in-cloud impaction scavenging on a global scale?

2)Are there certain geographic regions where in-cloud impaction contributes more to aerosol scavenging?

All results shown are for a 1-year simulation of the ECHAM5-HAM global aerosol-climate model, at T42 resolution, nudged to the meteorological conditions of the year 2001, and following a 3 months spin-up period.

SU:sulfate; BC:black carbon; POM:particulate organic matter; DU:dust; SS:sea salt

The 7 lognormal modes of the ECHAM5-HAM GCM:

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The current in-cloud scavenging in the ECHAM5-HAM GCM uses prescribed ratios. Since the ECHAM5-HAM GCM predicts aerosol size, we can replace these ratios with size-dependent in-cloud scavenging

NS KS AS CS KI AI CI

Prescribed in-cloud scavenging ratios of the standard ECHAM5-HAM (nucleation+impaction):

Size-Dependent Impaction Scavenging by Cloud Droplets:

Solid lines: Number scavenging coefficients Dashed lines: Mass scavenging coefficients Data sources described in Croft et al. (2009)

Example for CDNC 40 cm-3, assuming a gamma distribution

Prescribed coefficients of Hoose et al. (2008) prognostic scheme are shown with red steps

Impaction Scavenging by Column and Plate Ice Crystals:

Prescribed coefficients of Hoose et al. (2008) (red steps)

Assume columns for T<238.15KAssume plates for 238.15<T<273.15 K

(Data from Miller and Wang, (1991), and following Croft et al. (2009))

Diagnostic 2-Moment Nucleation Scavenging:

Assume each cloud droplet and ice crystal scavenge 1 aerosol by nucleation, and apportion this number between the j soluble modes,

based on the fractional contribution of each mode to the total number of soluble aerosols having radii greater than 35 nm, which are the aerosols that participate in the Ghan et al. (1993) activation scheme.

nm

jjjact

N

NfracICNCCDNCN

35, )(

Find the radius that contains exactly Nact,j in the lognormal tail, using cumulative lognormal size-distribution,

Scavenge all mass above this radius for nucleation scavenging. Thus, we typically scavenge a higher fraction of the mass versus number distribution.

))))(2(1(ln2exp(35

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jggcrit

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fracICNCCDNCerfrr

Find rcrit that contains Nact,j in the lognormal tail.

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SU BC POM SS DU KS AS CS KI AI CI

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Percent Change in Global Aerosol Mass and Number Burdens (With versus Without In-Cloud Impaction):

The global and annual mean dust mass burden, and the number burden for the nucleation and accumulation mode aerosols are sensitive to in-cloud impaction scavenging.

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Change in Mass Burdens Change in Number Burdens

Annual and Zonal Mean Mass Mixing Ratios:

Black carbon, particulate organic matter, and dust concentrations reduce by near to 25% with inclusion of in-cloud (IC) impaction, particularly in the regions of mixed and ice clouds.

Sea salt and sulfate are changed by less than 10% (not shown).

Inclusion of in-cloud impaction scavenging increases the zonal and annual mean black carbon scavenged mass in the upper troposphere by up to 100%.

Zonal and Annual Mean Black Carbon Scavenged Mass:

Stratiform Nucleation Stratiform Impaction

Annual and Global Mean Dust and BC In-Cloud Scavenging:

Contributions to global and annual mass deposition by process (%)

T>273K 238<T<273K T<238K T>273K 238<T<273K T<238K

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Dust

Black carbon

Warm Mixed Ice Warm Mixed Ice

For dust, unlike black carbon, the inclusion of impaction scavenging increases the scavenged mass in the lower troposphere since a relatively larger fraction of dust resides in the insoluble modes, which are not scavenged by cloud droplet nucleation.

Zonal and Annual Mean Dust Scavenged Mass:

Standard: Prescribed impaction and nucleation scavenging (Stier et al.(2005))

DIAG1: 1-moment nucleation scavenging + prescribed impaction

DIAG2: 2-moment nucleation scavenging + prescribed impaction

DIAG2+Imp: 2-moment nucleation scavenging + size-dependent impaction

PROG: prognostic aerosol processing scheme with prescribed impaction (Hoose et al. (2008))

Global Burden Sensitivity to In-Cloud Scavenging Parameterizations

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Zonal and Annual Mean Aerosol Optical Depth:

Observations are a MODIS, MISR, AERONET composite from Kinne, (2009)

Diagnostic in-cloud scavenging reduces the AOD over the southern oceans, whereas the prognostic scheme increases the AOD, but AOD is not significantly changed by the in-cloud size-dependent impaction.

Summary and Outlook:

1) Size-dependent in-cloud impaction scavenging reduced zonal and annual mean carbonaceous and dust concentrations by up to 25% and 75%, respectively, in the regions of mixed phase and ice clouds.

2) Prediction of climate change due to absorbing aerosols requires consideration of in-cloud impaction scavenging.

3) Impaction scavenging enhanced scavenged mass of black carbon by up to 100% in the upper troposphere.

4) Impaction scavenging in convective clouds will be investigated in future work.

Acknowledgments:Thanks!

Questions ?