Influences of In-cloud Scavenging and Cloud Processing on Aerosol

Concentrations in ECHAM5-HAM

Betty Croft - Dalhousie University, Halifax, CanadaUlrike Lohmann - ETH Zurich, Zurich, Switzerland

Randall Martin - Dalhousie University, Halifax, Canada

Philip Stier - University of Oxford, Oxford, U.K.

Johann Feichter - MPI for Meteorology, Hamburg, Germany

Sabine Wurzler - LANUV, Recklinghausen, Germany

Corinna Hoose - University of Oslo, Oslo, Norway

Aaron van Donkelaar - Dalhousie University, Halifax, Canada ---------------------------------------------------------------------------------------------------------------------------

HAMMOZ Meeting, ETH Zurich, March 26, 2010

Motivation: Significant inter-model aerosol profile differences

Koch et al. (2009), ACP

Black carbon profiles differ by 2 orders of magnitude among global models.

Do in-cloud scavenging parameterizations contribute to these differences?

AEROSOLS CLOUDS PRECIPITATION

The aerosol-cloud-precipitation interaction puzzle:

This problem involves many processes. Isolating the effects of one on the other is difficult.

Aerosol Scavenging Processes:

(Figure adapted 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

Aerosols Cloud Droplets / Ice Crystals Precipitation

Processes:

1) Nucleation of droplets/crystals

2) Impaction with droplets/crystals

Processes:

In-cloud: (tuning parameters)

1) Autoconversion

2) Accretion

3) Aggregation

Below-cloud:

1) Impaction with rain/snow

Aerosol wet scavenging processes:

We will examine the relative contributions of nucleation and impaction to in-cloud scavenging.

Modeling of Aerosol In-Cloud Scavenging:

Methodologies -

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

2) Diagnostic - cloud droplet and ice crystal number concentrations are used to diagnose nucleation scavenging + size-dependent impaction scavenging (e.g. Croft et al. (2010))

3) Prognostic - in-droplet and in-crystal aerosol concentrations are prognostic species that are passed between model time-steps (e.g., Hoose et al. (2008))Using the ECHAM5-HAM GCM, we can compare the

strength/weaknesses of these 3 fundamental approaches, and examine the sensitivity of predicted aerosol profiles to differences in the parameterization of in-cloud scavenging.

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

T>273K

238<T<273K

T<238K

2) Diagnostic scheme: Size-Dependent Nucleation Scavenging

Assume each cloud droplet and ice crystal scavenge 1 aerosol by nucleation, and apportion this number between the j=1-4 soluble modes,

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

From the 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.

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

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))

3) Prognostic scheme: Aerosol-cloud processing approach (Hoose et al. (2008))

Stratiform in-droplet and in-crystal aerosol concentrations are additional prognostic variables.

Two new aerosol modes

In-droplet (CD)

In-crystal (IC)

Histograms of diagnosed vs. prescribed scavenging ratios:

Aitken mode

Accumulation mode

Coarse mode

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

Uncertainty in global and annual mean mass burdens:

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DIAG-CTL

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SO4 BC POM DUST SS

Uncertainties in Aerosol Mass Mixing Ratios:

Zonal and annual mean black carbon mass is increased by near to one order of magnitude in regions of mixed and ice phase clouds relative to the simulation with prescribed scavenging ratios.

Uncertainties in Accumulation Mode Number:

Assuming 100% of the in-cloud aerosol is cloud –borne reduces the accumulation mode number burden by up to 0.7, but the diagnostic and prognostic scheme give increases up to 2 and 5 times, respectively relative to the prescribed fractions.

Uncertainties for Nucleation Mode Number:

Increased new particle nucleation is found for the simulation that assumes 100% of the in-cloud aerosol is cloud-borne.

Uncertainties in Aerosol Size: The size of the accumulation mode particles changes by up to 100%.

(nm)

Contributions of nucleation vs. impaction to annual and global mean stratiform in-cloud scavenging: Diag. scheme

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Nuc (Mixed)

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SO4 BC POM Dust SS Number

>90% of mass scavenging by nucleation (dust:50%); >90% of number scavenging by impaction.

Influence of impaction on black carbon scavenged mass:

Observed black carbon profiles from aircraft (Koch et al. 2009)

Observations of MBL size distributions (Heintzenberg et al. (2000))

Observations of AOD from MODIS MISR composite (van Donkelaar et al., subm.)

Observations of sulfate wet deposition (Dentener et al. (2006))

Observed 210Pb and 7Be concentrations and deposition (Heikkilä et al. (2008))

Current work: Coupled Stratiform-Convective Aerosol Processing:

CD CV

IC CV

Stratiform Clouds Convective Clouds

Detrainment

Detrainment

CDVC and ICCV will not be prognostic variables since the convective clouds entirely evaporate or sublimate after the above processes for each timestep.

Preliminary results: Zonal mean process transfer rates for the coupled stratiform-convective aerosol cloud processing:

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Aerosol Processing by Convective Clouds:

CCN0.6/CN

Red: 12 hours before convective system

Blue: 12 hours after convective system

Figure from Crumeyrolle et al. (2008), ACP - case study from Niger.

CN: Solid

CCN0.6: Dotted

Evidence for dust coating by sulfate above the boundary layer as a result of cloud processing.

Summary and Outlook:

1) Mixed /ice phase cloud scavenging was most uncertain between the parameterizations. Middle/upper troposphere black carbon concentrations differed by more than 1 order of magnitude between the scavenging schemes. Recommend: understanding nucleation and impaction processes for cloud temperatures T<273K.

2) In stratiform clouds, number scavenging is primarily (>90%) by impaction, and largely in mixed and ice phase clouds (>99%). Mass scavenging is primarily (>90%) by nucleation, except for dust (50%). Recommend: understanding of impaction processes for cloud temperatures <273K, and for dust at all cloud temperatures.

3) Better agreement with black carbon profiles for diagnostic and prognostic schemes. ↓ prescribed ratios for mixed phase clouds.

4) Recommend diagnostic and prognostic schemes over the prescribed ratio scheme, which can not represent variability of scavenging ratios.

5) Recommend further development of the prognostic aerosol cloud processing approach for convective clouds.

Acknowledgements: Thanks! Questions?