Towards Parameterization of Atmospheric Aerosol in Regional Forecast Models

David B. Mechem, and Yefim L. Kogan

Collaborators: Yi Lan, Paul Robinson, Yuri Shprits

The University of Oklahoma

Seminar presented at NRL, Monterey, 7 January 2005

Acknowledgements. This research was supported by the Office of Naval Research and the U. S. Department of Energy Atmospheric Radiation Measurement Program.

Aerosol dramatically influences the radiative characteristics of

PBL clouds

First indirect effect: cloud droplet radius and concentration influences albedo => ship tracks

Ship tracks in the eastern Atlantic

Photo credit: Robert Wood, University of Washington

Photo credit: Robert Wood, University of Washington

Aerosol affects the thermodynamic structure and

persistence of PBL clouds

Second indirect effect: drizzle may lead to cloud breakup=> Pockets of Open Cells (POCs)

POCs

Photo credit: Robert Wood, University of WashingtonPhoto credit: Robert Wood, University of Washington

Regional simulations of aerosol-cloud-drizzle interactions using the COAMPS mesoscale model coupled with the CIMMS

drizzle parameterization

Model setup•COAMPS v2.0.14

•18/6/2 km grid. Vertical grid spacing stretched from 10 to 800 m

•1.5-order subgrid closure (“Level 2.5” Mellor and Yamada 1982)

•24 h simulation. Two 12 h cycled pre-forecasts establish a reasonable boundary layer structure

•Bulk drizzle parameterization (Khairoutdinov and Kogan, 2000)

•Prognostic equations for qc, Nc, qr, Nr, and NCCN

•Initial and boundary condition CCN value of 45 cm-3

Compare drizzling (KK) and nondrizzling (ND) runs to evaluate the effect of drizzle on a mesoscale forecast.

Goal

5-moment scheme (KK) Operational microphysics

LWP [g m-2]

1800 UTC COAMPS LWP comparison of 5-moment drizzle parameterization (KK) and the operational (Kessler) microphysics scheme (18 km grid)

Three significant improvements of KK drizzle scheme:

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1 Reduced entrainment from drizzle-stabilization leads to a further northern extent of cloud wedge

Reduction in LWP and cloud coverage south and east of Point Conception.

Open oceanic LWP is better match with climatology

1800 UTC COAMPS LWP comparison of 5-moment drizzle parameterization (KK) and the operational (Kessler) microphysics scheme (2 km grid)

LWP[g m-2]

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5-moment scheme (KK)

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Operational microphysics

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Significant improvements from the KK drizzle scheme, inferred by LES results:

• More realistic cloud base structures and variability

• Improves ability of COAMPS to represent broken PBL cumulus fields

• Represents the transition from unbroken stratocumulus to PBL cumulus

A1-A1′ cuts across banded cloud structures with weak resolved vertical velocity. We take these bands to represent ensembles of PBL cumulus.

Comparison of surface bulk CCN concentration using the 5-moment drizzle parameterization (KK)

[cm-3]

1200 UTC 25 July 12 h later…

In these previous COAMPS simulations, aerosol characteristics were represented by a single parameter NCCN.

Attempting to distill aerosol characteristics to a single parameter often gives an incomplete and sometimes incorrect portrayal of aerosol properties

LWC [g m-3]

Droplet spectra at each grid point

Giant aerosol above the inversion: • Enhance drizzle production• Attenuate PBL turbulence• Accelerate stratocumulus breakup

When pollution above the inversion is predominantly fine-mode, drizzle production is suppressed.

Background sulfate plus giant aerosolBackground sulfate only

Effect of coarse mode/giant aerosols

Sea-salt case

Drop conc. [cm-3]

Sulfate + sea saltNon sea-salt caseSulfate aerosol only

Sea-salt caseNon sea-salt case

The presence of sea salt results in:

•Significant drizzle formation

•Reduction in mean drop concentration

•Large variations in cloud base

•Greater variability in cloud top

•More complex internal cloud structure

•Significant differences in overall cloud geometry — implying possible future breakup of cloud field

Effects of surface winds=sea salt aerosols

Susceptibility of cloud drop concentration to sea-salt addition

S=(Nss-N)/N

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3 LES simulations: • clean=low background

concentration• polluted with low and high

Aitken nuclei concentrations

Sea salt effect depends on the sulfate aerosol concentration, N:

When N is low, the effect of sea-salt is to significantly increase cloud drop concentration.

When N is high, the effect depends on the concentration of Aitken nuclei

• Advanced prediction of aerosol-cloud-drizzle feedbacks should include 3 main aerosol parameters:

Coarse mode (giant) aerosolsBackground (fine mode) sulfate aerosolsAitken nuclei

and

Parameterization of the effects of surface winds – sea-salt aerosols

Full system of equations describing coupled aerosol-cloud interactions

Equations for cloud drop parameters (4 equations in KK approach) need to be

complemented by 3 equations for major aerosol parameters

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t

qr

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r

jevap

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acc

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auto

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cond

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Cloud microphysics formulation

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xS

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t

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t

Nccn

,

,

,,,,

i=1,2,3

79.147.21350

ccauto

r Nqt

q 15.1)(67 ccacc

r Nqt

q

Prediction of Aerosol Parameters

Parameterization of cloud parameter conversion rates (for example):

Parameterization of aerosol-aerosol, aerosol-cloud conversion rates: yet TBD

Si,ccn represents (interstitial) source and sink terms of aerosol, e.g. transformation, sedimentation, production from DMS, sea-spray

What components are required for an accurate mesoscale forecast of aerosol-cloud-drizzle system?

•Specification of aerosol field (initial and boundary conditions)Size characteristicsSpatial distribution

•Cloud processingActivationCoagulation, rainout, diffusiophoresisRegeneration

•Specification of sources and sinksUrban sourcesSea saltHeterogeneous chemistryTransformation rates (fine↔coarse mode)

•TransportAdvectionSedimentationTurbulent mixing (entrainment)

Where we are now? •Specification of aerosol fieldObservations and data assimilation necessary for all 3 aerosol parameters

•Cloud processingProcessing via coagulation represented in some cloud physics schemesRecent activation parameterizations not yet linked to model SGS energetics

•Specification of sources and sinksSimple parameterizations exist for sea-spray aerosol sourceParameterizations of aerosol transformation rates have yet to be developed

•TransportAs accurate as the model’s advection schemeDepends on how accurately the model SGS represents entrainment

More understanding is needed of the relative role and importance of these various processes, sources, and sinks.

•Control experiments with different initial CCN concentrations

•Sensitivity runs with various CCN source mechanisms and magnitudes

Example of parameterization/formulation of cloud processing

Model setup — idealized

• Δx = Δy = 2 km; Δz = 25 m; Δt = 10 s• Domain size 1001001.5 km• Periodic horizontal boundary conditions• Imposed large scale divergence 5.010-6 s-1

• Sensible and latent heat fluxes (10 and 25 Wm-2)• Longwave only• KK bulk drizzle parameterization• Activation by Martin et al. (1994) and O’Dowd et al.

(1996)• Thermodynamic initial conditions from ASTEX

A209• Various initial CCN profiles and magnitudes

Time-height representation of qc and Nt

qc [g kg-1] Nccn+ Nc [cm-3]

Cloud top/base

LWP

Drizzle rate

Statistics for different initial CCN concentrations

Smaller values of CCN result in:

• Reduced entrainment and lower mean cloud top height

• Higher mean cloud base

• Reduced mean LWP

• Larger drizzle rates

• Increased variability

COAMPS aerosol budget

sinksource

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entrain

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cloud

tt

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PBL aerosol budget is calculated in terms of total particle concentration (CCN + droplet):

“Cloud processing”

•Calculate entrainment term from change of inversion height and magnitude of imposed divergence

•Any additional source/sink terms are known (i.e. imposed)

→ We can back-out the cloud processing rate

Cloud processing for two different CCN concentrations

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Cloud processing

Cloud processing + dilution

Sensitivity experiments — Entrainment source

•Assume NCCN = 200 cm-3 for z < zi, but various concentrations of free-tropospheric CCN at z > zi .

•As PBL entrains free tropospheric air, this CCN is mixed down into the boundary layer

Free-tropospheric concentration

When entrained into the PBL, free tropospheric CCN can:

•Suppress drizzle

•Counteract depletion via cloud processing

•Increase PBL Nt, given sufficient entrainment and free tropospheric CCN concentration

Validation and parameterization of cloud processing

• Cloud processing (depletion) is correlated to drizzle rates by simple power laws and largely independent of initial conditions

• Depletion can also be related to other model parameters (e.g. Nc, not shown)

• These relationships might serve as nexus of aerosol-cloud interactions in large-scale models

Validation of COAMPS cloud processing

•Results from LES show similar behavior

•Hoell et al. (2000) give larger cloud processing for given drizzle rates

•Albrecht’s estimate (1989) is for a strongly-drizzle, highly-depleting example

Summary of COAMPS cloud processing results

•Results respond predictably to changes in initial CCN

•Idealized COAMPS runs gauge the relative importance of various components of a mesoscale aerosol forecast

•Magnitude of the entrainment source is greater than any reasonable values of in-situ or surface sources… yet we know that sea-spray can play a vital role in PBL clouds

•Specification of vertical aerosol profile and species may be more vital than detailed knowledge of in-situ source rates — Importance of remote sensing.

Conclusions

•The general requirements of how to treat aerosol-cloud-drizzle interactions are becoming clear

•Absolute magnitudes of sources/sinks are poorly constrained

•Major effort to estimate these quantities and develop parameterizations, either from observations or process models (LES, CRM)

•Aerosol-cloud parameterization could be implemented gracefully (?) into the COAMPS aerosol-tracer module