An Interactive Aerosol-Climate Model based on CAM/CCSM: Progress and challenging issues

Chien Wang and Dongchul Kim (MIT) Annica Ekman (U. Stockholm)Mary Barth and Phil Rasch (NCAR)

Acknowledgments: NSF, NASA, Ford-MIT alliance, and MIT Global Change Joint Program

The MIT/NCAR Three-Dimensional Interactive Aerosol-Climate Model

References: Kim et al., 2006; Wang 2004; Ekman et al., 2005, 2006; Wilson et al., 2001; Barth et al., 2000; Mayer et al., 2000; Wang et al., 1998; Kiehl et al., 1998; Boville and Gent, 1998

Atmospheric Aerosol ModelAtmospheric Aerosol Model

6 Aerosol modes6 Aerosol modesAdvection, convection, mixing,Advection, convection, mixing,

as well as wet and dry depositionas well as wet and dry deposition

AGCMAGCM

NCAR CCM3/CAM + CLM NCAR CCM3/CAM + CLM

Circulation and State of AtmosphereCirculation and State of AtmosphereClouds and PrecipitationClouds and Precipitation

RadiationRadiation

MIT EPPA + Emission ProcessorMIT EPPA + Emission ProcessorNCAR DMS emissionsNCAR DMS emissions

Concentrations Concentrations of Aerosolsof Aerosols

Winds, T, HWinds, T, H22O,O,

Precipitation Precipitation & Vertical Fluxes& Vertical Fluxes

EmissionsEmissions

OGCM or SSTOGCM or SST DataData

A-O ExchangesA-O Exchanges

SO4 ait

SO4 nuc

SO4 acc

H2SO4(g) OC pure BC

BC/SO4 mixed

coagulation

condensation

dry deposition

wet deposition

emission

nucleation

growth

A Size-Resolving Aerosol Model Prognostic variables: Q and N for each mode + Qbc in mixed mode

WANG 12.806 05S

Size distributions of various modes

(a) (b)

(d)(c)

a) Air-rich fresh methane soot b) Air-rich fresh propane sootc) Fuel-rich propane soot after

exposure to H2SO4 vapord) Kerosene soot after exposure

to H2SO4 vapor.

Modeling the Mixed Aerosol:The “black carbon core” model; diffusive growth and coagulation are allowed;

radiative properties are calculated based on particle size and BC/acid volume ratio (Toon and Ackerman, 1981; revised by W. Wiscombe)

(Courtesy by R. Zhang, Texas A&M; unpublished; 2006)

An Example for the BC-Core Model:

TEM images of various soot-containing

aerosols

Modeled vs. Observed Surface Sulfate ConcentrationSeasonal means; Const. emissions; observations are from EPA monitoring stations;

Model results = accumulation sulfate + sulfate in mixed mode

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0.5

OBS

MODEL

Model vs. Observations: Vertical Sulfate Profile

Obs: ACE Asia, south Japan flights, late April; see Bahreini et al., 2003

Model: April-May mean; Kim et al., 2006

Model vs. Observations: Aerosol Optical Depth

(From selected AERONET stations; all are annual means)

Critical SSACalculated based on a

simple reflection model(Seinfeld and Pandis, 1998)

For: = 0.15; ocean = 0.06;20-yr mean surface albedo

TOA Forcing of Mixed Aerosols

Clear sky, no-feedback (W/m2)

Internal mixing scheme, size- and BC/acid volume

ratio dependent,BC core model

0.05

Atmospheric Forcing of Mixed Aerosols (W/m2)

Internal mixing scheme, size- and BC/acid volume

ratio dependent,BC core model

Atmospheric Forcing of Black Carbon (W/m2)

Mass based, external mixing scheme

BC mass = external BC + mixed mode BC

Challenges in connecting aerosol processes with global modelsRedistribution of Various Aerosols in CRM Simulations (Ekman et al., 2006)

Aitken Sulfate Accumulate Sulfate

BCBC3D: 3h

Nucleation scavenging of aerosols One of the major sinks of aerosols and the connecting point to the

indirect forcing Required information: supersaturation, aerosol size distribution Current assumption: assuming a given “typical supersaturation” to

calculate the minimum size of droplet activation

Recycling of aqueous S(VI) Determines a significant supply of sulfuric acids in the air for aerosol

nucleation and diffusive growth Required information: stored aqueous concentration of S(VI) and the

model (not net) evaporation measure of liquid particles Current assumption: assuming an arbitrary evaporation ratio of total

aqueous S(VI) based on CRM results

Conversion of cloud droplet to rain Critical process in simulating aerosol’s role in hydrological cycle Required information: precipitation efficiency in various clouds Current assumption: ?

Summary

Model is “functioning”; aerosol-forced integrations are undergoing Including size distribution and chemical composition in calculation

brings some interesting effects and would provide useful information for aerosol-rainfall and indirect forcing projects

Atmospheric forcing of BC/acid mixed mode aerosol is ~ 2x of that of BC estimated using external mixing scheme; Note their TOA forcings could differ significantly (SSA and surface albedo).

Interesting Issues

The role of aerosol in precipitation and more generally in hydrological cycle

How to use CRM and aerosol process models to derive parameterization of aerosol processes for the global models

Coupling with tropospheric chemistry model (oxidation; heterogeneous reactions, etc.)