Quantifying aerosol direct Quantifying aerosol direct radiative effect with MISR radiative effect with MISR

observationsobservations

Yang Chen, Qinbin Li, Ralph KahnJet Propulsion Laboratory

California Institute of Technology, Pasadena, CA

California Institute of Technology, June 05, 2007

OverviewOverview Aerosol direct radiative effect (Scattering, Absorption)

• Aerosol forcing (anthropogenic)• Aerosol radiative effect (natural + anthropogenic)

Method to estimate global aerosol direct radiative effect• Model-based, radiative transfer calculation• Satellite-based• -0.1~-0.9 W/m2 for annual mean global aerosol direct forcing

(IPCC,2007).

Previous satellite based studies• CERES (Clouds and the Earth’s Radiant Energy System) TOA fluxes ! Coarse resolution (~10x10 km2)

• MODIS (Moderate Resolution Imaging Spectroradiometer) Aerosols ! Has difficulty in retrieving aerosol properties over bright land

This study• Use MISR (Multi-angle Imaging SpectroRadiometer) to quantify global

aerosol shortwave direct radiative effect (SWDRE), on both land and oceans.

Introduction to MISR (Multi-angle Introduction to MISR (Multi-angle Imaging SpectroRadiometer)Imaging SpectroRadiometer)

On board satellite TERRA

9 view angles at earth surface:

• ±70.5º. ±60.0º, ±45.6º, ±26.1º, nadir

Four spectral bands at each angle:

• 446 nm (Blue) • 558 nm (Green)• 672 nm (Red)• 866 nm (NIR)

Global Mode:• 275 m sampling resolution for

nadir camera and red band of other cameras

• 1.1 km for the other channels• 400-km swath• Global coverage: 9 days at

equator, 2 days at poles

Continuous data retrieval since Feb 2000.

MISR products used

MISR products and imagesMISR products and images

Nadir view Cloud mask

AOD TOA albedo

1°x 1° grid

MISR images

TOA albedo TOA albedo (2.2x2.2 km2)

AOD AOD (17.6x17.6 km2)

Cloud mask Cloud mask (1.1x1.1 km2)

BHRPAR BHRPAR (1.1x1.1 km2)

MethodMethod

aerosolnoaerosolwithTOAskyclear IF __

fractionCloudFF skyclearskyall _1

Global distribution of MISR AOD, Global distribution of MISR AOD, albedo, and BHRPAR (July, 2002)albedo, and BHRPAR (July, 2002)

26 BHRPAR bins:

0~0.1: each 0.01 interval

0.1~0.4: each 0.02 interval

Above 0.4: 1 level

Global TOA albedo~AOD correlation Global TOA albedo~AOD correlation over oceanover ocean

List of selected regionsList of selected regions

Regions over land Regions over ocean

Each region has 10°x5° area

TOA albedo~AOD correlation over TOA albedo~AOD correlation over ocean regionsocean regions

The slopes indicate the ability of aerosols to affect TOA radiative flux.

Alternative method: do global regression for each solar zenith angle.

TOA albedo~AOD correlation TOA albedo~AOD correlation over remote ocean regionsover remote ocean regions

Global regression over ocean for each SZA

Global TOA albedo~AOD Global TOA albedo~AOD correlation over landcorrelation over land

List of selected regionsList of selected regions

Regions over land Regions over ocean

Each region has 10°x5° area

Albedo~AOD correlation over landAlbedo~AOD correlation over land

A

East US

East US

Albedo~AOD correlation over landAlbedo~AOD correlation over land

A

East US

Central Africa

Albedo~AOD correlation over landAlbedo~AOD correlation over land

A

East US

Sahara desert

Aerosol direct radiative effectAerosol direct radiative effect

(a) Clear-sky and (b) all-sky aerosol direct radiative effect (W/m2) for July 2002.

Aerosol direct radiative effectAerosol direct radiative effect

Aerosol DRE (Clear sky) (W/m2)

Aerosol DRE (All sky) (W/m2)

Global -4.70 -1.49

Over ocean -4.54 -1.95

Over land -4.88 -1.18

Source Aerosol DRE (W/m2) Spatial coverage Temporal coverage Satellite data source

Zhang and Christopher, 2005

-6.4 ± 2.6 Cloud-free oceans 09/2000-08/2001 CERES, MODIS

Christopher and Zhang, 2002

-6 Cloud-free oceans 09/2000 CERES, MODIS

Loeb and Kato, 2002

-4.6 ± 1 Cloud-free tropical oceans

01/1998-08/1998, 03/2000

CERES, TRMM VIRS

Loeb and Manalo-Smith, 2005

-5.5, -3.8 Cloud-free oceans 03/2000-12/2003 CERES, MODIS

-2.0, -1.6 All-sky oceans

From this study (July, 2002):

From previous satellite-based studies:

Correlation between aerosol Correlation between aerosol SWDRE and 0.56 SWDRE and 0.56 m AODm AOD

Correlation between aerosol SWDRE and AOD (a) Over ocean (b) Over land

UncertaintiesUncertainties Satellite retrieval of aerosol, TOA albedo and

surface properties.

Cloud contamination.

Diurnal variability.

TOA albedo narrow-to-broadband conversion.

Surface heterogeneity.

Use SVM classifier to calculate Use SVM classifier to calculate smoke aerosol effect - methodsmoke aerosol effect - method

SVM: Support Vector Machine

SVM classifiers• Clouds• Aerosols

Smoke Dust Other

• Ice/Snow• Water• Land

freeaerosolsmokeTOAaerosolssmoke IF __

Clouds

Land

Water

Smoke

Dust

Use SVM classifier to calculate Use SVM classifier to calculate smoke aerosol effect - resultsmoke aerosol effect - result

(a) Clear-sky aerosol SWDRE from smoke aerosols (W/m2)

(b) MODIS wild fire occurrence from Fire Information for Resource Management System (NASA/U of Maryland, 2002)

Use SVM classifier to calculate Use SVM classifier to calculate smoke aerosol effect - uncertaintysmoke aerosol effect - uncertainty

Threshold for differentiating ‘Aerosols’ and ‘Surface’ pixels is arbitrary, which may cause the underestimation of total number of ‘smoky’ pixels.

Since many ‘Surface’ pixels actually have some aerosol loading, the albedo for ‘Surface’ pixels is highly overestimated.

The SVM scene classification is still in provisional quality, and the aerosol sub-classification validation has not yet been completed.

Due to the above reasons, we chose not to use SVM scene classifiers in the global aerosol SWDRE estimation.

Conclusions and future workConclusions and future work Conclusions

• By using MISR datasets, first satellite-based attempt to estimate global aerosol direct radiative effect over both ocean and land has been made.

• Aerosols and TOA albedo show different correlations in absorptive aerosol dominated region and non-absorptive aerosol dominated region.

• Over land, the slope of AOD ~ TOA albedo decreases as BHRPAR increases, indicating the aerosol scattering and absorbing effect on TOA albedo is smaller over brighter surfaces.

Future work• Extend the approach to include seasonal and inter-

annual variability.

AcknowledgmentAcknowledgment MISR dataMISR data were obtained from the NASA Langley Atmospheric were obtained from the NASA Langley Atmospheric

Sciences Data Center (Sciences Data Center (http://eosweb.larc.nasa.gov/http://eosweb.larc.nasa.gov/).).