Development of the SEVIRI Aerosol Retrieval Algorithm (SARA) Aerosol retrievals using MSG-SEVIRI...

Development of the SEVIRI Aerosol Retrieval Algorithm (SARA)

Aerosol retrievals usingMSG-SEVIRI images

Y.S. Bennouna and G. de Leeuw

Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images2

Outline

• MSG-SEVIRI: Instrument characteristics • Aerosol retrievals over the ocean: algorithm principles

• Aerosol models and radiative transfer simulations

• Cloud screening process

• AOD results over the ocean for 2 case studies

• Conclusions and on-going work


Instrument characteristics of MSG-SEVIRI

Photo: ESA

Altitude: ~36 km

Nominal position: Equator, 0° Longitude

Geographic coverage: a full Earth hemisphere over Europe and Africa

Spatial resolution at the sub-satellite point: narrow band channels ~3 km, HRV ~1 km

Time sampling : 15 minutes


Retrievals over the ocean: Equations

• Assuming a plane parallele homogeneous atmosphere

- Over a sea target with directional effect [Tanré, 1979]

- Specular reflection of a wavy surface [Cox and Munk, 1954]

- Reflectance of oceanic whitecaps [Monahan and Muircheartaigh, 1980]

- Subsurface scattering

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• Atmosphere path reflectance splitted into individual path reflectance due to aerosol and and gas constituents:

• Hypothesis on reflectance for an external mixture of aerosols:[Wang and Gordon, 1994]

• Linear dependence approximation between TOA reflectance and AOD:[Durkee, 1986]

• Best mixture determined by a least square fitting method

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AOD retrievals: Equations

• Aerosol is considered to be an external mixture of a coarse (natural) and a fine (anthropogenic) aerosol type with monomodal lognormal size distribution

• Expansion coefficients of the scattering matrix and single scattering albedo calculated using:

- MIE code for spherical particles [De Haan et al., 1987]

- TMATRIX code for randomly oriented spheroids [Mischenko and Travis, 1998]


Aerosol models

NAM

OPAC

MODIS

[Hess et al., 1998]

[de Leeuw et al., 1989]

[Levy et al., 2006]

• LUT data generated using DAK radiative transfere model [Stammes,2001]

- Midlatitude Winter/Summer profile for gases [McClatchey et al., 1972]

- aerosol layer height: 2 km

- exponentially decreasing concentrations with height

- 15 solar and viewing zenith angles, 37 azimuth angles

- 11 aerosol optical depths at the reference wavelength of 500 nm (ranging between 0.02 and 6).


Radiative transfer simulations


Cloud Screening - Method

• Basic principle: “Clouds are generally colder and brighter than most surfaces”

• Method based on the cloud screening technique used in AATSR aerosol retrieval algorithm [Robles-Gonzalez, 2003]

Treshold technique based on image histogram analysis

• Detection tests:

- Infrared Gross Temperature Test (IGTT) – (over land and sea (T12.0)) - Dynamic Visible Test (DVT) – (over land (R0.6) and sea (R0.8))

- Ratio Test (DRT) – (over sea only (R0.8/R0.6))- Dynamic Spatial Coherence Test (SCT) – (over sea only (T10.8, R0.6))


Cloud Screening -Cloud mask comparisons over Europe

SEVIRI and SAFNWC/MFSEVIRI and SAFNWC/MF

SEVIRI and MODISSEVIRI and MODIS

4 week periods in (Jan/May/Aug/Oct) 2006:Most differences observed on cloud edges, and broken cloud fields.In general ~85% agreement with both products

Case study 1 (FF):Forest-Fire Smoke from the Iberian peninsula advected

over the Atlantic - 7 August 2006


Cabo da Roca True color scene from Terra-MODIS 11:10 UTC

SARA results for FF -7 August 2006, 11:15 UTC


AOD at 635 nmAOD at 635 nm

Angstrom 635-870 nmAngstrom 635-870 nm

Fine Mode Weight at 500 nmFine Mode Weight at 500 nm

Comparison with MODIS (MOD04) for FF


Comparison with MODIS for FFComparison with MODIS for FF

• Spatial variations well represented

• SEVIRI-AOD635 < < MODIS-AOD660

underestimation by ~ 50 %

MODIS - AOD at 660 nmMODIS - AOD at 660 nm

Validation with AERONET for FF


Validation with AERONET for FFValidation with AERONET for FF

• Diurnal variations well reproduced• Best results obtained for the visible channels• MODIS-AOD860 and SEVIRI-AOD810 overestimated

Case study 2 (DD):Dust Storm across the Eastern Mediterranean Sea

25 February 2006


Forth Crete

True color scene from Terra-MODIS 09:00 UTC

SARA results for DD -25 February 2006, 09:00 UTC


AOD at 1640 nmAOD at 1640 nm

Angstrom 870-1640 nmAngstrom 870-1640 nm

Fine Mode Weight at 500 nmFine Mode Weight at 500 nm

Comparison with MODIS (MOD04) for DD


Comparison with MODIS for DDComparison with MODIS for DD

•Very good agreement between SEVIRI-AOD1630 and MODIS-AOD1630

MODIS - AOD at 1630 nmMODIS - AOD at 1630 nm

Validation with AERONET for DD


Validation with AERONET for DDValidation with AERONET for DD

• Time series cloud contaminated• General trends well represented• Best results obtained for AOD in the near-infrared


Conclusion and on-going work

• The stand-alone algorithm for cloud detection with MSG-SEVIRI provided satisfying cloud masks over Europe

• Aerosol retrievals with SARA over the ocean have been validated for 2 case studies (FF and DD): favorable comparison with MODIS and AERONET data.

• Aerosol retrievals over land is an ongoing work. Preliminary results expected in winter 2008...

Thanks for your attention !

Development of the SEVIRI Aerosol Retrieval Algorithm (SARA) Aerosol retrievals using MSG-SEVIRI...

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