1. Development of the SEVIRI Aerosol Retrieval Algorithm (SARA) Aerosol retrievals using MSG-SEVIRI images Y.S. Bennouna and G. de Leeuw

2. 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

3. Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images3 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

4. Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images4 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

5. Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images5 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 AOD retrievals: Equations

6. 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] Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images6 Aerosol models NAM OPAC MODIS [Hess et al., 1998] [de Leeuw et al., 1989] [Levy et al., 2006]

7. 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). Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images7 Radiative transfer simulations

8. Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images8 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))

9. Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images9 Cloud Screening - Cloud mask comparisons over Europe SEVIRI and SAFNWC/MF SEVIRI 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

10. Case study 1 (FF): Forest-Fire Smoke from the Iberian peninsula advected over the Atlantic - 7 August 2006 Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images10 Cabo da Roca True color scene from Terra-MODIS 11:10 UTC

11. SARA results for FF - 7 August 2006, 11:15 UTC Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images11 AOD at 635 nm Angstrom 635-870 nm Fine Mode Weight at 500 nm

12. Comparison with MODIS (MOD04) for FF Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images12 Comparison with MODIS for FF Spatial variations well represented SEVIRI-AOD 635 < < MODIS-AOD 660 underestimation by ~ 50 % MODIS - AOD at 660 nm

13. Validation with AERONET for FF Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images13 Validation with AERONET for FF Diurnal variations well reproduced Best results obtained for the visible channels MODIS-AOD 860 and SEVIRI-AOD 810 overestimated

14. Case study 2 (DD): Dust Storm across the Eastern Mediterranean Sea 25 February 2006 Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images14 Forth Crete True color scene from Terra-MODIS 09:00 UTC

15. SARA results for DD - 25 February 2006, 09:00 UTC Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images15 AOD at 1640 nm Angstrom 870-1640 nm Fine Mode Weight at 500 nm

16. Comparison with MODIS (MOD04) for DD Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images16 Comparison with MODIS for DD Very good agreement between SEVIRI-AOD 1630 and MODIS-AOD 1630 MODIS - AOD at 1630 nm

17. Validation with AERONET for DD Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images17 Validation with AERONET for DD Time series cloud contaminated General trends well represented Best results obtained for AOD in the near-infrared

18. Helsinki, Tuesday 30 september 2008Aerosol retrievals using MSG-SEVRI images18 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 !