Development of the SEVIRI Aerosol Retrieval Algorithm (SARA)

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

2. 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 Aerosol retrievals using MSG-SEVRI images

3. Photo: ESA Instrument characteristics of MSG-SEVIRI 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 Aerosol retrievals using MSG-SEVRI images

4. 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 Retrievals over the ocean: Equations Aerosol retrievals using MSG-SEVRI images

5. 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 Aerosol retrievals using MSG-SEVRI images

6. Aerosol models • 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 [Mischenkoand Travis, 1998] NAM [de Leeuw et al., 1989] OPAC [Hess et al., 1998] MODIS [Levy et al., 2006] Aerosol retrievals using MSG-SEVRI images

7. Radiative transfer simulations • 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). Aerosol retrievals using MSG-SEVRI images

8. 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)) Aerosol retrievals using MSG-SEVRI images

9. Cloud Screening -Cloud mask comparisons over Europe 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 SEVIRI and SAFNWC/MF Aerosol retrievals using MSG-SEVRI images

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

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

12. Comparison with MODIS (MOD04) for FF • Comparison with MODIS for FF • Spatial variations well represented • SEVIRI-AOD635 < < MODIS-AOD660 • underestimation by ~ 50 % MODIS - AOD at 660 nm Aerosol retrievals using MSG-SEVRI images

13. Validation with AERONET for FF • Validation with AERONET for FF • Diurnal variations well reproduced • Best results obtained for the visible channels • MODIS-AOD860 and SEVIRI-AOD810 overestimated Aerosol retrievals using MSG-SEVRI images

14. Case study 2 (DD):Dust Storm across the Eastern Mediterranean Sea25 February 2006 Forth Crete True color scene from Terra-MODIS 09:00 UTC Aerosol retrievals using MSG-SEVRI images

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

16. Comparison with MODIS (MOD04) for DD MODIS - AOD at 1630 nm • Comparison with MODIS for DD • Very good agreement between SEVIRI-AOD1630 and MODIS-AOD1630 Aerosol retrievals using MSG-SEVRI images

17. Validation with AERONET for DD • Validation with AERONET for DD • Time series cloud contaminated • General trends well represented • Best results obtained for AOD in the near-infrared Aerosol retrievals using MSG-SEVRI images

18. 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 ! Aerosol retrievals using MSG-SEVRI images