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Jan-Peter Muller* (UCL) Carsten Brockmann, Marco Z ü hlke, Norman Fomferra (BC)

MERIS land surface Albedo from data fusion with MODIS BRDFs, its validation using MISR, POLDER and MODIS (gap-filled albedo) and Data Dissemination using DDS and OGC. Jan-Peter Muller* (UCL) Carsten Brockmann, Marco Z ü hlke, Norman Fomferra (BC)

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Jan-Peter Muller* (UCL) Carsten Brockmann, Marco Z ü hlke, Norman Fomferra (BC)

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  1. MERIS land surface Albedo from data fusion with MODIS BRDFs, its validation using MISR, POLDER and MODIS (gap-filled albedo) and Data Dissemination using DDS and OGC Jan-Peter Muller* (UCL) Carsten Brockmann, Marco Zühlke, Norman Fomferra (BC) Jürgen Fischer, Réné Preusker, Thomas Schröder (FUB) Peter Regner (ESA/ESRIN) *Professor of Image Understanding and Remote Sensing MISR & MODIS Science Team Member (NASA EOS Project) HRSC Science Team Member (ESA Mars Express 2003) Chair, CEOS-WGCV Terrain mapping sub-group

  2. Overview • Context • Objectives • BRDF/Albedo retrieval approach • BRDF/Albedo algorithm details • Initial Results • Validation approach • Preliminary Validation results • Future Prospects

  3. Context • All governments with space agencies agreed in Brussels in February 2005 on a common strategy for Earth Observation called • GEOSS (Global EO System of Systems) which has 9 societal benefit areas including climate modelling, biodiversity, ecology and hazard monitoring • ESA and the European Union have now established the funding for their GMES (Global Monitoring of Environment and Security) programme which embodies these GEOSS principles • CEOS (Committee on Earth Observing Systems) is now the provider of the space segment including setting ISO-level standards for cal/val • Main push is to improve interoperability between products from the same agency and products between different space agencies • Development of MERIS spectral and broadband albedo is the first example of trying to fuse at the processing chain/algorithm level between products from different space agencies • Albedo required for climate GCM model verification by Hadley Centre

  4. Objectives • Derivation of a one-year (2003) land surface albedo from MERIS for • 13 of the 15 MERIS wavelengths (excluding 2 inside O2 absorption bands) • 2 broadband albedos (0.4-0.7µm, 0.4-3µm) • 16-day and MONTHLY time step for 2003 • Input Level 2 Rayleigh+O3 corrected • 0.05º and 10km sinusoidal spatial resolutions • Publication of MERIS albedo browse images (as Web Map Services layers) within the CEOS-WGISS EO Data Portal (http://iceds.ge.ucl.ac.uk) • Publication of the associated albedo files downloadable through a cascaded Web Coverage Server • Main driver is to improve the retrieval of atmospheric parameters from MERIS. Hence, spectral albedos at the MERIS wavelengths are required • Secondary driver is the production of spectral and broadband albedos for use by the European climate and weather forecasting bureaus • Processsing software incorporated into the platform-independent (Java-based) BEAM software so that anyone can produce their own albedo products for any other time periods • Validation by inter-comparison with other EO sensors only envisaged at present

  5. BRDF/albedo approach(1) • Inputs are orthorectified, cloud-cleared, atmospherically-corrected Spectral/Surface Directional Reflectances (SDRs) from level 2 data at 1.2km spatial resolution and a typical sampling of every 2-3 days • BRDF retrieval is NOT directly performed on these SDRs as sampling of the bi-directional plane is insufficient for most land surfaces given the narrower swath (1130km) and lower temporal sampling (every 2-3 days at the equator) of MERIS cf. instruments such as MODIS (2550 km and daily sampling) • Instead the BRDF shape and BRDF models are taken for the 4 common spectral bands from the MOD43C2 (0.05º) product (see below for an intercomparison). N.B. Bands also common with MISR/POLDER

  6. BRDF/albedo approach(2) • Using magnitude inversion, MERIS BRDFs are calculated for each set of SDRs which are co-located with the MODIS 0.05º pixel where MODIS returns a value • Linear spectral interpolation is performed for the isotropic component of the BRDF for the remaining 9 MERIS spectral bands. (In future, it is planned to use spectral databases such as ASTER or SDRs from CHRIS/PROBA or CAR data to refine this approach) • Currently spectral interpolation for the 2 sets of broadband albedos (0.4-0.7µm, 0.4-3µm) is performed using the MISR-equivalent bands. Work is in progress to refine this approach • QC information is provided for 4 common spectral albedos and Nadir BRDF Adjusted Reflectances (NBAR) through statistical summaries of intercomparison with MOD43C1 (albedo)/MOD43C3 (NBAR)

  7. BRDF retrieval: vegetation Kernel-Driven Semiempirical BRDF Model • BRDF Model • Linear combination of two BRDF shapes and a constant • BRDF shapes described by kernels, which are • Trigonometric functions of incidence and view angles • Derived from physical models for surface scattering (Ross-Thick Li-Sparse Model Reciprocal (RTLSMR) for leaf “cloud” and shadows) • Analytical Form: where • is a constant for isotropic scattering ; • are trigonometric functions providing shapes for geometric-optical and volume-scattering BRDFs; and • are constants that weight the two BRDFs • MOD43C2 • Product supplies values of f for each 0.05º pixel and separate C code to calculate k

  8. BRDF retrieval: vegetation Magnitude inversion • We determine a on a per-band basis by • a least squares minimisation of the difference between directional reflectances (SDRs) predicted by the MOD43C2 BRDF parameters and those actually measured by the MERIS sensor • The predicted measurements are found by running the RTLSMR model in the forward mode using the MOD43C2 BRDF parameters under the same view and • illumination angles as the MERIS measurements available • Performed on 4 common spectral bands between MODIS (469,555,645,859nm) and MERIS (490,560,665,865nm)

  9. Albedo retrieval: vegetation Black-sky, White-sky and solar zenith dependence • Direct Hemispherical Reflectance, is given by Black-sky (NO diffuse) albedo, is given by • Diffuse bi-Hemispherical reflectance, is given by White-sky (diffuse ONLY) albedo, is given by

  10. Albedo retrieval: vegetation Black-sky, Blue-sky and solar zenith dependence • The solar angle dependence can be approximated by, • Under actual atmospheric conditions given the aerosol optical depth, the blue-sky albedo is given by Where is the fraction of diffuse skylight

  11. Albedo retrieval: vegetation Narrow-to-broadband conversion • Gao et al. (2003) derived a first approximation to broadband albedo conversion factors based on those from MISR which are taken from his paper with VIS (0.4-0.7µm), NIR (0.7-3µm) and Shortwave (0.4-3µm)

  12. Meris L2 SDRs Albedo retrieval scheme MOD43C2 BRDF (0.05º) + QA#1 flags BIN MERIS SDRs (0.05º x 0.05º) over 16-day MOD43C2 MOD43C3 NBAR (0.05º) QA#2 Nsamps, ave± stddev, min, max MAGNITUDE INVERSION with MOD43C2 CALCULATE <MERIS> NBAR OVER MODIS 16 DAY PERIOD INTERCOMPARE WITH MOD43C3 MERIS 0.05º 16- DAY NBAR CALCULATE MERIS NBAR 0.05º DAILY INTEGRATE MERIS ALBEDO FOR 16-DAY PERIOD INTERCOMPARE WITH MOD43C1 DIFF STATS MERIS 0.05º 16- DAY ALBEDOS MONTHLY/ SEASONAL AVERAGE RE-PROJECT TO 10Km/0.05º QA3 Nsamps, ± std.dev. MOD43C1 ALBEDO (0.05º) N.B. Status: Sample products produced for Europe. Global production completion due by end January 2006. INTERPOLATE ALBEDO VALUES TO 9 OTHER BANDS + INTEGRATE TO VIS, NIR, SW Broadbands MERIS 10KM/005º 13- SPECTRAL + 4 BROADBAND MONTHLY+ SEASONAL ALBEDOS

  13. First MERIS albedo product: DoY 257 (16-day time period : 14/9/03-29/9/03): all bands

  14. First MERIS albedo product: DoY 257 (16-day time period : 14/9/03-29/9/03): Band 5 (green)

  15. Validation approach(1) • Difference statistics between MERIS-Albedo and MODIS gap-filled albedo product (Moody et al., 2005) for common bands being analysed for the same 16-day time periods • Inter-comparisons are also being performed with • MISR 0.5º “true monthly” level-3 product (2003) • POLDER2 0.05º resampled from 6km sinusoidal gridded 30-day products reported on the 15th of each month (Apr03-to-Oct03) • MODIS gap-filled albedo product sampled for weighted average of constituent 16-day time periods within the months of Jan, Feb, Sep, Oct, Nov-03 • Initial inter-comparisons follow with POLDER2, MODIS gap-filled and MISR • Detailed inter-comparison also shown for MODIS gap-filled and MISR

  16. Validation issue: finding temporal coincidences for 16-day products to match them up against monthly climate modelling requirements

  17. MERIS (16-day,DoY=257-272) cf. POLDER2 (30-day, DoY=244-273) at 0.05º resolution N.B. Poor atmospheric correction of POLDER-2

  18. MERIS (16-day,DoY=257-272) cf. POLDER2 (30-day, DoY=244-273) at 0.05º resolution with coastlines MERIS: 865nm POLDER2: 865nm N.B. Very poor geocoding of POLDER-2. Decided NOT to perform any further inter-comparisons with MERIS and MISR until this problem is fixed

  19. MERIS cf. MODIS gap-filled albedo for common bands (16-day, DoY=257-272) at 0.05º resolution MERIS: 665, 560, 490 MODIS: 665, 560, 470 N.B. Noticeable differences in colour and bright albedo patterns

  20. MERIS vs MODIS gap-filled albedo for common bands (16-day, DoY=257-272) at 0.05º resolution 490 vs 470 665 vs 665 865 vs 869 N.B. 2D correlation improves with increasing wavelength

  21. MERIS (weighted average DOY 241(13), 257(16), 273(1)) cf. MISR (30-day, DoY=244-273) at 0.5º resolution 665,560,443nm 672,558,443nm 867,672,558nm 865,665,560nm N.B. MISR higher Albedo cf. MERIS MISR MERIS

  22. MERIS [weighted average DOY 241(13/30), 257(16/30), 273(1/30)] vs MISR (30-day, DoY=244-273) at 0.5º resolution 560 vs 558 443 vs 443 665 vs 672 865 vs 857 867,672,558nm N.B. MISR albedo values higher than MERIS but overall good correlation. Plan to compare instantaneous MISR albedo at 1.1km with MERIS 16-day. This requires modification of BEAM ingest for MISR Level 2AS data. This is planned later in 2006.

  23. MODIS gap-filled product [weighted average DOY 241(13/30), 257(16/30), 273(1/30)] MINUS MISR (30-day, DoY=244-273) at 0.5º resolution 470nm 555nm 665nm 859nm (MODIS-MISR)/MISR normalised difference albedo . MISR always HIGHER than MODIS ±2%±5%±10%±20%±50%±100%

  24. Conclusions • First demonstration of data fusion of MERIS and MODIS • Substantial interest in user community for monthly (and seasonal albedo products. Little interest in 16-day products • Simple weighted average of number of days within a 16-day cycle appears to provide reasonable values of monthly albedos • Significant differences between MISR and gap-filled MODIS albedos with MISR consistently higher than either MODIS or MERIS albedos • Some differences can be explained due to the derivation of snow-free MODIS gap-filled product • Good agreement (as expected) between MERIS and MODIS gap-filled products for common spectral bands

  25. Planned Prospects • Improvement in POLDER georeferencing so POLDER can be used to compare against MISR and MERIS • Intercomparisons of monthly MISR vs MODIS gap-filled albedo for 5 years of data • Intercomparisons of MISR L2AS with MODIS gap-filled albedo, POLDER and MERIS • Improvement of spectral interpolation using CHRIS/PROBA and GSFC-CAR measurements including development of CHRIS/PROBA processing chain within BEAM based on MERIS • Production of further years of MERIS spectral albedos (2002, 2004, 2005, 2006) at current resolutions • Development of modified processing chain for production of MERIS 300m spectral albedos for 2005 using MOD43B1 (500m, Collection 5) BRDFs including dealing with snow (explicitly) • Publication of MERIS spectral albedo browse products as WMS layers within ICEDS and for use by other WMS browsers as cascaded datasets • Publication of underlying MERIS spectral albedos as WCS layers at BC including direct linkage to BEAM and subsetting via WMS

  26. Product CD-ROM, DVD-ROM large capacity media USERS DDS broadcast (in Europe) ( deployment for Africa ) 3- via telecom satellite (NRT) Data delivery : current options 1- Physical media 2- Internet ftp or http Near Real Time (NRT): Rolling Archive or On request Archived data : gradual availability

  27. ENVISAT data accessto Near Real Time data

  28. Eutelsat W1 footprint Data Dissemination System (DDS) DDS Europe DDS Africa C-band Typical 1.2 m DDS receiving antenna About 2.5 m receiving antenna Dissemination of global MERIS RR Level 1 & Level 2 products

  29. ICEDS portal (http://iceds.ge.ucl.ac.uk)

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