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Satellite Ocean Color Overview

Satellite Ocean Color Overview. Dave Siegel – UC Santa Barbara With help from Chuck McClain, Mike Behrenfeld, Bryan Franz, Jim Yoder, David Antoine, Gene Feldman, Claudia Mengalt , Bob Evans, Norm Nelson, Stéphane Maritorena & many more. After ASLO 2011 Tutorial Talk.

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Satellite Ocean Color Overview

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  1. Satellite Ocean Color Overview Dave Siegel – UC Santa Barbara With help from Chuck McClain, Mike Behrenfeld, Bryan Franz, Jim Yoder, David Antoine, Gene Feldman, Claudia Mengalt, Bob Evans, Norm Nelson, StéphaneMaritorena& many more After ASLO 2011 Tutorial Talk

  2. What is Satellite Ocean Color? • The spectrum of the light reflected from the sea • Water-leaving photons are backscattered & not absorbed (ocean optics & relationship to ecology) • To see from space, we must account for the atmosphere (radiative transfer in the atmosphere) • Ocean color signals are small (great measurements require great care…) atmosphere ocean

  3. Bright Atmosphere – Dark Ocean How do we correct for the atmosphere?? TOA Radiance Factor of >10 difference Log(Radiance) Water-Leaving Radiance

  4. Atmospheric Correction Radiance budget for satellite radiance Measure Lt(l) Model Lr(l), Lf(l) & Lg(l) Unknowns are Lw(l), La(l) & Lar(l) It is Lw(l) we need to know…

  5. Atmospheric Correction Basics • Goal: Subtract off the atmospheric path signals from the satellite measurement • Model Rayleigh scattering, molecular absorption & interface reflectance terms • Hard part is aerosol radiances • Use near-infrared bands to model aerosol radiances in the visible • Requires detailed models of aerosol optical properties that can be diagnosed from NIR

  6. Ocean Color Sensor Requirements • An ocean color sensor must… • Have necessary spectral resolution • Accurate(gains must be well known) • Stable (changes in gains must be known) • Well characterized (polarization, spectral, etc.) • Devil is in the details…

  7. Comparison of Spectral Coverage 86 “hyperspectral” bands + 3 SWIR “Multispectral” Ocean Bands CZCS: 4 SeaWiFS: 8 MODIS: 9 VIIRS: 7 OES: 26 ACE plan CZCS (1978-1985) SeaWiFS (1997-2010 ) MODIS (2002- )* Products VIIRS Absorbing aerosols Dissolved organics Phytoplankton pigments Functional groups Particle sizes Physiology Pigment fluorescence Coastal biology Atmospheric correction (clear ocean) Atmospheric Correction (coastal) & Aerosol/cloud properties Ultraviolet Products No Measurements Ultraviolet Total pigment or Chlorophyll-a but major errors due to absorption by dissolved organics Atmospheric Correction/ MODIS chlorophyll fluorescence Visible Visible 5 nm resolution (345 – 775 nm) 26 required “multispectral” bands Atmospheric Correction (clear ocean) NIR NIR Atmospheric Correction (coastal)** SWIR 3 SWIR bands SWIR * MODIS on Terra was launched in 2000, but does not yet provide science quality ocean data MODIS/Visible Infrared Imaging Radiometer Suite (VIIRS) SWIR bands are not optimized for oceans ** Chuck McClain (GSFC)

  8. SeaWiFS: 1997-2010

  9. Satellite Sensor Gains • Accuracy requirements mean that satellite gains need to be known to better than 0.005 • Accurate ground data are required End-to-end test -> vicarious calibration • Changes in these gains must be monitored Lunar viewing or multiple on-board sources • Other “issues” creep in (like changes in polarization sensitivity or spectral responses)

  10. SeaWiFS Lunar Calibration Used to monitor sensor gain changes over time Uncertainty about trend lines is ~0.1% Fred Patt, GSFC

  11. Marine Optical BouY • Accurate source for vicarious calibration • Used with models to set absolute gains • Located off Hawaii and operational since 1996 • Difficulty with glint & nadir looking satellites (MODIS) • Other ground obs are used as well

  12. Satellite Sensor Issues • Accuracy requirements mean that satellite gains need to be known to better than 0.005 • Accurate ground data are required - MOBY+ End-to-end test -> vicarious calibration • Changes in these gains must be monitored Lunar viewing or multiple on-board sources • Other “issues” can creep in (like changes in polarization sensitivity or spectral responses) • Reprocessing is key…

  13. Bio-Optical Modeling • Goal: Relate water-leaving radiance spectra to useful in-water properties • Both empirical & semi-analytical approaches • Need simultaneous measurements of water-leaving radiance & useful in-water properties

  14. Global In Situ Data - SeaBASS All Data AOT Optical Profiles Chl

  15. Bio-Optical Algorithm OC4v6 used for SeaWiFS • Empirical • Maximum Band Ratio of LwN(l)’s (443/555, 489/555 & 510/555) From GSFC reprocessing page following O’Reilly et al. [1998] JGR

  16. GSM Semi-Analytic Model • Retrieves three relevant properties (CDM, BBP, Chl) • Assumptions… • Relationship between LwN(l) & properties is known • Component spectral shapes are constant - only their magnitudes vary • Water properties are known • Results in a least squares problem for the 3 properties • Model coefficients determined through a global optimization using field observations Maritorena et al. Applied Optics [2002]

  17. End-to-End Validation • OC4v6 w/ SeaWiFS • Global match-up data set of SeaWiFS & in situ Chl’s • Regression & the fit slope are very good http://oceancolor.gsfc.nasa.gov/REPROCESSING/R2009/ocv6/

  18. Ocean Color Components • Ocean color signals are small (great measurements require great care…) • We must account for the atmosphere (radiative transfer in the atmosphere) • Relate water-leaving radiance to bio-optical properties(ocean optics & relationship to ecology) • Validate we can do this end-to-end through the entire system (this requires periodic reprocessing)

  19. Where do we go now… • Use what we have to its fullest… • New algorithms and approaches with data in hand • Make VIIRS a success (this is a community responsibility) • Help in the planning for future missions • PACE/ACE, GeoCAPE, HyspIRI, … • Make satellite ocean color “carbon savvy” • Develop techniques for Net Community Production, C Export, N2 Fixation, etc.

  20. Thank You for Your Attention!! Special Thanx to the NASA Goddard Ocean Biology Processing Group and the NRC Committee on Sustained Ocean Color Obs

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