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Overview of Ocean Color: Theoretical background, sensors and application

Overview of Ocean Color: Theoretical background, sensors and application. Jian Wang, Ph.D IMCS Rutgers University. Introduction Theoretical Background Sensors and Platforms Applications Summary. Definition. Ocean Color :

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Overview of Ocean Color: Theoretical background, sensors and application

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  1. Overview of Ocean Color: Theoretical background, sensors and application Jian Wang, Ph.D IMCS Rutgers University

  2. Introduction • Theoretical Background • Sensors and Platforms • Applications • Summary

  3. Definition Ocean Color: Refers to the characteristic hue of the ocean according to the presence and concentration of specific minerals or substances, such as chlorophyll.

  4. Scope • Ocean color covers passive optical remote sensors (sun=light source) • Focus on digital image sensors, especially airborne/satellite radiometers

  5. Theoretical Background • Radiative transfer theory • Atmospheric correction • IOPs and AOPs • Chlorophyll retrieval

  6. Photon paths to sensor SUN SENSOR • Single and double scattering (air/int./water) photon paths • Only photons from via water (1-35%: Sturm, 1981) are useful, the rest is noise Aerosols and molecules Interface (with foam) Water, phyto., NAP, CDOM

  7. Atmospheric Correction • Solar radiation absorbed or scattered by the atmosphere before it reaches a sensor. • The ground surface receive not only the direct solar radiation but also sky light, or scattered radiation from the atmosphere. • A sensor will receive not only the direct reflected or emitted radiation from a target, but also the scattered radiation from a target and the scattered radiation from the atmosphere, which is called path radiance

  8. Atmospheric Correction • The method using the radiative transfer equation • The method with ground truth data • Other method: A special sensor to measure aerosol density or water vapor density

  9. Remote sensing reflectance: AOP f: empirical factor Q: ratio of upwelling irradiance to radiance t: transmittance of the air-sea interface n: refraction index of seawater

  10. Inherent optical properties (IOPs)Independent of light field • c = a + b • at = aw + ap + as • ap= aa + an t: total w: water p: particulate s: soluble a: phytoplankton n: nonpigmented particles

  11. Backscattering

  12. Chlorophyll Retrieval To go back from the light detected at the sensor to deduce marine phytoplankton (e.g. represented by CHL) • Remove/correct for atmospheric and air-sea interface effects (atmospheric correction) • Deduce CHL from water-leaving reflectance spectrum (CHL retrieval)

  13. Current SeaWiFS Chl algorithm:OC4V4 logChl=a+bR+cR2+dR3+eR4 R=log(Rrs443>490>510/Rrs555)

  14. Current satellite sensors SeaWiFS MODIS-T MODIS-A Agency OSC/NASA NASA NASA Launch 1997 1999 2002 Sp.Res.(m) 1100 250-1000 250-1000 Swath(km) 2800 2330 2330 VIS/NIR/ 6/2/0 11/5/20 11/5/20 other bands Tilt(less glint) Yes No No

  15. Websites • SeaWiFS http://oceancolor.gsfc.nasa.gov/SeaWiFS/ • MODIS http://modis.gsfc.nasa.gov/about/

  16. SeaWiFS data product • Level 1A: at-spacecraft raw radiance counts with calibration and navigation information available separately in the data file • Level 2: five normalized water-leaving radiance, and seven geophysical parameters derived from the radiance data. • Level 3: geophysical parameters binned to a 9x9 km (81 km2) global, equal-area grid at daily, 8-day, monthly, and annual intervals http://daac.gsfc.nasa.gov/oceancolor/dataprod/OC_Dataproducts.shtml

  17. Ocean color applications • Carbon cycle and climate change • Linking ocean ecosystem and the physical parameters • Coastal zone protection and marine resources management

  18. Surface distribution of chlorophyll a using SeaWiFS data sets: Note physical forcing effects: Coastal, Equator, North Atlantic SeaWiFS Team/GSFC/NASA

  19. New Jersey Coastal Upwelling July 6, ’98 - AVHRR July 11, ‘98 - SeaWiFS Chlor-a (mg/m3) Temperature oC 19 20 21 22 24 .1 .3 .5 1 2 4 40N 40N Historical Hypoxia/Anoxia Field Station Field Station LEO LEO 39N 39N 75W 74W 75W 74W Barnegat Cape May

  20. Wind driven coastal upwelling Note spatial scales of variability CZCS Images Island-Induced Upwelling Coastal Upwelling Off Africa Coastal Upwelling Off Peru SH NH Wind CZCS Team/GSFC/NASA

  21. Summary • Retrieve CHL from signal detected by sensors - Atmospheric correction - IOPs based algorithms • Broad applications

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