Jian Wang, Ph.D IMCS Rutgers University 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: Refers to the characteristic hue of the ocean according to the presence and concentration of specific minerals or substances, such as chlorophyll.

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

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

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

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

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

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

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

Backscattering

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)

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

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

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

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

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

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

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

Note spatial scales of variability 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

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