Slide 1Ocean color sensorsWhat makes the ocean colored?Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9We distinguish between: Inherent and Apparent Optical propertiesOcean color sensors: characteristicsSlide 12Slide 13Slide 14Slide 15Slide 16Slide 17Forward and Inverse modelsSlide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Remote Sensing of the Oceanand Atmosphere: John L. WilkinOcean [email protected] Building Room 214C609-630-0559 (g-voice)Michael Whitehead“Oceans Colour Scene”2Ocean color sensors• Operate in visible wavelengths• Typically passive, not active, instrumentsWhat makes the ocean colored?•Phytoplankton pigments–Chlorophyll-a: absorbs in the blue, so as its concentration increases the water appears increasingly green–Chlorophyll-b, c and carotenoids–Different species use different pigments, so spectrally resolved color observations provide information and phytoplankton community composition •Colored dissolved organic matter (CDOM)–Terrestrial CDOM: dissolved humic and fulvic acids from decaying vegetation–Ocean CDOM produced when phytoplankton are grazedWhat makes the ocean colored?•Organic particulates – detritus–Phytoplankton and zooplankton cell fragments–Zooplankton fecal pellets•Inorganic particulates–Sand, dust: from river runoff or atmospheric deposition–Sediments suspended form the seafloor•CDOM and particulates absorb strongly in blue–Create brownish-yellow color in the waterWhat makes the ocean colored?•Case 1 waters:–Phytoplankton pigments and their covarying detrital pigments dominate the optical characteristics–Open ocean •Case 2 waters:–Other substances that do not co-vary with chlorophyll-a dominate–Suspended sediment; other organic particles; CDOM–Coastal watersWhat makes the ocean colored?Scattering: Mostly determined by particle size–Viruses: 10-100 nm diameter (concentration ~1015 m-3)•Rayleigh scatter–Bacteria: 0.1-1 micron•Absorb blue light–Phytoplankton: 2-200 micron•Mie scatter–Zooplankton: 100 micron to 20 mm–Inert organic particles: size ~ phytoplankton–Inorganic particles: dust, clay, metal oxides 1-10 micronScattering and water leaving radiance depends on the angle of light illumination10We distinguish between:Inherent and Apparent Optical propertiesIOP: are not influenced by the light fielde.g., absorption coefficients in units m-1AOP: are influenced by the available light fielde.g., reflectance, backscattering11Ocean color sensors: characteristics• First sensors: B&W• Temporal resolution determined by orbit and swath width• Optical resolution defined in terms of numbers of spectral channels and bandwidth• Spatial resolution set by instrument FOV, altitude, and scan methodDifferences between measuring SST and ocean color:Infrared radiometers (like AVHRR) measure radiation emitted from the ocean surfaceAssumes ocean is like a black-body emitter with TB related to actual temperatureMeasures skin temperature only Ocean color sensors do not measure emission – they measure reflectanceHow do we know we’re measuring reflectance, not emission?How do we know we’re measuring reflectance, not emission? Emission by the Earth in the visible is zero.Reflectance of the ocean in the thermal infrared is almost zeroReflectance of the ocean is not only a “skin” phenomenon. Its signal is more complex because the optical depth is much greater and depends on wavelength.Ocean color sensors: characteristicshttp://www.ioccg.org/reports_ioccg.htmlOcean color sensors: characteristics16Ocean color sensors: characteristicsOcean color sensors: characteristicsHyperion hyperspectral sensor on EO-1220 channels18Forward and Inverse modelsForward: IOP Rrs(Hydrolight or non-commercial software)Given what we know isin the water, what do weexpect it to look like?Inverse Rrs(Empirical, analytical, statistical)Given what we see, what can we tell about what is in the water? IOPCDOM490 nm 670 nmConsiderratio 490/670Low: mostly CDOMHigh:mostly phytoplanktonRatio of water leaving radiance ratios in wavelengths 490 nm and 670 nm•Proxy sensitive to relative amount of CDOM vs. phytoplankton (Cahill et al. 2008, Schofield et al. 2009). •In river source plumes, CDOM is dominant optical constituent•As the plume ages, CDOM decreases relative to phytoplankton producing a spectral shift in the remote sensing reflectance CDOMThe MODIS empirical algorithm for chlorophyll-a concentration:RL = log10 ( max [ Rrs – ratio(443/551, 488/551) ] )where Rrs is the remote sensing reflectance, which is the ratioof the water leaving radiance Lw to the solar irradiance at thesurface log10 (Chl-a) = 0.283 – 2.753 RL + 0.659 RL2 + 0.649 RL3 – 1.403 RL424MODIS Sea Surface Temperature, 2000 December 6, 17:05 and MODIS Surface Chlorophyll ConcentrationCZCS image of the Gulf Stream obtained on April 1, 1982, showing a prominent warm-core ring. http://disc.sci.gsfc.nasa.gov/oceancolor/additional/science-focus/classic_scenesRrs(412)/Rrs(555) band ratio yields a reasonably consistent relationship with in situ observations of CDOM absorption across several regions in the Mid-Atlantic continental shelfCan also derive empirical relationship between backscatter and particulate matter in the water. This allows estimation, by satelite, or Particulate Organic Carbon (POC) in the ocean.3435Phytoplankton Bloom in the Arabian SeaNormally a picturesque blue lake surrounded by steep volcanoes and Mayan settlements, Guatemala’s Lake Normally a picturesque blue lake surrounded by steep volcanoes and Mayan settlements, Guatemala’s Lake Atitlán acquired a film of green scum in Oct-Nov 2009. A bloom of cyanobacteria (blue-green algae) spread Atitlán acquired a film of green scum in Oct-Nov 2009. A bloom of cyanobacteria (blue-green algae) spread across the lake in green filaments and strands visible in this simulated-natural-color image from 22 Nov 2009 across the lake in green filaments and strands visible in this simulated-natural-color image from 22 Nov 2009 from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) on NASA’s Terra satellite.from ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) on NASA’s Terra