Remote Sensing of the Ocean and Atmosphere John L Wilkin jwilkin rutgers edu IMCS Building Room 214C 609 630 0559 g voice Ocean Color Michael Whitehead Oceans Colour Scene Ocean color sensors Operate in visible wavelengths Typically passive not active instruments 2 What 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 grazed What 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 water What 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 waters What 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 micron Scattering and water leaving radiance depends on the angle of light illumination We distinguish between Inherent and Apparent Optical properties IOP are not influenced by the light field e g absorption coefficients in units m 1 AOP are influenced by the available light field e g reflectance backscattering 10 Ocean color sensors characteristics First sensors B W Optical resolution defined in terms of numbers of spectral channels and bandwidth Temporal resolution determined by orbit and swath width Spatial resolution set by instrument FOV altitude and scan method 11 Differences between measuring SST and ocean color Infrared radiometers like AVHRR measure radiation emitted from the ocean surface Assumes ocean is like a black body emitter with TB related to actual temperature Measures skin temperature only Ocean color sensors do not measure emission they measure reflectance How 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 zero Reflectance 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 characteristics http www ioccg org reports ioccg html Ocean color sensors characteristics Ocean color sensors characteristics 16 Hyperion hyperspectral sensor on EO 1 220 channels Ocean color sensors characteristics Forward and Inverse models Inverse Forward IOP Rrs Rrs IOP Hydrolight or non commercial software Empirical analytical statistical Given what we know is in the water what do we expect it to look like Given what we see what can we tell about what is in the water 18 490 nm CDOM Consider ratio 490 670 Low mostly CDOM High mostly phytoplankton 670 nm Ratio 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 CDOM The 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 ratio of the water leaving radiance Lw to the solar irradiance at the surface log10 Chl a 0 283 2 753 RL 0 659 RL2 0 649 RL3 1 403 RL4 MODIS Sea Surface Temperature 2000 December 6 17 05 and MODIS Surface Chlorophyll Concentration 24 http disc sci gsfc nasa gov oceancolor additional science focus classic scenes CZCS image of the Gulf Stream obtained on April 1 1982 showing a prominent warm core ring Rrs 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 shelf Can 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 34 Phytoplankton Bloom in the Arabian Sea 35 Normally a picturesque blue lake surrounded by steep volcanoes and Mayan settlements Guatemala s Lake Atitl n 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 from ASTER Advanced Spaceborne Thermal Emission and Reflection Radiometer on NASA s Terra satellite
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