Radiative Transfer Theory, Atmospheric Correction and Ocean Color

• Locates and enables monitoring of regions of high and low bio-activity.
  • Food (phytoplankton associated with chlorophyll)
  • Climate (phytoplankton possible CO₂ sink)
• Reveals ocean current structure and behavior.
  • Seasonal influences
  • River and Estuary influences
  • Boundary currents
• Reveals Anthropogenic influences (pollution)
• Remote sensing reveals large and small scale structures that are very difficult to observe from the surface.

Correcting for Effects due to the Atmosphere

• The Problem

  When viewed from above the atmosphere (from a satellite) the ocean appears dark. (A low level signal) The color and brightness variation in the image of the ocean, which (among other things) reveals the variation of the concentration of chlorophyll, is swamped by the sun's light scattering in the atmosphere. To see the chlorophyll in the ocean the images must have the light coming from the atmosphere removed. (A more extreme but similar example of this problem is our inability to see the stars in the daytime sky. The daylight sky is bright enough that one cannot see all but the brightest stars (close supernova) or brightest planets (Venus at its most brilliant). Another familiar example of this problem is looking outside at night from inside a well lit room through a window. The window will reflect a small amount (4%) of the light from the inside, but even this small signal, because of it's relative brightness, will wipe out your ability to see much of the poorly lit outside image.)
  
  This figure shows four typically observed wavelength bands and the water leaving radiance in high (dotted) and low (solid) chlorophyll waters without the atmospheric signal (lower curves) and with the atmospheric signal (upper curves). The satellite observes the water leaving radiance with the signal due to the atmosphere (upper curves).

  The atmosphere produces 80-90% of the radiance seen by a satellite when looking at the ocean, only 10-20% of the signal comes from the ocean.

  
  In this figure, several different light pathways in the atmosphere are illustrated:
  a) The light path of the water-leaving radiance. b) Shows the attenuation of the water-leaving radiance. c) Scattering of the water-leaving radiance out of the sensor's FOV. d) Sun glint (reflection from the water surface). e) Sky glint (scattered light reflecting from the surface). f) Scattering of reflected light out of the sensor's FOV. g) Reflected light is also attenuated towards the sensor. h) Scattered light from the sun which is directed toward the sensor. i) Light which has already been scattered by the atmosphere which is then scattered toward the sensor. j) Water-leaving radiance originating out of the sensor FOV, but scattered toward the sensor. k) Surface reflection out of the sensor FOV which is then scattered toward the sensor. L_w Total water-leaving radiance. L_r Radiance above the sea surface due to all surface reflection effects within the IFOV. L_p Atmospheric path radiance. (This figure is adapted from Robinson, I.S., 1983: Satellite observations of ocean colour, Philo. Trans. Royal Soc. of London, Series A, Volume 309, 338-347.)

• Atmospheric Correction Methods

  • Develop Theoretical Atmosphere. Include:
    • Rayleigh Scattering - (Strongest in Blue region)
    • Ozone
    • Aerosols - (Absorption and Scattering Characteristics)
  • Use Data from Infrared (IR) band and assume that all of this signal comes from the atmosphere to get knowledge of aerosols.
  • Solve Radiative Transfer Equation
    • Geometry
    • Location (types of aerosols possible)
  Other considerations:
  • Sun Glint. Avoid - Use wind speed to estimate surface roughness.
  • White Caps. Measure - Use wind speed to estimate coverage.
  A comparison of raw and corrected images.

• Some Results

  These methods were first used on the CZCS (Coastal Zone Color Scanner) instrument flying on the Nimbus-7 satellite from Nov '78 to Jun '86 producing these
  
  and other classic images from CZCS

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• NASA's Earth Observatory

  • Links to images of various atmospheric, oceanic, terrestrial and biospheric observations by NASA and its international partners.
    • Build your own animation (requires Quicktime) of global sea surface temperatures from NASA's (AVHRR) data sets.

• Sea-viewing Wide Field-of-view Sensor (SeaWiFS)

• Ocean Color and Temperature Sensor (OCTS)

• Moderate Resolution Imaging Spectroradiometer (MODIS)

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