Task 11 Remote Sensing of Ocean Color and Aerosol Properties NASA Grant (NAS5-00203) Assessment, Validation, and Refinement of the Atmospheric Correction Algorithm for the Ocean Color Sensors Menghua Wang The JCET Annual Report Retreat Port Deposit

1
 Atmospheric Correction using the MODIS SWIR
Bands (1240 and 2130 nm)
Menghua Wang (PI, NASA NNG05HL35I) NOAA/NESDIS/OR
A Camp Springs, MD 20746, USA Support from Wei
Shi UMBC, NOAA/NESDIS/ORA Camp Springs, MD
20746, USA The MODIS Science Team Meeting
January 4-6, 2006, Radisson Plaza Lord Baltimore
Hotel, Maryland
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Status of the Algorithm Modifications and
Refinements

• 1. Wang, M. and W. Shi, Estimation of ocean
  contribution at the MODIS near-infrared
  wavelengths along the east coast of the U.S. Two
  case studies, Geophys. Res. Lett., 32, L13606,
  doi10.1029/2005GL022917 (2005).
• 2. Wang, M., A refinement for the Rayleigh
  radiance computation with variation of the
  atmospheric pressure, Int. J. Remote Sens. (In
  press).
• Status Implemented into the MODIS/SeaWiFS data
  processing.
• 3. Wang, M., Effects of ocean surface
  reflectance variation with solar elevation on
  normalized water-leaving radiance, App. Opt. (In
  press).
• Status Implemented into the MODIS/SeaWiFS data
  processing.
• 4. Wang, M. and W. Shi, Cloud masking for ocean
  color data processing in the coastal regions,
  IEEE Trans. Geosci. Remote Sens. (Submitted).
• Status Developed cloud masking using MODIS SWIR
  bands (1240/1640/2130 nm). Scheme can be easily
  implemented into the MODIS data processing
  system.
• 5. Developed schemes using idea of Wang and
  Gordon (1994) to identify cases for the strongly
  absorbing aerosols and turbid waters with the
  MODIS data.
• Status A poster is presented in this meeting.
  Work is in progress.
• 6. Atmospheric correction using the MODIS SWIR
  bands.
• Status This presentation. Work is in progress.

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Atmospheric Correction
MODIS and SeaWiFS algorithm (Gordon and Wang 1994)

• ?w is the desired quantity in ocean color remote
  sensing.
• T?g is the sun glint contributionavoided/masked/c
  orrected.
• T?wc is the whitecap reflectancecomputed from
  wind speed.
• ?r is the scattering from moleculescomputed
  using the Rayleigh lookup tables (atmospheric
  pressure dependence).
• ?A ?a ?ra is the aerosol and Rayleigh-aerosol
  contributions estimated using aerosol models.
• For Case-1 waters at the open ocean, ?w is
  usually negligible at 750 865 nm. ?A can be
  estimated using these two NIR bands. Ocean is
  usually not black at NIR for the coastal regions.
• Gordon, H. R. and M. Wang, Retrieval of
  water-leaving radiance and aerosol optical
  thickness over the oceans with SeaWiFS A
  preliminary algorithm, Appl. Opt., 33, 443-452,
  1994.

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Atmospheric Correction Longer NIR

• In general, to effect the atmospheric correction
  operationally using the NIR bands at 748 and 869
  nm, or using the spectral optimization with
  measurements from 412-865nm, Case-2 bio-optical
  model that has strongly regional dependence is
  needed.
• At the longer NIR wavelengths (gt1000 nm), ocean
  water is much strongly absorbing and ocean
  contributions are significant less. Thus,
  atmospheric correction may be carried out at the
  coastal regions without using the bio-optical
  model.
• Examples using the MODIS Aqua 1240 and 2130 nm
  data to derive the ocean color products.
• We use the longer NIR (2130 nm) for the cloud
  masking. This is necessary for the coastal
  region waters.

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Water Absorption
6
Water Absorption Relative to 865 nm
Black ocean at the longer NIR bands Absorption
at the longer NIR bands is at least an order
larger than that at the 865 nm
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The Rayleigh-Corrected TOA Reflectance
748 nm
869 nm
1240 nm
1640 nm
Rayleigh-Removed
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Aerosol Single-Scattering Epsilon (l0 865 nm)
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Aerosol Single-Scattering Epsilon (l0 2130 nm)
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Data Processing Using the SWIR Bands

• Software Modifications
• Atmospheric correction package has been
  significantly modified based on SeaDAS 4.6.
• Data structure and format of aerosol lookup
  tables and diffuse transmittance tables have been
  changed.
• With these changes, it is flexible now to run
  with different aerosol models (e.g., absorbing
  aerosols) and with various band combinations for
  atmospheric correction.
• Lookup Tables Generation and Implementation
• Rayleigh lookup tables for the SWIR bands (for
  all MODIS 16 bands).
• Aerosol optical property data (scattering phase
  function, single scattering albedo, extinction
  coefficients) for the SWIR bands (12 models).
• Aerosol radiance lookup tables (12 aerosol
  models) for the SWIR bands. Table structures are
  completely changed (different from the current
  ones).
• Data Processing
• Regenerated MODIS L1B data including all SWIR
  band data (for SeaDAS).
• Developed cloud masking using the MODIS
  1240/1640/2130 nm band.
• For MODIS Aqua, atmospheric correction can be
  operated using 1240/2130 bands, 869/1240 bands,
  and 869/2130 bands.
• Current 8 bands 412, 443, 488, 531, 551, 869,
  1240, and 2130 nm.

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We have carried out vicarious calibration using a
MOBY scene from the standard processing
Vicarious Gains
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Initial Results

• We compare the current MODIS results (downloaded
  directly from Web) and results from algorithm
  using SWIR bands.

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Chlorophyll-a (2004071.1825)
New Processing (1240, 2130 nm)
Standard Processing (748, 869 nm)
March 12, 2004
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Chlorophyll-a (2004071.1825)
New Processing (1240, 2130 nm)
Standard Processing (748, 869 nm)
March 12, 2004
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Three weeks late
Chlorophyll-a (2004096.1820)
New Processing (1240, 2130 nm)
Standard Processing (748, 869 nm)
April 6, 2004
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nLw(443) (2004071.1825)
New Processing (1240, 2130 nm)
Standard Processing (748, 869 nm)
March 12, 2004
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nLw(531) (2004071.1825)
New Processing (1240, 2130 nm)
Standard Processing (748, 869 nm)
March 12, 2004
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nLw(869) (2004071.1825)
nLw(869)
New Processing (1240, 2130 nm)
NIR ocean contributions
March 12, 2004
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Three weeks late
nLw(443) (2004096.1820)
New Processing (1240, 2130 nm)
Standard Processing (748, 869 nm)
April 6, 2004
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Three weeks late
nLw(531) (2004096.1820)
New Processing (1240, 2130 nm)
Standard Processing (748, 869 nm)
April 6, 2004
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nLw(869) (2004096.1820)
nLw(869)
NIR ocean contributions
New Processing (1240, 2130 nm)
Three weeks late
April 6, 2004
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Outer Banks
Outside of Outer Banks
HistogramnLw(412) (2004071.1825)
Standard Processing (748, 869 nm)
New Processing (1240, 2130 nm)
March 12, 2004
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Outer Banks
Outside of Outer Banks
HistogramnLw(443) (2004071.1825)
Standard Processing (748, 869 nm)
New Processing (1240, 2130 nm)
March 12, 2004
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Outer Banks
Outside of Outer Banks
HistogramnLw(488) (2004071.1825)
Standard Processing (748, 869 nm)
New Processing (1240, 2130 nm)
March 12, 2004
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Outer Banks
Outside of Outer Banks
HistogramnLw(531) (2004071.1825)
Standard Processing (748, 869 nm)
New Processing (1240, 2130 nm)
March 12, 2004
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HistogramnLw(869) (2004071.1825)
Chesapeake Bay
Outer Banks
New Processing (1240, 2130 nm)
Open Ocean
SC Coast
March 12, 2004
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An example from the west coast
Chlorophyll-a (2004130.2125)
Standard Processing (748, 869 nm)
New Processing (1240, 2130 nm)
May 10, 2004
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An example from the west coast
nLw(412) (2004130.2125)
Standard Processing (748, 869 nm)
New Processing (1240, 2130 nm)
May 10, 2004
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An example from the west coast
nLw(488) (2004130.2125)
Standard Processing (748, 869 nm)
New Processing (1240, 2130 nm)
May 10, 2004
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An example from the west coast
nLw(531) (2004130.2125)
Standard Processing (748, 869 nm)
New Processing (1240, 2130 nm)
May 10, 2004
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nLw(869) (2004130.2125)
New Processing (1240, 2130 nm)
NIR ocean contributions
May 10, 2004
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Effects of band noises
Chlorophyll-a (2004071.1825)
New Processing (1240, 2130 nm)
New Processing (1240, 2130 nm)
Fixed Model M90
Fixed Model C50
March 12, 2004
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Effects of band noises
nLw(531) (2004071.1825)
New Processing (869, 2130 nm)
New Processing (1240, 2130 nm)
March 12, 2004
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Effects of band noises
nLw(531) (2004071.1825)
New Processing (869, 1240 nm)
Standard Processing (748, 869 nm)
March 12, 2004
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Effects of Band NoiseHistogramnLw(531)(Open
Ocean) (2004071.1825)
Standard
1240, 2130 nm
STD ValueStandard 0.05091240, 2130
0.1177869, 2130 0.0704869, 1240 0.0786
869, 2130 nm
869, 1240 nm
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Conclusions

• It works!
• For the turbid waters in coastal regions, ocean
  is not black at the NIR bands. This leads to
  underestimation of the sensor-measured
  water-leaving radiances with current
  SeaWiFS/MODIS atmospheric correction algorithm.
• Ocean is black for turbid waters at wavelengths
  gt1000 nm, e.g., 1240 and 2130 nm. Thus, the
  longer NIR bands can be used for atmospheric
  correction over the turbid waters. No ocean
  model is needed!
• Future ocean color sensor needs to include
  wavelengths gt 1000 nm with high SNR values.