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Atmospheric Correction for Turbid Waters in Coastal Regions: Need Bands > 1000 nm. Menghua Wang NOAA/NESDIS/ORA E/RA3, Room 102, 5200 Auth Rd. Camp Springs, MD 20746, USA Menghua.Wang@noaa.gov The Coastal Ocean Applications and Science Team Meeting

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Atmospheric correction for turbid waters in coastal regions need bands 1000 nm

Atmospheric Correction for Turbid Waters in Coastal Regions: Need Bands > 1000 nm

Menghua Wang

NOAA/NESDIS/ORA

E/RA3, Room 102, 5200 Auth Rd.

Camp Springs, MD 20746, USA

Menghua.Wang@noaa.gov

The Coastal Ocean Applications and Science Team Meeting

September 7-8, 2005, Corvallis, Oregon


Solar irradiance
Solar Irradiance

Passive Remote Sensing: Sensor-measured signals are all originated from the sun!


Ocean color remote sensing
Ocean Color Remote Sensing

Sensor-Measured

“Green” ocean

Blue ocean

Atmospheric Correction (removing >90% signals)

Calibration (0.5% error in TOA >>>> 5% in surface)

From H. Gordon


Atmospheric windows
Atmospheric Windows

UV bandscan be used for detecting the absorbing aerosol cases

Two long NIR bands (1000 & 1240 nm)are useful for of the Case-2 waters


Atmospheric correction for turbid waters in coastal regions need bands 1000 nm

The Ocean Radiance Spectrum

TOA Reflectance

Open Ocean Waters

Coastal Waters


Atmospheric correction
Atmospheric Correction

MODISand SeaWiFS algorithm (Gordon and Wang 1994)

  • wis thedesired quantity in ocean color remote sensing.

  • Tgis the sun glint contribution—avoided/masked/corrected.

  • Twcis the whitecap reflectance—computed from wind speed.

  • risthe scattering from molecules—computed using the Rayleigh lookup tables (atmospheric pressure dependence).

  • A = a+rais the aerosol and Rayleigh-aerosol contributions —estimated using aerosol models.

  • For Case-1 waters at the open ocean,wis usually negligibleat750 & 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.


Seawifs chlorophyll a concentration october 1997 december 2003
SeaWiFS Chlorophyll-a Concentration (October 1997-December 2003)


Seawifs experiences demonstrate that the atmospheric correction works well in the open oceans
SeaWiFS experiences demonstrate that the atmospheric correction works well in the open oceans.


Seawifs chlorophyll a comparison
SeaWiFS Chlorophyll-a Comparison


Seawifs aerosol optical thickness comparison
SeaWiFS Aerosol Optical Thickness Comparison


Seawifs global deep ocean results
SeaWiFS Global Deep Ocean Results

Wang, M., K. Knobelspiesse, and C. R. McClain, “Study of the SeaWiFS aerosol optical property data over ocean in combination with the ocean color products,” J. Geophys. Res., 110, D10S06, doi:10.1029/2004JD004950.


Atmospheric correction for turbid waters in coastal regions need bands 1000 nm

Turbid Waters: Examples from MODIS Data

(Short Wave Infrared Bands for Coastal Regions)

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. Letters,32, L13606, doi:10.1029/2005GL022917 (2005).


Atmospheric correction for turbid waters in coastal regions need bands 1000 nm

Case-2 Water Complications

For productive ocean waters at coastal regions, the ocean is usually not black at the NIR wavelengths at 765 and 865 nm. In these cases, the ocean contributions at the NIR are often mistakenly accounted as radiance scattering from atmosphere, thereby leading to over-correction of atmospheric radiance and underestimation of water-leaving radiance at the visible.

Examples of atmospheric correction for the non-black ocean at the NIR bands.

Negative nLw


Atmospheric correction short wave infrared swir
Atmospheric Correction: Short Wave Infrared (SWIR)

  • In general, to effect the atmospheric correction operationally using the NIR bands at 765 and 865 nm, or using the spectral optimization with measurements from 412-865nm, Case-2 bio-optical model that has strongly regional & temporal dependences is needed.

  • At the short wave infrared (SWIR) wavelengths (>~1000 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 1240 and 1640 nm data to derive the ocean contributions at the NIR bands.

  • We use the SWIR (1640 nm) for the cloud masking. This is necessary for the coastal region waters.



Water absorption relative to 865 nm
Water Absorption Relative to 865 nm

Black Ocean at the SWIR bands:Absorption at the SWIR bands is an order larger than that at the 865 nm


The rayleigh corrected toa reflectance
The Rayleigh-Corrected TOA Reflectance

Identified as clouds using 869 nm

Cloud Masking:Need bands > ~ 1000 nm


The rayleigh corrected toa reflectance1
The Rayleigh-Corrected TOA Reflectance

Identified as clouds using 869 nm

Cloud Masking:Need bands > ~ 1000 nm


The rayleigh corrected toa reflectance2
The Rayleigh-Corrected TOA Reflectance

Identified as clouds using 869 nm

Cloud Masking:Need bands > ~ 1000 nm




Aerosol single scattering epsilon 0 1640 nm
Aerosol Single-Scattering Epsilon ( 0 = 1640nm)

Spectral aerosol contribution relative to wavelength 1640 nm for 12 models


Data processing using swir bands
Data Processing Using SWIR Bands

Lookup Tables Generation and Implementation:

  • Rayleigh lookup tables for the SWIR bands.

  • Aerosol optical property data (scattering phase function, single scattering albedo, extinction coefficients) for the SWIR bands (12 models same as for SeaWiFS/MODIS).

  • Aerosol lookup tables (12 aerosol models--same as for SeaWiFS/MODIS) for the SWIR bands.

    Data Processing:

  • Developed cloud masking using the 1640 nm band. This is necessary for the high-productive waters (e.g., coastal regions).

  • Implemented the sun glint mask.


Atmospheric correction for turbid waters in coastal regions need bands 1000 nm

MODIS-Measured TOAvs. TheoreticalTOA

Open Ocean, MODIS Terra Granule 20041071625

Need vicarious calibration for 1240 and 1640 nm bands:Adjusting the gains so that the intercept=0 and slope=1.


Atmospheric correction for turbid waters in coastal regions need bands 1000 nm

MODIS Terra L1b

Vicarious Calibration

B1240 and B1640 at open ocean

Aerosol Type from B1240 and B1640

Reflectance w/o Ocean

Contribution (B748, B865)

Ocean-Contributed

Reflectance (B748, B865)

Procedures For the MODIS 748 and 869 nm Bands

Ocean-Contributed Reflectance Estimation



The rayleigh corrected toa reflectance3

MODIS Terra Granule:20040711515 (March 11, 2004)

The Rayleigh-Corrected TOA Reflectance

748 nm

869 nm

1240 nm

1640 nm

Rayleigh-Removed


The nir ocean contributions
The NIR Ocean Contributions

Very large ocean contribution at the NIR bands in coastal regions with significant spatial & temporal variations.


Atmospheric correction for turbid waters in coastal regions need bands 1000 nm

Real values VS Theoretical Values

(20040711825)

MODIS Aqua 2130 nm

MODIS Aqua 1240 nm


Atmospheric correction for turbid waters in coastal regions need bands 1000 nm

Band 748

Band 869

Ocean-Contributed Reflectance

(AQUA 20040711825)

B1240-2130 Atmospheric Correction

B1240-2130 Atmospheric Correction


Atmospheric correction for turbid waters in coastal regions need bands 1000 nm

Outer Banks: Sediment Dominated Waters


Atmospheric correction for turbid waters in coastal regions need bands 1000 nm

Ocean-Contributed Reflectance at NIR

Very significant ocean contributions!


Conclusions
Conclusions

  • Both SeaWiFS/MODIS provide high quality ocean color products in the open oceans.

  • 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 >~1000 nm. Thus, the longer NIR bands can be used for atmospheric correction over the turbid waters.

  • In addition, we need longer NIR bands for cloud masking in the coastal regions.

  • We need longer NIR bands (> ~1000 nm) for GOES-R HES-CW for atmospheric correction for turbid waters in coastal regions.