Atmospheric Correction using the MODIS SWIR Bands ( 1240 and 2130 nm)

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

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, doi:10.1029/2005GL022917 (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.

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.

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 (>~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 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.

Water Absorption

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

MODIS Terra Granule:20040711515 (March 11, 2004) The Rayleigh-Corrected TOA Reflectance 748 nm 869 nm 1240 nm 1640 nm Rayleigh-Removed

Aerosol Single-Scattering Epsilon (l0 = 865 nm)

Aerosol Single-Scattering Epsilon (l0 = 2130nm)

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.

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

Initial Results We compare the current MODIS results (downloaded directly from Web) and results from algorithmusing SWIR bands.

Chlorophyll-a (2004071.1825) Standard Processing (748, 869 nm) New Processing (1240, 2130 nm) March 12, 2004

Three weeks late …… Chlorophyll-a (2004096.1820) Standard Processing (748, 869 nm) New Processing (1240, 2130 nm) April 6, 2004

nLw(443) (2004071.1825) Standard Processing (748, 869 nm) New Processing (1240, 2130 nm) March 12, 2004

nLw(531) (2004071.1825) Standard Processing (748, 869 nm) New Processing (1240, 2130 nm) March 12, 2004

nLw(869) (2004071.1825) nLw(869) New Processing (1240, 2130 nm) NIR ocean contributions March 12, 2004

Three weeks late …… nLw(443) (2004096.1820) Standard Processing (748, 869 nm) New Processing (1240, 2130 nm) April 6, 2004

Three weeks late …… nLw(531) (2004096.1820) Standard Processing (748, 869 nm) New Processing (1240, 2130 nm) April 6, 2004

nLw(869) (2004096.1820) nLw(869) NIR ocean contributions New Processing (1240, 2130 nm) Three weeks late …… April 6, 2004

Outer Banks Outside of Outer Banks HistogramnLw(412) (2004071.1825) Standard Processing (748, 869 nm) New Processing (1240, 2130 nm) March 12, 2004

HistogramnLw(869) (2004071.1825) Chesapeake Bay Outer Banks New Processing (1240, 2130 nm) Open Ocean SC Coast March 12, 2004

An example from the west coast … Chlorophyll-a (2004130.2125) Standard Processing (748, 869 nm) New Processing (1240, 2130 nm) May 10, 2004

An example from the west coast … nLw(412) (2004130.2125) Standard Processing (748, 869 nm) New Processing (1240, 2130 nm) May 10, 2004

nLw(869) (2004130.2125) New Processing (1240, 2130 nm) NIR ocean contributions May 10, 2004

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

Effects of band noises: nLw(531) (2004071.1825) New Processing (1240, 2130 nm) New Processing (869, 2130 nm) March 12, 2004

Effects of band noises: nLw(531) (2004071.1825) Standard Processing (748, 869 nm) New Processing (869, 1240 nm) March 12, 2004

Effects of Band Noise:HistogramnLw(531)(Open Ocean)(2004071.1825) Standard 1240, 2130 nm STD Value:Standard: 0.05091240, 2130: 0.1177869, 2130: 0.0704869, 1240: 0.0786 869, 2130 nm 869, 1240 nm

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 >~1000 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 > ~1000 nm with high SNR values.

Atmospheric Correction using the MODIS SWIR Bands ( 1240 and 2130 nm)