Menghua Wang, NOAA/NESDIS/ORA Refinement of MODIS Atmospheric Correction Algorithm Menghua Wang (PI,...

Menghua Wang, NOAA/NESDIS/ORA

Refinement of MODIS Atmospheric Correction Algorithm

Menghua Wang (PI, NASA NNG05HL35I)

NOAA/NESDIS/ORACamp Springs, MD 20746, USA

Support from: Wei Shi UMBC, NOAA/NESDIS/ORA

Camp Springs, MD 20746, USA

The Ocean Color Research Team Meeting April 11-13, 2006, Hyatt Regency Newport, Newport,

Rhode Island


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. and W. Shi, “Cloud masking for ocean color data processing in the coastal regions,” IEEE Trans. Geosci. Remote Sens. (Accepted). Status: Developed cloud masking using MODIS SWIR bands (1240/1640/2130 nm). Scheme can be easily implemented into the MODIS data processing system. 3. Developed schemes to identify cases for the strongly absorbing aerosols and turbid waters with the MODIS SWIR and visible data. Status: A poster is presented in this meeting. Work is in progress. 4. Atmospheric correction using the MODIS SWIR bands. Status: This presentation. Work is in progress. 5. Deriving the MODIS high spatial resolution ocean color data. Status: This presentation. Work is in progress.

Status of the Algorithm Modifications and Refinements


Cloud Masking in Coastal Regions


Atmospheric Correction

w is the desired quantity in ocean color remote sensing. Tg is the sun glint

contribution—avoided/masked/corrected. Twc is the whitecap reflectance—computed from wind speed. r is the scattering from molecules—computed 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 in 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.

t = ρr + ρ A + t ρ wc + Tρ g + tρ w, ρ = π L μ0 F0

MODIS and SeaWiFS algorithm (Gordon and Wang 1994)


Atmospheric Correction: SWIR Bands

At the short wave IR (SWIR) wavelengths (>~1000 nm), ocean water has much strongly absorption and ocean contributions are significantly less. Thus, atmospheric correction can be carried out for coastal regions without using the bio-optical model.

Water absorption for 869 nm, 1240 nm, 1640 nm, and 2130 nm are 5 m-1, 88 m-1, 498 m-1, and 2200 m-1, respectively.

Results from simulations were presented in the Hawaii Ocean Science Meeting in Feb. of 2004.

Examples using the MODIS Aqua 1240 and 2130 nm data to derive the ocean color products are provided.

We use the SWIR band (1240 nm) for the cloud masking. This is necessary for coastal region waters.


Water Absorption

10-4

10-3

10-2

10-1

100

101

102

103

400 600 800 1000 1200 1400 1600 1800 2000 2200

Hale & Querry (1973)Segelstein (1981)Kou et al. (1993)

Wavelength (nm)

(a)

1000 nm

865 nm

1240 nm

1640 nm

2130 nm


748 nm

1240 nm

869 nm

1640 nm

MODIS Terra Granule:20040711515 (March 11, 2004)

The Rayleigh-Corrected TOA Reflectance

Rayleigh-Removed


Aerosol Single-Scattering Epsilon (0 = 865 nm)

0.6

0.7

0.8

0.9

1

2

300 420 540 660 780 900

O99M50M70M90M99C50C70C90C99T50T90T99ε

(, 0)

( )Wavelength nm

0 = 865 nm, θ

0 = 60o, θ = 45o, Δφ = 90o

(a)


Aerosol Single-Scattering Epsilon (0 = 2130 nm)

1

10

300 500 700 900 1100 1300 1500 1700 1900 2100

O99 M50 M70 M90 M99 C50 C70 C90 C99 T50 T90 T99

( )Wavelength nm

0 = 2130 nm, θ

0 = 60

o, θ = 20

o, Δφ = 90

o

(d)


Data Processing Using the SWIR BandsSoftware 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 MODIS 16 bands). Aerosol optical property data (scattering phase function,

single scattering albedo, extinction coefficients) for the SWIR bands (12 models & 16 bands).

The vector aerosol lookup tables (including the polarization effects) (12 aerosol models) for the SWIR bands, and for the MODIS high-spatial resolution 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.

Aqua-MODIS 15 bands: 412, 443, 469, 488, 531, 551, 555, 645,

667, 678, 748, 859, 869, 1240, and 2130 nm.


Vicarious Gains

Wavelength (nm)

Gain= (1240, 2130)

Wavelength (nm)

Gain= (1240, 2130)

412 0.972 667 0.970

443 0.982 678 0.971

469 0.976 748 0.978

488 0.977 859 1.010

531 0.979 869 0.991

551 0.972 1240 1.0

555 0.971 1640 —

645 1.034 2130 1.0

= Derived from MOBY scene 2004039.2320.004

We have carried out vicarious calibration using a MOBY scene from the standard

processing……

We will use the MOBY in situ measurements for vicarious calibration.


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

Normalized water-leaving radiances are derived for 412, 443, 469, 488, 531, 551, 555, 645, 667, 678, 748, 859, and 869 nm.

High spatial resolution products are derived for nLw(469) and nLw(555) (0.5 km) and nLw(645) and nLw(859) (0.25 km).

Initial Results


Chlorophyll-a (2004096.1820)New Processing (1240, 2130 nm)Standard Processing (748, 869 nm)

April 6, 2004

Absorbing Aerosols


nLw(443) (2004096.1820)New Processing (1240, 2130 nm)Standard Processing (748, 869 nm)

April 6, 2004

Absorbing Aerosols


nLw(531) (2004096.1820)New Processing (1240, 2130 nm)Standard Processing (748, 869 nm)

April 6, 2004


Noise Effects: Chl-a (2004096.1820)

New Processing (SWIR 3 km)New Processing (SWIR 1 km)

April 6, 2004


Noise Effects: nLw(531) (2004096.1820)

New Processing (SWIR: 3 km)New Processing (SWIR: 1 km)

April 6, 2004


Ocean NIR Contributions (SWIR: 3 km)

nLw(869)nLw(748)

April 6, 2004


HistogramnLw(443) (2004096.18

20)

Outer Banks Outside of Outer Banks

April 6, 2004

Standard Processing(748, 869 nm)

New Processing(1240, 2130 nm)



20)


April 6, 2004




Histogram Chl-a in Outer Banks(2004096.1820)

April 6, 2004




MODIS High Spatial Resolution Measurements

Lt(645) (0.25 km)Lt(645) (0.25 km)


High Spatial Resolution ProductnLw(645) (0.25 km, SWIR:0.5 km)nLw(645) (1 km, SWIR:1 km)

Chesapeake Bay



Outer Banks


High Spatial Resolution Product

nLw(859)(0.5 km, SWIR: 0.5

km)


High Spatial Resolution ProductnLw(859) (0.25 km, SWIR:0.5 km)nLw(859) (0.25 km, SWIR:0.5 km)


0

1

2

3

4

5

400 500 600 700 800 900

A (Outer Banks)B (Outer Banks)C (Open Ocean)D (Chesapeake Bay)E (Chesapeake Bay)

( )Wavelength nm

. . , 6, 2006U S East Coast April SWIR Bands Method

nLw(645)

BAC

E D

Ocean Spectrum from Visible to NIR for

Various Ocean Waters


Conclusions 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 SWIR bands can be used for atmospheric correction over the turbid waters. No ocean model is needed!

For turbid waters, the Red-NIR nLw contributions are significant and can be useful to derive ocean properties in coastal regions.

To obtain high quality ocean color products, sensor with high (adequate) SNR values is crucial.

Future ocean color sensor needs to include wavelengths > ~1000 nm with high SNR values.

Future works: Calibration/Validation, Dealing with absorbing aerosols, some other details.


Some Backups


High Spatial Resolution Product (SWIR 3 km)

nLw(645) (0.25 km)nLw(645) (1 km)

QuickTime™ and aTIFF (LZW) decompressorare needed to see this picture.QuickTime™ and a

TIFF (LZW) decompressorare needed to see this picture.



nLw(555) (0.5 km)nLw(555) (1 km)



nLw(645) (0.25 km)nLw(645) (1 km)



20)

Chesapeake Bay Outside of Chesapeake Bay

April 6, 2004





20)


April 6, 2004



Menghua Wang, NOAA/NESDIS/ORA Refinement of MODIS Atmospheric Correction Algorithm Menghua Wang (PI,...

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