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

Menghua Wang, NOAA/NESDIS/ORA

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

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 MODIS Science Team Meeting January 4-6, 2006, Radisson Plaza Lord Baltimore Hotel,

Maryland


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.

Status of the Algorithm Modifications and Refinements


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 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.

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: 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

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


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

10-1

100

101

102

103

104

800 1000 1200 1400 1600 1800 2000 2200

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

Wavelength (nm)

1000 nm

1240 nm

1640 nm

2130 nm

(b)


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 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.


Vicarious Gains

Wavelength (nm)

Gain= (1240, 2130)

Gain= (869, 2130)

Gain= (869, 1240)

412 0.975 0.973 0.969

443 0.980 0.979 0.969

488 0.970 0.971 0.969

531 0.978 0.978 0.970

551 0.960 0.960 0.966

869 0.976 0.976 0.976

1240 1.0 1.0 1.0

2130 1.0 1.0 1.0

= Derived from MOBY scene 2004039.2320.004

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

processing……


We compare the current MODIS results (downloaded directly from Web) and results

from algorithm using SWIR bands.

Initial Results


Chlorophyll-a (2004071.1825)

New Processing (1240, 2130 nm)Standard Processing (748, 869 nm)

March 12, 2004



New Processing (1240, 2130 nm)Standard Processing (748, 869 nm)

Three weeks late ……

April 6, 2004


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

March 12, 2004



March 12, 2004


nLw(869) (2004071.18

25)

New Processing(1240, 2130 nm)

nLw(869)

March 12, 2004

NIR ocean contributions




April 6, 2004


nLw(869) (2004096.18

20)


nLw(869)

April 6, 2004




HistogramnLw(412) (2004071.18

25)Standard Processing(748, 869 nm)


Outer Banks Outside of Outer Banks

March 12, 2004






March 12, 2004



25)


Open Ocean

Outer Banks Chesapeake Bay

SC Coast

March 12, 2004



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




May 10, 2004



nLw(869) (2004130.21

25)


May 10, 2004



Chlorophyll-a (2004071.1825)New Processing (1240, 2130 nm)

March 12, 2004

New Processing (1240, 2130 nm)

Effects of band noises:

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Fixed Model: M90 Fixed Model: C50


nLw(531) (2004071.1825)New Processing (869, 2130 nm)

March 12, 2004

New Processing (1240, 2130 nm)



nLw(531) (2004071.1825)New Processing (869, 1240 nm)

March 12, 2004


Standard Processing (748, 869 nm)


Effects of Band Noise:

HistogramnLw(531)(Open Ocean)

(2004071.1825)

Standard 1240, 2130 nm

869, 2130 nm 869, 1240 nm

STD Value:Standard: 0.0509

1240, 2130: 0.1177

869, 2130: 0.0704

869, 1240: 0.0786


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.

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

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