atmospheric correction for turbid waters in coastal regions: need bands > 1000 nm

Menghua Wang, NOAA/NESDIS/ORA

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

Menghua WangNOAA/NESDIS/ORA

E/RA3, Room 102, 5200 Auth Rd. Camp Springs, MD 20746, USA

[email protected]

The Coastal Ocean Applications and Science Team Meeting September 7-8, 2005, Corvallis, Oregon


Solar Irradiance

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300 500 700 900 1100 1300

( )Wavelength nm

(2002)Thuillier

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


Ocean Color Remote Sensing

Sensor-Measured

Blue ocean

“Green” ocean

From H. Gordon

Atmospheric Correction (removing >90% signals)Calibration (0.5% error in TOA >>>> 5% in surface)


Atmospheric Windows

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TotalH

2O

Ozone

Wavelength (μ )m

UV bands can be used for detecting the absorbing aerosol casesTwo long NIR bands (1000 & 1240 nm) are useful for of the Case-2 waters


The Ocean Radiance Spectrum

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0.1

400 500 600 700 800 900

Case 1, C = 0.1 mg m-3

Case 2, Sediment Dominated

Case 2, Yellow Substance

TOA Reflectance

ρ t(λ)

( )Wavelength nm

80 , M modelτa(865) = 0.1

θ0 = 0o, θ = 45o, Δφ = 90o

TOA Reflectance

Coastal Waters

10-4

10-3

10-2

10-1

300 400 500 600 700 800 900

Mauritania Water (sediment)Alberni Inlet Water (yellow-subs)

( )Wavelength nm

10-5

10-4

10-3

10-2

10-1

400 500 600 700 800 900

C = 0.03 mg/m3

C = 0.10 mg/m3

C = 0.30 mg/m3

C = 1.00 mg/m3

( )Wavelength nm

1 : Case water . (1988) . (2000)Gordon et al and Siegel et al

Open Ocean Waters


Atmospheric Correction

ρw is the desired quantity in ocean color remote sensing. Tρg is the sun glint contribution—avoided/masked/corrected. Tρwc is the whitecap reflectance—computed from wind speed. ρr is the scattering from molecules—computed using the

Rayleigh lookup tables (atmospheric pressure dependence). ρA = ρa + ρra is the aerosol and Rayleigh-aerosol contributions

—estimated using aerosol models. For Case-1 waters at the open ocean, ρw is usually negligible at

750 & 865 nm. ρA 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.

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

MODIS and SeaWiFS algorithm (Gordon and Wang 1994)


SeaWiFS Chlorophyll-a Concentration(October 1997-December 2003)

0.010.101.0010.0Chlorophyll-a Concentration (mg/m3)


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

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412 nm443 nm490 nm510 nm555 nm

[Lw(λ)]

N ( )In situ Data

[Lw(λ)]

N : ( /Unit mW cm2 μ )m sr

= 0.9924, = 0.0536, = 0.9478Slope Intercept R

555 nm

510 nm


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0.1

1

10

100

0.01 0.1 1 10 100

Chl-a (In Situ Data)

Chlorophyll-a Unit: (mg/m3)

Slope = 0.9357, Intercept = 0.0129, R = 0.9225

SeaWiFS Chlorophyll-a Comparison


SeaWiFS Aerosol Optical Thickness Comparison

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1

0.01 0.1 1

443 nm 490 nm 555 nm 670 nm 865 nm

τa(λ) ( )Ground Data

= 0.8812, = 0.0250, = 0.8864Slope Intercept R


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1997 1998 1999 2000 2001 2002 2003 2004

Date

Chl-a

[Lw(443)]

N

τa(865)

α(510)

Global Deep Ocean

Aerosol Parameters

Ocean Color Parameters

- = 0.193 ( Mean Chl a mg m-3)

Mean [Lw(443)]

N = 1.503 (mW cm

-2 μm

-1 sr

-1)

Mean α(510) = 0.277

Mean τa(865) = 0.111 (a)

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.


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Date

Chl-a

[Lw(443)]

N

τa(865)

α(510)

North Central Pacific(b)

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1997 1998 1999 2000 2001 2002 2003 2004

Date

Chl-a

[Lw(443)]

N

τa(865)

α(510)

Equatorial Pacific(c)

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0.20.3

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1

10

1997 1998 1999 2000 2001 2002 2003 2004

Date

Chl-a

[Lw(443)]

N

τa(865)

α(510)

South Pacific(d)

00.1

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1997 1998 1999 2000 2001 2002 2003 2004

Date

Chl-a

[Lw(443)]

N

τa(865)

α(510)

North Pacific(a)


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Chl-a

[Lw(443)]

N

τa(865)

α(510)

South Atlantic


(f)

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Date

Chl-a

[Lw(443)]

N

τa(865)

α(510)

North Indian

Aerosol Parameters

(g)

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Chl-a

[Lw(443)]

N

τa(865)

α(510)

South Indian


(h)

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1997 1998 1999 2000 2001 2002 2003 2004

Date

Chl-a

[Lw(443)]

N

τa(865)

α(510)

North Atlantic

Aerosol Parameters

(e)


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Jan. Feb. Mar. April May June July Aug. Sep. Oct. Nov. Dec.

Month

Chl-a

[Lw(443)]

N

τa(865)

α(510)

North Central Pacific(b)


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Chl-a

[Lw(443)]

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τa(865)

α(510)

Equatorial Pacific

Aerosol Parameters

(c)

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τa(865)

α(510)

South Pacific(d)


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Chl-a

[Lw(443)]

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τa(865)

α(510)

North Pacific

Aerosol Parameters

(a)

α(510) ~ 0.7


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τa(865)

α(510)

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Chl-a

[Lw(443)]

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North Indian

Aerosol Parameters

(g)

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α(510)

North Atlantic

Aerosol Parameters

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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).


Case-2 Water Complications

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-0.001

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400 450 500 550 600 650 700

Wavelengthλ ( )nm

80 , M modelτa(865) = 0.1, θ

0 = 40o, θ = 40o, Δφ = 90ο

ρw(865) ~ 10-5

ρw(865) ~ 10-4

ρw(765) ~ 2 ρ

w(865)

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)

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

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100

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

Wavelength (nm)

Water Absorption (1/cm)


Water Absorption Relative to 865 nm

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800 1000 1200 1400 1600 1800

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

Wavelength (nm)

Ratio of Absorption at 865 nm

1000 nm

1240 nm

1640 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

QuickTime™ and aNone decompressor

are needed to see this picture.

QuickTime™ and aNone decompressor


Identified as clouds using 869 nm

Cloud Masking: Need bands > ~ 1000 nm


Aerosol Single-Scattering Epsilon (λ0 = 865 nm)

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O99M50M70M90M99C50C70C90C99T50T90T99ε

(λ, λ0)

( )Wavelength nm

λ0 = 865 nm, θ

0 = 60o, θ = 45o, Δφ = 90o

(a)



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O99M50M70M90M99C50C70C90C99T50T90T99ε

(λ, λ0)

( )Wavelength nm

λ0 = 1240 nm, θ

0 = 60o, θ = 45o, Δφ = 90o

(b)



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O99M50M70M90M99C50C70C90C99T50T90T99

ε(λ, λ0)

( )Wavelength nm

λ0 = 1640 nm, θ

0 = 60o, θ = 45o, Δφ = 90o

(c)

Spectral aerosol contribution relative to wavelength 1640 nm for 12 models


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.


MODIS-Measured ρTOA vs. Theoretical ρTOA

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.

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MODIS Measured ρt(λ) (%)

MODIS Terra, April 16, 2004λ = 1240 nm, Detector# 6Open Ocean, Granule: 20041071625

(a)

Int = -0.02, Slope = 0.9503, R = 0.97440

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MODIS Measured ρt(λ) (%)

MODIS Terra, April 16, 2004λ = 1640 nm, Detector# 5Open Ocean, Granule: 20041071625

Int = 0.04, Slope = 1.0453, R = 0.9533

(b)


Ocean-Contributed Reflectance (B748,

B865)

MODIS Terra L1b

Vicarious CalibrationB1240 and B1640 at open

ocean

Aerosol Type from B1240 and B1640

Reflectance w/o Ocean Contribution (B748,

B865)

Procedures For the MODIS 748 and 869 nm BandsOcean-Contributed Reflectance Estimation


Histogram of Ocean Contributed Reflectance(Open Ocean)

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748 nm 865 nm

t ρw (λ) (%)

MODIS Terra: 2004071.1515Granule

( )a ( 11, 2004)Open Ocean Regions March

-0.5 0 0.5 1

748 nm 865 nm

t ρw (λ) (%)

MODIS Terra: 2004107.1625Granule

( )b ( 16, 2004)Open Ocean Regions April


748 nm

1240 nm

869 nm

1640 nm

MODIS Terra Granule:20040711515 (March 11, 2004)

The Rayleigh-Corrected TOA Reflectance

Rayleigh-Removed


The NIR Ocean Contributions

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


Real values VS Theoretical Values(20040711825)

MODIS Aqua 1240 nm MODIS Aqua 2130 nm


Ban

d 74

8

Ocean-Contributed Reflectance(AQUA 20040711825)

B1240-2130 Atmospheric Correction B1240-2130 Atmospheric Correction

Ban

d 86

9


Outer Banks: Sediment Dominated Waters

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Outer BanksOpen Ocean

Wavelength (nm)

MODIS Terra (March 11, 2004)Granule: 20040711515

(b)


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Chesapeake Bay (748 nm) Chesapeake Bay (869 nm) Open Ocean (869 nm)

t ρw (λ) (%)

( 16, 2004)MODIS Terra April: 20041071625Granule

( )b

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(748 )Outer Banks nm (869 )Outer Banks nm (869 )Open Ocean nm

t ρw (λ) (%)

( 11, 2004)MODIS Terra March: 20040711515Granule

( )c

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(748 )Outer Banks nm (869 )Outer Banks nm (869 )Open Ocean nm

t ρw (λ) (%)

( 16, 2004)MODIS Terra April: 20041071625Granule

( )d

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t ρw (λ) (%)

( 11, 2004)MODIS Terra March: 20040711515Granule

( )a

Ocean-Contributed Reflectance at NIR

Very significant ocean contributions!


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.


Some Backups


Band Signal/Noise Comparison

QuickTime™ and aTIFF (LZW) decompressor








atmospheric correction for turbid waters in coastal regions: need bands > 1000 nm

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