Atmospheric Correction of Ocean Color RS Observations

Menghua Wang, IOCCG Lecture Series--Atmospheric Correction

!Atmospheric Correction of Ocean Color

RS Observations

Menghua Wang

NOAA/NESDIS/STARE/RA3, Room 102, 5200 Auth Rd.Camp Springs, MD 20746, USA

IOCCG Summer Lecture Series, Villefranche-sur-Mer, France, July 2-14, 2012

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1. Introduction! Brief history! Basic concept of ocean color measurements! Why need atmospheric correction

2. Radiometry and optical properties! Basic radiometric quantities! Apparent optical properties (AOPs)! Inherent optical properties (IOPs)

3. Optical properties of the atmosphere! Molecular absorption and scattering! Aerosol properties and models

— Non- and weakly absorbing aerosols— Strongly absorbing aerosols (dust, smoke, etc.)

4. Radiative Transfer! Radiative Transfer Equation (RTE)! Successive-order-of-scattering method! Single-scattering approximation

Lecture Outlines (1)

5. Atmospheric Correction! Define reflectance and examine the various terms! Sea surface effects! Atmospheric diffuse transmittance! Normalized water-leaving radiance! Single-scattering approximation! Aerosol multiple-scattering effects! Open ocean cases: using NIR bands for atmospheric correction! Coastal and inland waters

— Brief overviews of various approaches— The SWIR-based atmospheric correction

! Examples from MODIS-Aqua measurements

6. Addressing the strongly-absorbing aerosol issue! The issue of the strongly-absorbing aerosols! Some approaches for dealing with absorbing aerosols! Examples of atmospheric correction for dust aerosols using MODIS-Aqua

and CALIPSO data

7. Requirements for future ocean color satellite sensors

8. Summary

Lecture Outlines (2)

IOCCG Report #10 at http://www.ioccg.org/reports_ioccg.html“Atmospheric Correction for Remotely-Sensed Ocean ColourProducts”.

Reference citations from my presentation can be found from thereport.

1. Introduction! Brief history! Basic concept of ocean color measurements! Why need atmospheric correction

Introduction

" Color photographs of the oceans were obtained from spacecraft (Apollo, etc.,1960’s).

" Clarke et al. (1970) showed the systematic measurements of radiance spectra ofsea (ocean color) from aircraft, demonstrated the possibility of detecting thechlorophyll concentration within the ocean upper layers. They also showedatmospheric effects on the measured signals.

" Tyler & Smith (1970) performed field measurements of upward & downwardirradiance spectra in different water bodies. Thereafter, many similar in situ wateroptical measurements.

" Interpretation of in situ reflectance measurements was given in the frame ofradiative transfer by Gordon et al. (1975) and also in terms of optically-significantwater substances by Morel and Prieur (1977) and Smith and Baker (1978).

" Gordon (1978; 1980) developed a single-scattering atmospheric correctionalgorithm for processing the NASA CZCS ocean color data, demonstrating thefeasibility of satellite ocean color remote sensing.

" Advanced atmospheric correction algorithm (Gordon and Wang, 1994) has beendeveloped for various more sophisticated ocean color satellite sensors, e.g.,SeaWiFS, OCTS, MODIS, MERIS, etc., following the successful CZCS proof ofconcept mission.

" In recent years, atmospheric correction effort has been on dealing with morecomplex water properties in coastal and inland waters, as well as strongly-absorbing aerosols.

Brief History

IOCCG Report-10

Satellite ocean color remote sensing is possible because

" Ocean color (spectral radiance/reflectance) data--water-leaving radiance/reflectance spectra--can be related to waterproperties, e.g., chlorophyll-a concentration. In other words,there exist various relationships between water opticalproperty and water biological / biogeochemical et al.properties.

" It is possible to carry out atmospheric correction for derivingaccurate water radiance/reflectance spectra data from satellitemeasurements.

Satellite Ocean Color Measurements

Menghua Wang, IOCCG Lecture Series--Atmospheric Correction Chesapeake BayLake Taihu

Ocean Color Spectra

Satellite-Measured Reflectance (Radiance)

!t "( ) = !r "( )Rayleigh!

+ !A "( )Aerosols"#$

+ t "( )t0 "( )Transmittance" #% $%

!w "( )#$ %&N

Ocean" #% $%

Satellite-Measured Ocean Contribution" #%%% $%%%

! "( ) = # L "( ) µ0 F0 "( )

!path "( ) = !r "( ) + !A "( )!a "( )+!ra "( )!"#

Normalized water-leaving radiance:

Lw !( )"# $%N, nLw !( ) mWcm-2

µm-1 str-1

Normalized water-leaving reflectance:

!w "( )#$ %&N, !wN "( ) unitless

Remote sensing reflectance:

Rrs !( ) = "wN !( ) /# str-1

Ocean Color Remote Sensing

Sensor-Measured

Blue ocean

“Green” ocean

From H. Gordon

Atmospheric Correction (removing >90% sensor-measured signals)

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

Ocean Contributions:

Case-1 Waters

400 450 500 550 600 650 700

C = 0.03 mg/m 3

C = 0.10 mg/m 3

C = 0.30 mg/m 3

C = 1.00 mg/m 3

[ !w ("

Wavelength (nm)

Case-1 Water: Gordon et al. (1988)

400 500 600 700 800 900

Sediment WatersYellow-Substance

Wavelength (nm)

Examples of Case-2 Water

Ocean Contributions:

Case-2 Waters

IOCCG Report-10

Satellite Sensor Measured TOA Reflectance Spectra

400 500 600 700 800 900

Case 1, C = 0.1 mg/m 3

Case 2, Sediment DominatedCase 2, Yellow Substance

Wavelength (nm)

M80 model, #a(865) = 0.1

$0 = 60o, $ = 45o, %& = 90o

IOCCG Report-10

The TOA Ocean Contributions

400 500 600 700 800 900

Case 1, C = 0.1 mg/m 3

Case-2, Sediment DominatedCase-2, Yellow Substance

Wavelength (nm)

M80 model, #a(865) = 0.1

$0 = 60o, $ = 45o, %& = 90o

Ocean TOA Radiance Contributions for Case-1 Waters

400 500 600 700 800 900

Case-1, C = 0.1 mg/m 3

Rayleigh, Black Ocean

Wavelength (nm)

M80 model, !a(865) = 0.1

Ratio "r(#)/"

Ratio "path

(#)/"t(#)

$0 = 60o, $ = 45o, %& = 90o

Reflectance

RatioIOCCG Report-10

For a typical Case-1 (open ocean) water:Satellite-Measured water-leaving reflectancecontributions at the TOA are 6.4%, 9.8%, 12.1%,10.8%, 5.5%, and 1.3% at wavelengths of 412, 443,490, 510, 555, and 670 nm.

IOCCG Report-10

Ocean TOA Radiance Contributions for

Sediment-type Case-2 Waters

400 500 600 700 800 900

Case-2, Sediment

Rayleigh, Black Ocean

Wavelength (nm)

M80 model, !a(865) = 0.1

Ratio "r(#)/"

Ratio "path

(#)/"t(#)

$0 = 60o, $ = 45o, %& = 90o

flecta

IOCCG Report-10 17

For a typical sediment-dominated water:Satellite-Measured water-leaving reflectancecontributions at the TOA are 5.8%, 11.2%, 26.3%,30.6%, 39.0%, 16.4%, 3.7%, and 2.4% at wavelengthsof 412, 443, 490, 510, 555, 670, 765, and 865 nm.

IOCCG Report-10

400 500 600 700 800 900

Case-2, Yellow Subs.Rayleigh, Black Ocean

Wavelength (nm)

M80 model, !a(865) = 0.1

Ratio "r(#)/"

Ratio "path

(#)/"t(#)

$0 = 60o, $ = 45o, %& = 90o

Signals are

very small

IOCCG Report-1019

Ocean TOA Radiance Contributions for

Yellow Substance-type Case-2 Waters

For a yellow-substance-dominated water:Satellite-Measured water-leaving reflectancecontributions at the TOA are 0.2%, 0.4%, 1.2%, 1.7%,3.6%, 5.3%, 1.2%, and 1.0% at wavelengths of 412,443, 490, 510, 555, 670, 765, and 865 nm.

It is very difficult to do atmospheric correction at theblue bands.

IOCCG Report-10

At satellite altitude~90% of sensor-measured signal over ocean

comes from the atmosphere & surface!

Factor of 1:10 from TOA to the surface

• It is crucial to have accurate atmospheric correction andsensor calibrations.

• 0.5% error in atmospheric correction or calibrationcorresponds to possible of ~5% error in the derived oceanwater-leaving radiance.

• We need ~0.1% sensor calibration accuracy.

Atmospheric Correction

Ocean Color Remote Sensing: Derive theocean water-leaving radiance spectra byaccurately removing the atmospheric andsurface effects.

Ocean properties: e.g., chlorophyll-aconcentration, diffuse attenuation coefficientat 490 nm, can be derived from the oceanwater-leaving radiance spectra.

References

Austin, R. W. (1974), The remote sensing of spectral radiance from below the ocean surface, in Optical Aspects ofOceanography, edited by N. G. Jerlov and E. S. Nielsen, pp. 317-344, Academic, San Diego, Calif.

Bricaud, A., and A. Morel (1987), Atmospheric correction and interpretation of marine radiances in CZCS imagery: useof a reflectance model, Oceanologica Acta, SP, 33-50.

Clarke, G. K., G. C. Ewing, and C. J. Lorenzen (1970), Spectra of backscattered light from the sea obtained from aircraftas a measure of chlorophyll, Science, 167, 1119-1121.

Gordon, H. R., O. B. Brown, and M. M. Jacobs (1975), Computed relationship between the inherent and apparent opticalproperties of a flat homogeneous ocean, Appl. Opt., 14, 417-427.

Gordon, H. R. (1978), Removal of atmospheric effects from satellite imagery of the oceans, Appl. Opt., 17, 1631-1636.

Gordon, H. R. (1980), A preliminary assessment of the NIMBUS-7 CZCS atmospheric correction algorithm in ahorizontally inhomogeneous atmosphere, in Oceanography from Space, edited by J. F. R. Gower, pp. 281-294,Plenum Press, New York and London.

Gordon, H. R., and D. K. Clark (1981), Clear water radiances for atmospheric correction of coastal zone color scannerimagery, Appl. Opt., 20, 4175-4180.

Gordon, H. R., and A. Morel (1983), Remote assessment of ocean color for interpretation of satellite visible imagery: Areview, 114 pp., Springer-Verlag, New York.

Gordon, H. R., and M. Wang (1994), Retrieval of water-leaving radiance and aerosol optical thickness over the oceanswith SeaWiFS: A preliminary algorithm, Appl. Opt., 33, 443-452.

Morel, A. (1980), In-water and remote measurements of ocean color, Boundary-layer Meteorology, 18, 177-201.

Morel, A., and L. Prieur (1977), Analysis of variations in ocean color, Limnol. Oceanogr., 22, 709-722.

Smith, R. C., and K. S. Baker (1978), The bio-optical state of ocean waters and remote sensing, Limnol. Oceanogr., 23,247-259.

Smith, R. C., and W. H. Wilson (1981), Ship and satellite bio-optical research in the California Bight, in Oceanographyfrom Space, edited by J. F. R. Gower, pp. 281-294, Plenum Press, New York and London.

Tyler, J. E., and R. C. Smith (1970), Measurements of spectral irradiance underwater, 103 pp., Gordon and BreachScience Publishers.

2. Radiometry and optical properties! Basic radiometric quantities! Apparent optical properties (AOPs)! Inherent optical properties (IOPs)

Radiometry and Optical Properties

Radiant Power:

P !( ) = Nhc

!/ t = Energy / t N " # of photons

Radiance is Apparent Optical Property (AOP).Ocean color sensors measure Radiance!

Definition of Radiance L:

L !( ) ="

2P !( )

cos# "$"A

Apparent Optical Properties

Absorption coefficient a:

a !( ) ="P !( )

P !( )"l

Scattering coefficient b:

Inherent Optical Properties (IOPs) (1)

b !( ) ="P !( )

P !( )"l

Extinction coefficient c:

c !( ) ="P !( )

P !( )"l= a !( ) + b !( )

Volume Scattering Function:

! "( ) =#2

P $( )

P $( )#%#l

Scattering phase function P:

Inherent Optical Properties (IOPs) (2)

P !( ) =" !( )

! "( )4#$ d% =

&P '( )

P '( )&l( b '( )

Definition of Scattering Angle

The single scattering albedo is a dimensionlessparameter which represents the percentage of photonsbeing scattered in the single scattering case:

!0 "( ) =b "( )

c "( )

Single-Scattering Albedo

Optical Thickness (e.g., aerosol optical thickness (AOT),Rayleigh optical thickness, cloud optical thickness, etc.)

! "( ) = c #z ,"( )0

$ d #z

3. Optical properties of the atmosphere! Molecular absorption and scattering! Aerosol properties and models

— Non- and weakly absorbing aerosols— Strongly absorbing aerosols (dust, smoke, etc.)

Optical Properties of the Atmosphere

Molecular (Rayleigh) Scattering

" The scattering of air molecules is purely electric dipole incharacter.

" The scattering phase function is symmetry, i.e., equalamounts of radiance are scattered into the forward andbackward directions.

" The amount of scattered radiance varies nearly as theinverse fourth power of wavelength (blue sky).

Molecular (Rayleigh) Scattering

Rayleigh Phase Function: Pr !( ) "1+ cos2!

The Rayleigh optical thickness is (Hansen & Travis, 1974):

! r ",P0( ) = 0.008569"#4 1+ 0.0113"#2+ 0.00013"#4( )

P0 is the standard atmospheric pressure of 1013.25 hPa, andfor a pressure P,

! r ",P( ) = ! r ",P0( )P

Atmospheric Pressure

Variation

Usually, it is within~±3%

Haze C Aerosol Model

Shettle and Fenn (1979)Aerosol Models

Aerosol Optical Property

The complex refractive index of aerosol particles is:m = mr - i mi,

mr is the real refractive index, e.g., ~1.33 for water in the visible,and mi the index related to particle absorption.

" Aerosol particles can be characterized with particle sizedistribution and particle refractive index.

" Given the size distribution, refractive index of the individualparticles, and assuming that the particles are spherical,aerosol optical properties can be computed using the Mietheory. These properties are:• The scattering phase function P(!)

• The single scattering albedo "0

• The extinction coefficient c

Aerosol Phase Function

Aerosol Optical Thickness

Shettle and Fenn (1979)Aerosol Models

! a "( )

! a "0( )=

&'() " ,"0( )

) ","0( ) = ln! a "( )! a "0( )

The Ångström Exponent # (particle size)

Small aerosol particle size has large # value, while largeparticle has small # value.

2cos!P !( )

# sin!d! =1

2µ P µ( )

The Asymmetry Parameter g (phase function)

The value of g ranges between -1 for entirely backscatteredlight to +1 for entirely forward-scattered light. Generally, g = 0indicates scattering direction evenly distributed betweenforward and backward directions, e.g., isotropic scattering.

Shettle and Fenn (1979) aerosol models used (12 models):Oceanic with relative humidity (RH) of 99%Maritime with RH of 50%, 70%, 90%, and 99%Coastal with RH of 50%, 70%, 90%, and 99%Tropospheric with RH of 50%, 90%, and 99%

Other models from AERONET measurements (Ahmad et al., 2010)

Aerosol Models

Characteristics of the Aerosol Models

Aerosol Model Single Scattering Albedo a(865)

Asymmetry Parameter g

Ångström Exponent (510, 865)

Oceanic! 1.0 0.724-0.840 -0.087~ -0.016

Maritime! 0.982-0.999 0.690-0.824 0.09-0.50

Coastal!! 0.976-0.998 0.682-0.814 0.23-0.76

Tropospheric! 0.930-0.993 0.603-0.769 1.19-1.53

Urban! 0.603-0.942 0.634-0.778 0.85-1.14

Dust!!! 0.836-0.994 0.662-0.763 0.29-0.36

! Shettle and Fenn (1979) aerosol models. !!Gordon and Wang (1994) !!! Shettle (1984) and Moulin et al. (2001).

Strongly-absorbing aerosols

4. Radiative Transfer! Radiative Transfer Equation (RTE)! Successive-order-of-scattering method! Single-scattering approximation

Radiative Transfer

The RTE is simply an accounting for the changes in radiancealong a path.

First, attenuation produces a loss:

L + !L =P + !P

P 1# c!l( )

The Radiative Transfer Equation (RTE)

dL !̂( )dl

= "c L !̂( )

Second, scattering produces a gain:

dL !̂( )dl

= " ˆ#! $ !̂( )#%& L ˆ#!( )d% ˆ#!( )

The RTE:

dL !̂( )dl

= "cL !̂( ) + # ˆ$! % !̂( )$&' L ˆ$!( )d& ˆ$!( )

For the atmosphere-ocean system:

cos!dL z,!,"( )

dz= #c z( )L z,!,"( )

+ $ z, %! , %" &!,"( )%'( L z, %! , %"( )d %'

The RTE:

Can be written as:

cos!dL z,! ,"( )

dz= #c z( )L z,! ,"( ) + $ z, %! , %" & ! ,"( )

( L z, %! , %"( )d %'

cos!dL z,! ,"( )

c z( )dz= #L z,! ,"( ) + $

0z( ) P z, %! , %" & ! ,"( )

( L z, %! , %"( )d %'

cos!dL " ,! ,#( )

d"= $L " ,! ,#( ) + %

0"( ) P " , &! , &# ' ! ,#( )

) L " , &! , &#( )d &(

where " = c &z( )0

) d &z is the optical thickness

See section from Wang (1991).

Successive-order-of-scattering method

Wang, M. (1991), “Atmospheric Correction of the Second Generation of Ocean ColorSensors,” Ph.D. Dissertation, 135 pp., University of Miami, Coral Gables, Florida, USA.

See section from Wang (1991).

Single-Scattering Approximation

!as "( ) =#0 "( )$ "( )

4 cos% cos%0

P &' ,"( ) + r %( ) + r %0( )( )P &+,"( )() *+

,#0 "( )$ "( )P &' ,"( )

Fresnel reflectivity of the surface

Wang, M. (1991), “Atmospheric Correction of the Second Generation of Ocean ColorSensors,” Ph.D. Dissertation, 135 pp., University of Miami, Coral Gables, Florida, USA.

Aerosol reflectance (radiance)

!"a #( )$ a #( )Pa %&,#( )

AOT can be routinely measured, butNOT Phase Function

For non- and weakly absorbingaerosols, aerosol reflectance: !" a #( )Pa $

%,#( )

Aerosol Single-Scattering Epsilon

! ","0( ) =#

as"( )

a"( )%

a"( ) P

',"( ) + r (( ) + r (0( )( )P

+,"( )[ ]

$a"0( )% a

"0( ) Pa&

',"0( ) + r (( ) + r (0( )( )P

+,"0( )[ ]

Spectral variation of aerosol reflectance

Characteristics of the Aerosol Models

Aerosol Model Single Scattering Albedo a(865)

Asymmetry Parameter g

Ångström Exponent (510, 865)

Oceanic! 1.0 0.724-0.840 -0.087~ -0.016

Maritime! 0.982-0.999 0.690-0.824 0.09-0.50

Coastal!! 0.976-0.998 0.682-0.814 0.23-0.76

Tropospheric! 0.930-0.993 0.603-0.769 1.19-1.53

Urban! 0.603-0.942 0.634-0.778 0.85-1.14

Dust!!! 0.836-0.994 0.662-0.763 0.29-0.36

! Shettle and Fenn (1979) aerosol models. !!Gordon and Wang (1994) !!! Shettle (1984) and Moulin et al. (2001).

Strongly-absorbing aerosols Non- and weakly-absorbing aerosols

Aerosol Single-Scattering Epsilon (!0 = 865 nm)

300 420 540 660 780 900

O99M50M70M90M99C50C70C90C99T50T90T99! (

Wavelength (nm)

"0 = 865 nm, #

0 = 60

o, # = 45

o, $% = 90

Epsilon and Aerosol Reflectance

! ","0( ) =#

as"( )

"0( )$ exp c "0 % "( ){ }

"(!, !0) can be computed from two bands (e.g., NIR bands) #as(!).Thus, #as(!) values in other wavelengths (e.g., visible bands) canbe estimated:

!as "( ) = # ","0( ) $ !as "0( )

A simple atmospheric correction (aerosol correction) scheme(Wang and Gordon, 1994)--Single-scattering approximation.

Questions?

Radiance $ ?

Atmospheric Correction of Ocean Color RS Observations

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