Absorption properties Scattering properties
ocean (water) color
light within water medium Lu(ϴ,ϕ) (Voss et al 2007) (Kirk 1994) They are modulated by water constituents!
Sensor measures E d L w [Chl] [CDOM] [SPM] IOPs Inherent Optical Properties (IOPs) a: absorption coefficient = a w + a xi b b : backscattering coefficient = b bw + c: beam attenuation coefficient (a+b) b bxi
Boundary conditions Water constituents IOPs color IOPs (Inherent Optical Properties): The optical capability regardless of the ambient light environment. Absorption properties; Scattering properties
Definition of absorption and scattering coefficients Δr P P IOP process 1 P P process r a 1 P P absorption r b 1 P P scattering r Units: Δr: infinitesimal (m) a&b: m -1 a = 1.2 m -1 b = 3.5 m -1
Energy transfer processes: photons absorption Transfer of energy scattering Redistribution of energy backscattering Scattering has angular dependence
Scattering Rayleigh scattering Mie scattering d << λ d >> λ
Scattering Elastic scattering In-elastic scattering (e.g., Raman scattering)
Scattering (no wavelength change) (google) (wavelength change)
absorption coefficient: a (m -1 ) Volume Scattering Function (VSF): β (m -1 sr -1 ) Scattering coefficient: b (m -1 ) forward-scattering coefficient: b f (m -1 ) b f b 2 0 sin( ) d backward-scattering coefficient: b b (m -1 ) b b 2 / 2 2 0 / 2 sin( ) d sin( ) d beam attenuation coefficient: c = a + b (m -1 )
a = a w + IOPs are additive. a xi b = b w + b xi
1. absorption properties a = a w + a xi Very detailed: (Stramski et al 2001)
Practical (and common) division: a = a w + a p + a g (google) (google) a = a w + a ph + a d + a g
a = a w + a ph + a d + a g Pure water (seawater): a w Particulate: a p = a ph +a d Pigments of living phytoplankton: a ph Detritus: a d Gelbstoff (yellow substance; colored dissolved organic matter): a g
a w spectrum (Mobley 1994)
a w spectrum
a w (Mobley 1994)
a w [m -1 ] Uncertainties of a w : 0.020 0.015 0.010 P&F_97 S&F_97 M_07 C_11 This study smoothed 0.005 0.000 360 380 400 420 440 460 480 500 Wavelength [nm] (Lee et al 2014) (Pope and Fry, 1997)
(Mason et al, not published)
a w [m -1 ] NFD Spectrum Rrs [sr -1 ] In the NIR-SWIR range 200 150 S_81 K_93 H&Q_73 0.10 0.05 0.00 NFD Rrs x 5 NFD 1/S_81 NFD 1/K_93 sample Rrs 0.06 0.04 100-0.05 0.02 50-0.10 0 800 1000 1200 1400 Wavelength [nm] -0.15 0.00 700 800 900 1000 1100 1200 1300 S_81: Segelstein 1981 K_93: Kou et al 1993 H&Q_73: Hale and Querry 1973 Wavelength [nm] (Lee et al 2016)
a w is temperature and salinity dependent (Pegau et al 1997; Sullivan et al 2006)
a p spectrum Bricaud and Stramski (1990) (google)
a d spectrum a d a d ( ) e 0 S d ( ) 0 S d : ~0.005 0.015 nm -1 Bricaud and Stramski (1990)
a ph = a p a d spectrum Bricaud and Stramski (1990)
Separated by size (Ciotti et al 2002) fatness
By species or groups Coccolithophore diatoms (Balch et al 1991) (cyanobacteria) (Dupouy et al 2008) (Dierssen et al 2006)
Contribution of various pigments (Bricaud et al 2004)
Package effect Increase of absorption is NOT linearly proportionally to Chl concentration! a * ph a ph Chl Specific absorption/scattering coefficient = Concentration normalized absorption/scattering coefficient (Bricaud et al 1995) Chl specific optical property
Simplified case: S: cross section V: volume W: weight a W S V * a S 1 a ph W V d Size matters on efficiency!
a g spectrum Absorption spectra of yellow substance (gelbstoff) (Bricaud et al 1981)
a g spectrum a g a g ( ) e 0 S g ( ) 0 S g : ~0.01 0.03 nm -1 (Kirk 1994) (Carder et al 1989)
(Twardowski et al 2004)
Slope changes with wavelength range (Twardowski et al 2004)
Power-law model for a g spectrum: (Twardowski et al 2004)
Values of a ph and a g of natural waters (Kirk 1994)
0.7 0.6 0.5 0.4 0.25 0.2 0.15 0.1 0.05 aw adg Contrast of absorption spectra of optically active components aph a_tot 0.3 0.2 0.1 0 400 450 500 550 600 aw aph adg a_tot 0 400 450 500 550 600 650 700 Wavelength [nm] 0.7 0.6 0.5 0.4 0.3 0.4 0.35 aw aph 0.3 adg a_tot 0.25 0.2 0.15 0.1 0.05 0 400 450 500 550 600 0.2 0.1 aw aph adg a_tot 0 400 450 500 550 600 650 700 Wavelength [nm]
2. Scattering properties
Size distribution (Stramski and Kiefer 1991)
b = b w + Very detailed: b xi b b = b w + b bxi (Stramski et al 2001)
Commonly separated groups for scattering: Molecules Suspended particles Bubbles Turbulence Or, b b w b p b b w b PIM b POM
β w (Ω)/β w (90 o ) Scattering of water molecules VSF of pure water (β w ) 2 1.6 1.2 0.8 0.4 0 0 50 100 150 200 Scattering angle ( o ) b w = 2 b bw
Spectral dependence Morel 1974: Shifrin: 1988 w w 450 0 450 0 4.17 4.32 βw is also found salinity dependent; its value could be ~30% higher for marine waters.
Value and spectrum of seawater b bw : b bw 450 ( ) 0.0023 4.32 (Morel 1974) b bw 450 ( ) 0.0020 4.3 (Zhang et al 2009)
Volume Scattering Function with particles (Petzold 1972)
MVSM measurements (Lee and Lewis, 2003)
MASCOT measurements (Sullivan and Twardowski, 2009)
(Mobley 1994) 90 deg β shape changes in a narrow range in the backward domain Particles are strongly forward scatters! Backscattering ratio: b ~ b bb b ~ b ~ b bw bp 0.5; ~ 0.005 0.05
b ~ bp and refractive index Twardowski et al (2001)
Examples of β model Henyey-Greenstein (1941) 1 4 (1 g 2 2 1 g 2g cos ) 1.5 Beardsley and Zaneveld (1969) ~ (1 f 1 4 cos ) (1 b cos ) 4 Fournier and Forand (1994)
Spectrum of scattering coefficient (Bricaud et al 1988) weakly wavelength dependent b( ) b( ) r 0.00113 1.625 0.00113 1.625 r (Gould et al 1999)
Backscattering coefficient [m -1 ] b b spectrum contrast Wavelength [nm] 400 450 500 550 600 650 700 0.02 0.002 pure seawater particles particles 0.0002 b bw : ~0.0001 0.004 m -1 b bp ( ) b η: ~0-2.0 0 0
bubbles (Zhang et al 2002) Not known the spectral characteristics of bubble scattering, considered spectrally flat
Organic vs inorganic separation (Stavn and Richter 2008)
(Stramski et al 2001)
(Prog. Oceanog. 28, 343-383, 1991)
a w spectrum b bw spectrum Band averaged pure water properties R rs ( ) G a w ( ) b a bw x ( ) b ( ) b bp bw ( ) ( ) b bp ( ) 100.0000 0.006 aw 10.0000 bbw 1.0000 350 550 750 950 0.1000 0.0100 0.0010 0.005 0.004 0.003 0.002 0.001 0
0.015 0.012 MERIS R rs [sr -1 ] 0.009 0.006 MODIS SeaWiFS 0.003 0.000 400 450 500 550 600 650 700 750 800 Wavelength [nm] CZCS
OC Satellite band is not a narrow-sharp band 10 SeaWiFS 412nm band SeaWiFS 490nm Band 0.1 1 380 480 580 680 780 880 980 1080 0.01 0.001 0.0001 0.00001 Band-representing pure-water constants for remote sensing?
Band a w [m -1 ] If by this standard approach: a w (i)=integral a w (λ) RSRi λ dλ intergral RSR i λ dλ 0.16 0.12 0.08 SeaWiFS MODIS VIIRS Hyper 0.04 0 400 450 500 550 Wavelength [nm]
R rs [sr -1 ] Consequence is no closure of Rrs 0.025 0.02 0.015 0.01 0.005 Rrs of S.P. gyre Mod. Rrs with MODIS aw 0 400 500 600 700 Wavelength [nm]
) ( ) ( ) ( ) ( ) ( ) ( ) ( bp bw x w bp bw rs b b a a b b G R d RSR d a RSR B x i w i i w ) ( ) ( 1/ ) ( i w i w B x B a 1 (Lee et al 2016) ) ( ) ( ) ( d w rs E L R
Band a w [m -1 ] band-averaged a w for Rrs 0.08 SeaWiFS MODIS VIIRS Hyper 0.04 0 400 420 440 460 480 500 520 540 560 Wavelength [nm]
R rs [sr -1 ] Closure of Rrs 0.025 0.02 0.015 0.01 0.005 Rrs of S.P. gyre Mod. Rrs with MODIS aw 0 400 500 600 700 Wavelength [nm]
b bw B i b bw ( ) RSR RSR i i ( ) d ( ) d
Key points: 1. In addition to boundary conditions, IOPs play the key role in forming ocean/water color. 2. Primary IOPs include absorption and scattering coefficients; the latter is direction dependent. 3. Bulk IOPs are lump sum contributions of the many individual, dissolved and suspended, molecules and particles. 4. Absorption and scattering coefficients of pure (sea)water are considered constant (change with temperature/salinity), but uncertainties still exist, especially for absorption in the UV range.
5. In addition to water molecules, practically and generally, for absorption: there are three major optically active components: phytoplankton pigments, detritus and gelbstoff (CDOM); for scattering: there are organic and inorganic particulates, bubbles, and many times lumped into one term. 6. Spectrally, water molecules are strong absorber in the longer wavelengths; phytoplankton absorption generally has two distinct peaks with a stronger peak centered around 440 nm and weaker peak centered around 675 nm; have varying spectral shapes detritus and gelbstoff are strong absorbers in the shorter wavelengths, and gelbstoff has steeper spectral slope; Water molecules are strong scatter in the shorter wavelengths; particle scattering is weakly wavelength dependent. It is strongly dependent on size, refractive index, and abundance.