ZhongPing Lee, University of Massachusetts Boston

Transcription

1 ZhongPing Lee, University of Massachusetts Boston

2 Absorption properties Scattering properties

3 ocean (water) color

4 light within water medium Lu(ϴ,ϕ) (Voss et al 2007) (Kirk 1994) They are modulated by water constituents!

5 Boundary conditions Water constituents IOPs color(λ)

6 Primary IOPs: absorption Transfer of energy scattering Redistribution of energy backscattering Scattering has angular dependence

7 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 sin( ) d sin( ) d beam attenuation coefficient: c = a + b (m-1)

8 IOPs are additive. a = a w + a xi b = b w + b xi

9 1. absorption properties a = a w + a xi Very detailed: (Stramski et al 2001)

10 Practical (and common) division: a = a w + a p + 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

11 a w spectrum

12 aw

13 a w (Mobley 1994)

14 Uncertainties of a w : (Pope and Fry, 1997) (Morel et al 2007)

15 (Lee et al 2000)

16 a w is temperature and salinity dependent (Pegau et al 1997)

17 a p spectrum Bricaud and Stramski (1990)

18 a d spectrum a d a d ( ) e 0 S d ( ) 0 S d : ~ nm -1

19 a ph = a p a d spectrum

20 Separated by size (Ciotti et al 2002)

21 By species or groups Coccolithophore diatoms (Balch et al 1991) (cyanobacteria) (Dupouy et al 2008) (Dierssen et al 2006)

22 Separated by pigments (Bricaud et al 2004)

23 Modeling a ph spectrum Example of one parameter hyperspectral a ph (λ) model: a Bricaud et al (1995): ph ( ) A ph ( ) Chl 1 B ph ( ) Lee (1994); Lee et al (1998): ( ) a 0( ) a1( a ph )ln( P) P P = a ph (440)

24 Example of two parameter model: (Ciotti et al 2002)

25 Multiple parameter model: (Hoepffner and Sathyendranath, 1993)

26 Package effect Specific absorption/scattering coefficient = Concentration normalized absorption/scattering coefficient a * ph a ph Chl b * ph b ph Chl Chl specific optical property (Bricaud et al 1995)

27 Increase of absorption is NOT linearly proportionally to Chl concentration! (Bricaud et al 1995)

28 Simplified case: a S W V * a S 1 a ph W V d

29 a g spectrum Absorption spectra of yellow substance (gelbstoff) (Bricaud et al 1981)

30 a g spectrum a g a g ( ) e 0 S g ( ) 0 S g : ~ nm -1 (Kirk 1994) (Carder et al 1989)

31 (Twardowski et al 2004)

32 Slope changes with wavelength range (Twardowski et al 2004)

33 Power-law model for a g spectrum: (Twardowski et al 2004)

34 Values of a ph and a g of natural waters (Kirk 1994)

35 aw adg Contrast of absorption spectra of optically active components aph a_tot aw aph adg a_tot Wavelength [nm] aw aph 0.3 adg a_tot aw aph adg a_tot Wavelength [nm]

36 2. Scattering properties

37 Size distribution (Stramski and Kiefer 1991)

38 b = b w + Very detailed: b xi b b = b w + b bxi (Stramski et al 2001)

39 Commonly used terms for scattering: Molecules Suspended particles Bubbles Turbulence Or, b b w b p b b w b PIM b POM

40 Scattering of water molecules VSF of pure water (β w ) 2 β w (Ω)/β w (90 o ) Scattering angle ( o ) b w = 2 b bw

41 Spectral dependence Morel 1974: Shifrin: 1988 w w βw is also found salinity dependent; its value could be ~30% higher for marine waters.

42 Value and spectrum of seawater b bw : b bw 450 ( ) (Morel 1974) b bw 450 ( ) (Zhang et al 2009)

43 Volume Scattering Function with particles (Petzold 1972)

44 MASCOT measurements (Sullivan and Twardowski, 2009)

45 MVSM measurements (Lee and Lewis, 2003)

46 (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; ~

47 b ~ bp and refractive index Twardowski et al (2001)

48 Examples of β model Henyey-Greenstein (1941) 1 4 (1 g g 2g cos ) 1.5 Beardsley and Zaneveld (1969) ~ (1 f 1 4 cos ) (1 b cos ) 4 Fournier and Forand (1994)

49 Spectrum of scattering coefficient (Bricaud et al 1988) vary weakly with wavelength b( ) b( ) r r (Gould et al 1999)

50 b b spectrum contrast Backscattering coefficient [m -1 ] Wavelength [nm] pure seawater particles particles b bw : ~ m -1 b bp ( ) b η: ~

51 bubbles (Zhang et al 2002) Not known the spectral characteristics of bubble scattering, considered spectrally flat

52 Organic vs inorganic separation (Stavn and Richter 2008)

53 (Stramski et al 2001)

54 (Prog. Oceanog. 28, , 1991)

55 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.

56 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.