Post on 30-Dec-2015
description
Page 1bracher@uni-bremen.de
Ocean Color Remote Sensing from Space
Lecture in Remote Sensing at 7 May 2007
Astrid BracherRoom NW1 - U3215
Tel. 8958bracher@uni-bremen.de
www.iup.uni-bremen.de/~bracher
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Basic principles of Ocean Color Remote Sensing(Doerffer et al. 2006)
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Absorption, Scattering and Beam Attenuation
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Spectral color and wavelength in Nanometer [nm= m-9]
Attenuation by water and water constituents
awas = absorption by waterkwas = attenuation by waterksus = attenuation by suspended
particleskwas = attenuation by phytoplanktonkgelb = attenuation by yellow substance
(dissolved organic matter)
(Modelled with SIRTRAM by Doerffer 1992)
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Marine Phytoplankton
Falkowski et al. Science, 2004
Global Contribution:Plant biomass 1-2% Primary production ~50%
Functional Groups:-Build-up of biominerals (e.g. silicate by diatoms) - Calcifiers (e.g. Emiliania)- Cloud formation (via DMSP: Phaeocystis)- Nitrogen-Fixation (blue algae)- Toxic Algae
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Climate Change and Phytoplankton Composition:
Bering Sea: extraordinarily warm
summer 1997– the first time ever bloom of
calcifying algae
True Color from SeaWiFS
(Napp et al. 2001)
Page 7bracher@uni-bremen.de400 450 500 550 600 650
0.05
0.04
0.03
0.02
0.01
0Sp
ec. p
hyt
op
lan
kto
n a
bso
rpti
on
[m
2 /m
g]
MERISSeaWiFS
---- low chl a, mainly Picoplankton
---- diatom bloom
---- Phaeocystis bloom
Bracher & Tilzer 2001
400 450 500 550 600 650 700wavelength [nm]
PhytoplanktonAbsorb light by pigments (chlorophylls, carotenoids,...) Pigments are excited
Excitation energy used in photosynthesis to make O2 & organic compounds
Basis for marine ecosystem and carbon cycle
Phytoplankton absorption
variable among species and
location!
photoacclimation and community composit.
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Downwelling irradiance attenuation coefficient
Green: 5 mg/l Total substanc
Green: 5 mg/l Total substa m-1
Green: 5 mg/l Total Suspended Matter (TSM), 5 µg/l chl a (phytoplankton), yellow substance ag440= 0.4 m-1
Blue: 0.1 µg/m-3 chl a
(Doerffer et al. 2006)
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Signal depth
Coastal waters (= case-2)Blue-green: 5 mg/l TSM, 5 µg/l chl a, ag440= 0.4 m-1
Open Ocean (= case-1)Blue: 0.1 µg/m-3 chl a
z90 = 1/k
(Doerffer et al. 2006)
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Absorption spectra in case 1 waters forwater, yellow substance and phytoplankton
In case-1 waters: attenuation dominated by phytoplankton, ratio of yellow substance conc. to chl a is constant
while it is not for case-2 (=coastal) waters
Empirical Model for phytoplankton biomass from remote sensingfor case-1 waters
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Comparison of ratio of Reflectances (at 445 nm to 555 nm) to phytoplankton biomass (chl a) measurements
Morel & Antoine MERIS ATBD
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MERIS – Median Resolution Imaging Spectrometer- Ocean Color Sensor
Other Ocean Color Sensors: Coastal-Zone-Color-Scanner (1978-1986), SeaWiFS (1997-), Modis (1999- on TERRA, 2002- on AQUA)MOS, POLDER, GLI, OCTS
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MERIS – Median Resolution Imaging Spectrometer- Ocean Color Sensor
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MERIS true color picture:
A large aquamarine-coloured plankton bloom streches across the length of Ireland in the North Atlantic Ocean
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MERIS global chl a (phytoplankton biomass) distribution from algorithm using Rrs[443] / Rrs[560]
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Water leaving Radiance Reflectance Spectra of North Sea water with first 10 MERIS spectral bands
Chl a from ocean color:
Ratio of reflectance at certain wavebands (blue /green)
But: Differences in phyto- plankton absorption
photoacclimation + species composition
Requires higher spectral resolution!
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Global Models on Marine Primary production• Function of fixed organic carbon to biomass (chl a) & light
• Use data of ocean color satellite sensors (MERIS, MODIS, SeaWIFS,…) on chl a, surface water reflectance and light
penetration depth
• Rarely consider spectral dependency of photosynthesis
primary production modeling:Directly affected: light actually absorbed
Indirectly: influences chl a retrieval from ocean color data
Limited data base on specific phytoplankton absorption (in situ measurements)
Phytoplankton absorption and major phytoplankton groups from space using highly spectrally resolved remote sensing data!
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(Scanning Imaging Absorption Spectrometer for Atmospheric Cartography)UV-VIS-NIR spectrometer on Envisat since 2002 in orbit
•8 high resolution and 6 polarization channels •measures transmitted, reflected and scattered sunlight
• wavelength coverage 220 – 2380 nm at 0.24-1.48 nm resolution•global information within 6 days, >30 km X >30 km resolution
Delivers information on:
-distributions of geophysical parameters in atmosphere
from 0-100 km
ozone depletion, greenhouse effect, air
pollution, climate change
- but now on ocean optics: phytoplankton, vibrational
raman scattering
SCIAMACHY
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Processing of SCIAMACHY nadir spectra with DOASDOAS = Differential Optical Absorption Spectroscopy (Perner and Platt, 1979)Uses differential absorption signal of the molecular absorber in the earthshine spectrum wrt. extraterrestrial
solar irradiance
Ratio Earthshine / Solar irradiance removes instrumental and Fraunhofer features
Input: Absorption cross section for each molecular species in spectral intervalLeast squares fit of DOAS equation based on Beer`s law to observationsSeparation of high- and low frequency absorption features by low order polynomial
Output: Slant column density SCD = number of molecules along average photon path
Op
tica
l de
pth
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Phytoplankton absorption from hyperspectral sensor SCIAMACHY
Differential phytoplankton absorption at high chl a
Clear differential signal from phytoplankton pigments!
--- reference spectrum from in-situ meas. of mixed population (by Bracher & Tilzer 2001)__ DOAS-fit with SCIAMACHY meas.
DOAS fit from 430 to 500 nm - included in analysis: O3, NO2, H2O (both vapor and liquid), Ring and differential
phytoplankton absorption spectrum measured in situ
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DOAS fit of phytoplankton pigment absorption
in vivo Phytoplankton Absorption
Specific In vivo reference spectra yield much better fits than chl a
Clear differential signal from phytoplankton pigments!
Chl a Standard Absorption
(mixed population, dominated by <20µm)
from Bracher and Tilzer 2001
from 430 to 500 nm
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Global Phytoplankton Absorption Fits from SCIAMACHY
http://oceancolor.gsfc.nasa.gov
Compared toMODIS chl a level-3 product
SCIAMACHY DOAS-Fits of phytoplankton absorption
Schl (Fit-Factor)
Monthly Average: 15.Oct-14.Nov 2005
Strong correlation to ocean color chl a !
Schl = slant column of specific phytoplankton absorption
Bracher et al. 2006
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Vibrational Raman Scattering (VRS) from SCIAMACHY
Vountas et al. submitted to Ocean SciencesHigh sensititvity of VRS fitat low chl a
Averages over July 2005
--- model__ SCIA meas.
VRS always accompanied by an elastic scattering process
Proxy for light penetration depth (δ) (transformation to λ of phytoplankton absorption fit)
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Phytoplankton biomass from Ocean colorSCIAMACHY chl a conc. c
First SCIAMACHY phytoplankton biomass determined with DOAS (whole spectrum fit) shows good visual agreement to MERIS
algal-1 chl a product
http://www.enviport.org/merisVountas et al. submitted
from DOAS-Fits of phytopl. absorption (mixed community) and VRS: C = Schl / δ
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Ocean Color Satellite Information