Ocean Color Remote Sensing from Space

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Ocean Color Remote Sensing from Space

Lecture in Remote Sensing at 7 May 2007

Astrid BracherRoom NW1 - U3215

Tel. [email protected]

www.iup.uni-bremen.de/~bracher


Basic principles of Ocean Color Remote Sensing(Doerffer et al. 2006)


Absorption, Scattering and Beam Attenuation


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)


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


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 [email protected] 450 500 550 600 650

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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.


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)


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)


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


Comparison of ratio of Reflectances (at 445 nm to 555 nm) to phytoplankton biomass (chl a) measurements

Morel & Antoine MERIS ATBD


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


MERIS – Median Resolution Imaging Spectrometer- Ocean Color Sensor


MERIS true color picture:

A large aquamarine-coloured plankton bloom streches across the length of Ireland in the North Atlantic Ocean


MERIS global chl a (phytoplankton biomass) distribution from algorithm using Rrs[443] / Rrs[560]


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 phytoplankton absorption

photoacclimation + species composition

Requires higher spectral resolution!


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!


(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


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

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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


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


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


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)


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


Ocean Color Satellite Information

Documents

Ocean Color Remote Sensing from Space