ocean color
TRANSCRIPT
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SeaDAS Training ~ NASA Ocean Biology Processing Group
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Satellite observations of ocean color
NASA Ocean Biology Processing Group
Goddard Space Flight Center, Greenbelt, Maryland, USA
SeaDAS Training Material
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Ocean color:
monitoring the oceans in the visible range of the electromagnetic spectrum
Primary (historical) goal:
to extract concentrations of marine phytoplankton
Phytoplankton:
fix carbon dioxide into organic material
play a profound role in the global carbon cycle and climate
responsible for ~half of Earth net primary production
form the basis of the marine food chain
support various industries, primarily fisheries
Secondary (modern) goals:
separate phytoplankton species (e.g. coccolithophore, harmful algae)
monitor coastal environments
Satellite ocean color
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( play MODIS-Terra swath movie )
Satellite ocean color
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Near-polar orbits enable low-altitude imaging and global daily coverageOrbital plane crosses the poles and is situated at high inclination to the Earth's rotation
Sun-synchronous orbits cross the equator at the same local time Pass over any given latitude at almost the same local time during each orbital pass
Near-polar sun-synchronous orbits
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Ocean color instruments are passive sensors
They measure electromagnetic radiation reflected or emitted by the Earth surface
( Compare to an active sensor, such as a LIDAR )
Reflective solar bands (MODIS 20 bands: 0.41 – 2.1m)
Thermal emissive bands (MODIS 16 bands: 3.7 – 14.4m)
MODIS-Aquapassive sensor
Passive sensors
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• primary optical variable:• normalized water-leaving radiances (nLw)
– the subsurface upwelled radiance which propagates through the sea-air interface;
– units: W cm-2 sr-1 nm-1
• primary bio-optical variable:
• chlorophyll-a concentration (Chl)– main photosynthetic pigment of
phytoplankton, used as index of phytoplankton biomass;
– units: mg m-3
412 670 745 865555510490443 Primary products
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visible light
wavelength (nm)
near-infraredultra-violet
443412 490 510 555 670 765 865
8 SeaWiFS channels
radiance, L, in units of W cm-2 nm-1 sr-1
surface
0º
reflectance, R =
L
incident irradiance, E
Satellite ocean color
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Chesapeake Bay Program
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photosynthetic pigment reflectance hinge point
absorption peak for photosynthetic pigment (medium-high concentrations)
absorption peak for particles,detritus, and dissolved substances
case-1/2 separation, absorbing aerosols
sediments, turbidity
atmospheric correction
absorption peak for photosynthetic pigment, fluorescence of elevated chlorophyll
absorption peak for photosynthetic pigment (low-medium concentrations)
350 400 450 500 550 600 650 700 750 800 850 900
Spectral characteristics of oceanic waters
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IOP (Inherent Optical Properties)
Medium properties that depend only on the composition of this medium, regardless of light conditions.
Examples are scattering (b), absorption (a), and fluorescence.
AOP (Apparent Optical Properties)
Characteristics of the medium dependent on geometric distribution of the light field and on the medium IOPs. They change with varying illumination conditions, such as solar zenith and azimuth angles.
Examples are irradiance (E), radiance (L), reflectance (R), diffuse attenuation coefficient (K), which depend on the surface boundary conditions.
IOPs and AOPs
photons have two fates whenthey travel through a medium:
(1) absorbed, a(2) scattered, b (backwards, bb)
€
Rrs(λ ) ≈ bb (λ )a(λ )
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photons have two fates whenthey travel through a medium:
(1) absorbed, a(2) scattered, b (backwards, bb)
€
Rrs(λ ) ≈ bb (λ )a(λ )
Relative concentrations of water-column constituents
Ref
lect
ance
chlorophyll-a concentrations
pure sea water
phytoplankton
CDOM
pure sea water
Abs
orpt
ion
[arb
itrar
y un
its]
Abs
o rpt
ion
pure sea water
particulate material
nback
scat
t erin
g
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Case 1water where the optical properties are determined primarily by phytoplankton and their derivative products
Case 2everything else, namely water where the optical properties are significantly influenced by other constituents, such as mineral particles, CDOM, or microbubbles, whose concentrations do not covary with the phytoplankton concentration
Morel Case-1 versus Case-2 water
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SEA SURFACE
TOP-OF-THE-ATMOSPHERE
the satellite views the spectral light field at the top-of-the-atmosphere
SATELLITE
PHYTOPLANKTON
1. remove atmosphere from total signal to derive estimate of light field emanating from sea surface (water-leaving radiance, Lw)
2. relate spectral Lw to Chl (or geophysical product of interest)
3. spatially / temporally bin and remap satellite Chl observations
Satellite ocean color
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L1Auncalibrated raw digital counts
L1Bcalibrated and geolocated
radiances
L2normalized water-leaving
radiances
From digital counts to radiances
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F0
nLwnw =
wavelength (nm)
chlo
roph
yll r
efle
ctan
ce
412 nm 443 nm 488 nm 531 nm 551 nm 667 nm 678 nm
Water-leaving radiances
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SeaWiFS observes El Niño / La Niña transition
January 1998
July 1998
chlorophyll-a concentration
0.01-64 mg m-3
Local coverage chlorophyll-amap provided to fishermen
MODIS-Aqua daily global coverage, 1 August 2007
chl-a
SST
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( insert biosphere movie here )
Satellite ocean color
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atmosphere is 80-90% of the total top-of-atmosphere signal in blue-green wavelengths (400-600 nm)
Satellite ocean color
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different water masses, different Lw …one Chl algorithm?one atmospheric correction approach?
Satellite ocean color
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1. empirical (statistical) algorithms
2. semi-analytical algorithms
photons have two fates whenthey travel through a medium:
(1) absorbed, a(2) scattered, b (backwards, bb)
€
Rrs(λ ) ≈ bb (λ )a(λ )
€
Rrs(λ ) → bb , a → Ca
least squares empirical
Bio-optical algorithms
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general form of algorithmlog10(Ca) = (c0 + c1 R + c2 R2 + c3 R3 + c4 R4)where R is log10(Rrs / Rrs555)
wavelengths usedOC4 = 443 > 490 > 510 / 555OC3 = 443 > 490 / 555OC2 = 490 / 555Clark = 490 / 555Carder = 490 / 555
principle differencesdevelopment data set (Rrs and Ca)coefficients / regression
Empirical algorithms
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⎟⎟⎠⎞
⎜⎜⎝⎛
++⎟⎟⎠
⎞⎜⎜⎝⎛
+=
b
b
b
brs ba
bgba
bgR 10
2
Rrs == remote sensing reflectancea == absorption coefficientbb == backscattering coefficientg == constant
a separated into contributions by:water (w) , dissolved + non-algal detrital material (dg), and phytoplankton ()
bb separated into contributions by:water (w), and particles (p)
(simplification of the radiative transfer equation)
Semi-analytical algorithms
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a = aw + adg e -S( + a* Chl
bb = bbw + bbp ⎟⎠⎞⎜⎝
⎛
443
⎟⎟⎠⎞
⎜⎜⎝⎛
++⎟⎟⎠
⎞⎜⎜⎝⎛
+=
b
b
b
brs ba
bgba
bgR 10
2
Rrs
S, , g0, g1, & a*
from satellite(s)
are constants
adg bbp Chl are unknown
Semi-analytical algorithms
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some challenges to remote sensing of coastal and inland waters:
temporal and spatial variabilitylimitations of satellite sensor resolution and repeat frequencyvalidity of ancillary data (reference SST, wind)varied resolution requirements and binning options
straylight contamination from land
non-maritime aerosols (dust, pollution)region-specific models requiredabsorbing aerosols
suspended sediments and CDOMcomplicates estimation of Lw(NIR), model not a function of Ca
complicates correction for non-uniform subsurface light field (f/Q)saturation of observed radiances
anthropogenic emissions (NO2 absorption)
Ocean color on regional scales
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In-water algorithms ...
Ocean color on regional scales
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1. empirical (statistical) algorithms
2. semi-analytical algorithms
photons have two fates whenthey travel through a medium:
(1) absorbed, a(2) scattered, b (backwards, bb)
€
Rrs(λ ) ≈ bb (λ )a(λ )
€
Rrs(λ ) → bb , a → Ca
least squares empirical
regional tuning?
Bio-optical algorithms
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N
1 km
South Brittany, France
Vilaine river plume – June 99
Spectral bands: 412, 443, 467, 489, 509, 531, 555, 599, 621, 667, 681, 705, 753, 772, 865, 899
RGB (705-555-489 nm)
Noctiluca scintillans Clear water
1
Increasing suspended
matter conc.
1
3
6
2
4 5
2
3
6
4
5
RGB (620-555-489 nm) Mouth of the Vilaine river
R2b
47°N
2°W Regional chlorophyll algorithms
chl-a
IFREMER Centre de Brest, France F. GOHIN, J.N. DRUON, and L. LAMPERT
modified algorithmSeaWiFS OC4
imaged aerial
transect
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Lw(412)
1
2
-1
-2
0
negative
Does coverage vary by algorithm?
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O
100
10
1
0.1
empirical semi-analyticalregional semi-analytical
Coverage varies by algorithm
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Atmosphere issues …
Satellite ocean color
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OMI-Aura Tropospheric NO2
Other unaccounted atmospheric gasses: CO2, sulfates
MODIS-Aqua RGB
Correction for NO2 absorption
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Global aerosol models
NIR (765 and 865-nm) used to estimate aerosol contributons to the total top-of-atmosphere radiance.
Is the operational suite of aerosol models sufficient to describe all potential geophysical scenarios?
models defined using:scattering phase functionsingle-scattering albedoaerosol optical thicknessrelative humidities (particle size)
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Single scattering albedoMaritime and absorbing dust aerosols
• Operational aerosol models− purely reflective or very weakly
absorbing− overestimate the atmospheric
contribution in the VIS when absorbing aerosols are present
• Absorbing aerosols − most are eliminated by the cloud albedo
threshold on band 869nm − the ones which pass the test cause
negative water leaving radiances and increased chlorophyll levels
• Absorbing aerosol flagging
0 = b / ( a + b )
Absorbing aerosols
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nLw(412) and in situ measurements Chl and in situ measurements
The scene passed 869nm cloud threshold criterionnLw(412) is decreased and Chl is elevated compared to in situ measurements
Effects of absorbing aerosols
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Location of AERONET CIMEL sun-photometers
Coastal aerosols
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Satellite vs. In Situ
AERONET sites
operational aerosol models
Ångström exponent – spectral shape of aerosols
Operational models do not exhibit the spectral distributions exhibited by coastal aerosolsLarge share of coastal aerosols are composed of small particles (soot, biomass burning)Large share of coastal aerosols are moderately and strongly absorbing
Coastal aerosols
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Satellite vs. in situ AOT time-series in Chesapeake Bay
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Satellite vs. in situ AOT match-ups in Chesapeake Bay
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Case-1 waters and absorbing aerosols* application of the entire VIS and NIR spectrum (412-865 nm)* simultaneous derivation of ocean optical properties and a set of aerosol models
including weakly and strongly absorbing types
Gordon, Du, Zhang, Applied Optics, vol. 36, no. 33, 1997 (best fit for A, ta, chl-a, bb)Chomko and Gordon, Applied Optics, vol. 37, no. 24, 1998 (nonlinear spectral
optimization based on simplified aerosol models) Chomko and Gordon, Applied Optics, vol. 40, no. 18, 2001 (nonlinear spectral
optimization applied to SeaWiFS data)Chomko, Gordon, Maritorena, Siegel, Remote Sensing of Environment, vol. 5775, 2002
(spectral optimization based on simplified aerosol models and complex water-reflectance models)
Turbid coastal waters• sequential atmospheric correction and water-leaving radiance retrieval
Gao, Montes, Ahmad, Davis, Applied Optics, vol. 39, no. 6, 2000
Simultaneous retrieval of atmospheric and oceanic properties
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Satellite ocean color