Optical closure –
Are we there yet?
Ina Lefering
David McKee
Marine Optics and Remote Sensing Group
University of Strathclyde
MASTS ASM, 3rd - 5th September 2014
Introduction
Methods
Results
Conclusion
Optical closure
in-situ IOPs
depth profile
absorption,
attenuation,
backscattering
In-situ radiometry
Ina Lefering
Hydrolight
well established
radiative transfer
model
above surface Ed
sub-surface Eu
MASTS ASM, 4th Sep 2014
modelled
light fields
Ed, Eu, Lu
depth profiles
2
Introduction
Methods
Results
Conclusion
Inherent Optical Properties (IOPs)
•
absorption a(λ), attenuation c(λ),
backscattering bb(λ)
•
Properties of the medium, i.e.
water, phytoplankton, minerals and
coloured dissolved organic
materials (CDOM)
Yellow colour of CDOM
0.6
•
IOPs determine how light
propagates under water and how
light is reflected from the surface
Determine ocean colour remote
sensing signals
0.5
absorption [m -1]
•
PSICAM
photometer
0.4
0.3
0.2
0.1
0
400
500
600
wavelength [nm]
700
800
CDOM absorption
Ina Lefering
MASTS ASM, 4th Sep 2014
3
Introduction
Methods
Results
Conclusion
Absorption coefficients
• absorption is a fundamental physical property of seawater;
important for photochemistry, primary production and solar
heating
• absorption of natural water samples is hard to measure
quantitatively due to scattering from particles
• existing methods, such as spectrophotometer, filter pads or ac-9
are limited by systematic scattering errors
Coloured solutions
(no scattering)
Ina Lefering
Particles in suspension
scatter light in all directions
MASTS ASM, 4th Sep 2014
4
Introduction
Methods
Results
Conclusion
Submersible IOP sensors
ac-9 – reflective tube
absorption meter,
9 wavelengths
Ina Lefering
MASTS ASM, 4th Sep 2014
5
Introduction
Methods
Results
Conclusion
Scattering correction for ac-9
Flat
NIR absorption is negligible
wavelength-independent scattering
Zaneveld
NIR absorption is negligible
wavelength-independent phase function
Semi-empirical based on estimation of ‘true absorption’ from PSICAM
- NIR absorption is NOT zero
Monte Carlo
Ina Lefering
iterative approach
estimation of wavelength dependent VSF based on
scattering and backscattering measurements
measurements
- NIR absorption is NOT zero
MASTS ASM, 4th Sep 2014
6
Introduction
Methods
Results
Conclusion
West Coast of the UK – June 2012
Wide range of different water masses:
CDOM-rich
fresh water
(Loch Lochy)
coccolithophore bloom
(Rockall trough)
sediment dominated
coastal waters
Reasonably clear
case 1 waters
Ina Lefering
MASTS ASM, 4th Sep 2014
7
Introduction
Methods
Results
Conclusion
In-situ radiometry
surface Reflectance
0.025
TriOS – hyperspectral radiometers
Ina Lefering
0.02
0.015
0.01
0.005
0
MASTS ASM, 4th Sep 2014
400
600
wavelength [nm]
800
8
Introduction
Methods
Results
Conclusion
Model performance
10
10
-1
10
-2
10
-3
Zaneveld
-1
modelled R
Monte Carlo
-2
-2
10
measured R
-3
-2
10
measured R
modelled R
modelled R
10
10
10
-1
10
-2
10
-3
Semi-empirical
-2
10
measured R
Ina Lefering
MASTS ASM, 4th Sep 2014
9
Introduction
Methods
Results
Conclusion
Model performance
0.12
Zaneveld
0.1
Monte Carlo
modelled R
0.12
0.08
0.06
0.04
0.02
0
0
0.06
0.12
0.04
0.05
measured R
0.1
Semi-empirical
0.1
0.02
coccolithophore bloom
0
0
0.05
measured R
0.1
modelled R
modelled R
0.1
0.08
0.08
0.06
0.04
0.02
0
0
Ina Lefering
MASTS ASM, 4th Sep 2014
0.05
measured R
0.1
10
Introduction
Methods
Results
Conclusion
IOP corrections
a @ 715nm [m-1]
0.06
Monte Carlo
semi-empirical
0.04
0.02
0
-0.02
1
bb @715
0.02
2
3
4
5
6
7 8 9 10 11 12 13 14 15
station number
3
4
5
6
7 8 9 10 11 12 13 14 15
station number
bb715
0.01
0
1
Ina Lefering
2
MASTS ASM, 4th Sep 2014
11
Introduction
Methods
Results
Conclusion
Conclusion
• Generally good agreement between modelled and measured light
fields over a wide range of different water masses
• ac-9 scattering corrections provide similar levels of optical
closure in most cases
• Underestimation of modelled surface reflectance for stations
with high backscattering coefficient e.g. coccolithophore bloom
• Corresponding results from Monte Carlo ac9 correction point to
scattering phase function as possible source of error
Ina Lefering
MASTS ASM, 4th Sep 2014
12
Introduction
Methods
Results
Conclusion
Future work
• Investigating the degree of optical closure for depth profiles of
Ed, Lu and Eu
• Improving the Monte Carlo correction for waters with high
backscattering coefficient
• Integrating a correction for the backscattering into Monte Carlo
correction
Ina Lefering
MASTS ASM, 4th Sep 2014
13
Introduction
Methods
Results
Conclusion
Thank you
Acknowledgements
References
•
McKee D, Piskozub J, Roettgers R, Reynolds RA (2014). Evaluation and Improvement of an Iterative
Scattering Correction Scheme for in situ Absorption and Attenuation Measurements. Journal of
atmospheric and oceanic technology 30: 1527 - 1541, http://dx.doi.org/10.1175/JTECH-D-1200150.1
•
Röttgers R, McKee D, Wozniak SB (2013). Evaluation of scatter corrections for ac-9 absorption
measurements in coastal waters. Methods in Oceanography 7: 21-39
•
WETLabs, Absorption and attenuation meter (ac-9), User’s Guide.
•
Zaneveld, J. R. V., Kitchen, J. C., Moore, C. M., 1994. The scattering error correction of reflecting-tube
absorption meters. Ocean Optics XII, J. S. Jaffe, Ed., International Society for Optical Engineering (SPIE
Proceedings, Vol. 2258), 44–55.
Ina Lefering
MASTS ASM, 4th Sep 2014
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