A Multi-Sensor (MERIS - SeaWiFS - MODIS) Ocean Colour Satellite Matchup Analysis
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Transcript of A Multi-Sensor (MERIS - SeaWiFS - MODIS) Ocean Colour Satellite Matchup Analysis
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
A Multi-Sensor (MERIS - SeaWiFS - MODIS) Ocean Colour Satellite Matchup Analysis
in the Mediterranean Sea (BOUSSOLE Project)
David Antoine1, Fabrizio D'Ortenzio1, Stanford B Hooker2, Guislain Bécu1,3, Bernard Gentili1, Dominique Tailliez1, Alec J. Scott1
1: Laboratoire d’Océanographie de Villefranche, Villefranche sur mer, France2: NASA / GSFC, Greenbelt, MD, USA3: ACRI-st, Sophia Antipolis, France
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
A reminder about BOUSSOLE
- Scientific objective: IOPs et AOPs documentation and understanding (bio-optics research), short-time changes...
- Operational objective: vicarious radiometric calibration of ocean color satellite observations, and validation of the Level-2 geophysical products derived from these observations (e.g., chlorophyll, reflectances...).
Motivation, objectives: establishing a long-term time series of optical properties (IOPs and AOPs), with two parallel objectives:
Strategy: combination of 3 elements:
- A Deep-sea mooring, for continuous collection of data at the surface
- Monthly cruises for the buoy servicing and the collection of data complementary to the buoy data
- An AERONET coastal station, to provide the necessary aerosol parameters
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
The BOUSSOLE sitein the Ligurian Sea (northwestern Mediterranean)
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Water at BOUSSOLE are permanently Case 1 waters
Buoy data
Morel & Maritorena, 2001Upper limit for Case 1 waters
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
The range of bio-optical properties at BOUSSOLE is representative of global Case 1 waters
Thick line : BOUSSOLEThin line and thin dotted line : SeaWiFS global or Med Sea onlyGray : NOMAD
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Range of variability in optical properties:“A field look”
Chl ~ 0.05 mg m-3 (march 2006)
Chl ~ 3 mg m-3 (April 2006)
-35 meters
-4 meters
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Feb March Apr May Jun Jul Sept Oct Nov Dec
2001
2002
2003
2004 SeaWiFS/SIMBIOS « diagnostic data sets »
Range of variability in optical properties:“The satellite view”
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
The BOUSSOLE buoy“transparent-to-swell” taut mooring
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
The BOUSSOLE buoy, some pictures
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Buoy data are in agreement with more“classical data”, i.e., profiling radiometer’s data
Slope is 1r2 is 0.98
Bias is 8 10-4
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Status of deployments
- 3 ½ years of deployment, with a quasi permanent data collection
- Bi-monthly servicing since July of 2007
- One system lost in Feb 2007 (likely a ship collision) – immediately replaced by the companion system only 2 weeks of interruption
- Next rotation is scheduled Sept 2007
- Evolution towards hyper-spectral measurements will start in fall of 2007. Nominal set of data is however preserved for continuity
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Matchup procedures
Essentially follows: Bailey S.W., and P.J. Werdell, 2006: A multi-sensor approach for the on-orbit validation of ocean color satellite data products. Remote Sens. Environ., 102, 12-23. - MERIS: level-2 RR, last reprocessing
- SeaWiFS: level-2 Merged Local Area Coverage (MLAC; until Dec. 2004) or Global Area Coverage (GAC; 2005-2006) data from reprocessing 5 (completed March 18, 2005)
- MODIS-A: level-2 GAC data from reprocessing 1 (completed in February 2005)
- 5x5 pixel box
- Flags: glint, clouds, haze, s<70°, v<60°, clear sky
- Spatial homogeneity is checked
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
3-year time series of w
MERIS
SeaWiFS
MODIS-A
Field data (10am – 2pm, clear sky)
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Full matchup set, using data from Sept 2003 to Sept 2006
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Full matchup set, using data from Sept 2003 to Sept 2006
Band per band
MERIS SeaWiFS MODIS
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
N Number of points
r Mean ratio, defined as
Ni
1i i
i
y
x
N
1
RPD Average relative (signed) percent difference (%), defined as
Ni
1i i
ii
y
)yx(
N
1100
|RPD| Average relative (unsigned) percent difference (%), defined as
Ni
1i i
ii
y
yx
N
1100
r2, m and y Coefficient of determination, slope and intercept of the linear fit
RMSE Root mean square error (or root means square of differences), defined as
Ni
1i
2ii )yx(
N
1
Note the Chl data are log-transformed before computing r2, m, y and RMSE.
Statistics
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Table 1. Match-up statistics for MERIS
l N RPD |RPD| r2m y RMSE
412 19 1.59 59.1 61.7 0.20 0.91 0.00790 0.00893443 66 1.25 24.9 30.9 0.30 0.63 0.00830 0.00492490 69 1.14 13.9 17.9 0.38 0.64 0.00650 0.00273
nLw 510 69 1.21 21.2 23.3 0.21 0.54 0.00630 0.00254560 66 1.22 22.1 25.1 0.34 0.62 0.00300 0.00153665 69 1.67 66.6 75.7 0.05 0.55 0.00070 0.00057681 70 1.59 59.4 64.2 0.29 0.50 0.00080 0.00058
all 428 1.36 36.0 40.7 0.86 1.11 0.00070 0.00316Chl 31 1.17 17.0 46.0 0.78 0.56 -0.60804 0.60432
15 1.09 9.3 41.7 0.87 0.58 -0.48613 0.61219Kd 234 1.00 -0.3 28.2 0.51 0.41 0.02649 0.02212
67 0.89 -10.6 20.4 0.66 0.51 0.02090 0.01745
For Chl and Kd, the first line reports statistics for all possible correspondances between field data and satellite observations
and the second line reports statistics for the set of points that have successfully passed the matchup criteria
Table 2. Match-up statistics for SeaWiFS
l N RPD |RPD| r2m y RMSE
412 79 0.90 -9.7 23.3 0.41 0.79 0.00160 0.00492443 212 0.99 -1.0 16.4 0.41 0.68 0.00470 0.00333490 215 0.97 -2.9 12.7 0.37 0.63 0.00490 0.00227
nLw 510 215 0.92 -7.9 14.9 0.15 0.41 0.00550 0.00204555 215 0.88 -11.7 19.4 0.15 0.42 0.00260 0.00147670 199 0.76 -24.2 50.1 0.02 0.25 0.00030 0.00035
all 1135 0.91 -9.4 22.4 0.87 0.92 0.00010 0.00244Chl 55 1.39 38.9 61.1 0.68 0.61 -0.41214 0.60522
44 1.69 68.5 84.9 0.51 0.45 -0.66199 0.68610Kd 330 1.05 4.5 34.1 0.35 0.45 0.02678 0.02561
240 1.07 6.7 35.2 0.28 0.29 0.03180 0.02387
Table 3. Match-up statistics for MODIS-A
l N RPD |RPD| r2m y RMSE
412 87 0.88 -11.8 24.2 0.55 0.85 0.00050 0.00437443 182 0.97 -3.0 16.8 0.57 0.73 0.00350 0.00320488 188 0.97 -2.9 11.9 0.54 0.70 0.00370 0.00211
nLw 551 188 0.93 -6.6 16.6 0.38 0.59 0.00190 0.00122667 177 0.78 -21.5 42.5 0.05 0.25 0.00030 0.00031
all 822 0.91 -8.7 21.9 0.89 0.91 0.00030 0.00238Chl 58 1.20 19.6 50.2 0.72 0.62 -0.49363 0.61499
31 1.21 20.7 40.9 0.82 0.77 -0.24622 0.42352Kd 344 1.09 9.1 33.0 0.44 0.50 0.02685 0.02344
187 1.08 8.1 35.5 0.31 0.34 0.03143 0.02307
r
r
r
Matchup statistics, using data from Sept 2003 to Sept 2006
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Kd(490) time series
MERIS
SeaWiFS
MODIS-A
Field data
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Kd(490) matchups
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Chlorophyll time series
MERIS
SeaWiFS
MODIS-A
Field data
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Chlorophyll matchups
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Conclusions
- The requirements in terms of accuracy of the atmospheric correction are only met at 443 and 490 nm by the SeaWiFS and MODIS-A products.
- The MERIS products never meet the requirements.
- The water-leaving radiance reflectances provided by the three sensors at 412 nm are severely affected by atmospheric correction errors.
- The uncertainty is significantly reduced for the “blue-to-green” reflectance ratio.
- These results and the matchup statistics are in agreement with the results obtained by two other similar efforts carried out at a coastal site [Zibordi et al., 2006] and globally [Bailey and Werdell, 2006].
Questions:
- Atmospheric corrections are still an issue. Are they however considered as the unique source of these uncertainties?
- Should we move to a vicarious calibration of MERIS?
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
BOUSSOLE web site and data basehttp://www.obs-vlfr.fr/Boussole
ENVISAT symposium, Montreux, Suisse, April 23-27, 2007
Thank you for your attention