Fabrizio D’Ortenzio, David Antoine, Guislain Bécu

A Multi-Sensor Ocean Color Satellite Matchup Analysis in the Mediterranean Sea (BOUSSOLE Project)

A Multi Sensor (MERIS, SeaWiFS, MODIS-A) A Multi Sensor (MERIS, SeaWiFS, MODIS-A) Ocean Color Satellite Matchup Analysis in the Ocean Color Satellite Matchup Analysis in the

Mediterranean SeaMediterranean Sea

BOUSSOLE ProjectBOUSSOLE Project

Fabrizio D’Ortenzio, David Antoine, Guislain BécuFabrizio D’Ortenzio, David Antoine, Guislain BécuLaboratoire d’Océanographie de Villefranche, CNRS et Université Pierre et Marie CurieLaboratoire d’Océanographie de Villefranche, CNRS et Université Pierre et Marie Curie

Stanford B. HookerStanford B. HookerNASA/GSFC, Greenbelt, MD 20771 USANASA/GSFC, Greenbelt, MD 20771 USA


Overview of the presentationOverview of the presentation

1. Some elements about the BOUSSOLE project

2. Matchup Procedure and evaluation of the results

3. Conclusions


Description of the BOUSSOLE ProjectDescription of the BOUSSOLE Project

Establish a time series of inherent and apparent optical properties (IOPs and AOPs).

Motivations:

Combination of 3 elements :• A deep-sea mooring, collecting data on a “continuous”

basis• Monthly cruises for collecting data that are not

accessible to the mooring (vertical profiles, water sampling)

• A coastal AERONET station, providing the necessary information about the aerosol properties

Strategy:


Characteristics of the buoy site: Characteristics of the buoy site: physical dynamicsphysical dynamics

Located in the middle of the Cyclonic Circulation of the NW Med

The physical dynamic is prevalently vertical i.e. 1-dimensional

Surface currents are generally weak

Millot, 1999

Mixed-layer depth varies from ~10 to ~200 meters

Marty et al. 2002

BOUSSOLE


Characteristics of the buoy site: Characteristics of the buoy site: biological dynamicsbiological dynamics

Biomass concentration is generally low.

Intense spring bloom is however observed.

Chlorophyll concentration varies from 0.05 to 5 mg/m3

SeaWiFS Chlorophyll Concentration

BOUSSOLE

13 April 199928 May1998


Why a new mooring type?Why a new mooring type?

Irradiance profilers provide Irradiance profilers provide highly noisy data near the highly noisy data near the surface because of the surface because of the effect of capillary and effect of capillary and gravity wavesgravity waves

the horizontal gradient in the horizontal gradient in E or L is often an order-of-E or L is often an order-of-magnitude larger than the magnitude larger than the vertical gradient vertical gradient

(see, e.g., (see, e.g., Zaneveld et al., Zaneveld et al., Appl. Opt., 40(9), 2001Appl. Opt., 40(9), 2001).).

The data near the surface are often unusable whereas they are the ones that should be used when satellite validation

is concerned.

Depth

Dep

th

Irradiance 490Irradiance 490


The new type of buoy ensures:

•the verticality of the instruments,

•the stability of the instruments,

•the minimisation of shading of the instruments by the buoy structure.

Why a new mooring type?Why a new mooring type?

The solution we have adopted:

Integration over time with a sensor maintained at a fixed position and depth

See Poster #204 of Bécu et al. for a complete description of the BOUSSOLE programme.


Surface irradiance (Es) Downwelling irradiance (Ed) Upwelling irradiance (Eu)Upwelling radiance at nadir (Lu)

Measurement suite, instrumentationMeasurement suite, instrumentation

Attenuation coefficientBackscattering coefficient Chlorophyll fluorescenceTemperature,PressureSalinityBuoy tilt

At 4 and 9 meters At 9 meters

Wetlabs C-star2 s, Hobilabs’ HydroscatChelsea MiniTrackaSeaBird’ SBE37

7 s Satlantic’ OCR-OCI 200

Series

Fully normalized reflectances w (defined as nLw/Es )

Water leaving radiances Lwn

+4 meters

-20 meters


nLw Time SeriesnLw Time Series

All data collected every

15 minutes during daylight

One day of data collection (15-min. resolution)

nLnLww443443

nLnLww490490

nLnLww560560

nLnLww670670

nLnLww490490


nLw Time SeriesnLw Time Series

Data further filtered for :

1. Excessive buoy tilt or depth

2. Sun zenith angle > 70°

nLnLww443443

nLnLww490490

nLnLww560560

nLnLww670670


SeaWiFS (2003-2004)

level-2 MLAC data from reprocessing 5 Obtained at : http://oceancolor.gsfc.nasa.gov/cgi/browse.pl?sen=am

Satellite DataSatellite Data

MERIS (2003-2005)

level-2 1-Km reduced resolution data processed by the MERIS prototype version 7.4.1 (called “MEGS7.4.1”)Obtained at: http://merci-srv.eo.esa.int/merci/welcome.do

AMODIS (2003-2005)level-2 data from reprocessing 1Obtained at : http://oceancolor.gsfc.nasa.gov/cgi/browse.pl?sen=am


The Matchup procedureThe Matchup procedureThe matchup process follows four steps (Bailey and Werdell, 2006, with additional considerations related to the specificity of the buoy-derived

observations):

Filtering of buoy data

sun zenith angles < 70°, buoy tilts < 15°, water leaving radiances > 0

Selection of satellite data

a 5 pixel by 5 pixel box, centered on the closest pixel to the buoy location

Filtering / averaging of satellite data

all 25 pixels must be valid, based on Level-2 processing flags

Selection of matchups

time lag between the satellite pass and a valid buoy measurement +/-1 hour

62 Points for MERIS

50 Points for MODIS

72 Points for SeaWiFS


Results: MERISResults: MERIS Satellite Data

• , • , • , • Buoy Dataww560560

ww490490

ww443443

ww443/ 443/ ww555555 ChlChl


Results: MODISResults: MODIS

nLnLww551551

nLnLww448448

nLnLww443443

nLnLww443/ 443/ nLnLww555555

Satellite Data

• , • , • , • Buoy Data


Results: SeaWiFSResults: SeaWiFS

nLnLww551551

nLnLww448448

nLnLww443443

nLnLww443/ 443/ nLnLww555555

Satellite Data

• , • , • , • Buoy Data


Results: All sensors, all bandsResults: All sensors, all bands

MerisMeris

In SituIn Situ

MODISMODIS SeaWiFSSeaWiFS

Sate

llite

Sate

llite


Results: StatisticsResults: Statistics

lambda npointsMeanratio

Absolutemean %err

r2 m y0

412 20 1,33 37,66 0,62 0,78 0,0067

443 62 1,11 18,40 0,61 0,80 0,0043

490 62 1,07 12,68 0,66 0,76 0,0040

MERIS 510 62 1,12 16,49 0,33 0,55 0,0054

560 62 1,11 17,19 0,21 0,42 0,0037

665 62 3,26 226,06 0,00 0,21 0,0009

681 62 3,62 261,61 0,56 1,27 0,0007

All 392 0,93 1,01 0,0009

412 22 0,83 20,65 0,79 1,11 -0,2508

443 50 0,98 9,69 0,69 0,95 0,0323

MODIS 488 50 1,00 7,07 0,64 0,78 0,1975

551 50 1,04 14,28 0,05 0,38 0,2052

667 49 1,55 71,30 0,02 -0,37 0,0235

All 221 0,95 0,95 0,0139

412 26 0,98 17,26 0,74 1,08 -0,0841

443 72 1,00 10,71 0,78 1,00 -0,0018

490 72 0,99 8,54 0,68 0,87 0,1059

SeaWiFS 510 72 0,97 10,17 0,39 0,61 0,2228

555 72 0,98 13,95 0,16 0,47 0,1588

670 71 1,57 76,35 0,06 0,74 0,0101

All 385 0,95 0,99 0,0001


Results: comparison with other Results: comparison with other studiesstudies

The overall agreement between the satellite-derived and the in situ reflectances or radiances is similar for

the three sensors, and it is close to what has been already reported elsewhere:

This study Zibordi et al. (2006) Bailey and Werdell (2006)and site web SEABASS

490 SeaWiFS

Npoints 72 208 242Mean Absolute % error 8.54 8 12.13r2 0.87 0.94 0.744

443 MERIS

Npoints 62 215Mean Absolute % error 18.4 29r2 0.61 0.79

551 MODIS AQUA

Npoints 50 120Mean Absolute % error 14.29 12.255r2 0.50 0.94


ConclusionsConclusions

BOUSSOLE provides a suitable set of data to validate Level-2 satellite Ocean Color products.

An overall agreement between the in situ and the SeaWiFS, MODIS and MERIS reflectances and radiances is obtained.


ConclusionsConclusions

Some specific points:

1) the MERIS data products in the “red part” of the spectrum (i.e., > 600 nm) are overestimated by a factor of two to four;

2) the MERIS data products for the blue wavelengths appear to be overestimated;

3) the SeaWiFS and AMODIS data products exhibit a slight tendency of underestimation in the blue domain;

4) AMODIS data for July and August 2005 appear to be underestimated but the cause of the error is at the moment unidentified.


Thank you for your attention

Fabrizio D’Ortenzio, David Antoine, Guislain Bécu

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