Transcript Document

Joint Research Centre
Traceability, quality control and uncertainties
of AERONET-OC data
for satellite ocean color applications
G. Zibordi 1, B. Holben 2, M.Gergely 1, M.Talone 1, F.Melin 1 and J.-F. Berthon 1
1
2
European Commission, JRC, Ispra, IT
National Aeronautics Space Administration, GSFC, Greenbelt, MD
AERONET-OC
Joint Research Centre
AERONET – Ocean Color
is a sub-network of the Aerosol Robotic Network
(AERONET), relying on modified sun-photometers to support ocean color validation activities
with highly consistent time-series of LWN() and a().
Rationale:
•Autonomous radiometers operated on fixed platforms in coastal regions;
•Identical measuring systems and protocols, sensors calibrated using a single reference source
and method, and data processed with the same code;
•Standardized products of normalized water-leaving radiance and aerosol optical thickness.
G.Zibordi et al. A Network for Standardized Ocean Color Validation Measurements. Eos Transactions, 87: 293, 297, 2006.
Joint Research Centre
AERONET-OC (2002-present)
Current sites
Planned sites
Potential sites
Current management and responsibilities
•NASA manages the network infrastructure (i.e., handles the instruments calibration and, data collection,
processing and distribution within AERONET).
• JRC has the scientific responsibility of the processing algorithms and performs the quality assurance of
data products (in addition to the management of 5 out of 15 sites).
•PIs establish and maintain individual AERONET-OC sites.
Joint Research Centre
Examples of AERONET-OC Sites
Site: AAOT
Location: Northern Adriatic Sea
Water type: Case-1/Case-2
Period: 2002-present
Site: GDLT
Location: Northern Baltic Proper
Water type: Case-2
Period: 2005-present (summer)
Site: AABP
Location: Persian Gulf
Water type: Case-1 (?)
Period: 2005-2008
G.Zibordi et al. AERONET-OC: a network for the validation of Ocean Color primary radiometric products.
Journal of Atmospheric and Oceanic Technology, 26, 1634-1651, 2009.
Joint Research Centre
Validation of MODIS-A
radiometric data
Scatter plots of MODIS-A (MOD-A) versus AERONET-OC (PRS) LWN match-up values at selected centerwavelengths for the GLR site. N indicates the number of match-ups, LWN and rmsd are in units of mW cm−2 μm−1
sr−1, || is the mean of absolute percent differences while  is the mean of percent differences, and r2 is the
determination coefficient. The right panel in the second row displays the AERONET-OC LWN spectra utilized to
construct match-ups.
Joint Research Centre
Validation of MODIS-A aerosol
optical depth data
Scatter plot of MODIS-A (MOD-A) versus AERONET-OC (PRS) a match-ups at selected center-wavelengths for the
GLR site. N indicates the number of match-ups, a and rmsd are dimensionless, || is the mean of absolute percent
differences while  is the mean of percent differences, and r2 is the determination coefficient. The right panel in
the second row displays frequency distributions of α determined with τa at 785 and 869 nm for MODIS-A, and 667
and 870 nm for PRS. The black characters and lines in the frequency distribution plot, indicate results from the
analysis of MODIS-A data while grey characters and solid bars indicate results from the analysis of AERONET-OC
data (m is the median and σ the standard deviation).
Intra-annual climatology
The intra-annual climatology confirms
the existence of two bio-optical cycles:
i. one occurring in winter and affecting
the regions starting from the shelf and
extending to the open waters;
ii. the other mostly affecting the shelf
and showing a maximum in spring and a
successive one less pronounced in fall.
𝐿𝑀𝑂𝐷−𝐴
(547/488)
𝑊𝑁
𝐿𝑃𝑅𝑆
𝑊𝑁 (547/488)
Joint Research Centre
Intra-annual climatology at GLR for the band-ratios LWN (547)/LWN(488) for AERONET-OC (PRS) an MODIS-A
(MOD-A) data, determined with all available data for the period 2010-2014.
Traceability
Each individual radiometer is calibrated at NASA-GSFC using an integrating sphere.
Sample radiometers are re-calibrated at the JRC for quality assessment using an
FEL-1000 Watts quartz-halogen lamp, and Spectralon 99% reflectance plaques.
All calibrations are traceable to the National Institute for Standards and Technology
(NIST).
NASA-JRC differences in absolute radiance calibrations are generally better 2% in
the 400-700 nm spectral interval and 3% beyond 700 nm.
This result has been largely confirmed by an inter-comparison involving NIST in
addition to NASA and JRC.
NASA calibration rely on GSFC
integrating sphere.
JRC calibration rely on a NIST
traceable FEL-C 1000W lamp and a
99% Spectralon Reflectance panel.
G.Zibordi, B.Holben, I.Slutsker, D.Giles, D.D’Alimonte, F.Mélin, J.-F. Berthon, D. Vandemark, H.Feng, G.Schuster, B.Fabbri, S.Kaitala,
J.Seppälä. AERONET-OC: a network for the validation of Ocean Color primary radiometric products. Journal of Atmospheric and
Oceanic Technology, 26, 1634-1651, 2009.
Quality Control
AERONET-OC products are classified at different levels:
o Level 1.0->  LWN() determined from complete measurement sequences.
o Level 1.5->  Cloud screened aerosol optical thickness data exist;
o
 Replicate sky and sea radiance measurements exhibit low variance;
o
 Empirical thresholds are satisfied (e.g., exceedingly negative
o
values or high reflectance in the near infrared);
o Level 2.0->  Pre- and post-deployment calibration coefficients exhibit
o
justifiable differences within 5%;
o
 LWN() spectral shapes are consistent based on statistical
o
approaches (i.e., high statistical representativeness within the data
set itself (self-consistency) or non-anomalous features with respect
to a reference set of quality-assured data (relative-consistency)) ;
o
 A final spectrum-by-spectrum screening is passed.
G.Zibordi, B.Holben, I.Slutsker, D.Giles, D.D’Alimonte, F.Mélin, J.-F. Berthon, D. Vandemark, H.Feng, G.Schuster, B.Fabbri, S.Kaitala,
J.Seppälä. AERONET-OC: a network for the validation of Ocean Color primary radiometric products. Journal of Atmospheric and
Oceanic Technology, 26, 1634-1651, 2009.
D.D’Alimonte and G.Zibordi. Statistical assessment of radiometric measurements from autonomous systems. IEEE Transactions in
Geoscience and Remote Sensing, 44, 1-11, 2006.
LWN uncertainties
Joint Research Centre
Considering the measurement equation for LWN
LWN= LW CA CQ with LW= LT -  Li
where LT is the total radiance measured above the sea surface, Li is the sky radiance,  is the surface reflectance,
CQ removes the dependence from the viewing geometry and bidirectional effects, and CA removes the basic
dependence on sun zenith, atmosphere and sun-earth distance.
The combined standard uncertainty of the normalized water-leaving radiance u(LWN) is the
composition in quadrature of any independent uncertainty due to sources i affecting LWN.
2
𝑢2(𝐿𝑊𝑁) = 𝑢 𝐿𝑊𝑁(𝑖)
Applying Guide to the Expression of Uncertainty in Measurement (GUM) and neglecting
correlations, the combined standard uncertainty of the normalized water-leaving radiance
u(LWN) is
2
with
2
𝑢2 𝐿WN = 𝐶𝑄 𝐶𝐴 𝑢2 𝐿W + 𝐿W 𝐶𝐴 2 𝑢2 𝐶𝑄 + 𝐿W 𝐶𝑄 𝑢2 𝐶𝐴
𝑢2 𝐿W = 𝑢2 𝐿T + 𝐿2i 𝑢2 𝜌 + 𝜌2 𝑢2 𝐿i .
The uncertainties u(LWN) and u(LT,i), indicating either u(LT) or u(Li), should account for contributions due to
the following sources i: absolute calibration, sensitivity change during the deployment period of the
measuring system, and environmental perturbations caused by sea surface roughness and environmental
changes during measurements.
M. Gergely and G. Zibordi, “Assessment of AERONET LWN uncertainties,” Metrologia 51, 40–47 (2014).
LWN relative uncertainties
at different AERONET-OC sites
Joint Research Centre
GUM
4.8
4.9
7.3
Relative combined uncertainties u(LWN )/LWN (%) and (in square brackets) combined standard uncertainties
u(LWN ) and median LWN (mW cm−2 sr−1 μm−1), respectively, at different λ (nm) for various AERONET-OC sites.
M. Gergely and G. Zibordi, “Assessment of AERONET LWN uncertainties,” Metrologia 51, 40–47 (2014).
G.Zibordi and K.J.Voss, Field Radiometry and Ocean Color Remote Sensing. In Oceanography from Space, revisited. V.Barale, J.F.R.Gower and
L.Alberotanza Ed.s, Springer, Dordrecht, pp. 365-398, 2010.
Joint Research Centre
Conclusions
AERONET-OC is an operational network delivering globally distributed and cross-site
consistent measurements of a and LWN in coastal and occasionally at open sea sites.
Qualifying element is the capability of delivering both, a and LWN based on metrology
principles and standardization of its components.
Major application is the validation of satellite ocean color primary data products.
However, it also offers the capability of supporting climatological studies on
atmospheric and marine processes related to primary or derived quantities.
Thanks