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Ocean Sci., 15,199-214, 2019
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to yield data that reflect the diurnal cycle without the effect
of heat conduction from the buoy and heating of the sensor
by direct solar radiation. Third, a better-documented protocol
is needed for initial sensor calibration, allowing postmission
recalibration, to avoid introducing additional uncertainty
through the use of unspecified calibration procedures.
Fourth, traceability to national metrological standards needs
to be established.
These findings were taken on-board to design a novel sensor
package for SVP-B, for the sake of providing FRM SST
data for the calibration and validation of satellite SST. The
new buoy, called S VP-BRST, carries two SST sensors: one of
standard manufacture, the other of absolute uncertainty better
than 0.01 K (absolute uncertainty refers here to expanded uncertainty).
In addition to measuring SST with improved calibration,
the HRSST sensor also includes a hydrostatic water
pressure sensor. The present paper indicates the initial design,
which may evolve slightly as experience is gained from
expected future deployments in greater numbers.
The two prototypes deployed in the Mediterranean Sea
feature, before release, deviations within 0.01 K from a reference
SBE-35 thermometer. Once freely drifting, the buoys
observe that the SST spread within 5 min is usually smaller
than 0.1 K, especially when the sea state is well mixed and
the buoys are within an eddy core. The availability of percentiles
from the 5 min distribution of SST sampled at 1 Hz
(by a sensor with a fast response time) should help users improve
their data processing chain to move towards an ensemble
approach. The results in this paper suggest that it is important
to consider the sea-state mixing and the ocean surface
circulation to understand the representativeness of the in situ
SST data, as they both affect observed SST variations (within
the day and within 5 min). Consequently, they may both be
worth considering in the process of satellite SST cal/val.
In addition, a fairly standard analysis, where ocean dynamics
behavior can be inferred from the buoy data, suggests that
the high-resolution SST data hold a wealth of information.
Properly analyzed and interpreted, these data can provide a
useful insight of the dynamics of the sampled area, especially
when the Supplement is brought into the picture to consider
sea state and ocean surface circulation. Even more interesting
may be to collect full samples of 1 Hz data, when possible,
in addition to the summaries of the distribution with five percentiles.
Such a high-frequency HRSST dataset (HFHRSST)
may serve other applications beyond satellite SST ca/val,
such as fine-scale model developments and enhanced understanding
of SST variability.
Future efforts include evaluation of the HRSST sensor
drift. This will be done by keeping one SVP-BRST buoy at
post in a monitored environment, and by recovering as many
SVP-BRST buoys as possible. The goal will be to assess
whether the temporal stability of SST from drifting buoys is
within ±0.01 Kyear -1 after manufacture. This is important
for climate monitoring, as initial results from past HRSST-2
buoys, presented in this paper, suggest temporal drifts that
are systematically negative and close to this figure, though
the very small number of drifting buoys surveyed (three) is
not significant enough to be conclusive. At least 100 SVP-BRST
buoys are expected to be deployed in the next 3 years,
with a view to cover a wide range of atmospheric and oceanographic
conditions.
Data availability. The HRSST-2 SVP-B and SVP-BS data are
available from the Copernicus In Situ Thematic Assembly Center
(http://marine.copernicus.eu/situ-thematic-centre-ins-tac/, Copernicus
Marine Environment Monitoring Service, 2019). The SVP-BRST
prototype drifter data used in this publication are available
in open access: https://doi.org/10.5281/zenodo.1410401 (Poli et al.,
2018). Reanalysis ERA5 data are available from the Copernicus Climate
Change Service.
Supplement. The supplement related to this article is available
online at: https://doi.org/10.5194/os-15-199-2019-supplement.
Author contributions. PP drafted the main text of this paper, prepared
several of the figures and corresponding scientific analysis,
and prepared the manuscript for submission. ML drafted the introduction
and, along with GC, contributed results and scientific analysis.
AD and MLM contributed to the buoy design and calibration.
PB, DM, AO’C, KH, and CM contributed to the buoy design and
scientific analysis.
Competing interests. Paul Poli, Marc Lucas, Anne O’Carroll,
Marc Le Menn, Arnaud David, Gary K. Corlett, Pierre Blouch,
Mathieu Belbeoch, and Kai Herklotz are participants in the
TRUSTED project (see acknowledgements), which funds the development
of the SVP-BRST buoy, as well as manufacturing of a
series of units, deployments, and data and metadata acquisition and
processing. David Meldrum and Christopher J. Merchant declare no
conflict of interest.
Acknowledgements. The authors are funded by their respective
institutions. Additional support, including the development of the
SVP-BRST prototypes and the resulting data analyses, is provided
by the European Union’s Copernicus program for funding the
development of the SVP-BRST drifting buoys under the project
“Towards Fiducial Reference Measurements from HRSST drifting
buoys for Copernicus satellite validation” as part of the TRUSTED
project led by CLS, with buoy manufacturing by NKE, calibration
by SHOM, coordination of deployments by Météo-France, provision
of a tethered reference measurement by BSH, and metadata
processing and deployment monitoring visualization tools by
JCOMMOPS. This publication contains modified Copernicus
Climate Change Service Information (2018) (C3S, 2017).
Edited by: Piers Chapman
Reviewed by: two anonymous referees
