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Assessment of the FES2014 Tidal Currents on the shelves around Australia

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The accuracy of the ocean tide correction is crucial for altimetry data, especially in the coastal regions, where the ocean tide signal shows the largest amplitudes. The FES2014 global tidal model benefits from a high resolution mesh, improved hydrodynamic modelling and data assimilation techniques, as well as a 20-year-long altimeter time series and a large dataset of tide gauge observations for data assimilation. The validation of the FES2014 tidal elevations has shown the very good performance of this atlas, and particularly on the shelves. In addition to the tidal elevations, the FES2014 global tidal atlas provides the tidal current velocity, which is of particular interest for many scientific (ocean circulation analysis, ocean dynamics modelling…) and industrial (offshore activities, tidal energy site assessment…) applications. Validation of the tidal currents is challenging as it requires long and accurate time series of current-meter observations. Luckily, for more than 10 years Australia has been maintaining a network of about 50 ADCP instruments all around the country, principally through its government-supported Integrated Marine Observing System (IMOS). The Australian continental shelf has a wide range of tidal regimes ranging from macro-tidal to micro-tidal, thus providing ideal conditions to thoroughly test a model. This paper presents an assessment of the FES2014 tidal current atlas against the tidal constituents computed from the IMOS current meter data around Australia. The results show the very good agreement between the FES2014 tidal currents and the measured tidal currents, in most of the regions around Australia. Some differences have also been noticed and analysed and some potential improvements to the model have been identified. This study provides new insights on the FES2014 tidal model quality and possibilities of improvements that would benefit to both the tidal currents and the tidal elevations, and subsequently to the coastal altimetry observations.
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22/02/2017 - 10th Coastal Altimetry Workshop - Florence, Italy - 1
Assessment of the FES2014 Tidal Currents
on the shelves around Australia
M. Cancet, F. Lyard, D. Griffin, L. Carrère, N. Picot
Motivation
FES2014 global tidal model
High resolution mesh
Improved hydrodynamic modelling
Assimilation of harmonic constants derived from 20-year-long altimeter
time series and tide gauges
Provides tidal elevations and currents
Main validation performed on the tidal elevations
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Motivation
Many applications for ocean tidal currents
Ocean circulation analysis and modelling
Industrial applications: offshore activities, tidal energy site assessment…
Challenging validation: requires long and accurate time
series of current-meter observations
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Region of interest
Australian continental shelf: wide range of tidal regimes from
micro-tidal to macro-tidal ideal conditions to thoroughly test a model
Australian ADCP network
48 stations all around Australia
Maintained for more than 10 years through IMOS
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Datasets
FES2014 model tidal currents
Barotropic model
Maps of tidal harmonic constituents (amplitude and phase lag)
for each tidal current direction (U eastward and V northward)
for 35 tidal components (M2, K1, O1, S2, etc)
In situ currentmeter observations
48 stations located all around Australia
Time series measured between 2007 and 2015 (measurement period
depends on the station)
Total current data available through IMOS website as time series in
each current direction (U eastward and V northward), at various depths
Specific computation of the total current at a mean depth (CSIRO)
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Comparison methodology
Equivalent physical content
Harmonic analysis on ADCP data Tidal harmonic constituents
(amplitude and phase lag) for each wave and for U and V
Tidal prediction from 5 main waves (M2, K1, S2, O1, N2) for FES2014
and the ADCP data Tidal prediction time series for U and V
Comparison
In the frequency domain
Vector differences between the tidal harmonic contituents of the model and of the
ADCP data, for U and V and for each main tidal component
Tidal current ellipses: for each tidal component, synthesis of the velocity amplitude
and direction
In the temporal domain
Comparison of the tidal prediction time series
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FES2014 tidal currents
assessment
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Good agreement in general for the main tidal wave in the region (M2)
A few stations show large misfits
(cm/s)
U
(cm/s)
V
Comparison in the frequency domain: vector differences for M2
FES2014 tidal currents
assessment
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Comparison in the frequency domain: current ellipses for M2
Very good agreement in general in both micro-
and macro-tidal regions
A few stations show large misfits
FES2014 tidal currents
assessment
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FES2014 tidal currents
assessment
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At most ADCP stations in
macro-tidal regions, very
good agreement (amplitude,
phase and direction) between
FES2014 and the in situ data.
FES2014 tidal currents
assessment
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FES2014 tidal currents
assessment
Darwin ADCP station
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Very shallow and coastal region
(ADCP station ~15 km from coast)
FES2014 global mesh resolution
too coarse (~15 km) to solve the tidal
current velocities accurately.
Bathymetry
FES2014 tidal currents
assessment
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FES2014 tidal currents
assessment
Great Barrier Reef stations (1/2)
GBRLSL station: open ocean side, low tidal
current amplitudes (2cm/s), poor consistency
model/obs
GBRLSH station: inside the Great Barrier, tidal
current amplitudes of ~20cm/s
Potential lack of accuracy in the bathymetry + mesh resolution
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FES2014 tidal currents
assessment
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FES2014 tidal currents
assessment
Great Barrier Reef stations (2/2)
GBRELR station: open ocean side, low tidal
current amplitudes (2cm/s), poor consistency
model/obs
Other stations: inside the Great Barrier, tidal
current amplitudes of ~20cm/s
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Bathymetry
(m)
FES2014 tidal currents
assessment
Great Barrier Reef stations (2/2)
GBRELR station: open ocean side, low tidal
current amplitudes (2cm/s), poor consistency
model/obs
Other stations: inside the Great Barrier, very shallow
region, tidal current amplitudes of ~20cm/s
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FES2014 tidal currents
assessment
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FES2014 tidal currents
assessment
Great Barrier Reef stations (2/2)
GBRELR station: open ocean side, low tidal
current amplitudes (2cm/s), poor consistency
model/obs
Other stations: inside the Great Barrier, very shallow
region, tidal current amplitudes of ~20cm/s
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The model and the ADCP data are in phase, but
the model under-estimates the current amplitude
Increase of the misfits between model tidal
elevations and tide gauges in the same region
Very shallow and complex region
Need for regional model (high resolution
bathymetry, high resolution mesh, local tuning of
the bottom friction parameter
FES2014 tidal currents
assessment
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FES2014 tidal currents
assessment
Coffs Harbour stations
Very strong FES2014 eastward
component at both stations
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FES2014 tidal currents
assessment
Coffs Harbour stations
Very strong FES2014 eastward
component at both stations
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(cm/s)
FES2014 U tidal current M2 wave
(cm)
FES2014 tidal elevation M2
FES2014 tidal currents
assessment
Coffs Harbour stations
Very strong FES2014 eastward
component at both stations
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Bathymetry (m)
Artefacts in the FES2014 eastward tidal component:
Local lack of resolution in the FES2014 mesh in
a region of steep bathymetry slope
Combined with discontinuous discretization
(DNP1) for the computation of the velocities
No impact on the FES2014 elevations
(continuous LGP2 discretization)
Conclusions
22/02/2017 - 10th Coastal Altimetry Workshop - Florence, Italy -
First validation of the FES2014 tidal current velocities
Ideal case: many ADCP stations, long time series, quality-checked data
Validation both in frequency and temporal domains complementary
Very good agreement between the FES2014 model and the in situ data
at most stations
Some large discrepancies observed at a few stations
All are explained
Mesh model lacks resolution (but global model should consider regional
configurations)
Need for high resolution bathymetry data
Very encouraging results, complementarity between elevations and
velocities validation
Looking for other in situ datasets to validate the model in other macro-
tidal regions, before the implementation of the next version 24
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