|
Who are we?
We
are scientists from two laboratories of Observatoire Midi-Pyrénées
(Laboratoire d'Aérologie
and Legos
from CNRS and University ofToulouse) who develop regional
oceanic models. INSU (an institute from CNRS for Universe
Sciences) gave us the mission to distribute our models and
to form scientists and engineers to their use. This service
is named SIROCCO.
SIROCCO also develops forecasting systems mainly as a decision-making
support during scientific cruises. Meanwhile, we develop research
activities in the frame of physical oceanography, sediment
transport and marine biogeochemistry. Our research group is
named POC
(Pôle d'Océanographie Côtière).
At the request of the
International Atomic Energy Agency (IAEA, March 14, 2011),
SIROCCO
is delivering every day a real time 6-day forecast bulletin
of the dispersion in seawater of radionuclides emitted by
the Fukushima nuclear plant. The simulations are based on
the
S2010.18
release of the 3D SIROCCO ocean circulation model. The system
is operational since March 24 and the bulletin is available
on an "open-access" basis since March 28.
The model uses a stretched
horizontal grid with a variable horizontal resolution: from
600m x 600m at the nearest grid point from Fukushima, to 5km
x 5km offshore. The initial fields (T,S,U,V,SSH) and the lateral
open boundary conditions are provided by the Mercator
PSY4V1R3 system (one field per day, horizontal resolution
1/12 ° x 1/12 °). At the sea surface, the ocean model is forced
by the meteorological fluxes delivered every 3hours by ECMWF.i
The tidal forcing at the lateral open boundaries is provided
by the T-UGO
model, implemented for this purpose by the SIROCCO
team on the Japonese Pacific coast.
Some details are given below on our methodology.
Bathymetry
The first and often critical step in building-up a new model
implementation is to extract, rebuild, check an accurate bathymetry
from which model bathymetry will be derived. In the case of
the Japanese waters, the availability of high resolution bathymetric
data released by the Japan
Oceanographic Data Center as more or less regularly distributed
point-wise values, packed in 2°x2° boxes, has been a great
help to reconstruct a quality ocean terrain model (see Figure
1a). As these data do not cover the full modeling area,
the JODC-derived database has been merged with the
GEBCO-08 30" database (30 arc-seconds resolution).
This bathymetry
has been projected on the curvilinear grid of the Sirocco
model (Figure 1b)
3D view of the
continental shelf bathymetry
- click to enlarge the picture - |
3D view of the
continental shelf bathymetry
- click to enlarge the picture - |
Initialization
and large scale forcing
Our
limited-area model needs initial fields and boundary conditions.We
use daily outputs of sea surface height, temperature, salinity
and currents given by Mercator which is a system providing analyses and forecasts
in real time for the global ocean. For this application, we
use fields at the resolution of 1/12° from the PSY4V1R3 prototype
providing a two-week analysis and seven days of forecast.
Tides
Global,
high accuracy tidal atlases are available for the global ocean
and could have provided directly the inputs needed by the
3-D circulation model. However, they usually offer tidal elevation
only, i.e. tidal currents, needed for an efficient treatment
of the open boundary conditions, are missing. In addition,
the low spatial resolution of the global atlases usually triggers
consistency issues if used at the open boundaries of an high
resolution model, and thus is damaging to the model simulations.
Therefore a specific, regional tidal model has been developed,
which include the 3-D circulation model implementation (see
Figure 2). It is
based on the finite element model T-UGOm,
routinely used in tidal applications. Thanks to an embedded
quasi-linear spectral solver, T-UGOm
can rapidly reconstruct adequate open boundary conditions
(.i.e. consistent with the local bathymetry and mesh resolution
of the regional model) to feed its sequential-in-time solver.
As the region of study do not include shelf seas, a limited
spectrum of about 20 tidal constituents has been modeled,
from which the tidal forcing has been extracted for the 3-D
circulation model ( Symphonie).
Regional tidal atlas validation
The T-UGOm tidal atlas, created to feed the SYMPHONIE coastal
ocean circulation model with accurate tidal boundary conditions,
has been validated against tide gauge data. In addition to
this routine validation, a special effort has been made by
the CTOH/INSU service (http://ctoh.legos.obs-mip.fr)
to process satellite altimeter data from the joint CNES/NASA
missions Topex/Poseidon and Jason-1/2 to build sea level time
series with a 6km along-track sampling. These time series
have been harmonically analyzed and corrected from solid and
loading tides to extract the ocean tides constants. On Figure
6, a zoom of the M2 amplitude has been displayed with the
misfits computed between the altimeter-derived data and the
M2 regional solution. Misfits are less than one centimeters,
except in the Kuroshio path where its meso-scale dynamics
interfers with the tidal sea level signal. This is due to
the fact that true tidal frequencies are aliased in the lower
part of the spectrum because of the temporal under-sampling
by the satellite instruments. Apparent period of M2 tide is
about 62 days in the T/P and Jason (~10 days repetitivity).
The methodologies and softwares deployed for tidal simulation
and analysis have been mostly supported by the CNES through
the OSTST science working group and COMAPI project funding.
Atmospheric
forcing
The
3D model is forced at the surface by air/sea fluxes calculated
from air parameters as well as radiative fluxes and precipitation
given by the ECMWF
(European Centre for Medium-Range Weather Forecasts). Every
day, a six-day forecast is downloaded. The spatial resolution
is about 25km and the time resolution is 3 hours. As the contaminated
water is emitted from the shoreline at Fukushima nuclear plant,
its advection by the ocean dynamic is controlled by the coastal
circulation, and thus might be sensitive to the short scales
in the surface wind forcing. The Sirocco group has decided
to examine the feasability of using a regional downscaling.
A forecasting system has been rapidly implemented, based on
the WRF model at a 10km resolution, with NCEP initialization
and lateral boundary conditions. A forecasting exercise has
been performed for the March 17th to March 24th period. As
it can be seen in Figure 7, large scale patterns in ECMWF
and in the regional WRF are very similar, but interestingly
the regional atmospheric model displays significant short
scales structures. Because further validations of the regional
forecasts are still needed, it has been decided to maintain
the ECMWF products for the atmospheric forcing in our operational
system. However, in the perspective of contaminated water
dispersion re-analysis, the question of atmospheric downscaling
could be examined in collaboration with the atmospheric modeling
expert groups.
Forecast
protocol
The 3D model SYMPHONIE
is at the heart of our modelling system. It receives the different
forcing listed before. Every wednesday, a hindcast run of
the previous seven days (red segments on Figure 8) is carried
out followed by a six-days forecast. Every other days, a six-day
forecast (blue segment) is run begining from the forecast
of the day before. This protocol allows us using always the
more accurate information available. Dynamic fields (temperature,
salinity, currents) are archived every 6 hours. Afterwards,
these three-dimensional currents are used to disperse radionuclides
following different
Scenarios
for radioactive tracers
The Fukushima nuclear plant has injected in the atmosphere
and in the seawater radionuclides at different times and in
variable quantities. A model of dispersion needs a good knowledge
of the source terms to be able to correctly calculate the
dispersion. Of course, very few information is available to
build elaborate scenarios of radionuclides emssion. Besides,
the evolution of dispersion in the sea also requires to know
very well the behaviour of the radionuclides, for example
the fraction which is dissolved in the seawater, the particulate
fraction and its associated sedimentation velocity. Even if
we knew all this information, radionuclides can aggregate
with marine particles and then their sedimentation velocity
can evolve. Finally, the oceanic currents computed by our
model are not the reality: they are the result of mathematical
equations too simple to fully represent the complexity of
nature. The wind which strongly drives the oceanic currents
is also a forecast whose accuracy is not known.
We
do not know how much radionuclides have been injected, when
they have been injected and how they behave once they reach
the sea. That is why we do not claim that our simulations
are able to provide an accurate quantification of radioactivity
in the sea. However, in order to build our scenarios, special
attention has been paid to the measurements of Cesium 137
concentration taken several times every day by TEPCO at 30
and 300m in front of the nuclear plant.
Two
sources of radionuclides are considered. One corresponds to
a direct emission in the sea in front of the nuclear plant
(migration of water contaminated by the reactors), the other
one corresponds to fallout of atmospheric particles. In the
first case, we introduce a flux at the grid point corresponding
to the nuclear plant. This flux is adjusted to produce a concentration
close to the values of Cesium 137 measured (see our
page of validation). In the last case, we used 1-h outputs of the
atmospheric transport model Polyphemus/Polair3D (0.25° horizontal resolution)
(pers. comm. Marc Bocquet & Victor Winiarek, Ecole des Ponts ParisTech/CEREA).
For each source (direct release and atmospheric), we consider
two cases: one corresponds to dissolved elements, the other
one to particles that fall into the sea with a velocity of
10meters per day. Obviously dispersion in the first case will
happen at larger scale than for the second case for which
deposition of particles on the sea floor reduces the dispersion.
Deposition of particles is cumulated over time and will be
mapped later on.
|
Click
to enlarge the picture
Figure
1a: Regional bathymetry. 500m resolution data have been collected
on 2°x2° boxes from the Japan
Oceanographic Data Center.
Click
to enlarge the picture
Figure 1b: bathymetry projected on the curvilinear grid of
the SIROCCO
model
Click
to enlarge the picture
Figure
2: T-UGOm
finite element mesh. Coastal resolution is a about 500 meters
in Fukushima neighborhood and along small islands, 2 kilometers
eslsewhere. Open ocean resolution is about 15 km, except on
steep bathymetric slopes where it increases up to a few kilometers.
Click
to enlarge the picture
Figure
3: M2 tide amplitude (cm) frome T-UGOm
simulation. Amplitude tends to amplify from North-east to
South-west. The 140° longitude seamounts barrier effects are
clearly visible in the tidal amplitude.
Click
to enlarge the picture
Figure
4: Mean tidal transport after 1 year of T-UGOm
integration. The Northern mean tidal transport follows the
coast toward the South of the island and disaggregate in the
vicinity of the Tokyo Bay, where it merges with off shore
transport cells due to the presence of the 140° longitude
seamount chain.
Click
to enlarge the picture
Figure
5: Mean tidal current after 1 year of T-UGOm
integration (m/s). The Northern mean tidal current follows
the coast toward the South of the island and disaggregates
in the vicinity of the Tokyo Bay. Off shore cells are due
to the presence of the seamount chain.
Click
to enlarge the picture
Figure 6: M2 regional tide validation: map
of the amplitude (in centimeters) and misfits with altimetry-derived
tidal constants (proportional to red dots size). Data have
been obtained from the harmonic analysis of the Topex/Poseidon
and Jason-1 time series (~17 years). The altimetry data used
in this work were developed, validated, and distributed by
the CTOH/LEGOS, France (http://ctoh.legos.obs-mip.fr/products/coastal-products/coastal-products).
The mean misfit level is about one centimeter, i.e. close
to the data accuracy itself. It is less than a few millimeters
in the Fukushima region. The larger misfits levels correspond
to regions where the presence of strong meso-scale circulation
tends to degrade the tidal analysis accuracy.
Figure 7: ECMWF surface wind forecast (left)
and regional WRF forecast (right, courtesy of P. Marchesiello)
March 18th, 00:00:00. ECMWF and regional WRF fields have similar
large scales pattern, WRF downscaling shows small scales structures
not present in ECMWF fields.
Click
to enlarge the picture
Figure 8: Protocol of simulation
|
Contact
us 
