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Improving the altimeter derived geostrophiccurrents using sea surface temperature images: feasibility study and application on real datasets
Marie-Helene Rio(1), Rosalia Santoleri (2) , Daniele Ciani (2)
(1) CLS, France,
(2) ISAC-CNR, Italy,
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Limitations➢ of the altimetry system for ocean current estimation
❖Only the geostrophic component of the surface current is obtained❖For a limited part of the spatio-temporal spectra
x
h
f
gv
y
h
f
gu
In order to go beyond the altimeter system limitations, new sensors and new methodologies must be exploredOur aim is to use SST field data to improve altimeter current enstimates
CONTEXT
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Can SST information be used to improvealtimeter current enstimates?
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Require the velocity field (u,v) to obey the SST evolution equation and inverse it for the velocity vector:
F(x,y,t) represents the source and sink terms (insolation, net infrared radiation,latent and sensible heat fluxes)
Challenge: only along-gradient velocity information can be retrieved from thetracer distribution at subsequent times in strong gradients areas.
)t,y,x(Fy
SSTv
x
SSTu
t
SST
Piterbarg et al, 2009; Mercatini et al, 2010 : Use a background velocity information(ubck, vbck) so that the satellite tracer information is used to obtain an optimized‘blended’ velocity (uopt, vopt).
METHOD
We applied the methodology on successive SST images using the low resolution,geostrophic altimeter velocities as background velocities
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Perfectly known forcing F
Require the velocity field (u,v) to obey the SST evolution equation and inverse it for the velocity vector:
)t,y,x(Fy
SSTv
x
SSTu
t
SST
Piterbarg et al, 2009
Change of coordinates (x,y) -> (xsst,ysst)
0x
SST
sst
sst
sst
y
SSTt
SSTF
v
²B²A
)EBvAu(Auu bckbck
bckopt
²B²A
)EBvAu(Bvv bckbck
bckopt
Ft
SSTE
y
SSTB
x
SSTA
Where:
ubck, vbck -> altimer geostrophic velocity
usst :infinite solutions
use of a background
information (ubck,vbck)
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The source and sink terms F
)TT(H
w
HC
)H(Qdz
²z
)z(T²
Hdz
²y
)z(T²
Hdz
²x
)z(T²
Hdz
z
)z(T)z(w
H
1F bm
e
p
H
0z
H
0z
z
H
0z
yx
Hz
0z
Vertical advection Diffusion
Heatfluxes Entrainment
velocity(0 in case of shoaling)
FromRio et al, 2016
~0 ~0
The Heat conservation Equation
Fy
SSTv
x
SSTu
t
SST
Large spatial
scale of 𝜕𝑆𝑆𝑇′
𝜕𝑡= Fbck
Forcing term estimate (Fbck)
𝜕𝑆𝑆𝑇
𝜕𝑡=
𝜕𝑆𝑆𝑇′
𝜕𝑡+
𝜕𝑆𝑆𝑇′′
𝜕𝑡𝐸 =𝜕𝑆𝑆𝑇
𝜕𝑡−𝜕𝑆𝑆𝑇′
𝜕𝑡
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Test of the Method: OSSE Experiment
Method tested using model data of the Mercator Ocean Global Model Forecsting system.Mercator • ORCA12: model resolution 1/12°, one year (2013)Model data: • sea surface height, temperature, and surface velocity
• Altimeter surface geostrophic currents simulated from the modeled SSH Maps• of SST simulated from model SST Model • surface currents –> reference velocity
everywhere, >30% on the meridional component in the equatorial band
Rio et al JTECH 2016
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▪Altimetry: DUACS L4 gridded products: « twosat » and « allsat »
▪Sea Surface temperature: L4 OI maps from REMSS (100km, 4 days):MW: based on microwave sensors only
resolution ¼°, daily mapsMW_IR: based on both microwave and infrared sensors
resolution ~9 km, daily
1 year of SST DUACS data: 2013
For validation (independent data):• Drifting buoy velocities, SVP drogued, 6 hourly resolution along the buoy
trajectory▪ Chlorophyll L4 maps distributed by CMEMS, resolution 4km, daily
Test method with real SST data: DATA used
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Example on September, 20th 2010
km500scalesbck
t
SSTF
105°C/s
Forcing term estimate (Fbck)
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RESULTS
Example 1: Gulfstream, September 21st 2010
Alti (2SAT) Alti (ALLSAT)SST REMSS MW + Buoy #88532
CMEMS CHL+Buoy #88532
Alti (2SAT) + SST MW Alti (ALLSAT) + SST MW
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CHL SST MW_IR
CHL
SSH isocontour
Optimal velocitiesAltimeter velocities
ALTI 2SAT
ALTI 2SAT Alti ALL SAT
Example 2: Gulfstream, August, 8th 2015
RESULTS
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SST
10-5°cm-1
VALIDATION
1 year (2003) of global maps of optimal velocities have been calculated and compared to SVP-drogued drifting buoy velocities
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Perfect Forcing F=Fbck
Deterioration➢ in weak gradients areas(<10-5°/s)
Light degradation for the zonal component
Deterioration➢ in weak gradients areas(<10-5°/s)
➢Strong improvement in strong gradients areas2SAT+SST equivalent to ALLSAT
zonal
meridional
RM
SU m
/sR
MSV
m/s
Altimetry 2satAltimetry AllSatOptimal
VALIDATION
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Altimetry 2satAltimetry AllSatOptimal unknown F (background=2SAT)Optimal unknown F (backgound=Allsat)
zonal
meridional
RM
SU m
/sR
MSV
m/s
SST L4 MWIR REMSS Products 9kmOI maps (100km, 4 days)
Improvement expected at medium scales only
Further improvements expected usinghigher resolution SST products
Unknown Forcing F=Fbck +εVALIDATION
Further improvementobtained on the meridional component
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1/20°
What is the impact of high resolution SST?Test on the Mediterranean Sea
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CONCLUSIONS
➢A method has been implemented which successfully combines SST and altimeter datato improve the altimeter derived surface currents.
Systematic➢ application for one year over the global ocean has been done andvalidated through comparison to independent drifting buoy velocities.
Significant➢ improvements (up to 20-30% locally) are obtained in strong SST gradientsareas for the meridional component of the velocity.
In➢ low gradients areas and for the zonal component of the velocity, weakerimprovement is expected by construction. Still, a few % of improvements is obtainedlocally. In these areas, taking into account the forcing and background errors is essential.
Further➢ improvements are expected by:Using➢ higher quality, higher resolution (spatial and temporal) SST productsBetter➢ estimating the forcing term F and its error ε.