ostst 2007 - march, 12-15 - hobart, tasmania
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Ocean Mean Dynamic Topography from altimetry and GRACE: Toward a realistic estimation of the error field. Marie-Helene Rio(1), Philippe Schaeffer(1), Jean-Michel Lemoine(2), Gilles Larnicol(1) - PowerPoint PPT PresentationTRANSCRIPT
OSTST 2007 - March, 12-15 - Hobart, Tasmania
Ocean Mean Dynamic Topography from altimetry and GRACE:
Toward a realistic estimation of the error field
Marie-Helene Rio(1), Philippe Schaeffer(1), Jean-Michel Lemoine(2), Gilles Larnicol(1)
(1) CLS, 8-10 rue Hermes, 31256 Ramonville saint agne, France (2) GRGS, 14 avenue Edouard Belin, 31400 toulouse, France
ContextMSS Geoid- = MDT
- Oceanographic analysis- Assimilation into operationnal ocean forecasting systems
Combination of MSS and geoid for MDT computation
Computation of high resolution MDT (Rio et al, 2004, 2005)
A number of key issues
+SLA=ADT
Computation of a realistic error field
Error on the geoid, error on the MSS, error on the MDT computation method
?
MDT
Large scale (400 km)
High resolution (<50 km)
?
Combination of MSS and geoid for MDT computation
MSS CLS01 – EIGEN-GL04S
Rc=133 km
Rc=200 km
Rc=300 kmRc=400 km
Gaussian filter
A
Limits of the gaussian filtering
No error estimate on the resulting MDT
Creation of spurious strong gradients in specific areas (around islands, along coasts, in strong subduction areas…)
rc=133km
rc=200km rc=300km rc=400km
cm
Optimal combination method
)r(Hα)r(H iobs
N
1ii
~
N
1jrj
1iji CA
N
1iriirr
2 CC)r(~
MDT Hobs=MSS-Geoid
Eigen-4SEigen-3S
Eigen-3C
GGM02S
GGM02C
EGM96
19 cm RMS
A: Covariance matrix of the observations:<Hobs,Hobs> = <H2>Fc(r) + <ε2
obs>
MSS CLS01 error field
cm
The CMDT RIO05 (Rio et al, 2005) field is low-pass filtered to 400 km and its variance is computed in 600 km radius domains.
cm
<H2>=A-priori variance
ε2obs= ε2
MSS+ ε2
Geoidcrar
e*)3cr
3r3a61
2cr
2r2a61
crr
a(1(r)cF
Rc=133km
Rc=400km
Different correlation functions have been tested
ResultsMDT Estimated error field
cm
cmcm
rc=133km
rc=200km
rc=300km
rc=400km
Validation
Is the estimated error field realistic?
A- Method: Comparison to independent synthetic MDT estimates
-In-situ temperature and salinity from XBT and CTD are used to compute dynamic heights relative to 1000m for the period 1993-2005.
hinsitu
geoid
z=-1000mh1000
-the dynamic topography at 1000m as estimated by (Willis et al, 2007) is added to the dynamic heights.
h
z=0
22insitu 3ε
22willis 4ε
22sla 3ε
22sla
2willis
2insitu
2synth 6εεεε
-Altimetric Sea Level Anomalies from AVISO are then subtracted to compute synthetic estimates of the Mean Dynamic Topography.
h’alti
Synthetic MDT estimates
MDTsynth
B- Results RMS differences between synthetic heights and:
22221 omsynthMDTRMS
1- Gaussian filtered MDTs
Dashed line: comparison to unfiltered synthetic heights:
133 km filter
2222 fsynthMDTRMS
Solid line: comparison to filtered synthetic heights:
400 km filter
22222
21
2 658 RMSRMSom
2- Optimal MDT RMS difference to unfiltered synthetic heights = 8.5 cm
~ 8.5²
222omsynthMDT 2
RMS
6.0²2² 6.0²+ +
Conclusions
We investigated the efficiency of optimal method to compute realistic large scale MDT from altimetry and geoid and associated error field.We showed consistency between the obtained error field and how the large scale MDT compares to independent synthetic estimates of the MDT.
Improvements need to be made for the better estimation of the altimetric data error (on MSS and SLA - error on the different corrections used during altimeter data processing)
Impact of using the covariance error information in the optimal MDT computation needs to be investigated (available in the case of future GOCE data)
Method based on the knowledge of the observation errors and a-priori statistics of the error field Further improvements are possible:
Future workMDT=MSS-Geoid
Improved estimates of high resolution « combined » MDT
EIGEN05S, EGM07,… GOCE!New MSS estimations and realistic error field
First Guess:EIGEN-GRACE03S 400 kmIn-situ data: drifters and dynamic heights 1993-2002Global: 1/2° resolution grid
The Combined Mean Dynamic Topography RIO05
Rio et al, 2005
cm