hf processes assimilation in a 1/12° regional model (ibi12)godae-data/oceanview/events/... ·...
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HF processes assimilation in a 1/12° regional model (IBI12)
M. Benkiran, C. Dufau (CLS)
and Mercator Ocean Team
http://www.mercator-ocean.fr
Motivation
Mercator Ocean is working on the development of a global model which takes into account the forcing by Tide and Atmospheric pressure. To prepare the assimilation of altimeter data
in accord with these high frequency processes,
We are testing a new assimilation in the regional model which includes Iberia, Biscay and Ireland (IBI12).
Sea Surface Temperature snapshot
IBI12 : Iberia-Biscay-Ireland (1/12°)
• explicit free surface, « time splitting » + Variable volume formulation • k-ε : the Generic Length Scale (GLS) formulation (Umlauf and Burchard, 2003) • Atmospheric Forcing : 3-h ECMWF ; Wind + pressure • Tides (including potential) : • Open boundaries from GLORYS2V1 ¼° reanalysis (daily)
• In situ profiles ,T & S from CORA3.1 data base • Reynolds AVHRR ¼° SST • Along track SLA observations From CMEMS/AVISO , usual
corrections applied filtering of tides in the Observation Operator H
• Reduced order Kalman Filter (SEEK formulation) • 3D-VAR Bias corrections : for T and S • Incremental Analysis Updates (IAU) : Analysis J-2.5 • SST Correction in Bulk • Quality Control of in situ observation
IBI-1/12° NEMO:
Data Assimilation :
Assimilated Observations :
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SLA :Observation
SLA Observation (CMEMS/AVISO), Corrected from geophysical signal
Tide : GOT model (Goddard Ocean Tide model) DAC (Dynamic Atmospheric Correction):
• Objectives of the DAC remove high frequencies signals forced by the atmosphere (pressure and wind) and aliased in altimetric data because of bad temporal sampling of altimeters
• DAC
optimal combination of high frequencies of barotropic model (MOG2D model) and low frequencies of IB: cut-off freq= 20 days
DAC = MOG2D_HF + IB_LF
LWE (Long Wavelength Error) : Residual orbitER, IBER, Residual Tide Corr and Aliased HF signal
SLA = SLAsat - Tide - DAC - LWE
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SLA = SLAsat - DAC - LWE - TIDE
Page 5
-0.4 0 0.4
-0.4 0 0.4
S N
Sla & Correction along track J2(163)
-0.4 0 0.4
(DAC=IBLF+MOG2DHF)
Dac : not negligible; Large Scale Lwe is negligible compared to Dac
(GOT002 model)
-1.0 0 1.0
20
09
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High frequency impact
We use a specific altimeter dataset dedicated to assimilation developed within CMEMS (“SLA-assim” issued from the TAPAS initiative). It provides at each along-track point the geophysical corrections We also adapt the model background for each experiment (2009)
Different experiences
LWETIDESLA SLA : Sim_Dac
DACLWETIDESLASLA : Sim_Ref
sat
obs
sat
obs
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High frequency impact
Different experiences
LWETIDESLA SLA : Sim_Dac
DACLWETIDESLASLA : Sim_Ref
sat
obs
sat
obs
25h
To remove a DAC
like correction
Estimation of the correction large-scale high frequency (From barotropic part (wind reponse…))
Mean Dynamic
Topography (RIO; CLS)
Sim_Dac
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Std (Innovation) in box (0.25x0.25), 2009
0. 8.0 12.0 20.0 Cm
Std(Innovation) in box (0.25x0.25), 2009
0. 8.0 12.0 20.0 Cm 0. 8.0 12.0 20.0 Cm
Impact of the assimilation of HF (SLA):
• Std(Innovation) :
Increase but this Std is not
calculated with the same data
Sim_Ref
Innovation = Data – Model Forecast
Mean = 5.88Cm Mean = 6.84Cm
Std(Innovation)/Std(Data) in box (0.25x0.25), 2009
0.4 0.8 1.2 1.6
Std(Innovation)/Std(Data) in box (0.25x0.25), 2009
0.4 0.8 1.2 1.6
Mean=0.80 Mean=0.51
• Std(Innovation)/Std(Data): The model is able to better
explain the observed signal with the HF data.
An improvement overall the basin In Open Ocean (Ratio <0.4 )
Sim_Dac
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Impact on insitu data (T & S) Mean & Std (Innovation)
Mean Innovation(Temp,Sim_Ref )
-1.0 0.0 1.0
0 -50 -100 -150 -200
Mean Innovation (Temp, Sim_Dac )
-1.0 0.0 1.0
0 -50 -100 -150 -200
0 -50 -100 -150 -200
• Temperature : No Change for Mean and Std (over all domain)
Mean Innovation (Sal, Sim_Ref )
-0.4 0.0 0.4
0 -50 -100 -150 -200
Std Innovation (Sal, Sim_Ref)
0 -50 -100 -150 -200
0 0.25 0.5
Mean Innovation (Sal, Sim_Dac )
0 -50 -100 -150 -200
-0.4 0.0 0.4
Innovation = Data – Model Forecast; Data From CORA base
Std Innovation (Temp, Sim_Ref)
0 1.25 2.5
Std Innovation (Temp, Sim_Dac)
0 1.25 2.5
0 -50 -100 -150 -200
Std Innovation (Sal, Sim_Dac )
0 0.25 0.5
0 -50 -100 -150 -200
• Salinity : improvement in both Mean and Std especially near the surface (0-100m)
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LWETIDEsat
SLAobsSLA
Std Sla Increment (Sim_Dac) in 2009
Sim_Dac
Std Sla Increment (Sim_Ref) in 2009
Impact LWE !!!
Role of LWE ? DAC contains residual errors ?
Impact of the assimilation of HF (SLA Increment):
DACLWETIDEsatSLAobsSLA
• Large increment in the highest-eddy-energy regions (present more variability): North Atlantic Drift….
• Position of the tracks
Sim_Ref
• It means that the assimilation step does not integrate the total difference (innovation) between the model and data
Increment = Result of analysis (correction)
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Stddev LWE (2009)
Stddev Lwe (EN) in each point during 2009 Stddev Lwe (J2) in each point during 2009 Sttdev Lwe (J1g) in each point during 2009
0 4 8 Cm
• More variability in the barotropic region : North Sea • Important variabiliry for En (Med, Bob…)
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LWE impact
High frequency impact
Different experiences
Correction of Innovation in analysis
bubble (removing inter-tracks bias)
InnovationLWE
TIDESLA SLA : Sim_Lwe2
TIDESLASLA : Sim_Lwe1
LWETIDESLA SLA : Sim_Dac
DACLWETIDESLASLA : Sim_Ref
sat
obs
sat
obs
sat
obs
sat
obs
We adapt the background model for each experiment (2009)
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Std Sla Increment (Sim_Lwe1) in 2009
not apply LWE
TIDEsatSLAobsSLA
Sim_Lwe1
Std Sla Increment in 2009 Std Sla Increment (Sim_Dac) in 2009
Amplification the variability of increment sea level anomaly
DACLWETIDEsatSLAobsSLA LWETIDEsatSLAobsSLA
Impact LWE for assimilation (Sla Increment,2009)
Sim_Ref Sim_Dac
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Std Sla Increment in 2009 Std Sla Increment (Sim_Dac) in 2009
DACLWETIDEsatSLAobsSLA LWETIDEsatSLAobsSLA
Impact LWE for assimilation (Sla Increment,2009)
Std Sla Increment (Sim_Lwe2) in 2009
not apply LWE apply LWEInnovation(bubble)
InnovationLWETIDE
satSLA
obsSLA
Significant reduction in variability of the (Sla) increments
Sim_Ref Sim_Dac Sim_Lwe2
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Conclusion & prospects
• Improvement of DA diagnostic without the correction of the Dac in the model and data.
• More energy even at small scales
• Salinity improvement 0-100 m.
• On line Lwe correction (bubble, Innovation) gives better results….
Why it is more efficient to not applied the DAC correction in altimetry?
– the S1 S2 atmospheric waves are removed from the atmospheric pressure forcing used for DAC computation (while they are not removed for the IBI model forcing)
– ECMWF 6h analyses are used for DAC computation ( while ECMWF 3h analyses are used in the IBI model forcing)
– DAC = MOG2D_HF + IB_LF using a centered filter (cut-off =20 days) while a centered filtering is not possible with forecasting systems.
Multi-mission cross-calibration
Step 2 : Empirical reduction of Long-Wave Errors (LWE)
•Purpose : Correction of the long-wavelength errors (LWE) due to residual errors on tides, aliased HF residual signals, …
•Method : Reduction of the inter-track bias with Objective Analysis (Le Traon and al., 1998, JAOT 15)
Spatial scales considered for LWE : 300 to 800 km
A-priori mean LWE Variance : 8.5 (TP/J1) to 9 (EN) cm². Can locally reach 30-40 cm² or more
From SSH to altimeter products
LWE correction
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Impact Sla(HF) on Surface Relative Vorticity…
DACLWETIDEsat
SLAobsSLA LWETIDEsat
SLAobsSLA
Vorticity (Sla With Dac) fro 05/12/2009 Vorticity (Sla Withou Dac) fro 05/12/2009
More energy : Eddies, filements we seem to increase the resolution of the model
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Mercator-Ocean :
• Operational system (Global, Regional) • Reanalysis (GLORYS, IBIRYS)
Convential SLA Observations (e.g CMEMS/AVISO): VFEC : Verified, Filtered, SubSampled and Corrected)
LWEDACTIDESLASLA satvfec
“TAPAS” Sla Data Assimilation
Non along-track filtered
Non sub-sampled (i.e. 7km instead of 14Km)
Without Dynamical Atmospheric Correction (DAC= IBLF + MOG2DHF)
TAPAS : Tailored Altimetric Products for Assimilation Systems
LWEDACTIDESLASLA satvxxc
TAPAS SLA Observations, verified and Corrected
we use this data with different corrections
Residual orbitER, IBER, Residual Tide Corr and Aliased HF signals