improved satellite altimeter data dedicated to coastal areas :

Validation over the Northwestern Mediterranean

Sea (MAP team)

Jérôme BouffardJérôme Bouffard

Y. Ménard, L. Roblou, F. Birol, F. Lyard, R. MorrowY. Ménard, L. Roblou, F. Birol, F. Lyard, R. Morrow

Improved Satellite Improved Satellite Altimeter data dedicated Altimeter data dedicated

to coastal areas :to coastal areas :

2

ContextContext

Western Mediterranean Sea

Longitude

Latitudes

Gulf of Genoa

Altimetric tracks: - Topex/poseidon- Envisat- jason 1- GFOTide gauges:

LPCLPCCurrentCurrent

Corsica Channel

Gulf of Lion

Catalan Sea

Ligurian Sea

Balear islands

Algerian Bassin

Liguro Provencal Catalan (LPC) currentLiguro Provencal Catalan (LPC) currentComplex small mesoscale dynamics (Send, 1999)

LPC Instability + meanders (Conan and Millot, 1995) Seasonnal variability (Millot 1991)

Are altimeter data valuable in coastal areas ?Are altimeter data valuable in coastal areas ?How to improve altimeter data in coastal areas?How to improve altimeter data in coastal areas?

3

Available dataAvailable data

Features:• Standard editing• MOG2D Global (w+P) + FES2004 (tide) corrections• Along-track sampling every 7km (~ 1HZ)• Large scale and orbit error reduction (Le Traon et Ogor,

1998)

Standard distributed data:Standard distributed data:

AVISO regional along-track product: DT-(M)SLA “Upd”

Improved coastal data:Improved coastal data:Margins Altimetry Projects (MAP) : Xtrack SLA (see Lyard et al OSTST 2007)

Features:• Specific editing and correction re-building• MOG2D -Medsea (w+P) + MOG2D-Medsea (tide)

corrections• High resolution sampling every 350 or 700 m (~ 10HZ/

20 HZ)

Example at the Nice TGExample at the Nice TG

Impact of the processing Impact of the processing featuresfeatures

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Comparisons at the Nice TG:% of RMS explained: Impact of Impact of regional de-aliasingregional de-aliasing

correctionscorrectionsCorrelation: 0.75Residual RMS: 4.1 cmRMS explained: 33%

NiceTG

MAP-Xtrack GlobalGlobal de-aliasingGlobal de-aliasing

Correlation: 0.78Residual RMS: 3.9 cmRMS explained: 37%

MAP-Xtrack Medsea

Regional de-aliasingRegional de-aliasing

NiceTG

%10 40

MOG2D Medsea (Regional configuration) significantly improves the consistency beween the altimeter and the Nice tide gauge time series.

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MAP-xtrack MedseaHF no editing

NiceTG

Standard editingStandard editing


NiceTG

MAP-Xtrack Medsea HF

Specific editingSpecific editing

%10 40

Comparisons at the Nice TG:% of RMS explained: Impact of the Impact of the datadata editing methodologyediting methodology

The specific data editing methodology allows to decrease the noise in the altimeter time series

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MAP-Xtrack Medsea

1HZ sampling1HZ sampling

NiceTG


NiceTG


HF samplingHF sampling

%10 40

Comparisons at the Nice TG:% of RMS explained: Impact of the Impact of the High Frequency samplingHigh Frequency sampling

The high frequency sampling allows to go closer to the coast

Example at the Nice Tide GaugeExample at the Nice Tide Gauge

Comparisons of the Map-Comparisons of the Map-Xtrack data with a regional Xtrack data with a regional

standard productstandard product

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The MAP-Xtrack processing allows to recover more data far more data far and close to the coastand close to the coast

Comparisons at the Nice TG:Number of dataNumber of data MAP-Xtrack Medsea HFMAP-Xtrack Medsea HF vsvs DT-(M)SLA UpdDT-(M)SLA Upd

NiceTG


NiceTG

DT-(M) SLA Upd

AVISOproduct

MAPproduct

115 125 115 125

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NiceTG

NiceTG

• Better statistical results for the the Xtrack-HF dataBetter statistical results for the the Xtrack-HF data

Comparisons at the Nice TG:% of RMS explained:% of RMS explained: MAP-Xtrack Medsea HFMAP-Xtrack Medsea HF vsvs DT-(M) SLA UpdDT-(M) SLA Upd


DT-(M) SLA Upd

Correlation: 0.74% of RMS explained: 32.9




Detection of small coastal spatial structure: are they physically reallistic ?Are they Linked with the LPC dynamics ?

LPC

1

2

AVISOproduct

MAPproduct

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Hovmuller plots of across track geostrophic Hovmuller plots of across track geostrophic velocity anomalies (TP track-222 in 2001)velocity anomalies (TP track-222 in 2001)

latitude

Tim

e

Aviso product MAP product

1

LPC current LPC current Close to the LPC and to Close to the LPC and to the coast:the coast: The two signals are well The two signals are well phased:phased:

The altimetric signal has The altimetric signal has a lower amplitude:a lower amplitude:Intrinsic seasonal variability of the LPC is shifted toward the steric signal.

1

Tim

e

Eddy ?Eddy ?Meander ?Meander ?A

ugust

Janu

ary

Far from the LPC:Far from the LPC:Dephasing between the Dephasing between the two signals:two signals: the Offshore and the coastal don’t « see » the same dynamics (eddy ?) .The altimetric and TG The altimetric and TG signals have equivalent signals have equivalent amplitude:amplitude: The steric effect is a large wavelenght signal

2

August January

2

Coastoffshore

cm/s

Example at the Sete Tide GaugeExample at the Sete Tide Gauge



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Comparisons at the Sète TG:Number of available dataNumber of available data MAP-Xtrack Medsea HF vs DT-(M)

Upd

MAP-Xtrack medsea HF processing allows to recover more data far and close to the coastmore data far and close to the coast

MAP-Xtrack Medsea HF DT-(M) SLA Upd

MAPproduct

Avisoproduct

95 120 95 120

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Comparisons at the Sète TG:% of RMS explained:% of RMS explained: MAP-Xtrack Medsea HF vs DT-(M) SLA Upd

Correlation: 0.89% of rms explained: 52.8



Correlation: 0.77% of rms explained: 35.9Sète TG Sète TG

DT-(M) SLA UpdMAP-Xtrack Medsea

HF

Better statistical results for the the Xtrack-HF dataStronger improvement than at Nice TG

MAPproduct

Avisoproduct

10 40 10 40% %

Mean statistics at Tide gauges Mean statistics at Tide gauges over the whole areaover the whole area



Tide Gauges

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DT-(M) SLA 90 73 0.77 4.4 34 %

Xtrack 99 61 0.83 3.7 44 %

Number of dataNumber of dataDistance to Distance to TGs (km)TGs (km)

CorrelationCorrelation RMS differenceRMS difference(cm)(cm)

% of RMS% of RMS explainedexplained

MAP

Multi-satellite: Mean Statistics at TGsMulti-satellite: Mean Statistics at TGsTopex / PoseidonTopex / Poseidon

Xtrack

Std. editing39 50 0.82 3.9 43 %

DT-(M) SLA 39 60 0.75 4.8 34 %

GFOGFO

DT-(M) SLA 38 69 0.80 4.2 36 %

Jason 1Jason 1

Xtrack

Std. editing

+ orbit adjust.

39 59 0.81 3.9 39 %

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• Processings– Specific editings allow to eliminate spurious data and improve the

quality of coastal altimetric products – Regional de-aliasing models strongly improves altimetric data – High frequency along track sampling allow to highlight small spatial

dynamical processes

• Comparisons with a standard product– More coastal data with the MAP processings – Closer to the coastline – Better quality of the altimetric data

• Applications– Validation of Regional 3D models

– Cal/Val– Monitoring of transport in marginal areas

ConclusionConclusionSea level anomalies comparisons in the Bay of Biscay along Jason-1 ground track 137 (July-August 2004).

Blue: altimeter data

Red: Symphonie coastal model

• Good agreements on instantaneous sea levels

• Satisfying correlation for synoptic scales and meso-scales dynamics

• Altimeter data exhibit short scales processes not represented in the model simulations

See also Roblou et al, « x-track a new processing tool for altimetry in coastal oceans »

Correlation SYMPHONIE model elevations – altimetry SLA: TOPEX + GFO

Sea level anomalies comparisons in the Mediterranean Sea along Multi-satellite ground track (2001-2003).

See also Bouffard et al, « Improved Altimetry in the Northwestern Mediterranean : Comparison of Ocean Dynamics with a Regional Circulation Model »

Correlation SYMPHONIE model elevations – altimetry SLA: Jason + GFO

Correlation SYMPHONIE model elevations – altimetry SLA: Jason + GFO + Envisat

2001

2002

2003

0.7 0.9

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0,18

0 20 40 60 80 100 120 140 160

Cycle

SSH

abs

olut

e bi

as (m

)

SSH absolute bias at M3 tide gauge, Jason-1 pass 085

Mean=0.1046m

Std dev=0,0252m

Jason-1 pass 222

Jason-1 pass 085

M3 tide gauge (Senetosa)

CAL/VAL at the Senetosa Tide Gauge

CrossoverEnvisat 257

Jason85


Jason44

CrossoverEnvisat 588 Envisat 257


GFO 257


Jason 44

CrossoverGFO 257Jason 85

CrossoverJason 44Jason 85

Crossover pointsCrossover points

Merid

ion

al dirrectio

n

Ligurian Sea

Corsica C

hannel

In-situstation

Comparisons with the normalized transport (from in-situ data)

Multi-satellite crossover geostrophic velocities

Ellipses of geostrophic velocity anomalies at altimetric crossovers

See also poster Bouffard et al, «A view from multi-mission satelite altimetry over the coastal ocean: a study in the Ligurian Sea and the Corsica channel. » (OSTST 2007)

improved satellite altimeter data dedicated to coastal areas :

Documents