ODINAFRICA/GLOSS ODINAFRICA/GLOSS Sea Level Training CourseSea Level Training Course

TIDE GAUGES AND ALTIMETRY IN THE GULF OF GUINEA

13-24 November 2006, Oostende

Angora AMAN

ContentsContents

• The systemThe system• - Principles of altimetry- Principles of altimetry• - sampling characteristics- sampling characteristics• ApplicationApplication- - Mean sea surfaces- Mean sea surfaces- - Sea Level variability- Sea Level variability• Why TG in the age of Altimetry?Why TG in the age of Altimetry?• - Comparison SL derived from T/P signal - Comparison SL derived from T/P signal

and TG and TG records (Pointe Noire, Sao Tome, records (Pointe Noire, Sao Tome, San Pedro)San Pedro)

• - Propagation of coastal upwelling in the - Propagation of coastal upwelling in the GG using GG using T/P signal and TG recordsT/P signal and TG records

• ConclusionConclusion

Tide gauges limitsTide gauges limits

• 2 fundamental problems2 fundamental problems

• Tide gauges have limited spatial Tide gauges have limited spatial distribution and suboptimal coastal distribution and suboptimal coastal locations and thus provide poor locations and thus provide poor sampling of the open oceansampling of the open ocean

• Tide gauge measures sea level relative Tide gauge measures sea level relative to a crustal reference point, which may to a crustal reference point, which may be moving vertically at rates be moving vertically at rates comparable to the true sea level signalscomparable to the true sea level signals

What is satellite altimetry?What is satellite altimetry?

By means of a nadir looking radar we measure the reflection of short pulse in the footprint. This footprint is about 4 to 8 kilometer in diameter.

Source: JPL

Principles satellite altimetryPrinciples satellite altimetry

• Orbit DeterminationOrbit Determination– The position of the radar altimeter satellite is The position of the radar altimeter satellite is

derived from observations acquired from a derived from observations acquired from a network of ground stationsnetwork of ground stations

– Newer satellites carry their own GPS receiver, Newer satellites carry their own GPS receiver, but in principle the method remains the samebut in principle the method remains the same

• Radar data processingRadar data processing– The radar observes a waveform samples The radar observes a waveform samples – As scientists we get: range, significant wave As scientists we get: range, significant wave

height and a radar backscatter value, and height and a radar backscatter value, and scalar wind speed estimatesscalar wind speed estimates

– Great effort are made to calibrate/validate Great effort are made to calibrate/validate this data this data

• Geophysical corrections are applied the Geophysical corrections are applied the sea surface to remove all unwanted sea surface to remove all unwanted effectseffects

Altimeter Crossover

Measurement Coverage:

TOPEX/POSEIDON, Jason: 660 latitude coverageERS-1/2, Envisat820 latitude coverageGeosat, GFO720 latitude coverage

Radar Altimeter GeometryRadar Altimeter Geometry

Altimeter measures geocentric sea level and ice sheet elevation change

• Mean Sea Surface: –100 m to +80 m

• Geoid ~ MSS• Ocean topography:

~ several meters• Ellipsoid: ~6378 km• Altimeter altitude:

800 – 1300 km

• Satellite rangeSatellite range: It characterizes the : It characterizes the distance from the satellite to the sea distance from the satellite to the sea surfacesurface

• 2. 2. Orbital heightOrbital height of the satellite: a distance of the satellite: a distance from the satellite to a reference ellipsoidfrom the satellite to a reference ellipsoid

• 3. Conversion from time delay to distance. 3. Conversion from time delay to distance. The system requires an accurate The system requires an accurate measurements necessary to estimate the measurements necessary to estimate the index of refraction of the atmosphere index of refraction of the atmosphere (troposphere and ionosphere)(troposphere and ionosphere)

• The satellite transmits a radar The satellite transmits a radar pulse toward the ocean surfacepulse toward the ocean surface

• After passing through the After passing through the atmosphere, the pulse arrives at atmosphere, the pulse arrives at the Atmosphere/ocean boundary, the Atmosphere/ocean boundary, interacts with the ocean, and is interacts with the ocean, and is then reflected back toward the then reflected back toward the satellite, again through the satellite, again through the atmosphere.atmosphere.

• H= (Ta-Tt)C/2H= (Ta-Tt)C/2

• Tt: time of the pulse Tt: time of the pulse transmissiontransmission

• Ta: time the pulse arrival Ta: time the pulse arrival back at the back at the satellitesatellite

• C : speed of lightC : speed of light

• What the altimeter measures is the What the altimeter measures is the average waveform of thousands of average waveform of thousands of returned pulses as function of time.returned pulses as function of time.

Error Sources in Satellite Error Sources in Satellite AltimetryAltimetry

• Error due to the orbit determination Error due to the orbit determination • The estimate of the index of The estimate of the index of

refraction is bit complicated with refraction is bit complicated with regard to the wet tropospheric regard to the wet tropospheric correction (0.5 cm for the ionospheric correction (0.5 cm for the ionospheric correction and 1.1 cm for the wet correction and 1.1 cm for the wet tropospheric contribution)tropospheric contribution)

• Surface errorsSurface errors• The tide model error is ~1-2 cm The tide model error is ~1-2 cm

(Shum et al.,1997) in the open ocean(Shum et al.,1997) in the open ocean

Past and Current altimeter satellitesPast and Current altimeter satellites

• SatelliteSatellite Years Years OrganisationOrganisation AccuracyAccuracy

• SKYLABSKYLAB 1972 1972 NASANASA 20 m20 m• GEOS-3GEOS-3 1975-1978 1975-1978 NASANASA 3 m3 m• SEASATSEASAT 1978 1978 NASANASA 2 m2 m• GEOSAT 1985-1990GEOSAT 1985-1990 US NavyUS Navy 30 cm30 cm• ERS-1ERS-1 1991-1996 1991-1996 ESAESA 4-10 cm4-10 cm• ERS-2ERS-2 1995-2006 1995-2006 ESAESA 4 cm4 cm• T/P T/P 1992-2005 1992-2005 NASA/CNESNASA/CNES 2 .. 3 cm2 .. 3 cm• GFOGFO 2000- 2000- US Navy 2 .. 5 cmUS Navy 2 .. 5 cm• JASONJASON 2001- 2001- NASA/CNESNASA/CNES 2 .. 3 cm2 .. 3 cm• ENVISAT ENVISAT 2002- 2002- ESAESA 2 .. 3 cm2 .. 3 cm

Calibrating the measurementCalibrating the measurement

• One way to make an overall One way to make an overall assessment of the precision and assessment of the precision and accuracy of the satellite altimetry accuracy of the satellite altimetry system for producing sea surface system for producing sea surface heights is to compare these heights to heights is to compare these heights to sea level measurements from tide sea level measurements from tide gauges.gauges.

• However, it is not easily to attribute any However, it is not easily to attribute any errors so observed to a particular errors so observed to a particular component of the altimetric system. It component of the altimetric system. It provides an important end – to- end provides an important end – to- end assessment of all the system.assessment of all the system.

APPLICATIONAPPLICATION

• Sea Level variabilitySea Level variability

• Mean Sea Level variabilityMean Sea Level variability

Why Tide Gaugesin the “Age of Altimetry”?

Acoustic Gauge in Australia

• Principle of continuity, relative low cost of gauges

• Long records for secular trend/acceleration studies (e.g. for input to IPCC)

• Higher frequency sampling important in straits and other areas

• High latitude regions of ice coverage

• Altimeter calibrations (‘absolute’ and ‘relative’)

• Coastal applications (GOOS Coastal Module)

ALTIMETRY AND TG RECORDS IN ALTIMETRY AND TG RECORDS IN THE GULF OF GUINEATHE GULF OF GUINEA

• POINTE NOIRE POINTE NOIRE

• SAO TOMESAO TOME

• SAN PEDROSAN PEDRO

POINTE NOIREPOINTE NOIRE

• The first attempt to estimate sea The first attempt to estimate sea level using altimetric data was level using altimetric data was made by Menard (1988), Arnault et made by Menard (1988), Arnault et al. (1994) with GEOSAT altimeter.al. (1994) with GEOSAT altimeter.

• RMS difference of 7.1 cm (1988) RMS difference of 7.1 cm (1988) and 5.4 cm (1994)and 5.4 cm (1994)

The appearance of the upwelling event is detected by a The appearance of the upwelling event is detected by a drop of MSL starting May. This occurs 2 weeks prior to the drop of MSL starting May. This occurs 2 weeks prior to the

drop of SSTdrop of SST

moyenne par quinzaine de la TSM et du NM 1991 à 1996

18

20

22

24

26

28

30

J J F F M M A A M M J J J J A A S S O O N N D D Mois

TS

M (

°C)

95

100

105

110

115

120

NM

(cm

)

TSM

NM

Seasonal upwelling at Pointe Noire and San Seasonal upwelling at Pointe Noire and San Pedro using SST in situ measurementsPedro using SST in situ measurements

15.000

17.000

19.000

21.000

23.000

25.000

27.000

29.000

Janvie

r

Fevrier

Ma

rs

Avril

Ma

i

Juin

Juillet

Ao

ut

Se

pte

mb

re

Octo

bre

No

vem

bre

De

cem

bre

Month

SS

T Pointe Noire

San Pedro

PROPAGATION OF COASTAL UPWELLING PROPAGATION OF COASTAL UPWELLING SIGNAL USING SIGNAL USING

- SST DERIVED FROM SATELLITE - SST DERIVED FROM SATELLITE

- SEA LEVEL ANOMALIES FROM SATELLITE - SEA LEVEL ANOMALIES FROM SATELLITE ALTIMETERALTIMETER

ConclusionConclusion

ConclusionConclusion

• - Satellite altimeters could detect - Satellite altimeters could detect correctly the spatio-temporal correctly the spatio-temporal variability of SL in the GG with a variability of SL in the GG with a great confidence (RMS~ 2 cm).great confidence (RMS~ 2 cm).

- Description of the seasonal upwelling Description of the seasonal upwelling variability with great confidence variability with great confidence

- Analysis of the propagation of the Analysis of the propagation of the upwelling signal along the coastupwelling signal along the coast

ConclusionConclusion

• However, in spite of their accuracy, satellite However, in spite of their accuracy, satellite observations must be carefully processed and observations must be carefully processed and supported by in situ measurementssupported by in situ measurements

• The combination of altimetric signal and TG The combination of altimetric signal and TG measurements and numerical models will offer measurements and numerical models will offer an interesting way for climate study.an interesting way for climate study.

• PB???PB??? • Most of the tide gauges along the Gulf of Guinea Most of the tide gauges along the Gulf of Guinea

cost are abandoned or provide poor quality cost are abandoned or provide poor quality records.records.

• ODINAFRICA !!!!!!ODINAFRICA !!!!!!

Estimated Global Sea Level Rise Using Estimated Global Sea Level Rise Using Tide Gauges and Satellite Altimetry Tide Gauges and Satellite Altimetry

(1948–2003)(1948–2003)

Estimated Sea Level Rise = 1.74±0.24 mm/yrEstimated Sea Level Rise = 1.74±0.24 mm/yr 585 selected tide gauges, multiple satellite altimetry used585 selected tide gauges, multiple satellite altimetry used

CK Shum

PRODUCTS EXPECTED FROM PRODUCTS EXPECTED FROM ODINAFRICAODINAFRICA

Two types of products can be generated:Two types of products can be generated:

• - - Real timeReal time products such as detecting products such as detecting upwelling, forecasting storm surges,upwelling, forecasting storm surges,

• - D- Delay modeelay mode data product such as tidal data product such as tidal analysis, detecting extreme tide, analysis, detecting extreme tide, developing tide tables.developing tide tables.

Long term climate studiesLong term climate studies

High quality data for satellite calibrationHigh quality data for satellite calibration

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