ODINAFRICA/GLOSS Sea Level Training Course

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ODINAFRICA/GLOSS Sea Level Training Course

• TIDE GAUGES AND ALTIMETRY IN THE GULF OF GUINEA
• 13-24 November 2006, Oostende
• Angora AMAN

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Contents

• The system
• - Principles of altimetry
• - sampling characteristics
• Application
• - Mean sea surfaces
• - Sea Level variability
• Why TG in the age of Altimetry?
• - Comparison SL derived from T/P signal
  and TG records (Pointe Noire, Sao Tome, San
  Pedro)
• - Propagation of coastal upwelling in the
  GG using T/P signal and TG records
• Conclusion

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Tide gauges limits

• 2 fundamental problems
• Tide gauges have limited spatial distribution
  and suboptimal coastal locations and thus provide
  poor sampling of the open ocean
• Tide gauge measures sea level relative to a
  crustal reference point, which may be moving
  vertically at rates comparable to the true sea
  level signals

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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
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Principles satellite altimetry

• Orbit Determination
• The position of the radar altimeter satellite is
  derived from observations acquired from a network
  of ground stations
• Newer satellites carry their own GPS receiver,
  but in principle the method remains the same
• Radar data processing
• The radar observes a waveform samples
• As scientists we get range, significant wave
  height and a radar backscatter value, and scalar
  wind speed estimates
• Great effort are made to calibrate/validate this
  data
• Geophysical corrections are applied the sea
  surface to remove all unwanted effects

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• Satellite range It characterizes the distance
  from the satellite to the sea surface
• 2. Orbital height of the satellite a distance
  from the satellite to a reference ellipsoid
• 3. Conversion from time delay to distance. The
  system requires an accurate measurements
  necessary to estimate the index of refraction of
  the atmosphere (troposphere and ionosphere)

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

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• H (Ta-Tt)C/2
• Tt time of the pulse transmission
• Ta time the pulse arrival back at the
  satellite
• C speed of light
• What the altimeter measures is the average
  waveform of thousands of returned pulses as
  function of time.

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Error Sources in Satellite Altimetry

• Error due to the orbit determination
• The estimate of the index of refraction is bit
  complicated with regard to the wet tropospheric
  correction (0.5 cm for the ionospheric correction
  and 1.1 cm for the wet tropospheric contribution)
• Surface errors
• The tide model error is 1-2 cm (Shum et
  al.,1997) in the open ocean

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Past and Current altimeter satellites

• Satellite Years Organisation Accuracy
• SKYLAB 1972 NASA 20 m
• GEOS-3 1975-1978 NASA 3 m
• SEASAT 1978 NASA 2 m
• GEOSAT 1985-1990 US Navy 30 cm
• ERS-1 1991-1996 ESA 4-10 cm
• ERS-2 1995-2006 ESA 4 cm
• T/P 1992-2005 NASA/CNES 2 .. 3 cm
• GFO 2000- US Navy 2 .. 5 cm
• JASON 2001- NASA/CNES 2 .. 3 cm
• ENVISAT 2002- ESA 2 .. 3 cm

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Calibrating the measurement

• One way to make an overall assessment of the
  precision and accuracy of the satellite altimetry
  system for producing sea surface heights is to
  compare these heights to sea level measurements
  from tide gauges.
• However, it is not easily to attribute any errors
  so observed to a particular component of the
  altimetric system. It provides an important end
  to- end assessment of all the system.

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APPLICATION

• Sea Level variability
• Mean Sea Level variability

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Why Tide Gauges in the Age of Altimetry?

• 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)

Acoustic Gauge in Australia
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ALTIMETRY AND TG RECORDS IN THE GULF OF GUINEA

• POINTE NOIRE
• SAO TOME
• SAN PEDRO

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POINTE NOIRE

• The first attempt to estimate sea level using
  altimetric data was made by Menard (1988),
  Arnault et al. (1994) with GEOSAT altimeter.
• RMS difference of 7.1 cm (1988) and 5.4 cm (1994)

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The appearance of the upwelling event is detected
by a drop of MSL starting May. This occurs 2
weeks prior to the drop of SST
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Seasonal upwelling at Pointe Noire and San Pedro
using SST in situ measurements
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PROPAGATION OF COASTAL UPWELLING SIGNAL USING
- SST DERIVED FROM SATELLITE- SEA LEVEL
ANOMALIES FROM SATELLITE ALTIMETER
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Conclusion
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Conclusion

• - Satellite altimeters could detect correctly the
  spatio-temporal variability of SL in the GG with
  a great confidence (RMS 2 cm).
• Description of the seasonal upwelling variability
  with great confidence
• Analysis of the propagation of the upwelling
  signal along the coast

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Conclusion

• However, in spite of their accuracy, satellite
  observations must be carefully processed and
  supported by in situ measurements
• The combination of altimetric signal and TG
  measurements and numerical models will offer an
  interesting way for climate study.
• PB???
• Most of the tide gauges along the Gulf of Guinea
  cost are abandoned or provide poor quality
  records.
• ODINAFRICA !!!!!!

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Estimated Global Sea Level Rise Using Tide Gauges
and Satellite Altimetry (19482003)
CK Shum
Estimated Sea Level Rise 1.740.24 mm/yr 585
selected tide gauges, multiple satellite
altimetry used
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PRODUCTS EXPECTED FROM ODINAFRICA

• Two types of products can be generated
• - Real time products such as detecting
  upwelling, forecasting storm surges,
• - Delay mode data product such as tidal
  analysis, detecting extreme tide, developing tide
  tables.
• Long term climate studies
• High quality data for satellite calibration
• ..