1. ODINAFRICA/GLOSS Sea Level Training Course
TIDE GAUGES AND ALTIMETRY IN THE GULF OF GUINEA
13-24 November 2006, Oostende
Angora AMAN

2. 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

4. 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

5. 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

6. 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

8. 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)

9. 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.

10. 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.

11. 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

12. Past and Current altimeter satellites
Satellite Years Organisation Accuracy
SKYLAB NASA 20 m
GEOS NASA 3 m
SEASAT NASA 2 m
GEOSAT US Navy 30 cm
ERS ESA cm
ERS ESA 4 cm
T/P NASA/CNES cm
GFO US Navy cm
JASON NASA/CNES cm
ENVISAT ESA cm

13. 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.

14. APPLICATION
Sea Level variability
Mean Sea Level variability

20. in the “Age of Altimetry”?
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

21. ALTIMETRY AND TG RECORDS IN THE GULF OF GUINEA
POINTE NOIRE
SAO TOME
SAN PEDRO

23. 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)

27. 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

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

29. PROPAGATION OF COASTAL UPWELLING SIGNAL USING SST DERIVED FROM SATELLITE - SEA LEVEL ANOMALIES FROM SATELLITE ALTIMETER

32. Conclusion

33. 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

34. 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 !!!!!!

35. Estimated Global Sea Level Rise Using Tide Gauges and Satellite Altimetry (1948–2003)
Plot present the sum of all forcing factors. Sea level drop near Greenland. Higher sea level trends appear in equatorial area.
CK Shum
Estimated Sea Level Rise = 1.74±0.24 mm/yr
585 selected tide gauges, multiple satellite altimetry used

36. 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
…..