Transcript Document

ODINAFRICA/GLOSS
Sea Level Training Course
TIDE GAUGES AND ALTIMETRY IN THE
GULF OF GUINEA
13-24 November 2006, Oostende
Angora AMAN
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
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
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 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
Altimeter
measures
geocentric
sea level and
ice sheet
elevation
change
Measurement
Coverage:
Altimeter Crossover
TOPEX/POSEIDON,
Jason:
660 latitude coverage
ERS-1/2, Envisat
820 latitude coverage
Geosat, GFO
720 latitude coverage
• Mean Sea Surface:
–100 m to +80 m
• Geoid ~ MSS
• Ocean topography:
~ several meters
• Ellipsoid: ~6378 km
• Altimeter altitude:
800 – 1300 km
Radar Altimeter Geometry
• 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)
• 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.
• 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.
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
Past and Current altimeter satellites
• Satellite
•
•
•
•
•
•
•
•
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SKYLAB
GEOS-3
SEASAT
GEOSAT
ERS-1
ERS-2
T/P
GFO
JASON
ENVISAT
Years
1972
1975-1978
1978
1985-1990
1991-1996
1995-2006
1992-2005
200020012002-
Organisation
Accuracy
NASA
NASA
NASA
US Navy
ESA
ESA
NASA/CNES
US Navy
NASA/CNES
ESA
20 m
3m
2m
30 cm
4-10 cm
4 cm
2 .. 3 cm
2 .. 5 cm
2 .. 3 cm
2 .. 3 cm
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.
APPLICATION
• Sea Level variability
• Mean Sea Level variability
Why Tide Gauges
in the “Age of Altimetry”?
• Principle of continuity, relative low cost of
gauges
• Long
records
for
trend/acceleration
studies (e.g. for input to IPCC)
secular
• Higher frequency sampling important in
straits
and other areas
• High latitude regions of ice coverage
Acoustic Gauge in
Australia
• Altimeter
‘relative’)
• Coastal
Module)
calibrations
applications
(‘absolute’
(GOOS
and
Coastal
ALTIMETRY AND TG RECORDS IN THE
GULF OF GUINEA
• POINTE NOIRE
• SAO TOME
• SAN PEDRO
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)
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
moyenne par quinzaine de la TSM et du NM 1991 à 1996
30
120
NM
115
26
110
24
105
22
20
18
100
95
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
NM (cm)
TSM (°C)
28
TSM
Seasonal upwelling at Pointe Noire and San Pedro
using SST in situ measurements
29.000
27.000
25.000
23.000
SST
Pointe Noire
San Pedro
21.000
19.000
17.000
15.000
Decembre
Novembre
Octobre
Septembre
Aout
Juillet
Juin
Mai
Avril
Mars
Fevrier
Janvier
Month
PROPAGATION OF COASTAL UPWELLING SIGNAL
USING
- SST DERIVED FROM SATELLITE
- SEA LEVEL ANOMALIES FROM SATELLITE
ALTIMETER
Conclusion
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
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 !!!!!!
Estimated Global Sea Level Rise Using Tide
Gauges and Satellite Altimetry (1948–2003)
Estimated Sea Level Rise = 1.74±0.24 mm/yr
585 selected tide gauges, multiple satellite altimetry used
CK Shum
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
…..