Transcript Sea Level Slides

Sea Level
Change Measurements:
Estimates from Altimeters
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R. S. Nerem, University of Colorado
and a host of others
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Understanding Sea Level Rise and Variability
June 6-9, 2006
Paris, France
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What Are We Trying to Do?
• We would like to measure changes in the volume of
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the ocean water over time
This is difficult to do, so instead we measure
changes in the level of the top surface
When averaged globally, gives us changes in global
mean sea level
If possible, correct this for changes in the volume of
the ocean basins (GIA, etc.)
Why T/P and Jason?
• Every component of these measurement systems are
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of the highest possible fidelity (dual-frequency, POD,
atmospheric delays (ionosphere, troposphere), tidal
aliasing, etc.)
Altimeters in sun-synchronous orbits have
undesirable tidal aliasing characteristics, especially
for detecting climate signals
Nevertheless, combinations of sun-synchronous
altimeters (ERS, etc.) with T/P and Jason have been
used successfully to study sea level change in the
polar regions above ±66° latitude.
TOPEX/Poseidon and Jason
10-day Groundtrack
TOPEX Sea Level: January 1998
Global Mean Sea Level Variations
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TOPEX/Poseidon
Jason
²MSL (mm)
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Global Mean Sea Level Variations
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²MSL (mm)
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Global Mean Sea Level Variations
Seasonal variations removed
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²MSL (mm)
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Global Mean Sea Level Variations
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Seasonal variations removed
GIA correction applied (0.3 mm/year)
²MSL (mm)
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Rate = 3.2 ± 0.4 mm/year
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http://sealevel.colorado.edu
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Glacial Isostatic Adjustment
-7.0 -2.0 -1.5 -1.0 -0.5 0 0.5 1.0 1.5 2.0 25.0
Present-Day Radial Deformation (mm/year)
[Milne, 2005]
Effect of High Latitude Altimeter Data
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55° Cutoff
66° Cutoff
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MSL (mm)
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T/P/Jason GMSL Trends
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MSL (mm/year)
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Total versus Thermosteric Sea Level Change
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Total GMSL
Thermosteric (Willis)
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MSL (mm)
3.2 mm/year
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1.1 mm/year
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[Willis et al., 2005]
Altimeter - Thermosteric Sea Level
Sea Level Trend versus Latitude
(1993-2005)
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Total
Thermosteric
Difference
MSL (mm/year)
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Latitude
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Sea Level Variations versus Latitude
Global Sea Level Trends: 1993-2005
Global Mean Sea Level Variations
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Seasonal variations removed
GIA correction applied (0.3 mm/year)
²MSL (mm)
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Rate = 3.2 ± 0.4 mm/year
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http://sealevel.colorado.edu
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Platform Harvest Calibration Site
GPS ANTENNA (JPL)
WVR (JPL)
+ 54 m
MET SENSORS
LASER
SENSOR
(CU)
+ 27 m
EQUIP SHED
+6m
Point Arguello, CA
ACOUSTIC (NOAA)
BUBBLER (NOAA)
Harvest Altimeter Bias Results
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Altimeter
Epoch
TOPEX-A
POSEIDON-1
TOPEX-B
JASON
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LSQ Bias
(mm)
+4.3
–6.4
+3.1
+141.8
LAD Bias
(mm)
–0.4
+2.0
–1.2
+136.0
LSQ Drift
(mm/yr)
+2.7
–1.0
–3.0
–12.0
LAD Drift
(mm/yr)
+4.0
–3.1
–5.2
–9.2
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Bias (mm)
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[Haines et al., 2006]
Comparison of T/P and Jason-1
Map of Jason-1 relative bias (relative to global mean of 15.0 cm)
– A single calibration site can never accurately determine the true global
average
– SSH bias affected by multiple sources of geographically correlated
errors (SSB, orbit, troposphere, etc)
[Chambers, 2006]
Tide Gauges Available for Altimeter Calibration
Tide Gauges versus Satellite Altimetry
TOPEX and Jason Tide Gauge Calibrations
TOPEX
Jason
Error Analysis of the Trend
• Formal error = 0.05 mm/year
• After accounting for serial correlation of residuals,
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the error scales to 0.2 mm/year
Tide gauge calibration error = 0.4 mm/year
(dominated by errors in the land motion corrections)
This gives a total error in the trend of 0.44 mm/year
Thus our final trend estimate is +3.2 ± 0.4 mm/year
over 1993-2005
Ignores effects of decadal variability, other smaller
error sources
Estimating Biases Between Instruments
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TOPEX-A/B/J-1
Fit (a + b + c + dt)
Rate = 2.6 mm/year
MSL (mm)
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What Have We Learned from Altimetry?
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A great leap forward has occurred in sea level change science
in the last decade, largely due to the influx of satellite
altimeter measurements.
Satellite altimetry has observed an average increase in sea
level of 3.2 mm/year over 1993-2005, which probably reflects
an acceleration versus the 20th century rate (~1.8 mm/year).
Roughly half the current rise may be attributed to thermal
expansion, leaving half to the addition of water to the oceans
(likely from melting of mountain glaciers and polar ice).
Large spatial variability in the sea level rise signal has been
observed.
Together, altimetry and satellite gravity measurements provide
a powerful means of determining the amount of sea level
change and the causes of the change.
Sea Level Change Satellite Missions
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Medium accuracy RA (SSH) from high-inclination orbit
Geosat Follow-On
NPOESS
ERS-2 RA
OceanSat-3 AltiKa
Envisat RA-2
GMES S-3
High accuracy RA (SSH) from mid-inclination orbit
TOPEX/POSEIDON
JASON-1
Ice Altimetry
Jason-3 ?
JASON-2/OSTM
CRYOSAT-2
ICESAT
ICESAT-2 ?
Temporal Gravity (ocean mass)
GRACE
In orbit
Approved
GRACE follow-on ?
Pending approval
Dreams
Jason-3: New Orbit?
• The current T/P/J-1/J-2 orbit has a high 1336 km
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altitude (increased s/c costs), a relatively low 66°
inclination, and moderate spatial resolution (10-day
repeating orbit).
Lower altitude (800-1000 km) orbits exist that have
better latitude coverage (~78°), good tidal aliasing
characteristics, and variable repeat periods (10-21
days), which would provide a lower-cost mission with
broader scientific applications.
While continuing with Jason-3 in the T/P orbit is highly
desirable for sea level change applications, alternative
orbits could be considered.
Jason-3: New Orbit Considerations
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Sun-synchronous orbits undesirable
Non-repeating orbits undesirable
Continuation of the T/P/J-1/J-2 sea level time series
Tide gauge calibration of data from a new orbit
(offset with Jason-2)
Reference mean sea surface, geoid gradients
Tidal aliasing characteristics
Tide modeling errors
Tradeoffs of temporal/spatial sampling (repeat
period)
Latitudinal coverage (inclination)
Altitude
Conclusions
• We have made some great progress in sea level
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change science using satellite altimetry, but longer
datasets are needed to study climate signals.
The 13-year continuous satellite record of sea level
change is at significant risk, with the possibility of
gaps in the future, and no concrete plans for
ensuring the record after Jason-2 (20 years).
While it is highly desirable to continue in the T/P
orbit, alternative orbits with lower altitude and
higher inclination exist that could be considered to
continue the T/P/J-1/J-2 sea level time series.
Calibration of the altimeter instruments using tide
gauges is an indispensable component of any
altimeter-based sea level monitoring program.