ASIC**3 Workshop -- May 2006
Measuring Global Sea Level Rise With Satellite Radar Altimetry
ASIC**3 Workshop -- May 2006
ASIC**3 Workshop -- May 2006
Laury Miller
NOAA/NESDIS Lab for Satellite Altimetry
ASIC**3 Workshop -- May 2006
Special Thanks To:
Professor Gary Mitchum, Univ. of South Florida
Remko Scharroo, Altimetrics, LLP
John Lillibridge, NOAA Lab for Satellite Altimetry
ASIC**3 Workshop -- May 2006
Talk Outline
The Sea Level Rise Measurement Problem– Measuring a small trend or acceleration in the presence of large regional and decadal variability – Dealing with relatively short, possibly gappy records.
How the altimeter works
Relative calibration: island tide gauge network (poor geodetic control)
Absolute calibration: geodetically controlled sites at Harvest Platform, Lampadusa, etc.
In the Future?
ASIC**3 Workshop -- May 2006
Global Mean Sea Level From Multiple Altimeters
Decadal Rate (1992 to 2005)
2.97+/-0.4 mm/yr
20th Century Rate From Tide
Gauges 1.8+/-0.3 mm/yr
Altimeter observations show sea level rising nearly 50% faster over the past decade than over the 20th century, as determined from tide gauges. It is unclear whether this reflects a long-term change or evidence of decadal
variability.
ASIC**3 Workshop -- May 2006
Global Sea Level Trends: 1993 to Feb 2006From TOPEX & Jason-1 Altimetry
• Large regional variability.
• Largest average rise in southern hemisphere.
mm/year
ASIC**3 Workshop -- May 2006
The Principle of Satellite Altimetry
• Altimeter range
From radar round-trip time
• Satellite altitude
From from various tracking systems
• Sea surface height
Difference:satellite altitude – altimeter range – corrections
Sum of:geoid + dynamic topography + tides
ASIC**3 Workshop -- May 2006
Current Tide Gauge & Geodetic Sites
ASIC**3 Workshop -- May 2006
Miller & Douglas, Phil Trans. Roy. Soc., 2006
Relative Sea Level Trends & Distance From Hudson Bay
ASIC**3 Workshop -- May 2006
TOPEX vs. Christmas Island Sea Level
ASIC**3 Workshop -- May 2006
Altimeter Algorithm Error
The failure to detect this error early in the mission lead to the publication of an erroneous high rate of sea level rise in SCIENCE magazine.
TOPEX Drift (Altimeter - TideGauge Sea Level at ~50 Sites)
ASIC**3 Workshop -- May 2006
TOPEX Drift (+/-0.4 mm/yr)After Correction
ASIC**3 Workshop -- May 2006
Jason-1 Altimeter (2001 ->) vs. Tide Gauges Nominal Drift
ASIC**3 Workshop -- May 2006
Jason-1 Altimeter (2001 ->) vs. Tide Gauges After Correcting Water Vapor Radiometer Drift
ASIC**3 Workshop -- May 2006
TOPEX, Poseidon & Jason-1 Bias EstimatesThe Absolute Calibration Problem
14 CM ??
ASIC**3 Workshop -- May 2006
NASA/NOAA Absolute Altimetry Calibration Site Harvest Platform, Santa Barbara CA
Monitors absolute (geocentric) bias and bias drift through a collection of supporting measurements
• Platform Located directly beneath a Jason-1 track line.
• Sea height wrt platform determined with redundant tide gauge systems
• Platform height wrt reference ellipsoid determined with GPS and Very Long Baseline Interferometry (VLBI).
• Water vapor radiometer used to determine radar path length correction.
For TOPEX altimeter: bias 7.3 +/- 4.3 mm, bias drift -0.4 +/- 1.5 mm/yr
Bias drift error is comparable to rate of global sea level rise!
ASIC**3 Workshop -- May 2006
High Accuracy Altimetry: Past, Present, and Future
• 1991-2005: TOPEX Joint NASA/CNES mission
• 2001-->: Jason-1 Joint NASA/CNES mission
• Jason-2 (2008): NASA/CNES with NOAA & EUMETSAT as junior partners.
• Jason-3 (2013?): NOAA/EUMETSAT with NASA & CNES as junior partners.
Shift in Agency Responsibility Going From Research To Operations
ASIC**3 Workshop -- May 2006
The Calibration Challenge
• Need Overlap Between Missions -- Can’t Depend on Absolute Calibration
• Need To Maintain International Tide Gauge Network – NOAA Currently Supporting more than 40 gauges– Need to upgrade many sites with GPS.
• Need To Lower Current Error In Bias Trend Estimate To Improve Ability To Detect An Acceleration.
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