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.