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ON COMBINING BATHYMETRIC AND OCEAN CIRCULATION ALTIMETER MISSIONS

ON COMBINING BATHYMETRIC AND OCEAN CIRCULATION ALTIMETER MISSIONS. Frank M. Monaldo and David L. Porter The Johns Hopkins University Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel, MD 20723-6099 Frank.Monaldo@jhuapl.edu, David.L.Porter@jhuapl.edu.

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ON COMBINING BATHYMETRIC AND OCEAN CIRCULATION ALTIMETER MISSIONS

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  1. ON COMBINING BATHYMETRIC AND OCEAN CIRCULATION ALTIMETER MISSIONS Frank M. Monaldo and David L. Porter The Johns Hopkins University Applied Physics Laboratory 11100 Johns Hopkins Road, Laurel, MD 20723-6099 Frank.Monaldo@jhuapl.edu, David.L.Porter@jhuapl.edu

  2. Circulation v. Bathymetric Altimeter • Circulation (TOPEX/Poseidon) • High precision over long ocean basin scales for global circulation • Ionospheric / atmospheric corrections • Short repeat periods to obtain mean sea surface (10 days) • Bathymetry (ABYSS, Geosat) • High precision over short spatial scales • Tight ground track and long repeat period (180 days)

  3. Ground Track Coverage TOPEX/Poseidon Ground Track ERS-1 Ground Track

  4. Circulation (TOPEX) High-precision height Dual-frequency to correction for ionosphere On-board radiometer for atmospheric water vapor Short repeat period Higher Cost Bathymetry (ABYSS) High-precision height Single-frequency (modeled ionospheric correction) No radiometer, perhaps? (modeled water vapor correction) Long repeat period Lower cost Altimeter Comparison

  5. Orbit: JGM3-gravity Dry troposphere: ECMWF Wet troposphere: Radiometer Ionosphere: Smoothed dual-frequency measurement Ocean tide: GOT00.2 model Sea state bias: BM3/BM4 model Mean surface height: CLS01 Ocean depth / land elevation: ETOPO2 Orbit: JGM3-gravity Dry troposphere: ECMWF Wet troposphere: Radiometer or ECMWF Ionosphere: IRI95 Model Ocean tide: GOT00.2 model Sea state bias: BM3/BM4 model Mean surface height: CLS01 Ocean depth / land elevation: ETOPO2 Detailed Correction List ERS-1 TOPEX • ECMWF: European Centre for Medium-range Weather Forecasts, http://www.ecmwf.int • JGM-3: Tapley B.D., et al., The JGM-3 geopotential model. J. Geophys. Res., 101, 28,029-28,049, 1996 • GOT00.2: http://oceantopo.jpl.nasa.gov/science/invest-ray.html • IRI95: International Reference Ionosphere: http://nssdc.gsfc.nasa.gov/space/model/ionos/iri.html • SS Bias: Gaspar, P., F. Ogor, P.Y. Le Traon and O.Z. Zanife, Estimating the sea state bias of the TOPEX and POSEIDON altimeters from crossover differences. J. Geophys. Res., 99, 24,981-24,994, 1994. • CLS01: Herandez, F.,and P. Schaeffer, THE CLS01 mean sea surface, Ramonvile St. Agnes, France 2001 • ETOPO2: http://www.ngdc.noaa.gov/mgg/fliers/01mgg04.html All data from RADS (Radar Altimeter Data Base System)

  6. Conventional Wisdom • Circulation / bathymetry altimeters have been long divorced on the grounds of irreconcilable differences. • What if altimetry has matured more than is commonly recognized? • Can we now model the ionosphere and atmosphere well enough and do we know the mean sea surface well enough to monitor, at least, mesoscale circulation with an altimeter designed primarily for bathymetry? • Yes?

  7. Experiment • From 28 Sep 1994 to 21 March 1995, ERS-1 was in a 168-day repeat. • ERS-1 was a single-frequency altimeter with a water vapor radiometer. • During the same period, TOPEX was operating. • TOPEX is a dual-frequency altimeter with a water vapor radiometer • A comparison of mesoscale circulation fields is a way of assessing how well we might expect a bathymetric altimeter to perform in monitoring ocean mesoscale variability.

  8. Global Sea Level Anomaly Fields TOPEX ERS-1

  9. Standard Deviations in Sea Level Anomaly Fields TOPEX ERS-1

  10. TOPEX Sea Level Anomaly in North Atlantic 1994 Dec 15 to 1995 Jan 4

  11. ERS-1 Sea Level Anomaly in North Atlantic 1994 Dec 15 to 1995 Jan 4 Radiometer correction for water vapor Model correction for water vapor

  12. TOPEX & ERS-1 Sea Level Anomaly Color = TOPEX Lines = ERS-1 1994 Dec 15 to 1995 Jan 4

  13. Difference • Difference associated with orbits • Mean difference= -4.8 cm w/o radiometer, -2.6 cm with radiometer • Std Dev= 9.8 cm with and without radiometer • Correlation=0.72 with and without radiometer 1994 Dec 15 to 1995 Jan 4

  14. Standard Deviation Discussion • 9.8 cm std dev, lat/long cell point-by point • 4 ERS-1 points 7 TOPEX points • 7.6 cm smoothed and regularly sampled. • 4.6 cm when high pass filtered to eliminate difference variations greater than 500 km.

  15. Conclusions • A bathymetric altimeter should be able to measure SSH variability to within 9 to 5 cm. • The measured signal should have a correlation of 0.72 with respect to a TOPEX-class altimeter. • Additional questions: • Need to apportion errors as a function of wavelength. • Mean sea surface would improve through out a bathymetric mission reducing errors. • How do these errors propagate through underwater acoustic models? • Promising.

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