Article Type:
Research Article

Measurement of Directional Wave Spectra Using Aircraft Laser Altimeters

J. Sun National Center for Atmospheric Research, Boulder, Colorado

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S. P. Burns National Center for Atmospheric Research, Boulder, Colorado

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D. Vandemark NASA Goddard Space Flight Center, Wallops Island, Virginia

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M. A. Donelan RSMAS/Applied Marine Physics, University of Miami, Miami, Florida

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L. Mahrt College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon

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Timothy L. Crawford NOAA/ARL/Field Research Division, Idaho Falls, Idaho

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T. H. C. Herbers *Department of Oceanography, Naval Postgraduate School, Monterey, California

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G. H. Crescenti FLP Energy, Juno Beach, Florida

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J. R. French National Oceanic and Atmospheric Administration, Oak Ridge, Tennessee

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Print Publication:
01 Jul 2005
DOI:
https://doi.org/10.1175/JTECH1729.1
Page(s):
869–885
Restricted access

Abstract

A remote sensing method to measure directional oceanic surface waves by three laser altimeters on the NOAA LongEZ aircraft is investigated. To examine feasibility and sensitivity of the wavelet analysis method to various waves, aircraft motions, and aircraft flight directions relative to wave propagation directions, idealized surface waves are simulated from various idealized aircraft flights. In addition, the wavelet analysis method is also applied to two cases from field measurements, and the results are compared with traditional wave spectra from buoys. Since the wavelet analysis method relies on the “wave slopes” measured through phase differences between the time series of the laser distances between the aircraft and sea surface at spatially separated locations, the resolved directional wavenumber and wave propagation direction are not affected by aircraft motions if the resolved frequencies of the aircraft motion and the wave are not the same. However, the encounter wave frequency, which is directly resolved using the laser measurement from the moving aircraft, is affected by the Doppler shift due to aircraft motion relative to wave propagations. The wavelet analysis method could fail if the aircraft flies in the direction such that the aircraft speed along the wave propagation direction is the same as the wave phase speed (i.e., the aircraft flies along wave crests or troughs) or if two waves with different wavelengths and phase speed have the same encountered wavelength from the aircraft. In addition, the data noise due to laser measurement uncertainty or natural isotropic surface elevation perturbations can also affect the relative phase difference between the laser distance measurements, which in turn affects the accuracy of the resolved wavenumber and wave propagation direction. The smallest waves measured by the lasers depend on laser sampling rate and horizontal distances between the lasers (for the LongEZ this is 2 m). The resolved wave direction and wavenumber at the peak wave from the two field experiments compared well with on-site buoy observations. Overall, the study demonstrates that three spatially separated laser altimeters on moving platforms can be utilized to resolve two-dimensional wave spectra.

@@ Deceased

Corresponding author address: Dr. Jielun Sun, NCAR/MMM, P.O. Box 3000, 3450 Mitchell Lane, Boulder, CO 80307-3000. Email: jsun@ucar.edu

Abstract

A remote sensing method to measure directional oceanic surface waves by three laser altimeters on the NOAA LongEZ aircraft is investigated. To examine feasibility and sensitivity of the wavelet analysis method to various waves, aircraft motions, and aircraft flight directions relative to wave propagation directions, idealized surface waves are simulated from various idealized aircraft flights. In addition, the wavelet analysis method is also applied to two cases from field measurements, and the results are compared with traditional wave spectra from buoys. Since the wavelet analysis method relies on the “wave slopes” measured through phase differences between the time series of the laser distances between the aircraft and sea surface at spatially separated locations, the resolved directional wavenumber and wave propagation direction are not affected by aircraft motions if the resolved frequencies of the aircraft motion and the wave are not the same. However, the encounter wave frequency, which is directly resolved using the laser measurement from the moving aircraft, is affected by the Doppler shift due to aircraft motion relative to wave propagations. The wavelet analysis method could fail if the aircraft flies in the direction such that the aircraft speed along the wave propagation direction is the same as the wave phase speed (i.e., the aircraft flies along wave crests or troughs) or if two waves with different wavelengths and phase speed have the same encountered wavelength from the aircraft. In addition, the data noise due to laser measurement uncertainty or natural isotropic surface elevation perturbations can also affect the relative phase difference between the laser distance measurements, which in turn affects the accuracy of the resolved wavenumber and wave propagation direction. The smallest waves measured by the lasers depend on laser sampling rate and horizontal distances between the lasers (for the LongEZ this is 2 m). The resolved wave direction and wavenumber at the peak wave from the two field experiments compared well with on-site buoy observations. Overall, the study demonstrates that three spatially separated laser altimeters on moving platforms can be utilized to resolve two-dimensional wave spectra.

@@ Deceased

Corresponding author address: Dr. Jielun Sun, NCAR/MMM, P.O. Box 3000, 3450 Mitchell Lane, Boulder, CO 80307-3000. Email: jsun@ucar.edu

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