Measurement of Directional Wave Spectra Using Aircraft Laser Altimeters Public Deposited

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  • 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.
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  • Sun, J., and Coauthors, 2005: Measurement of Directional Wave Spectra Using Aircraft Laser Altimeters. Journal of Atmospheric and Oceanic Technology, 22, 869–885.
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  • 22
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  • 7
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  • Jielun Sun and Sean Burns were supported by ONRGrant N00014-98-1-0245 for SHOWEX and by ONRGrant N00014-01-C-0108 for CBLAST-Low. Mark Donelan was supported by ONR Grant N00014-97-1-0348. Larry Mahrt was supported by ONR GrantN000140110084. The NOAA LongEZ aircraft groupwas supported by ONR Contract N00014-97-F-0123 forthe SHOWEX field experiment and by ONR ContractN00014-01-F-0008 for CBLAST-Low. Thomas Herberswas supported by ONR Grant N0001404WR20065.
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