o Does the background wave field modify wave growth? Are its
directional characteristics important?
o Is the directional wave spectrum a sufficiently complete
descriptor? How important are wave groups, which may give
rise to non-Gaussian statistics?
o How good are the models? How efficiently can they assimi-
late the data?
o How well can we remotely measure the wave field? How well
can we measure it from orbital altitude?
o Can we learn any new physics from the measurements?
o What is the optimum strategy for global wave monitoring?
Is is practical? Is its implementation likely to improve
wave forecasting?
We do not yet know the answers to these questions, but we are begin-
ning to accumulate a data base which is offering some major contributions
toward the solutions.
III. The SIR-B Extreme Waves Experiment off Chile
From October 8 to 12, 1984, as part of the SIR-B experiment, simultan-
eous and coincident measurements of the ocean wave directional energy spec-
trum were made in a NASA P-3 aircraft flying off the southwest coast of
Chile. Sea states during the experiment ranged from 1.5 m to 4.5 m, and
included fresh seas, decaying swells, and both unimodal and bimodal wave
fields. The two primary aircraft instruments were the Surface Contour Radar
(SCR; Walsh, 1985) and the Radar Ocean Wave Spectrometer (ROWS; Jackson,
1985), each of which was independently capable of measuring the directional
energy spectrum.
Figure 1 illustrates the geometry of the Chile experiment, and Table I
summarizes the various means of estimating (or predicting) properties of the
wave field. The SIR-B was operated through a variety of off-nadir incidence
angles, from 40? to 18?. Independent spectral estimates also were made with
the airborne optical laser (AOL) and the "AAFE" altimeter, a nadir-looking
altimeter yielding estimates of significant wave height. In addition to
the aircraft and spacecraft estimates, the U.S. Navy's Global Spectral Ocean
Wave Model (GSOWM), which only recently began operating in the Southern Hem-
isphere, yielded separate estimates of the directional wave spectrum.
IV. Experimental Results
Although a complete set of spectral comparisons is available for each
of the last four days of the experiment, only one set will be shown here,
that of October 11, 1984. Preliminary comparisons are also reported in Beal
et al, 1986a, and more complete comparisons of the entire data set will be
forthcoming.
Figure 2 shows the four-way comparison of wave number spectra for each
of the aforementioned methods. The spectra from the SCR, ROWS, and GSOWM are
all displayed in relative spectral energy density units of m^. The SAR
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