OPERATIONAL WAVE FORECASTING WITH SPACEBORNE SAR:
PROSPECTS AND PITFALLS
R. C. Beal
Applied Physics Laboratory
The Johns Hopkins University
Johns Hopkins Road
Laurel, Maryland 20707
1. Introduction
In April 1965, now more than 20 years ago, a small number of radio
scientists and oceanographers congregated at the Woods Hole Oceanographic
Institute to consider how the new tools of remote sensing might enhance our
understanding of the global ocean environment. Among the techniques consid-
ered were radar altimetry, radar scatterometry, and synthetic aperture ra-
dar. Of course, at that time, good data sets were scarce, and many of the
recommendations were by necessity somewhat speculative in nature. Neverthe-
less, the compliment of active microwave instruments that was identified was
very similar to the suite later flown on SEASAT in 1978.
Of particular interest to the topic of ocean wave forecasting is the
following quotation from Gifford Ewing, the conference editor. With respect
to the value of SAR in monitoring the global wave spectrum, he said:
"What is needed is the directional energy spectrum of the waves on
a two dimensional surface, and for this, the vantage point offered
by a satellite is ideal .... It is within the capability of pre-
sent day radar technology to give a complete description of the
sea surface".
That statement was made over 21 years ago. Since then, we have had
SEASAT, SIR-A, and SIR-B, all containing high resolution synthetic aperture
radars, and all collecting varying "descriptions of the sea surface". The
descriptions, however, are only more or less complete, and we are just re-
cently beginning to accumulate the evidence necessary to assess the true
value of SAR for obtaining useful estimates of the global directional energy
spectrum.
II. The Problem of Wave Prediction
Ocean wave prediction over global scales has been a fond hope of ocean-
ographers for several decades. Significant progress was made during and soon
after World War II, particularly when the concept of a "wave spectrum" was
advanced to describe the statistical properties of the wave field. The idea
of an "equilibrium spectrum" gained support from both theoretical and exper-
imental perspectives in the late 1950's, and has served as a framework for
departure even up to the present. Moreover, theories of wave growth are now
well developed in terms of the source wind field properties, such as fetch
and duration, and to some extent even in terms of the "background" wave
field. And yet, in the perspective of the global wave prediction problem,
there are still several fundamental unanswered questions. These questions
center on both the models and on the means for verifying them. For example:
0 How often must we measure the wave field in order to
predict it?
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