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 The Momentum Flux Balance at the Sea Surface
Donald Resio and Charles Long
Coastal and Hydraulics Laboratory
Army Engineer Research and Development Center
3909 Halls Ferry Road, Vicksburg, MS
1. Introduction
Today, second - and third -generation wave models are used for many applications
around the world. From the design of coastal structures to proper regional sediment
management and from estimates of mixing in the upper ocean to the efficient the routing
of ships across oceans, the accuracy of these models has critical importance. However,
following the development of the WAM model in the mid to late 1980's, there has been
little effort focused directly on wave model development. Instead, most research in this
area has involved re-calibrating various source terms to obtain better fits to observations
(satellite and in situ). Two likely reasons for the lack of focus in this area are 1) the
general acceptance outside of the wave community that the WAM model physics
provided a good "detailed -balance" description of the wave generation process and 2)
wave researchers cannot agree on necessary modifications to the WAM physics.
The purpose of this paper is to revisit the general framework of the physics of wave
generation and attempt to formulate a concept for wave generation that seems to "fit"
some important constraints implicit in observations over a wide range of generation
scales (time and space). Toward this end, data from three very different sites will be
used:
1. Lake George a small, shallow site located in Southeast Australia (Figure 1);
2. FRF Gage #630 a coastal site located in the Atlantic Ocean in a depth of
about 18 meters of water approximately 5 km off the coast of Duck, North
Carolina (Figure 2); and
3. NDBC Buoy 46035 an open-ocean site located in deep -water in the Bering
Sea (Figure 3).
These sites cover a wide range of practical wave generation scales, from shallow to deep
and from very small fetch to extremely large fetch.
2.
Theoretical Framework
Ever since the mid 1970's, most simulations of the evolution of wave spectra
under the action of the wind have included the effects of wave-wave interactions.
Unfortunately, this nonlinearity effectively couples the entire wave spectrum into a single
dynamic system, making it diffic ult to assess the precise roles of different forcing
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