3) How can surface wave disturbances be eliminated from the signal?
3.2.2 Wave-following versus fixed coordinate system
Since the wave breaking energy substantially dissipates above the trough
line, and the dissipation rate profile is a nonlinear function of depth, then the
difference between fixed and wave-following measurements can be
significant. For instance, in a fixed coordinate system it is practically
impossible to study near-surface layers with a thickness less than the surface
wave height. Any observational point between the maximum wave trough
and crest will alternate between water and air. Therefore, in order to study
turbulence close to the ocean surface, a wave following coordinate system
should be used. Csanady (1984) suggested analyzing near-surface data in a
coordinate system connected to the ocean surface; this system is, however,
efficient only close to the ocean surface (within a few significant wave
heights). It is easy to see that for depths where the surface wave motion
diminishes, this coordinate system is irrelevant. The coordinate system
proposed in Section 3.1.1 (equations (3.1)-(3.2)) resolves this difficulty.
The Craig and Banner (1994) and Benilov (2002) models of waveenhanced turbulence that are discussed throughout this chapter are consistent
with the coordinate system (3.1)-(3.2). The Terray et al. (1996) model,
which was originally fitted to the data collected in a fixed coordinate system,
would produce a different dissipation profile in the wave-following
coordinate system.
The coordinate system (3.1)-(3.2) at a first glance seems to be infeasible.
However, it can be easily implemented with a free-ascending instrument by
a proper choice of its hydrodynamic characteristics (Section 3.2.4).
3.2.3 Disturbances from surface-waves
The orbital velocities of surface waves influence turbulence
measurements by
a) Generating an additive fluctuation velocity signal, and
b) Modulating the relative speed (and direction) of the flow.
We should mention here that (a) is an additive (that is a linear) process while
(b) is a nonlinear process, which resembles the process of frequency
modulation in radio techniques (Dozenko, 1974). Recall the difference
between AM (amplitude modulated) and FM (frequency modulated) ranges
in your automobile radio).
An additional complication is that the time scales of surface waves and
near-surface boundary-layer turbulence substantially overlap.
Chapter 3: NEAR-SURFACE TURBULENCE
153
(
3.2.2 Wave-following versus fixed coordinate system
Since the wave breaking energy substantially dissipates above the trough
line, and the dissipation rate profile is a nonlinear function of depth, then the
difference between fixed and wave-following measurements can be
significant. For instance, in a fixed coordinate system it is practically
impossible to study near-surface layers with a thickness less than the surface
wave height. Any observational point between the maximum wave trough
and crest will alternate between water and air. Therefore, in order to study
turbulence close to the ocean surface, a wave following coordinate system
should be used. Csanady (1984) suggested analyzing near-surface data in a
coordinate system connected to the ocean surface; this system is, however,
efficient only close to the ocean surface (within a few significant wave
heights). It is easy to see that for depths where the surface wave motion
diminishes, this coordinate system is irrelevant. The coordinate system
proposed in Section 3.1.1 (equations (3.1)-(3.2)) resolves this difficulty.
The Craig and Banner (1994) and Benilov (2002) models of waveenhanced turbulence that are discussed throughout this chapter are consistent
with the coordinate system (3.1)-(3.2). The Terray et al. (1996) model,
which was originally fitted to the data collected in a fixed coordinate system,
would produce a different dissipation profile in the wave-following
coordinate system.
The coordinate system (3.1)-(3.2) at a first glance seems to be infeasible.
However, it can be easily implemented with a free-ascending instrument by
a proper choice of its hydrodynamic characteristics (Section 3.2.4).
3.2.3 Disturbances from surface-waves
The orbital velocities of surface waves influence turbulence
measurements by
a) Generating an additive fluctuation velocity signal, and
b) Modulating the relative speed (and direction) of the flow.
We should mention here that (a) is an additive (that is a linear) process while
(b) is a nonlinear process, which resembles the process of frequency
modulation in radio techniques (Dozenko, 1974). Recall the difference
between AM (amplitude modulated) and FM (frequency modulated) ranges
in your automobile radio).
An additional complication is that the time scales of surface waves and
near-surface boundary-layer turbulence substantially overlap.
Chapter 3: NEAR-SURFACE TURBULENCE
153
(