THE NEAR-SURFACE LAYER OF THE OCEAN
disturbance is six orders of magnitude greater than the useful signal. Such
exceptionally strong disturbance from surface waves imposes special
requirements on the measurement techniques and sensors for observations of
near-surface turbulence. Buoy devices (Jones and Kenney, 1977), ship- or
submarine-mounted instruments (Stewart and Grant, 1962; Volkov et al.,
1989; Osborn et al., 1992; Soloviev and Lukas, 2003), free ascending
profilers (Soloviev et al., 1988), tower-based instruments (Terray et al.,
1996), and autonomous underwater vehicles (Thorpe et al., 2003a) have been
utilized to obtain measurements in the upper few meters of the ocean.
Each approach offers different insights into near-surface physics and
suffers from different limitations. For example, tower-based measurements
yield temporal measurements. However, there is no clear separation in
frequency space of the surface wave velocity field and that due to
turbulence. The lack of a dependable mean speed prevents conversion to the
spatial domain, which complicates correct estimation of the kinetic energy
dissipation rate. Vertical profiling methods can provide such estimates if the
vertical speed of the profiler is much greater than the surface wave orbital
velocities. However, vertical profiling is inefficient for obtaining large
sample sets of turbulence statistics in the near-surface region due to the
change of the turbulent statistics as a function of depth. It might also be
difficult to detect and adequately measure regions of large horizontal
gradients by vertical profiling.
Among the key factors that can affect the quality and interpretation of
measurements in the near surface layer from a vessel are surface wave
perturbations, influence of the ship wake, variation of the sensor motion,
impact of bubbles on conductivity (and hence salinity and density)
measurements, and strong electrical currents coupled to the water near the
vessel due the ship’s electrical field. Sharp vertical gradients in the nearsurface physics can also be a factor depending on the measurement
approach.
Towed methods can efficiently generate large sample sets of dissipation
estimates, but they are typically degraded by broadband motion
contamination due to the non-stationary push/pull motion of these devices.
The large area of influence of the ship’s wake can also affect near-surface
measurements. Ship’s bow-, submarine-, or AUV-mounted devices can
efficiently produce large sample sets, but offer unique challenges - such as
the need to assess and minimize ship motion-induced effects and flow
perturbations in the vicinity of the sensors due to interaction of the vehicle
with the wave field.
Three principal questions associated with collecting turbulence data near
the ocean surface are as follows:
1) What reference system should be used?
2) How can the flow disturbance from the vehicle carrying the sensor
be reduced?
152
disturbance is six orders of magnitude greater than the useful signal. Such
exceptionally strong disturbance from surface waves imposes special
requirements on the measurement techniques and sensors for observations of
near-surface turbulence. Buoy devices (Jones and Kenney, 1977), ship- or
submarine-mounted instruments (Stewart and Grant, 1962; Volkov et al.,
1989; Osborn et al., 1992; Soloviev and Lukas, 2003), free ascending
profilers (Soloviev et al., 1988), tower-based instruments (Terray et al.,
1996), and autonomous underwater vehicles (Thorpe et al., 2003a) have been
utilized to obtain measurements in the upper few meters of the ocean.
Each approach offers different insights into near-surface physics and
suffers from different limitations. For example, tower-based measurements
yield temporal measurements. However, there is no clear separation in
frequency space of the surface wave velocity field and that due to
turbulence. The lack of a dependable mean speed prevents conversion to the
spatial domain, which complicates correct estimation of the kinetic energy
dissipation rate. Vertical profiling methods can provide such estimates if the
vertical speed of the profiler is much greater than the surface wave orbital
velocities. However, vertical profiling is inefficient for obtaining large
sample sets of turbulence statistics in the near-surface region due to the
change of the turbulent statistics as a function of depth. It might also be
difficult to detect and adequately measure regions of large horizontal
gradients by vertical profiling.
Among the key factors that can affect the quality and interpretation of
measurements in the near surface layer from a vessel are surface wave
perturbations, influence of the ship wake, variation of the sensor motion,
impact of bubbles on conductivity (and hence salinity and density)
measurements, and strong electrical currents coupled to the water near the
vessel due the ship’s electrical field. Sharp vertical gradients in the nearsurface physics can also be a factor depending on the measurement
approach.
Towed methods can efficiently generate large sample sets of dissipation
estimates, but they are typically degraded by broadband motion
contamination due to the non-stationary push/pull motion of these devices.
The large area of influence of the ship’s wake can also affect near-surface
measurements. Ship’s bow-, submarine-, or AUV-mounted devices can
efficiently produce large sample sets, but offer unique challenges - such as
the need to assess and minimize ship motion-induced effects and flow
perturbations in the vicinity of the sensors due to interaction of the vehicle
with the wave field.
Three principal questions associated with collecting turbulence data near
the ocean surface are as follows:
1) What reference system should be used?
2) How can the flow disturbance from the vehicle carrying the sensor
be reduced?
152