temperature ‘T’-sensors along its central axis. More principally, the Reynolds
decomposition requirement of a spectral gap between mean flow and turbulence is
not found in the ocean. The method was thus also found inadequate for the deep
ocean.
Quantifying Internal Wave Turbulent Mixing:
Moored High-Resolution T-sensors
As an alternative, NIOZ developed high-resolution T-sensors to be moored with
relatively many and supported by some current and pressure measurements. Initially
T-sensors were connected via cables to a central data logger and power supply unit
[22]. After considerable connection problems it was decided to work with stand-alone
self-contained sensors [49]. The high-resolution <0.1 mK and high precision <0.5
mK sensors have a response time of 0.25 s in water and standard sample at a rate of
1 Hz (adjustable) for a duration of one year on one AA-battery. Typically, 100 sensors
are taped to a plastic-coated steel cable of several 100 m long. This offers great
flexibility and sensors have been taped at intervals between 0.04 and 16 m depending
on the required configuration for study. This is adequate to resolve most of the
stratified turbulence and internal wave overturning scales as mean Ozmidov scales are
between 0.1 and a few 10’s of meters while large-scale internal wave amplitudes reach
100 m. The steel cable is used to synchronize the sensor clocks so that a profile of up to
600 m long (tested) is measured within 0.02 s. Such a snapshot over this vertical range
is not achievable using 0.7–1.0 m s
−1 free-falling or lowered profilers. The large
number of sensors in a profile is also adequate to correct for sensor drift, which
amounts about 1 mK/mo after aging. For an averaging period longer than the buoyancy period, but commonly taken longer than the inertial period to be sure, turbulent
overturns cannot last and the mean ocean profile is statically stably stratified. Any
remaining unstable deviations are thus instrumental (or salinity driven, see below) and
need to be corrected. This is achieved by forcing the mean observed profile to a
smooth, stable profile using a constant correction per sensor. In practice, an averaging
period of 2–7 days is chosen for this local correction and repeated for different times in
the (up to yearlong) record.
Considerable effort is put in improving the mooring design by minimizing its
motions to obtain almost truly Eulerian measurements with negligible artifacts in the
frequency range of interest. At NIOZ it is preferred to have all buoyancy near the top
so that recovery after successful release of the bottom weight is smooth without
entanglement of cables. As large-scale ocean currents are almost horizontally, the
associated drag forces by any object obstructing the flow are nearly perpendicular to
gravity (negative buoyancy). A balance of forces is thus impossible and the mooring
will inevitably be deflected by a non-zero flow. We impose the criterion that the
vertical deflection may never exceed half the distance between any of the instruments along the mooring line, so typically <0.5 m for a mooring with a string of 1-m
132
H. van Haren
decomposition requirement of a spectral gap between mean flow and turbulence is
not found in the ocean. The method was thus also found inadequate for the deep
ocean.
Quantifying Internal Wave Turbulent Mixing:
Moored High-Resolution T-sensors
As an alternative, NIOZ developed high-resolution T-sensors to be moored with
relatively many and supported by some current and pressure measurements. Initially
T-sensors were connected via cables to a central data logger and power supply unit
[22]. After considerable connection problems it was decided to work with stand-alone
self-contained sensors [49]. The high-resolution <0.1 mK and high precision <0.5
mK sensors have a response time of 0.25 s in water and standard sample at a rate of
1 Hz (adjustable) for a duration of one year on one AA-battery. Typically, 100 sensors
are taped to a plastic-coated steel cable of several 100 m long. This offers great
flexibility and sensors have been taped at intervals between 0.04 and 16 m depending
on the required configuration for study. This is adequate to resolve most of the
stratified turbulence and internal wave overturning scales as mean Ozmidov scales are
between 0.1 and a few 10’s of meters while large-scale internal wave amplitudes reach
100 m. The steel cable is used to synchronize the sensor clocks so that a profile of up to
600 m long (tested) is measured within 0.02 s. Such a snapshot over this vertical range
is not achievable using 0.7–1.0 m s
−1 free-falling or lowered profilers. The large
number of sensors in a profile is also adequate to correct for sensor drift, which
amounts about 1 mK/mo after aging. For an averaging period longer than the buoyancy period, but commonly taken longer than the inertial period to be sure, turbulent
overturns cannot last and the mean ocean profile is statically stably stratified. Any
remaining unstable deviations are thus instrumental (or salinity driven, see below) and
need to be corrected. This is achieved by forcing the mean observed profile to a
smooth, stable profile using a constant correction per sensor. In practice, an averaging
period of 2–7 days is chosen for this local correction and repeated for different times in
the (up to yearlong) record.
Considerable effort is put in improving the mooring design by minimizing its
motions to obtain almost truly Eulerian measurements with negligible artifacts in the
frequency range of interest. At NIOZ it is preferred to have all buoyancy near the top
so that recovery after successful release of the bottom weight is smooth without
entanglement of cables. As large-scale ocean currents are almost horizontally, the
associated drag forces by any object obstructing the flow are nearly perpendicular to
gravity (negative buoyancy). A balance of forces is thus impossible and the mooring
will inevitably be deflected by a non-zero flow. We impose the criterion that the
vertical deflection may never exceed half the distance between any of the instruments along the mooring line, so typically <0.5 m for a mooring with a string of 1-m
132
H. van Haren