13
Lagrangian method in which a parcel of water is tracked by means of a drifting
body. Each method has its strengths and weaknesses so that use of either method is
governed by the needs of a particular application. For coastal operations, concern is
focused on velocity (incorporating a directional component) and dispersion, characterized by stochastic drift of individual Lagrangian bodies embedded in turbulent
flow. Stringent operational requirements of ocean observing and the difficulty of
deploying and recovering individual drifting bodies favor the Eulerian method. A
variety of current meters based on different principles are available incorporating
electropotential, acoustic, and Bragg radio scattering methods. Lagrangian methods are not particularly favored for routine operational observation but find use
during field campaigns to provide validation for shore-based radio frequency observation of ocean currents to be discussed below.
Eulerian Current Observation
Historic: Ekman, Savonius Rotors In situ Eulerian current meters have been a
fixture in the ocean sciences for over 100 years since the advent of the Ekman current meter in 1903. This ingenious apparatus consisted of a small propeller-like
rotor mounted inside an annular duct that protects the rotor and at the same time
dampens the effect of vertical movements caused by wave action or platform heave.
Propeller rotation was fed to a clockwork mechanism ending in three mechanical
dials that recorded units, tens and hundreds of revolutions. A large vane and a swivel
mount allowed orientation of the apparatus to the current axis. To measure direction,
steel balls were periodically dropped into a cup separated into pie-shaped
chambers.
The first modern electronic instrument was introduced in the 1960s by Ivar
Aanderaa incorporating a Savonius rotor, which more efficiently dampened the
effect of vertical wave-induced motion, a strong pressure housing, and an electronic
package incorporating an analog to digital converter (ADC). Digital data received
from the ADC was recorded on tape using onboard recorders within the pressure
housing. This development can be recognized as a significant milestone in the adoption of digital electronic data systems for ocean observing with greatly increased
capacity for data handling and data processing. Despite the great advance provided
by the Savonius-rotor meters, on the other hand, these mechanical systems were still
subject to corrosion, material wear, and biological fouling and were rapidly replaced
by newer technology using systems free of moving parts.
Ocean Current Measurements Through Electromagnetic Induction
Physicochemical effects such as those used to measure salinity can be effectively
inverted to measure currents. The moving ions in seawater induce current in a stationary orthogonal conductor. Existing undersea cables have been used successfully
to document large volume transport. The long-term measurements of Florida
Current variability using retired submarine telephone cables (Baringer and Larsen
2001; Peng et al. 2009) are a remarkable example.
2.2 Electronic Sensors and Instruments for Ocean Observing
Lagrangian method in which a parcel of water is tracked by means of a drifting
body. Each method has its strengths and weaknesses so that use of either method is
governed by the needs of a particular application. For coastal operations, concern is
focused on velocity (incorporating a directional component) and dispersion, characterized by stochastic drift of individual Lagrangian bodies embedded in turbulent
flow. Stringent operational requirements of ocean observing and the difficulty of
deploying and recovering individual drifting bodies favor the Eulerian method. A
variety of current meters based on different principles are available incorporating
electropotential, acoustic, and Bragg radio scattering methods. Lagrangian methods are not particularly favored for routine operational observation but find use
during field campaigns to provide validation for shore-based radio frequency observation of ocean currents to be discussed below.
Eulerian Current Observation
Historic: Ekman, Savonius Rotors In situ Eulerian current meters have been a
fixture in the ocean sciences for over 100 years since the advent of the Ekman current meter in 1903. This ingenious apparatus consisted of a small propeller-like
rotor mounted inside an annular duct that protects the rotor and at the same time
dampens the effect of vertical movements caused by wave action or platform heave.
Propeller rotation was fed to a clockwork mechanism ending in three mechanical
dials that recorded units, tens and hundreds of revolutions. A large vane and a swivel
mount allowed orientation of the apparatus to the current axis. To measure direction,
steel balls were periodically dropped into a cup separated into pie-shaped
chambers.
The first modern electronic instrument was introduced in the 1960s by Ivar
Aanderaa incorporating a Savonius rotor, which more efficiently dampened the
effect of vertical wave-induced motion, a strong pressure housing, and an electronic
package incorporating an analog to digital converter (ADC). Digital data received
from the ADC was recorded on tape using onboard recorders within the pressure
housing. This development can be recognized as a significant milestone in the adoption of digital electronic data systems for ocean observing with greatly increased
capacity for data handling and data processing. Despite the great advance provided
by the Savonius-rotor meters, on the other hand, these mechanical systems were still
subject to corrosion, material wear, and biological fouling and were rapidly replaced
by newer technology using systems free of moving parts.
Ocean Current Measurements Through Electromagnetic Induction
Physicochemical effects such as those used to measure salinity can be effectively
inverted to measure currents. The moving ions in seawater induce current in a stationary orthogonal conductor. Existing undersea cables have been used successfully
to document large volume transport. The long-term measurements of Florida
Current variability using retired submarine telephone cables (Baringer and Larsen
2001; Peng et al. 2009) are a remarkable example.
2.2 Electronic Sensors and Instruments for Ocean Observing