The inherent variability of wave breaking at the sea surfaces has motivated the design and construction of a series of specialized buoy systems designed for air-sea interaction measurements of heat, momentum, and gas fluxes, directional wave spectra,
and the near-surface turbulence and bubble fields. Our experience from previous field efforts have identified a number of instrument design criteria for air-sea interaction measurements. We incorporated these criteria into the design of the buoys used for SHOWEX, and continuously work to improve and enhance each platform. A few of the key design considerations are listed below:
Turbulence Buoy Drawing
(click to enlarge)
The desire to obtain bubble, turbulence, and wave measurements over several storm cycles often eliminates the option of tethered instruments because of the high costs of a support vessel remaining on station for long durations.
Ease of deployment and recovery:
The desire to obtain bubble and turbulence measurements during periods of high sea states presents the difficulty of deploying and recovering instruments from a research vessel. While these difficulties can be reduced if measurement campaigns are restricted to larger vessels, the added costs of a larger vessel for a multi-day research cruise can be prohibitive.
High sampling rates:
Previous measurements in both the laboratory and field suggest that sampling rates of O(1)Hz are required to prevent aliasing with the temporal variability of wave breaking. Furthermore, the event like nature of wave breaking suggests that measurements of bubbles or turbulence at a moored location (or from a slowly drifting platform) must be sufficiently long in duration to arrive at meaningful statistics.
Our buoy systems are typically composed of three main sections: a central buoyancy section, a rigid subsurface spar section, and an instrument case fixed above the waterís surface.
A light weight buoy hull was custom built by the Gilman Corporation (Gilman, Connecticut). The hull shape is based on an existing hull design named the Abrupt Topography Buoy hull but lengthened to 0.86 m from the standard 0.69 m height. This modification increased the hullís load rating significantly. The hemispherical shape is designed to follow the surface in heave but remain unresponsive to roll. The hull material is an ionomer closed-cell foam (DuPont Surlyn) with a density of 0.064 g/cm3, providing an extremely high volume-to-weight ratio.
The hull has an outer diameter of 1.21 m, an inner diameter of 0.32 m, and a total weight of 52 kg. The maximum displacement of the hull shape is approximately 725 kg.
Bubble Size Distribution Buoy
(click to enlarge)
A top structure built of 6061 aluminum was designed to fit inside the well of the buoy hull and connect the spar section to the flotation through two sets of aluminum clamps. An instrument case which houses the data acquisition, telemetry, motion packages, and control systems is bolted to the top structure. Above-water meteorological sensors are also mounted to the top structure. These include a sonic anemometer, air temperature, barometric pressure.
The subsurface spar, built of 6061 aluminum tubing and typically ranging from 5-10m in length, provides a mounting location for various sensors.
Two such instrumented platforms have been constructed, each carrying a range of sensors. Their general descriptions are shown in the schematics presented on this page. Photographs may be found in the Point Conception, July 1998 and Torrey Pines, March 1999 test deployments. A description of the sensors follows in the next section.
A high-resolution thermistor chain composed of 8 separate channels was custom built to our specifications by Precision Measurement Engineering, Encinitas, California. Each node is sampled with 16 bit resolution. The response time of the thermistor bead is approximately 1/3 of a second and the calibrated accuracy of the system typically ranges from 0.003-0.005 degrees C.
5 MHz Acoustic Doppler Velocimeter, Sontek ADV OceanProbe
Measurements of turbulence in the water column are made with a vertical array of Sontek Acoustic Doppler Velocimeters (ADV's). The ADVs measure three components of the local velocity field at rates up to 25 Hz. The earth-referenced velocity field is obtained through the removal of platform motion using data obtained with a 6 DOF motion package and three-axis magnetometer.
1.7 MHz pulse-to-pulse coherent Doppler profiler, Sontek DopBeam
The Sontek 1.7 MHz single beam pulse-to-pulse coherent system is used to measure high resolution velocity profiles at several Hertz rate to ranges of O(1)m. The resulting profiles are used to directly calculate turbulent wavenumber spectra without invoking Taylor's frozen turbulence hypothesis.
Broadband Acoustic Bubble Measurements
A broad-band acoustical technique to measure bubble size distributions in the upper ocean has recently been developed under funding by ONR (acoustics and DURIP) and NSF (ocean instrumentation). The technique is simple in principle: changes in sound speed and attenuation caused by the bubbles are directly measured using a broad-band pulse (2kHz-200kHz) over path lengths of O(.1-1)m using a pair of transmit and receive transducers (see publications list for Terrill & Melville 1998, Terrill 1998). The resulting attenuation data can be inverted for the bubble sizes present using numerical algorithms developed by Commander & McDonald (1991). Internal consistency checks of the quality of data are provided by measurements of both the sound speed and attenuation. The broad-band technique has been developed and tested in the laboratory with independent optical measurements of bubble size distributions beneath breaking waves in seawater and found to give excellent results. The range of void fractions currently resolved by the instrument is O(10E-8) to O(10E-4) for bubbles with radii of 15-1600 microns. Higher concentrations of bubbles can be resolved with conductivity probes and optical techniques.
3 MHz Acoustic Doppler Profiler (ADP)
A high frequency acoustic Doppler current profiler obtained from Sontek has been adapted for mounting to the base of the spar pointing upwards. This instrument provides high-resolution (bin-bin resolutions of O(10)cm) measurements of the current structure in the surface wave layer. The high-frequency acoustic backscatter information obtained from the upward looking sonar can be used to provide measurements of the bubble cloud morphology, penetration depths, and surface identification
High resolution wind velocity measurements (0.01 m/s resolution) are made with an ultrasonic anemometer which can be sampled at a rate up to 10 Hz. Compensation of the wind measurements for platform motion is made using information from a 6-axis motion package and three-axis magnetometer.