M. Laan

A camera and image-analysis system for in situ observation of flocs in natural waters

Abstract An in situ suspension camera in combination with an image-analysis system was developed at NIOZ to measure the in situ particle size of suspended matter. It differs from other methods in that in sit particle size is measured from ∼ 4 μm upwards in a relatively simple and direct way. It can be used in any waters down to ∼ 4000 m depth (with some adjustments to 7000 m) and in water with a suspended matter concentration up to 200 mg·dm −3 . In very clear ocean water the system becomes inconvenient because of the large number of photographs that have to be taken to obtain a reliable size distribution. This paper describes the camera and the image-analysis system and gives some results of measurements in the Scheldt river and estuary in April 1989. These measurements show a continuous size distribution by volume between 3.6 μm and 644 μm and a good agreement of the data obtained with the 1:1 and 1:10 cameras.

NIOZ3: Independent Temperature Sensors Sampling Yearlong Data at a Rate of 1 Hz

Some 110 independent sensors form the NIOZ3-thermistor ldquostringrdquo to study waves in the ocean interior sampling at a rate of 1 Hz during at least one year. The string operates without connecting cables between the newly designed sensors, which are programmed and synchronized via induction. The accuracy of previous custom-made high-sampling rate thermistor strings is maintained, being better than 1 mK. This is demonstrated here using data from three recent field trials, two above seamounts and one in the ocean interior that occasionally show vigorous (nonlinear) internal wave motions.

Internal Wave Turbulence Near a Texel Beach

A summer bather entering a calm sea from the beach may sense alternating warm and cold water. This can be felt when moving forward into the sea (‘vertically homogeneous’ and ‘horizontally different’), but also when standing still between one’s feet and body (‘vertically different’). On a calm summer-day, an array of high-precision sensors has measured fast temperature-changes up to 1°C near a Texel-island (NL) beach. The measurements show that sensed variations are in fact internal waves, fronts and turbulence, supported in part by vertical stable stratification in density (temperature). Such motions are common in the deep ocean, but generally not in shallow seas where turbulent mixing is expected strong enough to homogenize. The internal beach-waves have amplitudes ten-times larger than those of the small surface wind waves. Quantifying their turbulent mixing gives diffusivity estimates of 10−4–10−3 m2 s−1, which are larger than found in open-ocean but smaller than wave breaking above deep sloping topography.

Construction of a 3D Mooring Array of Temperature Sensors

AbstractA small-scale 3D mooring array comprising up to 550 high-resolution temperature sensors was custom designed and constructed. The stand-alone array samples ocean temperature to depths of about 3000 m, limited by the buoyancy elements, at a rate of 1 Hz, with a precision better than 0.5 mK, a noise level better than 0.1 mK, and an endurance of 1 year. Its purpose serves quantitative studies on the development of turbulent mixing by internal wave breaking above deep-ocean topography. The 3D array consists of five parallel cables 105 m long with 3.2 mm inner diameter during the first deployment, now 5.5 mm. The cables are 4 and 5.6 m apart horizontally and are held under tension of 1000 N each using heavy buoyancy elements in a single line above. The entire array is folded into a 6-m high, 3-m-diameter structure above a 750-kg weight. It is deployed in a single overboard operation similar to that of a free-falling mooring. New here is the unfolding of the compacted array when hanging overboard and pri...