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W. A. Oost

Abstract

A new instrument is presented to measure the wind vector under fouling circumstances, such as spray conditions just above the sea. It is in principle a combination of six pressure tubes, arranged in three mutually orthogonal sets of two. The tubes in each set point in opposite directions and are connected to the ports of a differential pressure transducer. From the readings of the three pressure transducers both the wind speed and direction can be derived. In order to keep the tubes clean, a continuous airflow is forced through them. In its present configuration, the pressure anemometer has a lower speed limit of 4 m s−1 and a frequency range of 0–35 Hz.

The instrument was compared with a sonic anemometer at a research platform off the Dutch coast. The results of this intercomparison show satisfactory agreement between the instruments. The new instrument, however, has a better high-frequency response.

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W. A. Oost

Abstract

A model for the upwind influence of bluff bodies on a flow developed by Wyngaard is extended to include bodies without axial symmetry; i.e., ellipsoids with three unequal axes. The requirements and limits for the application of the model are discussed.

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W. A. Oost
and
G. J. Komen

Abstract

One of the important characteristics of an anemometer is its spatial resolution. A three-dimensional generalization is given of a method to calculate the transfer function as a function of the wavenumber, devised by Kaimal et al. for a sonic anemometer. The method has been applied to the sensor system of the pressure anemometer, a new type of wind vector measuring instrument. The results, given in a number of figures, show an extremum before the transfer function falls off to its final value.

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C. Kraan
and
W. A. Oost

Abstract

To obtain accurate results from flux measurements with the eddy-correlation method, one has to be very sure about the causes of any mean vertical wind speed. Furthermore, it is generally not sufficient to use simple analytical expressions for the relation between the signals of the anemometer and the wind speed components. We therefore introduce a new calibration/interpretation method for anemometers with fixed sensors, which provides the desired extra accuracy, and then consider the results of the application of this method to eight successive calibrations of a sonic anemometer.

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G. Kraan
,
W. A. Oost
, and
P. A. E. M. Janssen

Abstract

With video equipment and image processing software the authors have measured the whitecap coverage of the sea surface near the Dutch coast. Reality proved the initial idea about the simplicity of this way of whitecap measurement to be overly optimistic. A model was developed that produces a theoretical estimate of the whitecap percentage as a function of the wave age, which in turn depends on the wave peak frequency and the friction velocity in the air. The comparison between the measured whitecap coverage and the model results shows an order of magnitude correspondence for most data. A group of outliers appears to be connected to the strength of the current.

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W. A. Oost
,
E. H. W. Worrell
,
J. W. Schaap
,
C. van Oort
, and
C. Kraan

Abstract

An earlier paper introduced a new type of fast-responding vector wind meter, designed for adverse conditions, such as those that exist low over the sea surface. The most critical part of this instrument, its sensor head, was difficult to construct and was suitable for wind speeds above 8 m s−1 only. In this article a new type of sensor head is introduced that is simple to construct and extends the wind speed range of the instrument downward to less than 2 m s−1.

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Wiebe A. Oost
,
Christopher W. Fairall
,
James B. Edson
,
Stuart D. Smith
,
Robert J. Anderson
,
John A.B. Wills
,
Kristina B. Katsaros
, and
Janice DeCosmo

Abstract

Several methods are examined for correction of turbulence and eddy fluxes in the atmospheric boundary layer, two of them based on a potential-flow approach initiated by Wyngaard. If the distorting object is cylindrical or if the distance to the sensor is much greater than the size of the body, the undisturbed wind stress can be calculated solely from measurements made by the sensor itself; no auxiliary measurements or lengthy model calculations are needed. A more general potential-flow correction has been developed in which distorting objects of complex shape are represented as a number of ellipsoidal elements.

These models are applied to data from three turbulence anemometers with differing amounts of flow distortion, operated simultaneously in the Humidity Exchange over the Sea (HEXOS) Main Experiment. The results are compared with wind-stress estimates by the inertial-dissipation technique; these are much less sensitive to local flow distortion and are consistent with the corrected eddy correlation results. From these comparisons it is concluded that the commonly used “tilt correction” is not sufficient to correct eddy wind stress for distortion by nearby objects, such as probe supports and neighboring sensors.

Neither potential-flow method is applicable to distortion by larger bodies of a scale comparable to the measuring height, such as the superstructure of the Meetpost Noordwijk (MPN) platform used in HEXOS. Flow distortion has been measured around a model of MPN in a wind tunnel study. The results were used to correct mean winds, but simulation of distortion effects on turbulence levels and wind stress turned out not to be feasible.

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