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  • Author or Editor: Andreas Münchow x
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Andreas Münchow

Abstract

A three-dimensional data interpolation technique is proposed that efficiently removes tidal currents from spatial velocity surveys. The least squares method extends prior two-dimensional detiding methods to three spatial dimensions using biharmonic splines. Biharmonic splines are fitted to velocity data from acoustic Doppler current profiler (ADCP) surveys, moorings, and ocean surface current radar (OSCR). The data are used to predict diurnal and semidiurnal tidal currents on the inner shelf off New Jersey that vary between 1 and 15 cm s−1 at spatial scales of about 20 km. The (tidal) signal to (subtidal) noise is thus O(1) in the study area. Although the main task of this study is to remove tidal variance from the ADCP survey data, an attempt is made to accurately“predict” tidal currents from the data. The latter task is more difficult. Both artificial data with known signal-to-noise properties and actual measurements indicate that the method estimates both diurnal and semidiurnal tidal currents to within about 3.5 cm s−1 rms, or 30% of the true tidal signals. While the biharmonic splines remove tidal currents successfully, the prediction of the vertical structure of tidal currents is only fair. Some experimentation guided by physical intuition and prior knowledge of the tidal fields is necessary in order to obtain an accurate and stable solution. While this ambiguity constitutes the main disadvantage of the method, its simple algebraic expression to predict tidal currents in space and time is its main advantage. Properly weighting velocity data from different sources, such as moorings, surface current radar, and ADCP surveys of different quality, improves the quality of the fit.

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Andreas Münchow
,
Charles S. Coughran
,
Myrl C. Hendershott
, and
Clinton D. Winant

Abstract

A towed acoustic Doppler current profiler (ADCP) system was tested. The instrument was deployed from ships of opportunity and towed at depths between 5 and 25 m. The towed system carries upward- and downward-looking ADCPs. The instrument platform is stable in most operating conditions at ship speeds up to 4.5 m s−1. Large discrepancies are found, however, between the ship's velocity obtained from bottom-tracking ADCP pulses and that from navigational data. These are explained with a magnetic compass bias that varies with the ship's heading direction. Both the ship and the tow platform induce magnetic fields that bias the ADCP compass. An in situ compass calibration scheme is thus necessary and requires accurate navigational data. In our main study area, it is found that the Global Position System provides absolute and relative positions to within 88 and 4 m, respectively. These accuracies are sufficient for calibration purposes. With our calibration scheme the towed ADCP system performs as well as vessel-mounted systems. The case of deployment from ships of opportunity and the capacity of the tow system to carry additional instruments makes it a valuable research tool. Furthermore, the capability of our system to profile the water column above and below the platform with different frequencies and thus different vertical resolutions enhances its flexibility and usefulness, especially to study surface and bottom boundary-layer processes.

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