A detailed comparison of the data recorded by Aanderaa Model 4 and Plessey MO21 recording current meters moored in two shelf-sea locations, each with strong tidal currents

Abstract

In this paper the results of two intercalibration exercises carried out in the North and Irish Seas are described. Detailed comparisons are made of values interpolated at standard 10-minute intervals from measurements of tidal velocity and direction recorded by Aanderaa Model 4 and Plessey MO21 current meters respectively. The comparisons are made over most of the 7-day North Sea exercise (“Gaus” data) and at key times in the 40-day Irish Sea exercise (BISOP data), such as equinoctial springs and neaps and the beginning and end of the measuring period. In the latter case the four sets of data are divided into near-surface and near-bottom pairs. Estimates of residual drift from each set of data have been made and these are compared also. The “Gaus” data reveal a systematic difference in direction measurement between the two current meters being compared; the most probable source of this is a mechanical defect in one of them. This difference gives rise to an incorrect assessment of the residual drift that would not normally have been detected. The BISOP data provide an example of sustained consistency of performance from all the four instruments involved. Some differences occur in the near-surface layer when surface waves develop, and at depth when spring tides run. Explanations of these anomalies and possible ways of preventing their appearance in the future are provided. The paper ends with a plea for the establishment of a European current meter calibration site, so that future cooperative moored current meter exercises are placed on the most scientific footing possible.