Ship ADCP measurements and tidal models of the North Sea

Abstract

Depth-averaged currents calculated from ship Acoustic Doppler Current Profiler (ADCP) measurements in a continental shelf sea have been compared with two-dimensional numerical model tidal currents, with the aim of checking the measurements and the model calculations. A scheme for calculating the residual (non-tidal) currents is outlined. The measurements were made during a set of 14 cruises at monthly intervals in the southern North Sea which followed the same cruise track. This data set has provided a more rigorous test of the tidal model predictions than could have been obtained from combining all the previous recording current meter observations. The nature of the data set was such that both temporal (over the 15 month cruise period) and spatial variations in the gain (observed divided by model amplitudes) and timing differences were studied. Spatial variations were more significant than temporal variations. Timing differences were less than 15 min over half the region but there was a phase shift along the European continental coast. The gain was less than 1, indicating that the model currents were greater (by of order 20%) than the observed, with the difference being largest in the Southern Bight. Both the phase and gain differences were consistent with model water depths being too shallow. The relative effect of changing the grid size and the number of tidal constituents on the accuracy of the model currents was studied.