Factors affecting the quality of shipboard acoustic Doppler current profiler data

Acoustic Doppler current profilers (ADCPs) are now commonplace on many ships and have had a considerable impact on modern oceanography. During a recent cruise on R.R.S. Discovery, however, the presence of a large spurious shear in the direction of the ship's motion was noted on a 150 kHz profiler, particularly when steaming into heavy seas. It is thought that this results from the trapping of bubbles near the ship's hull. Parameters representing the state of the wind and sea, and the ship's motions, are combined in a semi-empirical way to produce a function (E) that estimates the severity of this effect and so gives insights into the factors affecting the data quality. The results are compared with a further cruise upon which the transducers were extended beyond the bubble layer. The estimating function allows a meaningful intercomparison for the different weather conditions on the second cruise and shows that the spurious shear was effectively eliminated. Similar problems are now being observed on other vessels, and it is recommended that ADCP transducers should be mounted below the bubble layer whenever possible.