A comparison of near-surface current measurement methods

A solution to measuring near-surface currents via a single-cell acoustic Doppler current meter with horizontally oriented acoustic beams positioned very close to the sea surface is presented. The current meter was deployed on a surface buoy on the 25 m isobath of the west Florida continental shelf. Near-surface velocity observations from the current meter (1.1 m depth) are compared to estimates from a surface mounted acoustic Doppler velocimeter (0.8 m depth) and the first several cells of a downward looking acoustic Doppler current profiler (4.0 m to 13.0 m depth). Rotary auto spectra and cross spectra analyses are used to examine the velocity gain and veering angle at different frequencies as a function of water depth. Results indicate that velocity measurements at 1.1 m were 7% higher in the M2 tidal band and 18% higher in the synoptic band than measurements at 7.0 m. There was negligible near-surface velocity rotation in both the tidal band and the synoptic band compared with measurements at 7.0 m.