Estimates of sea level, waves and winds from a bottom-mounted ADCP in a shelf sea

Abstract

Sea level and atmospheric parameters are estimated using an upward-looking 600 kHz acoustic Doppler current profiler (ADCP) at ranges close to the sea surface, known to be unsuitable for measuring ocean currents. The ADCP is moored at the bottom, 45 m below the surface in the central North Sea under occasionally severe winter conditions with significant wave heights ( H s) up to 12 m and wind speeds exceeding 20 m s −1. ADCP estimates of sea level (relative accuracy <0.02 m) reveal that a bottom pressure recorder shows an artificial datum depression down to about — 0.03 H s m due to wave action just above the bottom. The proper determination of the sea surface location turned out to be crucial for ADCP estimates of waves and winds. Significant wave height is estimated better than 0.7 m using ADCP's back-scattered amplitude (‘echo intensity’) signal from depths a few metres below where the acoustic signal directly hits the surface. At these depths, echo intensity is related to air bubble amounts, proportional to H s. In contrast, echo intensity observed close to and at the surface is inversely proportional to H s, due to wave smearing. It is demonstrated that high-frequency velocity variance (noise) in ADCP current data is not instrumental but predominantly induced by surface wave aliasing. It is suggested that this variance be used to infer significant wave number and frequency. Wind stress is estimated after subtracting mid-depth ADCP current data from those obtained close to the surface, in a surface following co-ordinate system. Wind stress magnitude is estimated better than 0.1 Pa (with values exceeding 1 Pa) and wind direction better than 10°. It is concluded that the ADCP ‘measures’ the downwind component related to Langmuir circulation, rather than Ekman surface currents.