Turbulence measurement with a moored instrument

A new autonomous and moored microstructure measuring instrument has been tested in coastal waters. The instrument measures shear in the dissipation range of the wavenumber spectrum using four shear probes; temperature fluctuations using two FP-07 thermistors; temperature and salinity using three pairs of Seabird sensors; horizontal currents using two ducted rotors and a fluxgate compass; and body motions using three accelerometers and a pressure transducer. Data are sampled for 128 s every 5 min and reduced into ensemble and band-averaged spectra and statistical parameters. The processed data are transferred to a disk every 6 h along with one 128-s set of unprocessed data. In a 2-day experiment, the instrument experienced peak flows of 0.15 m s^-1 and resolved dissipation rates in currents as slow as 0.03 m s^-1. Spectra of velocity fluctuations agree closely with the Nasmyth universal spectrum. For dissipation rates, the lower limit of detection is determined by electronic noise, while the upper limit is set by the angle of attack of cross-stream velocity fluctuations. Both constraints become less restrictive with increasing current. The electronic noise falls below 10^-10 W kg^-1 in flows faster than 0.1 m s^-1 for which the authors detected dissipation rates as low as 4 × 10^-10 W kg^-1. The autonomous instrument makes it possible to obtain long time series at a much reduced cost compared to conventional profiling from a ship.