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Simultaneous measurements of the rates of turbulent kinetic energy (TKE) dissipation () and production (P) have been made over a period of 24 h at a tidally energetic site in the northern Irish Sea in water of 25-m depth. Some profiles from ϳ5 m below the surface to 15 cm above the seabed were obtained using a fast light yoyo (FLY) microstructure profiler, while P profiles were determined from a bottom-mounted high-frequency acoustic Doppler current profiler (ADCP) using the variance method. In homogeneous flow of the kind observed, the turbulence regime should approximate to local equilibrium so that, with no buoyancy forces involved, and P are expected to covary with mean values that are equal. The results show a close tracking of and P for most of the observational period. For the second tidal cycle, when there was no significant surface wave activity, a mean ratio of /P Ӎ 0.63 Ϯ 0.17 was obtained. Although this is a significant deviation from unity, it is within the range of uncertainty previously reported for the measurements. A marked phase lag of between 5 and 20 min between the maximum P and the maximum is interpreted using a simple model in terms of the decay rate of TKE. Consideration of inherent instrument noise has enabled an estimate of the lowest P threshold measurable using the variance technique. For the chosen averaging parameters a value of P min ϳ 7 ϫ 10 Ϫ5 W m Ϫ3 is estimated. Two other significant differences between the two sets of measurements are attributed to errors in the stress estimate. The first is a bias in the estimate of stress resulting from a combination of instrument tilt (1Њ-3.5Њ) and surface wave activity. The second are anomalously high stress estimates, covering nearly one-half of the water column at times, which are thought to be due to instrument noise associated with the large wave orbital velocities.

The use of acoustic Doppler current profilers (ADCPs) to measure turbulent parameters via the variance method involves uncertainties due to instrument noise and flow-related errors in measurement. For weak flows, the uncertainty in Reynolds stress measurements arises mainly from instrument noise and is proportional to the square of the velocity standard deviation, while the uncertainty in the corresponding estimates of the rate of production of turbulent kinetic energy (TKE) is proportional to the cube of the velocity standard deviation. For stronger flows, the principal determining parameter is the number of individual independent velocity measurements over which the variance is calculated. These results are validated by detailed analyses of two datasets from an RD Instruments 1.2-MHz Workhorse ADCP, using a ping rate of 2 Hz with ensemble averaging at 0.5 Hz, and a ping rate of 10 Hz with ensemble averaging at 1 Hz, respectively. While increasing ping rate generally reduces the effects of instrument noise, it will not alleviate the influence of flow-related noise once the sampling interval is less than the autocovariance time scale of the turbulence. Using the fast ping rate, the uncertainty in the Reynolds stress due to instrument noise is reduced by a factor of more than 3 to ϳ0.02 Pa; in higher energy environments there is a reduction in the uncertainty of about 30%. The observational and theoretical estimates for the reduction in the uncertainty using the fast ping rate are in good agreement.

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