ADCP estimates of Reynolds and bottom stress

Field measurements of Reynolds and bottom stress are rare and difficult to make but are required for studies of large scale dynamical balances and of small scale dissipation and exchange processes. Recently a simple to apply technique has been reported in which Reynolds stress profiles can he calculated from the variance of along beam data recorded by fast sample, O(1 Hz), high frequency Acoustic Doppler Current Profilers (ADCPs). The technique has been applied in eight experiments in the north-west European continental shelf seas involving the deployment of 0.6 and 1.2 MHz standard broadband ADCPs mounted in sea-bed frames. The sites ranged from high tidal energy, shallow (20 in deep) to low tidal energy, deeper (110 in). The ADCPs recorded data with a variety of sample regimes, from 2 to 0.5 Hz; bin sizes ranged from 0.25 to 1 in. In two of the experiments the ADCP near bed Reynolds stress estimates were tested against independent estimates from toroidal electro-magnetic current meters measuring the three components of current (vertical and both horizontal) at 8 Hz. In all cases the correlation coefficient squared between the two sets of Reynolds stress estimates were 0.7. In addition the three components of turbulence intensity (and hence the degree of anisotropy) were estimated from the electromagnetic current meter records. This cannot be estimated from the ADCP data, unless the ADCP is fitted with a vertical beam, but is important for interpretation of the results. One objective of these studies is to test and improve representation of dissipation processes in two and three-dimensional numerical models, including the concept of the constant stress layer. At its very simplest bottom stress can be related to the depth-averaged flow via a quadratic drag coefficient - estimates from the different sites were 0.001 ± 0.0003, smaller than the value used in most depth-averaged numerical models (0.0025).