Reynolds stress observations in continental shelf seas

Abstract

The three basic observational approaches to estimating turbulence parameters in continental shelf seas are free-fall micro-structure probes from which dissipation is inferred; fast-sample (10-20 Hz) current meters in sea-bed frames measuring turbulence intensity directly; and fast-sample O(1 Hz) high-frequency Acoustic Doppler Current Profilers (ADCPs) where Reynolds stress profiles and hence turbulence production can be estimated from the variance of the along beam data. Uncertainties are associated with each approach since turbulence is a small-scale, high-frequency phenomenon and since estimates can easily be contaminated by the presence of surface waves. This paper concentrates on the latter two approaches, particularly the ADCP method, focussing on the degree of confidence that can be placed on the estimates.Results are presented from nine experiments from six sites in the North and Irish Seas and one in the Gulf of California, involving the deployment of 0.6 and 1.2 MHz standard broadband ADCPs mounted in sea-bed frames. The sites ranged from very tidally energetic, shallow (20 m deep) to low tidal energy, deeper (110 m). The ADCPs recorded data with a variety of sample regimes, from 2 to 0.5 Hz; bin sizes ranged from 0.25 to 1 m. 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, deployed on a nearby frame. In all cases the correlation coefficient squared between the two sets of Reynolds stress estimates was 0.7. In a further three recent deployments, an Acoustic Doppler Velocimeter (ADV) was deployed on the bottom frame with the ADV measuring volume located within the first ADCP bin and sampling at 20 or 25 Hz. The ADV measurements also show an explained variance of about 80% and a transfer function of about 1 during periods where waves were not present.One objective of these studies was to test and improve representation of dissipation processes in two-and threedimensional numerical models, including the concept of the constant stress layer. At its very simplest, bottom stress is estimated from the depth-averaged flow via a quadratic drag law. Calculations from these measurements give values for the drag coefficient between 0.0006 and 0.0019, averaging 0.0011, smaller than the value used in most depth-averaged numerical models (0.0025). There is some evidence that the value of the drag coefficient is dependent on the tidal current speed. r

Estimating turbulent diffusion in a benthic boundary layer

“…D T_F and D T_T compared well with the highly variable diffusivities reported previously by Lorke (2007) for a shallow and stably stratified station in Lake Constance using Osborn's approach (D T_O ). However, most hydrodynamic studies in the BBL focus on estimating bottom stress (Dewey and Crawford 1988;Howarth and Souza 2005;Kim et al 2000;Trowbridge et al 1999) from which diffusivities may be derived using the log-law. Diffusivity measurements in the BBL that are not…”

Measuring Turbulent Dissipation Using a Tethered ADCP

“…In the ''variance method,'' the shear stress in the flow above a bottommounted ADCP is estimated from the difference of velocity variance in the opposing ADCP beams (Stacey et al 1999;Lu et al 2000;Rippeth et al 2002;Howarth and Souza 2005;Williams and Simpson 2004) in a technique analogous to that used in stress measurements in radar meteorology. The rate of turbulent energy production is obtained from the product of the shear stress and the vertical shear of velocity, which is also measured by the ADCP.…”

Current and turbulence measurements at the FINO1 offshore wind energy site: analysis using 5-beam ADCPs

“…ADCPs, as a common ocean current profiling technique, use short-term bursts of along-beam velocities to extract vertical profiles of currents and turbulence parameters (Lu and Lueck 1999;Howarth and Souza 2005;Nystrom et al 2007). In the presence of surface gravity waves, however, contributions from the wave orbital velocities may be an order of magnitude larger than the turbulence fluctuating velocities (Trowbridge 1998;Shaw and Trowbridge 2001;Gerbi et al 2008).…”