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
The UK National Oceanography Centre has maintained an observatory in Liverpool Bay since August 2002. Over 8 years of observational measurements are used in conjunction with regional ocean modelling data to describe the physical and dynamical oceanography of Liverpool Bay and to validate the regional model, POLCOMS. Tidal dynamics and plume buoyancy govern the fate of the fresh water as it enters the sea, as well as the fate of its sediment, contaminants and nutrient loads. In this context, an overview and summary of Liverpool Bay tidal dynamics are presented. Freshwater forcing statistics are presented showing that on average the bay receives 233 m 3 s −1 . Though the region is salinity controlled, river input temperature is shown to significantly modulate the plume buoyancy with a seasonal cycle. Stratification strongly influences the region's dynamics. Data from long-term moored instrumentation are used to analyse the stratification statistics that are representative of the region. It is shown that for 65% of tidal cycles, the region alternates between being vertically mixed and stratified. Plume dynamics are diagnosed from the model and are presented for the region. The spring-neap modulation of the plume's westward extent, between 3.5 • W and 4 • W, is highlighted. The rapid eastward erosion of the plume during spring tides is identified as a potentially important freshwater mixing mechanism. Novel climatological maps of temperature, salinity and density from the CTD surveys are presented and used to validate numerical simulations. The model is found to be sensitive to the freshwater forcing rates, temperature and salinities. The existing CTD survey grid is shown to not extend sufficiently near the coast to capture the near coastal and vertically mixed component the plume. Instead the survey grid captures the westward spreading, shallow and transient, portion of the plume. This transient plume feature is shown in both the long-term averaged model and observational data as a band of stratified fluid stretching between the mouth of the Mersey towards the Isle of Man. Finally the residual circulation is discussed. Long-term moored ADCP data are favourably compared with model data, showing the general northward flow of surface water and southward trajectory of bottom water.
Sea level comprises a mean level, tidal elevation and a residual elevation. Knowledge of whatcauses maximum water levels is often key in coastal management. However, different methods to extract deviations in water level (residuals) from modelled and observed elevation can give different results. The Dee Estuary, northwest England, is a macrotidal estuary that undergoes periodic stratification. It is used here to demonstrate methods to extract the residual water level in response to the following interactive processes: tidal, river induced stratification and flow, meteorological and waves. Using modelling techniques the interaction and contribution of different physical processes are investigated. Classical harmonic tidal analysis, model simulations and filtering techniques have been used to "de-tide" the total elevation for short-term (~month long) records. Each technique gives a different result highlighting the need to select the ~ 2 ~ correct method for a required study. Analysis of the residual components demonstrates that all processes inducing residuals interact with the tide generating a semi-diurnal residual component.It is suggested that modelling methods enable the full effect of tidal interaction to remain in the residual, while harmonic tidal analysis (partly) modify and filtering methods (fully) remove this component of the residual. The analysis methods presented and their influences on the resultant residual are applicable to other study sites. However, when applied specifically to the mouth of the Dee Estuary, the external surge is found to be the main contributor to the total residual, while local wind and stratification effects are of secondary importance.
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