Graig Sutherland

Tidal current energy assessment for Johnstone Strait, Vancouver Island

Abstract The maximum tidal power potential of Johnstone Strait, BC, Canada is evaluated using a two-dimensional finite element model (TIDE2D) with turbines simulated in certain regions by increasing the drag. Initially, side channels are closed off so that the flow is forced through one channel to test the validity of a general analytic theory [1] with numerical results. In this case, the modelled power potential of 886 MW agrees reasonably well with the analytic estimate of 826 MW. In reality, two main channels, Discovery Passage and Cordero Channel, connect the Pacific Ocean to the Strait of Georgia. Turbines are simulated in Johnstone Strait, northwest of the two main channels, and separately for Discovery Passage and Cordero Channel. Northwestern Johnstone Strait is similar to the one channel case as the flow must go through this channel, but Discovery Passage and Cordero Channel are different as the flow can be diverted away from the channel with the turbines and into the other channel. The maximum extractable power in northwestern Johnstone Strait is found to be 1335 MW, which agrees well with the theoretical estimate of 1320 MW. In Discovery Passage and Cordero Channel, the maximum extractable power is modelled to be 401 and 277 MW, respectively, due to the flow being partly diverted into the other channel. In all cases, the current is reduced to between 57 and 58 per cent of the undisturbed flow, close to the 56 per cent predicted by the analytic theory. All power calculations are for the M2 constituent alone, as this is the largest current in the region. The total power from the eight major constituents (M2, S2, N2, K2, K1, 01, P1, and Q1) can be obtained by multiplying the power estimates for M2 by 1.12.

The Air-Sea Interaction Profiler (ASIP): An Autonomous Upwardly Rising Profiler for Microstructure Measurements in the Upper Ocean

AbstractThe upper few meters of the ocean form a critical layer for air–sea interaction, but because of observational challenges this region is undersampled. However, the physical processes controlling momentum transfer, gas exchange, and heat transfer are all concentrated in the uppermost region of the ocean. To study this region, the Air–Sea Interaction Profiler (ASIP) was developed. This is an autonomous microstructure vertical profiling instrument that provides data from a maximum depth of 100 m to the ocean surface and allows measurements to be performed in an undisturbed environment. The core sensor package on ASIP includes shear probes, microstructure and CTD-quality temperature and conductivity sensors, a photosynthetically active radiation (PAR) sensor, and an oxygen optode providing a repeated high-resolution dataset immediately below the air–sea interface. Autonomous profiling is accomplished with thrusters that submerge the positively buoyant instrument. Once the desired depth is reached, ASIP...

Tidal Resonance in Juan de Fuca Strait and the Strait of Georgia

Abstract The resonant period and quality factor Q are determined for the semienclosed sea comprising Juan de Fuca Strait, Puget Sound, and the Strait of Georgia. The observed tidal elevation gain and phase change, from the Pacific Ocean to this inland sea, are fitted to the predictions of simple analytic models, which give a resonant period of 17–21 h and a Q of about 2. The low Q value, indicative of a highly dissipative system, is consistent with the need for numerical models for the area to employ large bottom friction coefficients. These include the effects of form drag.

Evaluating Langmuir turbulence parameterizations in the ocean surface boundary layer

It is expected that surface gravity waves play an important role in the dynamics of the ocean surface boundary layer (OSBL), quantified with the turbulent Langmuir number ( La=u*/us0, where u* and us0 are the friction velocity and surface Stokes drift, respectively). However, simultaneous measurements of the OSBL dynamics along with accurate measurements of the wave and atmospheric forcing are lacking. Measurements of the turbulent dissipation rate ϵ were collected using the Air-Sea Interaction Profiler (ASIP), a freely rising microstructure profiler. Two definitions for the OSBL depth are used: the mixed layer derived from measurements of density (hρ), and the mixing layer (hϵ) determined from direct measurements of ϵ. When surface buoyancy forces are relatively small, ϵ∝La−2 only near the surface with no dependency on La at mid-depths of the OSBL when using hρ as the turbulent length scale. However, if hϵ is used then the dependence of ϵ with La−2 is more uniform throughout the OSBL. For relatively high destabilizing surface buoyancy forces, ϵ is proportional to the ratio of the OSBL depth against the Langmuir stability length LL. During destabilizing conditions, the mixed and mixing layer depths are nearly identical, but we have relatively few measurements under these conditions, rather than any physical implications. Observations of epsilon are compared with the OSBL regime diagram of Belcher et al. (2012) and are generally within an order of magnitude, but there is an improved agreement if hϵ is used as the turbulent length scale rather than hρ.

Mixed and mixing layer depths in the ocean surface boundary layer under conditions of diurnal stratification

A comparison between mixed (MLD) and mixing (XLD) layer depths is presented from the SubTRopical Atlantic Surface Salinity Experiment (STRASSE) cruise in the subtropical Atlantic. This study consists of 400 microstructure profiles during fairly calm and moderate conditions (2 < U10 < 10 m s−1) and strong solar heating O(1000 W m−2). The XLD is determined from a decrease in the turbulent dissipation rate to an assumed background level. Two different thresholds for the background dissipation level are tested, 10−8 and 10−9 m2 s−3, and these are compared with the MLD as calculated using a density threshold. The larger background threshold agrees with the MLD during restratification but only extends to half the MLD during nighttime convection, while the lesser threshold agrees well during convection but is deeper by a factor of 2 during restratification. Observations suggest the use of a larger density threshold to determine the MLD in a buoyancy driven regime.

Measurements of Waves in Landfast Ice Using Inertial Motion Units

Inertial motion units (IMUs) are used to perform measurements of waves in water covered by landfast ice close to the ice edge in Svalbard. The effective noise level of the instruments is assessed in controlled wave tank experiments. A set of measurements collected in Tempelfjorden, Svalbard in March 2015 is presented, and the ability of the sensors to operate in the field is validated. Several characteristics of the recorded signals, including correlation between the different sensors, are analyzed. Horizontal and vertical motions are of the same order of magnitude. A clear transition in the signal properties is observed in relation with changes in incoming wave field and the development of cracks in the ice layer. We show that complex physics takes place when waves propagate in landfast ice and that the use of times series containing information on the full three-dimensional linear acceleration, rather than spectra, is required to capture the underlying phenomena.

Large-Scale Errors in ERS Altimeter Data

A method is described for mapping time-uncorrelated large-scale errors in satellite altimeter sea surface heights. Standard deviations of differences between pairs of successive measurements at track crossovers are computed, and the functional dependence of these deviations on absolute time difference is used to estimate the errors of individual measurements. This is first applied to all of ERS-1,2 altimeter data in the Pacific Ocean, yielding average errors of 3.2 cm in the deep ocean (>1 km) and 4.7 cm in the shallow seas (<1 km). The procedure is repeated for variable latitude bands, each with a full range of possible time differences, yielding a meridional profile of computed errors, ranging from 2.6 cm near the Antarctic continent (67–60S) and South Subtropical regions (25–5S) to 3.5 cm in the Antarctic Circumpolar Current (60–45S) and the Northern Hemisphere Subtropical and Subpolar Gyres. Finally, coarse-resolution maps of these errors are produced by subdividing the Pacific Ocean into latitude-longitude bins, each large enough to contain a sufficient number of samples for the functional fits. The larger errors are in Northwest and Subtropical Pacific, especially in South China Sea (4.3 to 4.5 cm) and off northern Australia (5.4 cm), while the smaller errors (2.5 to 3 cm) are in Northeast Pacific, central Tropical Pacific and near Antarctica in Southeast Pacific Ocean. These are lower bounds on altimeter errors, as they do not include contributions from time-correlated errors. We find that the computed error fields are not correlated with sea level standard deviations, thus disproving the notion that altimeter error variance can be scaled with the variance of sea surface height data.