Acoustic Doppler Current Profiler Velocity Research Articles

Mesoscale Eddies and Near-Inertial Internal Waves Modulate Seasonal Variations of Vertical Shear Variance in the Northern South China Sea

Abstract Diapycnal mixing in the South China Sea (SCS) is commonly attributed to the vertical shear variance S2 of horizontal ocean current velocity, but the seasonal modulation of S2 is still poorly understood due to the scarcity of long-term velocity observations. Here, this issue is explored in detail based on nearly 10-yr-long acoustic Doppler current profiler (ADCP) velocity data from a mooring in the northern SCS. We find that S2 in the northern SCS exhibits significant seasonal variations at both the near-surface (90–180 m) and subsurface (180–400 m) layers, but their seasonal cycles and modulation mechanisms are quite different. For the near-surface layer, S2 is stronger in late summer, autumn, and winter but weaker in spring and early summer, while in the subsurface layer, it is much stronger in winter than in other seasons. Further analysis suggests that in the near-surface layer, the stronger S2 in autumn and winter is primarily caused by typhoon-induced near-inertial internal waves (NIWs) and the large subinertial (SI) velocity shear of the baroclinic mesoscale eddies, respectively. With respect to the subsurface layer, the enhanced wintertime S2 is primarily associated with the “inertial chimney” effect of anticyclonic eddies, trapping wind-forced downward-propagating NIWs and significantly increasing the near-inertial shear at the critical layer. The findings in this study highlight the potentially important roles of mesoscale eddies and NIWs in modulating the seasonality of upper-ocean mixing in the northern SCS. This modulation is attributed not only to the strong shear of these features but also to their interactions. Significance Statement Vertical shear variance of velocity S2 significantly modulates turbulent mixing in the thermocline, but its climatologically seasonal variations and the associated mechanisms are still obscure due to the scarcity of long-term in situ velocity data. By analyzing nearly a decade of velocity data, we reveal significant seasonal variations in S2 at different ocean layers in the northern SCS and uncover different seasonal cycles and modulation mechanisms. The study sheds light on the pivotal roles of mesoscale eddies and near-inertial internal waves in modulating seasonality of S2 in the upper ocean. These findings have important implications for improving mixing parameterizations in numerical models.

Eddy Tracking From In Situ and Satellite Observations

AbstractMesoscale eddies are a dominant source of spatial variability in the surface ocean and play a major role in the biological marine carbon cycle. Satellite altimetry is often used to locate and track eddies, but this approach is rarely validated against in situ observations. Here we compare measurements of a small (under 25 km radius) mode water anticyclonic eddy over the Porcupine Abyssal Plain in the northeastern Atlantic Ocean using CTD and Acoustic Doppler Current Profiler (ADCP) measurements from three ships, two gliders, two profiling floats, and one Lagrangian float with those derived from sea level anomaly (SLA). In situ estimates of the eddy center were estimated from maps of the thickness of its central isopycnal layer, from ADCP velocities at a reference depth, and from the trajectory of the Lagrangian float. These were compared to three methods using altimetric SLA: one based on maximizing geostrophic rotation, one based on a constant SLA contour, and one which maximizes geostrophic velocity speed along the eddy boundary. All algorithms were used to select CTD profiles that were within the eddy. The in‐situ metrics agreed to 97%. The altimetry metrics showed only a small loss of accuracy, giving >90% agreement with the in situ results. This suggests that current satellite altimetry is adequate for understanding the spatial representation of even relatively small mesoscale eddies.

In-Stream Laser Diffraction for Measuring Suspended Sediment Concentration and Particle Size Distribution in Rivers: Insights from Field Campaigns

This study evaluates the laser in situ scattering and transmissometry (LISST) instrument LISST-SL2, a laser diffraction instrument for suspended sediment sampling in rivers, with concurrent physical measurements of suspended sediment concentration (SSC) and particle size distribution (PSD) as well as velocity measurements by an acoustic Doppler current profiler (ADCP). We collected 136 LISST-SL2 samples along with 61 physical samples for SSC measurement, of which 24 physical samples included PSD measurement during 2018–2020 from 11 sites in Washington state and Virginia. An effective density is required to convert the measured volumetric SSC by the LISST-SL2 into a reported mass SSC, and by default the LISST-SL2 assumes a value of 2.65 g/mL. From our data set, we computed effective densities (mass SSC/volumetric SSC) that ranged from 0.5 to 5.4 g/mL, with a best-fit value of 2.05 g/mL. Additionally, the LISST-SL2 was not able to measure the finest sediment sizes in suspension, which affects the resulting PSD. Therefore, we propose some adjustments of the LISST-SL2 data with a supporting physical sample to account for these effective density and PSD issues. When doing so, we were able to reduce the root-mean square relative error (RMSRE) to 18% from 117% for SSC, and to 26% from 78% for PSD. LISST-SL2 velocities were generally higher than ADCP velocities with a 21% RMSRE. Our results and guidance will allow for more accurate sampling by the LISST-SL2, which has potential for studying spatial and temporal variation of suspended sediment characteristics in rivers.

Improving Accuracy of Horizontal Flow Field Using Acoustic Tomography With Real-Time Station Position Correction

High-precision and spatiotemporal flow field measurement in small-scale waters has a decisive impact on navigation and water resource management. Multitransmission rays and position drift, as two main difficulties in improving the accuracy of small-scale flow field, need to be solved urgently. In this article, four coastal acoustic tomography (CAT) systems are arranged in Qingyang Lake, Hunan, China, on March 1, 2022, for a short-term observation. As the GPS error and shipping caused station drift contaminate the reconstructed flow field, a real-time station position correction method, based on travel time and temperature, depth (TD) sensors, is applied in our inversion. The inversion range average current (RAC) is in agreement with forward calculation, which proves the reliability of the reconstructed horizontal flow field. Besides, comparisons of net flow and acoustic Doppler current profiler (ADCP) velocity are performed to validate the improvement of the real-time station position correction algorithm. The errors of the corrected net flow are less than 3.818 m 2/s, with a minimum of 0.0042 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{2}$ </tex-math></inline-formula> /s. The root-mean-square error (RMSE) between the CAT inversion results and the ADCP velocity is 0.123 m/s after correction, relatively reduced 5.95% than before correction. The smaller the RMSE, the higher the inversion accuracy, so the CAT method with real-time station position correction is feasible for high-precision horizontal flow field reconstruction.

Diagnosing Frontal Dynamics From Observations Using a Variational Approach

AbstractIntensive hydrographic and horizontal velocity measurements collected in the Alboran Sea enabled us to diagnose the three‐dimensional dynamics of a frontal system. The sampled domain was characterized by a 40 km diameter anticyclonic eddy, with an intense front on its eastern side, separating the Atlantic and Mediterranean waters. Here, we implemented a multi‐variate variational analysis (VA) to reconstruct the hydrographic fields, combining the 1‐km horizontal resolution of the Underway Conductivity‐Temperature‐Depth (CTD) system with information on the flow shape from the Acoustic Doppler Current Profiler velocities. One advantage of the VA is given by the physical constraint, which preserves fine‐scale gradients better than the classical optimal interpolation (OI). A comparison between real drifter trajectories and virtual particles advected in the mapping quantified the improvements in the VA over the OI, with a 15% larger skill score. Quasi‐geostrophic (QG) and semi‐geostrophic (SG) omega equations enabled us to estimate the vertical velocity (w) which reached 40 m/day on the dense side of the front. How nutrients and other passive tracers leave the mixed‐layer and subduct is estimated with 3D advection from the VA, which agreed with biological sampling from traditional CTD casts at two eddy locations. Downwelling warm filaments are further evidence of subduction, in line with the w from SG, but not with QG. SG better accounted for the along‐isopycnal component of w in agreement with another analysis made on isopycnal coordinates. The multi‐platform approach of this work and the use of variational methods improved the characterization and understanding of (sub)‐mesoscale frontal dynamics.

In Situ Calibration of Underwater Glider Flight Model Using Acoustic Doppler Current Profilers

Abstract The angle of attack (AOA) is the difference between the underwater glider’s path and pitch angle and is necessary to accurately estimate dead-reckoned position and depth-averaged velocity. The AOA is also important for any sensor measurements that are affected by the glider’s velocity through water, such as ocean turbulence measurement. A glider flight model is generally used to accurately estimate AOA and glider’s actual velocity based on the knowledge of lift and drag coefficients optimized for each glider. This paper examines the AOA of a Slocum glider using an acoustic Doppler current profiler (ADCP) to demonstrate a regression method to estimate these coefficients. Since the current shear was sufficiently small on average, it was reasonable to assume that the ADCP velocity at the nearest bin could capture the glider’s motion during flight and was used to calculate AOA. The lift and drag coefficients were optimized so the flight model estimated the observed pitch–AOA relationship derived from the ADCP and the glider’s pitch observations. The resultant coefficients also satisfied the vertical and horizontal constraints of glider motion and gave unbiased estimates of turbulence intensity derived from the flight model and ADCP. Our method was also applied to a SeaExplorer glider to derive the lift and drag coefficients for the first time. The observed pitch–AOA relationship was reasonably captured by the flight model with the resultant coefficients, suggesting that our method to estimate the lift and drag coefficient of underwater gliders can be applied to any type of underwater glider equipped with an ADCP.

Unpiloted Aerial Vehicle (UAV) image velocimetry for validation of two-dimensional hydraulic model simulations

Non-intrusive image-based techniques for measuring surface river velocities have rapidly evolved as a cost-effective and safe means for quantifying flow patterns. Large-scale particle image velocimetry (LSPIV) can provide instantaneous surface velocities over a large spatial footprint rapidly and with little pre-calibration as compared to traditional techniques. Assessment of the spatial distribution of flow velocities in hydraulic models has been comparatively harder to achieve than assessment of depth due to logistical challenges but would be aided using large observational datasets that represent the variability and distribution of flow hydraulics. Additionally, the efficacy of image velocimetry in assessing the accuracy of outputs from 2D hydraulic models has not been addressed. Here, we demonstrate how LSPIV can be used to calibrate and validate 2D model predictions in a gravel bed river reach. LSPIV velocities are depth-averaged using standard velocity coefficients (α) and then using the Probability Concept (PC) - a probabilistic formulation of velocity distributions that accounts for non-standard velocity profiles, typical in field settings. UAV surveys were used to acquire video for LSPIV and imagery for Structure from Motion (SfM) topographic modelling. We use spatially dense acoustic doppler current profiler (aDcp) velocity data for benchmark assessment of the velocity outputs of HEC-RAS 2D model simulations. 2D model prediction error, based on seeded LSPIV velocities, was within range (4.2%) of the aDcp parametrised model, with improvements in modelled versus predicted velocity correlations (up to 7.7%) when using PC to depth average LSPIV velocities. Validation bias reduced significantly (11%) with tighter error distributions when compared to the aDcp based model. Although additional hydraulic measurements are required to parametrise the Probability Concept algorithm, the performance of 2D hydraulic models calibrated/validated with LSPIV velocities is on par with traditional techniques, demonstrating the potential of this non-intrusive, low-cost approach.

Identifying optimal position for a fish collection system for endemic fishes in Hong River, China

Establishing a suitable location for a fish collection system is crucial for luring and collecting fish and requires the swimming characteristics of fish to be considered. This study aimed to identify a suitable location to lure fish to the fish collecting system (or barge) on the Hong River. For this purpose, a 3D computational fluid dynamics model for the downstream of dam was verified using Acoustic Doppler current Profiler (ADCP) velocity data to simulate the flow field of river channel considering of seven typical operation scenarios. The swimming performance of five endemic fish (Bagarius rutilus, Cyprinus carpio rubrofuscus, Semilabeo obscures, Channa argus, and Opsariichthys bidens) were evaluated using three indexes, i.e., induced flow velocity, critical swimming speed, burst swimming speed, to determine their migration behavior zones by analyzing different flow velocities. By adapting the simulated flow fields in each operation scenario, potential fish migration aggregation zones were analyzed. The results suggested that the most optimal position for placing the FCP was in the left bank of river channel at X = 400–500 m in typical operation scenarios. This study provides a reference for designing FCP for fish pass facilities that combine fish swimming behavior and hydraulic variables of the river channel.

Vorticity Recirculation and Asymmetric Generation at a Small Headland With Broadband Currents

AbstractFixed acoustic Doppler current profiler (ADCP) velocity measurements are used to investigate headland vorticity generation and recirculation in ∼20 m depth around the small (∼1 km) central California headland Pt. Sal. To reduce vorticity estimation noise, velocities are reconstructed from the first two Empirical Orthogonal Function modes representing ≈73% of the variance. Using fixed ADCPs, depth‐averaged vorticity is estimated west and south of Pt. Sal. Only one west‐location vorticity component is estimated, leading to negative vorticity bias for northward flow. The south location vorticity is consistent with estimates from parallel vessel transects on one day. The observed depth‐averaged flow V was primarily along‐bathymetric contours and varied ±0.2 ms−1 across subtidal and tidal frequency bands. The depth‐averaged normalized vorticity varied ±8 across all frequency bands. Vorticity distributions are skewed with opposite sign at west and south locations, and is more likely at the west location. At both locations, depth‐averaged vorticity and velocity are inversely related, with relationship asymmetric with sign of V, indicating headland and farther upstream vorticity generation. Binned‐mean depends on both V and its time‐derivative, and indicates vorticity recirculation across the headland. The ∼2 h vorticity adjustment timescale and the associated short excursion distances indicate vorticity generation between south and west locations. Potential vorticity changes across the headland are different for positive and negative V indicating headland asymmetric vorticity generation. Pt. Sal occupies a nondimensional parameter space that is unique relative to other well studied headlands.

Shelfbreak Jet Structure and Variability off New Jersey Using Ship of Opportunity Data From the CMV Oleander

AbstractRepeat measurements of velocity and temperature profiles from the Container Motor Vessel (CMV) Oleander provide an unprecedented look into the variability on the New Jersey Shelf and upper continental slope. Here 1362 acoustic Doppler current profiler (ADCP) velocity sections collected between 1994 and 2018 are analyzed in both Eulerian and stream coordinate reference frames to characterize the mean structure of the Shelfbreak Jet, as well as its seasonal to decadal variability. The Eulerian mean Shelfbreak Jet has a maximum jet velocity of 0.12 m s−1. The maximum jet velocity peaks in April and May and reaches its minimum in July and August. In a stream coordinate framework, the jet is only identified in 61% of transects, and the mean stream coordinate Shelfbreak Jet has a maximum jet velocity of 0.32 m s−1. Evidence is found that Warm Core Rings, originating from the Gulf Stream arriving in the Slope Sea adjacent to the New Jersey Shelf, shift the Shelfbreak Jet onshore of its mean position or entirely shutdown the Shelfbreak Jet's flow. At interannual timescales, variability in the Shelfbreak Jet velocity is correlated with the temperature on the New Jersey Shelf 2 months later. When considered in a stream coordinate framework, Shelfbreak Jet have decreased over the time period considered in the study.

Design of Acoustic Doppler Current Profiler Calibration System

In order to trace and transmit the velocity value of acoustic Doppler current profiler (ADCP), an ADCP velocity measurement method based on deep water channel and unmanned ship is proposed. An unmanned ship carrying ADCP moves at a uniform speed along the planned route. High-speed camera and total station are used as the main standard for time and length measurement. The average velocity of ADCP in the measurement section is measured as a reference value for velocity. Compared with ADCP velocity indication, the error of velocity indication is calculated. Real-time kinematic positioning technology (GPS-RTK) is used to map the trajectory and yaw error of unmanned ship and verify the instantaneous velocity stability. The test results show that, under the condition of unmanned ship setting speed of 1 m/s, the error of ADCP velocity indicated by inspection is -0.029 m/s, and the extended uncertainty of this test system is evaluated as U=0.071m/s, k=2.

Incorporating real-time acoustic ranging and glider-based Doppler measurements to aid vehicle navigation

In the summer of 2017, two Seagliders equipped with low frequency acoustic recorders and 1 MHz acoustic Doppler current profilers (ADCPs) were deployed in the Canada Basin as part of a large-scale acoustic tomography experiment. Acoustic tomography sources were moored at approximately 175 m depth within an acoustic duct enabling acoustic transmission to be received at long ranges. The sources transmitted at a center frequency of approximately 250 Hz every four hours, and ranges between the sources and Seaglider were estimated in real time using a WHOI MicroModem. These ranges can be incorporated into an extended Kalman filter for navigation. In addition, glider-mounted upward-looking ADCPs recorded ~26.5 m shear profiles every 15 seconds. These overlapping profiles can be used to estimate (in post-processing) both the local current profile on a per-dive basis and the glider's relative velocity through the water. In real time, the close-range ADCP velocity measurements can be used to estimate the glider's relative (Through The Water, TTW) velocity to improve the glider's subsea position estimate. We will describe our recent developments in using these Doppler measurements to aid glider navigation, comparing the results to previous developments in range-aided navigation.

Application of an Automated Discharge Imaging System and LSPIV during Typhoon Events in Taiwan

An automated discharge imaging system (ADIS), which is a non-intrusive and safe approach, was developed for measuring river flows during flash flood events. ADIS consists of dual cameras to capture complete surface images in the near and far fields. Surface velocities are accurately measured using the Large Scale Particle Image Velocimetry (LSPIV) technique. The stream discharges are then obtained from the depth-averaged velocity (based upon an empirical velocity-index relationship) and cross-section area. The ADIS was deployed at the Yu-Feng gauging station in Shimen Reservoir upper catchment, northern Taiwan. For a rigorous validation, surface velocity measurements were conducted using ADIS/LSPIV and other instruments. In terms of the averaged surface velocity, all of the measured results were in good agreement with small differences, i.e., 0.004 to 0.39 m/s and 0.023 to 0.345 m/s when compared to those from acoustic Doppler current profiler (ADCP) and surface velocity radar (SVR), respectively. The ADIS/LSPIV was further applied to measure surface velocities and discharges during typhoon events (i.e., Chan-Hom, Soudelor, Goni, and Dujuan) in 2015. The measured water level and surface velocity both showed rapid increases due to flash floods. The estimated discharges from ADIS/LSPIV and ADCP were compared, presenting good consistency with correlation coefficient R = 0.996 and normalized root mean square error NRMSE = 7.96%. The results of sensitivity analysis indicate that the components till (τ) and roll (θ) of the camera are most sensitive parameters to affect the surface velocity using ADIS/LSPIV. Overall, the ADIS based upon LSPIV technique effectively measures surface velocities for reliable estimations of river discharges during typhoon events.

The ADCP's bottom track capability for bedload prediction: Evidence on method reliability from sandy river applications

ABSTRACT River processes understanding requires the investigation of sediment transported by the streamflow. To this aim, the use of physical samplers is particularly challenging in large sandy rivers, which pushed the thriving of surrogate methods. The acoustic Doppler current profiler (ADCP) has been recently used to quantify the suspended-load as well as the bed-load. The latter is related to the apparent velocity of particles at the bedload layer as resulting by comparing the ADCP's velocity from its capability to acoustically track the bottom and from accurate GPS recordings. The reliability of this technique depends on instrument frequency, acoustic pulse length used and site-specific properties, such as riverbed composition and bedforms presence that eventually require further research. This work presents ADCP-DGPS measurements in the large sandy Parana River in Argentina, performed with ADCPs operating at 600 and 1200 kHz frequencies and using different acoustic pulse lengths to eventually assess the bedload rate. Combining theoretical parameters and apparent velocity of bed-particles from ADCP, assessed values agree on bedload rate values obtained with the dune tracking method with a discrepancy of 29% on average. The measurements obtained with 1200 kHz and longer pulse lengths deliver better agreements, although variation with longest pulse length was 15% off at most.

Observations of inner shelf cross-shore surface material transport adjacent to a coastal inlet in the northern Gulf of Mexico

Motivated by the Deepwater Horizon oil spill, the Surfzone and Coastal Oil Pathways Experiment obtained Acoustic Doppler Current Profiler (ADCP) Eulerian and GPS-drifter based Lagrangian “surface” (<1m) flow observations in the northern Gulf of Mexico to describe the influence of small-scale river plumes on surface material transport pathways in the nearshore. Lagrangian paths are qualitatively similar to surface pathlines derived from non-traditional, near-surface ADCP velocities, but both differ significantly from depth-averaged subsurface pathlines. Near-surface currents are linearly correlated with wind velocities (r =0.76 in the alongshore and r =0.85 in the cross-shore) at the 95% confidence level, and are 4–7 times larger than theoretical estimates of wind and wave-driven surface flow in an un-stratified water column. Differences in near-surface flow are attributed to the presence of a buoyant river plume forced by winds from passing extratropical storms. Plume boundary fronts induce a horizontal velocity gradient where drifters deployed outside of the plume in oceanic water routinely converge, slow, and are re-directed. When the plume flows west parallel to the beach, the seaward plume boundary front acts as a coastal barrier that prevents 100% of oceanic drifters from beaching within 27km of the inlet. As a result, small-scale, wind-driven river plumes in the northern Gulf of Mexico act as coastal barriers that prevent offshore surface pollution from washing ashore west of river inlets.

Optimizing attraction flow for upstream fish passage at a hydropower dam employing 3D Detached-Eddy Simulation

Restoration of upstream fish passage requires construction of efficient fishways. Selection of attraction flow rates and entrance velocities is one of the fundamental research tasks on medium-sized German rivers as general recommendations are ambiguous.We used a transient 3D Computational Fluid Dynamics model of a hydropower dam tailrace calibrated with Acoustic Doppler Current Profiler velocity data and Detached-Eddy Simulation turbulence modeling to produce seven flow fields. Hydraulic results were linked to fish performance by means of fish-size-speed relations (ethohydraulic scale).Resulting attraction flow relationships agree well with literature recommendations if the competing flow is defined as the adjacent turbine flow. Further, we found that entrance velocity clearly determines the downstream influence of the attraction flow plume over the attraction flow rate if no rapid mixing is present.

Pathways and Water Mass Properties of the Thermocline and Intermediate Waters in the Solomon Sea

AbstractThe semienclosed Solomon Sea is the final passage in the equatorward transit of the South Pacific western boundary currents (WBCs) that play a key role in heat and mass budgets of the equatorial Pacific. The Solomon WBCs and their associated water properties are examined using data from two oceanographic cruises undertaken during the contrasting trade wind seasons: July 2012 and March 2014. The mean circulation and associated transports with uncertainties is determined from the cruise data using a unique configuration of an inverse box model formulated based on measured shipboard acoustic Doppler current profiler velocities. An intense inflow of 36 Sv is found entering the Solomon Sea in July–August 2012 that falls by 70% to 11 Sv in March 2014. Large differences are also found in the total transport partitioning through each of the major exit passages during each season. Different water masses are found in the WBC stream northeast of the Solomon Islands that are likely related to a northern stream of the South Equatorial Current. Within the Solomon Sea, isopycnal salinity gradients are gradually stronger than within the subtropical Pacific, likely induced by stronger diapycnal mixing processes. WBC pathways exhibit distinct water mass signatures in salinity, oxygen, and nutrients that can be traced across the Solomon Sea, associated with significant water mass modifications at the northern exit straits and south of the Woodlark Island.

Larval fish habitats in a mesoscale dipole eddy in the gulf of California

The three-dimensional distribution of larval fish habitats in a mesoscale dipole eddy was analyzed in the Southern Gulf of California in July 2010. Detection of the characteristic mushroom-shape using chlorophyll and sea surface temperature satellite images was used to direct sampling of two transects across the structure with conductivity, temperature, depth casts and stratified oblique zooplankton hauls with opening-closing net. The latter sampled every 17m layer down to the thermocline and every 50m layer from the thermocline down to 200m depth. Lowered Acoustic Doppler Current Profiler velocities revealed that the dipole structure persisted down to 800m depth. The “cap” showed two opposite poles: one cyclonic with diameter ~50km, and the other anticyclonic with diameter~80km. It had a vigorous central jet with speeds>25cm/s. The Θ-SA properties were consistent with formation of the structure outside the Gulf of California in Tropical Surface and Subtropical Subsurface water masses. In this context, three larval fish habitats were statistically defined (P<0.001). One habitat, located in the surface mixed layer (~15m depth), was characterized by the highest species richness, high abundance of the mesopelagic species Vinciguerria lucetia and the presence of epipelagic species like Auxis sp.; this habitat was correlated with the warm (24–25°C) and oxygenated (4–5mL/L) Tropical Surface Water. The other two larval fish habitats were defined below the thermocline (between ~15 and 50m depth) and comprised mainly by mesopelagic species. One of them occurred in the cyclonic pole and central jet zone with the highest larval abundance, dominated by Diogenichthys laternatus; the other habitat was located in the dipole margins, or zones of return flow, with the lowest larval abundance, characterized by Psenes sio and Bathylagoides wesethi, species frequent outside of the Gulf of California. These two habitats were correlated with cooler (~13–22°C) and hypoxic (0.03–1mL/L) Subtropical Subsurface water masses. The results suggest that during the formation of the dipole, water masses dominated by different species were trapped, forming larval fish habitats.

Using an ADCP to Estimate Turbulent Kinetic Energy Dissipation Rate in Sheltered Coastal Waters

AbstractTurbulent microstructure and acoustic Doppler current profiler (ADCP) data were collected near Tacoma Narrows in Puget Sound, Washington. Over 100 coincident microstructure profiles have been compared to ADCP estimates of turbulent kinetic energy dissipation rate (ϵ). ADCP dissipation rates were calculated using the large-eddy method with theoretically determined corrections for sensor noise on rms velocity and integral-scale calculations. This work is an extension of Ann Gargett’s approach, which used a narrowband ADCP in regions with intense turbulence and strong vertical velocities. Here, a broadband ADCP is used to measure weaker turbulence and achieve greater horizontal and vertical resolution relative to the narrowband ADCP. Estimates of ϵ from the Modular Microstructure Profiler (MMP) and broadband ADCP show good quantitative agreement over nearly three decades of dissipation rate, 3 × 10−8–10−5 m2 s−3. This technique is most readily applied when the turbulent velocity is greater than the ADCP velocity uncertainty (σ) and the ADCP cell size is within a factor of 2 of the Thorpe scale. The 600-kHz broadband ADCP used in this experiment yielded a noise floor of 3 mm s−1 for 3-m vertical bins and 2-m along-track average (≈four pings), which resulted in turbulence levels measureable with the ADCP as weak as 3 × 10−8 m2 s−3. The value and trade-off of changing the ADCP cell size, which reduces noise but also changes the ratio of the Thorpe scale to the cell size, are discussed as well.

Measurement and numerical modeling studies of the highest bottom shear stress in the Randle Reef area

The purpose of this study was to investigate the highest bottom shear stress, induced by wind in an area of Hamilton Harbour, Ontario, Canada known as Randle Reef. The study was conducted in support of a component of a contaminated sediment remediation plan utilizing a thin layer of sand to manage contaminated sediments. Toward this end, four acoustic Doppler current profilers (ADCPs) were deployed at two locations in the study region to measure velocity profiles for the purpose of indirectly measuring bottom shear stress (BSS) and model verification. There is no easy way to directly measure BSS in the field. As a result, the use of the logarithmic-profile method from the ADCP measured high resolution velocity profiles in the bottom layer was explored. This approach, according to our best knowledge, has not been published for a wind driven flow in a small open water body. To use the indirectly measured BSS to estimate the highest BSS in the study area, a three-dimensional hydrodynamic model was adopted to provide the spatial and temporal information of the bottom flow. The results showed that the modeled and measured flow velocity components agreed reasonably well at most of the water depths with the correlation coefficients being greater than 0.6. However, agreements between the modeled and measured bottom flow speeds were worse than expected due to the error contributions from both the modeled velocity components. Therefore, the modeled flow speed required rescaling based on ADCP velocity measurements before it could be deemed reliable. This is especially important in estimation of the BSS with a quadratic formula because the calculated BSS is proportional to the square of the speed.