This paper describes the instrumentation and techniques for long-term targeted observation of the centimeter-scale velocity structure within the oceanic surface boundary layer, made possibl...

Observing Finescale Oceanic Velocity Structure with an Autonomous Nortek Acoustic Doppler Current Profiler

Broadband Acoustic Doppler Current Profiler (BBADCP) is a widely used technology in velocity measurements. To adapt to the varied water environment and different measurement requirements, flexible tuning of transmitted signal parameters will improve the feasibility and accuracy of velocity measurement. Compared with the conventional signal, the orthogonal combined signal designed in this paper can generate a wealth of signal combination examples and improve the accuracy of the velocity measurement under the same conditions. The proposed orthogonal combined signal consists of two orthogonal sub-signals with a symmetrical spectrum. Each is designed based on time delay to eliminate or weaken the current velocity ambiguity. Then, the processing method of the received signal when the pulse signals are the same or different coded signal is discussed. The numerical simulation results show that, when using the proposed method, the standard deviation of the estimated current velocity has different degrees of reduction at different current velocities. Our simulation also shows that, compared to the convention method, the proposed method can improve the SNR by 10 dB. This can help significantly increase the scope of the configuration.

Using Orthogonal Combined Signals in Broadband ADCP for Improving Velocity Measurement

The recent integration of Acoustic Doppler Current Profilers (ADCPs) onto underwater gliders changes the way current and sediment dynamics in the coastal zone can be monitored. Their endurance and ability to measure in all weather conditions increases the probability of capturing sporadic meteorological events, such as storms and floods, which are key elements of sediment dynamics. We used a Slocum glider equipped with a CTD (Conductivity, Temperature, Depth), an optical payload, and an RDI 600 kHz phased array ADCP. Two deployments were carried out during two contrasting periods of the year in the Rhone River region of freshwater influence (ROFI). Coastal absolute currents were reconstructed using the shear method and bottom tracking measurements, and generally appear to be in geostrophic balance. The responses of the acoustic backscatter index and optical turbidity signals appear to be linked to changes of the particle size distribution in the water column. Significantly, this study shows the interest of using a glider-ADCP for coastal zone monitoring. However, the comparison between suspended particulate matter dynamics from satellites and gliders also suggests that a synoptic view of the processes involved requires a multiplatform approach, especially in systems with high spatial and temporal variability, such as the Rhone ROFI area.

Glider-Based Active Acoustic Monitoring of Currents and Turbidity in the Coastal Zone

Caribbean through-flow accounts for up to two-thirds of the Florida Current and consequently is an important conduit of heat and salt fluxes in the Atlantic branch of the Meridional Overturning Circulation (MOC). While high-latitude sinking and interior mixing processes have a first order control on the magnitude of the MOC, low-latitude wind-driven processes determine and modify the subsurface density structure of the water masses flowing through the Caribbean Sea. Considering there is evidence that up to one-half of the Florida Current originates as South Atlantic Waters (SAW), determining the distribution of SAW throughout the Caribbean Island passages is important as this constitutes the major pathway for cross-equatorial MOC return flow. Ship-based observations in the 1990’s revealed the Windward Island passages as a dominant SAW inflow pathway. However, there is still a significant amount of SAW that is taking an unknown, alternate route northward. The Anegada Passage (AP) is a major location for subtropical gyre inflow and suggested to be an alternate SAW inflow pathway. Here, we present the first co-located observations of temperature, salinity, and subsurface velocity in the AP in nearly 20 years. These observations provide evidence that the total transport (4-5 Sv) and the transport of SAW through the AP (1-2.55 Sv) is larger than previously estimated. This result implies that the AP is a significant pathway for cross-equatorial MOC return flow. The results presented here also provide evidence that gliders with acoustic doppler profilers are viable, cost-effective method for measuring island passage transport. 

/pdf/upper-ocean-transport-in-the-anegada-passage-from-multi-year-3g7fy429.pdf

Upper Ocean Transport in the Anegada Passage from Multi-Year Glider Surveys

Upper Ocean Transport in the Anegada Passage From Multi‐Year Glider Surveys

Ocean gliders are autonomous underwater vehicles typically used to sample spatial variations in scalar variables (e.g., temperature, salinity, and bio-optical water properties) along transects. More recently, gliders have been equipped with ADCPs for measuring current profiles along transects. Accurate measurement of velocity profiles from a moving platform requires knowledge of the platform motion over the earth. Determination of glider motion over the earth relies on glider GPS positions available only at times of glider surfacing. Glider surfacing intervals typically range from 10’s of minutes to hours, precluding accurate instantaneous knowledge of glider motion. This is a major challenge for measuring velocity profiles from ocean gliders. One approach for determining vertical velocity profiles from glider-mounted ADCPs relies on estimating depth-integrated currents averaged between glider surfacings. Once estimated, vertically averaged velocity can be combined with horizontal glider velocities relative to the water to obtain depth-resolved velocities using methods of Visbeck (2001) and Todd et al. (2011). The vertically averaged velocity is calculated from the distance the glider strays from its projected position during the time between surfacings. This distance is computed as the difference between the dead-reckoned surfacing location and the actual surfacing location as measured by GPS. There are numerous methods for computing the dead-reckoned position of glider surfacings, but these have not been evaluated to determine which best predicts surfacing locations. Here, three methods for calculating vertically averaged horizontal current velocities from gliders are evaluated. Two Slocum Coastal G1 gliders (manufactured by Teledyne Webb Research), each with an upward looking 1 MHz ADCP (manufactured by Teledyne RD Instruments), were deployed off the California coast during the summer of 2012. The gliders flew 500 m square patterns around a bottom-mounted, upward-looking 600 kHz ADCP (manufactured by Teledyne RD Instruments) moored at a depth of 26 m. The moored ADCP data are key to evaluating methods for computing vertically averaged velocity from gliders and ultimately assessing depth-resolved current velocities obtained from glider-mounted ADCPs. The first method calculates the glider’s horizontal velocity using pitch and vertical velocity. Vertical velocity is calculated by taking the derivative of pressure with respect to time. It is often assumed that the flight path is in the direction of the glider’s long axis and α, the angle of attack or the angle between the glider's path through the water and its long axis, is zero. This assumption can result in errors of 2.5 cm s-1 (Merckelbach et al. 2001) which are on the order of 10% of the horizontal velocity. Previous studies have estimated angle of attack (e.g. Sherman et al. 2001) using model results for internally-mounted ADCPs on Spray gliders. The gliders in this study carried externally-mounted ADCPs, so errors may also result from extra drag and non-zero values of α. The value of α is found by maximizing the r2 of vertically averaged velocities found by this method with vertically averaged velocities from the mooring. The second method directly measures currents using the ADCPs mounted on the gliders. Water velocity relative to the glider is obtained from the ADCP bin nearest the glider which is 1m long and begins 1.2 m from the glider. This velocity is calculated on glider upcasts and downcasts in east-north-up (ENU) coordinates. Velocities in ENU coordinates are derived from software provided with the ADCP. The third method uses velocities measured along the ADCP beams in so-called beam coordinates. Directly measured velocity can then be used (see Todd et al. 2011) from the component of the ADCP beams oriented in the direction parallel to the glider’s long axis — this can be 2 or 3 beams depending on the transducer head on the ADCP. Experimentation with these data and this method is ongoing and will be compared with the previously mentioned methods. The three methods are used to estimate vertically averaged currents from the gliders, and these are compared with vertically averaged currents from the bottom-mounted ADCP. Of the three methods, the first produces the highest correlations (r2 values). However, it is necessary to estimate the angle of attack when using the first method.

Improved methods to calculate depth-resolved velocities from glider-mounted ADCPs

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Improved methods to calculate depth-resolved velocities from glider-mounted ADCPs