Processing explorer ADCP data collected on slocum gliders using the LADCP shear method

Abstract

A publicly available implementation of the shear method for processing horizontal velocities acquired with Lowered Acoustic Doppler Current Profiler (LADCP) systems has been applied to measurements obtained with TRDI Explorer ADCPs mounted on Webb Slocum gliders, in order to determine whether LADCP methods are suitable for processing glider- ADCP data. Quality of the resulting profiles was assessed primarily by comparison with simultaneous velocity profiles collected near the gliders using a vessel-mounted (vmADCP) system. Due to the way the Explorer ADCPs are installed on the gliders, the ADCP data from the upward portions of the dives are degraded. As a consequence, the standard LADCP method of using surface GPS fixes and time-series measurements of horizontal flow past the instrument cannot be applied to reference the baroclinic velocity profiles calculated with the shear method. For dives within range of the seabed, bottom-tracking (BT) can be used to reference the velocities instead, which implies velocity errors increasing with increasing distance from the seabed. During a short test cruise, a set of comparatively shallow glider dive profiles were collected during two primary groups of missions off the California coast. During the so-called A-mission dives, gliderattitude measurements, updated every 4 s, were used to transform the ADCP beam to earth coordinates, resulting in rms velocitycomponent differences between glider- and vmADCP-derived velocities of approximately 6cm/s. During the B-mission dives, attitude data from a external sensor were used instead, resulting in reduced rms velocity-component differences of 3–4 cm·s−1, similar to what is achievable for typical deep-water LADCP profiles. These tests indicate that useful absolute velocity profiles can be obtained with Explorer ADCPs mounted on Slocum gliders, as long as i) a suitable external attitude measurement system is used to transform the ADCP measurements into earth coordinates, and ii) the dives are deep enough to measure the glider motion over ground using bottom tracking.Without BT data, the ADCP-glider system can still provide useful measurements of vertical shear of horizontal velocity.