Vertical Water Velocities from Underwater Gliders

Abstract

The underwater glider is set to become an important platform for oceanographers to gather data within oceans. Gliders are usually equipped with a conductivity-temperature-depth (CTD) sensor, but a wide range of other sensors have been fitted to gliders.In the present work, the authors aim at measuring the vertical water velocity. The vertical water velocity is obtained by subtracting the vertical glider velocity relative to the water from the vertical glider velocity relative to the water surface. The latter is obtained from the pressure sensor. For the former, a quasi-static model of planar glider flight is developed. The model requires three calibration parameters, the (parasite) drag coefficient, glider volume (at atmospheric pressure), and hull compressibility, which are found by minimizing a cost function based on the variance of the calculated vertical water velocity.Vertical water velocities have been calculated from data gathered in the northwestern Mediterranean during the Gulf of Lions experiment, winter 2008. Although no direct comparison could be made with water velocities from an independent measurement technique, the authors show that, for two different heat loss regimes ('0 and '400 W m 22 ), the calculated vertical velocity scales are comparable with those expected for internal waves and active open ocean convection, respectively. High noise levels resulting from the pressure sensor require the water velocity time series to be low-pass filtered with a cutoff period of 80 s. The absolute accuracy of the vertical water velocity is estimated at 64 mm s 21.

Determining Vertical Water Velocities from Seaglider

“…In this paper, we describe a technique for estimating vertical velocities from Seaglider, an autonomous underwater vehicle, to better than 1 cm s 21 (Frajka-Williams 2009). This technique is similar to that given by Merckelbach et al (2010) for a Slocum glider. The flight equations for the two glider models are similar; however, the number of flight parameters that are allowed to vary differs, and several relevant engineering details are different.…”

“…The overall range of frequencies for which the spectrum is calculated is much lower than that in Merckelbach et al (2010). This is because our sample rate was variable and lower overall than that for the Slocum glider missions they used.…”

Importance of the variability of hydrographic preconditioning for deep convection in the Gulf of Lion, NW Mediterranean

“…Sinking occurs in plumes of horizontal scale O(1 km) in which the dense water sinks at vertical speeds up to 10 cm/s (Voorhis and Webb, 1970;Schott and Leaman, 1991;Merckelbach et al, 2010). These plumes are mixing agents (Send and Marshall, 1995) associated with a small-scale circulation sometimes modeled as paired, discrete point vortices called hetons (Hogg and Stommel, 1985;Legg and Marshall, 1993).…”

Measurements of Near-Surface Turbulence and Mixing from Autonomous Ocean Gliders

“…The flight dynamics of ocean gliders have been the subject of many studies. One such study specific to the Slocum platform is that of Merckelbach et al (2010). Gliders typically fly at a pitch angle of 25 to 30 degrees.…”