Vertical water velocities from underwater gliders

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 has been tted to gliders.
In the present work, we 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
ight is developed. The
model requires three calibration parameters, the (parasite) drag coecient, glider volume
(at atmospheric pressure) and hull compressibility, which are found by minimising 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 com-
parison could be made with water velocities from an independent measurement technique,
we show that for two dierent heat loss regimes ( 0 and 400 W/m2), the calculated
vertical velocity scales are comparable with those expected for internal waves and active
open ocean convection, respectively. High noise levels due to the pressure sensor require the
water velocity times series to be low-pass ltered with a cut-o period of 80 seconds. The
absolute accuracy of the vertical water velocity is estimated at 4 mm/s.