Recovery of Near-Surface Velocity from Undrogued Drifters

Measuring surface currents with Surface Velocity Program drifters: the instrument, its data, and some recent results

“…Several drogue designs were examined and various problems identified. For example, windowshade drogues could twist and sail across a current; parachute drogues could collapse and subsequently provide very little drag; the line-and-chain FGGE drogue provided too little area compared to the surface float, resulting in significant slip with respect to the currents at the drogue depth (Niiler et al, 1987Niiler and Paduan, 1995;Pazan and Niiler, 2001). Other factors were also considered; for example the rigid three-dimensional tristar drogue was found to have somewhat better water-following characterists than the holey-sock drogue developed at AOML, but the increased manufacturing and shipping costs and more difficult deployments for the tristar outweighted this benefit.…”

“…JIMO supervises the industry, acquires the drifters from the various manufacturers, upgrades the technology, develops new sensors, and creates enhanced data sets (Pazan and Niiler, 2004) to the research community. The manufacturers build the SVP drifters according to closely-monitored specifications.…”

“…At a drag area ratio of 40, the resulting downwind slip (see Section 4.a.) is 0.7 cm/s in 10 m/s winds; for comparison, a standard FGGE-type drifter had a drag area ratio of 10-12 and a downwind slip of 8 cm/s in 10 m/s winds (Niiler and Paduan, 1995;Pazan and Niiler, 2001). In practice, the manufacturer-provided drag area ratios range from 37.5-45.9.…”

“…For example, as noted earlier, many observations have been collected by non-SVP drifters, including the dense data set collected by FGGE drifters in the Southern Ocean. Recently, Pazan and Niiler (2004) announced a data set of ocean currents derived by combining these data after applying a drifter-dependent slip correction model (Pazan and Niiler, 2001). The types of drifters in this data set include SVP (both with and without drogue), FGGE (including later FGGE-type drifters deployed by the International Ice Patrol), and Navy-AN/WSQ-6.…”

“…15 of Rio and Hernandez, 2003). Bottom: time-mean slip of undrogued SVP drifters, using the parameterizations of Niiler and Paduan (1995) and Pazan and Niiler (2001). Drogued drifters experience a slip which is smaller by an order of magnitude.…”