Vertical velocity from LADCP data

Abstract

Vertical velocity is important for ocean dynamics on a vast range of scales, from isotropic turbulence to the global overturning circulation, and directly affects transport of biogeochemical tracers. In spite of this importance, vertical-velocity measurements in the ocean are scarce. In an effort to remedy this situation, a new method has been developed to obtain full-depth profiles of vertical velocity from data collected with standard Lowered Acoustic Doppler Current Profiler (LADCP) systems, such as the ones used during the CLIVAR repeat hydrography sections. Data from LADCP systems, which consist of CTDs and ADCPs lowered on hydrographic wires, are typically processed to obtain full-depth profiles of horizontal velocity. The fundamental difficulty underlying LADCP data processing is that the velocity measurements are relative to the moving instrument package. In order to obtain absolute ocean velocities, the instrument motion must be removed from each ADCP velocity profile. One method for achieving this consists in vertically integrating vertical shear of velocity, which can easily be obtained from LADCP velocity records and is independent of instrument motion, and to reference the resulting baroclinic velocity profiles with external constraints, such as package motion derived from bottom tracking. While this method can, in principle, be applied both to horizontal and to vertical velocity data the resulting uncertainties of ≈3-5 cm·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> are larger than the typical signal expected for vertical velocity in the ocean. In the new method presented here, vertical instrument motion is estimated from the temporal derivative of CTD pressure. While conceptually extremely simple, practical difficulties arise because vertical package motion (winch speed plus surface-wave induced heave) is usually associated with velocities on the order of 1 m·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , whereas instantaneous vertical velocities in the ocean are typically 2 orders of magnitude smaller. Nevertheless, it is found that absolute vertical velocities accurate to ≈θ.5 cm·s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> can be obtained with available off-the-shelf instrumentation (Teledyne/RDI Workhorse ADCP, SeaBird 9plus CTD), as long as suitably lowpass-filtered high-frequency CTD pressure data are matched carefully to the corresponding ADCP time series. The new method can potentially be applied to available CTD/LADCP data from thousands of profiles collected all over the world's oceans. It is expected that the resulting vertical velocity data will provide novel insights into many dynamical processes, including internal waves, boundary currents, hydraulics, mesoscale and sub-mesoscale eddies, fronts, etc. It may furthermore be possible to use the vertical velocity data to improve "finestructure parameterization methods" that are increasingly being used to study turbulence and mixing from CTD/LADCP data.