Measurement of Vertical Kinetic Energy and Vertical Velocity Skewness in Oceanic Boundary Layers by Imperfectly Lagrangian Floats

Abstract

The effects of upward buoyancy on the accuracy with which Lagrangian floats can measure the Eulerian mean variance ?ww?E and skewness SwE of vertical fluid velocity w in the wind-driven upper-ocean boundary layer is investigated using both simulated floats in large-eddy simulation (LES) models and two float datasets. Nearly neutrally buoyant floats are repeatedly advected by the turbulent velocities across the boundary layer. Their vertical position Z is determined from pressure measurements; their W variance ?WW?F and skewness SWE are determined from the time series of float W = dZ/dt. If the float buoyancy is small, then the simulated floats measure the Eulerian velocity accurately; that is, δW2 = ?WW?F ? ?ww?E and δSW = SWF ? SwE are small compared to ?ww?E and SwE respectively. If the floats are buoyant, and thus have an upward vertical velocity Wbias relative to the water, then δW2 and δSW can become significant. Buoyancy causes the floats to oversample both shallow depths and strong vertical velocities, leading to positive values of δW2 and negative values of δSW. The skewness SZ?F of depth measures the degree to which shallow depths are oversampled; it is shown to be a good predictor of Wbias/?WW?F1/2, δW2/?WWF?, and δSW/SWF over a wide range of float buoyancies and boundary layer forcings. Float data collected during two deployments confirm these results, but averaging times of several float days are typically required to obtain stable statistics. Significant differences in the magnitude of the effect may occur between datasets from different ocean forcing regimes and float designs. Other measures of float buoyancy are also useful predictors. These results can be used to correct existing float measurements of ?ww?E for the effects of buoyancy and also can be used as a means to operationally assess and control float buoyancy.