Measuring overturns with gliders
Abstract:The accuracy of the estimation of the vertical size of eddies in turbulent stratified shear flows in the ocean from measurements obtained by gliders is examined. It is assumed that gliders move along paths inclined at moderate angles to the horizontal. Comparison is made with measurements by probes falling or lowered vertically through billows resulting from Kelvin-Helmholtz (K-H) or Holmboe instability and through the statically unstable regions formed at early stages of convective breaking of internal waves. The probable errors involved in estimating the overturn scale of a K-H billow along the track of a glider are greatest when the ratio of the billow's vertical to horizontal scale, b/a, is greatest and when a glider's inclination angle, α, is moderate, but the errors are generally relatively small. At small angles, α, the glider path may intersect more than one billow, reducing the errors. Larger errors are possible, however, in measuring eddies in turbulent stratified shear flows, and their magnitude depends on the orientation of eddies relative to the trajectory of the glider.
False overturns may be apparent using gliders with small inclination angles, α, in internal waves, and consequently erroneous estimates of the displacement scales obtained, even when the slope of the waves, s, < 1 and convective overturn is entirely absent. Quantification of overturns from glider measurements of the apparent vertical size of the regions in which the density increases upward can result in misleading estimates of the scale of overturns. Although, because of the wave-induced horizontal and vertical motions, the trajectory of free-fall probes will not be vertical when passing through an internal wave field, and nor will it be steady, the mean square displacements obtained from measurements are found to be the same as those that would be made by a probe passing vertically through a frozen wave field. Attention is drawn to the paucity of information about the structure of naturally occurring eddies in the stratified ocean.
Document Type: Research Article
Publication date: January 1, 2012
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