Resolving Velocity Profiles with the Multi-Scale Profiler

The Multi-Scale Profiler MSP, a freely falling dropsonde, has been used over the last 12 years to resolve oceanic shear variance at vertical scales from a few hundred meters down to nearly a centimeter. Because MSP yielded the first complete oceanic shear spectra., it is important to document the methods by which they were produced. Large scales are measured by an electromagnetic current meter ECM, microscales by airfoil probes, and intermediate scales by an acoustic current meter ACM. The ACM detects velocity relative to the instrument, so the platform motion must be known to determine the water velocity. Primarily, the ACM measurements are affected by tilt oscillations and by the gross point-mass motion of the vehicle the former is inferred from accelerometer data, and the latter is constructed from a model of the vehicles response to oceanic shear. Horizontal forcing on the array of drag brushes and turning blades at the tall complicates the response by causing MSP to react strongly to fluctuations with scales near the instruments length of 4.3 m. We examine the effects of this response on spectra of the ACM measurements, noting particularly a deep notch near 0.2 cpm cycles per meter. To account for such spectral features, the model of Hayes et al. for the TOPS dropsonde was modified so that it correctly parameterized our large tail force. We discuss the dynamics, data processing, and model formulation relevant to production of oceanic velocity profiles from the ACM data,and present analytic transfer functions-derived from Fourier transforms of the model equations-which guide selection of optimal values for the model parameters.