Resolving Oceanic Shear and Velocity with the Multi-Scale Profiler

Abstract

The Multi-Scale Profiler (MSP), a freely falling dropsonde, has been used over the past 12 years to measure oceanic shear variance. Complete resolution of oceanic shear spectra is achieved by combining the measurements of MSP?s acoustic current meter (ACM), electromagnetic current meter (ECM), and airfoil probes. The ACM detects flow relative to MSP, so the platform motion must be known to determine the water velocity. The vechicl's tilt oscillation is inferred from accelerometer data, and its gross (point mass) horizontal motion is simulated by modeling MSP's response to the relative flow. Forcing on its tail array causes MSP to react as a point mass to fluctuations with scales as small as 2-3 m. The model of Hayes et al. for the TOPS dropsonde was modified so that it reasonably parameterized the large MSP tail force. Relevant dynamics and data processing are discussed, and the point-mass model is presented along with the analytic transfer functions that are used to select parameter values, assess sensitivities, and estimate uncertainties. Because they are unaffected by MSP's horizontal motion, the ECM measurements directly reflect the flow structure and, consequently, provide an onboard reference against which the large-scale corrections to the ACM measurements are validated. Uncorrected ACM data provide a direct check on the airfoil data, which resolve microscale shear variance to within a factor of 2, aside from some noted exceptions in warm, turbulent waters. The motion-corrected ACM profiles are shown to resolve shear variance to within 10%?15% at vertical scales from over 200 m down to 1 m (with minor anomalies at 5-m and 2-3-m scales).