Determine the speed of profiling
The speed of profiling can be determined with a variety of different methods. It must be determined because it is required for the conversion of the shear-probe samples into physical units and for transforming frequency spectra into wavenumber spectra. Methods for determining the speed of profiling include, but are not limited to:
- Using the rate-of-change of pressure for a vertical profiler.
- Using the rate-of-change of pressure, the pitch angle, and the angle of attack for a glider.
- Using a flight model for a glider.
- Using an onboard current meter.
- Using a constant speed.
- Using the speed recorded by an independent instrument after its record has been carefully aligned in time with the shear-probe data.
The rate of change of pressure is often used to determine the speed of profiling of a vertical profiler. Likewise, the rate of change of pressure, the pitch angle and the angle of attack are often used to determine the speed of a glide. There are circumstances when rate of change of pressure may not provide a reliable estimate of the speed of profiling with respect to the local water surrounding an instrument. In vinously turbulent waters that contains eddies with scales larger than your instrument, the up- and down-drafts created might modify the vertical velocity of your instrument with respect to an inertial (or fixed-earth) reference. For example, if an updraft equals the nominal vertical speed of a profile, the profiler will stall, and its rate of change of pressure will go to zero. However, there is still flow past the shear probes and the speed of this flow will be close to the nominal vertical speed of your instrument because most profilers adjust quickly (in about one body length if travel) to their surrounding conditions. You can also get an erroneous indication of the speed of profiling from the rate of change of pressure when your instrument travels through a soliton. In fact, anicdodal information (L. St. Laurent, personal communication) indicates that a profiler may momentarily reverse its course when traveling through a very energetic soliton. For such unusual conditions it may be wise to set the speed of profiling to a constant equal to the nominal vertical velocity of your profiler.
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