Instrument Depth at OSNAP 53N

The OSNAP data were made available in a collection of NETCDF files, one file per current meter or CTD record. Each file contained the estimated depth of the instrument. It was immediately apparent that the depths given in these files were not and could not be accurate.

We begin with an understanding that the profile of a mooring under the action of varying currents at different depths is complex. One way of understanding this is to model it with mooring design software. For many years WHOI employed the famous NOYFB. The present writer designed many of the the OSU Buoy Group's moorings using a program named MOORAN. These programs take into account line stretch, the weight and buoyancy of all elements attached to the line, as well as the line itself, and the drag of the current. With a program like this, one can predict the depth of any instrument in a particular current regime. Of course, currents generally vary in time, so if a single number is desired the best one can do is select a single representative depth corresponding to a particular current profile.

All of the OSNAP 53N instruments contained pressure sensors. Comparing the pressure data and the given depths, it became clear that virtually all of the given depths were in error.

In most cases the PI of a given dataset will provide estimates of the minimum depth of each instrument - the depth during still water. Under this assumption, we used the pressure records to calculate a single adjustment for each mooring, which we added to the given depth of each instrument. The advantage of using a single adjustment for the whole mooring is that it preserves the vertical distances between instruments, which we assume are correct. Thus, with the first assumption above, the depths we provide here can be taken as instrument depths in the absence of blowover.

This is in line with our general practice in this archive. The time series that people send us sometimes are quite clean but often are full of unrealistic spikes and other obvious instrument errors. Our goal in this archive is not to make available whatever the data owner sends us. It is to add value by presenting clean data that in our opinion represents as closely as possible what a perfect measurement might show. We always either replace bad data with very short interpolations (sometimes linear, sometimes predictive) or we excise it entirely. The depth we assign to each instrument is either the depth provided to us, or one that we think is better.

All of this is by way of answering criticism from some who believe the depths we provide are wrong because they disagree with those that appear in some published sources.

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