What is the wave velocity cell and how is it used?Follow
For the AWAC AST there is only one cell measuring velocities during a wave burst using only the slanted beams. The size and position of this cell is not straight forward: The position of the cell is determined by a fraction (10%) of the the minimum pressure measured in the current profile that immediately precedes the wave wave burst. This is indicated in the "Wave header" tab of the AWAC software, it is also output in the wave header file (*.WHD) when you do the file conversion to ASCII.
The values shown in the "Wave Vel" tab in the AWAC software are the velocities measured in the single wave cell of each beam during the wave burst mode.
You can see the Current Speed and Current Direction measured in the wave cell in the *.WAP file, these two values are the mean velocities (from the East and North components). Bare in mind that this cell size is often much larger than the cell sizes of the current profile. Furthermore the position (and size to a lesser extent) of this cell will change with variations in the minimum sea level (from tides and sea state). This current estimate in the wave burst turns out to be an estimate with very low uncertainty since the averaging interval is the length of the wave burst and that the cell is large; so treat it as a good, but "gross" estimate.
When the AWAC measures the orbital velocities during the wave burst mode it measures in the BEAM coordinate system. This means that we are really measuring the orbital velocities as they are projected onto the beams direction. This is why you often see a sinusoidal time series of the velocity that is slightly out of phase with the other beams, and has a slightly different amplitude. The phase difference is a result of the time lag as the wave passes through the "array" of velocity measurement cells. The amplitude difference is attributed to how much of the waves direction is in line with the beam, projected onto the beam.
The velocities that are written out to the *.WAD file are always in the Beam coordinate system. This is in fact indicated in the *.HDR file, which provides a description of each one of the fields of the converted files. The coordinate system for the *.WAD file is always Beam no matter if you select SUV (a variant of ENU) data collection or standard data collection
The coordinate system that you select in the configuration applies only to the current profiles. Therefore if you select ENU it will not change the way the waves are measured or written to the *.WAD file.
The velocities are located in the first bin above the blanking distance. In fact this is the only bin collected during the wave burst mode.
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