AWAC wave velocity cell and usage

Below is a question regarding current data from an AWAC during the wave burst mode

I have a number of questions regarding some AWAC AST data I have.

First, when I access the "*.wpr" data from the AWAC and before I do any
processing I can view the data using the AWAC software. When I view the
wave data, there is a tab on the bottom of the program that says "wave vel".
Are these the values for the orbital velocity?
If they are, from what depth below the surface are they taken? For example
are they taken from a certain ADCP bin? I ask this as I would like to have
perform some data analysis on the orbital velocitis.

The values shown in the "Wave Vel" tab in the AWAC software is the velocities measured in the single wave cell of each beam during the wave burst mode.  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 differerent amplitude.

The phase difference is a result of the time lag as the wave passes through the "array" of velocity measaurement cells.  The amplitude difference is attributed to how much of the waves direction is in line with the beam, projected onto the beam.

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.

Second, I have used the Quickwave software to process the AWAC AST data
which has worked very well and given me wave parameters.
In the output file "*.wap", there are two columns of data called "current
speed" and "current direction" in columns 21 and 22 respectively. In
brackets after it says wave cell. Can you tell me what these data are and
how they are measured?

These two values of current speed and direction are the mean velocities (from the East and North components) that are measured in the wave velocity cell.  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

kind regards,

Torstein

Hi Torstein,

Thank you very much for the reply.

To clarify, the velocity values I see in the "*.wad" ASCII file are the orbital velocities as measured using the wave cell.  Have the values in "*.wad" file already been transformed from BEAM coordinates to the user-defined coordinates?

I chose ENU coordinates, so are the orbital velocities in the "*.wad" file in ENU coordinates?

All the best,

Adrian

Hi Adrian,

The velocities that are written out to the *.WAD file are always in the Beam coordinate sytem.  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

Th 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.

kind regards,

Torstein