I was having some trouble understanding the VELPTMN (mean point water velocity) data product from the Southern Ocean Apex surface mooring for deployment 2 and 3.
First, I want to confirm with you that in both deployments the single point velocity meter was implemented at depth of approximately 13m as indicated by the pressure measurement. If I didn’t interpret the depth wrong, then I noticed that the velocity measurements for deployment 2 (year 2016) are almost an order of magnitude larger than that for deployment 3 (year 2017 and 2018).
I doubled checked SST and air temperature data from the Bulk Meteorology Instrument package but they seemed much more continuous than the velocities. I wonder are there some instrument changes/calibrations or data scalings that I wasn’t aware of or should I interpret the results physically?
I further noticed that the VELPTMN data product I acquired from the single velocity meter has frequency of 15 minutes whereas the same VELPTMN data from the Bulk Meteorology Instrument package has frequency of 1 minute. I wonder whether the 1-minute data is from linear interpolation?
To begin with, there is only one VELPT (Nortek Aquadopp) instrument deployed on the Surface Mooring, and it is on the Near Surface Instrument Frame (NSIF) at ~12 m water depth. That is the same for all deployments.
Also, the Bulk Meteorology (METBK) data products incorporate the VELPT data in the calculations. The METBK samples at a frequency of 1 min, but the VELPT samples once every 15 minutes, so the VELPT data is interpolated to match the METBK data.
As for the change in magnitude of the velocity data from Deployment 2 to Deployment 3, we’ll have to dig into that a bit further and get back to you.
We have looked into this a bit – checking on calibration sheets, instrument history, and such – but have not yet determined a reason why the velocity magnitude would be so different. It is still in our queue to investigate further.
We have dug into the velocity magnitude issue and have not found a root cause. There are no obvious issues with the instrument calibrations, the configuration, or the sampling scheme.
We also found that we see a similar issue for the VELPT instrument on the Irminger Sea and Argentine Basin Surface Moorings – the velocity magnitudes for the 2nd deployment seem higher than they should be.
We have added annotations to the data sets to note this problem and flag the data as suspect. It is possible that there may be a conversion factor or offset that would make the data more reasonable. This could be determined by looking at the ADCP data from that mooring.
Thanks and stay safe,
Sheri N. White
OOI/CGSN Data Team
