WDirDiff/Compass data from Speyside / Angel's Reef, 2013-present

This post is part of a series of posts to share the results of my recent evaluation of data produced by all of the CREWS/CCCCC buoys over their lifetimes, from 2013 to the present.  This post will discuss the offsets (WDirDiffs) between the wind directions reported by the analog anemometer manufactured by RM Young (RMY) and the sonic wind sensors on Vaisala's Weather Transmitter (WXT).  Ideally these offsets should be less than 5° in absolute value.  This post will further discuss the raw directions reported by the buoy's Compass.

For reference, some important milestones in this station's lifetime are as follows:

• 11/25/2013: initial deployment
• 11/10/2014 - 11/13/2014: buoy brought to land for a maintenance operation
• 2/4/2015 - 2/5/2015: buoy brought to land for a maintenance operation

The following graph shows the differences in wind directions reported by the two wind sensors (red, on the left axis) and the raw directions reported by the compass (blue, on the right axis).  All directions are reported in degrees of compass but note where the scales are different by a factor of 6x and the zeroes offset, with the WDirDiff axis running on the left from -30° to +30° but the Compass axis running on the right from 0° to 360°.  A negative WDirDiff would indicate that the reported WXT wind directions are lower than the corresponding analog anemometer values.

Please click on this image to see it in larger form.

First of all the Compass averages suggest that this buoy has been deployed in the same orientation throughout its entire lifetime to date.  See the report of WDirDiff/Compass averages for the Buccoo Reef station for an example where this does not appear to be the case.

The second thing to note from this graph is that the WDirDiffs average over the buoy's lifetime is -11.4°.  This is of concern because it falls outside of a range explainable by the specifications of the anemometer (± 5° accuracy) and the WXT (± 3° accuracy).  It suggests that one or both of the wind sensors are not properly oriented on the buoy in a manner consistent with correction to magnetic north using the direction offsets measured by the compass.  As of this writing it is not known which of the two reported wind directions is likely to be (more) accurate.

Similar analyses carried out at this buoy's sister stations at Buccoo Reef, Tobago (BUTO1) and Little Cayman, Cayman Islands (CCMI2) found that the BUTO1 Compass directions can be divided into four distinct "regimes" with subsequent regime averages offset from one another by roughly 180°, and the CCMI2 Compass directions were stable throughout its deployment lifetime to date.  At BUTO1 the lifetime WDirDiff average is -18.6°, which suggests that the BUTO1 wind instruments may not be properly oriented and are even more divergent than the ARTO1 instruments.  At CCMI2 the WDirDiffs average through the end of 2014 (after which time WXT wind directions are not available for comparison) is +1.5°, which is entirely reasonable and consistent given the specifications of the two wind sensors.

The complete analyses for the other WDirDiff/Compass averages, including graphs, may be found at this link for BUTO1 and at this link for CCMI2.

(signed)
Mike Jankulak