WDirDiff/Compass data from Buccoo 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:

• 11/27/2013: initial deployment
• 5/15/2014 - 8/11/2014: station offline due to a power failure
• 11/12/2014 - 11/13/2014: buoy brought to land for a maintenance operation
• 2/2/2015 - 2/3/2015: 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, note the four different "regimes" of compass directions.  They alternate between averages that are roughly 180° offset from one another, with the transition times coinciding with periods when the buoy was believed to have been brought back to land and then redeployed (details below).  I am not familiar with CREWS/CCCCC buoy design but I believe these buoys are deployed at the midpoint of mooring lines attached to two anchor points on the ocean floor, and this pattern suggests to me that the Buccoo Reef buoy has on occasion been attached to its mooring in a manner that is 180° twisted from its prior attachment.  To my knowledge this is not indicative of any problem, however, no such pattern has been noted at this station's sister buoys (see below) to date.

The second thing to note from this graph is that the WDirDiffs average over the buoy's lifetime is -18.6°.  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.  The WDirDiff averages are slightly different during the different compass "regimes" described above but these differences are within the range of accuracies of the wind sensors and may not be significant.

The exact statistics of WDirDiff averages and Compass averages per "regime" are as follows:

• 11/27/2013 - 5/15/2014: WDirDiff -19.8°, Compass 205.1°
• 8/12/2014 - 11/12/2014: WDirDiff -17.7°, Compass 30.0°
• 11/12/2014 - 2/2/2015: WDirDiff -19.3°, Compass 204.6°
• 2/2/2015 - 6/9/2015 (present): WDirDiff -17.1°, Compass 32.3°

Similar analyses carried out at this buoy's sister stations at Speyside / Angel's Reef, Tobago (ARTO1) and Little Cayman, Cayman Islands (CCMI2) found that the both the ARTO1 and CCMI2 Compass directions were stable throughout their deployment lifetimes to date.  At ARTO1 the lifetime WDirDiff average is -11.4°, which suggests that the ARTO1 wind instruments may not be properly oriented although they are not as divergent as the BUTO1 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 ARTO1 and at this link for CCMI2.

(signed)
Mike Jankulak