WDirDiff/Compass data from Little Cayman, 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:

• 10/23/2013: initial deployment
• 10/14/2014 - 10/29/2014: buoy brought to land for a maintenance operation
• 12/29/2014 - 3/12/2015: station offline due to a power failure, brought to land before redeployment
• 3/12/2015 - 6/9/2015 (present): station's WXT non-operational (no redundant wind data)

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 through the end of 2014 (after which time WXT wind directions are not available for comparison) is +1.5°.  This is entirely reasonable and falls within a range explainable by the specifications of the anemometer (± 5° accuracy) and the WXT (± 3° accuracy).

Similar analyses carried out at this buoy's sister stations at Buccoo Reef, Tobago (BUTO1) and Speyside / Angel's Reef, Tobago (ARTO1) 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 ARTO1 Compass directions were stable throughout its deployment lifetime to date.  At BUTO1 the lifetime WDirDiff average is -18.6° and at ARTO1 the lifetime WDirDiff average is -11.4°, which suggests that at both Tobago buoys the wind instruments may not be properly oriented with the divergence being more significant at BUTO1 compared to ARTO1.

The complete analyses for the other WDirDiff/Compass averages, including graphs, may be found at this link for BUTO1 and at this link for ARTO1.

(signed)
Mike Jankulak