Improvements in cold weather condition flux measurements with Gill 3 axis ultrasonic

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Wednesday, 7 January 2015
Richard L. McKay, Gill Instruments Limited, Lymington, Hampshire, United Kingdom; and T. Stickland

Eddy Covariance (EC) systems are gaining growing acceptance as repeatable and a qualitatively good method to measure changes in carbon, water vapour and other trace gases in the atmosphere. With increasing global coverage these systems are moving away from ideal conditions to more extreme weather and complex terrain environments to try and gain a complete energy budget picture in all conditions. In these new and challenging conditions the equipment is pushed to the limits and invariably some issues are found with all of the instrumentation. There has been a discernible push to measure the onset of flux conditions for these gases in cold temperatures around 0C where Gill 3 axis ultrasonic anemometers have been found to have a unique non-linear response for speed of sound (SoS) and therefore sonic temperature (Ts) accuracy between -5C and 0C. Although this does not affect instantaneous Ts accuracy, this was of increasing concern as trace samples quantities are already small where this additional error was a big concern and any data collected would be hard to correlate and later integrate into models and advance our understanding of the environment, but also because the community wishes to use Ts as an ambient temperature marker for longer duration, slower time domain temperature measurement for other parameters which would then have additional error. This study presents information on what has caused this non-linear response along with some other undesirable characteristics that have been identified and corrected during investigations over the last 2 years, both in the laboratory at Gill as well as in the field over several seasons in both North America and Europe at 3 locations with independent verification of the improvements. Results will show improvements to the manufacture and fitting of transducers used by Gill to shift this undesired non-linear response down to much lower temperatures of -30C where this will not affect flux measurements. Additionally, preliminary results show this new manufacturing technique also improves the overall Ts accuracy.