Wind Direction

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CH-AWS, Alp Weissenstein

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Flux footprint CH-AWS, from Zeeman et al. (2010)

Wind direction CH-AWS, from Hiller et al. (2008)

The flux footprint from Zeeman et al. (2010) shows the main wind directions coming from approx. 300° and 120°.Hiller et al. (2008) show the same wind directions during day and night.

The main wind directions for the adaptive EC system at CH-AWS (with Gill HS-50, yoke-mounted) should be the same like in the previous publications. I tested in EddyPro which settings are needed to achieve the same wind direction results for 2018 data. In CH-AWS, the spar where the sonic is mounted points exactly to the north.The sonic is operated in AXIS configuration.

In EddyPro, it is necessary to define North alignment (AXIS or SPAR) and North off-set (0-359°) for the EC system. The same main wind directions like in the mentioned publications were achieved with the settings SPAR / 0° or AXIS / 30° (both yield same results). 

In the EddyPro help files, it is mentioned

North alignment: For Gill anemometers, specify whether the spar or axis of the anemometer is oriented toward north. This field only applies to Gill anemometers.

Source

After tests by ICOS ETC and us in the lab it seems like AXIS / 0° (SA is running in AXIS configuration and the spar/boom where the SA is mounted now points exactly to the north) is the correct setting and the old wind directions are potentially wrong by 30°.

Going by this description, the best setting would be SPAR / 0°, since the spar points exactly to the north.

In summary

• SPAR / 0°: correct wind direction
• AXIS / 30°: correct
• AXIS / 0°: too low by 30°
• SPAR / 330°: too low by 30°
• AXIS / 330°: too low by 60°

Downloads

2019-02-19 comparison wind direction EC vs wind vane.xlsx