Optimizing the Physical Implementation of an Eddy-covariance System to Minimize Flow Distortion

Abstract:

The eddy-covariance technique is widely applied to observe the exchange of energy and scalars between the earth’s surface and its atmosphere. In practice, fast (≥10 Hz) sonic anemometry and enclosed infrared gas spectroscopy are used to determine fluctuations in the 3-D wind vector and trace gas concentrations, respectively. Here, two contradicting requirements need to be fulfilled: (i) the sonic anemometer and trace gas analyzer should sample the same air volume, while (ii) the presence of the gas analyzer should not affect the wind field measured by the 3-D sonic anemometer. To determine the optimal positioning of these instruments with respect to each other, a trade-off study was performed.

Theoretical formulations were used to determine a range of positions between the sonic anemometer and the gas analyzer that minimize the sum of (i) decorrelation error and (ii) wind blocking error. Subsequently, the blocking error induced by the presence of the gas sampling system was experimentally tested for a range of wind directions to verify the model-predicted placement: In a controlled environment the sonic anemometer was placed in the directed flow from a fan outfitted with a large shroud, with and without the presence of the enclosed gas analyzer and its sampling system.

Blocking errors were enhanced by up to 10% for wind directions deviating ≥130° from frontal, when the flow was coming from the side where the enclosed gas analyzer was mounted. Consequently, we suggest a lateral position of the enclosed gas analyzer towards the aerodynamic wake of the tower, as data from this direction is likely affected by tower-induced flow distortion already. Ultimately, this physical implementation of the sonic anemometer and enclosed gas analyzer resulted in decorrelation and blocking errors ≤5% for ≥70% of all wind directions. These findings informed the design of the National Ecological Observatory Network’s (NEON) eddy-covariance system, which is currently being deployed at all NEON sites.