Sonic anemometry is fundamental to all eddy-covariance studies of surface energy and ecosystem carbon and water balance. Recent studies have shown some anemometers underestimate vertical wind. Using a replicated inter-comparison experiment between several different anemometer designs we tested the hypothesis that this underestimation is due to a lack of transducer and structural shadowing correction.
This project involved experimental research in the field and subsequent data analysis. Bayesian analysis was used to test differences between half-hourly anemometer measurements of the standard deviation of wind and temperature, turbulent kinetic energy (TKE), and the ratio between vertical/horizontal TKE. Data sets were analyzed with various applications of transducer shadow correction.
Results from this experiment demonstrated that the vast majority of ecosystem flux sites around the world measure 10% too little atmospheric exchange of carbon dioxide, water vapor, and energy. Observed differences between different sonics were mitigated (but not necessarily fully eliminated) by adding the shadowing correction to anemometers that had not used it previously. Other sonic configuration tests confirmed that the shadowing issue is complex and three-dimensional in nature, which does not lend itself to a simple shadowing correction evaluated by this study.
Considering the ubiquity of non-orthogonal anemometers, these results are consequential across all worldwide flux networks. Until more appropriate corrections for non-orthogonal designs are developed and the winds between sites are considered, the broader impact across all flux sites can only be surmised.
Frank, JM, WJ Massman, E Swiatek, HA Zimmerman, BE Ewers. 2015 (in review). Transducer/structure shadowing explains observed underestimates in vertical wind velocity from some non-orthogonal sonic anemometers. Journal of Atmospheric and Oceanic Technology.