Evaluation of the Flow Distortion around the Campbell Scientific CSAT3 Sonic Anemometer Relative to Incident Wind Direction

Abstract

In June 2002, a high-frequency air-sea momentum system was deployed in the surf zone for 3 days as part of an experiment to quantify air-sea momentum transfer when the wind and wave direction were at angles. The system obtained measurements in the nearshore via a high-resolution Campbell Scientific CSAT3 3D sonic anemometer and five high-frequency saltwater wave staffs. An advantage of the air-sea momentum system is that direct measurements of the atmospheric turbulent fluctuations can be obtained and applied to the calculation of momentum transfer at the air-sea interface. The Campbell Scientific CSAT3 sonic anemometer was postcalibrated under turbulent wind conditions to determine incident wind direction measurements influenced by the geometry of the instrument. Measurement results are compared to a pre-established benchmark, constant tow speed; and the mean wind speed, incident wind direction, and spectral density characteristics are evaluated to resolve specific instrument orientations in which the measurements are corrupted by the head and probe supports of the sonic anemometer.Calibration testing of the sonic anemometer determined that the mean wind speeds are reduced by 16% over a 408 range for incident wind angles of 1608-2008 relative to the head of the anemometer. Tilting the anemometer is found to decrease mean wind speed reduction influenced by the geometry of the anemometer. Variations in the measured wind directions were found to be greater than 18 for incident wind angles between 1608 and 2008 for 08 and 108 of tilt. Spectral characteristics were highly repeatable for all wind angles except for incident wind angles of 1808 for 08 and 108 of tilt.

Note on the application of planar-fit rotation for non-omnidirectional sonic anemometers

“…1b, any direction except flow from the back side). The disturbed sector for CSAT3 was confirmed to be approximately ± 20 • in wind tunnel measurements (Friebel et al, 2009). Field measurements typically show a standard deviation for wind direction of ± 20 • , which is also found in the data sets used for this study.…”

A Bayesian model to correct underestimated 3-D wind speeds from sonic anemometers increases turbulent components of the surface energy balance

“…Bayesian statistics is based on Bayes' theorem (Bayes and Price, 1763), which in modern applications relates the posterior probability of a model parameter conditioned on data to the product of the likelihood of the data and the prior probability of that parameter (Gelman et al, 2014). In essence, the prior represents an initial educated guess or belief in the value of a model parameter; the likelihood is the probability of observing the data if they were deterministically generated from a model; and the posterior is an updated belief in the model parameter considering each the prior, the model, and the data.…”

“…A second problem is that in turbulent flow fields there are few standards available to use as a calibration reference. Advancements in Bayesian techniques (Gelman et al, 2014) have created the potential to resolve both of these issues by incorporating prior knowledge of transducer flow distortions with a model that evaluates the omnidirectionality of a sonic anemometer to produce a posterior 3-D correction.…”

Using sonic anemometer temperature to measure sensible heat flux in strong winds

“…The 21.5 m CSAT was oriented so the alongsonic axis was pointed 203 • from true north. Strong winds at NWT are almost exclusively from the west (e.g., so that the angle between the wind vector and CSAT axis was ≈ 67 • , well within the CSAT ± 170 • acceptance angle and also avoiding influence of the tower structure (Friebel et al, 2009). Winds are rotated from sonic to planar-fit streamwise coordinates prior to the flux calculations (Wilczak et al, 2001).…”