Correction of a Non-orthogonal, Three-Component Sonic Anemometer for Flow Distortion by Transducer Shadowing

Abstract

We propose that flow distortion within a non-orthogonal CSAT3 sonic anemometer is primarily due to transducer shadowing, which is caused by wakes in the lee of the acoustic transducers impinging on their measurement paths. The dependence of transducer shadowing on sonic path geometry, wind direction and atmospheric stability is investigated with simulations that use surface-layer data from the Horizontal Array Turbulence Study (HATS) field program and canopy roughness-sublayer data from the CHATS (Canopy HATS) field program. We demonstrate the efficacy of correcting the CSAT3 for transducer shadowing with measurements of its flow distortion in the NCAR wind tunnel, combined with 6 months of data collected in the atmospheric surface layer with adjacent CSAT3 and orthogonal ATI-K sonic anemometers at the NCAR Marshall field site. CSAT3 and ATI-K measurements of the variance of vertical velocity σ 2 w and the vertical flux of sonic temperature agree within 1 % after correction of both sonics for transducer shadowing. Both the simulations of transducer shadowing and the comparison of CSAT3 and ATI-K field data suggest a simple, approximate correction of CSAT3 surface-layer scalar fluxes with an increase on the order of 4-5 %, independent of wind direction and atmospheric stability. We also find that σ w /u * (where u * is the friction velocity) and r uw (the correlation coefficient) calculated with corrected CSAT3 data are insensitive to wind direction and agree closely with known values of these dimensionless variables for neutral stratification, which is evidence for the efficacy of the correction of the horizontal wind components for transducer shadowing as well.

Field intercomparison of prevailing sonic anemometers

“…Considering flow distortion errors of the order of 5 % or more that are reported in the literature (Frank et al, 2016;Horst et al, 2015;Huq et al, 2017), the very good agreement between all sonic anemometers in this field experiment is nevertheless somewhat surprising. A contribution by changes in the firmware of the different sonic anemometers over the last 10 years is likely but not fully documented.…”

“…For comparison, Gash and Dolman (2003) report about 90 % of their data to be within ±20 • for the Horstermeer peat bog site, and Grare et al (2016) report their data to be in a range of ±15 • , most of times even within ±10 • , measuring at 10 m above shrubland. Horst et al (2015) report their angles of attack to be mostly within ±8 • for measurements above low weeds and crop stubble with an aerodynamic roughness length of 0.02 m. Since the spread of angles of attack is at the upper end of the values reported in the literature, our comparison results can be considered as a conservative estimate for the random instrument-related uncertainty of typical applications of eddy covariance measurements over vegetation canopies. A common significant systematic error of all tested instruments is quite possible, as suggested by Frank et al (2016).…”

“…The persisting lack of energy balance closure at many sites around the world (Stoy et al, 2013) and the emerging indications of a general flux underestimation of non-orthogonal sonic arrays were the primary motivation of a special field experiment by Horst et al (2015). They conducted an intercomparison at an almost ideal site, which was flat, even and with a homogeneous fetch.…”

Scalar turbulent behavior in the roughness sublayer of an Amazonian forest

“…For the CSAT3, these corrections may change w estimates by approximately 8 % (Frank et al, 2013) and σ w by approximately 5-6 % (Frank et al, 2013;Horst et al, 2015).…”

A study of turbulent fluxes and their measurement errors for different wind regimes over the tropical Zongo Glacier (16° S) during the dry season

“…To evaluate the degree to which this error could affect our measurements, we selected runs during specific events for which wind conditions were representative of the three main wind regimes. For each event we compared the covariances of w with θ and q, obtained without any correction related to the attack angle, and covariances obtained after correcting the wind components for transducer shadowing following the method proposed by Horst et al (2015). The relative difference between these two covariances, plotted against ω, the attack angle relative to the u − v plane of the instrument, helps us to evaluate the degree of flux underestimation in each wind regime.…”