Improved wind and turbulence measurements using a low-cost 3-d sonic anemometer at a low-wind site

Inexpensive 2-D sonic anemometers have become popular for routine wind monitoring because of their low or no maintenance requirements. Until recently, only expensive research sonic anemometers were available for estimating vertical turbulent wind variables in order to estimate vertical dispersion and heat, evaporative and momentum fluxes. The Terrestrial and Atmospheric Monitoring and Modeling (TAMM) Group at the Lawrence Livermore National Laboratory (LLNL) acquired and installed an inexpensive 3-D sonic anemometer at the 10-m level on one of its meteorological towers to supplement its monitoring program. Goals include acquiring data to make accurate estimates of evaporation (evaporative heat flux), vertical heat and momentum flux, and improved vertical turbulent fluctuation data. This instrument also serves as a redundant sensor to co-located mechanical sensors at the same height.

A year of data from the sonic anemometer and mechanical wind sensors were analyzed and compared. Results indicate that 15-minute average and peak 1-second wind speeds (u) from the sonic agree well with data derived from a co-located cup anemometer over a wide range of speeds. Wind direction data derived from the sonic also agree closely with those from a wind direction vane. Values of standard deviation of longitudinal wind speed fluctuations (σu) are about 5% greater from the sonic than from the cup anemometer and the correlation is very good. The standard deviation of wind direction fluctuations (σΘ) from the sonic and vane agree well, although the vane underestimates at very low wind speeds and this variable shows more scatter. The most significant differences are associated with the standard deviation of vertical wind fluctuations (σw): the co-located vertical propeller anemometer yields values increasingly less than those measured by the sonic anemometer as horizontal wind speeds decrease from 2.5 to near 0 m/s. The underestimation of (σw) by the vertical propeller and to a lesser extent u by the cups at low wind speeds compounds the errors for the standard deviation of vertical wind angle fluctuations (σφ), an indicator of vertical dispersion that is often used to calculate the P-G stability category. The sonic anemometer routinely indicates larger σφ values than the propeller vane, with the sonic values typically 5° to 10° higher when the propeller indicates σφ is less than about 5°. The errors in the propeller anemometer, caused by its inability to capture the higher frequency (smallest scale) turbulent fluctuations, could therefore lead to large (factors of 2 to 10 or more) errors in the vertical dispersion during stable conditions with light winds.

The low-cost 3-D anemometer has reliably measured the 3 components of wind during an entire year during this study. The ability of this anemometer to more accurately measure vertical wind fluctuations can provide more accurate vertical dispersion and flux data, including evaporation. The major drawbacks include invalid or lost data from wetting during or after rainfall and relatively large power requirements from a battery backup in case the tower experiences an AC power loss. Therefore this instrument is ideally suited to supplement routine wind measurements by equaling or improving most measurements, especially in the vertical, and simultaneously providing a low-maintenance redundant instrument during dry conditions.