Identification of Tower Wake Distortions in Sonic Anemometer Measurements during XPIA

The eXperimental Planetary boundary layer Instrumentation Assessment (XPIA) field campaign compared state-of-the-art measurement technologies from March to May of 2015 at the Boulder Atmospheric Observatory (BAO) in Erie, CO. The centerpiece of this campaign was the 300-m meteorological tower instrumented with sonic anemometers, mounted on opposite sides of the tower at six heights. The BAO tower triangular lattice sides are 3 m long, with most sonic anemometer booms 4.3 m long; at the 250-m level, the SE boom was only 3.3 m. Due to the booms orientation and sufficient length, at any time, one sonic anemometer could receive free-stream winds, independent of the tower wake influence. Using the measured data, we now compare the sonic anemometer data from opposing booms to identify times when each instrument was waked from the BAO tower obstructing the flow. By analyzing the BAO tower data collected throughout the diurnal cycle for three months, we are able to explore the impact of ambient atmospheric stability on the tower wake. To independent verify the measurements, we also combine profiling and scanning lidar measurements of winds in the vicinity of the tower. We determine the characteristics of the tower wake with the aforementioned instruments, and expand our analysis using turbulence measurements. We also examine turbulent kinetic energy to quantify the angular range of BAO tower wake directions. A wider range of wind directions experience tower wake impacts when turbulent kinetic energy is considered; the wake angular width expands from a range of 35 degrees for mean wind speeds to a range of 65 degrees for turbulent kinetic energy. Comparisons of wind speed from profiling and scanning lidars confirmed the angular extent of the tower wake, with approximately a 50% reduction in wind speed observed by the waked sonic anemometer, and a speed-up effect for wind directions around the wake boundaries. Wind direction differences between pairs of sonic anemometers and between sonic anemometers and lidars also show substantial biases, indicating deflected flow around the tower structure. These results indicate that tower wakes are likely more consequential than past studies previously acknowledge. The goal is to offer stability-dependent recommendations for diagnosing and filtering tower wake contamination in the XPIA and other datasets using meteorological towers.