Abstract
Abstract. The eXperimental Planetary boundary layer Instrumentation Assessment (XPIA) field campaign took place in March through May 2015 at the Boulder Atmospheric Observatory, utilizing its 300 m meteorological tower, instrumented with two sonic anemometers mounted on opposite sides of the tower at six heights. This allowed for at least one sonic anemometer at each level to be upstream of the tower at all times and for identification of the times when a sonic anemometer is in the wake of the tower frame. Other instrumentation, including profiling and scanning lidars aided in the identification of the tower wake. Here we compare pairs of sonic anemometers at the same heights to identify the range of directions that are affected by the tower for each of the opposing booms. The mean velocity and turbulent kinetic energy are used to quantify the wake impact on these first- and second-order wind measurements, showing up to a 50 % reduction in wind speed and an order of magnitude increase in turbulent kinetic energy. Comparisons of wind speeds from profiling and scanning lidars confirmed the extent of the tower wake, with the same reduction in wind speed observed in the tower wake, and a speed-up effect around the wake boundaries. Wind direction differences between pairs of sonic anemometers and between sonic anemometers and lidars can also be significant, as the flow is deflected by the tower structure. Comparisons of lengths of averaging intervals showed a decrease in wind speed deficit with longer averages, but the flow deflection remains constant over longer averages. Furthermore, asymmetry exists in the tower effects due to the geometry and placement of the booms on the triangular tower. An analysis of the percentage of observations in the wake that must be removed from 2 min mean wind speed and 20 min turbulent values showed that removing even small portions of the time interval due to wakes impacts these two quantities. However, a vast majority of intervals have no observations in the tower wake, so removing the full 2 or 20 min intervals does not diminish the XPIA dataset.
Highlights
Sonic anemometry is a pivotal tool for measuring highfrequency motions and fluxes in the atmosphere, and tall towers are commonly used to mount these instruments in order to observe the surface layer of the atmosphere
Separating the sonic anemometer observations into daytime and nighttime conditions, we find only small differences: the peak wind speed ratio when the northwest booms were in the tower wake was 15 % larger during the daytime than at night and 4 % larger in the daytime when the southeast booms were in the wake
When wind speeds from the sonic anemometers mounted on opposing sides of the 300 m BAO tower are compared to each other (Fig. 5), the effects of the tower wakes on the downstream instrumentation and speed-up around the tower are quantified
Summary
Sonic anemometry is a pivotal tool for measuring highfrequency motions and fluxes in the atmosphere, and tall towers are commonly used to mount these instruments in order to observe the surface layer of the atmosphere. The current study stands uniquely as a comprehensive method of determining any meteorological tower wake, using either a set of in situ anemometers in and out of the wake or independent wind measurements (e.g., profiling lidars). This thorough analysis for the BAO tower eliminates, hereafter, the need to repeatedly quantify the angles of the tower wake for each subsequent measurement campaign or deployment of sonic anemometry on the BAO tower. All data are publicly available in the Data Archive and Portal, found at https://a2e.energy.gov/projects/xpia
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