1Cooperative Institute for Research in Environmental Sciences, University of Colorado at Boulder, Boulder, Colorado, USA
2National Oceanic and Atmospheric Administration, Earth Systems Research Laboratory Physical Sciences Division, Boulder, Colorado, USA
3Department of Atmospheric and Oceanic Sciences, University of Colorado at Boulder, Boulder, Colorado, USA
4National Oceanic and Atmospheric Administration, Earth Systems Research Laboratory Chemical Sciences Division, Boulder, Colorado, USA
5National Center for Atmospheric Research, Boulder, Colorado, USA
6Department of Mechanical Engineering, University of Texas at Dallas, Dallas, Texas, USA
7National Renewable Energy Laboratory, Golden, Colorado, USA
Received: 23 May 2016 – Discussion started: 22 Jul 2016
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.
Revised: 30 Dec 2016 – Accepted: 03 Jan 2017 – Published: 02 Feb 2017
McCaffrey, K., Quelet, P. T., Choukulkar, A., Wilczak, J. M., Wolfe, D. E., Oncley, S. P., Brewer, W. A., Debnath, M., Ashton, R., Iungo, G. V., and Lundquist, J. K.: Identification of tower-wake distortions using sonic anemometer and lidar measurements, Atmos. Meas. Tech., 10, 393-407, doi:10.5194/amt-10-393-2017, 2017.