NREL 2011: Comparing remote sensing and in-situ tower observations

When monitoring winds and atmospheric stability for wind energy applications, remote sensing instruments present some advantages to in-situ instrumentation such as larger vertical extent, in some cases easy installation and maintenance, measurements of vertical humidity profiles throughout the boundary layer, and no restrictions on prevailing wind directions. In this study, we compare remote sensing devices, Windcube lidar and microwave radiometer, to meteorological in-situ tower measurements (NWTC M2 tower) as shown in Fig. 1 to demonstrate the accuracy of these measurements and to assess the utility of the remote sensing instruments in overcoming tower limitations. We compare temperature and wind observations, as well as calculations of Brunt-Väisälä frequency and Richardson numbers for the instrument deployment period in May-June 2011 at the U.S. Department of Energy National Renewable Energy Laboratory’s National Wind Technology Center near Boulder, Colorado. The study reveals that a lidar and radiometer measure wind and temperature with the same accuracy as tower instruments, while also providing advantages for monitoring stability and turbulence. We demonstrate that the atmospheric stability is determined more accurately when the liquid-water mixing ratio derived from the vertical humidity profile is considered under moist-adiabatic conditions.

We examined the advantages of remote sensing of turbulence and stability within the atmospheric boundary layer by comparing Windcube lidar and microwave radiometer measurements to in-situ tower observations. The investigation focused on three main issues: 1) the accuracy of wind and temperature measurements from the remote sensing instruments, 2) advantages of remote sensing instruments for monitoring stability and turbulence in the atmospheric boundary layer, and 3) the influence of the humidity profile on atmospheric stability and Richardson number.

The Windcube lidar and Radiometrics microwave radiometer provided temperature and wind speed measurements as accurate as the in-situ tower observations. Differences in temperature ranged between 0.7–1.7°C between the tower and radiometer. Slightly larger values were observed at the surface, which were likely related to thermal turbulence rather than instrument accuracy. Wind observations from the Windcube lidar and in-situ tower indicated a spread of 1.2 ms-1 for wind speed. As a result, the differences in squared Brunt-Väisälä frequency and the Richardson numbers based on in-situ tower and remote sensing observations showed median values of 0.2x10-4 s-2 and 0.013 and spread values of 3.2x10-4 s-2 and 0.13, respectively (Fig. 2).

While the importance of humidity measurements has yet to be established for wind energy applications,
we demonstrated that the atmospheric stability, and therefore Ri, were determined more accurately when the liquid-water mixing ratio derived from the vertical humidity profile was considered in a saturated atmosphere (Figs. 2,3).  Under those conditions, derived from the radiometer humidity profile indicated that the atmosphere was unstable, instead of stable as indicated when using . In those cases, changes in the squared Brunt-Väisälä frequency affected the accuracy of Ri and therefore, the relationship between the Ri and the turbulence intensity.

    This comparison study also showed the ability of the remote sensing instruments in overcoming the limits of traditional tower measurements by measuring wind, temperature and humidity beyond 100 m AGL (Fig. 3). In addition, the small footprint of the remote sensing instruments provides flexibility in choosing deployment locations. Wind measurements and, therefore, the calculation of Ri, are not limited to cases when the orientation of an anemometer is appropriate for the wind direction but can instead be accomplished under any wind direction. Further observational studies are necessary to quantify the role of vertical humidity profiles on the stability during moist-adiabatic conditions and to determine how humidity impacts the formation and maintenance of low-level wind maxima and wind turbine performance.

 

Friedrich, K., J. K. Lundquist,M. Aitken, E. A. Kalina, and R. F. Marshall, 2012: Stability and turbulence in the atmospheric boundary layer: A comparison of remote sensing and tower observations. Geophys. Res. Lett., Vol. 39, No. 3, L03801, doi:10.1029/2011GL050413.