Investigation of Lidar Vertical Wind Variance Errors at the Boulder Atmospheric Observatory

Doppler wind lidars (DWLs) have been increasingly used in boundary layer research and assessment of wind resources for the wind energy industry. DWLs are capable of accurately measuring the mean horizontal wind, which has been well-documented in literature. Recently, DWLs have been used more often to investigate and quantify boundary-layer turbulence. However, there are still open questions regarding how accurately lidars can measure turbulence. Compared to sonic anemometers that are typically used to measure turbulence, DWLs have a much lower sampling rate and a larger sampling volume. Additionally, data from DWLs can be noisy leading to an overestimate of atmospheric turbulence, especially when aerosol loading is low. Differences in lidar and sonic measured turbulence were investigated during the Lower Atmospheric Thermodynamics and Turbulence Experiment (LATTE).

LATTE took place at the Boulder Atmospheric Observatory in Erie, Colorado during February and March 2014. A Halo Doppler scanning lidar, WindCube v2 lidar, ZephIR 300 lidar, and a 449-MHz wind profiling radar were deployed at the site. Additionally, a 300-m tower was instrumented with sonic anemometers at six different levels spaced evenly at and between 50 and 300 m AGL. An unmanned aerial system also operated during parts of the experiment. Various DWL scanning strategies were tested during the experiment to measure turbulence, including the six-beam strategy discussed in a companion presentation. During part of the experiment, the WindCube and the Halo lidars were situated a few meters away from the base of the 300-m tower and the Halo lidar collected vertical velocity data continuously for direct comparison with the sonic anemometers. This paper will focus on data collected during this time frame.

The primary focus of this paper is the comparison of vertical velocity variances and spectra measured by the Halo lidar compared with those from the collocated sonic anemometers above the DWL. Preliminary results indicate that the lidar underestimates vertical velocity variance measured by the sonic anemometers a significant amount. The causes of these turbulence discrepancies are investigated and compared for both stable and unstable conditions.