Investigating low-level jet wind profiles using two different lidars

Nocturnal winds aloft in the central United States are typically characterized by the low-level jet (LLJ), an elevated wind maximum occurring near the top of the stable boundary layer. The height of the LLJ wind speed maximum varies over the course of the night, and from night to night, but is typically found at elevations around 200-500 m. LLJs impact moisture transport into the Great Plains, can affect air quality, and provide a tremendous resource for the harvesting of wind energy. At the same time, the shear and turbulence induced by the LLJ can adversely impact wind turbines.

We present a characterization of the nocturnal LLJ using two Doppler lidar systems. As part of the 2013 Crop Wind Energy Experiment (CWEX), we deployed three Leosphere Windcube V1 lidars in a wind farm in central Iowa in the United States. The V1 lidar uses the Doppler beam swinging technique to generate profiles of u, v, and w wind components with a rated height range of 40-200 m. We also deployed the 200S scanning version of the Windcube lidar, which was colocated with one of the V1 models. The 200S supports multiple scanning strategies and features a rated maximum line-of-sight range of 8500 m. In practice, the effective range of both models depends on the aerosol concentration of the measured sample volumes. To generate wind profiles using the 200S, we use 360° plan position indicator scans to sample the winds aloft at azimuth angles spaced 3° apart. Then, using the velocity azimuth display technique, we compute wind profiles with a nonlinear least-squares fitting algorithm.

The colocation of a Windcube V1 and the Windcube 200S enables us to perform a side-by-side comparison of each lidar in the 100-200 m height range. Quantities relevant to wind energy, such as wind speed, direction, vertical wind shear, and approximations of turbulence (based on the standard deviation of the wind speed retrievals), are computed using data from each lidar. Additionally, the unique coverage of the V1 lidar below 100 m and the 200S above 200 m allows for compilation of a more comprehensive wind profile that includes the both the wind-turbine-rotor layer and the entire extent of the nocturnal LLJ. Approximately 65% of nights in the CWEX campaign featured wind speed maxima above the maximum height of the V1 lidar, but within the observation range of the 200S. Together, the two lidars provide us with an exceptionally detailed overview of the interaction between the momentum field encountered by the wind turbines and boundary-layer wind profile above.