Investigating low-level jet wind profiles using two different lidar scanning strategies

Nocturnal winds aloft in the central United States are often characterized by the low-level jet (LLJ), an elevated wind maximum that occurs 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 facilitate 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 collocated a Leosphere V1 profiling lidar and a Leosphere 200S scanning lidar. The V1 lidar uses the Doppler beam swinging technique to generate profiles of u, v, and w wind components at heights ranging from 40-220 m. The 200S supports multiple scanning strategies and features an operational line-of-sight range of between 100 and 5000 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 collocation 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 enables investigation of 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.

We use the Whiteman et al. jet detection scheme to identify weak and strong LLJs from the 200S profiles. Our results demonstrate that the LLJ is a common feature of summer nights in central Iowa. Jet observations indicate the signature of inertial wind veering over time, a dependence of jet speed on the height of the maximum, and veering with height in strong jets. The majority of measured nocturnal LLJs exist during periods of southerly synoptic flow associated with a westerly position of the Atlantic subtropical high. The strongest jets, with maximum wind speeds above 20 m/s, are particularly biased toward southwesterly wind directions.

We also perform a model analysis in which different configurations of the Weather Research and Forecasting (WRF) model v3.4.1 are evaluated to determine their efficacy in reproducing observed LLJ characteristics. We find that WRF broadly reproduces the regular pattern of LLJs over an eight day subset of the data collection period. The intensity and timing of modeled low-level jets is more sensitive to the choice of boundary and initial condition data than to planetary boundary layer scheme.