Yelena L. Pichugina*1, R. M. Banta2, N. D. Kelley3, 1Science and Technology Corporation, Boulder, Colorado, USA 2Environmental Technology Laboratory (ETL), NOAA, Boulder, Colorado, USA 3National Renewable Energy Laboratory, Golden, Colorado, USA
The Low-Level Jet (LLJ) that forms in the stable mixed layer during night time is very important to wind energy operations. LLJs occur frequently at the site to the south of Lamar, Colorado, in the southeastern corner of the state, particularly during the period of April-September (Kelley et al. 2004).
To obtain detailed information about the LLJ characteristics and turbulence environment in which large GE wind turbine rotors were being installed, an intensive field-measurement campaign was carried out in early September 2003 at this site. Instrumentation included a 120-m tower instrumented at 4 levels, and a 3-component Doppler sodar, which were in operation for two summers, and ETL’s high resolution Doppler lidar (HRDL), which was deployed from 1 to 16 September.
Analyses of 15-min wind speed profiles derived from conical and vertical-slice scans shows that LLJs were present in 86% of the cases. During the period of HRDL measurements, only 22% of LLJs occurred within the layer occupied by the turbine rotor (54-116 m) with a maximum of wind speed 10-11 m/s, about 56% LLJs occurred within the layer 50-200m, and almost all observed jets were within layer 10-500m (94.5%) with a maximum of wind speed 16-17 m/s.
Histogram distributions of LLJ wind speed direction indicates a very narrow range of prevalent wind directions (160-180) with a maximum at 170 degrees. This is consistent with the high frequency of southerly jets found by Whiteman et al. (1997) and Kelly et al. (2004) during the warm season (April-September) months.
Besides providing strong winds for wind energy production, LLJ can create intense vertical shear within the layer occupied by a turbine rotor. Distribution of the shear exponent corresponding to the rotor lower half (54-85 m), the rotor upper half (85-116 m), and entire rotor depth (54-116m) indicates that in most cases shear exponent was greater than 0.2.
The observed mean turbulence levels (wind speed standard deviation) at the heights of the turbine remain below the IEC (International Electrotechnical Comission) ‘A’ and ‘B’ specifications. These findings are important to wind energy applications and to a complete understanding of nocturnal boundary layer processes up to 200 m and above using HRDL data.