Nocturnal low-level jet characteristics over southern Colorado

Shear generated below a nocturnal low-level jet (LLJ) may have an important role regulating near-surface turbulence and fluxes in the stable boundary layer (SBL), based on measurements over southeastern Kansas during CASES-99 (Banta et al. 2002, 2003). Properties of the LLJ are also critical to wind energy applications (Kelley et al. 2004). An important question is, how general are the CASES-99 results, which were in the central Great Plains in October.

A site to the south of Lamar, Colorado, in the southeastern corner of the state, has been chosen as a wind energy site. An intensive field-measurement campaign in early September 2003 was designed to characterize mean and turbulent LLJ properties at this site. Instrumentation included a 120-m 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. This instrument deployment provides an opportunity to test the generality of the CASES-99 results against a High Plains site in late summer.

Histogram distributions of LLJ speed and height indicate that wind-speed maxima were stronger and higher above ground level (AGL) than the CASES-99 results (Banta et al. 2002). The most frequent speeds were 18-20 m/s, as opposed to 8-10 m/s for CASES-99, and the most probable heights were 180-200 m AGL, compared with 80-100 m for CASES-99.

Other important aspects of the LLJ were its spatial variability and temporal evolution. On most HRDL vertical-slice-scans, the jet was horizontal across the 2-3 km expanse of the scan, but at times the height varied significantly over the scan, often apparently following the local terrain. The jet generally took ~3 h to form after sunset, with the jet speed often reaching several temporal maxima at 3-5 h interval through the night. Plots of mean wind speed vs. bulk Richardson number Ri across the tower indicated that the highest wind speeds were associated with the smallest Ri, as expected. These findings are important to wind energy applications and to a complete understanding of SBL processes, including evaluation of the effects of near surface fluxes in the vertical distribution of quantities through a nighttime period.