Low-level jet properties and turbulence below the jet during the Lamar Low-Level Jet Project

A relationship between the properties (speed and height) of the nocturnal low-level jet (LLJ) and the magnitude of the turbulence kinetic energy (TKE) in the stable boundary layer (SBL) below the jet was demonstrated by Banta et al. (2003; JAS, pp.2549ff) using CASES-99 data in southeastern Kansas. This relationship is potentially important for determining near-surface fluxes in the SBL and is also important for applications such as wind energy, where one issue is the premature failure of turbine hardware as a result of significant bursts of turbulence (Kelley et al. 2004; NREL report). An uncertainty in the CASES-99 results is how general they are—do they apply to other locations and other seasons?

To address this problem as well as other wind-energy issues, a late-summer field project was organized at a High Plains location in southeastern Colorado (Kelley et al. 2004; Pichugina et al. 2004: this symposium). Instrumentation included a 120-m tower instrumented at four levels, a 3-component Doppler sodar, and ETL’s high-resolution Doppler lidar (HRDL). Data were analyzed in a manner similar to that described in Banta et al. (2003, 2002; BLM, pp221ff). LLJ properties were significantly different for this early-September period, with the LLJs significantly stronger and higher than during the October CASES-99 project (Pichugina et al. 2004).

During CASES-99 most LLJs were ~10 m/s and only three nights had jets of 15 m/s or stronger. During the stronger-jet cases of Lamar, __ nights exhibited LLJs of 15 m/s (20?) or more, providing an opportunity to further investigate the low-stability, high-wind (low RiJ) regime, and see if the sensitivity of the subjet layer to decreasing RiJ found in CASES-99 also applied to the Lamar cases. Significantly, preliminary analyses of the data reinforce the CASES-99 findings in the low RiJ regime, which has been classified as the “moderately-stable boundary layer” regime by Mahrt (1999). In this regime the shear is supposed to be strong enough to maintain continuous turbulence in the surface layer, with the implication that Monin-Obukhov scaling applies.

The research reported here will present the results of in-depth analysis of the TKE dependence on RiJ, and will test the applicability of M-O scaling for the high-speed LLJ, low RiJ cases. It will also include an analysis of “coherent TKE,” a quantity representing that portion of the TKE that contributes to Reynolds’ stresses (Kelley et al., 2004). This quantity is of potential importance as an indicator of turbulence that adversely affects wind turbine operations.