Turbulent velocity-variance profiles in the stable boundary layer generated by a nocturnal low-level jet

Profiles of mean winds and turbulence were measured by the High Resolution Doppler lidar in the strong-wind stable boundary layer (SBL) with continuous turbulence. The turbulence quantity measured was the variance of the streamwise wind velocity component sigma(2)(u). This variance is a component of the turbulence kinetic energy (TKE), and it is shown to be numerically approximately equal to TKE for stable conditions-profiles of sigma(2)(u) are therefore equivalent to profiles of TKE. Mean-wind profiles showed low-conditions-profiles of sigma(2)(u) level jet (LLJ) structure for most of the profiles, which represented 10-min averages of mean and fluctuating quantities throughout each of the six nights studied. Heights were normalized by the height of the first LLJ maximum above the surface Z(X), and the velocity scale used was the speed of the jet U-X, which is shown to be superior to the friction velocity it, as a velocity scale. The major results were 1) the ratio of the maximum value of the streamwise standard deviation to the LLJ speed sigma(u)/U-X was found to be 0.05, and 2) the three most common sigma(2)(u) profile shapes were determined by stability (or Richardson number Ri). The least stable profile shapes had the maximum sigma(2)(u) at the surface decreasing to a minimum at the height of the LLJ; profiles that were somewhat more stable had constant sigma(2)(u) through a portion of the subjet layer; and the most stable of the profiles had a maximum of a, aloft, although it is important to note that the Ri for even the most stable of the three profile categories averaged less than 0.20. The datasets used in this study were two nights from the Cooperative Atmosphere-Surface Exchange Study 1999 campaign (CASES-99) and four nights from the Lamar Low-Level Jet Project, a wind-energy experiment in southeast Colorado, during September 2003.