Turbulence Profiles in the Weakly Stable Boundary Layer with a Strong LLJ

Understanding processes that control the vertical profiles of turbulence quantities beneath the low-level jet (LLJ) in the nocturnal stable boundary layer (SBL) has been an area of active concern for nearly three decades. This understanding is important for being able to represent turbulent mixing processes properly in numerical weather prediction (NWP) models. These turbulence quantities have been thought to be a result of the mean wind at a level just above the surface interacting with surface roughness elements, so that the appropriate scaling velocity would be the surface-layer friction velocity u_*. In the strong-wind, weakly stable boundary layer, profiles of the stress and other turbulence momentum quantities normalized by u_* have been found to be at a maximum at the surface, decreasing approximately linearly to 0 at h, the top of the SBL, defined in various ways. This paper will present recent analyses of measurements of the streamwise velocity variance (or equivalently, the streamwise standard deviation σ_u) taken with NOAA's High-Resolution Doppler Lidar (HRDL) over two sites in the Great Plains of the United States. The streamwise variance is shown to be quantitatively equivalent to TKE in stable conditions.

These analyses have shown that the speed of the first LLJ maximum, i.e., the first speed maximum encountered above the ground (usually 100-250 m above the surface) was a more appropriate velocity scale than u_*. The appropriate length scale was the height of this first speed maximum of the LLJ, i.e., the height of the LLJ. These measurements further show that the turbulence profile just described, with maximum at the surface decreasing to a minimum at the LLJ height, was appropriate for the least stable conditions (strongest LLJ speeds). Quantitatively, the peak value of σ_u near the surface was found to be 5% of the speed of the LLJ maximum. For slightly more stable conditions, however, the peak in the turbulence profile was elevated above the surface (but still 5 % of the LLJ maximum speed), leading to an ‘upside down' structure, in which turbulence is generated aloft and transported downwards. Other findings were that the minimum in turbulence at the LLJ maximum did not necessarily go to 0, and that for LLJs with a distinct maximum or ‘nose' profile, turbulence zones existed both above and below the level of the minimum value of turbulence at the jet nose, whereas for LLJs with a more constant wind-speed profile above the subjet shear zone, a maximum in turbulence existed only below the LLJ

These findings indicate that the control of the turbulence in the layer below the LLJ is the speed of the jet, which in turn constrains the jet height. This is significant because the speed of the LLJ is controlled by large-scale meteorological processes, rather than the details of the turbulent interactions near the surface.