Turbulent Velocity-Variance Profiles in the Stable Boundary Layer Generated by a Nocturnal Low-Level Jet

Abstract

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 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 2 u are therefore equivalent to profiles of TKE. Mean-wind profiles showed lowlevel 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 u * as a velocity scale. The major results were 1) the ratio of the maximum value of the streamwise standard deviation to the LLJ speed u /U X was found to be 0.05, and 2) the three most common 2 u profile shapes were determined by stability (or Richardson number Ri). The least stable profile shapes had the maximum 2 u at the surface decreasing to a minimum at the height of the LLJ; profiles that were somewhat more stable had constant 2 u through a portion of the subjet layer; and the most stable of the profiles had a maximum of 2 u 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.

Estimate of Boundary-Layer Depth Over Beijing, China, Using Doppler Lidar Data During SURF-2015

“…In general, the urban boundary-layer structure and evolution observed for our limited number of cases were similar to what has been observed previously. However, the nocturnal boundary layer remained near-neutral: the widely documented "upside-down boundary layer", "downward mixing regime" (e.g., Banta et al 2006;Sun et al 2012), and "weak turbulence regime" (Sun et al 2012) were not observed in our cases. Although a longer-term dataset is needed to determine the frequency of such events, urban nocturnal heat sources likely produce a more frequent near-neutral (or slightly convective) NBL and hence a lower frequency of a more stable NBL, compared to natural landscapes.…”

Boundary layer dynamics over London, UK, as observed using Doppler lidar during REPARTEE-II

“…The minimum in turbulence at z ∼ 1.3z MH is consistent with a nocturnal jet structure, where there is least shear near the jet maximum. Banta et al (2006) observed very similar structure in σ u values using Doppler lidar, and it is reasonable to assume that σ w will exhibit similar structure. The appearance of a jet-type structure was highly unusual -this was the most prominent example, there being a much weaker, marginal case overnight on 1/2 November.…”

Low-level mixing height detection in coastal locations with a scanning Doppler lidar

“…Beyrich, 1997;Seibert et al, 2000;Emeis et al, 2008), Doppler lidar (e.g. Harvey et al, 2013;Schween et al, 2014) and radar wind profiler (e.g.…”

“…Scanning Doppler lidars can partially overcome a minimum height limitation by retrieving radial wind velocity measurements at a low elevation angle (Banta et al, 2006). In this paper we present a method whereby scanning Doppler lidars can identify the presence of turbulent mixing from the instrument level up, making it possible to cover the full range of potential MLHs with an appropriate selection of scan types from one instrument.…”