Stable Boundary-Layer Turbulence Profile Shape beneath a Low-Level Jet

The vertical structure of turbulence profiles through the stable boundary layer (SBL) is important for SBL dynamics, because the relevant terms in the momentum and scalar budget equations involve the vertical derivatives of the turbulence terms. Velocity variance or covariance profiles have been found to exhibit different shapes, including a traditional vertical structure where turbulence magnitudes are at a maximum value near the surface and drop monotonically to small values at the top h of the SBL, and the so-called upside down structure, where the magnitudes of turbulence quantities increase with height through at least the lower part of the SBL before decreasing to h. The structure of the turbulent streamwise velocity-variance profiles (as well as the mean-wind profile) was measured by NOAA's High Resolution Doppler Lidar during two field campaigns in the Great Plains of the central United States. HRDL data have been shown to be capable of determining streamwise variance values to a precision that can be used to determine variance profile shapes. The mean-wind profiles exhibited a low-level jet (LLJ) structure, and when the jet had a distinct maximum or nose, that nose was found to be equivalent to h. HRDL profile data revealed three dominant profile shapes: traditional structure, upside-down structure, and a profile where the peak was in a layer of constant variance over a depth of 30-60 m or so. Nearby tower measurements documented the thermodynamic structure and allowed Richardson numbers to be calculated (using shear from the HRDL mean-wind profiles). The traditional structure was characteristic of the least stable conditions, with a mean Ri of 0.15. Upside-down structure was characteristic of more stable conditions, with mean Ri= 0.19, a value found to be significantly different statistically from that of the traditional sample. The constant maximum profile had a mean Ri between the others, at Ri= 0.17. An interesting aspect of the upside down sample was that the maximum in variance tended to be near 60 m (+/- 20 m) height, regardless of LLJ and SBL depth. Other quantitative aspects of the relationship between LLJs and SBL properties, such as the ratio of the LLJ maximum speed to the peak turbulence standard deviation ( square root of the variance or TKE maximum value), which was found to be 0.05, the magnitude of the shear below the LLJ, the magnitude of Ri for the strongest wind cases, the relationship between the streamwise variance and TKE, and the implications of these observations to SBL dynamics will be discussed.