Vertical Structure and Intermittent Turbulence of Nocturnal Stable Boundary Layer

Abstract: The vertical structure of the stable boundary layer has been investigated using the data taken during the winter Phoenix Air Flow Experiment (PAFEX I). Tethered balloon data indicated the presence of three vertical layers in the nocturnal boundary layer. The stable layer closest to the ground grew to 50 meters from 1600 to midnight and then dropped to about 10 meters. In this layer, the Brunt-Vaisala frequency appeared to be constant at 0.05 rad/s. Just above this very stable layer and below the neutrally stable upper layer, another less stable layer with Brunt-Vaisala frequency about 0.025 rad/s developed as a transition layer. At times, these two layers had different wind directions, indicating that this layered structure may be due to the intrusion of different air masses originating at different slopes. Sonic anemometer measurements were collected at 10 meters above the surface during a three-day period. Turbulence observation in the near-ground layer showed strong intermittency. The Orthogonal Wavelet Transform (OWT) was used to investigate the influence of intermittency on the spectral slope of the inertial subrange. The results of this OWT analysis were in agreement with those of previous work, in that the spectral slopes in the inertial subrange deviate from the -5/3 power law especially when the variance of the turbulence signal is small. The results also revealed that this deviation is directly linked to the atmospheric stability. The more stable the atmosphere, the smaller the variance of signal and therefore stronger the deviation from the -5/3 power law. The wavelet statistics Flatness Factor (FF) and dimensionless coefficient of spatial variation of energy (CVe) of each scale were calculated. A conditional wavelet sampling scheme was used to eliminate the influence of small-scale intermittency on the spectral slope or structure function. This thresholding method can recover the Gaussian property under any stability regime since it could reduce the FF value to 3 when the thresholding value K is selected. The FF value did not have any obvious relationship with the atmospheric stability, but the CVe values are significantly affected by z/L, where L is the Monin-Obukhov length scale. Further studies are underway to seek alternative conditioning criteria to achieve the goal of eliminating the effect of large-scale intermittency.

Keywords: stable boundary layer, intermittent turbulence, wavelet analysis, nocturnal boundary layer