A conditional analysis of spanwise vortices within the lower atmospheric log layer

The current study is an initial step towards the full characterization of coherent structures within the turbulent atmospheric boundary layer (ABL). A conditional-analysis technique to identify spanwise vortices is applied on data acquired throughout January-August 2012. This study begins with an approximation of the ABL stability and thickness (δ). Subsequently the measured Reynolds stresses are compared to those measured by Klebanoff (1955) over a flat plate and similarity formulations developed by Marusic and Kunkel (2003). The ABL data is shown to be neutral with a thickness of approximately 1000 m. The statistical descriptors of the turbulent fluctuations are found to qualitatively agree with Klebanoff (1955) and Marusic and Kunkel (2003). From the conditional analysis, it is found that the height of spanwise vortices follows a Rayleigh-like distribution with a mean and standard deviation of 0.065δ and 0.038δ, respectively. The circulation distribution of spanwise vortices is found to be bimodal about Γ/U0δ=0 with a standard deviation of 0.036U0δ. The magnitude of the positive peak of the bimodal distribution tends to be 80-100% that of the negative peak, which demonstrates that prograde vortices are more frequent than retrograde vortices.