Stability Effects on the Evolution of Very Large Scale Motions as the Atmospheric Surface Layer Transitions through Neutral Thermal Stability

Research has shown that the near-neutral atmospheric surface layer (ASL) exhibits the same turbulent characteristics as that observed in much lower Reynolds number laboratory-generated boundary layers. This body of work has also lead to new understandings about asymptotic Reynolds number trends in such quantities as: the peak in the inner-normalized fluctuating streamwise velocity profile, viscous sublayer streak spacing, Taylor microscale, velocity spectra, and pressure spectra. However, comparisons between laboratory turbulence data and that from the near-neutral ASL reveal noticeable differences in the one-dimensional streamwise velocity spectra, particularly in the low frequency regime typically associated with the signature of Very Large Scale Motions (or VLSMs). These are long meandering structures with streamwise lengthscales of 10 – 20 times the turbulent boundary layer height. They contain a significant amount of turbulent kinetic energy and Reynolds shear stress; and, as such, they are typically characterized by a peak occurring at low frequency in the pre-multiplied energy spectra.

It is hypothesized in the present study that the introduction of buoyancy forces alters the structure of the turbulent boundary layer, compared to the canonical neutral case, by limiting the growth of VLSMs as the ASL transitions between different thermal stability regimes. Of particular interest is understanding the extent to which, if any, the finite-time duration of the near-neutral period affects the evolution of VLSMs in the ASL. To test the above hypothesis, existing hot-wire and sonic anemometry data sets from the Surface Layer Turbulence and Environmental Science Test (SLTEST) facility located in Utah's western desert were utilized. This experimental effort involved thirty-one single-element hot-wire probes spaced logarithmically in the wall-normal direction between 1 mm and 5 m, nine sonic anemometers (CSAT3) spanning a vertical distance of 1.4 to 26 m (spaced logarithmically), and a spanwise array of ten sonic anemometers, spaced 3 m apart and located at 2.1 m above the desert floor. The combined hot-wire and sonic anemometer tower measurements provides a unique opportunity to examine how the near-surface turbulent velocity spectrogram evolves with time of day.

Results clearly show variations in the structure of the turbulence, especially in the intermediate to low frequencies. The most prevalent difference is a shift in energy from the intermediate frequencies during slightly unstable conditions to low frequencies during neutral, and back to higher frequencies again in the slightly stable regime. In both the slightly unstable and stable regimes, this energy peak resides at about a height of 0.2 m and spans a frequency range of 0.03 to 3 Hz. During the transition through neutral, however, this energy peak moves into a narrow frequency band centered at about 0.01 Hz and extending over the upper portion of the logarithmic layer, from a height of 0.1 to 10 m.