Turbulence organization and mean profile shapes in the stably stratified boundary layer: Zones of uniform momentum and air temperature

A persistent spatial organization of eddies is identified in the lowest portion of the stably-stratified planetary boundary layer. The analysis uses flow realizations from published large-eddy simulations (Sullivan et al., J Atmos Sci 73(4):1815–1840, 2016) ranging in stability from near-neutral to almost z-less stratification. The coherent turbulent structure is well approximated as a series of uniform momentum zones (UMZs) and uniform temperature zones (UTZs) separated by thin layers of intense gradients that are significantly greater than the mean. This pattern yields stairstep-like instantaneous flow profiles whose shape is distinct from the mean profiles that emerge from long-term averaging. However, the scaling of the stairstep organization is closely related to the resulting mean profiles. The differences in velocity and temperature across the thin gradient layers remain proportional to the surface momentum and heat flux conditions regardless of stratification. The vertical thickness of UMZs and UTZs is proportional to height above the surface for near-neutral and weak stratification, but becomes thinner and less dependent on height as the stability increases. Deviations from the logarithmic mean profiles for velocity and temperature observed under neutral conditions are therefore predominately due to the reduction in zone size with increasing stratification, which is empirically captured by existing Monin-Obukhov similarity relations for momentum and heat. The zone properties are additionally used to explain trends in the turbulent Prandtl number, thus providing a connection between the eddy organization, mean profiles, and turbulent diffusivity in stably stratified conditions.

Methods
The dataset is a collection of post-processing results based on large-eddy simulations (LES) of the planetary boundary layer.  The simulations include a neutrally-stratified case and four stably-stratified cases with different surface cooling rates. Additional details of the LES are given elsewhere (Sullivan et al., J Atmos Sci 73(4):1815–1840, 2016). The dataset includes statistics for zones of uniform momentum and air temperature detected from instantaneous realizations of the turbulent LES flow volumes. Also included are sample scripts for detecting uniform zones from flow volumes and computing the zone statistics. The scripts adhere to the detection methodology and processing discussed in the related article.