The Active Role of Streamwise Velocity Organization in Near-Surface Turbulent Phenomena

Turbulent processes in the atmospheric surface layer (ASL) are often reduced to statistics including averages and covariances, and there is limited understanding of how instantaneous turbulent structures lead to these statistics. Point instruments such as sonic anemometers only provide a temporal snapshot of the flow, and there is a scarcity of spatially-resolved field-scale measurements to investigate the shape and organization of turbulent structures in the ASL. In this contribution, recent super-large-scale particle image velocimetry measurements are introduced and validated. The two-dimensional field-scale measurements reveal the spatial organization of atmospheric turbulence under neutral stratification. Consistent with previous laboratory-scale studies, the flow field is observed to be composed of regions of coherent streamwise velocity known as uniform momentum zones (UMZs) separated by thin layers where vorticity and shear are primarily concentrated (see Heisel et al., J. Fluid Mech. 857, 2018). Comparison of UMZs in the ASL with laboratory-scale results reveals a direct relationship between the spatial organization of the ASL and the hypothetical eddies of Prandtl’s mixing length used to derive the logarithmic mean velocity profile. In addition to uncovering the turbulent structures that lead to the mean flow statistics, the findings provide a potential framework for modeling turbulent phenomena in atmospheric applications. By replicating the characteristics and variability of representative turbulent structures such as UMZs, near-surface turbulent transport processes could perhaps be modeled stochastically with limited computational resources.