The effect of Rossby number on the characteristics of superstructures and their impact on interscale energy exchange in the atmospheric boundary layer

Thursday, 18 December 2014
Mohammad Ahsanuzzaman Khan, University of Utah, Salt Lake City, UT, United States and Rob Stoll II, University of Utah, Mechanical Engineering, Salt Lake City, UT, United States
In both experiments in the atmosphere and simulated flows, large meandering structures are observed to exist as streaky structures of positive and negative stream wise velocity fluctuations. These “superstructures” are hypothesized to play an important role in the exchange of momentum and energy between the surface layer and the outer layer of the atmospheric boundary layer (ABL). Although these structures have been observed in a variety of flows, their dependence on external forcing parameters and the nature of transport between them and small scale turbulent flow features is still unresolved. A series of large-eddy simulations of the ABL with different forcing parameter in a high aspect ratio domain (128 Km x 1.5 Km in the horizontal and vertical directions, respectively) was used to examine the distribution of these structures and the role they play in accumulating or transferring energy to other scales. To begin to address the dependence of these structures on atmospheric forcing and how this dependence manifests in the ABL, the effect of large-scale rotation was studied by simulating ABL flows with different Rossby numbers. Correlations and wavenumber spectra of the resolved velocity field were studied to identify the signatures and spatial length scales associated with these superstructures. The energy transfer to and from these flow structures was explored by examining the nature of triadic interaction with other flow scales. It was observed that energy transfer between large-scale structures was non local when superstructure scales interacted with scales that where much larger than the boundary layer height and that these interactions had characteristics of two-dimensional turbulence. Conversely when the other two flow scales involved in the exchange process were smaller than the order of boundary layer height, the exchange was characteristic of local energy transfer through non-local interactions.