Very-large-scale motions in the neutral atmospheric boundary layer: a numerical study

In the last few decades, laboratory experiments and direct numerical simulations of turbulent boundary layers, performed at low to moderate Reynolds numbers, have revealed the existence of very-large-scale motions (VLSM) in the logarithmic region. The size of VLSMs was found to be 10-50 times as large as the boundary-layer thickness. Recently, few studies based on field experiments examined the presence of VLSMs in neutral atmospheric boundary layer flows, which are invariably at very high Reynolds numbers. Very large scale structures similar to those observed in laboratory scale experiments have been found and characterized. However, it is known that field measurements are more challenging than laboratory-based measurements, and can lack resolution and statistical convergence. Such challenges could have implications on the robustness of the analysis, which may be further deteriorated by the use of Taylor's hypothesis to convert time series to spatial data.

We use large eddy simulation (LES) to investigate the very-large-scale motions in the neutral atmospheric boundary layer at a very high friction Reynolds number. The vertical height of the computational domain is Lz = 1000 m, which corresponds to the thickness of the boundary layer. The horizontal dimensions of the simulation domain are chosen to be Lx = 32Lz and Ly = 4Lz respectively, in order to contain a sufficient number of large-scale structures. We will present flow visualization and statistical analysis of the spatially coherent structures associated with VLSMs. The instantaneous velocity fields in streamwise/spanwise planes give evidence of streamwise-elongated zones of low speed fluid with negative streamwise velocity fluctuation, which are flanked on either side by similarly elongated high speed ones. The pre-multiplied power spectra and two-point correlations indicate that the scales of these streak-like structures are very large, up to 20Lz in the streamwise direction and Lz in the spanwise direction. These features are similar to what had been found in the logarithmic region of laboratory-scale boundary layers by direct numerical simulations and experiments conducted at low to moderate Reynolds numbers. Thanks to the LES results available in fully 3D space and time, the three dimensional correlation map and conditional average of the three components of velocity are calculated by choosing the conditional event at different heights. The so-obtained 3D information further indicates that the low-speed and high-speed regions possess the same elongated ellipsoid-like structure, which is inclined upward along the streamwise direction, and they are accompanied by counter-rotating roll modes in the cross section perpendicular to the streamwise direction. These findings are in agreement with recent observations made from field campaigns in the atmospheric boundary layer and enrich the global picture of the VLSM phenomenon.

Furthermore, we will present results on the effect that the Coriolis force has on the very-large-scale motions in the neutral atmospheric boundary layer.