Behavior of hairpin shaped vortex structures in the lower atmosphere

What is it about?

The lower part of the atmosphere, which is in contact with the Earth's surface, is known to be turbulent. Commonly referred to as the atmospheric boundary layer, a source of complexity in studying this part of the atmosphere is due to the cycle of heating (during daytime conditions) and cooling (during nighttime conditions) of the surface which previous observations have shown to have a direct impact on the intensity of turbulence. During night-time conditions turbulence is known to feature low intensity turbulence and is spread out in pockets, that are interspersed with stretches of nearly laminar flow. On the basis of direct numerical simulations, the authors recently observed an abundance of hairpin-shaped patterns or structures oriented in similar directions within the turbulent flow pockets. In this paper, utilizing a well-established computational thin-filament model, we study the behavior of these hairpin structures in neutrally, intermediate, and strongly stratified boundary layers, the latter corresponding to typical night-time conditions. By initializing the hairpins at three different heights and observing their motion, we suggest that the horizontal orientation of a hairpin is linked causally to its initial starting height under night-time conditions. With increasing initial height, the hairpin exhibits a change in orientation from a clockwise to an anticlockwise rotation relative to the direction of the flow outside the boundary layer. This change in orientation with increasing height is, however, not observed under neutral conditions. The paper also presents a new method for following coherent structures in time directly in well-resolved simulation data and compares them with the results obtained from the thin-filament model. It is found that both show similar behavior, thereby corroborating our results. Finally, by retracing the steps of the matched asymptotic derivation of the filament model, it is shown that under the conditions considered in this study buoyancy does not affect the filament motion. The computational model used in the study has been simplified accordingly.

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Photo by Simon Berger on Unsplash

Photo by Simon Berger on Unsplash

Why is it important?

Our work not only sheds new light on the fundamental aspects of turbulence through the behavior of hairpin structures but also improves our understanding of turbulence in the atmospheric boundary layer during night-time conditions. Eminent researchers within the field of boundary layer meteorology have repeatedly pointed out that the night-time boundary layer is not very well understood due to complicated dynamics arising as a result of weak turbulence spread over pockets, interactions among numerous physical processes, etc.. Accurate representation of the atmospheric boundary layer is important for numerical weather prediction and climate models whose performance is known to be sensitive to the details of boundary layer formulation.

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