Scale-dependent inclination angle of turbulent structures in stratified atmospheric surface layers

TL;DR

This study investigates how the inclination angles of turbulent structures in the atmospheric surface layer vary with stability conditions, revealing scale-dependent angles and aspect ratios through detailed velocity and temperature measurements.

Contribution

It extends the understanding of self-similar wall-attached turbulence structures to unstable and near-neutral atmospheric conditions, including their statistical structure in multiple directions.

Findings

Inclination angles increase with instability, reaching up to 65° for small scales.

Aspect ratios between different scales are approximately 1, indicating self-similarity.

Angles are highly sensitive to stability parameters under near-neutral conditions.

Abstract

A large-scale spanwise and wall-normal array of sonic anemometers in the atmospheric surface layer is used to acquire all three components of instantaneous fluctuating velocity as well as temperature in a range of stability conditions. These data permit investigation of the three-dimensional statistical structure of turbulence structures. The present work extends the view of a self-similar range of wall-attached turbulence structures to the atmospheric surface layer under unstable and near-neutral stability conditions, and includes the statistical structure in both the wall-normal and spanwise directions in relation to the streamwise wavelength. Results suggest that the self-similar wall-attached structures have similar aspect ratios between streamwise/wall-normal scales and streamwise/spanwise scales such that $\lambda_x/\Delta z : \lambda_x/\Delta y \approx 1$ for both near-neutral and unstable conditions. By analysing the phase shift between synchronized measurements, in the spectral domain, it is quantified how the structure inclination angle varies with stability. Under the most unstable conditions, coherent structures of $\lambda_x/\delta = 1$ are inclined at angles as high as $65^\circ$ relative to the solid boundary, while larger scales of $\lambda_x/\delta = 6$ exhibit inclination angles of approximately $35^\circ$. For near-neutral stability conditions, the angle tends towards $12^\circ$ for all scales. It is noted that in the near-neutral condition, the structure inclination angle and the aspect ratio -- and thus the statistical modeling of coherent structures in the ASL -- are highly sensitive to the value of the stability parameter.