Effects of the Coriolis force on the coherent structures in conventionally neutral atmospheric boundary layers

TL;DR

This study investigates how the Coriolis force associated with Earth's rotation shapes turbulent coherent structures in conventionally neutral atmospheric boundary layers (CNBL) using high-fidelity large-eddy simulations. By varying latitude and geostrophic wind speed, the authors quantify deflections of streamwise, spanwise, and vertical velocity structures toward the geostrophic wind and demonstrate a near-universal relation between structure deflection and the mean wind veer, expressed as $\theta-\theta_0$. They further show that the inclination angles of large-scale structures increase with stronger Coriolis influence (lower latitude or higher $U_g$), linking deflection dynamics to vertical hairpin-packet distortion. The results enhance understanding of CNBL dynamics under rotation and suggest avenues for improved parameterizations, while acknowledging the limitations of the $f$-plane approximation and the need to include the full Coriolis terms in future work.

Abstract

It is well known that the Coriolis force due to Earth's rotation can induce wind veer in the mean flow velocity of an atmospheric boundary layer (ABL), but much less is known about its effects on turbulent coherent structures. In this work, large-eddy simulation (LES) is employed to investigate the effects of the Coriolis force on the characteristics of turbulent coherent structures in the conventionally neutral atmospheric boundary layers (CNBL). Variation of the Coriolis force is realized by changing latitude or geostrophic wind speed.We found that the Coriolis force causes distinct deflection of coherent velocity structures to the geostrophic wind direction, which is not aligned with the direction of either the mean wind or the mean shear. By plotting against the difference between the local wind veer angle and the global cross-isobaric angle, the structure deflection angle under different conditions can be well collapsed, indicating a possible universal relationship. Moreover, we also studied the effect of the Coriolis force on the inclination angle of large-scale turbulent structures. It is found that as latitude decreases or geostrophic wind speed increases, the inclination angle in the surface layer increases, probably due to the deflection of turbulent structures caused by the Coriolis force.

Figures (33)

• Figure 1: Temporal evolution of the friction velocity $U_\tau$ with dimensionless time $f_c t$, and the shaded area represents the time-averaging window.
• Figure 2: Comparison of the CNBL statistics with different grids. (a) Mean streamwise velocity, (b) Reynolds normal stress, and (c) Reynolds shear stress.
• Figure 3: Comparison of the present simulation and Narasimhan et al.narasimhan2022effects. (a) Mean streamwise velocity; (b) mean total velocity. $z_{h=100}$ is the height of 100 m, and $U_{h=100}$ is the mean streamwise wind velocity at $z_{h=100}$.
• Figure 4: Comparison of the CNBL LES results with theoretical predictions by Liu et al.liuUniversalWindProfile2021liu2022vertical. $\hat{U}$ and $\hat{V}$ are the mean streamwise and lateral wind velocity components in a transformed coordinate system with the new streamwise direction aligned with that of the mean surface shear stress. (a) Mean streamwise wind velocity deficit, (b) mean spanwise wind velocity deficit, (c) streamwise turbulent shear stress, and (d) spanwise turbulent shear stress. The solid lines represent the theoretical predictions, and the symbols represent the current LES results.
• Figure 5: Comparison of (a) the mean streamwise wind speeds, (b) the mean spanwise wind speeds, (c) the total mean wind speeds, (d) the Ekman spirals, and (e) the wind veer angles to the streamwise direction (the geostrophic wind direction) in the CNBLs at different latitudes and the TNBL.