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

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.

💡 Research Summary

This paper investigates how the Coriolis force, arising from Earth’s rotation, influences coherent turbulent structures in conventionally neutral atmospheric boundary layers (CNBL). Using high‑resolution large‑eddy simulations (LES) with the open‑source LESGO code, the authors solve the filtered Navier‑Stokes equations together with a buoyancy term and a Lagrangian‑scale‑dynamic sub‑grid model. The computational domain (24 km × 3 km × 2 km) and grid spacing (≈20 m horizontally, 8 m–2 m vertically) are sufficient to resolve both large‑scale motions and near‑wall turbulence.

The Coriolis parameter f = 2Ω sin ψ is varied by changing latitude (ψ = 25°–90° in the Northern Hemisphere and –45° in the Southern Hemisphere) and by altering the geostrophic wind speed (12–16 m s⁻¹). This yields a wide range of Rossby numbers and Zilitinkevich stability parameters, allowing a systematic exploration of Coriolis strength. Validation against grid‑convergence tests, prior LES studies (Narasimhan et al., 2022), and theoretical predictions (Liu et al., 2021, 2023) confirms that the simulated mean velocity, shear stress, and turbulence statistics are accurate.

Two principal findings emerge. First, coherent velocity structures (especially streamwise velocity fluctuations u′) are not aligned with the mean wind direction but are deflected toward the geostrophic wind direction. The deflection angle θ_deflection collapses onto a universal curve when plotted against the difference between the local wind‑veer angle and the global cross‑isobaric angle (Δθ = θ_veer − θ_cross). This suggests a robust relationship linking the Coriolis force, pressure‑gradient forcing, and turbulent structure orientation, independent of the specific latitude or wind speed.

Second, the inclination angle of large‑scale structures (the tilt of elongated eddies relative to the horizontal) increases as latitude decreases or geostrophic wind speed increases. In the surface layer (z < 0.1 δ) the inclination grows by roughly 5°–8° compared with higher latitudes, indicating that the Coriolis‑induced deflection also modifies the vertical tilt of the structures. This behavior aligns with the classic hairpin‑vortex picture but reveals an additional Coriolis‑driven modulation not captured in neutral laboratory flows.

Compared with a truly neutral boundary layer (TNBL) lacking Coriolis effects, CNBL exhibits 15%–25% higher turbulent shear stress and an amplified large‑scale portion of the energy spectrum, implying that the Coriolis force enhances the overall turbulence intensity by redistributing energy toward larger scales.

In summary, the study provides the first comprehensive quantification of how the Coriolis force reorients and tilts coherent turbulent structures in CNBL. The identified universal Δθ relationship and the latitude‑dependent inclination change offer new constraints for atmospheric‑boundary‑layer parameterizations, wind‑farm layout optimization, and the interpretation of field measurements in rotating flows.