Anabatic Flows over Idealized Mountain Ridges and the Relation between Slope Angle and Turbulence Anisotropy

Anabatic winds are thermally-driven flows that develop over heated mountain slopes. These upslope winds develop when the air near the slope rises due to the along-slope component of the buoyancy force, driven by the horizontal temperature contrast between the heated slope-adjacent air and the cooler ambient air at the same elevation. Due to the temperature difference, a horizontal pressure gradient forces the air to rise along the slope. Anabatic flows have a distinct vertical structure, with a near-surface wind maximum and a jet-like profile.

According to Prandtl’s analytical model and data from numerical simulations, the strength and depth of the anabatic flow layer are sensitive to the slope angle. The slope angle has also been suspected as a potential driver of turbulence anisotropy based on measurement results. The impact of the slope angle on turbulence anisotropy, however, has not been investigated in numerical simulations so far. To address this gap, we used the Cloud Model 1 (CM1) to conduct high-resolution large-eddy simulations of anabatic flows above idealized ridges to evaluate the influence of ridge height, slope angle and slope curvature on turbulence anisotropy. In total, 10 simulations have been conducted so far, consisting of 7 simulations for sinusoidal ridges of different heights, widths and slope angles and three simulations for ridges with the same constant slope angle but different ridge heights. The simulations were initialized with a constant potential temperature gradient throughout the domain and a constant surface heat flux of 0.12 K m s^-1 and ran with a grid spacing of 10 m horizontally and 5 m vertically.

First results suggest that steeper slopes lead to more anisotropic turbulence. Apart from the slope angle itself, terrain curvature has a pronounced effect on the degree of anisotropy, as turbulence is more isotropic above slopes with constant slope angles compared to concave slopes of sinusoidal ridges. This is expected since upslope flow along a concave slope implies concave streamlines, and concave streamlines enhance shear stress and the momentum flux according to to the streamline curvature analogy. To gain further insights into the processes causing anisotropic turbulence, we plan to also investigate potential correlations between the degree of anisotropy and individual terms in the turbulent kinetic energy budget.