Oscillatory flow regimes in turbulent katabatic flows retrieved from direct numerical simulations

Persistent katabatic winds, which in basic terms can be described as buoyantly driven, turbulent boundary layer flows along cooled sloping surfaces in a stratified environment, are typical for vast areas of the earth and play an important role in the local and regional climate. Even in their most idealized or elemental forms, these flows conflate two notoriously difficult aspects of atmospheric dynamics: turbulence and natural convection. Although much progress has been made in the conceptual understanding and numerical simulation of these flows, long-standing difficulties with turbulence modeling in stably-stratified flows along slopes, and the variety of flow interactions that can occur with complex topography make katabatic flow dynamics a rich area of study.

We apply Direct Numerical Simulation (DNS) to investigate heat and momentum transfer properties in idealized downscaled turbulent katabatic flows in an environment stratified with constant ambient buoyancy frequency. An intriguing result is the discovery of low-frequency modulations of turbulent flow fluctuations with an oscillation frequency given by the product of the ambient buoyancy frequency and the sine of the slope angle. Such behavior of the flow is reminiscent of laminar natural convection flow of a stably-stratified fluid along a slope induced by a temporally-periodic change in surface temperature (as was revealed by one of our preceding studies). In the present case, however, the oscillations result from interactions between turbulent motions and ambient stable stratification in the presence of a temporally constant surface forcing. Our findings may provide a particularly helpful framework for understanding the generation of low-frequency oscillations or surges in katabatic flows, a phenomenon that has long been observed in atmospheric field experiments.