The Interaction of Katabatic Flow and Mountain Waves. Part II: Case Study Analysis and Conceptual Model

Abstract

Via numerical analysis of detailed simulations of an early September 1993 case night, the authors develop a conceptual model of the interaction of katabatic flow in the nocturnal boundary layer with mountain waves (MKI). A companion paper (Part I) describes the synoptic and mesoscale observations of the case night from the Atmospheric Studies in Complex Terrain (ASCOT) experiment and idealized numerical simulations that manifest components of the conceptual model of MKI presented herein. The reader is also referred to Part I for detailed scientific background and motivation.The interaction of these phenomena is complicated and nonlinear since the amplitude, wavelength, and vertical structure of the mountain-wave system developed by flow over the barrier owes some portion of its morphology to the evolving atmospheric stability in which the drainage flows develop. Simultaneously, katabatic flows are impacted by the topographically induced gravity wave evolution, which may include significantly changing wavelength, amplitude, flow magnitude, and wave breaking behavior. In addition to effects caused by turbulence (including scouring), perturbations to the leeside gravity wave structure at altitudes physically distant from the surface-based katabatic flow layer can be reflected in the katabatic flow by transmission through the atmospheric column. The simulations show that the evolution of atmospheric structure aloft can create local variability in the surface pressure gradient force governing katabatic flow. Variability is found to occur on two scales, on the meso-␤ due to evolution of the mountain-wave system on the order of one hour, and on the microscale due to rapid wave evolution (short wavelength) and wave breaking-induced fluctuations. It is proposed that the MKI mechanism explains a portion of the variability in observational records of katabatic flow.

The role of advection and of work done by forces in the gustiness of the Bora flow

“…Poulos et al (2007), except for velocity and not KE when investigating the flow in the eldorado Canyon in Colorado. They thus had to consider more terms: advection, pressure gradient, gravity, coriolis effects, and turbulent momentum flux contributing to local changes in wind velocity.…”

Strong wind events across Greenland�s coast and their influence on the ice sheet, sea ice and ocean

“…Durran [1990]) suggest that downslope wind storms can be forced by mountain waves or a combination of katabatic flows and mountain waves [Poulos et al, 2000[Poulos et al, , 2007. Gravity waves were also observed during a katabatic wind event in West Greenland [Heinemann, 1999] and Doyle et al [2005] observed large-amplitude wave breaking during an individual DWE in Ammassalik.…”

“…Interactions between gravitationally driven flows and mountain waves have been studied by Poulos et al [2000Poulos et al [ , 2007 for different Froude number regimes. Using an upstream wind speed of 30 m s −1 for the strong wind event, a stability of = 0.02 s −1 and a mountain height of 3000 m, the Froude number is close to 0.5 for the strong wind event.…”

“…The mountain wave separation point delimits the region where the mountain wave dominates the flow. According to Poulos et al [2000Poulos et al [ , 2007 the coupling of the katabatic wind with the mountain wave downstream of the separation point can deepen the temperature deficit layer by turbulence relative to the case without mountain waves. Indeed, for both wind events, I find very deep deficit layers of more than 1000 m (e.g.…”

“…Based on idealized simulations, Poulos et al [2000] find that radiative cooling can enhance the mountain wave flow speed in the lee for Froude numbers up to 0.75. For Froude numbers above ∼0.5 the gravity current can be scoured such that the mountain wave flow penetrates to the surface while for Froude numbers less than ∼0.5 the katabatic flow and the mountain wave couple, resulting in a complex mutually interdependent evolution [Poulos et al, 2000[Poulos et al, , 2007. While mountain waves can influence the intensity, depth and local variability of katabatic winds through turbulence and dynamic pressure perturbations [Mursch-Radlgruber, 1995, Jin et al, 1996, Poulos et al, 2000, the stability and reduced turbulence of a lower stratified layer can in turn affect the dynamics of mountain waves [Scorer, 1967, Poulos et al, 2000.…”