The Interaction of Katabatic Flow and Mountain Waves. Part I: Observations and Idealized Simulations

Abstract

The mutual interaction of katabatic flow in the nocturnal boundary layer (NBL) and topographically forced gravity waves is investigated. Due to the nonlinear nature of these phenomena, analysis focuses on information obtained from the 1993 Atmospheric Studies in Complex Terrain field program held at the mountain?canyon?plains interface near Eldorado Canyon, Colorado, and idealized simulations. Perturbations to katabatic flow by mountain waves, relative to their more steady form in quiescent conditions, are found to be caused by dynamic pressure effects. Based on a local Froude number climatology, case study analysis, and the simulations, the dynamic pressure effect is theorized to occur as gravity wave pressure perturbations are transmitted through the atmospheric column to the surface and, through altered horizontal pressure gradient forcing, to the surface-based katabatic flows. It is proposed that these perturbations are a routine feature in the atmospheric record and represent a significant portion of the variability in complex terrain katabatic flows. The amplitude, wavelength, and vertical structure of mountain waves caused by flow over a barrier are themselves partly determined by the evolving structure of the NBL in which the drainage flows develop. For Froude number Fr > ?0.5 the mountain wave flow is found to separate from the surface at higher altitudes with NBL evolution (increasing time exposed to radiational cooling), as is expected from Fr considerations. However, flow with Fr < ?0.5 behaves unexpectedly. In this regime, the separation point descends downslope with NBL evolution. Overall, a highly complicated, mutually evolving, system of mountain wave?katabatic flow interaction is found, such that the two flow phenomena are, at times, indistinguishable. The mechanisms described here are expanded upon in a companion paper through realistic numerical simulations and analysis of a nocturnal case study (3?4 September 1993).