Mountain Waves and Turbulent Exchange Processes: Progress and Remaining Challenges (Invited Presentation)

The airflow past orography generates a broad range of phenomena, which encompass all scales of atmospheric motion, from planetary-scale waves to mesoscale organized motions, all the way down to micro-scale turbulent mixing. Orographic flows have been the subject of extensive research over several past decades and considerable progress has been made in terms of understanding how complex terrain influences and shapes local winds and precipitation patterns and, more generally, local and large-scale weather and climate.

Orographically-generated internal gravity waves, or mountain waves, are created by the restoring action of buoyancy force as the stably-stratified air is forced over a terrain barrier. Their importance in redistributing momentum and energy in the atmosphere has long been recognized. A wide range of dynamical processes related to mountain waves, such as wave breaking and atmospheric rotors, can significantly modulate the exchange of momentum and scalar quantities between the atmospheric boundary layer and the underlying surface, between the atmospheric boundary layer and the free troposphere as well as between different atmospheric layers higher up in the atmosphere. In this presentation we will provide a brief overview of recent progress in understanding orographic flows in which mountain waves play a key role in influencing turbulent exchange processes and identify some of the remaining challenges in observing and quantifying mountain-wave-induced turbulent exchange processes. Special emphasis will be placed on the role of observations and the progress that has resulted from research within the context of large observational field campaigns in recent years as well as the design of future observing systems needed to advance the understanding of mountain-wave-induced turbulent exchange processes.