The role of tributary valley outflows in the development of a nocturnal mountain boundary layer (TribuNAL)

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Over horizontally homogeneous and flat terrain, the exchange of heat, momentum, mass, aerosols, dust and various other harmful pollutants is governed by turbulent motions within the atmospheric boundary layer, the lowest part of the troposphere. If a heterogeneous mountainous area is considered instead, the turbulent motions involved in the exchange greatly increase in complexity. Of particular interest are nighttime, stable conditions, which give rise to phenomena and interactions specific only to mountains. All these phenomena typically converge in low points of the terrain, most often in valleys where the majority of human activities are located. During nighttime, pollutants may get trapped inside very shallow layers adjacent to the valley surface, therefore affecting visibility, traffic, and most important of all, health and safety of people living in valleys. One problem lies in adequately sampling the interactions in question, owing to the broad range of spatio-temporal scales involved. As a result, weather forecast models struggle to properly predict the evolution of local weather in such regions of complex terrain, since their grid resolution is still too coarse. The present proposal addresses the confluence of flows in a parent valley with its tributary valleys during stable nighttime conditions. This aspect of nighttime interactions has received little attention to date, despite the advancement of remote sensing technologies such as lidars over the past four decades. Due to an on-going lack of adequate observations of such interactions, both our understanding, and a proper representation in numerical weather prediction models remain incomplete. This proposal intends to produce several types of deliverables, ranging from high-quality post-processed observational products to improvements of the physical understanding of the involved processes that can potentially lead to a better representation of nighttime phenomena within numerical models.