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Observational and numerical study of daytime flows in an alpine valley Rucker, Magdalena

Abstract

Thermally-driven valley wind systems are an integral part of the meteorology in mountain valleys and hence play an important role in environmental issues of human habitation. While we have a basic understanding of the evolution and mechanisms of valley flows, very little is known about the spatial structures of these flows and how they relate to topography. This lack of understanding has been largely due to the lack of spatial resolution with conventional in-situ instrumentation. This study utilizes high spatially-resolved Doppler lidar measurements to examine the spatial structure of daytime thermally-driven valley flows in the Wipptal, Austria. Results are presented for a number of days which provides insight into which flow aspects are influenced externally and which ones are induced locally. Numerical simulations are conducted to elucidate the flow dynamics in the Wipptal and to explore the role of topography. Observations show that, regardless of external conditions, the wind speed increases with up-valley distance in the narrow section of the Wipptal. Analysis of the along-valley volume flux shows that the increase in wind speed cannot be explained as a Venturi effect due to the horizontal contraction of the valley sidewalls. Furthermore, significant subsidence must occurs in this part of the valley in order to balance the along-valley volume flux divergence. Numerical modeling of the flow in the Wipptal supports the notion that the observed flow structure is linked to the valley geometry. Advection processes, however, are also found to play a significant role. From these findings, it may be inferred that the along-valley kinematic structure is influenced by the geometry of the valley and that localized subsiding motion can occur over localized valley segments as a result of changes in the intra-valley pressure gradient. This calls into question the concept that heating in an entire catchment area is relayed back to the main valley through subsidence at the mouth of a tributary.

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