TExSMBL (Near-surface turbulent exchange in the stable mountain boundary layer)

Picture rights: Manuela Lehner.

Turbulent motions in the atmospheric boundary layer, that is, the lowest layer of the atmosphere, play a crucial role in transporting momentum, heat, moisture, and other atmospheric components between the earth’s surface and the atmosphere and within the boundary layer. The correct representation of these turbulent transport processes in weather forecasting and climate models is thus critical for producing accurate forecasts.

Turbulence in the boundary layer is produced by wind shear and, during daytime, by buoyancy due to the heating of the surface. During nighttime, on the other hand, cooling of the surface and the resulting stable stratification of the atmosphere suppress turbulence. Turbulence and turbulent transport are thus typically weak and intermittent under stable conditions and existing theories to describe turbulent exchange in forecasting models become invalid, so that the models have often problems with very stable conditions.

In this project, we want to analyze near-surface turbulent exchange in the Austrian Inn Valley. Mountain valleys, such as the Inn Valley, are characterized by strong diurnal wind circulations under undisturbed, clear-sky conditions. Due to the stronger cooling of the valley atmosphere compared to the surrounding plain, down-valley winds form during nighttime, which are directed along the valley axis towards the valley exit. In addition, the stronger cooling of the surface along sloping mountain sidewalls compared to the free valley atmosphere causes a down-slope flow. Both down-valley and down-slope flows are characterized by jet-like profiles and thus strong vertical wind shear. The horizontal structure of the down-valley wind and the interaction between the two flow systems causes additional wind shear, all of which contribute to turbulence production. On the other hand, cold-air pools or temperature inversions, that is, strongly stable layers, form frequently in valleys, which suppress turbulence.

The objective of this project is to analyze the relative contributions of these mountain-specific processes to turbulence production and damping in the stable boundary layer of a mountain valley. Data will be analyzed from six eddy-covariance stations in the Inn Valley, which are part of the i-Box project of the University of Innsbruck. The stations are located at different locations in the valley, specifically, at the valley floor, on the south- and north-facing sidewalls, and at a mountain top. We will quantify the spatial variations of near-surface turbulent exchange at the different sites, determine whether persistent wind shear associated with the characteristic flow systems suffices to produce continuous in contrast to intermittent turbulence, determine the impact of observed oscillations in the nocturnal flow, and describe turbulent exchange through the morning and evening transition periods.

TExSMBL is funded by the the Austrian Science Fund (FWF): V 791-N. The project runs from July 2020 to December 2023 and is led by Manuela Lehner at the Department of Atmospheric and Cryospheric Sciences of the University of Innsbruck.