Turbulent exchange in the stable mountain boundary layer

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 earths 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.

Turbulent motions play a crucial role in the transport of momentum, heat, moisture, and other atmospheric components between the earth's surface and the atmosphere and within the atmospheric boundary layer, that is, the lowest layer of the atmosphere. Representing turbulent transport correctly in weather and climate models is thus critical for producing accurate forecasts. Turbulence in the atmospheric boundary layer is produced by wind shear and, during daytime, by buoyancy due to surface heating by solar irradiation. During nighttime, cooling of the surface results in a stable stratification of the near-surface atmosphere, which suppresses turbulence. Turbulence is thus typically weak and intermittent during stable, nighttime conditions and existing theories to describe turbulent exchange in forecasting models become invalid, so that models often struggle to represent stable conditions correctly. In TExSMBL, we analyzed near-surface turbulent exchange over mountainous terrain, using data from five eddy-covariance stations in the Inn Valley, Austria, which are part of the i-Box infrastructure of the University of Innsbruck. The results show a strong dependence of turbulence strength and intermittency on the location in the valley and on the processes present at the different locations. At the valley floor, the atmosphere is typically characterized by low wind speeds and a strong temperature inversion, that is, a very stably stratified layer, during undisturbed, clear-sky nights. The observed turbulence is thus weak and strongly intermittent. In contrast, turbulence is observed to be higher and intermittency weaker at sites located on the valley sidewall, where downslope flows continue throughout the night, which are characterized by a jet profile and thus strong wind shear that contributes to turbulence production. Since both slope winds and valley winds undergo a pronounced diurnal cycle, their impact on near-surface turbulence also leads to a characteristic temporal evolution of turbulent exchange throughout the night. In addition, meandering motions, that is, oscillations of the horizontal wind, occur frequently near the valley floor, in particular during conditions with high stability and very low wind speeds. During these periods, the wind direction varies strongly and turbulent exchange is further reduced. Analysis of turbulent motions in the atmosphere relies on removing larger-scale motions from the signal by applying a high-pass filter. During stable conditions, many small-scale non-turbulent motions, like meandering, occur with time scales close to turbulent motions. A statistical relation was determined between the time scale separating turbulent and non-turbulent motions and near-surface wind speed and stability. This relationship can be used to apply a non-constant filter time scale. Analysis of the data from the Inn Valley revealed that a filter time of about 2 to 3 min is suited best for this location to capture most of the turbulent motions, while removing most of the non-turbulent motions.