Flow during the evening transition over steep Alpine slopes

A field campaign, the Slope Experiment near La Fouly (SELF-2010), was conducted to monitor the evening transition of slope flows on clear-sky days from July to September 2010 in a narrow valley of the Swiss Alps. A steep west-facing slope with inclinations ranging from 25◦ to 45◦ was instrumented from 1900mto 2200mabove sea-level. Detailed measurements were made along a linear transect of the slope with two turbulence towers, two weather stations, five surface temperature measurement stations and a tethered balloon system. The present study focuses on nine exemplary ‘convective’ days, characterized by weak synoptic flow and clear-sky conditions, during which thermal circulations prevail. The analysis of the observational data shows that topographic shading triggers the evening transition. The topographic configuration around the experimental site results in a sharply defined ‘shading front’ propagating up slope, causing a sudden decrease in incoming short-wave radiation on the order of several hundreds of Wm−2 within a few minutes. The slope surface rapidly responds to the advancing shading front; in some cases, reductions in surface temperatures of some 10◦C in less than 10 min are observed. This is rapidly followed by an early-evening calm period with very small turbulent kinetic energy (TKE< 0.05m2 s−2) and extremely light wind speeds (< 0.5ms−1). When the inertia-driven upslope flow is fully stopped by the katabatic acceleration, a shallow local drainage flow forms and reaches a quasi-equilibrium 1.5 h after the local sunset. An analysis of the TKE budget close to the surface shows that the buoyancy flux is much greater than the shear production in the last hours before the local sunset, possibly due to valley curvature effects. Copyright c 2012 Royal Meteorological Society

Thermal Circulation Patterns and Turbulent Fluxes along Steep Mountain Slopes

Fabian Bärenbold, Chad Higgins, Wolf Hendrik Huwald, Daniel Nadeau, Eric Richard Pardyjak, Marc Parlange, Eulalie Sauthier

In hydrology, it is crucial to understand the atmospheric flow dynamics in mountainous terrain to predict turbulent exchanges of heat and moisture accurately at the regional scale. Under clear sky and weak synoptic conditions, these land–atmosphere interactions are driven by thermal circulations that take place over a strong diurnal cycle. During the day, winds travel up the mountain slopes and at night, they travel down toward to the bottom of the valley. Little is known about how the transition between these two regimes takes place over steep slopes. The Slope Experiment at La Fouly (SELF) in the Swiss Alps was designed to investigate these transition periods throughout summer 2010. In this paper, we will present the first results obtained from this field campaign. Data from a network of 16 wireless surface stations is used to define catchment wide micrometeorological processes such as slope and valley wind system development, while detailed measurements of the turbulent processes on a steep idealized slope (20 to 45 degrees) were also made. The slope was instrumented along a transect with four towers (including a surface energy budget station and 10 m tower with sonic anemometers), 13 surface temperature measurement stations and a tethered balloon system to capture the complex interplay between surface and atmosphere. Initial data presented will include basic circulation pattern development and measurements of the turbulent fluxes of water vapor, heat and momentum on the slope.

2010

Thermal Circulation Patterns and Turbulent Fluxes along Steep Mountain Slopes

Fabian Bärenbold, Chad Higgins, Wolf Hendrik Huwald, Daniel Nadeau, Eric Richard Pardyjak, Marc Parlange

In hydrology, it is crucial to understand the atmospheric flow dynamics in mountainous terrain to predict turbulent exchanges of heat and moisture accurately at the regional scale. Under clear skies and weak synoptic conditions, these land–atmosphere interactions are principally driven by thermal circulations that take place over a strong diurnal cycle. During the day, winds travel up the mountain slopes and at night, they travel down toward to the bottom of the valley. Little is known about how the transition between these two wind regimes takes place over steep slopes. The Slope Experiment at La Fouly (SELF) in the Swiss Alps was designed to investigate these transition periods throughout summer 2010. In this paper, we will present the first results obtained from this field campaign. Detailed measurements of the turbulent processes on a steep idealized slope (20 to 45 degrees) were made with four towers (including a surface energy budget station and 10 m tower with sonic anemometers), 15 surface temperature measurement stations and a tethered balloon system to capture the complex interplay between surface and atmosphere. Initial data presented will include basic circulation pattern development and measurements of the turbulent fluxes of water vapor, heat and momentum on the slope.

2011

Atmospheric Boundary Layer Dynamics of Transitional Flows over Complex Terrain

Daniel Nadeau

Recent advances in boundary-layer meteorology are beginning to allow the study of atmospheric flow phenomena that have previously been poorly understood. In this dissertation, we study the effects of complex terrain and unsteady regimes on the atmospheric boundary layer (ABL) dynamics, with the help of field measurements and theoretical analyses. We consider transitions generated by sharp surface discontinuities and by the daily cycle of solar heating. When air flows over an inhomogeneous landscape, it encounters a series of parcels (vegetated areas, mountains, cities, etc.) with different mechanical and thermal properties. Each parcel's boundary triggers a transition layer in the atmosphere. Substantial changes in momentum and heat exchanges are found when air flows over an urban area, due to the complex arrangement of buildings and the various thermal properties of the surface materials. To study the daytime heat exchanges between the built environment and the atmosphere, we conducted a field campaign over the EPFL campus in 2006-07 with a wireless network of 92 weather stations and an atmospheric profiler. In this analysis, the heat exchanges are successfully estimated with Monin-Obukhov similarity and the thermal roughness length method. We also illustrate how one carefully-selected station inside the urban canopy can provide a satisfying estimate of the sensible heat flux over the campus. The diurnal cycle of solar heating also induces transitions in the ABL. During the day, the ABL is usually characterized by an unstable stratification and large turbulent exchanges of momentum, heat and moisture. At night, the ABL is stably stratified and weak turbulent exchanges with the surface are typically observed. This dissertation presents a simple model to track the decay of atmospheric turbulence during the evening transition period, when the ABL shifts from its daytime to its nighttime regime. First, we describe a function to model the sensible heat flux during the transition period. This function is then inserted into a simplified version of the turbulent kinetic energy (TKE) budget, which we validate with several eddy covariance datasets. This study shows that the decay of convective turbulence over flat and unobstructed terrain occurs in three different steps: (i) the TKE is relatively constant for several convective eddy turnover time scales; (ii) the TKE decays with a t-2 rate, where t is time after the start of the decay; (iii) an abrupt decay rate of TKE near the sunset is observed indicating a rapid collapse of turbulence. We also study the evening transition period for slope flows developing over alpine terrain under clear skies and weak synoptic conditions. Slope winds travel upslope during the day and downslope at night. Little is known about the transition between these two wind regimes over steep slopes, mostly because they are extremely challenging to monitor. For this reason, in summer 2010, we deployed a suite of meteorological stations on a 25 to 45 degrees slope of the Swiss Alps. The results show that a 'shading front' induced by surrounding topography triggers the evening transition. The impact on the surface temperatures is substantial and in some cases, drops of 10 °C in less than 10 min are found. This is usually followed by an early evening calm period with low turbulence levels and small wind speeds (< 0.5 ms-1). At night, a shallow layer of 'skin' downslope flow forms, with the maximum wind velocity just above the surface (< 1.5 m).

EPFL2011

Atmospheric Boundary Layer Dynamics of Transitional Flows over Complex Terrain

Daniel Nadeau

EPFL2011