Atmospheric Boundary Layer Dynamics of Transitional Flows over Complex Terrain

Abstract

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).