Modeling Snow Saltation: The Effect of Grain Size and Interparticle Cohesion

The surface of the Earth is snow-covered at least seasonally over large areas. This snow surface is highly dynamic, particularly under the influence of strong winds. The motion of snow particles driven by the wind not only changes the snow cover but has important consequences for the atmosphere in that it adds mass and moisture and extracts heat. Large scale meteorological and climatological models neglect these surface dynamics or produce conflicting results from too simplified process representation. With recent progress in the detailed understanding of the saltation process, in particular with respect to sand saltation, and the advancement of numerical models, we can systematically investigate the influence of snow properties on saltation. This contribution uses a Large Eddy Simulation model with full surface particle dynamics to investigate how snow cohesion and size distribution influence saltation dynamics and in particular the total mass flux. The model reproduces some known characteristics of the saltation system such as a focus point or a constant near surface particle speed. An interesting result is that cohesion and grain size heterogeneity can increase the overall saltation mass flux at high friction velocities. Moreover, some simplified models agree reasonably well with the simulations for given bed characteristics, while others clearly do not. These results are valid for continuous saltation while intermittent saltation, which often occurs in nature, needs further investigation. In order to successfully parameterize saltation in large scale models, progress must be made in correctly representing snow surface properties in these models, in particular cohesion.

On the Diurnal Soil Water Content Dynamics during Evaporation using Dielectric Methods

Ivan Lunati, Marc Parlange, Scott W. Tyler

The water content dynamics in the upper soil surface during evaporation is a key element in land–atmosphere exchanges. Previous experimental studies have suggested that the soil water content increases at the depth of 5 to 15 cm below the soil surface during evaporation, while the layer in the immediate vicinity of the soil surface is drying. In this study, the dynamics of water content profiles exposed to solar radiative forcing was monitored at a high temporal resolution using dielectric methods both in the presence and absence of evaporation. A 4-d comparison of reported moisture content in coarse sand in covered and uncovered buckets using a commercial dielectric-based probe (70 MHz ECH2O-5TE, Decagon Devices, Pullman, WA) and the standard 1-GHz time domain reflectometry method. Both sensors reported a positive correlation between temperature and water content in the 5- to 10-cm depth, most pronounced in the morning during heating and in the afternoon during cooling. Such positive correlation might have a physical origin induced by evaporation at the surface and redistribution due to liquid water fluxes resulting from the temperature-gradient dynamics within the sand profile at those depths. Our experimental data suggest that the combined effect of surface evaporation and temperature-gradient dynamics should be considered to analyze experimental soil water profiles. Additional effects related to the frequency of operation and to protocols for temperature compensation of the dielectric sensors may also affect the probes response during large temperature changes.

2010

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

Wind Tunnel Experiments and Modelling of Particle-Laden Boundary Layer Flows

Michael Lehning, Benjamin Andreas Walter

Particle-laden boundary flows occur in many geophysical and industrial environments yet are difficult to understand and quantitatively describe because the interactions of an often turbulent boundary layer flow with surface and particle dynamics are complex. The SLF wind tunnel allows the investigation of snow and sand particle laden boundary layer flows over their natural surfaces with and without the presence of a grass canopy.The experimental results are used to investigate the two possible approaches in describing the surface dynamics: (i) Models of particle transport, which assume a stationary flow situation and predict a mass flux for an hypothetical equilibrium; (ii) Models that take the temporal variability of the mass flux explicitly into account such as Lagrangian Stochastic particle tracking Models (LSM) on the basis of large eddy simulation (LES) or direct numerical simulation (DNS) of flow and turbulence. This presentation shows that wind tunnel data support the form of semi-empirical equilibrium models, which predict mass flux, q, as a function of the mean wind speed or the friction velocity, u, and a threshold velocity, uth: q=a(u-uth)x. For the exponent "x", a value of approximately 3, as based on theoretical considerations, is consistent with the data. This simple form of equilibrium models as well as more complicated equilibrium models are all based on the hypothesis that the surface shear stress induced by a fluid on the ground during sediment saltation is constant, i.e. independent of the magnitude of the particle mass flux (Owen's second hypothesis). Our surface shear stress measurements in a drifting-sand wind tunnel show a constant value of the fluid shear stress for saltation layers of various mass-flux magnitudes, directly validating Owen’s second hypothesis for the first time. The equilibrium models, however, only insufficiently describe the full dynamics of particle-laden flows. The second part of the presentation therefore discusses non-equilibrium features such as a high variability of the particle mass flux caused by flow turbulence and surface heterogeneity. Mass flux intermittency is primarily observed around the threshold value uth. Using a combination of LES and LSM models, we show how the simulation of individual feed-back processes leads to a more complete understanding of the mechanisms behind the flux variability.