Search

Classic Monin–Obukov similarity scaling states that in a stationary, horizontally homogeneous flow, in the absence of subsidence, turbulence is dictated by the balance between shear production and buoyancy production/destruction, whose ratio is characterized by a single universal scaling parameter. An evident breakdown in scaling is observed though, through large scatter in traditional scaling relations for the horizontal velocity variances under unstable stratification, or more generally in complex flow conditions. This breakdown suggests the existence of processes other than local shear and buoyancy that modulate near-surface turbulence. Recent studies on the role of anisotropy in similarity scaling have shown that anisotropy, even if calculated locally, may encode the information about these missing processes. We therefore examine the possible processes that govern the degree of anisotropy in convective conditions. We first use the reduced turbulence-kinetic-energy budget to show that anisotropy in convective conditions cannot be uniquely described by a balance of buoyancy and shear production and dissipation, but that other terms in the budget play an important role. Subsequently, we identify a ratio of local time scales that acts as a proxy for the anisotropic state of convective turbulence. This ratio can be used to formulate a new non-dimensional group. Results show that building on this approach the role of anisotropy in scaling relations over complex terrain can be placed into a more generalized framework.

Monin-Obukhov similarity theory (MOST) stands at the center of understanding of atmospheric surface-layer turbulence. Based on the hypothesis that a single length scale determined by surface fluxes governs the turbulent exchange of momentum, mass, and energy between the Earth's surface and the atmosphere, MOST has been widely used in parametrizations of turbulent processes in virtually all numerical models of atmospheric flows. However, its inherent limitations to flat and horizontally homogeneous terrain, the non-scaling of horizontal velocity variances, and stable turbulence have plagued the theory since its inception. Thus, the limitations of MOST, and its use beyond its intended range of validity contribute to large uncertainty in weather, climate, and air-pollution models, particularly in polar regions and over complex terrain. Here we present a first generalized extension of MOST encompassing a wide range of realistic surface and flow conditions. The novel theory incorporates information on the directionality of turbulence exchange (anisotropy of the Reynolds stress tensor) as a key missing variable. The constants in the standard empirical MOST relations are shown to be functions of anisotropy, thus allowing a seamless transition between traditional MOST and its novel generalization. The new scaling relations, based on measurements from 13 well-known datasets ranging from flat to highly complex mountainous terrain, show substantial improvements to scaling under all stratifications. The results also highlight the role of anisotropy in explaining general characteristics of complex terrain and stable turbulence, adding to the mounting evidence that anisotropy fully encodes the information on the complexity of the boundary conditions.

While advances in computation are enabling finer grid resolutions in numerical weather prediction models, representing land-atmosphere exchange processes as a lower boundary condition remains a challenge. This partially results of the fact that land-surface heterogeneity exists at all spatial scales and its variability does not `average' out with decreasing scales. The work here presented uses large-eddy simulations and the concept of dispersive fluxes to quantify the effect of surface thermal heterogeneity with scales $\sim$ 1/10th the height of the atmospheric boundary layer and characterized by uniform roughness. Such near-canonical cases describe inhomogeneous scalar transport in an otherwise planar homogeneous flow when thermal stratification is weak or absent. Results illustrate a regime in which the flow is mostly driven by the surface thermal heterogeneities, in which the contribution of the dispersive fluxes can account for more than 40\% of the total sensible heat flux at about 0.1$z_i$, with a value of 5 to 10\% near the surface, regardless of the spatial distribution of the thermal heterogeneities, and with a weak dependence on time averaging. Results also illustrate an alternative regime in which the effect of the surface thermal heterogeneities is quickly blended, and the dispersive fluxes match those obtained over an equivalent thermally homogeneous surface. This character seems to be governed by a new non-dimensional parameter representing the ratio of the large-scale advection effects to the convective turbulence. We believe that results from this research are a first step in developing new parameterizations appropriate for non-canonical atmospheric surface layer conditions., Comment: Manuscript submitted to Boundary-Layer Meteorology

In high-altitude alpine catchments, diurnal streamflow cycles are typically dominated by snowmelt or ice melt. Evapotranspiration-induced diurnal streamflow cycles are less observed in these catchments but might happen simultaneously. During a field campaign in the summer 2012 in an alpine catchment in the Swiss Alps (Val Ferret catchment, 20.4 km2, glaciarized area: 2%), we observed a transition in the early season from a snowmelt to an evapotranspiration-induced diurnal streamflow cycle in one of two monitored subbasins. The two different cycles were of comparable amplitudes and the transition happened within a time span of several days. In the second monitored subbasin, we observed an ice melt-dominated diurnal cycle during the entire season due to the presence of a small glacier. Comparisons between ice melt and evapotranspiration cycles showed that the two processes were happening at the same times of day but with a different sign and a different shape. The amplitude of the ice melt cycle decreased exponentially during the season and was larger than the amplitude of the evapotranspiration cycle which was relatively constant during the season. Our study suggests that an evapotranspiration-dominated diurnal streamflow cycle could damp the ice melt-dominated diurnal streamflow cycle. The two types of diurnal streamflow cycles were separated using a method based on the identification of the active riparian area and measurement of evapotranspiration.

In realistic urban environments, where buildings experience variable heights, the tallest structures have a disproportional impact on drag, mean flow and turbulence. Although wind-engineering studies document well the effects of individual high-rise buildings on the immediate surrounding, the impact of varying building heights on the larger horizontally averaged flow has not been quantified systematically for realistic urban configurations. We use Large Eddy Simulation (LES) as a mean to study the fully developed turbulent flow over and within a 512 x 512m subset of truthful urban geometry in the city of Basel, Switzerland. A periodic LES domain is centered on the location of a tower, where measurements of turbulence were performed in 2001/02, which allows a direct validation of the LES at a specific location in the domain. The Lagrangian scale-dependent LES model is adopted to parametrize the subgrid stresses in the bulk of the flow and buildings are taken into account adopting a discrete-forcing-approach immersed boundary method (IBM), with the geometry taken from a highly accurate digital building model. A series of high-resolution LES runs are performed for various directions of the approaching flow, and with all buildings included and then buildings above a certain height threshold progressively removed, to isolate the impact of tall structures. Results show how the presence of isolated tall buildings strongly modifies the roughness properties of the entire urban roughness sublayer, causing an increase in resolved pressure forces, which contributes to several percentages of the overall surface induced drag. In presence of tall buildings the local structure of the roughness sublayer is partitioned in two regimes: fine scale wake turbulence and elongated, high speed streak-like motions, locked between the position of isolated structures, with their axis aligned in the streamwise direction. For arrays with variable building heights statistics significantly differ from standard surface layers’ ones and are strongly inhomogeneous in space. Dispersive momentum fluxes are shown to significantly increase with the presence of isolated tall buildings, mainly due to the spatial heterogeneity of the flow in the spanwise direction.

The wind energy industry is increasingly focusing on optimal power extraction strategies based on layout design of wind farms and yaw alignment algorithms. Recent field studies by Mikkelsen et al. (Wind Energy, 2013) have explored the possibility of using wind lidar technology installed at hub height to anticipate incoming wind direction and strength for optimizing yaw alignment. In this work we study the benefits of using remote sensing technology for predicting the incoming flow by using large eddy simulations of a wind farm. The wind turbines are modeled using the classic actuator disk concept with rotation, together with a new algorithm that permits the turbines to adapt to varying flow directions. This allows for simulations of a more realistic atmospheric boundary layer driven by a time-varying geostrophic wind. Various simulations are performed to investigate possible improvement in power generation by utilizing upstream data. Specifically, yaw-correction of the wind-turbine is based on spatio-temporally averaged wind values at selected upstream locations. Velocity and turbulence intensity are also considered at those locations. A base case scenario with the yaw alignment varying according to wind data measured at the wind turbine's hub is also used for comparison. This reproduces the present state of the art where wind vanes and cup anemometers installed behind the rotor blades are used for alignment control.

We study the diurnal streamflow cycles in a high altitude catchment where a large field campaign has been undertaken. In one monitored sub-basin, the cycle is evapotranspiration-dominated, in another, the cycle is icemelt-dominated. The two processes happen at the same time of the day but with opposite signs. Our results suggest that the amplitude of icemelt dominated streamflow is damped by evapotranspiration in the riparian area.

The influence of surface heterogeneities extends vertically within the atmospheric surface layer to the so-called blending height, causing changes in the fluxes of momentum and scalars. Inside this region the turbulence structure cannot be treated as horizontally homogeneous; it is highly dependent on the local surface roughness, the buoyancy and the horizontal scale of heterogeneity. The present study analyzes the change in scalar flux induced by the presence of a large wind farm installed across a heterogeneously rough surface. The change in the internal atmospheric boundary-layer structure due to the large wind farm is decomposed and the change in the overall surface scalar flux is assessed. The equilibrium length scale characteristic of surface roughness transitions is found to be determined by the relative position of the smooth-to-rough transition and the wind turbines. It is shown that the change induced by large wind farms on the scalar flux is of the same order of magnitude as the adjustment they naturally undergo due to surface patchiness.

The movement of air in natural porous media is complex and challenging to measure. Yet gas transport has important implications, for instance, for the evolution of the seasonal snow cover and for water vapor transport in soil. A novel in situ multi-sensor measurement system providing high-resolution observation of gas transport in snow is demonstrated. Carbon monoxide was selected as the tracer gas for having essentially the same density as air, low background concentration, low water solubility, and for being detectable to ≤ 1 ppmv with small, low-cost, low-power sensors. The plume of 1% CO injections 30 cm below the snow surface was monitored using 28 sensors (4 locations, 7 depths). The CO breakthrough curves obtained at distances of 0.5–1 m were in good agreement with a simple analytical advection-diffusion model. The tracer system appears suitable for a wide range of applications in experimental soil science and hydrology addressing moisture transport and evapotranspiration processes.

Parametrization of momentum surface roughness length in mountainous regions continues to be an active research topic given its application to improved weather forecasting and sub-grid scale representation of mountainous regions in climate models. A field campaign was conducted in the Val Ferret watershed (Swiss Alps) to assess the role of topographic variability and snow cover on momentum roughness. To this end, turbulence measurements in a mountainous region with and without snow cover have been analyzed. A meteorological mast with four sonic anemometers together with temperature and humidity sensors was installed at an elevation of 2500 m and data were obtained from October 2011 until May 2012. Because of the long-term nature of these experiments, natural variability in mean wind direction allowed a wide range of terrain slopes and snow depths to be sampled. A theoretical framework that accounted only for topographically induced pressure perturbations in the mean momentum balance was used to diagnose the role of topography on the effective momentum roughness height as inferred from the log-law. Surface roughness depended systematically on wind direction but was not significantly influenced by the presence of snow depth variation. Moreover, the wind direction and so the surface roughness influenced the normalized turbulent kinetic energy, which in theory should not depend on these factors in the near-neutral atmospheric surface layer. The implications of those findings to modeling momentum roughness heights and turbulent kinetic energy (e.g. in conventional K-epsilon closure) in complex terrain are briefly discussed.