Mass consistent models for wind distribution in complex terrain — Fast algorithms for three dimensional problems

Mass consistent models for wind distribution in complex terrain are extremely useful and readily applied to many practical situations, such as the siting of wind turbines, or as input in the estimation of diffusion and transport of pollutants in complex terrain. These models are based on the numerical solution of the steady state three dimensional continuity equation for the mean wind components. The momentum and energy equations are not solved explicitly, but considered indirectly using parametric relations and wind data. In practical applications the equations must be solved numerous times (e.g. for each time interval). Standard techniques for numerical solution of three dimensional problems are frequently very expensive and thus not suitable for practical needs. In the present work, great emphasis is given to the development of fast algorithms, and techniques based on the multigrid approach are shown. Two mass consistent programs are described, the first based on the parametric representation of one of the wind components, and the second based on a few wind measurements and a variational principle. To verify the reliability of the variational approach, a measurement program related to a project of wind energy, is being performed at Har-Ahim, a site located in the Galilee.     

Water organisation at the solid–aqueous solution interfaceOrganisation de l'eau à l'interface solide–solution aqueuse

The aim of this review is to examine the present knowledge about water structure close to an interface or confined in porous spaces. First, the structure of liquid water is briefly described. Though its understanding remains incomplete, it appears that it is largely dominated by the hydrogen bond network and its dynamical evolution. The presence of any ‘foreign’ substance in water perturbs such a structure by changing at least locally the hydrogen bond network. For this reason, the presence of a solid interface significantly modifies the structure of the first adsorbed layers. Whatever the support, it is now clearly evidenced that structural perturbations are limited to distances lower than 10–15 Å from the interface. The nature, energetic heterogeneity and hydrophilicility/hydrophobicity of the solid surface influence the arrangement of water molecules. This surface organisation must definitely be considered when studying adsorption phenomena at the solid–aqueous solution interface. The relevance of such problems to geosciences is illustrated by a few situations in which water structure plays a prominent role. To cite this article: L.J. Michot et al., C. R. Geoscience 334 (2002) 611–631.     

Absolute seafloor vertical positioning using combined pressure gauge and kinematic GPS data

Knowledge of the position and motion of points on the seafloor can be critically important in both fundamental research (for example, global geodesy and plate tectonics) and for more practical applications such as seismic risk evaluation, off-shore construction and pipeline monitoring. In the Vanuatu subduction zone, for example, measuring deformation underwater could provide valuable information for modeling deformation and understanding the seismic cycle. We report a shallow water experiment in Vanuatu to measure the relative and absolute depth of seafloor points. The experiment differs from previous efforts mainly in that it uses the height of the sea surface determined by kinematic GPS, allowing us to locate the points in a global reference frame. The ITRF2005 ellipsoidal height of a seafloor benchmark was determined with a 1-sigma uncertainty of 0.7–2.1 cm. The estimated ellipsoidal height differs only by a few tenths of a centimeter between measurements made in 2004 and another set made in 2006. These results are encouraging and open new perspectives for vertical underwater deformation monitoring in shallow water areas. Sea-surface GPS measurements can also help to reduce the uncertainty in depth difference determination for relative measurements.     

Effects of Wormhole Configurations on Performance of Cyclic Solvent Injection in Heavy Oil Systems

Natural Resources Research - In this study, the effect of three different wormhole configurations: single-linear, single-branched, and double-branched wormholes on the performance of cyclic solvent...     

Controls on the aerodynamic roughness length and the grain‐size dependence of aeolian sediment transport

Estimates of the wind shear stress exerted on Earth's surface using the fully rough form of the law‐of‐the‐wall are a function of the aerodynamic roughness length, z₀. Accurate prediction of aeolian sediment transport rates, therefore, often requires accurate estimates of z₀. The value of z₀ is determined by the surface roughness and the saltation intensity, both of which can be highly dynamic. Here we report field measurements of z₀ values derived from velocity profiles measured over an evolving topography (i.e. sand ripples). The topography was measured by terrestrial laser scanning and the saltation intensity was measured using a disdrometer. By measuring the topographic evolution and saltation intensity simultaneously and using available formulae to estimate the topographic contribution to z₀, we isolated the contribution of saltation intensity to z₀ and document that this component dominates over the topographic component for all but the lowest shear velocities. Our measurements indicate that the increase in z₀ during periods of saltation is approximately one to two orders of magnitude greater than the increase attributed to microtopography (i.e. evolving sand ripples). Our results also reveal differences in transport as a function of grain size. Each grain‐size fraction exhibited a different dependence on shear velocity, with the saltation intensity of fine particles (diameters ranging from 0.125 to 0.25 mm) saturating and eventually decreasing at high shear velocities, which we interpret to be the result of a limitation in the supply of fine particles from the bed at high shear velocities due to bed armoring. Our findings improve knowledge of the controls on the aerodynamic roughness length and the grain‐size dependence of aeolian sediment transport. The results should contribute to the development of improved sediment transport and dust emission models. © 2018 John Wiley & Sons, Ltd.     

Which way do you lean? Using slope aspect variations to understand Critical Zone processes and feedbacks

Soil‐mantled pole‐facing hillslopes on Earth tend to be steeper, wetter, and have more vegetation cover compared with adjacent equator‐facing hillslopes. These and other slope aspect controls are often the consequence of feedbacks among hydrologic, ecologic, pedogenic, and geomorphic processes triggered by spatial variations in mean annual insolation. In this paper we review the state of knowledge on slope aspect controls of Critical Zone (CZ) processes using the latitudinal and elevational dependence of topographic asymmetry as a motivating observation. At relatively low latitudes and elevations, pole‐facing hillslopes tend to be steeper. At higher latitudes and elevations this pattern reverses. We reproduce this pattern using an empirical model based on parsimonious functions of latitude, an aridity index, mean‐annual temperature, and slope gradient. Using this empirical model and the literature as guides, we present a conceptual model for the slope‐aspect‐driven CZ feedbacks that generate asymmetry in water‐limited and temperature‐limited end‐member cases. In this conceptual model the dominant factor driving slope aspect differences at relatively low latitudes and elevations is the difference in mean‐annual soil moisture. The dominant factor at higher latitudes and elevations is temperature limitation on vegetation growth. In water‐limited cases, we propose that higher mean‐annual soil moisture on pole‐facing hillslopes drives higher soil production rates, higher water storage potential, more vegetation cover, faster dust deposition, and lower erosional efficiency in a positive feedback. At higher latitudes and elevations, pole‐facing hillslopes tend to have less vegetation cover, greater erosional efficiency, and gentler slopes, thus reversing the pattern of asymmetry found at lower latitudes and elevations. Our conceptual model emphasizes the linkages among short‐ and long‐timescale processes and across CZ sub‐disciplines; it also points to opportunities to further understand how CZ processes interact. We also demonstrate the importance of paleoclimatic conditions and non‐climatic factors in influencing slope aspect variations. Copyright © 2017 John Wiley & Sons, Ltd.     

Simulating vegetation effects on flows in 3D using an unstructured grid model: model development and validation

Ocean Dynamics - Prevalence of vegetation (either submerged or emergent) in shallow water significantly affects the flow and turbulence structure in this environment. In this paper, we develop a...     

IMAGE: a multivariate multi-site stochastic weather generator for European weather and climate

Capturing the spatial and temporal correlation of multiple variables in a weather generator is challenging. A new massively multi-site, multivariate daily stochastic weather generator called IMAGE is presented here. It models temperature and precipitation variables as latent Gaussian variables with temporal behaviour governed by an auto-regressive model whose residuals and parameters are correlated through resampling of principle component time series of empirical orthogonal function modes. A case study using European climate data demonstrates the model’s ability to reproduce extreme events of temperature and precipitation. The ability to capture the spatial and temporal extent of extremes using a modified Climate Extremes Index is demonstrated. Importantly, the model generates events covering not observed temporal and spatial scales giving new insights for risk management purposes.     

FracKfinder: A MATLAB Toolbox for Computing Three-Dimensional Hydraulic Conductivity Tensors for Fractured Porous Media

Fractures in porous media have been documented extensively. However, they are often omitted from groundwater flow and mass transport models due to a lack of data on fracture hydraulic properties and the computational burden of simulating fractures explicitly in large model domains. We present a MATLAB toolbox, FracKfinder, that automates HydroGeoSphere (HGS), a variably saturated, control volume finite-element model, to simulate an ensemble of discrete fracture network (DFN) flow experiments on a single cubic model mesh containing a stochastically generated fracture network. Because DFN simulations in HGS can simulate flow in both a porous media and a fracture domain, this toolbox computes tensors for both the matrix and fractures of a porous medium. Each model in the ensemble represents a different orientation of the hydraulic gradient, thus minimizing the likelihood that a single hydraulic gradient orientation will dominate the tensor computation. Linear regression on matrices containing the computed three-dimensional hydraulic conductivity (K) values from each rotation of the hydraulic gradient is used to compute the K tensors. This approach shows that the hydraulic behavior of fracture networks can be simulated where fracture hydraulic data are limited. Simulation of a bromide tracer experiment using K tensors computed with FracKfinder in HGS demonstrates good agreement with a previous large-column, laboratory study. The toolbox provides a potential pathway to upscale groundwater flow and mass transport processes in fractured media to larger scales.     

Asymmetry of weathering‐limited hillslopes: the importance of diurnal covariation in solar insolation and temperature

Hillslope asymmetry, i.e. variation in hillslope form as a function of slope aspect and/or mean solar insolation, has been documented in many climates and geologic contexts. Such patterns have the potential to help us better understand the hydrologic, ecologic, and geomorphologic processes and feedbacks operating on hillslopes. Here we document asymmetry in the fraction of hillslope relief accommodated by cliffs in weathering‐limited hillslopes of drainage basins incised into the East Kaibab Monocline (northern Arizona) and Raplee Ridge Monocline (southern Utah) of the southern Colorado Plateau. We document that south‐ and west‐facing hillslopes have a larger proportion of hillslope relief accommodated by cliffs compared with north‐ and east‐facing hillslopes. Cliff abundance correlates positively with mean solar insolation and, by inference, negatively with soil/rock moisture. Solar insolation control of hillslope asymmetry is an incomplete explanation, however, because it cannot account for the fact that the greatest asymmetry occurs between southwest‐ and northeast‐facing hillslopes rather than between south‐ and north‐facing hillslopes in the study sites. Modeling results suggest that southwest‐facing hillslopes are more cliff‐dominated than southeast‐facing hillslopes of the same mean solar insolation in part because potential evapotranspiration rates, which control the soil/rock moisture that drives weathering, are controlled by the product of solar insolation and a nonlinear function of surface temperature, together with the fact that southwest‐facing hillslopes receive peak solar insolation during warmer times of day compared with southeast‐facing hillslopes. The dependence of water availability on both solar insolation and surface temperature highlights the importance of the diurnal cycle in controlling water availability, and it provides a general explanation for the fact that vegetation cover tends to exhibit the greatest difference between northeast‐ and southwest‐facing hillslopes in the Northern Hemisphere and between southeast‐ and northwest‐facing hillslopes in the Southern Hemisphere. Copyright © 2017 John Wiley & Sons, Ltd.