Répercussions météorologiques découlant de modifications naturelles ou délibérées de la surface. Principes généraux et prospectives, région de la Baie James

Les régions arctique et subarctique se prêtent merveilleusement à l'expérimentation du fait de la grande variabilité saisonnière de l'état de leurs surfaces glacées et neigeuses. Ces régions se prêtent bien aux études d'évolution climatique car en changeant les paramètres de surface on arrive à modifier significativement les processus d'échange par chaleur sensible et chaleur latente.À l'Université McGill on a élaboré un modèle numérique, utilisant aussi bien les données synoptiques des stations météorologiques, les données de point de grille que les données climatiques moyennes afin de calculer les différents termes du bilan énergétique. La région de la Baie de James est intéressante à étudier dans cette optique. L'article décrit un traitement numérique expérimental de la région.The arctic and sub-arctic regions are very suitable for model experimentation because their snow and ice covered surfaces change markedly from season to season. These regions lend themselves well to studies of climatic change because, by changing the surface parametres, it is possible to modify significantly the exchange processes for sensible and latent heat.A numerical model has been designed at McGill University, which can use either synoptic data, grid point data or climatic average data, for the calculation of the various terms in the energy balance. The James Bay region is interesting for studies of this kind, and the paper describes a numerical experiment from that region

Evaluation of North Water Spring Ice Cover from Satellite Photographs

Satellite photographs for 2 years (March-September) have been used to study ice cover in the polynia called "North Water," and to determine whether reliable ice maps could be made from satellite data without computer analysis. After early July the clouds became opaque and distinction between cloud and ice is impossible. It was concluded that ice distribution for short periods could best be obtained by careful photograph interpretation. The most persistent open water is found at the northern edge, at about 78° N. The southern ice edge is diffuse. The changes in ice cover in the North are mainly caused by freezing and melting, whereas ice transport is important in the southern area

Rheology of mobile sediment beds sheared by viscous, pressure-driven flows

We present a detailed comparison of the rheological behaviour of sheared sediment beds in a pressure-driven, straight channel configuration based on data that was generated by means of fully coupled, grain-resolved direct numerical simulations and experimental measurements reviously published by Aussillous {\it et al.} (J. Fluid Mech., vol. 736, 2013, pp. 594-615). The highly-resolved simulation data allows to compute the stress balance of the suspension in the streamwise and vertical directions and the stress exchange between the fluid and particle phase, which is information needed to infer the rheology, but has so far been unreachable in experiments. Applying this knowledge to the experimental and numerical data, we obtain the statistically-stationary, depth-resolved profiles of the relevant rheological quantities. The scaling behavior of rheological quantities such as the shear and normal viscosities and the effective friction coefficient are examined and compared to data coming from rheometry experiments and from widely-used rheological correlations. We show that rheological properties that have previously been inferred for annular Couette-type shear flows with neutrally buoyant particles still hold for our setup of sediment transport in a Poiseuille flow and in the dense regime we found good agreement with empirical relationships derived therefrom. Subdividing the total stress into parts from particle contact and hydrodynamics suggests a critical particle volume fraction of 0.3 to separate the dense from the dilute regime. In the dilute regime, i.e., the sediment transport layer, long-range hydrodynamic interactions are screened by the porous media and the effective viscosity obeys the Einstein relation

Settling of cohesive sediment: particle-resolved simulations

We develop a physical and computational model for performing fully coupled, particle-resolved Direct Numerical Simulations of cohesive sediment, based on the Immersed Boundary Method. The model distributes the cohesive forces over a thin shell surrounding each particle, thereby allowing for the spatial and temporal resolution of the cohesive forces during particle-particle interactions. The influence of the cohesive forces is captured by a single dimensionless parameter in the form of a cohesion number, which represents the ratio of cohesive and gravitational forces acting on a particle. We test and validate the cohesive force model for binary particle interactions in the Drafting-Kissing-Tumbling (DKT) configuration. The DKT simulations demonstrate that cohesive particle pairs settle in a preferred orientation, with particles of very different sizes preferentially aligning themselves in the vertical direction, so that the smaller particle is drafted in the wake of the larger one. To test this mechanism in a system of higher complexity, we perform large simulations of 1,261 polydisperse settling particles starting from rest. These simulations reproduce several earlier experimental observations by other authors, such as the accelerated settling of sand and silt particles due to particle bonding. The simulations demonstrate that cohesive forces accelerate the overall settling process primarily because smaller grains attach to larger ones and settle in their wakes. For the present cohesion number values, we observe that settling can be accelerated by up to 29%. We propose physically based parametrization of classical hindered settling functions proposed by earlier authors, in order to account for cohesive forces. An investigation of the energy budget shows that the work of the collision forces can substantially modify the relevant energy conversion processes.Comment: 39 page