Brian Hanson

Flow mechanism of glaciers on soft beds.

Subhourly measurements of bed deformation, bed shear strength, subglacial water pressure, and surface speed at Storglaci�ren, a glacier in northern Sweden, showed that the shear-strain rates of the bed decrease during periods of high water pressure and surface speed. High water pressures appear to be accompanied by a reduction in the coupling of ice with the bed that is sufficient to reduce or eliminate shearing. The instability of large ice masses may result from similar decoupling rather than from pervasive bed deformation, as has been commonly thought.

Rheology of till beneath Storglaciären, Sweden

In order to study, in situ, the rheology of a deforming subglacial till, various instruments were emplaced in till beneath Storglaciaren, Sweden. Boreholes were used to gain access to the till beneath about 100 m of ice. Tiltmeters provided an estimate of the shear strain rate in the till. Two other instruments yielded measures of till strength. In addition, water pressures were recorded in boreholes and in the till, a computer-controlled distance meter provided an effectively continuous record of the surface velocity and data from frequent surveys of a stake network were used to estimate the mean basal drag, based on a force-balance calculation. Tilt rates varied directly with effective pressure, so decreases in water pressure apparently increased the coupling between the glacier and the bed. Surface speed was either out of phase with tilt or varied independently of tilt. Thus, increases in speed were apparently a consequence either of longitudinal coupling or of reduced coupling between the glacier and the bed; they were not a result of till deformation! Till strength varied directly with effective pressure, which is consistent with it being a Mohr-Coulomb, or frictional material. The devices measuring till strength are presumed to have been pulled through the till at a speed that varied in phase with the surface speed but till strength did not vary systematically with surface speed. This implies that the residual strength of the till is insensitive to strain rate. Thus, the appropriate constitutive equation for till rheology may be of the form: e˙∞ e kτ where k is a constant. This is consistent with experimental data reported in the geotechnical literature.

Glacier Calving: A Numerical Model of Forces in the Calving-Speed/Water-Depth Relation

Empirical data suggest that the rate of calving of grounded glaciers terminating in water is directly proportional to the water depth. Important controls on calving may be the extent to which a calving face tends to become oversteepened by differential flow within the ice and the extent to which bending moments promote extrusion and bottom crevassing at the base of a calving face; Numerical modelling suggests that the tendency to become oversteepened increases roughly linearly with water depth. In addition, extending longitudinal deviatoric stresses at the base of a calving face increase with water depth. These processes provide a possible physical explanation for the observed calving-rate/water-depth relation.

A fully three-dimensional finite-element model applied to velocities on Storglaciären, Sweden

A finite-element model that solves the stress-balance equations for glacier dynamics in three dimensions has been developed by extending previous flow-plane models. This model retains all terms of the stress tensor and uses a Glen-type power law for viscosity calculations. In the paper, the model is applied to Storglaciaren, Sweden, to explore both the glaciers dynamics and the models characteristics. Values of the stiffness parameter B of 0.20-0.22 MPa year 1/n were required to match observed strain rates on Storglaciaren. Overall velocities required imposition of a problematically small sliding speed. Stress fields implied by the model simulation showed that the glacier receives its largest driving force from a high-slope zone near the equilibrium line, and that a large proportion of the resistive stress comes from lateral drag. Lateral drag is enhanced on this glacier by being frozen to ts sidewalls and by a turning of the main flow as it comes out of the main, or northern, cirque

Short-term velocity and water-pressure variations down-glacier from a riegel, Storglaciären, Sweden

During the 1991-94 summer field seasons, time-correlated measurements of water pressure and surface speed were made over and down-glacier from a major riegel on Storglaciaren, Sweden. Measurements were made at sub-hourly time-scales in order to discern details in the diurnal cycle. Large water-input events, typically associated with rain storms, produced coherent, lagged surface-velocity responses that could be understood in terms of till deformation or decoupling, and these have been discussed elsewhere. The consequences of smaller diurnal water-pressure events were more enigmatic, in that acceleration of ice flow generally preceded the onset of the local water-pressure rise. From consideration of these data and other work done on the hydrology of Storglaciaren, we infer that the ice in this area is generally pushed from behind via a relaxation in extensional strain across the riegel. Hence, accelerations occur in response to increases in water pressure that occur up-glacier and that precede local water-pressure rises. In addition, following a period of large storm events, surface speeds became more spatially coherent and were in phase with the diurnal ater-pressure cycle. This suggests that the large water-pressure events lead to a spatially more homogeneous subglacial drainage system. Sliding laws need to take into accoun such temporal changes in spatial coherence of the subglacial drainage system.

Buckling Rate and Overhang Development at a Calving Face

Using the finite-element we have modeled the stress field near the calv- ing face of an idealized tidewater glacier under a variety of assumptions about submarine calving-face height, subaerial calving-face height, and ice rheology. These simulations all suggest that a speed maximum should be present at the calving face near the waterline. In experiments without crevassing, the decrease in horizontal velocity above this maximum culminates in a zone of longitudinal compression at the surface somewhat up-glacier from the face. This zone of compression appears to be a consequence of the non-linear rheology of ice. It disappears when a linear rheology is assumed. Explorations of the near-surface stress field indicate that when pervasive crevassing of the surface ice is accounted for in the simulations (by rheological softening), the zone of compressive strain rates does not devel- op.Variations in the pattern of horizontal velocity with glacier thickness support the con- tention that calving rates should increase with water depth at the calving face. In addition, the height of the subaerial calving face may have an importance that is not visible in current field data owing to the lack of variation in height of such faces in nature. Glaciers with lower calving faces may not have sufficient tensile stress to calve actively, while ten- sile stresses in simulated higher faces are sufficiently high that such faces will be unlikely to build in nature.

A Teleconnection between Forced Great Plains Snow Cover and European Winter Climate

Abstract Anomalies in Siberian snow cover have been shown to affect Eurasian winter climate through the North Atlantic Oscillation (NAO). The existence of a teleconnection between North American snow cover and the NAO is far less certain, particularly for limited, regional snow cover anomalies. Using three ensembles of the Community Atmosphere Model, version 2 (CAM2), the authors examined teleconnections between persistent, forced snow cover in the northern Great Plains of the United States and western Eurasian winters. One ensemble allowed the model to freely determine global snow cover, while the other two forced a 72-cm snowpack centered over Nebraska. Of the forced ensembles, the “early-season” (“late season”) simulations initiated the snowpack on 1 November (1 January). The additional snow cover generated lower (higher) sea level pressures and geopotential heights over Iceland (the Azores) and warmer (cooler) temperatures over northern and western (eastern and southeastern) Europe, which suggests the...

Circulation Response to Eurasian versus North American Anomalous Snow Scenarios in the Northern Hemisphere with an AGCM Coupled to a Slab Ocean Model

AbstractThe difference between snow-covered and snow-free conditions is the most climatically significant natural seasonal change the land surface can experience. Most GCM studies investigating snow–atmosphere interactions have focused on impacts of Eurasian snow anomalies caused by the magnitude of snow mass, while North American snow has been shown to have a weaker relationship with downstream climate. Experiment design of recent snow–atmosphere interactions studies has been limited to atmosphere-only models, with sea surface temperature (SST) and sea ice extent represented as boundary conditions. The authors explore the circulation response to anomalous snow scenarios, for both North America and Eurasia, using a slab ocean model. Surface response include significant SST cooling directly downstream of each individual forcing region in addition to upstream centers of remote cooling under maximum snow conditions. Atmospheric response to anomalous snow conditions is consistent through multiple levels in th...

Influence of heterogeneous land surfaces on surface energy and mass fluxes

SummaryLand-surface heterogeneity affects surface energy fluxes. The magnitudes of selected land-surface influences are quantified by comparing observations with model simulations of the FIFE (First ISLSCP Field Experiment) domain. Several plausible heterogeneous and homogeneous initial and boundary conditions are examined, although soilmoisture variability is emphasized. It turns out that simple spatial averages of surface variation produced biased flux values. Simulated maximum latent-heat fluxes were approximately 30 to 40 W m−2 higher, and air temperatures ≃ 0.4 °C lower (at noon), when computations were initialized with spatially averaged soil-moisture and leaf-area-index fields. The planetary boundary layer (PBL) height and turbulent exchanges were lower as well. It additionally was observed that (largely due to the nonlinear relationship between initial soil-moisture availability and the evapotranspiration rate), “real” latent-heat flux can be substantially less than simulated latent-heat flux using models initialized with spatially averaged soil-moisture fields. Differences between “real” and simulated fluxes also vary with the resolution at which “real” soil-moisture heterogeneity is discretized.

Simulating the surface energy budget over the Konza Prairie with a mesoscale model

Abstract Using FIFE observations on four golden days and a coupled soil-canopy-atmosphere (mesoscale) dynamic model, we simulate the biophysical influences of a prairie on the surface energy budget. Atmospheric dynamics within the model are fully three-dimensional. Canopy and soil submodels were modified to include the aggregate effects of standing brown vegetation on stomatal conductance and surface energy partitioning, and litter cover on soil thermal and hydraulic properties. Model-simulated energy fluxes compare reasonably well to in situ measurements. It was found that the spatial variation of simulated latent-, sensible-, and ground-heat fluxes is very sensitive to soil-moisture and biomass distributions over the FIFE domain. High latent-heat fluxes are related to high occurrences of green leaf-area indices (LAI green ), low brown leaf-area indices (LAI brown ), and high soil-moisture contents, as well as to sufficient available energy. High sensible-heat fluxes often are coincident with a low LAI green , high LAI brown , and relatively dry soils. High ground-heat fluxes occur with lower values of total LAI and high soil-moisture contents. As a result of soil-canopy-atmosphere interactions, simulated air temperatures vary spatially up to 2°C. In addition, it was found that—for a prairie environment like the Konza Prairie with green leaf-area indices less than 2—simulated latent-heat fluxes from the canopy are about two times greater than those from the soil beneath the canopy.