flow structure and turbulence characteristics of the daytime atmosphere in a steep and narrow alpine valley aircraft measurements radio soundings and sonic data obtained during the map riviera field campaign in autumn 1999 in southern switzerland are used to investigate the flow structure temperature profiles and turbulence characteristics of the atmosphere in a steep and narrow alpine valley under convective conditions on all predominantly sunny days of the intensive observation periods a pronounced valley wind system develops in the southern half of the valley the daily up valley winds have a jet like structure and are shifted towards the eastern slope these up valley winds advect potentially colder air a process which appears to be balanced by vertical warm air advection from above the profiles of potential temperature show that with the onset of up valley winds the mixed layer consistently stops growing or on days with very strong up valley winds even stabilizes almost throughout the entire valley atmosphere this is probably due to a pronounced secondary circulation in the southern part of the valley which induces advection of warm air from above the secondary circulation appears to be a consequence of sharp curvature in the along valley topography turbulence variables are calculated from flight legs in the along valley direction turbulent kinetic energy tke scales surprisingly well i if a tke criterion tke 0 5 m2s 2 is employed as a definition of the boundary layer height and ii if the surface fluxes which exhibit a substantial spatial variability from the slope sites are used rather than those from directly beneath the profile considered significant site to site differences in incoming solar radiation seem to be the reason for this characteristic behaviour profiles of momentum flux scaled with a surface friction velocity reveal more scatter than the tke profiles but still show a consistent behaviour a surprisingly strong shear in the cross valley direction can be observed and is probably a result of the secondary circulation copyright 2004 royal meteorological society

greenland flow variability from ice sheet wide velocity mapping using radarsat synthetic aperture radar data we have mapped the flow velocity over much of the greenland ice sheet for the winters of 2000 01 and 2005 06 these maps provide a detailed view of the ice sheet flow including that of the hundreds of glaciers draining the interior the focused patterns of flow at the coast suggest a strong influence of bedrock topography differences between our two maps confirm numerous early observations of accelerated outlet glacier flow as well as revealing previously unrecognized changes the overall pattern is one of speed up accompanied by terminus retreat but there are also several instances of surge behavior and a few cases of glacier slowdown comprehensive mappings such as these at regular intervals provide an important new observational capability for understanding ice sheet variability

observations of cross ridge flows across steep terrain a field experiment gaudex has been conducted to address the need for quantitative measurements of turbulence in the vicinity of steep terrain a dense network of automatic weather stations and turbulence towers was deployed along cross ridge transects over gaudergrat a steep triangular cross section ridge in eastern switzerland a new feature whereby ridge normal cross ridge flows develop at the crest even when the flow in each valley is oriented parallel to the ridge axis is identified this occurs independently of whether the flow is thermally or synoptically driven pressure measurements across the ridge show that this flow is driven by a cross ridge pressure gradient two mechanisms for generating cross ridge flows have been identified from measurements of pressure wind and temperature in most cases the pressure gradient arises from a difference in flow speeds between the two sides of the ridge caused by different valley geometries less commonly the cross ridge flow is explained by linear speed up of the ridge normal flow copyright 2008 royal meteorological society

observations of downslope winds and rotors in the falkland islands a field campaign aimed at observing the near surface flow field across and downwind of a mountain range on the falkland islands south atlantic is described the objective was to understand and eventually predict orographically generated turbulence the instrumentation was based primarily on an array of automatic weather stations awss which recorded 30 s mean surface pressure wind speed and direction at 2 m temperature and relative humidity for approximately one year these measurements were supported by twice daily radiosonde releases the densest part of the aws array was located to the south of the wickham mountain range across mount pleasant airfield mpa in northerly flow the array provides a detailed study of the flow downwind of the mountain range the dataset contains several episodes in which the flow downwind of the mountains is accelerated relative to the upwind flow during some of these episodes short lived typically 1 hour periods of unsteady flow separation are observed and these are associated with the formation of rotors aloft such events present a significant hazard to aviation at mpa examination of radiosonde profiles suggests that the presence of a strong temperature inversion at a height similar to the mountain height is a necessary condition for both downwind acceleration and the formation of rotors the data are used to show that the downwind fractional speed up is proportional to the non dimensional mountain height based on upstream near surface winds and a depth averaged brunt vaisala frequency diagnosed from radiosonde data similarly a relationship is established between a quantity that describes the spatial variability of the flow downwind of the mountains and the upstream wind and depth averaged brunt vaisala frequency the dependence of the flow behaviour on the froude number defined in the usual way for two layer shallow water flow and ratio of mountain height to inversion height is presented in terms of a flow regime diagram

greenland ice sheet surface melt extent and trends 1960 2010 observed meteorological data and a high resolution 5 km model were used to simulate greenland ice sheet surface melt extent and trends before the satellite era 1960 79 and during the satellite era through 2010 the model output was compared with passive microwave satellite observations of melt extent for 1960 2010 the average simulated melt extent was 15 5 for the period 1960 72 simulated melt extent decreased by an average of 6 whereas 1973 2010 had an average increase of 13 with record melt extent in 2010 the trend in simulated melt extent since 1972 indicated that the melt extent in 2010 averaged twice that in the early 1970s the maximum and mean melt extents for 2010 were 52 9 5 10 5 km 2 and 28 5 2 10 5 km 2 respectively due to higher than average winter and summer temperatures and lower than average winter precipitation for 2010 the southwest greenland melt duration was 41 60 days longer than the 1960 2010 average while the northeast greenland melt duration was up to 20 days shorter from 1960 to 1972 the melting period with a 10 melt extent decreased by an average of 3 days a 1 after 1972 the period increased by an average of 2 days a 1 indicating an extended melting period for the ice sheet of about 70 days 40 and 30 days in spring and autumn respectively

properties and parameterization of the stable boundary layer over moderate topography numerical simulations are used to investigate the impact on the stable boundary layer of moderate topography with hill heights in some cases comparable to the undisturbed boundary layer depth area averaged properties of the resulting boundary layers which are often highly inhomogeneous are diagnosed the presence of the hills leads to enhanced turbulence and drag and a deepening of the area averaged boundary layers over and above that due to a simple displacement effect the ability of well established formulas for the depth of the boundary layer over homogeneous terrain to predict this deepening is investigated finally the implications of the results for the use in large scale weather and climate prediction models of effective roughness length parameterizations of the effects of hills are discussed while not capturing some of the more detailed effects the simplest approach of using a roughness length independent of stability is found to perform reasonably well in predicting the total surface drag

strength of frozen sand containing tetrahydrofuran hydrate cylindrical samples of frozen sand containing tetrahydrofuran hydrate were tested under uniaxial compression at 267 k and strain rates between 0 000001 s and 0 001 s in the low strain rate region the compressive strength of the samples was higher than that of frozen sand containing ice for example at 267 k and a strain rate of 0 000001 s the peak stress for the frozen sand containing hydrate was about 16 mpa whereas the corresponding value for the frozen sand containing ice was only 10 5 mpa the strain rate dependence of stress for the frozen sand containing hydrate was much smaller than that of frozen sand containing ice so that at higher strain rates the compressive strengths of the two materials become almost the same cold regions such as siberia alaska and the northwest territories of canada contain hydrate deposits within the soil mass which in view of the problems experienced during drilling and excavations are a possible source of energy because of this the physical and mechanical properties of hydrates and frozen soils containing hydrates is being investigated

mechanical properties of sand silt and clay containing tetrahydrofuran hydrate 1 the mechanical behavior of hydrate bearing sediments subjected to large strains has relevance for the stability of the seafloor and submarine slopes drilling and coring operations and the analysis of certain small strain properties of these sediments for example seismic velocities this study reports on the results of comprehensive axial compression triaxial tests conducted at up to 1 mpa confining pressure on sand crushed silt precipitated silt and clay specimens with closely controlled concentrations of synthetic hydrate the results show that the stress strain behavior of hydrate bearing sediments is a complex function of particle size confining pressure and hydrate concentration the mechanical properties of hydrate bearing sediments at low hydrate concentration probably 50 of pore space the behaviorbecomesmoreindependentofstressbecausethehydratescontrolbothstiffnessand strength and possibly the dilative tendency of sediments by effectively increasing interparticle coordination cementing particles together and filling the pore space the cementation contribution to the shear strength of hydrate bearing sediments decreases with increasing specific surface of soil minerals the lower the effective confining stress the greater the impact of hydrate formation on normalized strength

fine scale modeling of the boundary layer wind field over steep topography 1 this paper describes the adaptation of wind fields to steep and complex terrain using fine scale numerical modeling the work is motivated by the need of high resolution flow fields to predict snow transport and snow cover development for avalanche warning purposes applying the nonhydrostatic and compressible atmospheric prediction model advanced regional prediction system arps to steep alpine topography the boundary layer flow was simulated and evaluated against measurements the adaptation of the wind field to steep terrain for specific initial and boundary conditions was simulated the topography used in our study has a length scale of 500 m a typical height of 150 m and maximum slopes of 45 numerical experiments with idealized triangular ridges were conducted to find an adequate model configuration this analysis indicates that a high resolution grid with horizontal spacing of at least 25 m and vertical spacing of 3 m near the surface is necessary to reproduce small scale flow features such as speed up separation and recirculation the onset of flow separation is highly sensitive to initial and boundary conditions slope angle and surface roughness the results of the comparison between the model simulations and measurements on our experimental site show that typical wind field characteristics are well reproduced the simulated wind fields have been used to drive a numerical three dimensional snow drift model which is presented in a companion paper by lehning et al 2008

the effect of the subglacial water pressure on the sliding velocity of a glacier in an idealized numerical model in order to interpret observed short term variations of the sliding velocity of a glacier the effect of a variable subglacial water pressure on the sliding velocity has been studied using an idealized numerical model in particular the transient stages of growing or shrinking water filled cavities at the ice bedrock interface were analysed it was found that the sliding velocity was larger when cavities were growing than when they had reached the steady state size for a given water pressure the smallest sliding velocities occurred while cavities were shrinking when cavitation is substantial a small drop of water pressure below the steady state value e g by 0 5 bar can temporarily cause backward sliding a limiting water pressure at which sliding becomes unstable is derived the consequences of more realistic assumptions than those of the model are discussed

orographic gravity waves close to the nonhydrostatic limit of vertical propagation abstract orographic gravity waves excited by a narrow mountain ridge are investigated with the aid of numerical simulations when the nondimensional mountain half width na u is around 1 n a and u being the brunt vaisala frequency the dimensional half width and the ambient wind speed respectively only part of the gravity wave spectrum excited by the mountain is able to propagate vertically in this case linear theory as well as numerical simulations with low mountains show two wind maxima one at the mountain crest and one in the lee of the mountain as na u is reduced below 1 the wind maximum in the lee weakens and moves farther downstream and the maximum at the crest intensifies rapidly with decreasing na u simulations in which a gap with a level axis is embedded in the mountain ridge demonstrate that the wind perturbations along the gap axis are qualitatively similar to those over the adjacent mountain ridge however their magnitude is substantially lower and the tendency to form a wind max

links between acceleration melting and supraglacial lake drainage of the western greenland ice sheet 1 the impact of increasing summer melt on the dynamics and stability of the greenland ice sheet is not fully understood mounting evidence suggests seasonal evolution of subglacial drainage mitigates or counteracts the ability of surface runoff to increase basal sliding here we compare subdaily ice velocity and uplift derived from nine global positioning system stations in the upper ablation zone in west greenland to surface melt and supraglacial lake drainage during summer 2007 starting around day 173 we observe speedups of 6 41 above spring velocity lasting 40 days accompanied by sustained surface uplift at most stations followed by a late summer slowdown after initial speedup we see a spatially uniform velocity response across the ablation zone and strong diurnal velocity variations during periods of melting most lake drainages were undetectable in the velocity record and those that were detected only perturbed velocities for 1 day suggesting preexisting drainage systems could efficiently drain large volumes of water the dynamic response to melt forcing appears to 1 be driven by changes in subglacial storage of water that is delivered in diurnal and episodic pulses and 2 decrease over the course of the summer presumably as the subglacial drainage system evolves to greater efficiency the relationship between hydrology and ice dynamics observed is similar to that observed on mountain glaciers suggesting that seasonally large water pressures under the ice sheet largely compensate for the greater ice thickness considered here thus increases in summer melting may not guarantee faster seasonal ice flow

seasonal speedup of the greenland ice sheet linked to routing of surface water we use interferometric synthetic aperture radar observations recorded in a land terminating sector of western greenland to characterise the ice sheet surface hydrology and to quantify spatial variations in the seasonality of ice sheet flow our data reveal a non uniform pattern of late summer ice speedup that in places extends over 100 km inland we show that the degree of late summer speedup is positively correlated with modelled runoff within the 10 glacier catchments of our survey and that the pattern of late summer speedup follows that of water routed at the ice sheet surface in late summer ice within the largest catchment flows on average 48 faster than during winter whereas changes in smaller catchments are less pronounced our observations show that the routing of seasonal runoff at the ice sheet surface plays an important role in shaping the magnitude and extent of seasonal ice sheet speedup

evolution of subglacial water pressure along a glacier s length observations from along the length of bench glacier alaska usa show that the subglacial water pressure field undergoes a multiphase transition from a winter mode to a summer mode data were collected at the glacier surface the outlet stream and in a network of 47 boreholes spanning the length of the 7 km long glacier the winter pressure field was near overburden with low magnitude centimeter to meter scale and long period days to weeks variations during a spring speed up event boreholes showed synchronous variations and a slight pressure drop from prior winter values diurnal pressure variations followed the speed up with their onset associated with a glacier wide pressure drop and flood at the terminus stream diurnal variations with swings of up to 80 of overburden pressure were typical of mid summer several characteristics of our observations contradict common conceptions about the seasonal development of the subglacial drainage system and the linkages between subglacial hydrology and basal sliding 1 increased water pressure did not accompany high sliding rates 2 the drainage system showed activity characteristic of the spring season long before abundant water was available on the glacier surface 3 the onset of both spring activity and diurnal variations of the drainage system did not show a spatial progression along the length of the glacier

wind borne redistribution of snow across an antarctic ice rise redistribution of snow by the wind can drive spatial and temporal variations in snow accumulation that may affect the reconstruction of paleoclimate records from ice cores in this paper we investigate how spatial variations in snow accumulation along a 13 km transect across lyddan ice rise antarctica are related to wind borne snow redistribution lyddan ice rise is an approximately two dimensional ridge which rises about 130 m above the surrounding ice shelves local slopes on its flanks never exceed 0 04 despite this very smooth profile there is a pronounced gradient in snow accumulation across the feature accumulation is highest on the ice shelf to the east climatologically upwind of the ice rise and decreases moving westward with the lowest accumulation seen to the west climatologically downwind of the ice rise crest superimposed on this broad scale gradient are large 20 30 localized variations in accumulation on a scale of around 1 km that appear to be associated with local variations in surface slope of less than 0 01 the broad scale accumulation gradient is consistent with estimates of wind borne redistribution of snow made using wind speed observations from three automatic weather stations the small scale variability in accumulation is reproduced quite well using a snow transport model driven by surface winds obtained from an airflow model providing that both the wind shear and static stability of the upwind flow are taken into account we conclude that great care needs to be exercised in selecting ice core sites in order to avoid the possibility of blowing snow transport confounding climate reconstructions

coherent radar ice thickness measurements over the greenland ice sheet we developed two 150 mhz coherent radar depth sounders for ice thickness measurements over the greenland ice sheet we developed one of these using connectorized components and the other using radio frequency integrated circuits rfics both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice we used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers we operated both radar systems on the nasa p 3b aircraft equipped with gps receivers radar data are tagged with gps derived location information and are collected in conjunction with laser altimeter measurements we have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of program for regional climate assessment parca investigations and the north greenland ice core project radar echograms and derived ice thickness data are placed on a server at the university of kansas http tornado rsl ukans edu greenlanddata htm for easy access by the scientific community we obtained good ice thickness information with an accuracy of 10 m over 90 of the flight lines flown as a part of the parca initiative in this paper we provide a brief description of the system along with samples of data over the interior along the 2000 m contour line in the south and from a few selected outlet glaciers

basal drainage system response to increasing surface melt on the greenland ice sheet surface meltwater reaching the bed of the greenland ice sheet imparts a fundamental control on basal motion sliding speed depends on ice bed coupling dictated by the configuration and pressure of the hydrologic drainage system in situ observations in a four site transect containing 23 boreholes drilled to greenland s bed reveal basal water pressures unfavorable to water draining conduit development extending inland beneath deep ice this finding is supported by numerical analysis based on realistic ice sheet geometry slow meltback of ice walls limits conduit growth inhibiting their capacity to transport increased discharge key aspects of current conceptual models for greenland basal hydrology derived primarily from the study of mountain glaciers appear to be limited to a portion of the ablation zone near the ice sheet margin

rapid glacier sliding reverse ice motion and subglacial water pressure during an autumn rainstorm measurements of basal water pressure from 15 boreholes located at both local tens of meters and regional kilometers length scales were used to elucidate the pressure sliding relationship during an autumn rapid motion event on bench glacier alaska usa the 8 day event had two distinct phases each with a ten fold speed up with respect to winter velocity the water pressure in all 15 boreholes varied synchronously during the speed up the first phase of rapid sliding began after a peak in basal water pressure and continued while the pressure was elevated and stable or decreasing the second phase of rapid sliding occurred when the basal water pressure was low but increasing and terminated before the pressure peaked pressure and velocity do not appear unrelated but the pressure sliding relationship was not consistently linked to increasing decreasing or a critical water pressure the pressure variations and sliding accelerations are a response to a warm rainstorm although equally large input events occurred in weeks prior with no apparent response drainage system evolution therefore appears to play a key role in both the acceleration and the pressure velocity relationship basal cavity dynamics are likely responsible for three episodes of reverse up valley motion observed after enhanced sliding

seasonal speedup along the western flank of the greenland ice sheet it has been widely hypothesized that a warmer climate in greenland would increase the volume of lubricating surface meltwater reaching the ice bedrock interface accelerating ice flow and increasing mass loss we have assembled a data set that provides a synoptic scale view spanning ice sheet to outlet glacier flow with which to evaluate this hypothesis on the ice sheet these data reveal summer speedups 50 to 100 consistent with but somewhat larger than earlier observations the relative speedup of outlet glaciers however is far smaller 15 furthermore the dominant seasonal influence on jakobshavn isbrae s flow is the calving front s annual advance and retreat with other effects producing outlet glacier speedups an order of magnitude larger seasonal melt s influence on ice flow is likely confined to those regions dominated by ice sheet flow

surficial glaciology of jakobshavns isbrae west greenland part i surface morphology jakobshavns isbrae is a large fast moving ice stream outlet glacier in west greenland which ends at a floating calving front it drains about 6 5 of the area of the greenland ice sheet studies of its surface morphology are described in this paper the surface is relatively steep 0 01 0 03 and the thickness is large up to 2600 m along the center line clarke and echelmeyer 1989 indicating very high driving stresses 200 300 kpa the ice stream is about 6 km wide and 85 90 km long all of which is in an area of surface melting the base of the ice stream and of much of the drainage area is below sea level marginal crevasse zones have a width on the order of the width of the ice stream itself unique surficial features are ice blisters and lakes the latter have a sequence of ogive like features on their floating ice cover which can be used to determine velocity there is a pinning point near the terminus which may act as a stabilizing influence possibly playing a role in halting at least temporarily a recent retreat of the terminus ice thickness estimates at the terminus lead to a flux which is less than previously assumed by others e g bindschadler 1984 pelto and others 1989 when estimating jakobshavns isbrae s drainage basin to be nearly in balance the driving stresses on jakobshavns isbrae are an order of magnitude higher than those of the ice streams of west antarctica its crevasse patterns are much less localized its relatively unconfined terminus is more comparable to that of relatively unbuttressed ice streams such as pine island and thwaites glaciers than it is to other west antarctic ice streams which terminate in large confined ice shelves

measurements of the near surface flow over a hill the near surface flow over a hill with moderate slope and height comparable with the boundary layer depth is investigated through field measurements of the mean flow at 2 m surface pressure and turbulent momentum flux divergence between 8 and 15 m the measurements were made along an east west transect across the hill tighvein height 458 m approximate width 8 km on the isle of arran south west scotland during two separate periods each of around three weeks duration radiosonde ascents are used to determine the variation of a froude number fl u nl where u is the wind speed at the middle layer height hm n is the mean brunt vaisala frequency below this height and l is a hill length scale measurements show that for moderately stratified flows for which fl 0 25 a minimum in the hill induced surface pressure perturbation occurs across the summit and this is accompanied by a maximum in the near surface wind speed in the more strongly stratified case fl 0 25 the pressure field is more asymmetric and the lee slope flow is generally stronger than on the windward slope such a flow pattern is qualitatively consistent with that predicted by stratified linear boundary layer and gravity wave theories the near surface momentum budget is analysed by evaluating the dominant terms in a bernoulli equation suitable for turbulent flow measurements during periods of westerly flow are used to evaluate the dominant terms and the equation is shown to hold to a reasonable approximation on the upwind slope of the hill and also on the downwind slope away from the summit immediately downwind of the summit however the bernoulli equation does not hold possible reasons for this such as non separated sheltering and flow separation are discussed copyright 2002 royal meteorological society

atmospheric gravity waves and the rissaga phenomenon the atmospheric forcing of large sea level oscillations up to 1 5 m amplitude with about a 10 min period in some bays and inlets of the balearic islands is described using simultaneous measurements of pressure and sea level elevation in the port of ciutadella at the end of a large and narrow inlet during 5 7 july 1989 the influence of atmospheric pressure oscillations on these large sea level variations locally known as rissaga is investigated coherence and cross correlation functions reveal that the large oscillations in the inlet are associated with and probably forced by a 10 min gravity wave in the atmosphere the effect of longer period waves on the inlet is seen in the well known inverted barometric effect forcing small oscillations in the water level of just some centimetres these latter waves have a period of around 50 min and an unusually large amplitude of some 3 mb the phase spectrum between sea level and pressure suggests that the inlet may behave as a damped harmonic oscillator which during rissaga is externally forced by the atmospheric pressure oscillations

compressive strength and creep behavior of hydrate consolidated sand cylindrical samples of sand consolidated with tetrahydrofuran hydrate were tested for their compressive strength and creep behavior under uniaxial compression the samples were 15 cm in length and 7 5 cm in diameter and were tested at 10 c the results when combined with our previous measurements on similar samples at 6 c show that the material becomes stronger by about 10 with decrease in temperature otherwise the slopes of the peak stress strain rate curves are the same these results are similar to those of sand consolidated with ice except that in the latter case the increase in strength over the same temperature range is about 30 furthermore the slope of the peak stress strain rate curve for the hydrate consolidated sand is almost zero whereas for the ice consolidated sand it is quite steep consequently at strain rates below 10 5 s 1 the hydrate consolidated sand is stronger whereas at strain rates above 10 5 s 1 the ice consolidated sand is the stronger material noticeable diff

stably stratified shear flow over low hills summary an analysis is presented for the mean flow over hills with low slopes for various kinds of stable stratification in the approach flow the froude number f based on the height is always greater than unity the analysis is based on dividing up the flow into different regions with different dynamics we concentrate on the flow in the inviscid outer region of the flow it is shown that in the lower part of the outer region the middle layer where the shear is large the upwind velocity profile and the shape of the hill rather than the upwind density profile largely determine the vertical perturbation velocity in the upper layer the structure of the flow depends largely on the form of the upwind density profile for example this largely determines the location of maximum surface wind new solutions for stable conditions are obtained when there is a constant density gradient when n ub 0 z2 and when there is an elevated inversion layer in the first and third of these cases we compare the results with the hydrostatic approximation and also obtain new general formulae for the near surface winds an approximate formula is developed for surface winds at arbitrary values of f 1 for uniform stable stratification

evolution of glacial flow and drainage during the ablation season proglacial discharge is controlled by the geometry of the glacial drainage network and by meltwater processes the glacial drainage systems of some alpine glaciers have been characterised using a model based on a proglacial discharge analysis in this paper we apply high frequency systemic analysis to data collected from the baounet glacier french alps during two successive ablation seasons our approach is based on an analysis of the evolution of daily recession curves during the ablation period the observed data are fitted by a single coefficient dependant recession law changes in are compared to variations in the daily discharge amplitude and the time lag between air temperature and proglacial discharge the changes in the values do not appear to be related to changes in the time lag and the amplitude of the daily discharge there were significant variations in the coefficient during the two ablation periods studied here even if there was no time lag or the daily discharge amplitude change high values of correspond to high drainage velocities therefore increases in values can be used as an indicator of the evolution of the glacier s internal drainage network

modeling channelized and distributed subglacial drainage in two dimensions 1 we present a two dimensional glacier drainage system model glads that couples distributed and channelized subglacial water flow distributed flow occurs through linked cavities that are represented as a continuous water sheet of variable thickness channelized flow occurs through rothlisberger channels that can form on any of the edges of a prescribed unstructured network of potential channels water storage is accounted for in an englacial aquifer and in moulins which also act as point sources of water to the subglacial system solutions are presented for a synthetic topography designed to mimic an ice sheet margin for low discharge all the flow is accommodated in the sheet whereas for sufficiently high discharge the model exhibits a channelization instability which leads to the formation of a self organized channel system the random orientation of the network edges allows the channel system geometry to be relatively unbiased in contrast to previous structured grid based models under steady conditions the model supports the classical view of the subglacial drainage system with low pressure regions forming around the channels under diurnally varying input water flows in and out of the channels and a rather complex spatiotemporal pattern of water pressures is predicted we explore the effects of parameter variations on the channel system topology and mean effective pressure the model is then applied to a mountain glacier and forced with meltwater calculated by a temperature index model the results are broadly consistent with our current understanding of the glacier drainage system and demonstrate the applicability of the model to real settings

subglacial basins their origin and importance in glacial systems and landscapes closed topographic basins are found beneath contemporary ice masses and within the footprint of former ice masses in all glaciated regions we present the first integrated review of subglacial basin occurrence and formation and the implications of such basins for glaciological processes and the evolution of landscape our purpose is to motivate research in areas where understanding of basin origin and process significance is weak basins on the order of 10 102 m deep and 102 103 m long are produced by glacial erosion of subglacial rock and or sediment and are known as overdeepenings outlet and valley glaciers can overdeepen their beds far below sea level or local fluviatile base level larger basins typically in ice sheet contexts may have a pre glacial usually tectonic origin subglacial basins are important glaciologically because they require ice water and sediment to ascend an adverse subglacial slope in order to exit the glacial system the efficiency of which is dependent upon the gradient of the adverse slope and that of the ice surface basins thus influence subglacial drainage system morphology and transmissivity the thickness and distribution of basal ice and sediment layers and the mechanisms and dynamics of ice flow adverse gradients that exceed 11 times that of the ice surface may even permit the formation of subglacial lakes we speculate that in comparison to ice masses with few or no subglacial basins those with numerous or very large basins may respond to climatic changes with unexpected vigour in addition erosion rates and transport pathways of water and sediment through the glacial system and the expression of these processes in the sediment and landform record may be unexpectedly complex further our review shows that in a warming climate ice masses resting on adverse slopes will be vulnerable to rapid and potentially catastrophic retreat new lakes in subglacial basins exposed by mountain glacier retreat will present an increasing hazard and subglacial lakes may drain catastrophically on even longer time scales we speculate that the glacial excavation and post glacial filling of basins in mountainous regions should contribute importantly to climate related changes in isostasy and relief although the controls on overdeepening and their influence on other glacial and landscape processes remain uncertain we hypothesise that overdeepened glacial systems reflect an equilibrium ice bed geometry that maximises the efficiency of ice discharge improved understanding of overdeepening processes especially overdeepened bed hydrology is therefore necessary to understand fully the dynamic behaviour of valley and outlet glaciers and thus the fate of earth s largest ice masses

decadal slowdown of a land terminating sector of the greenland ice sheet despite warming ice flow along land terminating margins of the greenland ice sheet gis varies considerably in response to fluctuating inputs of surface meltwater to the bed of the ice sheet such inputs lubricate the ice bed interface transiently speeding up the flow of ice greater melting results in faster ice motion during summer but slower motion over the subsequent winter owing to the evolution of an efficient drainage system that enables water to drain from regions of the ice sheet bed that have a high basal water pressure however the impact of hydrodynamic coupling on ice motion over decadal timescales remains poorly constrained here we show that annual ice motion across an 8 000 km 2 land terminating region of the west gis margin extending to 1 100 m above sea level was 12 slower in 2007 14 compared with 1985 94 despite a 50 increase in surface meltwater production our findings suggest that over these three decades hydrodynamic coupling in this section of the ablation zone resulted in a net slowdown of ice motion not a speed up as previously postulated increases in meltwater production from projected climate warming may therefore further reduce the motion of land terminating margins of the gis our findings suggest that these sectors of the ice sheet are more resilient to the dynamic impacts of enhanced meltwater production than previously thought

underlying structures in orographic flow an analysis of measurements made on the isle of arran in a recent field experiment on the isle of arran scotland measurements of surface pressure and near surface wind fields were made along a roughly west east transect on the hill tighvein a principal components analysis of these measurements has been made to determine any significant atmospheric modes of variability and this objective method successfully identifies several physically meaningful flows copyright 2006 royal meteorological society

identification and control of subglacial water networks under dome a antarctica 1 subglacial water in continental antarctica forms by melting of basal ice due to geothermal or frictional heating subglacial networks transport the water from melting areas and can facilitate sliding by the ice sheet over its bed subglacial water flow is driven mainly by gradients in overburden pressure and bed elevation we identify small median 850 m water bodies within the gamburtsev subglacial mountains in east antarctica organized into long 20 103 km coherent drainage networks using a dense 5 km grid of airborne radar data the individual water bodies are smaller on average than the water bodies contained in existing inventories of antarctic subglacial water and most are smaller than the mean ice thickness of 2 5 km reflecting a focusing of basal water by rugged topography the water system in the gamburtsev subglacial mountains reoccupies a system of alpine overdeepenings created by valley glaciers in the early growth phase of the east antarctic ice sheet the networks follow valley floors either uphill or downhill depending on the gradient of the ice sheet surface in cases where the networks follow valley floors uphill they terminate in or near plumes of freeze on ice indicating source to sink transport within the basal hydrologic system because the ice surface determines drainage direction within the bed constrained network the system is bed routed but surface directed along flow variability in the structure of the freeze on plumes suggests variability in the networks on long 10s of ka timescales possibly indicating changes in the basal thermal state

seasonal variations in greenland ice sheet motion inland extent and behaviour at higher elevations abstract we present global positioning system observations that capture the full inland extent of ice motion variations in 2009 along a transect in the west greenland ice sheet margin in situ measurements of air temperature and surface ablation and satellite monitoring of ice surface albedo and supraglacial lake drainage are used to investigate hydrological controls on ice velocity changes we find a strong positive correlation between rates of annual ablation and changes in annual ice motion along the transect with sites nearest the ice sheet margin experiencing greater annual variations in ice motion 15 18 than those above 1000 m elevation 3 8 patterns in the timing and rate of meltwater delivery to the ice bed interface provide key controls on the magnitude of hydrologically forced velocity variations at each site in the lower ablation zone the overall contribution of variations in ice motion to annual flow rates is limited by evolution in the structure of the subglacial drainage system at sites in the upper ablation zone a shorter period of summer melting and delayed establishment of a hydraulic connection between the ice sheet surface and its bed limit the timeframe for velocity variations to occur our data suggest that land terminating sections of the greenland ice sheet will experience increased dynamic mass loss in a warmer climate as the behaviour that we observe in the lower ablation zone propagates further inland findings from this study provide a conceptual framework to understand the impact of hydrologically forced velocity variations on the future mass balance of land terminating sections of the greenland ice sheet

transmission of basal variability to a glacier surface 1 transmission of basal variability to a glacier surface is investigated using analytical models for a linearly viscous medium the three dimensional transient response of the surface to both bedrock undulations and spatial variations in basal slipperiness for perturbations of arbitrary wavelengths is determined using perturbation methods both information transfer toward the surface and lateral transmission of horizontal stresses are strongly affected by the slip ratio that is the ratio of basal sliding to deformational velocity for any mean bedrock slope and above a minimum value of slip ratio the amplitude transfer of bedrock undulations toward the surface has a local maximum at undulation span corresponding to about 3 8 times the mean ice thickness the transmission of basal variability to a glacier surface increases quite significantly with increasing slip ratio this explains why the surfaces of fast flowing ice streams are more undulating than the slower moving bordering areas at slip ratios higher than about 100 the flow of glaciers and ice sheets becomes nonlocal in the sense that surface velocities and buildup and propagation of surface undulations cannot be calculated accurately on the basis of local thickness and slope using linearized long wave theories at these slip ratios instead of the more accurate arbitrary wavelength theory gives estimates of decay times that are an order of magnitude too small and phase velocities several times too large the problem of the propagation and decay of small amplitude surface undulations on glaciers in three dimensions is solved small amplitude surface waves on glaciers are strongly diffusive and dispersive redistribution of mass on ice sheets and glaciers is a diffusion process and it is misleading albeit not mathematically incorrect to describe the reaction of glaciers to surface perturbations in terms of a wave propagation

observed near storm environment variations across the southern cumberland plateau system in northeastern alabama abstractthe sand mountain and lookout mountain plateaus in northeastern alabama have been established as a regional relative maximum in tornadogenesis reports within the southeastern united states

gravity wave drag produced by madeira the gravity wave drag exerted on the atmosphere by an isolated three dimensional mountain is here evaluated from observational data the representation of such drag in numerical weather prediction models is known to be of considerable importance and this study provides some rare field measurements for input and verification of parametrization schemes the results of a microbarograph experiment performed in the spring of 1993 on the island of madeira are presented the data obtained with thermostatically adjusted microbarographs located on the two sides of the island are analysed in terms of the gravity wave drag produced by the mountain the time series obtained show a high degree of coherency consistent with the idea that they represent a mesoscale signal not significantly affected by small scale perturbations the pressure signal at each station shows a superposition of different oscillations including a 5 7 day synoptic oscillation a 12 hour tide and an irregular oscillation with an amplitude of about 2 hpa which can be associated with gravity wave drag the pressure differences across the island essentially contain the gravity wave drag signal which somewhat surprisingly correlates rather well with linear estimates the estimated drag has an average value similar to those observed in the pyrex experiment despite the lower wind speeds typical of madeira

evolution of the subglacial drainage system beneath the greenland ice sheet revealed by tracers predictions of the greenland ice sheet s response to climate change are limited in part by uncertainty in the coupling between meltwater lubrication of the ice sheet bed and ice flow1 2 3 this uncertainty arises largely from a lack of direct measurements of water flow characteristics at the bed of the ice sheet previous work has been restricted to indirect observations based on seasonal and spatial variations in surface ice velocities4 5 6 7 and on meltwater flux8 here we employ rhodamine and sulphur hexafluoride tracers injected into the drainage system over three melt seasons to observe subglacial drainage properties and evolution beneath the greenland ice sheet up to 57 km from the margin tracer results indicate evolution from a slow inefficient drainage system to a fast efficient channelized drainage system over the course of the melt season further inland evolution to efficient drainage occurs later and more slowly an efficient routing of water was established up to 41 km or more from the margin where the ice is approximately 1 km thick overall our findings support previous interpretations of drainage system characteristics thereby validating the use of surface observations as a means of investigating basal processes

glaciohydraulic supercooling a freeze on mechanism to create stratified debris rich basal ice i field evidence debris laden ice accretes to the base of matanuska glacier alaska u s a from water that supercools while flowing in a distributed drainage system up the adverse slope of an overdeepening frazil ice grows in the water column and forms aggregates while other ice grows on the glacier sole or on substrate materials sediment is trapped by this growing ice forming stratified debris laden basal ice growth rates of 0 1 m a 1 of debris rich basal ice are possible the large sediment fluxes that this mechanism allows may have implications for interpretation of the widespread deposits from ice that flowed through other overdeepenings including heinrich events and the till sheets south of the laurentian great lakes

rerouting of subglacial water flow between neighboring glaciers in west greenland investigations of the greenland ice sheet s subglacial hydrological system show that the connectivity of different regions of the system influences how the glacier velocity responds to variations in surface melting here we examine whether subglacial water flow paths can be rerouted beneath three outlet glaciers in the ablation zone of western greenland we use lamont doherty and center for remote sensing of ice sheets of university of kansas cresis ice penetrating radar data to create a new ice thickness map we then use a simple subglacial water flow model to examine whether flow paths can be rerouted and identify the topographic conditions that are sensitive to subglacial rerouting by varying water pressures within an observationally constrained range we show that moderate changes in pressure can cause flow paths to reroute and exchange water from one subglacial catchment to another flow across subglacial overdeepenings is particularly sensitive to rerouting these areas have low hydraulic gradients driving flow so subtle water pressure variations have a strong influence on water flow direction based on correlations between water flow paths and ice velocity changes we infer that water piracy between neighboring catchments can result in a different spatial pattern of hydrologically induced ice velocity speedup depending on the amount and timing of surface melt the potential for subglacial water to reroute across different catchments suggests that multiple hydrographs from neighboring glaciers are likely necessary to accurately ascertain melt budgets from proglacial point measurements the relationship between surface runoff ice dynamics and proglacial discharge can be altered by rerouting of subglacial water flow within and across outlet glaciers

the relationship between subglacial water pressure and velocity of findelengletscher switzerland during its advance and retreat findelengletscher switzerland advanced about 250 m between 1979 and 1985 and retreated thereafter subglacial water pressure surface velocity and surface strain rate were determined at several sites the measurements were made early in the melt seasons of 1980 1982 1985 and 1994 and in the autumn of 1983 and the winter of 1984 changes of surface geometry were assessed from aerial photographs the estimated basal shear stress changed little between 1982 and 1994 nevertheless large changes in the relationship of subglacial water pressure and surface velocity were observed which cannot be reconciled with the most commonly used sliding law unless it is modified substantially consideration of possible reasons indicales that a change in the subglacial drainage system occurred probably involving a change in the degree of cavity interconnection isolated cavities damp the variations in sliding velocity that normally result from changes in water pressure because the pressure in isolated cavities decreases as the sliding speed increases in contrast by transmitting water pressure fluctuations to a larger area of the bed interconnected cavities amplify the effect of water pressure fluctuations on sliding speed thus we suggest that an observed decrease in velocity for a given water pressure between 1982 and 1994 was a consequence of a decrease in the interconnectedness of the subglacial cavity system

melt induced speed up of greenland ice sheet offset by efficient subglacial drainage fluctuations in surface melting are known to affect the speed of glaciers and ice sheets but their impact on the greenland ice sheet in a warming climate remains uncertain although some studies suggest that greater melting produces greater ice sheet acceleration others have identified a long term decrease in greenland s flow despite increased melting here we use satellite observations of ice motion recorded in a land terminating sector of southwest greenland to investigate the manner in which ice flow develops during years of markedly different melting although peak rates of ice speed up are positively correlated with the degree of melting mean summer flow rates are not because glacier slowdown occurs on average when a critical run off threshold of about 1 4 centimetres a day is exceeded in contrast to the first half of summer when flow is similar in all years speed up during the latter half is 62 16 per cent less in warmer years consequently in warmer years the period of fast ice flow is three times shorter and overall summer ice flow is slower this behaviour is at odds with that expected from basal lubrication alone instead it mirrors that of mountain glaciers where melt induced acceleration of flow ceases during years of high melting once subglacial drainage becomes efficient a model of ice sheet flow that captures switching between cavity and channel drainage modes is consistent with the run off threshold fast flow periods and later summer speeds we have observed simulations of the greenland ice sheet flow under climate warming scenarios should account for the dynamic evolution of subglacial drainage a simple model of basal lubrication alone misses key aspects of the ice sheet s response to climate warming

a background investigation of tornado activity across the southern cumberland plateau terrain system of northeastern alabama abstractthe effects of terrain on tornadoes are poorly understood efforts to understand terrain effects on tornadoes have been limited in scope typically examining a small number of cases with li

seasonal changes in ice sheet motion due to melt water lubrication abstract a numerical model is used to calculate how the motion of an idealized ice sheet margin is affected by the subglacial drainage of melt water from its surface the model describes the evolution of the drainage system and its coupling with ice flow through a sliding law that depends on the effective pressure the results predict ice acceleration during early summer when the inefficient drainage system is temporarily overwhelmed the growth of a more efficient drainage system leads to a subsequent slowdown of the ice very close to the margin but high water pressure and ice velocity are maintained through much of the summer further inland annual mean ice velocity increases with the total quantity of melt water except close to the margin where it is almost insensitive to the amount of melting short term variability of melt water input leads to rapid changes in ice velocity that result in a slight increase in the mean velocity relative to a smoother input linked cavity and poroelastic models for the distributed drainage system are compared and their relative merits discussed two different sliding laws are considered and the need for a holistic description of hydraulically controlled sliding is highlighted

response of glacier basal motion to transient water storage the speed of a glacier is affected most by sudden jumps in the water supply to the glacier but it goes back to previous levels if high water inputs are sustained because the glacier s plumbing system adjusts

progress in stability analysis of submarine slopes considering dissociation of gas hydrates gas hydrates have the potential to be a new energy source and a submarine geohazard though researchers generally agree about the association between gas hydrate dissociation and submarine slope failures the processes and mechanism of submarine slope failure caused by gas hydrate dissociation are not clearly understood in the last few years some authors have tried to analyse submarine slope stability by considering the existence and dissociation of gas hydrate and a few researchers have presented quantitative models this paper presents a review of the various causes of submarine slope failures associated with gas hydrate dissociation also analysis models of submarine slope stability associated with gas hydrate dissociation that are documented from the literatures including the infinite slope model wedge model slump and retrogressive failure model are interpreted and illustrated respectively

direct observations of evolving subglacial drainage beneath the greenland ice sheet simultaneous observations of moulins and boreholes in western greenland show that water delivery to the base of the ice sheet via moulins affects short term ice velocity fluctuations but not late season ice velocity decelerations increased meltwater delivery to the base of the greenland ice sheet will increase the ice sheet velocity accelerating its inevitable rush towards the ocean and subsequent sea level rise or will it debate on this topic is at the forefront of cryospheric research but has been hampered by the lack of simultaneous observations of hydraulic head in moulins vertical shafts that deliver water to the base of the ice sheet and boreholes which monitor basal water pressure lauren andrews and colleagues now provide these observations from a small region in western greenland and reveal that water delivery by moulins into channelized basal flow is indeed linked to short term fluctuations in ice velocity late season decelerations in ice velocity however seem to be controlled by changes in unchannelized flow rather than any shifts in the moulin system seasonal acceleration of the greenland ice sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed1 2 via crevasses and moulins vertical conduits connecting supraglacial water to the bed of the ice sheet as the melt season progresses the subglacial hydrologic system drains supraglacial meltwater more efficiently1 2 3 4 decreasing basal water pressure4 and moderating the ice velocity response to surface melting1 2 however limited direct observations of subglacial water pressure4 5 6 7 mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby 0 3 2 kilometres away boreholes indicating that moulins connect to an efficient channelized component of the subglacial hydrologic system which exerts the primary control on diurnal and multi day changes in ice velocity our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the paakitsoq region of western greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin connected channels to convey supraglacial melt instead these observations suggest that decreasing late season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt season dynamics affect ice velocity beyond the conclusion of the melt season8 9 10

gas hydrates in permafrost distinctive effect of gas hydrates and ice on the geomechanical properties of simulated hydrate bearing permafrost sediments

high sensitivity of subglacial hydrological pathways in antarctica to small ice sheet changes 1 the flow of water beneath the antarctic ice sheet has recently been shown to involve drainage from and exchange between subglacial lakes with flow routing over distances of 100km as subglacial water is known to be important to ice sheet dynamics changes to the flow routing of basal water may yield an ice sheet response here we show that the directions of subglacial hydrological flowpaths are highly sensitive to the ice sheet surface topography at certain sites in antarctica surface elevation changes of between 5 and 15 m are found to be sufficient to produce significant changes to the subglacial water flowpaths which affect the potential drainage routes of large subglacial lakes we identify the reasons for this high sensitivity and recognise several locations in east antarctica where changes to the drainage regime could have important effects on large scale basal water distribution

stagnation of ice stream c west antarctica by water piracy the dynamic behavior of the west antarctic ice sheet is of interest because of the possibility that it may change and cause rapid sea level rise attention is focused on the fast moving and rapidly responding ice streams that drain the ice sheet one of these ice stream c largely stopped about a century ago and some models for this shutdown postulate negative feedbacks that would tend to stabilize the ice sheet here new data are presented indicating that the slowdown of the ice stream is restricted to its lower part and occurred because of loss of lubrication on localized sticky spots at the bed of the ice stream the increased friction probably arises from a topographic accident of the glacier bed that has directed lubricating water to the neighboring ice stream b together with slow drawdown of the ice sheet rather than from any general stabilizing feedbacks

a case study of lee waves over the lake district in northern england the structure and intensity of stationary orographically forced over the lake district and northern pennines on he afternoon of 26 november 1991 is determined through a combination of field observations and high resolution numerical modelling observations were provided by the meteorological office s c130 aircraft at four different levels and by five radisondes of differing ascent rates released in rapid succession from an observing station located on the cumbrian coast vertical velocity was inferred from both of these data sources and with the help of satellite imagery it was possible to build up a comprehensive picture of the lee wave structure in addition a non hydrostatic primitive equation model was run with a 2 km grid covering a 180 km square region of the northern pennines in an attempt to simulate the gravity wave motion observed the model was initialized with an upstream profile formed by merging a descent profile obtained from the aircraft with one from the sondes the model orography was defined using a terrain height data set with 500 m resolution and with some smoothing to the model grid after about four hours of integration the model achieves a quasi steady state with strong lower tropospheric lee waves verification on the model response was sought by comparing the vertical velocity in the model along the paths of the aircraft and sondes with that actually deduced from the observations the overall agreement of the simulated amplitude and phase of the lee waves with that observed is very encouraging as found in a different case study shutts 1992 the success of the model in predicting lee wave motion gives strong support for the use of these models for the development of gravity wave drag parametrization schemes the domain averaged surface frictional stress was found to be much larger than the wave drag and consistent with the high roughness lengths likely in the case of mountainous terrain

air flow over a two dimensional hill studies of velocity speed up roughness effects and turbulence wind tunnel measurements have been made of the streamwise mean and turbulent velocities over a rough bell shaped two dimensional hill with height h and maximum slope 0 26 placed in a neutrally stable boundary layer of thickness 10 h and roughness length zo 0 02 h close agreement is found between the mean velocity and predictions obtained from taylor s 1977 computational model and jackson and hunt s 1975 analytical linearized model for locations at or upwind of the hill top but not in the wake examination of the models shows that the shear stresses are only important in an inner region close to the hill surface so that as suggested by jackson and hunt 1975 the perturbation to the mean flow outside this region is essentially inviscid the theory shows that over very rough surfaces such as wooded or urban terrain the height of this inner region can be of the same order as the height of the roughness elements so that in our experiments no measurements could be made in this region in a second experiment flow over a smooth hill on a rough surface was studied the additional increase of wind speed over the hill top can be estimated by assuming a linear superposition of the velocity changes produced by the changes in elevation and in surface roughness in this case rough to smooth in the lee of a hill however the change in roughness significantly alters the flow with flow separation being suppressed and here a linear superposition is not appropriate finally we consider why observed changes in turbulence structure close to the surface differ from those well above the surface calculations of these changes based on the simple theoretical arguments of equilibrium shear layers and rapidly distorted turbulent flows agree well with turbulence measurements in wind tunnels and in the field

greenland ice sheet mass balance reconstruction part iii marine ice loss and total mass balance 1840 2010 abstractgreenland ice sheet mass loss to the marine environment occurs by some combination of iceberg calving and underwater melting referred to here as marine ice loss lm this study quantifies the relation between lm and meltwater runoff r at the ice sheet scale a theoretical basis is presented explaining how variability in r can be expected to govern much of the lm variability over annual to decadal time scales it is found that r enhances lm through three processes 1 increased glacier discharge by ice warming softening and basal lubrication sliding 2 increased calving susceptibility through undercutting glacier front geometry and reducing ice integrity and 3 increased underwater melting from forcing marine convection applying a semiempirical lm f r parameterization to a surface mass balance reconstruction enables total ice sheet mass budget closure over the 1840 2010 period the estimated cumulative 171 yr net ice sheet sea level contribution is 25 10 mm the rise punctuated by periods

combined measurements of subglacial water pressure and surface velocity of findelengletscher switzerland conclusions about drainage system and sliding mechanism interpretation des mesures de pression dans les forages et des vitesses superficielles la comparaison avec les modeles suggere que l on a neglige le frottement du aux debris dans les modeles et qu il n existe jamais de chenaux cylindriques rectilignes