atmospheric rotors and severe turbulence in a long deep valley abstractthe conceptual model of an atmospheric rotor is reexamined in the context of a valley using data from the terrain induced rotor experiment t rex conducted in 2006 in the southern sierra nevada and owens valley california all t rex cases with strong mountain wave activity have been investigated and four of them iops 1 4 6 and 13 are presented in detail their analysis reveals a rich variety of rotorlike turbulent flow structures that may form in the valley during periods of strong cross mountain winds typical flow scenarios in the valley include elevated turbulence zones downslope flow separation at a valley inversion turbulent interaction of in valley westerlies and along valley flows and highly transient mountain waves and rotors the scenarios can be related to different stages of the passage of midlatitude frontal systems across the region the observations from owens valley show that the elements of the classic rotor concept are modulated and at times almost completely offset

the owles iop2b lake effect snowstorm dynamics of the secondary circulation abstractintense lake effect snowfall results from a long lake axis parallel llap precipitation band that often forms when the flow is parallel to the long axis of an elongated body of water such as lake ontario the intensity and persistence of the localized precipitation along the downwind shore and farther inland suggests the presence of a secondary circulation that helps organize such a band and maintain it for some time as the circulation is advected inland unique airborne vertical plane dual doppler radar data are used here to document this secondary circulation in a deep well organized llap band observed during intensive observing period iop 2b of the ontario winter lake effect systems owles field campaign the circulation centered on a convective updraft intensified toward the downwind shore and only gradually weakened inland the question arises as to what sustains such a circulation in the vertical plane across the llap band wrf model simulations indicate that the primary llap band a

a high resolution dual doppler technique for fixed multiantenna airborne radar a new technique is presented for the analysis of data collected by pairs of aircraft ac mounted fixed radar beams directed 30deg apart leading to two dimensional wind field syntheses in the horizontal and vertical planes the technique was applied to the 95 ghz wyoming cloud radar wcr configurations aboard the university of wyoming king air and the national center for atmospheric research c 130 research ac the construction of the data grid allows to closely follow the ac flight attitude and thus the beam scanned surfaces with a resolution between 30 and 45 m a weighted least squares method is employed to solve the velocity inverse decomposition problem for every valid grid cell the effect on the solution of the nonmeasured cross plane wind component is minimized by using an external estimate of the ambient wind the error sources are discussed and maximum expected values for the velocity field errors are provided with reference to the radar system the data collection process and specific aspects of the target under investigation these uncertainties 1 2 m s are quantified for two case studies that show the potential of the technique in retrieving the kinematic field in transects of cumulus and stratus clouds the technique produces smooth fine scale kinematic fields although each velocity point is the result of an independent calculation and no binding constraint is imposed among neighboring points as is done in other dual doppler techniques

numerical and observational case study of a deep adriatic bora a detailed analysis of bora winds is presented that were observed on 28 march 2002 in the north eastern part of the adriatic sea very high resolution numerical simulations are compared with airborne surface and balloon observations the key instrument for the verification of the numerical results is an aerosol backscatter lidar on board the dlr falcon research aircraft the high spatial resolution of the model and of the observational dataset allows several small scale aspects of the bora winds to be explored the study emphasises the great impact of boundary layer effects including convective mixing and surface friction on the structure of the bora flow the numerical model reveals that the diurnal cycle of the planetary boundary layer causes a diurnal variation of the gravity wave amplitude and consequently a variation in the bora strength the downslope flow is strongest during the night time when the impinging air mass is stably stratified and exhibits a low level jet the bora strength weakens during the daytime due to the evolution of a neutrally stratified convective boundary layer lidar observations and simulations both suggest that the observed bora winds are driven by the dynamics of orographic gravity waves in the deep north easterly cross mountain flow steeply amplified but non breaking gravity waves as well as trapped lee waves are found throughout the troposphere vertical separation of the boundary layer over the steep leeward terrain slope prevents the bora flow from reaching the coast as a result the flow is kept aloft in a series of at least three unsteady trapped lee waves before it reattaches to the surface over the sea downstream of the mountains no clear evidence is found for the existence of low level rotors underneath these lee waves the ability of the model to capture the trapped waves is found to depend on the horizontal model resolution bora winds are identified as jets emanating from several mountain gaps the physical mechanism for the gap flow formation is boundary layer separation controlled by surface friction flow separation is more effective over the highest terrain where a broad wake forms downstream with weak winds compared to the mountain gaps where the strong bora flow eventually reattaches to the surface the strongest gap jet belongs to the vratnik pass upstream of the town senj and forms the primary shear line in the northern adriatic with a corresponding potential vorticity pv banner the numerical model reveals several secondary orographic pv banners and embedded pv filaments the vertical pv distribution exhibits a two layer structure with pv anomalies at lower higher elevations originating from mountain gaps isolated mountain peaks surface friction has a minor direct effect on pv generation but has a major indirect effect in controlling the mechanism for the generation of pv in a simulation without surface friction the primary pv banner forms as a result of gravity wave breaking in the more realistic simulation however the primary mechanism for pv generation is flow separation controlled by surface friction copyright 2005 royal meteorological society

bounded influence magnetotelluric response function estimation author posting blackwell 2004 this article is posted here by permission of blackwell for personal use not for redistribution the definitive version was published in geophysical journal international 157 2004 988 1006 doi 10 1111 j 1365 246x 2004 02203 x

the epica dronning maud land deep drilling operation we report on the epica dronning maud land east antarctica deep drilling operation starting with the scientific questions that led to the outline of the epica project we introduce the setting of sister drillings at northgrip and epica dome c within the european ice coring community the progress of the drilling operation is described within the context of three parallel deep drilling operations the problems that occurred and the solutions we developed modified procedures are described such as the monitoring of penetration rate via cable weight rather than motor torque and modifications to the system e g closing the openings at the lower end of the outer barrel to reduce the risk of immersing the drill in highly concentrated chip suspension parameters of the drilling e g corebreak force cutter pitch chips balance liquid level core production rate and piece number are discussed we also review the operational mode particularly in the context of achieved core length and piece length which have to be optimized for drilling efficiency and core quality respectively we conclude with recommendations addressing the design of the chip collection openings and strictly limiting the cable load drop with respect to the load at the start of the run

airborne radar observations of lake effect snowbands over the new york finger lakes abstractthe vast majority of lake effect snow research throughout the years has focused on the north american great lakes since they are often associated with strong lake effect events that produce heavy downstream snowfall this study investigates a lake effect snow event that instead occurred over two smaller lakes the new york finger lakes which are just o 5 km wide and o 50 km long a pair of well defined snowbands that formed over seneca and cayuga lakes the two largest of the finger lakes were sampled from above by a vertically pointing doppler radar and lidar on board the university of wyoming king air uwka with typical widths matching the widths of the lakes and depths of less than 1000 m the long lake axis parallel bands were actually quite intense for their size for example updrafts of 2 3 m s 1 or greater within the band cores were common and reflectivity occasionally exceeded 5 dbz airborne dual doppler data show that both bands were sometimes accompanied by a well defined therm

blowing snow as a natural glaciogenic cloud seeding mechanism abstractwinter storms are often accompanied by strong winds especially over complex terrain under such conditions freshly fallen snow can be readily suspended most of that snow will be redistributed across the landscape e g behind obstacles but some may be lofted into the turbulent boundary layer and even into the free atmosphere in areas of boundary layer separation near terrain crests or in hydraulic jumps blowing snow ice crystals mostly small fractured particles thus may enhance snow growth in clouds this may explain why shallow orographic clouds with cloud top temperatures too high for significant ice initiation may produce usually light snowfall with remarkable persistence while drifting snow has been studied extensively the impact of blowing snow on precipitation on snowfall itself has not airborne radar and lidar data are presented to demonstrate the presence of blowing snow boundary layer separation and the glaciation of shallow supercooled orographic clouds further evidenc

parameterization of the annual surface temperature and mass balance of antarctica this study entails the parameterization by means of a linear multiple regression analysis of the annual surface temperature and mass balance of antarctica the analysis was performed for the entire ice cap as well as for three separate regions ice shelves elevation less than 200 m the interior elevation above 1500 m and the escarpment region in between it was found that temperature can be parameterized very well in terms of elevation and latitude the latitudinal gradient on the ice shelves can be explained by the super adiabatic lapse rate along the surface and latitudinal temperature gradient in the interior assuming adiabatic descent of air in the inversion layer from the interior region towards the coast and an axisymmetric spreading over the ice shelves the surface mass balance can be parameterized reliably only in the interior where it has a strong positive correlation with the saturation vapour pressure of the free atmosphere and a significant correlation with the shape of the dome the convex shape of the dome contributes to the mass balance by inducing subsidence of the relatively moist air of the free atmosphere into the inversion layer this results in precipitation as radiative cooling in the inversion exceeds adiabatic warming an estimate is made of the annual horizontal and vertical advective velocities in the free atmosphere above the interior based on regression results and a physical analysis of the precipitation processes in this region a temperature sensitivity analysis was performed for the current mass balance distribution for a 1 k rise in surface temperature the regression estimate of the increase in accumulation on the grounded ice sheet is equivalent to a rate of sea level lowering of 0 2 mm a 1 this is about 30 less than estimates based on the current mass balance perturbated by the increase in saturation vapour pressure of the free atmosphere

surface topography and ice flow in the vicinity of the edml deep drilling site antarctica interpretation of ice core records requires accurate knowledge of the past and present surface topography and stress strain fields the european project for ice coring in antarctica epica drilling site 75 00258 s 0 06848 e 2891 7 m in dronning maud land antarctica is located in the immediate vicinity of a transient and forking ice divide a digital elevation model is determined from the combination of kinematic gps measurements with the glas12 datasets from the icesat based on a network of stakes surveyed with static gps the velocity field around the drilling site is calculated the annual mean velocity magnitude of 12 survey points amounts to 0 74ma 1 flow directions mainly vary according to their distance from the ice divide surface strain rates are determined from a pentagonshaped stake network with one center point close to the drilling site the strain field is characterized by along flow compression lateral dilatation and vertical layer thinning

the influence of inversions on rotors abstract rotors are small scale circulations about a horizontal or quasi horizontal axis that usually form in conjunction with high amplitude mountain waves the moderate to severe turbulence often found within rotors is a hazard to aviation observations and numerical model studies have revealed two types of mountain wave rotor systems the first type is associated with trapped waves whereas the second less common type resembles a hydraulic jump it has long been known that an upstream near mountaintop inversion plays an important role in mountain wave rotor formation in this study the role of the upstream inversion strength and height in an environment with sheared flow and over a barrier with a steep lee slope is investigated it is found that the second mountain wave rotor type is more likely to form when a strong near mountaintop inversion is present baroclinic generation of horizontal vorticity within the inversion along the lee slope leads to overturning in an upstream direction and spreadin

detailed dual doppler structure of kelvin helmholtz waves from an airborne profiling radar over complex terrain part i dynamic structure abstractkelvin helmholtz kh waves are remarkably common in deep stratiform precipitation systems associated with frontal disturbances at least in the vicinity of complex terrain as is evident f

application of a continuum mechanical model for the flow of anisotropic polar ice to the edml core antarctica we present an application of the newly developed caffe model continuum mechanical anisotropic flow model based on an anisotropic flow enhancement factor to the epica ice core at kohnen station dronning maud land antarctica referred to as the edml core a one dimensional flow model for the site is devised which includes the anisotropic flow law and the fabric evolution equation of the caffe model three different solution methods are employed 1 computing the ice flow based on the flow law of the caffe model and the measured fabrics 2 solving the caffe fabric evolution equation under the simplifying assumption of transverse isotropy and 3 solving the unrestricted caffe fabric evolution equation method 1 demonstrates clearly the importance of the anisotropic fabric in the ice column for the flow velocity the anisotropic enhancement factor produced with method 2 agrees reasonably well with that of method 1 even though the measured fabric shows a girdle structure which breaks the transverse isotropy in large parts of the ice core for method 3 we find that the measured fabric is reproduced well by the model down to 2100 m depth systematic deviations at greater depths are attributed to the disregard of migration recrystallization in the model

grain growth in polar ice i theory many observations regarding grain growth in ice sheets are glaciologically interesting but imperfectly understood here we develop the theory of grain growth in ice that is not deforming rapidly and in the succeeding paper we use this theory to explain observations from glacial ice in the absence of significant strain energy the driving force for grain growth arises from grain boundary curvature grain growth is slowed by the interaction of grain boundaries with extrinsic materials microparticles bubbles and dissolved impurities if the driving force for growth is not large enough to cause boundaries to separate from an extrinsic material then the grain boundary velocity is determined by the velocity characteristic of the extrinsic material low velocity regime if the driving force is large enough to cause separation then boundaries migrate more rapidly than the extrinsic material high velocity regime but the net driving force is reduced through transient pinning by the extrinsic material polar ice is typically in the low velocity regime relative to dissolved impurities and the high velocity regime relative to microparticles and bubbles cross sectional area of grains is predicted to increase linearly with time under most but not all circumstances

ice flow of the antarctic ice sheet we present a reference comprehensive high resolution digital mosaic of ice motion in antarctica assembled from multiple satellite interferometric synthetic aperture radar data acquired during the international polar year 2007 to 2009 the data reveal widespread patterned enhanced flow with tributary glaciers reaching hundreds to thousands of kilometers inland over the entire continent this view of ice sheet motion emphasizes the importance of basal slip dominated tributary flow over deformation dominated ice sheet flow redefines our understanding of ice sheet dynamics and has far reaching implications for the reconstruction and prediction of ice sheet evolution

on the onset of bora and the formation of rotors and jumps near a mountain gap this study investigates the onset phase of a strong adriatic bora windstorm that occurred on 4 april 2002 the target area is a gap about 20 km wide embedded in the coastal mountain barrier of the dinaric alps that favours strong jet like winds airborne aerosol back scatter lidar measurements on board the dlr falcon research aircraft together with surface and upper air observations are used to verify high resolution numerical experiments conducted with the mesoscale atmospheric model rams and a single layer shallow water model swm especially during the breakthrough phase of the bora the flow at the gap exit exhibits a complex spatial structure and temporal evolution on a transect through the centre of the gap a hydraulic jump forms this is located close to the coast throughout the night and starts to propagate downstream in the early morning on a transect through the edge of the gap a lee wave induced rotor becomes established due to boundary layer separation it starts to propagate downstream about two hours after the jump this flow evolution implies that the onset of strong winds at the coast occurs several hours earlier downstream of the centre of the gap than downwind of the edge of the gap consequently the wind field in the vicinity of rijeka airport located downwind of the gap is strongly inhomogeneous and transient and represents a potential hazard to aviation measured bora winds at the surface exceed 20 ms 1 and the simulated wind speed in the gap wind layer exceeds 30 ms 1 the simulated turbulent kinetic energy exceeds 10 m2 s 2 rams indicates that wave breaking near a critical level is the dominant mechanism for the generation of the windstorm gap jets can be identified downstream of several mountain passes the simulated wave pattern above the dinaric alps the wave decay with height due to directional wind shear and the strong flow descent on the leeward side of the barrier are supported by measured back scatter intensities basic bora flow features including gap jets and jumps are remarkably well reproduced by swm simulations the rams reference run captures observed flow phenomena and the temporal flow evolution qualitatively well a cold low level bias an overestimated bora inversion strength and a slightly too early bora onset are probably related to insufficient turbulent mixing in the boundary layer the amplitude of trapped gravity waves the time of the bora breakthrough and the inversion strength are found to be quite sensitive to the turbulence parametrization copyright 2008 royal meteorological society

an enthalpy formulation for glaciers and ice sheets polythermal conditions are ubiquitous among glaciers from small valley glaciers to ice sheets conventional temperature based cold ice models of such ice masses cannot account for that portion of the internal energy which is latent heat of liquid water within temperate ice so such schemes are not energy conserving when temperate ice is present temperature and liquid water fraction are however functions of a single enthalpy variable a small enthalpy change in cold ice is a change in temperature while a small enthalpy change in temperate ice is a change in liquid water fraction the unified enthalpy formulation described here models the mass and energy balance for the three dimensional ice fluid for the surface runoff layer and for the subglacial hydrology layer together in a single energy conserving theoretical framework it is implemented in the parallel ice sheet model results for the greenland ice sheet are compared with those from a cold ice scheme this paper is intended to be an accessible foundation for enthalpy formulations in glaciology

physical analysis of an antarctic ice core towards an integration of micro and macrodynamics of polar ice microstructures from deep ice cores reflect the dynamic conditions of the drill location as well as the thermodynamic history of the drill site and catchment area in great detail ice core parameters crystal lattice preferred orientation lpo grain size grain shape mesostructures visual stratigraphy as well as borehole deformation were measured in a deep ice core drilled at kohnen station dronning maud land dml antarctica these observations are used to characterize the local dynamic setting and its rheological as well as microstructural effects at the edml ice core drilling site european project for ice coring in antarctica in dml the results suggest a division of the core into five distinct sections interpreted as the effects of changing deformation boundary conditions from triaxial deformation with horizontal extension to bedrock parallel shear region 1 uppermost approx 450 m depth with still small macroscopic strain is dominated by compression of bubbles and strong strain and recrystallization localization region 2 approx 450 textendash 1700 m depth shows a girdle type lpo with the girdle plane being perpendicular to grain elongations which indicates triaxial deformation with dominating horizontal extension in this region approx 1000 m depth the first subtle traces of shear deformation are observed in the shape preferred orientation spo by inclination of the grain elongation region 3 approx 1700 textendash 2030 m depth represents a transitional regime between triaxial deformation and dominance of shear which becomes apparent in the progression of the girdle to a single maximum lpo and increasing obliqueness of grain elongations the fully developed single maximum lpo in region 4 approx 2030 textendash 2385 m depth is an indicator of shear dominance region 5 below approx 2385 m depth is marked by signs of strong shear such as strong spo values of grain elongation and strong kink folding of visual layers the details of structural observations are compared with results from a numerical ice sheet model pism isotropic for comparison of strain rate trends predicted from the large scale geometry of the ice sheet and borehole logging data this comparison confirms the segmentation into these depth regions and in turn provides a wider view of the ice sheet this article is part of the themed issue textquoteleft microdynamics of ice textquoteright

impurity influence on normal grain growth in the gisp2 ice core greenland intercept analysis of approximately bi yearly vertical thin sections from the upper part of the gisp2 ice core central greenland shows that grain size ranges increase with increasing age this demonstrates that something in the ice affects grain growth rates and that grain size cannot be used directly in paleothermometry as has been proposed correlation of grain growth rates to chemical and isotopic data indicates slower growth in ice with higher impurity concentrations and especially slow growth in forest fire layers containing abundant ammonium however the impurity grain growth relations are quite noisy little correlation is found between growth rate and isotopic composition of ice

polar ice structure and the integrity of ice core paleoclimate records abstract polar ice is a unique archive of the climatic conditions in the past however ice sheets flow and this flow may affect the integrity of paleoclimate records a useful method to analyze the effect of ice flow upon climate records is the combination of microstructure mapping with ice core line scanning microstructure and stratigraphy have been mapped along the entire epica dml ice core with this combined method on the macroscale the stratigraphy seems perfectly preserved down to ca 1700 m depth mis4 below which minor undulations start to develop layers inclined up to 15 and millimeter scale z folds are observed below 2050 m depth notwithstanding the epica dml climate record appears not seriously disturbed down to ca 2400 m depth which marks the climatic transition from the last interglacial mis5e to the mis6 glacial period below this depth the synchronization with the epica dome c record is lost and stratigraphic disturbances appear up to the meter scale on the microscopic scale we observe dynamic recrystallization already in deep firn leading to substantial microstructural changes prior to bubble close off the concentration of visible micro inclusions in the holocene part of the core seems to increase with depth which could possibly indicate post depositional formation of salts in glacial period ice the concentration of visible micro inclusions in certain layers is so high that these strata appear as light scattering bands often called cloudy bands another interesting stratigraphic feature is what we call bubble free bands viz millimeter thick strata deprived of bubbles and rich in clathrate hydrates which are found within the bubble hydrate transition zone 800 1200 m depth and are probably caused by anomalously fast clathration while there is hardly any interaction between visible micro inclusions and grain boundaries down to 2500 m depth in the deeper ice warmer than 10 c many grain boundaries seem to harvest micro inclusions here we discuss these observations and their relevance for the interpretation of climate records

the microstructure of polar ice part ii state of the art an important feature of natural ice in addition to the obvious relevance of glaciers and ice sheets for climate related issues is its ability to creep on geological time scales and low deviatoric stresses at temperatures very close to its melting point without losing its polycrystalline character this fact together with its strong mechanical anisotropy and other notable properties makes natural ice an interesting model material for studying the high temperature creep and recrystallization of rocks in earth s interior after having reviewed the major contributions of deep ice coring to the research on natural ice microstructures in part i of this work faria et al 2014 here in part ii we present an up todate view of the modern understanding of natural ice microstructures and the deformation processes that may produce them in particular we analyze a large body of evidence that reveals fundamental flaws in the widely accepted tripartite paradigm of polar ice microstructure also known as the three stage model cf part i these results prove that grain growth in ice sheets is dynamic in the sense that it occurs during deformation and is markedly affected by the stored strain energy as well as by air inclusions and other impurities the strong plastic anisotropy of the ice lattice gives rise to high internal stresses and concentrated strain heterogeneities in the polycrystal which demand large amounts of strain accommodation from the microstructural analyses of ice cores we conclude that the formation of many and diverse subgrain boundaries and the splitting of grains by rotation recrystallization are the most fundamental mechanisms of dynamic recovery and strain accommodation in polar ice additionally in fine grained high impurity ice layers e g cloudy bands strain may sometimes be accommodated by diffusional flow at low temperatures and stresses or microscopic grain boundary sliding via microshear in anisotropic ice sheared at high temperatures grain boundaries bulged by migration recrystallization and subgrain boundaries are endemic and very frequent at almost all depths in ice sheets evidence of nucleation of new grains is also observed at various depths provided that the local concentration of strain energy is high enough which is not seldom the case as a substitute for the tripartite paradigm we propose a novel dynamic recrystallization diagram in the three dimensional state space of strain rate temperature and mean grain size which summarizes the various competing recrystallization processes that contribute to the evolution of the polar ice microstructure dedicated to sepp kipfstuhl on occasion of his 60th birthday

wave induced boundary layer separation in the lee of the medicine bow mountains part i observations abstracttwo cases of mountain waves rotors and the associated turbulence in the lee of the medicine bow mountains in southeastern wyoming are investigated in a two part study using aircraft observations and numerical simulations in part i observations from in situ instruments and high resolution cloud radar on board the university of wyoming king air aircraft are presented and analyzed measurements from the radar compose the first direct observations of wave induced boundary layer separation the data from these two events show some striking similarities but also significant differences in both cases rotors were observed yet one looks like a classical lee wave rotor while the other resembles an atmospheric hydraulic jump with midtropospheric gravity wave breaking aloft high resolution 30 30 m2 dual doppler syntheses of the two dimensional velocity fields in the vertical plane beneath the aircraft reveal the boundary layer separation the scale and structure of the attendant rotors and downsl

ice sheet model sensitivities to environmental forcing and their use in projecting future sea level the searise project ten ice sheet models are used to study sensitivity of the greenland and antarctic ice sheets to prescribed changes of surface mass balance sub ice shelf melting and basal sliding results exhibit a large range in projected contributions to sea level change in most cases the ice volume above flotation lost is linearly dependent on the strength of the forcing combinations of forcings can be closely approximated by linearly summing the contributions from single forcing experiments suggesting that nonlinear feedbacks are modest our models indicate that greenland is more sensitive than antarctica to likely atmospheric changes in temperature and precipitation while antarctica is more sensitive to increased ice shelf basal melting an experiment approximating the intergovernmental panel on climate change s rcp8 5 scenario produces additional first century contributions to sea level of 22 3 and 8 1 cm from greenland and antarctica respectively with a range among models of 62 and 14 cm respectively by 200 years projections increase to 53 2 and 26 7 cm respectively with ranges of 79 and 43 cm linear interpolation of the sensitivity results closely approximates these projections revealing the relative contributions of the individual forcings on the combined volume change and suggesting that total ice sheet response to complicated forcings over 200 years can be linearized

observations and numerical simulations of subrotor vortices during t rex high resolution observations from scanning doppler and aerosol lidars wind profiler radars as well as surface and aircraft measurements during the terrain induced rotor experiment t rex provide the first comprehensive documentation of small scale intense vortices associated with atmospheric rotors that form in the lee of mountainous terrain although rotors are already recognized as potential hazards for aircraft it is proposed that these small scale vortices or subrotors are the most dangerous features because of strong wind shear and the transient nature of the vortices a life cycle of a subrotor event is captured by scanning doppler and aerosol lidars over a 5 min period the lidars depict an amplifying vortex with a characteristic length scale of 500 1000 m that overturns and intensifies to a maximum spanwise vorticity greater than 0 2 s 1 radar wind profiler observations document a series of vortices characterized by updraft downdraft couplets and regions of enhanced reversed flow that are generated in a layer of strong vertical wind shear and subcritical richardson number the observations and numerical simulations reveal that turbulent subrotors occur most frequently along the leading edge of an elevated sheet of horizontal vorticity that is a manifestation of boundary layer shear and separation along the lee slopes as the subrotors break from the vortex sheet intensification occurs through vortex stretching and in some cases tilting processes related to three dimensional turbulent mixing the subrotors and ambient vortex sheet are shown to intensify through a modest increase in the upstream inversion strength which illustrates the predictability challenges for the turbulent characterization of rotors

the microstructure of polar ice part i highlights from ice core research polar ice sheets play a fundamental role in earth s climate system by interacting actively and passively with the environment active interactions include the creeping flow of ice and its effects on polar geomorphology global sea level ocean and atmospheric circulation and so on passive interactions are mainly established by the formation of climate records within the ice in form of air bubbles dust particles salt microinclusions and other derivatives of airborne impurities buried by recurrent snowfalls for a half century scientists have been drilling deep ice cores in antarctica and greenland for studying such records which can go back to around a million years experience shows however that the ice sheet flow generally disrupts the stratigraphy of the bottom part of deep ice cores destroying the integrity of the oldest records for all these reasons glaciologists have been studying the microstructure of polar ice cores for decades in order to understand the genesis and fate of ice core climate records as well as to learn more about the physical properties of polar ice aiming at better climate record interpretations and ever more precise models of ice sheet dynamics in this part i we review the main difficulties and advances in deep ice core drilling in antarctica and greenland together with the major contributions of deep ice coring to the research on natural ice microstructures in particular we discuss in detail the microstructural findings from camp century byrd dye 3 grip gisp2 northgrip vostok dome c edml and dome fuji besides commenting also on the earlier results of some pioneering ventures like the jungfraujoch expedition and the norwegian british swedish antarctic expedition among others in the companion part ii of this work faria et al 2014 the review proceeds with a survey of the state of the art understanding of natural ice microstructures and some exciting prospects in this field of research

conditions for bubble elongation in cold ice sheet ice highly elongated bubbles are sometimes observed in ice sheet ice elongation is favored by rapid ice deformation and opposed by diffusive processes we use simple models to show that vapor transport dominates diffusion except possibly very close to the melting point and that latent heat effects are insignificant elongation is favored by larger bubbles at pore close off but is nearly independent of bubble compression below close off the simple presence of highly elongated bubbles indicates only that a critical ice strain rate has been exceeded for significant time and provides no information on possible disruption of stratigraphic continuity by ice deformation

the epica ice core from dronning maud land first results from stable isotope measurements the european project for ice coring in antarctica epica focuses on the drilling of two deep ice cores the first at dome c and the second at kohnen station 75 00 s 0 04 e in dronning maud land dml this paper deals with stable isotope records from ice cores drilled in dml in the first season the deep epica dml core reached a depth of 450 m recovering ice approximately 7000 years old generally the d 18 o record indicates a stable holocene climate and shows low variability however during the last 4000 years based on a preliminary time scale the d 18 o values decrease continuously by about 0 6 and the deuterium excess values increase by about 0 5 t he correlation between d 18 o and the deuterium excess d is investigated for a 50 m long core section and the near surface snow high pass filtered profiles are positively correlated whereas the correlation between low pass filtered profiles is negative a post depositional effect due to diffusion processes can be seen in a sub annually resolved profile from snow pit samples changes in the seasonality of the evolution of the snow cover and the consequences for stable isotope content are demonstrated with data from ice core b31

turbulence in breaking mountain waves and atmospheric rotors estimated from airborne in situ and doppler radar measurements atmospheric turbulence generated in flow over mountainous terrain is studied using airborne in situ and cloud radar measurements over the medicine bow mountains in southeast wyoming usa during the nasa orographic clouds experiment nasa06 in 2006 two complex mountain flow cases were documented by the university of wyoming king air research aircraft carrying the wyoming cloud radar the structure of turbulence and its intensity across the mountain range are described using the variance of vertical velocity w2 and the cube root of the energy dissipation rate 1 3 edr for a quantitative analysis of turbulence from the cloud radar the uncertainties in the doppler wind retrieval have to be taken into account such as the variance of hydrometeor fall speed and the contamination of vertical doppler velocity by the horizontal wind a thorough analysis of the uncertainties shows that 25 accuracy or better can be achieved in regions of moderate to severe turbulence in the lee of the mountains while only qualitative estimates of turbulence intensity can be obtained outside the most turbulent regions two nasa06 events exhibiting large amplitude mountain waves mid tropospheric wave breaking and rotor circulations are examined moderate turbulence is found in a wave breaking region with w2 and edr reaching 4 8 m2 s 2 and 0 25 m2 3 s 1 respectively severe turbulence is measured within the rotor circulations with w2 and edr respectively in the ranges of 7 8 16 4 m2 s 2 and 0 50 0 77 m2 3 s 1 a unique result of this study is the quantitative estimation of the intensity of turbulence and its spatial distribution in the interior of atmospheric rotors provided by the radar derived turbulence fields

exact solutions to the thermomechanically coupled shallow ice approximation effective tools for verification we describe exact solutions to the thermomechanically coupled shallow ice approximation in three spatial dimensions although artificially constructed these solutions are very useful for testing numerical methods in fact they allow us to verify a finite difference scheme that is to show that the results of our numerical scheme converge to the correct continuum values as the grid is refined in three dimensions comparison of numerical results with exact solutions has helped us to precisely quantify and understand some of the numerical errors we are making our verified numerical scheme shows the basal temperature spokes which arose in the eismint european ice sheet modelling initiative ii intercomparison payne and others 2000 a careful analysis describes these warm spokes as numerical errors which occur when the derivative of the strain heating term with respect to the temperature is large on the other hand the appearance of basal temperature spokes in a verified numerical scheme strongly suggests that they are a feature of the eismint ii experiment f continuum problem in fact they are clear evidence of an unstable equilibrium point of the continuum problem this paper is a sequel to bueler and others 2005 which addresses exact solutions and verification in the isothermal case

an anisotropic flow law for ice sheet ice and its implications abstract a new flow law or anisotropic polycrystalline ice is presented the strain rate tensor is related by a geometrical actor tensor g to the stress tensor the g factor tensor can be obtained from the c axis fabric data and stress condition this new flow law describes well the direction dependent mechanical properties of anisotropic ice which cannot be demonstrated by glen s flow law for example the new flow law can explain the fact that a strong single maximum fabric ice such as dye 3 vvisconsin ice can deform several times aster than isotropic ice under horizontal shear but can hardly deform under vertical or horizontal normal stress i ve also show that at a deeper part of an ice sheet where a single maximum fabric develops a positi e vertical strain rate can be produced with only a horizontal shear stress 1 introduction ice sheet modelling and studies on ice sheet dynamics carried out so far have been based on glen s flow law

crystalline texture of the 2083 m ice core at vostok station antarctica crystalline texture and c axis orien tation of the 2083 m ice core at vostok station covering more than 150 kyear reveal the existence of strong anisotropies changes in crystal size with depth are compatible with the growth of grains driven by the free energy of grain boundaries a smaller growth rate appears to be associated with cold periods a gradual increase in the horizontal elongation of grains was observed between 350 and 680 m but the mean value of the coefficient of the linear dimensional orientation of grains does not cha nge below 700 m the e axis orientation of ice grains tends to orientate perpendicular to the direction of the elongation of grains forming a vertical girdle patt ern this charac teristic fabric has been interpreted as resulting from the gradual rotation of grains by basal glide under uniaxial longit udi nal tension the rotation of grains was calcu lated with respect to the total strain simulating the form ation of the girdle fabric pattern the fabric enhance ment factor was calculated at various depths it appears that vostok ice hardens gradua lly