seasonal contrasts in snow cover characteristics on ross sea ice floes snow cover characteristics on ice floes in the ross sea antarctica were examined during cruises in autumn 1998 and summer 1999 the autumn snow cover was shallower colder and had higher and more variable salinity and smaller single and composite grain sizes than the summer snow cover the autumn snow cover was dominated by rounded particles in chains of grains and clusters while the summer snow cover was composed primarily of melt clusters there was extensive flooding of the summer snow cover at the snow ice interface the summer snow cover was nearly isothermal and close to the melting point it exhibited obvious signs of melting and refreezing in the form of ice lenses pipes and superimposed ice although no melt ponds were evident many of the ice lenses were located directly above the saline standing water found on most of the summer ice floes

faceted crystal formation in the northeast greenland low accumulation region snow pit analysis in the low accumulation region in northeast greenland indicated a mean annual accumulation of 12 1 6 cm it also revealed numerous faceted layers that were not related to the summer horizon of previous years in general the faceted crystals were found under a wind crust these crusts appear to originate during the winter snow and air temperature data from automatic weather stations suggest that the katabatic storms are responsible for the formation of the wind crusts and the faceted layers frequent abrupt temperature changes up to 30 c were observed within two days due to turbulent mixing of cold air inversions the near surface air temperature variations are correlated with wind speed the latter leading the former by 6 9 hours for increasing wind and 1 4 hours for decreasing wind because of low accumulation katabatic storms affect the same snow surface for several months causing repeated air temperature fluctuations this leads to strong vapor pressure gradients and faceted crystal growth in the top snow layer on average we found two faceted layers in addition to the summer layer per year for a 4 year record in addition to its climatological significance in the snow record the formation of these layers is especially important to understand for the interpretation of microwave satellite observations of the firn

field investigation of near surface metamorphism of snow buried surface hoar and near surface faceted crystals are known to lead to deadly avalanches over the course of three winter seasons a field investigation detailing the environmental conditions leading to the formation of these crystals was performed weather stations on north and south facing aspects were established the weather data were accompanied by detailed daily observations and grain scale photographs of the snow surface during the three seasons 35 surface hoar and 47 near surface facets events were recorded the mean weather conditions for the entire dataset all three seasons and both stations were compared to the nights when surface hoar formed the comparison yielded five parameters that were statistically linked to the formation of surface hoar incoming longwave radiation snow surface temperature wind velocity relative humidity and the air snow temperature difference a similar comparison between the daytime mean values for all days with near surface facet events revealed three parameters with statistically significant differences thus these parameters short and longwave radiation and relative humidity could be statistically linked to facet formation this research also suggests that environmental conditions in the daytime hours before and after surface hoar formation are statistically similar to the conditions causing near surface facet formation

a microstructural dry snow metamorphism model for kinetic crystal growth historically dry snow metamorphism has been classified by the thermal environment and thermodynamic processes in a snowpack snow experiencing predominantly macroscopic isothermal conditions develops different microstructure than snow subjected to large temperature gradients as such much previous research has been categorized by and limited to specific thermal conditions the current research expands a generalized approach for the movement of heat and mass to include a snow crystal kinetic growth model an existing spiral defect propagation theory for kinetic growth on simple faceted geometry is utilized primary crystal habit as a function of temperature is incorporated a model of heat and mass transfer through an ice and pore structure is coupled with phase change thermodynamics during kinetic growth a kinetic growth microstructure model is developed and integrated into heat and mass transfer representations which are solved using finite difference techniques the kinetic morphology model approximates frequently observed hopper type crystals the snow microstructure is allowed to change at every step resulting in a transient description of kinetic growth metamorphism variable kinetic growth rates are demonstrated based on temperature and on crystallographic orientation relative to a temperature gradient crystals preferentially aligned with the temperature gradient have significantly higher growth rates supporting previous observations of predominant crystal habits developing under temperature gradient conditions grain size dispersion increase with time is demonstrated and supported experimentally in the literature a dominant grain growth theory based on crystallographic orientation that has been previously postulated is supported a broad range of metamorphic geometric parameters and thermal conditions may now be simulated with a single model

formation of refrozen snowpack layers and their role in slab avalanche release 1 in a variety of snow climates numerous slab avalanches release over crusts consisting of refrozen snow slab avalanches sometimes release in weak layers of faceted crystals that developed while underlying wet layers froze into crusts often within a day weak layers of faceted crystals can also develop when less permeable and more conductive crusts alter the temperature and vapor pressure gradients these processes create interfaces where differences in grain radii can contribute to weak bonding in western canada layers of faceted crystals facet layers on crusts are most common in early and late winter when thaws and rain are more frequent also thin facet layers occur in spring when surface melting by solar radiation is common shear strength tests on facet layers show an initial strength loss during faceting followed by a slow strength increase the spatial distribution of poorly bonded crusts can be interpreted from the interaction of terrain and meteorology that caused the antecedent wet layer

formation and strengthening of layers of dry faceted crystals above artificial melt freeze crusts from overburden stress in a controlled environment layers of faceted crystals were grown from naturally fallen snow above wet snow layers in a temperature controlled laboratory static loads were then applied to the weak layers to represent overburden snow eleven experiments with constant loads were analyzed equivalent to 260 to 1500 pa of overburden stress the density of the slab above the weak layer increased with time following a power law relationship with an average exponent of 0 10 shear strength of the weak layer increased with time for all experiments also following a power law relationship early time rates of strength gain averaged 250 pa day 1 for the constant load experiments over the first 3 5 days decreasing to an average of 1 pa day 1 after approximately 30 days exponents for the power law relationships ranged between 0 09 and 0 35 with an average of 0 26 0 08 sintering was likely the dominant process for strength gain although densification probably contributed as well three experiments were conducted in which the overburden

modal variability in the southeast pacific basin energetics of the 2009 event abstract we study the barotropic variability in the southeast pacific basin in particular focusing on the extreme event during the fourth quarter of 2009 a 3 year integration of a barotropic shallow water model forced with wind stress anomalies generates localized variability that is similar in spatial extent and amplitude as the observed anomalous event an eigenmode analysis of the same model shows the presence of several free modes in the southeast pacific but projection of the modal patterns on the model output shows that their amplitudes are low instead the mode is interpreted as an almost free mode the modal excitation accounts for a considerable fraction 23 on average of the kinetic energy input by the wind stress in the southeast pacific basin increasing to 38 for the anomalous event in 2009 surprisingly a similar but weaker event during the third quarter of 2008 appears to have been more significant from an energetics point of view with almost 50 of the energy being input into the mode key areas of energetic dissipation appear to be the eltanin fracture zone the crest of the east pacific rise and the chile rise east pacific rise intersection

winter snow cover on the sea ice of the arctic ocean at the surface heat budget of the arctic ocean sheba temporal evolution and spatial variability 1 the evolution and spatial distribution of the snow cover on the sea ice of the arctic ocean was observed during the surface heat budget of the arctic ocean sheba project the snow cover built up in october and november reached near maximum depth by mid december then remained relatively unchanged until snowmelt ten layers were deposited the result of a similar number of weather events two basic types of snow were present depth hoar and wind slab the depth hoar 37 of the pack was produced by the extreme temperature gradients imposed on the snow the wind slabs 42 of the snowpack were the result of two storms in which there was simultaneous snow and high winds 10 m s 1 the slabs impacted virtually all bulk snow properties emphasizing the importance of episodic events in snowpack development the mean snow depth n 21 169 was 33 7 cm with a bulk density of 0 34 g cm 3 n 357 r2 of 0 987 giving an average snow water equivalent of 11 6 cm 25 higher than the amount record by precipitation gauge both depth and stratigraphy varied significantly with ice type the greatest depth and the greatest variability in depth occurring on deformed ice ridges and rubble fields across all ice types a persistent structural length in depth variations of 20 m was found this appears to be the result of drift features at the snow surface interacting with small scale ice surface structures a number of simple ways of representing the complex temporal and spatial variations of the snow cover in ice ocean atmosphere models are suggested

snow avalanche formation 1 snow avalanches are a major natural hazard endangering human life and infrastructure in mountainous areas throughout the world in many countries with seasonally snow covered mountains avalanche forecasting services reliably warn the public by issuing occurrence probabilities for a certain region however at present a single avalanche event cannot be predicted in time and space much about the release process remains unknown mainly because of the highly variable layered character of the snowpack a highly porous material that exists close to its melting point the complex interaction between terrain snowpack and meteorological conditions leading to avalanche release is commonly described as avalanche formation it is relevant to hazard mapping and essential to short term forecasting which involves weighting many contributory factors alternatively the release process can be studied and modeled this approach relies heavily on snow mechanics and snow properties including texture while the effect of meteorological conditions or changes on the deformational behavior of snow is known in qualitative or semiquantitative manner the knowledge of the quantitative relation between snow texture and mechanical properties is limited but promising developments are under way fracture mechanical models have been applied to explain the fracture propagation and micromechanical models including the two competing processes damage and sintering have been applied to explain snow failure there are knowledge gaps between the sequence of processes that lead to the release of the snow slab snow deformation and failure damage accumulation fracture initiation and fracture propagation simultaneously the spatial variability that affects damage fracture initiation and fracture propagation has to be considered this review focuses on dry snow slab avalanches and shows that dealing with a highly porous media close to its melting point and processes covering several orders of scale from the size of a bond between snow grains to the size of a mountain slope will continue to be very challenging

tomography based characterization of ice air interface dynamics of temperature gradient snow metamorphism under advective conditions snow at or close to the surface commonly undergoes temperature gradient metamorphism under advective flow which alters its microstructure and physical properties a functional understanding of this process is essential for many disciplines from modeling the effects of snow on regional and global climate to assessing avalanche formation time lapse x ray microtomography was applied to investigate the structural dynamics of temperature gradient snow metamorphism exposed to an advective airflow in controlled laboratory conditions experiments specifically analyzed sublimation and deposition of water vapor on the ice structure in addition an analysis of the ice air interface dynamics was carried out using a macroscopic equivalent model of heat and water vapor transport through a snow layer the results indicate that sublimation of the ice matrix dominated for flow rates 10 6 m3 s 1 while during increased mass flow rates the water vapor deposition supplied by the advective flow counteracted sublimation a flow rate dependence of water vapor deposition at the ice interface was observed asymptotically approaching an average estimated maximum deposition rate on the whole sample of 1 05 10 4 kg m 3 s 1 the growth of microsized whisker like crystals on larger ice crystals was detected on microscope photographs leading to an increase of the specific surface area and thus suggest a change of the physical and optical properties of the snow the estimated values of the curvature effect of the ice crystals and the interface kinetic coefficient are in good agreement with previously published values

evolution of the specific surface area of snow during high temperature gradient metamorphism the structural evolution of low density snow under a high temperature gradient over a short period usually takes place in the surface layers on clear cold nights in this paper x ray computed microtomography microct was combined with numerical simulations to investigate the temperature gradient metamorphism tgm on different types of snow precipitation particles pp small rounded particles rgsr and large rounded particles rglr were each observed in high temperature gradients 100 500 k m 1 at a mean temperature of 4 c the specific surface area ssa was used to characterize the tgm which were influenced by both the magnitude of the temperature gradient and the initial snow structures pp samples experienced a logarithmic decrease of ssa with time and the depth hoar structures created under high tgm 500 k m 1 have higher ssa compared to those under lower tgm unlike previous observations for initial rounded and connected structures like rglr samples the ssa increased during tgm simulated normal vapor flux distributions for different snow types were used to help understand the structural evolution under tgm understanding the ssa increase is important in order to predict the enhanced uptake of chemical species by snow or increase in snow albedo

burgess shale type biotas were not entirely burrowed away burgess shale type biotas occur globally in the cambrian record and offer unparalleled insight into the cambrian explosion the initial phanerozoic radiation of the metazoa deposits bearing except

snow crystal growth with varying surface temperatures and radiation penetration the temperature field is derived for a sinusoidally varying surface temperature with varying solar radiation penetration the growth rates of snow crystals are calculated to explain the rapidly growing layers of faceted crystals i e depth hoar that form just below the surface at high altitudes and in polar snow the solutions also show that a layer of wet snow can exist just below the surface even on days when the surface temperature remains sub freezing