air sea interactions over terra nova bay during winter simulation with a coupled atmosphere polynya model a preliminary simulation of the terra nova bay polynya has been performed with a coupled atmosphere polynya model the atmospheric model is a hydrostatic primitive equations model that has been validated previously by a simulation of the strong katabatic winds observed in that area the polynya model includes a representation of the free drift of frazil ice and simple sea ice dynamics and thermodynamics two and three dimensional experiments have been performed under polar night conditions two dimensional experiments show that an open warm water area influences significantly the atmospheric circulation in the antarctic coastal zone an additional ice breeze effect is simulated and is responsible for the strenghthening of the katabatic winds near the coast because of the important temperature difference between the continental air and the ice free ocean up to 40 c strong surface heat fluxes are simulated over the polynya finally a three dimensional experiment has been performed the integration domain includes terra nova bay the polynya observed in that region is well simulated it is found that heat losses from the polynya surface are stronger than previously thought but are probably constrained by the idealized representation of frazil ice which is assumed to be uniform in each grid box this stresses the need for having a better knowledge of frazil ice evolution in large polynyas

the reflectance of rough snow surfaces in antarctica from polder adeos remote sensing data the polder polarization and directionality of the earth s reflectances sensor was launched onboard the japanese adeos satellite in august 1996 it measured the energy reflected by the earth in the solar spectrum at 8 wavelengths from this sensor an earth target could be seen up to 14 times along a single path providing a good characterization of the brdf bidirectional reflectance distribution function from space a set of one month of polder data over east antarctica was analyzed after cloud masking and atmospheric corrections mean brdfs were derived for different areas in some parts of the ice sheet the brdf is assymetric with a strong reflectance peak in the half space oriented towards the sun backscatter this peak is due to the anisotropic roughness of the surface sastrugi and its position depends on the sastrugi direction

the confluence zone of the intense katabatic winds at terra nova bay antarctica as derived from airborne sastrugi surveys and mesoscale numerical modeling the surface wind field inland of the intense coastal kataba c wind regime at terra nova bay antarctica has been studied both observationally and numerically airborne surveys of windinduced features on the snow surface have been used to construct the time averaged winter surface airflow pattern the surface motion field has also been simulated by a mesoscale primitive equation model using terrain slopes with a horizontal resolution of 32 km both methods of analys s demonstrate that the intense katabatic airstream at terra nova bay is forced by converging air currents in the continental interior the broadscale confluence zone becomes organized into two regions within about 180 km of the coast the primary route for katabatic mass transport into the terra nova bay area is reeves glacier valley but an important secondary source is provided by airflow down david glacier the former is generated by the coriolis induced concentration of mass transport on the left side looking downwind of the broadscale confluence zone as well as by the near coastal mountain deflection of airflow into the valley head the confluence zone feeding into david glacier valley stretches over 100 km into the interior and is forced by the broadscale terrain configuration of the ice sheet source areas for the two airstreams differ with reeves glacier valley being fed by cold air formed well into the interior whereas the airflow down david glacier is sustained by radiative cooling over lower parts of the ice sheet which are much closer to the ross sea evaluation of the observational and modeling results reveals both the strengths and weaknesses of each approach airborne surveys of sastrugi orientations are a highly successful method for establishing the detailed pattern of surface airflow however a systematic examination of sastrugi dimensions suggests that such work could also be carried out using spot type satellite observations which is a more cost effective approach than aircraft surveying the double jet structure of katabatic airflow into the terra nova bay area is well represented by the primitive equation modeling but much finer grids are needed to resolve the near coastal drainage details

instrumented aircraft observations of the katabatic wind regime near terra nova bay abstract two aircraft missions to sample the boundary layer dynamics associated with the intense katabatic wind regime at terra nova bay antarctica were flown on successive days in early november 1987 light winds averaging 5 m s 1 were monitored at the 170 m flight level over the interior of the ice sheet dramatic acceleration of the airflow and abrupt 5 7 c cooling were encountered on both days near the head of reeves glacier just upslope from where the terrain steepens considerably these results suggest that much of the airflow convergence which sustains the coastal katabatic winds is forced by localized topographic channeling into reeves glacier and that the descending airstream is negatively buoyant the horizontally propagating katabatic winds were followed for 250 km directly offshore and for 200 km southward parallel to the victoria land coast the airstream momentum gradually decreased along both flight paths in conjunction with the descent of negatively buoyant air down reeves glacier and h

katabatic wind forcing of the terra nova bay polynya the terra nova bay polynya is a perennial winter feature in the western ross sea antarctica which occupies roughly 1000 km2 it is formed and maintained by the combined influence of persistent katabatic winds which advect newly formed bay ice eastward and the drygalski ice tongue which prevents northward drifting pack ice from entering terra nova bay existence of anomalously strong katabatic drainage along this coast is predicted by parish s 1982 simulation of wintertime airflow which reveals a pronounced confluence of surface winds upslope from the reeves glacier where the winds are further focused by local topography the simulation is strongly supported by regional sastrugi orientations average wintertime atmospheric conditions and ice sheet topography which control surface air drainage are stable on a climatic time scale therefore persistent wintertime katabatic winds should be an annual phenomenon further evidence comes from multi year landsat images which consistently show windswept snowfree areas on the reeves glacier in marked contrast to typical antarctic katabatic winds strong persistent winter winds are observed at sea level 25 km beyond the coastal slope break air probably descends as bora type winds and is likely to be significantly denser than the air at sea level conditions are not favorable for hydraulic jumps apparently typical of other katabatic regimes the horizontal density difference is maintained during airflow across the nansen ice sheet because relatively little air mass modification occurs there in contrast to situations where air moves over an ice laden ocean observations thus suggest that katabatic winds maintain their identity for some distance seaward of the coast qualitative trajectory calculations indicate that for representative geostrophic conditions this distance is on the order of the observed polynya width estimated ice freezing rates are 20 cm day 1 but wind generated waves and currents prevent ice from consolidating the blocking effect of the drygalski ice tongue is a consequence of its orientation with regard to western ross sea ice drift patterns ice tongue length controls polynya width absence of such blocking along other coasts experiencing strong katabatic outflow partly explains why similar polynyas do not form there

modulation of the semidiurnal cycle of turbulent dissipation by wind driven upwelling in a coastal embayment with two 25 hour series of turbulent microstructure and currents observations carried out in august 2013 during spring chaos 1 and neap tides chaos 2 we investigated the semidiurnal cycle of turbulent dissipation in an embayment affected by coastal upwelling ria de vigo nw iberia at the time of sampling the bay hosted a net wind driven bi directional positive exchange flow and thermal stratification turbulent kinetic energy tke dissipation e at the interface between upwelled and surface waters was enhanced by two orders of magnitude during the ebbs 10 6 w kg 1 with respect to the floods 10 8 w kg 1 this pattern was caused by the constructive interference of the shear associated with the upwelling and tidal currents the vertical structure of the tidal currents was consistent with a deformation of tidal ellipses by stratification which was tightly coupled to the intensity of upwelling this two pronged interaction resulted in a modulation of the semidiurnal cycle of turbulent dissipation by coastal upwelling thus as a result of the upwelling relaxation conditions experienced during chaos 1 depth integrated interior tke dissipation rates were higher by a factor of 2 compared to chaos 2 by using a simple model we determined that observed variations in turbulent mixing had a limited influence on the tidal variations of stratification which were dominated by straining and advection the mixing mechanism described here is potentially relevant for the ecology of upwelling bays as it can stimulate the transport of nutrients from deep upwelled waters to the sun lit surface layers where primary production takes place

an extraordinary katabatic wind regime at terra nova bay antarctica abstract three years of automatic weather station observations for the months of february to april show that intense katabatic winds persistently blow across the western shore of terra nova bay the data demonstrate that the anomalously strong katabatic winds of adelie land are not unique and thus strongly support the proposition that most of the cold boundary layer air from the ice sheet crosses the coastline in a small number of narrow zones furthermore the observations prove that katabatic winds can routinely blow for substantial distances across flat terrain in marked contrast to the abrupt dissipation previously monitored just offshore from east antarctica winter wind conditions onset suddenly in mid february and are characterized by negligible directional variations and by speeds mostly ranging between 10 and 30 m s 1 katabatic winds at terra nova bay both affect and are affected by the regional atmospheric circulation this katabatic airflow is a time averaged source of cold boundary layer air

on the forcing of planetary scale rossby waves by antarctica the propagation of rossby waves forced at high latitudes into middle and low latitudes is studied using the barotropic vorticity equation on the sphere ray tracing theory indicates that fundamental differences exist between the response of an atmosphere initialized with typical northern hemisphere winter flow and typical southern hemisphere winter flow to such a forcing with a substantial response more likely for the southern hemisphere case integrations with a fully nonlinear barotropic model confirm this suggestion and indicate that away from the local forcing region linear theory provides a good description of the response the most notable feature in the steady solutions is a split of the mid latitude jet close to new zealand the location of this split coincides well with a region where blocking is observed to be prevalent in the southern hemisphere

the forcing of antarctic katabatic winds abstract the temporal and spatial development of katabatic winds along an idealized slope representative of anmtarctic terrain is examined using a hydrostatic two dimensional primitive equation model with explicit longwave radiation parameterization a detailed diagnosis is made of a simulation in which katabatic flow develops from rest due to the strong radiational cooling of the underlying surface significant thermodynamic and dynamic differences are seen between the gravity driven flows over the gently sloping interior and over the steep ice slopes near the coast the strongest temperature inversions and largest static stabilities are found over the interior of the continent although the net cooling of the katabatic layer and magnitude of the downslope directed horizontal pressure gradient force are greatest at the coast the interior is characterized by low rossby number quasi geostrophic type flows while more intense near antitriptic winds occur at the coast model results are in reasonable agr

improving digital elevation models over ice sheets using avhrr based photoclinometry advanced very high resolution radiometer avhrr images and a radar altimetry based digital elevation model dem covering part of the northeast greenland ice stream are combined to create an improved topographic map of the area using photoclinometry in this application of photoclinometry a dem is used to establish the photometric relationship for two avhrr images of a snow surface slopes from the dem are compared with avhrr data that are filtered i e blurred to the resolution of the dem to give an empirical photometric determination this is then used to convert unfiltered avhrr data into quantitative slope measurements of the surface in the along sun direction in each image resolving features not present or poorly represented in the dem co registration of the images is based on the assumption that the two slope fields from the images describe one continuous smooth surface the combined slopes are then converted to topography in the test case the technique adds topographic details with spatial scales of 3 to 20 km a comparison of our results with airborne laser elevation profiles demonstrates that the new technique recovers most of the topography that is missed by the dem the improved topographic map reveals a ten fold increase in local surface relief over the ice stream feature and shows the presence of shallow troughs over the shear margins of the feature

observation and characterization of radar backscatter over greenland characterization of the microwave signature of the greenland snow surface enables delineation of the different snow facies and is a tool for tracking the effects of climate change a new empirical observation model is introduced that uses a limited number of parameters to characterize the snow surface based on the dependence of radar backscatter on incidence angle azimuth angle spatial gradient and temporal rate of change the individual model parameters are discussed in depth with examples using data from the nasa scatterometer nscat and from the c band european remote sensing ers satellite advanced microwave instrument in scatterometer mode the contribution of each model term to the overall accuracy of the model is evaluated the relative contributions of the modeled dependencies vary by region two studies illustrating applications of the model are included first interannual changes over the greenland ice sheet are investigated using nine years of ers data surface changes are observed as anomalies in the spl sigma spl deg model parameters second intraannual variations of the surface are investigated these changes are observed in the average backscatter normalized to a given observation geometry the results indicate that the model can be used to obtain a more complete understanding of multiyear change and to obtain low variance high temporal resolution observations of intraannual changes the model may be applied for increased accuracy in scatterometer synthetic aperture radar sar and wide angle sar studies

altimetric observations of surface characteristics of the antarctic ice sheet the aim of this paper is to investigate the geophysical characteristics of the antarctic ice sheet using radar altimetric observations to do this we use an altimetric waveform simulator in situ observations ers 1 european remote sensing satellite data and spot satellite pour l observation de la terre images the small scale study takes place at dome c terre adelie which is a relatively flat region with gentle undulations and low wind speed despite this the altimetric waveform parameters height energy leading edge and trailing edge are highly noisy the effect of undulations on the waveform parameters is found to be dominant the combination of a subsurface signal and a rough surface produces a linear effect on the altimetric backscattering or on the trailing edge of the waveform but a strongly non linear effect on the leading edge of the waveform or height estimation as a consequence the height measurement is very sensitive to the altimeter technical or orbital characteristics and is not reproducible from one mission to another observations show sastrugi fields that enhance the leading edge and affect the whole waveform observed local backscattering changes probably due to local variations in surface microroughness enhance the backscattered energy and may artificially create a topographic signal the continental scale study shows coherent patterns even if both surface and subsurface components affect the altimetric observation the large scale signal is mostly controlled by surface backscattering variations the surface or near subsurface characteristics of the snowpack may then be reached by altimetric observations

snow facies over ice sheets derived from envisat active and passive observations this paper aims to separate different snow regions over the terrestrial ice sheets based on their measured microwave signatures it takes advantage of coregistered data from passive and active sensors on the environmental satellite envisat to directly derive a snow facies indicator in a point by point basis this paper represents the first attempt of this kind in exploiting nadir viewing and dual frequency data from both altimeter and radiometer sensors the approach is based on a clustering method such representation of the data by means of fewer clusters necessarily loses fine details but achieves simplification in geographical representation and eases the description of the condition of the ice sheets in 2004 our approach broadens the description of the snow pack by taking into account characteristics such as surface roughness grain size stratification and snowmelt effects whereas the latter has often solely been considered in most previous work such partition of the ice sheets might help to better understand relationships between microwave signatures and snow morphology it could also be useful for estimating elevation uncertainty in altimeter data which in turn is essential to correctly interpret the significance of the rates of elevation change in a changing climate and to convert elevation change to snow mass change

testing self organized criticality in the crust using entropy a regionalized study of the cmt global earthquake catalogue 1 we test the notion of self organized criticality soc and the proximity to the critical point in the brittle crust if the system were strictly critical we would expect an infinite correlation length with minimal temporal or spatial predictability an alternative view is that the earth is in a self organized subcritical sosc state with a finite and systematically fluctuating correlation length this would imply a system that is sufficiently near critical to maintain power law scaling relations over a finite scale range but can intermittently reach criticality in the form of a single large earthquake when the correlation length becomes effectively infinite over the scale of the observed region here we address the question of proximity to criticality from the viewpoint of statistical physics by describing a regionalized study equivalent to the ensemble approach in thermodynamics flinn engdahl regionalization of global seismicity is used to calculate the expectation of the logarithm of energy lne and entropy s from centroid moment tensor cmt data for different seismic regions we compare a phase diagram for s and lne from the data with an analytical statistical mechanical solution and find that they are in good agreement the analysis shows systematic spatial heterogeneity in entropy that is associated with the tectonic deformation style oceanic ridges are seen to be low entropy relatively ordered and subduction zones have higher entropy less ordered with collision zones scattered at the two extremes statistically resolvable phase variations in the system as a whole point toward it being better described as subcritical in the spatial ensemble

impact of model resolution on simulated wind drifting snow and surface mass balance in terre adelie east antarctica this paper presents the impact of model resolution on the simulated wind speed drifting snow climate and surface mass balance smb of terre ad elie and its surroundings east antarctica we compare regional climate model simulations at 27 and 5 5 km resolution for the year 2009 the wind speed maxima in terre ad elie and the narrow glacial valleys of victoria land are better represented at 5 5 km resolution because the topography is better resolved drifting snow sublimation is 100mma 1 in regions with high wind speeds our results indicate a strong feedback between topography wind gradients and drifting snow erosion as a result smb shows much more local spatial variability at 5 5 km resolution that is controlled by drifting snow erosion whereas the large scale smb gradient is largely determined by precipitation drifting snow processes lead to ablation in the narrow glacial valleys of victoria land the integrated smb equals 86gt although wind climate drifting snow processes and smb variability are better represented at 5 5 km the area integrated smb is not significantly different between the simulations at 27 and 5 5km a horizontal resolution of 27 km is sufficient to realistically simulate ice sheet wide smb

envisat radar altimeter measurements over continental surfaces and ice caps using the ice 2 retracking algorithm abstract this study presents the first results obtained with the envisat satellite radar altimeter ra 2 over lands and ice caps as radar altimetry was firstly devoted to ocean study this paper starts with a detailed description of the radar altimetry principle over continental surfaces a general formulation of the radar altimeter echoes according to different surface types is given and the retracking procedure ice 2 specially implemented on the envisat satellite ground segment to provide output parameters over continents is clearly presented the envisat ra 2 datasets processed with the ice 2 procedure are then mapped given an overall description of the output parameters over various surfaces the behaviour of these envisat altimetric parameters are analysed and related to main land surface characteristics such as surface volume echo specularity roughness apart from few unexpected signals which will need to be further investigated the new envisat dataset confirms the potential of inland altimetry and provides a new opportunity to monitor continental surfaces the ku band measurements confirm previous results recently obtained with the altimeters on board ers or topex poseidon and offer a temporal continuity the s band altimeter measurements are in accordance with theoretical expected ones and confirm previous topex poseidon observations in c band the s band and the use of the dual frequency thus bring new capabilities for altimetric applications with important deliveries for temporal evolution survey for instance s band will be used over continents to better detect wetlands located under vegetated areas or to better quantify wave penetration in terrestrial snowpack to accurately retrieve snow depth evolution over ice sheets the dual frequency s and ku band waveforms parameters will provide a more accurate height estimation as it will be possible to quantify the error on height measurements induced by snowpack structure change over time this will then provide a more confident survey of the ice sheets surface topography evolution the combination and complementarities with the other altimetry missions such as ers topex poseidon and jason look already promising and offer a 10 year span dataset for continental altimetry applications

a numerical study of strong katabatic winds over antarctica abstract certain coastal sections of antarctica most notably adelie land and terra nova bay experience anomalously intense persistent katabatic winds the forcing of such katabatic outflow is believed to originate several hundred kilometers upslope in the interior of the continent where cold air drainage currents from a large area converge into a relatively narrow zone focused on the steeply sloping ice terrain near the coastline numerical simulations with a three dimensional hydrostatic model incorporating terrain features representative of adelie land reveal a significant topographical channeling of the surface airflow katabatic wind speeds as depicted by the model are greatly enhanced downslope of the convergence channel these results emphasize the importance of topography in the continental interior in shaping the character of coastal katabatic flow

modeling the effect of sastrugi on snow reflectance a snow reflectance model was developed to assess the bidirectional reflectance distribution function brdf of snow over anisotropic rough surfaces such as fields of sastrugi commonly encountered in some areas of the ice sheets of antarctica and greenland the sastrugi are modeled as regularly spaced identical rectangular protrusions with the same orientation interfacet multiple scattering is ignored each model sastruga has vertical walls and a small width compared to its length and to its height the input data for the model are the sun and the viewing directions the number of sastrugi per pixel their dimensions and the bidirectional reflectance for a flat snow surface the latter is computed by a multiple scattering model and snow particles are assumed to be spherical mie theory in order to compare the model results with ground measurements acquired at the south pole by grenfell et al 1994 and brandt et al 1991 atmospheric effects were taken into account the model reproduces the global influence of sastrugi on the shape of the brdf the influence of sastrugi is clearly visible for zenith viewing angles larger than 50

observations of the terra nova bay antarctica polynya by modis ice surface temperature imagery from 2005 to 2010 abstract ice surface temperature ist imagery from the thermal infrared bands of the satellite borne moderate resolution imaging spectroradiometer modis has been used to analyze the activity of the terra nova bay polynya from 2005 to 2010 in clear sky conditions the shape of the open water area of the polynya i e the portion of the ocean free of sea ice is revealed by the skin sea ice temperatures close to the water freezing point provided by the modis derived ist product the swaths of the modis sensors onboard terra and aqua satellites over the terra nova bay consist of about 5500 scenes per year useful to estimate the cloud cover too the open water area was extracted from clear sky scenes and from scenes affected by fog but providing continuity of the shape of the polynya using about 40 of the available swaths the polynya was detected for more than 80 of the sea ice period with a revisit time below 24 h and for more than 50 with a revisit time below 3 h the open water area was extracted using thresholds of ist based on the ist values detected within the open water area and within the band of sea ice surrounding the polynya obtained scene by scene an ice free area larger than 100 km 2 was observed near the nansen ice shelf for more than 70 of time larger than 500 km 2 for 50 of time the mean annual open water area around 900 km 2 was close to the lower limit of the reported range except during 2006 around 600 km 2 due to the limited activity in the second half of the cold season the largest extents between 6000 and 8500 km 2 occurred in different periods of the antarctic winter and exceeded maxima estimated in previous studies by thermal infrared data polynya openings developed from a few hours to several days in the latter case sustained by subsequent katabatic events visible in the ist sequence peaks of growing rate above 300 km 2 h were observed during intense katabatic events with gusts of 50 m s the fluctuations of the open water area resulted well correlated with wind records available from local automatic weather stations within the main katabatic routes and followed katabatic events at intervals between 0 and 6 h the analysis of the ist data set derived from the thermal infrared bands of modis provides new polynya observations of high horizontal resolution 1 km and frequent revisit time during the winter season complementing previously available observations with passive microwave imagery

snow megadunes in antarctica sedimentary structure and genesis 1 megadune fields occupy large areas in the interior of the east antarctic ice sheet and are the result of unusual snow accumulation and redistribution processes they therefore are important to surface mass balance and ice core interpretation field observations gps gpr and surface measurements have provided a detailed description of megadune sedimentation and morphology over a 70 km2 area located 200 km east of dome c a combination of remote sensing analysis using landsat and satellite radar altimetry and field measurements indicate that slope in the prevailing wind direction spwd and climatic conditions play a crucial role in megadune genesis the megadune areas tend to be characterized by slightly steeper regional slope and the presence of highly persistent katabatic winds the megadunes represent 2 to 4 m amplitude waves of 2 to 5 km wavelength formed by variable net accumulation ranging between 25 leeward faces to 120 windward faces of the accumulation in adjacent nonmegadune areas leeward faces are characterized by glazed sastrugi free surfaces and extensive depth hoar formation windward faces are covered by large rough sastrugi up to 1 5 m in height

relating microwave backscatter azimuth modulation to surface properties of the greenland ice sheet azimuth dependence of a normalized radar cross section over the greenland ice sheet is modeled with a simple surface scattering model the model assumes that azimuth anisotropy in surface roughness at scales of 3 300 m is the primary mechanism driving the modulation to evaluate the contribution of azimuth anisotropy in surface roughness to the radar backscatter the model is compared to models based on isotropic surface roughness the models are inverted to estimate snow surface properties using measurements from the c band european remote sensing satellite advanced microwave instrument in scatterometer mode results indicate that the largest mesoscale rms surface slopes are found in the lower portions of the dry snow zone estimates of the preferential direction in surface roughness are highly correlated with katabatic wind fields over greenland which is consistent with wind formed sastrugi as the dominant mechanism causing azimuth modulation of the maximum improvement of the azimuth modulation surface model compared to its isotropic counterparts occurs in the lower regions of the dry snow zone where the azimuth variability of is the largest in regions with azimuth modulation over 1 db the mean root mean square error estimate of the azimuth dependent surface scattering model is 0 46 db compared with 0 70 db for similar models using isotropic roughness

observation of diffusion processes in earthquake populations and implications for the predictability of seismicity systems scale invariance either in space or in time has been shown in many papers to characterize earthquake distributions unfortunately little work has been dedicated to looking at the general space time scaling invariance of seismicity systems even though a better understanding of how the two domains spatial and temporal link together could help the development of the stochastic dynamical modeling of earthquake populations in this paper we report the observation of diffusion processes of temporally correlated seismic activity for three different data sets a mine creighton mine canada the long valley caldera in eastern california and a 7 year period of recorded seismic activity in southern california the observed subdiffusion processes are indicative of the general space time scaling of the system taking the form of a slow power law growth r t similar to t h of the mean distance r t between the main event arid the temporally correlated afterevents occuring after a delay t h is found on average to be small 0 1 for creighton mine 0 22 for the long valley caldera and 0 22 for the southern california main events with magnitude greater than or equal to 1 5 but fluctuates significantly from one main event to the other the diffusion is found to be intermittent non gaussian and multiscaling and except for the long valley caldera a systematic correlation between the sizes of the main event and subsequent afterevents and the growth exponent h is observed while classical viscous relaxation models e g elastic listhosphere plastic asthenosphere coupling or fluid flow triggered by sudden changes in pore pressure have been proposed to characterize this relaxation by homogeneous i e nonintermittent normal h 0 5 diffusion processes the direct implication of the reported results is that seismicity systems at spatial scales from meters to hundreds of kilometers and small microearthquakes in a mine to intermediate magnitudes relax spatiotemporally in a nonelastic way revealing the stochastic space time scale invariant nature of such systems since these diffusion processes correspond to a loss of information with time on the location of the main event they can be used to investigate the limits of predictability at all spatial scales of seismicity systems in terms of the spatiotemporal clustering of temporally correlated earthquakes

snow dunes and glazed surfaces in antarctica new field and remote sensing data as part of the international trans antarctic scientific expedition project the italian antarctic programme undertook two traverses from the terra nova station to talos dome and to dome c along the traverses the party carried out several tasks drilling glaciological and geophysical exploration the difference in spectral response between glazed surfaces and snow makes it simple to identify these areas on visible near infrared satellite images integration of field observation and remotely sensed data allows the description of different mega morphologic features wide glazed surfaces sastrugi glazed surface fields transverse dunes and megadunes topography global positioning system ground penetrating radar and detailed snow surface surveys have been carried out providing new information about the formation and evolution of mega morphologic features the extensive presence up to 30 of glazed surface caused by a long hiatus in accumulation with an accumulation rate of nil or slightly negative has a significant impact on the surface mass balance of a wide area of the interior part of east antarctica the aeolian processes creating these features have important implications for the selection of optimum sites for ice coring because orographic variations of even a few metres per kilometre have a significant impact on the snow accumulation process remote sensing surveys of aeolian macro morphology provide a proven high quality method for detailed mapping of the interior of the ice sheet s prevalent wind direction and could provide a relative indication of wind intensity

fault behavior and characteristic earthquakes examples from the wasatch and san andreas fault zones paleoseismological data for the wasatch and san andreas fault zones have led to the formulation of the characteristic earthquake model which postulates that individual faults and fault segments tend to generate essentially same size or characteristic earthquakes having a relatively narrow range of magnitudes near the maximum analysis of scarp derived colluvium in trench exposures across the wasatch fault provides estimates of the timing and displacement associated with individual surface faulting earthquakes at all of the sites studied the displacement per event has been consistently large measured values range from 1 6 to 2 6 m and the average is about 2 m on the basis of variability in the timing of individual events as well as changes in scarp morphology and fault geometry six major segments are recognized along the wasatch fault on the basis of the most likely number of surface faulting events 18 that have occurred on segments of the wasatch fault zone during the past 8000 years an average recurrence interval of 400 666 years with a preferred average of 444 years is calculated for the entire zone geologic data on the distribution of slip associated with prehistoric earthquakes and slip rates along the south central segment of the san andreas fault suggest that the m 8 1857 earthquake is a characteristic earthquake for this segment comparisons of earthquake recurrence relationships on both the wasatch and san andreas faults based on historical seismicity data and geologic data show that a linear constant b value extrapolation of the cumulative recurrence curve from the smaller magnitudes leads to gross underestimates of the frequency of occurrence of the large or characteristic earthquakes only by assuming a low b value in the moderate magnitude range can the seismicity data on small earthquakes be reconciled with geologic data on large earthquakes the characteristic earthquake appears to be a fundamental aspect of the behavior of the wasatch and san andreas faults and may apply to many other faults as well

on entropy and clustering in earthquake hypocentre distributions summary the degree of clustering or disorder within earthquake distributions may be measured using the concept of entropy a method for calculating the entropy of any 3 d point set e g earthquake foci is presented this makes use of voronoi cells convex polyhedra representing nearest neighbour regions to measure point density in three dimensions an estimate of event density can be determined directly from the size of voronoi cells normalizations are introduced to the definition of entropy that allow data sets containing different numbers of events and occupying different volumes to be compared quantitatively for example earthquake catalogues from different tectonic regimes our results show a clear correlation between earthquake entropy and tectonic regime the most ordered are the mid ocean ridges followed by the subduction zones and finally intraplate seismicity we show how entropy may be used to quantify the simplification of earthquake distributions for example due to relocation procedures a recently published algorithm called the collapsing method is used as an example of a technique that reduces entropy while respecting data fit modifications to this method are made that reduce artefacts and use additional temporal information in the earthquake distribution these methods are applied to a global catalogue of 85 000 events and a local catalogue from the sil network in iceland containing 43 300 events the entropy of both catalogues is reduced results from the hengill region within the sil network show lineations whose orientations agree with independent studies using relative location techniques and surface faulting

abrupt climatic change related to unstable ice sheet dynamics toward a new paradigm ice streams are fast currents of ice that drain some 90 of present day ice sheets and probably drained a similar percent of ice from former ice sheets ice streams may have life cycles akin to but much longer than surge cycles of certain mountain glaciers the floating tongues of marine ice streams coalesce to form floating ice shelves that buttress marine ice sheets and the grounded lobes of terrestrial ice streams stabilize terrestrial ice sheets ice shelves and ice lobes form during the life cycles of ice streams and disintegrate rapidly to initiate new life cycles successive life cycles continue until the ice sheet collapses a new paradigm is postulated in which abrupt climatic change is driven by iceberg outbursts associated with life cycles of marine ice streams and dust storms associated with life cycles of terrestrial ice streams for pleistocene northern hemisphere ice sheets these mechanisms regulated north atlantic surface temperatures and salinity and perhaps planetary albedo and atmospheric co2 and thereby drove climatic change which tended to be abrupt because ice stream life cycles began and ended abruptly ice streams proliferated along the margins of northern hemisphere ice sheets after they advanced beyond the crystalline shields of north america and eurasia and spread over softer sedimentary rocks overlain by thick ice cemented sediments that progressively thawed especially along river valleys and marine troughs an increasing fraction of these ice sheets was then subjected to the unstable life cycles of ice streams as seen in the pronounced spikiness of the late glacial oxygen isotope stratigraphy in the greenland ice core at dye 3 life cycles of greenland ice streams during the holocene seem to be linked to episodes of regional climatic cooling most recently the little ice age when superimposed on the present milankovitch hemicycle of reduced insolation over the north atlantic this could abruptly initiate a new worldwide glaciation cycle in the aftermath of present day co2 greenhouse warming proliferation of antarctic ice stream life cycles during the period of greenhouse warming may also trigger global climatic change by forcing a reorganization of circumpolar thermohaline circulation in the southern ocean

katabatic wind intensity and direction over antarctica derived from scatterometer data although satellite radar scatterometers are initially designed to measure ocean surface wind speed and direction they also provide important observations over continental ice sheets data from the seasat scatterometer analysed over a sector of antarctica show that the signal is strongly dependent on the incidence angle of observation but that it also depends on azimuth ledroit et al 1992 the minimum values of the backscatter coefficient the ratio of backscattered versus incident power are always observed in the direction of katabatic winds a theoretical analysis shows that the backscatter coefficient must be very sensitive to the sastrugi slopes and orientations which are streamlined features formed on the snow surface in the wind direction parish and bromwich 1987 satellite scatterometers can thus map the characteristics of sastrugis of importance for the interpretation of microwave measurements above ice sheets this indirectly provides the intensity and direction of these very persistent and strong winds which play a important role in the behaviour of the atmosphere and ocean in high southern latitudes

effect of slab temperature on deep earthquake aftershock productivity and magnitude frequency relations deep earthquakes generally show fewer aftershocks and a larger variability in magnitude frequency relations than shallow earthquakes1 5 these characteristics and the factors that control them may place constraints on the mechanism of deep earthquakes which is still uncertain6 8 here we show that systematic variations in aftershock productivity and magnitude frequency relations are related to the temperature of the downgoing slab as inferred from the vertical velocity and age of the subducting lithosphere deep earthquakes with substantial aftershock sequences are found only in colder slabs and earthquakes in warmer slabs consistently show few aftershocks magnitude frequency relations for deep earthquakes also show systematic variations as a function of slab thermal structure with warmer slabs showing fewer small earthquakes lower b values the statistical properties of deep earthquakes thus appear to be temperature sensitive to an extent not observed for shallow earthquakes and not adequately explained by simple geometrical limitations on fault width

surface airflow over east antarctica abstract surface winds over the antarctic interior occur mainly due to the strong radiational cooling of the ice slopes as a consequence such winds exhibit a high degree of persistence with a predominant direction closely related to the terrain orientation using detailed contour maps of the interior ice topography and representative values of the mean wintertime strength of the temperature inversion it is possible to infer the terrain induced accelerations a simple diagnostic equation system is formulated from which a time averaged surface airflow pattern of east antarctica is generated the results appear consistent with observations the occurrence of localized anomalously strong katabatic winds is explained as a result of typographically forced patterns of cold air convergence depicted in the airflow analysis

the role of eolian processes in forming surface features of the martian polar layered deposits abstract the major topographic features of the martian polar layered deposits include the conspicuous spiral troughs subtle undulations broad reentrants chasma steep arcuate scarps and dune fields a prominent role for eolian processes in the formation of all of these features is proposed strong katabatic winds occur on the north polar cap producing widespread frost streaks these cold drainage winds are analogous to those on the terrestrial antarctic cap in the chasmae convergent katabatic winds create yardangs and erode arcuate scarps in the layered deposits that are also the source for local dune fields migrating outward from the polar cap the chasmae are postulated to originate by long term erosion by katabatic wind wind plays a role in formation of the troughs and scarps primarily through removal of dust freed from exposed layered deposits by ablation of ice on defrosted equator facing slopes the undulations are created by interaction between surface erosion or deposition of perennial ice and standing waves in the shallow katabatic windflow the south polar layered deposits exhibit topographic features similar to those at the north polar cap but erosion and depositional processes creating these features may be inactive during the current epoch because of the lack of an extensive perennial ice cover and associated katabatic wind

one slope or two detecting statistically significant breaks of slope in geophysical data with application to fracture scaling relationships the scaling of displacement as a function of length is important for a variety of applications which depend on the mechanical and hydraulic properties of faults and fractures recently it has been suggested that the power law exponent which has been found to characterise this relationship may change significantly at a characteristic length for a variety of reasons for example when cracks begin to interact or when faults grow to a length comparable to a characteristic size in the brittle layer such a break of slope requires a second straight line requiring two extra model parameters here we present a new method for analysing such data which penalises the extra parameters using a modified form of schwarz s information criterion and a bayesian approach which represents uncertainty in the unknown parameters we apply the method to data from the krafla fissure zone in the north of iceland and find a significant break of slope from 3 2 to 2 3 at a characteristic length of 12m

the influence of antarctica on the momentum budget of the southern extratropics the antarctic plateau acts as a strong heat sink for the global climate cooling the atmosphere and radiating energy to space a cold dense atmospheric boundary layer is formed strong surface winds are formed as the boundary layer drains off the plateau these drainage winds and the eddy fluxes necessary to maintain them are analysed in a general circulation model gcm the drainage flow is well represented in the gcm the associated mean meridional circulation is analysed in isentropic coordinates the momentum budget over antarctica reveals a balance between the eliassen palm flux convergence and the coriolis torque exerted by the mean meridional mass flux both vertical and horizontal components of the eliassen palm flux contribute the vertical component being the greater

observations of the antarctic ice sheet with the seasat scatterometer relation to katabatic wind intensity and direction the seasat a satellite scatterometer radar initially designed to measure ocean wind intensity and direction also provided observations on the antarctic ice sheet the signal of the back scatter coefficient decreases strongly from 10 to 20 db when the incidence angle of the observations increases from 0 to 65 an additional 5 db signal is found which is correlated with the direction and intensity of katabatic winds independent of the incidence angle and polarization of the signal by using simplified models of the volume scattering within the snowpack which is mostly sensitive to snow grain size and surface scattering from the air snow interface which depends on roughness it is evident that the signal of the scatterometer could result from the effects of snow dunes at low incidence angle and of micro roughness and volume back scatter at incidence angles greater than 25 the instrument therefore provides a means of measuring the direction and intensity of katabatic winds

polarimetric decomposition over glacier ice using long wavelength airborne polsar in recent years there has been increased interest in using synthetic aperture radar sar to study and monitor glaciers and ice sheets for glaciological and climate change research however due to the medium s complexity sar backscattering from ice remains poorly understood including the relative importance of scattering from surface and volume layers and also dependences on frequency and glacier zone extreme weather conditions can result in quickly changing surface conditions influencing backscatter signatures while leaving the underlying volume of interest unchanged surface and volume components must thus be separated in order to infer information regarding the properties of the ice volume this paper describes a three component scattering model to decompose polarimetric sar polsar images of glacier ice total backscatter is modeled as the incoherent summation of surface volume and sastrugi wind induced feature components the proposed model adapts and extends the freeman and durden decomposition for an ice volume scenario in which the volume is a dielectric medium forms of the model for both random and oriented volumes are considered and a new oriented sastrugi component is introduced which is able to explain backscatter behavior between different winter scenes validation is performed with airborne polsar data at l and p band collected using the e sar system of the german aerospace center over the austfonna ice cap in svalbard norway as part of the icesar campaign

configurational entropy of critical earthquake populations we present an approach to describe the evolution of distributed seismicity by configurational entropy we demonstrate the detection of phase transitions in the sense of a critical point phenomenon in a 2d site percolation model and in temporal and spatial vicinity to the 1992 m7 3 landers earthquake in southern california our findings support the assumption of intermittent criticality in the earth s crust we also address the potential usefulness of the method for earthquake catalogue declustering

the surface windfield over the antarctic ice sheets the intense radiative cooling of air over the ice slopes of antarctica generates a surface wind regime that is strongly controlled by topography and plays a key role in determining the behaviour of the atmosphere and ocean in high southern latitudes1 7 resultant surface winds are intimately linked to the orientation of the ice terrain fig 1 and display the highest degree of persistence found on earth the close coupling between wind and topography allows estimation of the former if the latter is known with some precision here we report on time averaged near surface airflow over the antarctic continent during winter diagnosed from a recent accurate synthesis of terrain slopes and from estimates of the lower atmospheric temperature structure the simulated drainage pat tern exhibits strong spatial variability with the airflow concentrated into several zones near the coastal margin these confluence regions are responsible for strong persistent katabatic winds over downstream coastal stretches and are indicative of zones of greatest katabatic potential