an automated time window selection algorithm for seismic tomography we present flexwin an open source algorithm for the automated selection of time windows on pairs of observed and synthetic seismograms the algorithm was designed specifically to accommodate synthetic seismograms produced from 3 d wavefield simulations which capture complex phases that do not necessarily exist in 1 d simulations or traditional traveltime curves relying on signal processing tools and several user tuned parameters the algorithm is able to include these new phases and to maximize the number of measurements made on each seismic record while avoiding seismic noise our motivation is to use the algorithm for iterative tomographic inversions in which the synthetic seismograms change from one iteration to the next hence automation is needed to handle the volume of measurements and to allow for an increasing number of windows at each model iteration the algorithm is sufficiently flexible to be adapted to many tomographic applications and seismological scenarios including those based on synthetics generated from 1 d models we illustrate the algorithm using data sets from three distinct regions the entire globe the japan subduction zone and southern california

automated seismic waveform location using multichannel coherency migration mcm ii application to induced and volcano tectonic seismicity locating microseismic events is essential for many areas of seismology including volcano and earthquake monitoring and reservoir engineering due to the large number of microseismic events in these settings an automated seismic location method is required to perform real time seismic monitoring the measurement environment requires a precise and noise resistant event location method for seismic monitoring in this paper we apply multichannel coherency migration mcm to automatically locate microseismic events of induced and volcano tectonic seismicity using sparse and irregular monitoring arrays compared to other migration based methods in spite of the often sparse and irregular distribution of the monitoring arrays the mcm can show better location performance and obtain more consistent location results with the catalogue obtained by manual picking our mcm method successfully locates many triggered volcano tectonic events with local magnitude smaller that 0 which demonstrates its applicability on locating very small earthquakes our synthetic event location example at a carbon capture and storage site shows that continuous and coherent drilling noise in industrial settings will pose great challenges for source imaging however automatic quality control techniques including filtering in the frequency domain and weighting are used to automatically select high quality data and can thus effectively reduce the effects of continuous drilling noise and improve source imaging quality the location performance of the mcm method for synthetic and real microseismic datasets demonstrates that the mcm method can perform as a reliable and automatic seismic waveform analysis tool to locate microseismic events

seismic arrival enhancement through the use of noise whitening abstract a constant feature in seismic data noise is particularly troublesome for passive seismic monitoring where noise commonly masks microseismic events we propose a statistics driven noise suppression technique that whitens the noise through the calculation and removal of the noise s covariance noise whitening is shown to reduce the noise energy by a factor of 3 5 resulting in microseismic events being observed and imaged at lower signal to noise ratios than originally possible whilst having negligible effect on the seismic wavelet the procedure is shown to be highly resistant to most changes in the noise properties and has the flexibility of being used as a stand alone technique or as a first step before standard random noise attenuation methods

observations theory and modeling of the differential accumulation of antarctic megadunes antarctic megadunes are characterized by significant spatial differences in accumulation rate with higher accumulation on the windward side and near zero accumulation on the lee side this leads to spatial differences in physical properties of snow and surface roughness as well as to the upwind migration of the megadunes while previous studies agree that megadunes are a result of complex interactions between wind topography and snow it is not clear how they form or why they accumulate on the windward side here we use icesat observations dimensional analysis and atmospheric flow modeling to investigate what conditions are responsible for the accumulation patterns and upwind migration of the megadunes first we use icesat data to quantify the pattern of differential surface elevation change across the megadunes we then use dimensional analysis based on supercritical flow theory and atmospheric flow modeling to show that the megadunes topography and a stable atmosphere will always lead to upwind dune migration we show that a combination of persistent katabatic winds strong stability and spatial variability in surface roughness is responsible for the accumulation on the upwind slope and hence the upwind migration of the megadunes we further show that spatial differences in surface roughness are the primary control on accumulation magnitudes and hence dune migration velocity the dune migration velocity in turn influences the degree of snow metamorphism and the physical properties of snow that are relevant for paleoclimate records our findings pertain to the ongoing evolution of the megadunes but their genesis remains an open question

automated seismic event location by waveform coherence analysis summary automated location of seismic events is a very important task in microseismic monitoring operations as well for local and regional seismic monitoring since microseismic records are generally characterized bylowsignal to noiseratio automated locationmethods arerequested to be noise robust and sufficiently accurate most of the standard automated location routines arebasedontheautomatedpicking identificationandassociationofthefirstarrivalsofpands waves and on the minimization of the residuals between theoretical and observed arrival times of the considered seismic phases although current methods can accurately pick p onsets the automatic picking of the s onset is still problematic especially when the p coda overlaps the s wave onset in this paper we propose a picking free earthquake location method based on the use of the short term average long term average sta lta traces at different stations as observed data for the p phases we use the sta lta traces of the vertical energy function whereas for the s phases we use the sta lta traces of a second characteristic function which is obtained using the principal component analysis technique in order to locate the seismic event we scan the space of possible hypocentral locations and origin times and stack the sta lta traces along the theoretical arrival time surface for both p and s phases iterating this procedure on a 3 d grid we retrieve a multidimensional matrix whose absolute maximum corresponds to the spatial coordinates of the seismic event a pilot application was performed in the campania lucania region southern italy using a seismic network irpinia seismic network with an aperture of about 150km we located 196 crustal earthquakes depth 20km with magnitude range 1 1 ml 2 7 a subset of these locations were compared with accurate manual locations refined by using a double difference technique our results indicate a good agreement with manual locations moreover our method is noise robust and performs better than classical location methods based on the automatic picking of the p and s waves first arrivals

continuous kurtosis based migration for seismic event detection and location with application to piton de la fournaise volcano la reunion abstract we present an automatic earthquake detection and location technique based on migration of continuous waveform data data are preprocessed using a kurtosis estimator in order to enhance the first arrival information then migrated onto a predefined search grid using precalculated p wave travel times and finally stacked local maxima in the resulting 4d space time grid indicate the locations and origin times of seismic events we applied our technique to earthquake swarms occurring on piton de la fournaise volcano la reunion france we located 5000 events from 12 different swarms that occurred between 2009 and 2011 our automated locations are consistent with those performed using manual picks and indicate that the seismicity concentrates around sea level multiplet analysis of the detected events and subsequent double difference relocation produce sharper images of the earthquake swarms our code waveloc is released in open source online material figures of seismicity distributions from waveloc synthetic test and stack amplitude values versus magnitudes

rapid changes of glacial climate simulated in a coupled climate model abrupt changes in climate termed dansgaard oeschger and heinrich events have punctuated the last glacial period approximately 100 10 kyr ago but not the holocene the past 10 kyr here we use an intermediate complexity climate model to investigate the stability of glacial climate and we find that only one mode of atlantic ocean circulation is stable a cold mode with deep water formation in the atlantic ocean south of iceland however a warm circulation mode similar to the present day atlantic ocean is only marginally unstable and temporary transitions to this warm mode can easily be triggered this leads to abrupt warm events in the model which share many characteristics of the observed dansgaard oeschger events for a large freshwater input such as a large release of icebergs the model s deep water formation is temporarily switched off causing no strong cooling in greenland but warming in antarctica as is observed for heinrich events our stability analysis provides an explanation why glacial climate is much more variable than holocene climate

effects of glacial meltwater in the giss coupled atmosphere ocean model 2 a bipolar seesaw in atlantic deep water production the relationship between the northern hemisphere and southern hemisphere deep water circulation systems is explored in experiments with gradual and impulsive freshwater input through the st lawrence with sufficient freshwater volume input 50 sv years north atlantic deep water nadw cessation occurs as does substantial cooling in the northern hemisphere the colder temperatures are accompanied by increased mass and sea level pressure in the northern hemisphere with corresponding lower pressure in the southern hemisphere the low pressure response occurs at high southern latitudes consistent with the antarctic annular mode the leading mode of variability in the current climate stronger winds associated with this increased cyclonicity intensify the antarctic circumpolar current acc with heat flux divergences in the south atlantic and convergences and warming in the indian ocean weddell sea bottom water production increases in response to the south atlantic high latitude cooling and sea ice growth hence acting as a seesaw with the decreasing nadw and even global antarctic bottom water increases although not as strongly the initial seesaw response occurs within a few years although it takes some 100 years to maximize due to the response time of the acc the south atlantic cooling occurs approximately in phase with the north atlantic so the seesaw is not in temperature within that ocean basin however warming in portions of the southern indian ocean occurs out of phase with the northern hemisphere cooling nadw does not resume of its own accord once complete cessation occurs even when freshwater input is stopped but when increased evaporation is used to force nadw formation temporarily weddell sea bottom water decreases accordingly

origin and consequences of cyclic ice rafting in the northeast atlantic ocean during the past 130 000 years deep sea sediment cores recovered from the northeast atlantic ocean were examined in order to elucidate the influence of the earth s orbital parameters on major ice rafting analyses of coarse grained ice rafted debris and planktonic foraminifers revealed a strong reaction to the precession signal since 130 000 yr b p dropstone layers have been deposited each half period of a precessional cycle 11 000 1000 yr ice rafting occurs during times of winter minimum summer maximum insolation and summer minimum winter maximum insolation in the first case high summer insolation forces meltwater discharge from the ice sheets into the polar seas which subsequently enhances formation of sea ice during the winter in the second case growth of continental ice enhances iceberg production which also leads to a salinity reduction of surface seawater both situations result in a southward penetration of polar water thus the marine record of dropstones documents ice rafting not only during weichselian stades but also during cold events within interstades the regularity of ice rafting yields a useful framework to calibrate and elucidate climatic changes not only in the region of the north atlantic ocean but also in remote areas such as the pacific ocean and the antarctic

thermohaline circulation hysteresis a model intercomparison we present results from an intercomparison of 11 different climate models of intermediate complexity in which the north atlantic ocean was subjected to slowly varying changes in freshwater input all models show a characteristic hysteresis response of the thermohaline circulation to the freshwater forcing which can be explained by stommel s salt advection feedback the width of the hysteresis curves varies between 0 2 and 0 5 sv in the models major differences are found in the location of present day climate on the hysteresis diagram in seven of the models present day climate for standard parameter choices is found in the bi stable regime in four models this climate is in the mono stable regime the proximity of the present day climate to the stommel bifurcation point beyond which north atlantic deep water formation cannot be sustained varies from less than 0 1 sv to over 0 5 sv

the effect of surface curvature on the deposition of loess a physical model summary a physical model for the deposition of fine dusty material loess on curved surfaces is presented the model is tested in a sedimentation experiment with a symmetrical hill with length axis perpendicular to the flow direction in a wind tunnel it appears both from the model and the experiment that loess deposition will occur preferably on windward slopes and not on leeward ones in contrast to the widespread wind shadow concept below the deflation threshold loess deposition increases as wind speed increases above the deflation threshold an effective accumulation of the loess layer on the surface will occur within a well defined wind speed interval the loess layer on the surface will erode away only if wind speed has exceeded this interval for flat and dry surfaces free of roughness elements the upper limit of the interval is 7 m s free stream velocity for wet surfaces however it may be considerably higher 15 m s and more

atmospheric flow over a succession of nearly two dimensional ridges and valleys observations of neutral static stability flow in a nearly two dimensional valley are presented upstream and downstream of the valley are a succession of similar ridges and valleys the valley sides have a slope of about 32 to the horizontal and when the wind is nearly perpendicular to the valley there is a marked flow separation the observations of mean flow and turbulence are compared with numerical simulations of flow over periodic two dimensional ridges and valleys

strong hemispheric coupling of glacial climate through freshwater discharge and ocean circulation the climate of the last glacial period was extremely variable characterized by abrupt warming events in the northern hemisphere accompanied by slower temperature changes in antarctica and variations of global sea level it is generally accepted that this millennial scale climate variability was caused by abrupt changes in the ocean thermohaline circulation here we use a coupled ocean atmosphere sea ice model to show that freshwater discharge into the north atlantic ocean in addition to a reduction of the thermohaline circulation has a direct effect on southern ocean temperature the related anomalous oceanic southward heat transport arises from a zonal density gradient in the subtropical north atlantic caused by a fast wave adjustment process we present an extended and quantitative bipolar seesaw concept that explains the timing and amplitude of greenland and antarctic temperature changes the slow changes in antarctic temperature and its similarity to sea level as well as a possible time lag of sea level with respect to antarctic temperature during marine isotope stage 3

bedforms in a turbulent stream formation of ripples by primary linear instability and of dunes by nonlinear pattern coarsening it is widely accepted that both ripples and dunes form in rivers by primary linear instability the wavelength of the former scaling on the grain size and that of the latter being controlled by the water depth we revisit here this problem in a theoretical framework that allows to give a clear picture of the instability in terms of dynamical mechanisms a multi scale description of the problem is proposed in which the details of the different mechanisms controlling sediment transport are encoded into three quantities the saturated flux the saturation length and the threshold shear stress hydrodynamics is linearized with respect to the bedform aspect ratio we show that the phase shift of the basal shear stress with respect to the topography responsible for the formation of bedforms appears in an inner boundary layer where shear stress and pressure gradients balance this phase shift is sensitive to the presence of the free surface and the related effects can be interpreted in terms of standing gravity waves excited by topography the basal shear stress is dominated by this finite depth effect in two ranges of wavelength when the wavelength is large compared to the flow depth so that the inner layer extends throughout the flow and in the resonant conditions when the downstream material velocity balances the upstream wave propagation performing the linear stability analysis of a flat sand bed the relation between the wavelength at which ripples form and the flux saturation length is quantitatively derived it explains the discrepancy between measured initial wavelengths and predictions that do not take this lag between flow velocity and sediment transport into account experimental data are used to determine the saturation length as a function of grain size and shear velocity taking the free surface into account we show that the excitation of standing waves has a stabilizing effect independent of the details of the flow and sediment transport models consequently the shape of the dispersion relation obtained from the linear stability analysis of a flat sand bed is such that dunes cannot result from a primary linear instability we present the results of field experiments performed in the natural sandy leyre river which show the formation of ripples by a linear instability and the formation of dunes by a nonlinear pattern coarsening limited by the free surface finally we show that mega dunes form when the sand bed presents heterogeneities such as a wide distribution of grain sizes

flow structure and resistance over subaquaeous high and low angle dunes a prominent control on the flow over subaqueous dunes is the slope of the downstream leeside while previous work has focused on steep 30 asymmetric dunes with permanent flow separation little is known about dunes with lower lee slope angles for which flow separation is absent or intermittent here we present a laboratory investigation where we systematically varied the dune lee slope holding other geometric parameters and flow hydraulics constant to explore effects on the turbulent flow field and flow resistance three sets of fixed dunes lee slopes of 10 20 and 30 were separately installed in a 15 m long and 1 m wide flume and subjected to 0 20 m deep flow measurements consisted of high frequency vertical profiles collected with a laser doppler velocimeter we show that the temporal and spatial occurrence of flow separation decreases with dune lee slope velocity gradients in the dune leeside depict a free shear layer downstream of the 30 dunes and a weaker shear layer closer to the bed for the 20 and 10 dunes the decrease in velocity gradients leads to lower magnitude of turbulence production for gentle lee slopes aperiodic strong ejection events dominate the shear layer but decrease in strength and frequency for low angle dunes flow resistance of dunes decreases with lee slope the transition being nonlinear over the 10 20 and 30 dunes shear stress is 8 33 and 90 greater than a flat bed respectively our results demonstrate that dune lee slope plays an important but often ignored role in flow resistance

the role of the north atlantic drift in the millennial timescale glacial climate fluctuations abstract the greenland ice core records show that the overall cold climate of the last glacial period was repeatedly interrupted by short rapid warmings the events known as dansgaard oeschger d o events are strongly imprinted in the north atlantic marine records suggesting that they were linked to north atlantic circulation changes however the causes of the d o events are poorly understood and they represent one of the most intriguing puzzles of the last glacial period in order to investigate a possible mechanism we have studied variations in the distribution of benthic and planktonic foraminifera oxygen isotopes and ice rafted debris during the last glacial period in eight cores from the north atlantic and the nordic seas taken at mid depth from 853 to 1760 m the parameters indicate in agreement with previous studies that the circulation system during the interstadials resembled the present system atlantic surface water flowed north into the nordic seas where most of it sank through convection and as cold deep water flowed back into the north atlantic during the stadials a halocline was established in the nordic seas and in the northernmost part of the north atlantic and the outflow from the nordic seas stopped however below the cold light surface layer the relatively warm water of the north atlantic drift continued to flow across the north atlantic and into the nordic seas here warming up the deep water masses we suggest that the warm water gradually made the water column unstable the pivotal abrupt climate warmings were caused by sudden upwelling of the deep warm water masses causing overturning of the entire water column and onset of convection

measurements of the flow structure around ailsa craig a steep three dimensional isolated hill observations of neutral flow around an isolated island are presented the island has a fairly smooth shape with slopes of about 30 and a horizontal section which is roughly elliptical with axis ratio 1 5 the flow field 4 m above the surface shows a speed up around the sides and over the summit with reverse flow on the lee slope measurements with an instrumented aircraft of the mean flow in the wake of the island reveal a single very powerful trailing vortex with axis close to the upstream wind direction there are indications that the single vortex was a consequence of the asymmetry caused by the island s elliptical shape measurements of fluctuating quantities with a tethered balloon system show marked changes in the components of the turbulence energy in the flow around the side of the hill

two climatic states and feedbacks on thermohaline circulation in an earth system model of intermediate complexity the mcgill paleoclimate model 2 mpm 2 is employed to study climate thermohaline circulation thc interactions in a pre industrial climate with a special focus on the feedbacks on the thc from other climate system components the mpm 2 a new version of the mpm has an extended model domain from 90s to 90n active winds and no oceanic heat and freshwater flux adjustments in the mpm 2 there are mainly two stable modes for the atlantic meridional overturning circulation moc under the present day forcing present day solar forcing and the pre industrial atmospheric co2 level of 280 ppm the on mode has an active north atlantic deep water formation while the off mode has no such deep water formation by comparing the off mode climate state with its on mode analogue we find that there exist many large differences between the two climate states which originate from large changes in the oceanic meridional heat transports by suppressing or isolating each process associated with a continental ice sheet over north america sea ice the atmospheric hydrological cycle and vegetation feedbacks from these components on the atlantic moc are investigated sensitivity studies investigating the role of varying continental ice growth and sea ice meridional transport in the resumption of the atlantic moc are also carried out the results show that a fast ice sheet growth and an enhanced southward sea ice transport significantly favor the resumption of the atlantic moc in the mpm 2 in contrast to this the feedback from the atmospheric hydrological cycle is a weak positive one the vegetation albedo feedback could enhance continental ice sheet growth and thus could also favor the resumption of the atlantic moc however before the shut down of the atlantic moc feedbacks from these components on the atlantic moc are very weak

spectra of velocity components over complex terrain spectra have been measured over a variety of types of complex terrain on tops of hills and escarpments over land downstream of a water surface and over rolling terrain differences between spectra over many types of complex terrain and over uniform terrain can be explained by these hypotheses when air moves over terrain with changed characteristics then 1 for wavelengths very short relative to the fetch over the new terrain the spectral densities are in equilibrium with the new terrain 1 for wavelengths long compared to this fetch spectral densities remain unchanged if the ground is horizontal and decrease when the flow is uphill for the longitudinal velocity component only since vertical velocity spectra contain relatively less low wavenumber energy than horizontal velocity spectra energetic vertical velocity fluctuations tend to be in equilibrium with local terrain

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

microseismic monitoring and stability analysis of the left bank slope in jinping first stage hydropower station in southwestern china risks associated with the left bank slope of jinping first hydropower station in southwestern china are growing as a result of excavation of the dam foundation ditch and various tunnels as well as grouting to soft strata to improve our understanding of the slope instability and resolve the complex subsurface conditions of the highly fractured rock mass a microseismic monitoring system was installed in june 2009 which is the first time such a technique has been introduced into stability analysis of a high steep rock slope in china the microseismic monitoring system consists of a data acquisition unit a data processing unit and a three dimensional array of uniaxial accelerometers in order to analyze the correlation between seismicity and potential slip surface of the slope a numerical model was implemented to analyze the failure processes and stability of the slope the monitoring results illustrate that microseismic events mainly occur as a strip along the spandrel groove of the dam which coincides with the potential slip surface obtained from numerical analysis especially at the bottom of the slope therefore the comprehensive method incorporating microseismic monitoring and numerical analysis has been proven to be very promising in instability prediction of rock slope subject to identification and delineation of potential slip surface and failure regions

simulation of abrupt climate change induced by freshwater input to the north atlantic ocean temperature records from greenland ice cores1 2 suggest that large and abrupt changes of north atlantic climate occurred frequently during both glacial and postglacial periods one example is the younger dryas cold event broecker3 speculated that these changes result from rapid changes in the thermohaline circulation of the atlantic ocean which were caused by the release of large amounts of melt water from continental ice sheets here we describe an attempt to explore this intriguing phenomenon using a coupled ocean atmosphere model in response to a massive surface flux of fresh water to the northern north atlantic of the model the thermohaline circulation weakens abruptly intensifies and weakens again followed by a gradual recovery generating episodes that resemble the abrupt changes of the ocean atmosphere system recorded in ice and deep sea cores4 the associated change of surface air temperature is particularly large in the northern north atlantic ocean and its neighbourhood but is relatively small in the rest of the world

solution of a model for the oceanic pycnocline depth scaling of overturning strength and meridional pressure difference 1 we present an analysis of the model by gnanadesikan 1999 for the pycnocline depth in the ocean an analytic solution for the overturning strength as a function of the meridional pressure difference is derived and used to discuss their mutual scaling we show that scaling occurs only in two unphysical regimes of the model in the absence of the southern ocean so processes i e for a northern overturning cell the volume transport is proportional to the square root of the pressure difference linear scaling is seen when the overturning is restricted entirely to the so i e when no northern downwelling exists for comparison we present simulations with the coupled climate model climber 3 which show linear scaling over a large regime of pressure differences in the north atlantic na we conclude that the pycnocline model is not able to reproducethelinearscalingbetweenitstwocentralvariables pressure and volume transport index terms 4532 oceanography physical general circulation 4203 oceanography general analytical modeling 4267 oceanography general paleoceanography citation levermann a and a griesel 2004 solution of a model for the oceanic pycnocline depth scalingofoverturningstrengthandmeridionalpressuredifference

oceanic processes as potential trigger and amplifying mechanisms for heinrich events marine sediments recorded a series of heinrich events during the last glacial period massive ice surges thatdeposited prominent layers of ice rafted debris in the north atlantic here we explore oceanic mechanisms thatcan potentially trigger and amplify the observed ice calving events simulations of abrupt glacial climate changewith a coupled ocean atmosphere sea ice model show a substantial regional sea level rise in the north atlantic inresponse to a collapse of the atlantic meridional overturning circulation moc the increased heat uptake ofthe global ocean after the moc collapse leads to an additional rise in global sea level we hypothesize that thesesea level changes have the potential to destabilize northern hemisphere ice shelves and ice sheets and to triggerice surges sea level rise due to ice calving and subsurface ocean warming provides two positive feedbackmechanisms contributing to further destabilization of ice shelves and ice sheets

application of a boundary layer model to flow over an eolian dune as part of a larger project to model sand transport over isolated dunes and the temporal evolution of such dunes the present paper describes a wind flow simulation that results from an application of the ms3djh 1 5 model walmsley et al 1982 to the dune studied by howard et al 1978 a comparison is made between the calculations and observations of surface wind direction and wind speed at 80 cm above terrain on the upwind side of the dune the mean error and rms deviation from the mean error respectively are about 0 2 and 7 for surface wind direction and 9 09 and 0 08 for normalized wind speed

11 11 dune morphology and dynamics this chapter discusses the key types of sand dunes and how they can be classified and assesses the major controls on their development the processes of dune dynamics are reviewed leading to a discussion of the concept of equilibrium in solitary dune form recent advances in modeling are reviewed that show how interactions between dunes may result in equilibrium at the dune field scale rather than at the single dune scale

temporal geographical meltwater influences on the north atlantic conveyor implications for the younger dryas the temporal and geographical roles of meltwater discharge from the laurentide ice sheet on north atlantic deep water nadw production are investigated utilizing a global realistic geometry coupled climate model which does not require the use of flux adjustments model results suggest that preconditioning by meltwater discharge to the mississippi prior to the younger dryas yd is capable of pushing nadw beyond the limit of its sustainability the diversion of meltwater to the st lawrence then merely serves to completely inhibit nadw production the modeled change in surface air temperature generally agrees with the global pattern and magnitude of temperature change seen in paleoclimatic reconstructions of the yd and is intimately linked to changes in nadw formation the global thermohaline circulation provides an interhemispheric teleconnection with the southern oceans while changes in the atmospheric heat transport reacting to a global redistribution of oceanic heat transport provide a mechanism for interbasin teleconnection although the primary thermodynamic and hydrological cycle feedback processes are included within the atmospheric model in the absence of additional feedbacks an equilibrium without the presence of nadw is possible the inclusion of the wind stress speed feedback is found to significantly contribute to the resumption of nadw production as suggested by previous studies contrary to these same studies however the coupled model indicates an advective spin up timescale is required for resumption of nadw production and hence the termination of the modeled yd like climate event as opposed to a decadal century timescale the reason for the discrepancy is unclear but may be associated with the use of fixed salt flux fields applied in previous studies or the duration strength and geographical location of the imposed meltwater applied

ocean subsurface warming as a mechanism for coupling dansgaard oeschger climate cycles and ice rafting events 1 heinrich events have been attributed to surging of the laurentide ice sheet every 7 thousand years or so during glacial time these massive ice rafting events only occurred during cold phases of millennial scale dansgaard oeschger climate cycles other observed ice rafting events sourced from ice streams at various locations around the northern north atlantic occurred during all dansgaard oeschger cold phases and led heinrich events when the latter took place here it is suggested that ocean subsurface warming in the northern north atlantic during the cold phases may provide the key to explaining these climate ice rafting phasings such warming would lead to ice shelf melting and breakup without buttressing by ice shelves ice streams may surge leading to increased iceberg production this interpretation is supported by results of a simplified coupled climate model and by available sediment and ice sheet data

field measurements of mean and turbulent airflow over a barchan sand dune advances in our knowledge of the aeolian processes governing sand dune dynamics have been restricted by a reliance on measures of time averaged airflow such as shear velocity u it has become clear that such measures are incapable of explaining the complete dynamics of sediment transport across dune surfaces past evidence from wind tunnel and modelling studies has suggested that in some regions on a dune s surface the sediment transport might be better explained through investigations of the turbulent nature of the airflow however to date there have been no field studies providing data on the turbulent characteristics of the airflow around dunes with which to support or refute such hypotheses the field investigation presented here provides mean and turbulent airflow measurements across the centre line of a barchan sand dune in namibia data were collected using arrays of sonic anemometers and were compared with sand flux data measured using wedge shaped traps results support previously published data derived from wind tunnels and numerical models the decline in mean wind velocity at the upwind toe of the dune is shown to coincide with a rise in turbulence whilst mean velocity acceleration on the upper slope corresponds with a general decline in measured turbulence analysis of the components of reynold shear stress u w and normal stresses u 2 and w 2 supports the notion that the development of flow turbulence along the dune centre line is likely to be associated with the interplay between streamline curvature and mean flow deceleration acceleration it is suggested that due to the nature of its calculation turbulence intensity is a measure of less practical use than direct assessments of the individual components of reynolds stress particularly the instantaneous horizontal streamwise component u 2 and shear stress u w whilst increases in reynolds shear stress and the horizontal streamwise component of stress in the toe region of the dune may effectively explain the maintenance of sand flux in a region of declining mean velocity they have much less explanatory power for sand flux on the upper windward slope and in the crestal region of the dune here it is suggested that mean flow acceleration is likely to provide the most significant driving force on sand flux possibly augmented by a rise in the horizontal streamwise component of reynolds stress u 2 in the crest brink region therefore although wind turbulence is considered to be of fundamental importance in explaining the sediment transport dynamics across the dune s surface it is recognised that the interaction between mean flow deceleration acceleration streamline curvature and individual components of reynolds stress is complex and the identification of a single element of flow that offers a panacea for accounting for sand flux and dune dynamics is difficult to find

linking microseismic event observations with geomechanical models to minimise the risks of storing co2 in geological formations abstract for carbon capture and storage ccs in geological formations to be scientifically viable we must be able to model and monitor the effects of geomechanical deformation on the integrity of the caprock excess deformation may open fractures providing pathways for co2 leakage from the reservoir an acceptable geomechanical model must provide a good match with field observations microseismic activity is a direct manifestation of mechanical deformation so it can be used to constrain geomechanical models the aim of this paper is to develop the concept of using observations of microseismic activity to help ground truth geomechanical models microseismic monitoring has been ongoing at the weyburn co2 storage and monitoring project since 2003 we begin this paper by presenting these microseismic observations less than 100 events have been recorded documenting a low rate of seismicity most of the events are located close to nearby producing wells rather than the injection well a pattern that is difficult to interpret within the conventional framework for injection induced seismicity many events are located in the overburden without geomechanical simulation it is difficult to assess what these observations mean for the integrity of the storage formation to address these uncertainties we generate numerical geomechanical models to simulate the changes in stress induced by co2 injection and use these models to predict the generation of microseismic events and seismic anisotropy the initial geomechanical model that we generate using material properties based on laboratory core measurements does not provide a good match with either event locations or s wave splitting measurements made on the microseismic events we find that an alternative model whose reservoir is an order of magnitude softer than lab core sample measurements provides a much better match with observation as it leads shear stresses to increase above the production wells promoting microseismicity in these areas and generates changes in effective horizontal stresses that match well with s wave splitting observations this agreement between geophysical observations and a softer than lab measurements reservoir model highlights the difficulties encountered in upscaling lab scale results there is a strong need to link geomechanical models with observable manifestations of deformation in the field such as induced seismicity for calibration only then can we accurately assess the risks of leakage generated by mechanical deformation

analytical models for the mean flow inside dense canopies on gentle hilly terrain simplifications and scaling arguments employed in analytical models that link topographic variations to mean velocity perturbations within dense canopies are explored using laboratory experiments laser doppler anemometry lda measurements are conducted in a neutrally stratified boundary layer flow within a large recirculating flume over a train of gentle hills covered by a dense canopy the hill and canopy configuration are such that the mean hill slope is small and the hill is narrow in relation to the canopy h l 1 and lc l 1 where h is the hill height l the half length and lc the canopy adjustment length scale the lda data suggest that the often criticized linearizations of the advective terms turbulent shear stress gradients and drag force appear reasonable except in the deep layers of the canopy as predicted by a previous analytical model the lda data reveal a recirculation region within the lower canopy on the lee slope adjusting the outer layer pressure perturbations by a virtual ground that accounts for the mean streamline distortions induced by this recirculation zone improves this model s performance for the velocity perturbations in the deeper layers of the canopy a new analytical model which retains a balance between mean horizontal advection mean pressure gradient and mean drag force but neglects the turbulent shear stress gradient is developed the proposed model reproduces the lda measurements better than the earlier analytical model which neglected advection but retained the turbulent shear stress gradient in the lower layers of the canopy and near the hill top this finding is consistent with the fact that the earlier model was derived for tall hills in which advection inside the canopy remains small in essence the newly proposed model for the narrow hill studied here assumes that in the deeper layers of the canopy the spatial features of the mean flow perturbations around their background state can be approximated by the inviscid mean momentum equation we briefly discuss how to integrate all these findings with recent advances in canopy lidar remote sensing measurements of general topography and canopy height copyright 2008 royal meteorological society

rapid bottom melting widespread near antarctic ice sheet grounding lines as continental ice from antarctica reaches the grounding line and begins to float its underside melts into the ocean results obtained with satellite radar interferometry reveal that bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed the melting rate is positively correlated with thermal forcing increasing by 1 meter per year for each 0 1 c rise in ocean temperature where deep water has direct access to grounding lines glaciers and ice shelves are vulnerable to ongoing increases in ocean temperature

separation of upslope flow over a uniform slope motivated by the importance of understanding mountain weather during periods of thermal convection a laboratory study was conducted to investigate the separation of an upslope anabatic flow on a two dimensional heated mountainous slope in the absence of a background mean flow three flow regimes were identified in the first at slope angles larger than a critical value the separated flow generated a rising plume completely fed by the anterior upslope flow for this case a simple model based on a balance between the opposing vorticities of baroclinicity and shear was proposed to predict the location of the separation point relative to the mountain base the model also predicts the velocity and length scales at separation as well as those of the rising plume after separation in the second regime the volume flow of the separated plume was not fully supplied by the upslope flow requiring entrainment of additional ambient fluid at the base of the plume source the third regime occurred when wherein the plume almost completely engulfed the slope similar to a buoyant plume emanating from a source of finite dimensions thus overshadowing the upslope flow measurements of the separation point conducted during the materhorn field research program were consistent with the results of the laboratory experiments and modelling

evidence for enhanced mixing over rough topography in the abyssal ocean the overturning circulation of the ocean plays an important role in modulating the earth s climate but whereas the mechanisms for the vertical transport of water into the deep ocean deep water formation at high latitudes and horizontal transport in ocean currents have been largely identified it is not clear how the compensating vertical transport of water from the depths to the surface is accomplished turbulent mixing across surfaces of constant density is the only viable mechanism for reducing the density of the water and enabling it to rise however measurements of the internal wave field the main source of energy for mixing and of turbulent dissipation rates have typically implied diffusivities across surfaces of equal density of only approximately 0 1 cm2 s 1 too small to account for the return flow here we report measurements of tracer dispersion and turbulent energy dissipation in the brazil basin that reveal diffusivities of 2 4 cm2 s 1 at a depth of 500 m above abyssal hills on the flank of the mid atlantic ridge and approximately 10 cm2 s 1 nearer the bottom this amount of mixing probably driven by breaking internal waves that are generated by tidal currents flowing over the rough bathymetry may be large enough to close the buoyancy budget for the brazil basin and suggests a mechanism for closing the global overturning circulation

time dependent response of the global ocean clathrate reservoir to climatic and anthropogenic forcing 1 buffett and archer 2004 showed a model of the steady state global ocean clathrate reservoir that reproduces drill data and predicts a global methane inventory of about 5000 gton c the model inventory is very sensitive to ocean temperature with factor of two changes in methane inventory in response to temperature changes of 1 5 c here we explore the potential time dependent behavior of the clathrate reservoir assuming simple first order rate constants for methane accumulation and meltdown in response to changes in the predicted steady state reservoir for melting we divide the reservoir into two parts one melting catastrophically and the other slowly evolution of clathrates through geologic time is very sensitive to the choice of these rate constants a positive feedback exists during a transient meltdown event stronger if the catastrophic melting fraction is high and its melting timescale is fast a clathrate reservoir governed by such kinetics would melt down periodically on a multimillion year timescale determined by the rate constant for clathrate accumulation ideally we would like a model that predicts runaway melting in the paleocene and stability in the pleistocene however the model is most unstable when climate is cold rather than warm opting for stability in the pleistocene constrains the melting kinetic parameters leaving one degree of freedom the grow in timescale must be 5 myr or longer to get the deglacial 13c right but shorter than 10 myr in order for clathrates to have built up through geologically recent cooling the model thus constrained predicts methane fluxes of 200 gton c or less on deglaciations but eventual releases of 2000 4000 gton c in response to a 2000 gton c anthropogenic carbon release anthropogenic climate change differs from a deglaciation in that it warms the ocean to temperatures not seen in millions of years

coriolis effects in mesoscale flows with sharp changes in surface conditions a general linearized shallow layer perturbation model where the approximately neutral lower layer of thickness h0 is situated below a stable upper layer i e an inversion with temperature change t is developed for steady mesoscale atmospheric flows over low lying topography whose height is less than h0 with the coriolis parameter f sharp changes in surface conditions surface roughness terrain elevation heat flux are modelled as a distributed body force through the lower layer the froude number of this layer is small typical cases of mesoscale discontinuities are examined the results are compared with those of a continuously stratified model and observations and with numerical mesoscale model results for a meteorological case study over the dover straits region of the english channel the main results are i if the wind direction is parallel to the edge line separating the change in surface roughness there are marked increases and decreases in these coastal winds whose maxima can occur over the sea within a distance of order h0 1 km of a coast the strength of these wind jets which do not occur in the absence of coriolis force decrease away from the edge line gradually over transverse length scales of the order of the rossby deformation radius changes to surface roughness lead to an increase in the wind speed perturbation in the downwind direction until limited by non linear effects when the wind is at an angle to a roughness change or coast the maxima occur at the coastline ii where there are sharp changes in the orientation of contours of constant roughness length e g at capes or bays on the coastline or wakes of high drag areas detached jets are formed in the downwind direction iii changes in surface elevation at a coast produce effects different from those of roughness a positive wind jet forms parallel to the coast in the direction of the wind when the coast is on the right looking downwind and a negative jet when the coast is on the left these jets do not increase in strength along the flow and do not persist downwind iv coriolis effects also determine how the inversion height varies near coastlines and surface roughness changes for example increasing decreasing inland over a distance lr when stable airflow approaches from the sea and the coast is on the right left of an observer looking downwind opposite in the southern hemisphere this mechanism is consistent with observed increasing decreasing cloudiness inland from a coast v other effects occur where the surface elevation changes gradually over a distance of order lr e g a wide shallow valley frictional effects are comparable with buoyancy and coriolis forces and flows perpendicular to the elevation change are deflected to the left in the northern hemisphere as observed in the rhine valley vi the shallow layer model simulates the major features of the low level flow field computed using the numerical mesoscale model with a horizontal resolution of 2 km i e of order h0 broad features were captured using a coarser resolution of 12 km vii the analysis provides a method of estimating errors associated with finite grid size in numerical mesoscale models copyright 2004 royal meteorological society

rapid identification of earthquake rupture plane using source scanning algorithm summary accurate identification of an earthquake s rupture plane is critical in many aspects ranging from scientific such as seismology and plate tectonics to societal such as emergency rescue planning we modify the recently developed source scanning algorithm ssa to rapidly image the rupture pattern of an earthquake using waveform data recorded at local and regional distances the method calculates the brightness function of a grid point by summing the observed amplitudes of the p wave envelopes at the corresponding predicted arrival times at all stations the composite image consisting of brightness functions of all grid points within a prescribed source volume and time interval illuminates the locations on the rupture plane where significant seismic energy is emitted synthetic tests indicate that the proposed method is robust even under the presence of hypocentral mislocation and origin time errors application of this method to the 2003 san simeon and 2004 parkfield earthquakes both occurred in central california clearly identifies the nw se striking planes as the actual rupture planes a conclusion consistent with all available evidences limited azimuthal distribution of seismic stations will not severely affect the identification result making this method ideal for offshore earthquake studies because the proposed method is able to identify the rupture plane using seismic waveform data only it is perfectly suited for near real time operation and may enable routine report of earthquake rupture planes by local seismic networks in the future

comparison of surface and borehole locations of induced seismicity monitoring of induced microseismic events has become an important tool in hydraulic fracture diagnostics and understanding fractured reservoirs in general we compare microseismic event and their uncertainties using data sets obtained with surface and downhole arrays of receivers we first model the uncertainties to understand the effect of different acquisition geometries on location accuracy for a vertical array of receivers in a single monitoring borehole we find that the largest part of the final location uncertainty is related to estimation of the backazimuth this is followed by uncertainty in the vertical position and radial distance from the receivers for surface monitoring the largest uncertainty lies in the vertical position due to the use of only a single phase usually p wave in the estimation of the event location in surface monitoring results lateral positions are estimated robustly and are not sensitive to the velocity model in this case study we compare event location solutions from two catalogues of microseismic events one from a downhole array and the second from a surface array of 1c geophone our results show that origin time can be reliably used to find matching events between the downhole and surface catalogues the locations of the corresponding events display a systematic shift consistent with a poorly calibrated velocity model for downhole dataset for this case study locations derived from surface monitoring have less scatter in both vertical and horizontal directions

atlantic subsurface temperatures response to a shutdown of the overturning circulation and consequences for its recovery abstract using the coupled climate model of intermediate complexity climber 3 changes in the vertical thermal structure associated with a shutdown of the atlantic meridional overturning circulation amoc are investigated when north atlantic deep water formation is inhibited by anomalous freshwater forcing intermediate depth ventilation can remain active and cool the subsurface water masses i e the cold case however if intermediate ventilation is completely suppressed relatively warm water coming from the south penetrates to a high northern latitude beneath the halocline and induces a strong vertical temperature inversion between the surface and intermediate depth i e the warm case both types of temperature anomalies emerge within the first decade after the beginning of the freshwater perturbation the sign of subsurface temperature anomaly has a strong implication for the recovery of the amoc once the anomalous freshwater forcing is removed while the amoc recovers from the cold case

the role of the atlantic freshwater balance in the hysteresis of the meridional overturning circulation we have studied the response of the atlantic meridional overturning circulation to surface freshwater forcing using an ocean gcm coupled to an energy moisture balance atmosphere model the overturning collapses rapidly when a slowly increasing forcing applied to the north atlantic passes a positive threshold and spins up equally quickly when the forcing falls below a negative threshold this well known behaviour is referred to as hysteresis because the thresholds in forcing are different for the transitions in opposite directions however we argue that the behaviour of the atlantic salinity is more fundamental than the forcing hysteresis as a function of freshwater forcing occurs because the states with north atlantic overturning on and off each tend to reinforce their associated salinity distributions and inhibit the transition to the other state during the collapse the atlantic becomes less saline because of the import of 80 sv year of freshwater by ocean transports across 30 s during the spin up this freshwater is exported again we show that qualitatively similar hysteresis behaviour can be produced by perturbing the system without any net freshwater forcing the salinity flip flop is associated with the appearance and disappearance of a shallow reverse overturning circulation south of the equator which is present while the northern overturning is absent and may provide the mechanism for the ocean freshwater influx during collapse

airflow over hills of moderate slope a three layer model is presented this describes the flow on an inversion capped boundary layer over hills of moderate size and slope the linearized vorticity equation is solved using non linear boundary conditions the pressure field developed in the boundary layer is used to calculate a modified inner region velocity field jackson and hunt 1975 a low level inversion with height i 2h is found to decrease the speed up near the ground when i 2h the flow of the inversion layer shows an increasing tendency to be subcritical this leads to a much increased speed up increasing the stability of air above the inversion increases the asymmetry of the flow this causes an upwind lull and a subsequent rapid increase in the flow speed with very high wind speeds at the summit and downwind comparison of the theory with simple observations made on great dun fell cumbria shows encouraging agreement

rapid freshening of the deep north atlantic ocean over the past four decades the overflow and descent of cold dense water from the sills of the denmark strait and the faroe shetland channel into the north atlantic ocean is the principal means of ventilating the deep oceans and is therefore a key element of the global thermohaline circulation most computer simulations of the ocean system in a climate with increasing atmospheric greenhouse gas concentrations predict a weakening thermohaline circulation in the north atlantic as the subpolar seas become fresher and warmer1 2 3 and it is assumed that this signal will be transferred to the deep ocean by the two overflows from observations it has not been possible to detect whether the ocean s overturning circulation is changing but recent evidence suggests that the transport over the sills may be slackening4 here we show through the analysis of long hydrographic records that the system of overflow and entrainment that ventilates the deep atlantic has steadily changed over the past four decades we find that these changes have already led to sustained and widespread freshening of the deep ocean

uturuncu volcano bolivia volcanic unrest due to mid crustal magma intrusion uturuncu volcano sw bolivia is a dormant stratovolcano 85 km3 dominated by dacitic lava domes and flows 39ar 40ar ages show that the volcano was active between 890 ka and 271 ka with the lavas becoming younger and less extensive at higher elevations there are current signs of unrest between 1992 and 2006 geodetic satellite measurements record an ongoing 70 km deformation field with a central uplift rate of 1 to 2 cm yr deformation indicates volume changes of 400 x 108 m3 over 14 years an average of 1 m3 s 10 2 km3 yr the deformation is attributed to magma intrusion into the altiplano puna regional crustal magma body deformation models indicate a source at depths of 17 to 30 km beneath current local relief in a reconnaissance survey persistent seismic activity mean of 2 6 earthquakes per hour with a maximum of 14 per hour was recorded at about 4 km depth below the center of the uplift 4 km sw of the volcano s summit the seismic events have a normal b value 1 04 and activity is attributed to brittle deformation in the elastic crust above the active deep magma intrusion the porphyritic dacite lavas 64 68 sio2 have a plagioclase orthopyroxene biotite magnetite ilmenite assemblage and commonly contain juvenile silicic andesite inclusions cognate norite nodules and crustal xenoliths temperature estimates are in the range 805 to 872 c for the dacites and about 980 c for the silicic andesites the dacite magmas formed by fractional crystallization of andesite forming norite cumulates and involving partial melting of crust compositions and zoning patterns of orthopyroxene and plagioclase phenocrysts indicate that compositional variation in the dacites is caused by magma mixing with the silicic andesite reversely zoned orthopyroxene phenocrysts in the andesitic end member are explained by changing oxidation states during crystallization fe3 fe2 ratios from orthopyroxene crystals and fe3 in plagioclase provide evidence for a relatively reduced melt that subsequently ascended degassed and became more oxidized as a consequence of degassing the geophysical and petrological observations suggest that dacite magma is being intruded into the altiplano puna regional crustal magma body at 17 km or more depth consistent with deformation models in the late pleistocene dacitic and andesitic magmas ascended from the regional crustal magma body to a shallow magma system at a few kilometers depth where they crystallized and mingled together the current unrest together with geophysical anomalies and 270 ka of dormancy indicate that the magmatic system is in a prolonged period of intrusion such circumstances might eventually lead to eruption of large volumes of intruded magma with potential for caldera formation

an insar based survey of volcanic deformation in the central andes we extend an earlier interferometric synthetic aperture radar insar survey covering about 900 remote volcanos of the central andes 14 27 s between the years 1992 and 2002 our survey reveals broad 10s of km roughly axisymmetric deformation at 4 volcanic centers two stratovolcanoes are inflating uturuncu bolivia and hualca hualca peru another source of inflation on the border between chile and argentina is not obviously associated with a volcanic edifice here called lazufre and a caldera cerro blanco also called robledo in northwest argentina is subsiding we explore the range of source depths and volumes allowed by our observations using spherical ellipsoidal and crack like source geometries we further examine the effects of local topography upon the deformation field and invert for a spherical point source in both elastic half space and layered space crustal models we use a global search algorithm with gradient search methods used to further constrain best fitting models inferred source depths are model dependent with differences in the assumed source geometry generating a larger range of accepted depths than variations in elastic structure source depths relative to sea level are 8 18 km at hualca hualca 12 25 km for uturuncu 5 13 km for lazufre and 5 10 km at cerro blanco deformation at all four volcanoes seems to be time dependent and only uturuncu and cerro blanco were deforming during the entire time period of observation inflation at hualca hualca stopped in 1997 perhaps related to a large eruption of nearby sabancaya volcano in may 1997 although there is no obvious relation between the rate of deformation and the eruptions of sabancaya we do not observe any deformation associated with eruptions of lascar chile at 16 other volcanoes that had recent small eruptions or fumarolic activity or associated with a short lived thermal anomaly at chiliques volcano we posit a hydrothermal system at cerro blanco to explain the rate of subsidence there for the last decade we calculate the ratio of the volume of magma intruded to extruded is between 1 10 and that the combined rate of intrusion and extrusion is within an order of magnitude of the inferred geologic rate

coalescence microseismic mapping earthquakes are commonly located by linearized inversion of discrete arrival time picks made from signals recorded at a network of seismic stations if mis picks are made these will contribute to the location therefore causing potential bias for data recorded by a dense seismic array direct imaging methods can be applied instead we describe the coalescence microseismic mapping method which is a bridge between the two approaches and will operate with seismic data recorded continuously on a sparse array by continuously mapping scalar signals derived from the envelope of seismic arrivals we derive robust estimates of the spatio temporal coordinates of the origins of seismic events noisy data are migrated away from the correct origin so do not contribute to errors in location the method is rooted in a bayesian formulation of event location traveltime inversion allows imaging of source locations and has the capacity to handle errors in modelled traveltimes it has the advantage of working with any 3 d velocity model which therefore may include anisotropy it also automatically incorporates both p and s wave data a multiresolution grid search leads to an efficient implementation with a search over a larger domain including joint inversion for location and velocity structure possible where warranted by the data quality we discuss the theory and implementation of this method and illustrate it with real data from microseismic events in iceland caused by melt intrusion in the crust

simultaneous microseismic event localization and source mechanism determination abstractmicroseismic monitoring has become a tool of choice for the development and optimization of oil and gas production from unconventional reservoirs the primary objective of micro seismic monitoring includes localization of micro seismic events and characterization of their source mechanisms most seismic events are of a nonexplosive nature and thus there are waveform polarity differences among receivers specifically double couple sources represented a challenge for migration based localization techniques we developed and applied a new migration type location technique combined with source mechanism inversion that allowed for constructive interference of signal in seismic waveforms the procedure included constructing image functions by stacking the amplitudes with compensated polarity changes the compensation weights were calculated by using moment tensor inversion this method did not require any picking of arrivals at individual receivers but it required receivers to be distributed i

migration of early aftershocks following the 2004 parkfield earthquake a significant number of the early aftershocks following large shallow earthquakes are missing in existing earthquake catalogues an analysis of the waveforms of 3 647 relocated earthquakes along the parkfield section of the san andreas fault reveals 11 times more aftershocks within three days of the 2004 parkfield earthquake than listed in a standard catalogue large shallow earthquakes are immediately followed by numerous aftershocks a significant portion of these events is missing in existing earthquake catalogues mainly because seismicity after the mainshock can be masked by overlapping arrivals of waves from the mainshock and aftershocks1 2 3 4 however recovery of the missing early aftershocks is important for understanding the physical mechanisms of earthquake triggering2 3 4 and for tracking postseismic deformation around the rupture zone associated with the mainshock5 6 7 here we use the waveforms of 3 647 relocated earthquakes8 along the parkfield section of the san andreas fault as templates9 10 to detect missing aftershocks within three days of the 2004 magnitude 6 0 parkfield earthquake we identify 11 times more aftershocks than listed in the standard catalogue of the northern california seismic network we find that the newly detected aftershocks migrate in both along strike and down dip directions with logarithmic time since the mainshock consistent with numerical simulations of the expansion of aftershocks caused by propagating afterslip11 12 the cumulative number of early aftershocks increases linearly with postseismic deformation in the first two days supporting the view that aftershocks are driven primarily by afterslip13 14

assessment of 4d seismic repeatability and co2 detection limits using a sparse permanent land array at the aquistore co2 storage site abstracttwo 3d time lapse seismic surveys were acquired in 2012 and 2013 at the aquistore co2 storage site prior to the start of co2 injection using these surveys we determined the background time lapse noise at the site and assessed the feasibility of using a sparse areal permanent receiver array as a monitoring tool applying a standard processing sequence to these data we adequately imaged the reservoir at 3150 3350 m depth evaluation of the impact of each processing step on the repeatability revealed a general monotonic increase in similarity between the data sets as a function of processing the prestack processing sequence reduced the normalized root mean squared difference nrms from 1 13 between the raw stacks to 0 13 after poststack time migration the postmigration cross equalization sequence further reduced the global nrms to 0 07 a simulation of the changes in seismic response due to a range of co2 injection scenarios suggested that co2 was detectable within the reservoir at the aquistor

a numerical model of flow over sand waves in water of finite depth summary a model of turbulent flow above sand waves in water of finite depth is described closure assumptions are based on an eddy viscosity proportional to the square root of the local value of turbulent kinetic energy and mixing length dependent upon distance from the lower boundary results are presented for some idealized cases to investigate the effects of wave slope water depth froude number and wave shape the implications of the model for the transport of sediment are discussed and the development of the wave investigated it is found that the crest of the wave will become sharper for lower flow rates as has been observed in the sea comparisons are made with recent measurements made over sand waves in the columbia river

the role of streamline curvature in sand dune dynamics evidence from field and wind tunnel measurements abstract field measurements on an unvegetated 10 m high barchan dune in oman are compared with measurements over a 1 200 scale fixed model in a wind tunnel both the field and wind tunnel data demonstrate similar patterns of wind and shear velocity over the dune confirming significant flow deceleration upwind of and at the toe of the dune acceleration of flow up the windward slope and deceleration between the crest and brink this pattern including the widely reported upwind reduction in shear velocity reflects observations of previous studies such a reduction in shear velocity upwind of the dune should result in a reduction in sand transport and subsequent sand deposition this is not observed in the field wind tunnel modelling using a near surface pulse wire probe suggests that the field method of shear velocity derivation is inadequate the wind tunnel results exhibit no reduction in shear velocity upwind of or at the toe of the dune evidence provided by reynolds stress profiles and turbulence intensities measured in the wind tunnel suggest that this maintenance of upwind shear stress may be a result of concave unstable streamline curvature these additional surface stresses are not recorded by the techniques used in the field measurements using the occurrence of streamline curvature as a starting point a new 2 d model of dune dynamics is deduced this model relies on the establishment of an equilibrium between windward slope morphology surface stresses induced by streamline curvature and streamwise acceleration adopting the criteria that concave streamline curvature and streamwise acceleration both increase surface shear stress whereas convex streamline curvature and deceleration have the opposite effect the relationships between form and process are investigated in each of three morphologically distinct zones the upwind interdune and concave toe region of the dune the convex portion of the windward slope and the crest brink region the applicability of the model is supported by measurements of the rate of sand transport and the change of the dune surface in the field

seismicity associated with magmatism faulting and hydrothermal circulation at aluto volcano main ethiopian rift the silicic volcanic centres of the main ethiopian rift mer play a central role in facilitating continental rifting many of these volcanoes host geothermal resources and are located in heavily populated regions insar studies have shown several are deforming but regional seismic networks have detected little seismicity a local network of 12 seismometers was deployed at aluto volcano from 2012 to 2014 and detected 2142 earthquakes within a 24 month period we locate the events using a 1d velocity model that exploits a regional model and information from geothermal boreholes and calculate local magnitudes b values and focal mechanisms event depths generally range from the near surface to 15 km with most of the seismicity clustering in the upper 2 km a significant amount of seismicity follows the artu jawa fault zone which trends in alignment with the wonji fault belt nne ssw and is consistent with previous studies of strain localisation in the mer focal mechanisms are mostly normal in style with the mean t axes congruent to the orientation of extension in the rift at this latitude some show relatively small left lateral strike slip components and are likely associated with the reactivation of ne ene structures at the southern tip of the aluto gedemsa segment events range from 0 40 to 2 98 in magnitude and we calculate an overall b value of 1 40 0 14 this relatively elevated value suggests fluid induced seismicity that is particularly evident in the shallow hydrothermal reservoir and above it subdividing our observations according to depth identifies distinct regions beneath the volcanic edifice a shallow zone 2 0 km of high seismicity and high b values that corresponds to the hydrothermal system and is influenced by a high fluid saturation and circulation a relatively aseismic zone 0 2 km with low b values that is impermeable to ascending volatiles a region of increased fluid induced seismicity 2 9 km that is driven by magmatic intrusion from below and a deeper zone below 9 km that is interpreted as a partially crystalline magmatic mush these observations indicate that both the magmatic and hydrothermal systems of aluto volcano are seismically active and highlight the need for dedicated seismic monitoring at volcanoes in the mer

a wind tunnel simulation and field verification of desert dust deposition avdat experimental station negev desert the deposition of natural dust in an area of 53 ha situated in the northern negev desert is investigated in detail both in the wind tunnel dust storm simulations over a topographic scale model and in the field the wind tunnel results and the field results show a high degree of agreement indicating that scale model simulation may be considered an important technique for future loess and desert research more dust settles on windward slopes than on leeward slopes which is in contradistinction with the widespread wind shadow concept air flow separation zones immediately downwind of steep windward slopes have an important impact on dust deposition too in the case of dust deposition on topographic scale models a restricted height distortion of the model will not necessarily lead to serious problems in addition wind tunnel blockage percentages up to 13 may be allowed in order to obtain acceptable dust deposition patterns for the scale model a mean gross dust deposition of about 200 250 g m 2 year 1 is calculated for the northern negev desert for 1987 thus if the settled dust can be protected against erosion in the cultivated areas in the negev the dust content of the top soil will markedly increase with time however it has to be borne in mind that cultivation activities themselves may also contribute to a higher soil erosion and hence to a higher dust content in the atmosphere at any rate a higher dust content in the top soil will unquestionably have an important positive effect on agricultural yields from the air dust concentration data and the dust deposition data a deposition velocity of 4 7cms 1 can be calculated for avdat dust

evidence for massive discharges of icebergs into the north atlantic ocean during the last glacial period sediments in the north atlantic ocean contain a series of layers that are rich in ice rafted debris and unusually poor in foraminifera1 here we present evidence that the most recent six of these heinrich layers deposited between 14 000 and 70 000 years ago record marked decreases in sea surface temperature and salinity decreases in the flux of planktonic foraminifera to the sediments and short lived massive discharges of icebergs originating in eastern canada the path of the icebergs clearly marked by the presence of ice rafted detrital carbonate can be traced for more than 3 000 km a remarkable distance attesting to extreme cooling of surface waters and enormous amounts of drifting ice the cause of these extreme events is puzzling they may reflect repeated rapid advances of the laurentide ice sheet perhaps associated with reductions in air temperatures yet temperature records from greenland ice cores appear to exhibit only a weak corresponding signal moreover the 5 10 000 yr intervals between the events are inconsistent with milankovitch orbital periodicities raising the question of what the ultimate cause of the postulated cooling may have been

were glacial iceberg surges in the north atlantic triggered by climatic warming high resolution physical mineralogical sedimentological and micropalaeontological studies were carried out on north atlantic cores from the reykjanes ridge at 59degreesn and from the region southwest of the faeroe islands all core sites are situated along the pathway of iceland scotland overflow water isow and the various parameters measured display similar features previously identified carbonate oscillations keigwin and jones 1994 j geophys res 99 12397 12410 in the time span back to the marine isotope stage 5 4 transition and late glacial lithic events bond and lotti 1995 science 267 1005 1010 such as the heinrich ice rafting events are all represented in the core records long term trends and higher frequency changes in isow intensity were reconstructed on the basis of various independent proxy records the long term trends in circulation match theoretical orbitally forced insolation changes our observed links between ice rafted detritus ird input variations in sea surface temperature sst and circulation at greater depth point to the need to re examine the origin of ird events we suggest that these events may have been triggered by enhanced partly sub surface heat transport to the north enhanced northward heat transport may have caused bottom melting of floating outlet glaciers and ice shelves leading to increased iceberg discharge and ice sheet destabilization this discharge resulted in lower sst s and a lower temperature over greenland thus as shown by our records this scenario implies a temporary de coupling of surface processes and circulation at greater depth a key feature is the occurrence of a saw tooth pattern in the marine data which is similar to the greenland ice core records moreover the warming theory of ird events would explain the observed out of phase relationship between the greenland and antarctic ice core records and also the rapid establishment of higher temperatures over greenland immediately after the cold phases stadials of the dansgaard oeschger cycles less

diurnal variations in flow over a succession of ridges and valleys observations of flow in a nearly two dimensional valley located within a succession of similar ridges and valleys are presented measurements of the mean flow have been made at all times of the day and are found to be well characterized by a suitably defined froude number the unstable and neutral daytime flow shows a flow separation whilst when the froude number has a value of about two usually around evening and dawn transitions the flow is attached and the valley well ventilated for smaller values of froude number the flow speed in the valley reduces and a characteristic drainage flow occurs in addition to mean flow measurements and soundings of temperature structure measurements were also made of turbulence energy and stress this allows an estimate of the effective value of the surface roughness zo and an indication of the stability dependence of the momentum transfer to be made

enso suppression due to weakening of the north atlantic thermohaline circulation changes of the north atlantic thermohaline circulation thc excite wave patterns that readjust the thermocline globally this paper examines the impact of a freshwater induced thc shutdown on the depth of the pacific thermocline and its subsequent modification of the el nino southern oscillation enso variability using an intermediate complexity global coupled atmosphere ocean sea ice model and an intermediate enso model respectively it is shown by performing a numerical eigenanalysis and transient simulations that a thc shutdown in the north atlantic goes along with reduced enso variability because of a deepening of the zonal mean tropical pacific thermocline a transient simulation also exhibits abrupt changes of enso behavior depending on the rate of thc change the global oceanic wave adjustment mechanism is shown to play a key role also on multidecadal time scales simulated multidecadal global sea surface temperature sst patterns show a large degree of similarity with previous climate reconstructions suggesting that the observed pan oceanic variability on these time scales is brought about by oceanic waves and by atmospheric teleconnections

intermediate depth warming in the tropical atlantic related to weakened thermohaline circulation combining paleoclimate data and modeling results for the last deglaciation 1 benthic foraminiferal oxygen isotope ratios from two sediment cores recovered at 426 and 1299 m water depth in the eastern and western tropical atlantic show that a slowdown of the thermohaline circulation thc during heinrich event h1 and the younger dryas was accompanied by rapid and intense warming of intermediate depth waters millennial scale covariations of low paleosalinities in the subpolar north atlantic with decreased benthic oxygen isotope ratios in the eastern tropical atlantic throughout the past 10 000 years suggest that thc weakening might be related to middepth warming during the holocene period as well climate model experiments simulating a strong reduction of the thc in the atlantic ocean under present day and glacial conditions reveal that the increase of temperature in the middepth tropical and south atlantic is a common feature for both climatic states caused by a reduced ventilation of cold intermediate and deep waters in conjunction with downward mixing of heat from the thermocline from the similarity of the paleoclimatic records with the model simulations we infer that the characteristic pattern of temperature change in the atlantic ocean related to weakened thermohaline circulation can serve as an indicator of present day and future thc slowdown

performance of a second order moments advection scheme in an ocean general circulation model 1 the reliability of ocean general circulation models ogcms strongly depends on the quality of their tracer advection schemes for the sake of simplicity and computing time tracer advection schemes most commonly used in large scale ogcms tend to be low order schemes which suffer from spurious numerical diffusion and dispersion that result in distorted solutions the application of high order schemes would reduce numerical errors but at a considerable cost in terms of computing time an alternative to the use of high order methods is the implementation of algorithms that take into account the sub grid distribution of tracers one such method is the second order moments som scheme of prather 1986 which is more accurate than a fourth order scheme but at the time consumption of a second order algorithm this article presents results from coarse resolution global ocean simulations with very low explicit diapycnal mixing in which active and passive tracers were advected with the som method we compare the performance of the method with that of more traditional schemes namely the fct flux corrected transport and quicker quadratic upstream interpolation for convective kinematics schemes in general the use of the som method significantly improves tracer distributions and transports compared to fct and quicker thus leading to a better representation of ocean currents notably boundary currents and frontal systems while model simulations employing the fct and quicker schemes recreate a global overturning circulation with strong upwelling occurring in low latitudes the som simulations admit a circulation pattern closer to that known as the reconfigured conveyor belt toggweiler and samuels 1993 in which the bulk of the global ocean upwelling occurs in the southern ocean

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

a decomposition of the atlantic meridional overturning circulation into physical components using its sensitivity to vertical diffusivity abstract the sensitivity of the atlantic ocean meridional overturning circulation to the vertical diffusion coefficient in the global coupled atmosphere ocean sea ice model climber 3 is investigated an important feature of the three dimensional ocean model is its low diffusive tracer advection scheme the strength mmax of the atlantic overturning is decomposed into three components 1 the flow ms exported southward at 30 s 2 the large scale upward flow that balances vertical diffusion in the atlantic and 3 a wind dependent upwelling flux wbound along the atlantic boundaries that is not due to vertical diffusion the export of water at 30 s varies only weakly with but is strongly correlated with the strength of the overflow over the greenland scotland ridge the location of deep convection is found to be mixing dependent such that a shift from the nordic seas to the irminger sea is detected for high values of the ratio r ms mmax gives a measure of the interhemispheric overturning efficienc

the effective roughness length for scalar transfer in neutral conditions over hilly terrain numerical weather prediction models are required to represent the area averaged effects of hills on temperature and water vapour in the atmospheric boundary layer the transfer of these scalars within the boundary layer is affected by bouyancy however we have simplified the problem by simulating the behaviour of a passive scalar in a neutrally stratified turbulent boundary layer over hills the numerical model used for these simulations was fully nonlinear and flows were simulated over a range of slopes including ones where separation occurred the area averaged effects of hills are small over low slopes but significant at steeper slopes due to the impact of the separated region the area averaged scalar concentration varies logarithmically with height but in a region that is shallower than the logarithmic wind region we have found that using an effective roughness length for scalar transfer zoseff the effects of hills on area averaged scalars may be parametrized zoseff is smaller than the local value of roughness length for scalar transfer and for hills of certain wavelengths and roughness it is three orders of magnitude smaller a simplified interpretation of the physical processes simulated by the numerical model has been used to derive an expression for zoseff in terms of hill characteristics only this parametrization should be useful for representing the influence of hills on scalar transfer in numerical weather prediction and climate models

shallow seismicity triggered seismicity and ambient noise tomography at the long dormant uturuncu volcano bolivia using a network of 15 seismometers around the inflating uturuncu volcano from april 2009 to 2010 we find an average rate of about three local volcano tectonic earthquakes per day and swarms of 5 60 events a few times per month with local magnitudes ranging from 1 2 to 3 7 the earthquake depths are near sea level more than 10 km above the geodetically inferred inflation source and the altiplano puna magma body the mw 8 8 maule earthquake on 27 february 2010 triggered hundreds of earthquakes at uturuncu with the onset of the love and rayleigh waves and again with the passage of the x2 x3 overtone phases of rayleigh waves this is one of the first incidences in which triggering has been observed from multiple surface wave trains the earthquakes are oriented nw se similar to the regional faults and lineaments the b value of the catalog is 0 49 consistent with a tectonic origin of the earthquakes we perform ambient noise tomography using love wave cross correlations to image a low velocity zone at 1 9 to 3 9 km depth below the surface centered slightly north of the summit the low velocities are perhaps related to the hydrothermal system and the low velocity zone is spatially correlated with earthquake locations the earthquake rate appears to vary with time a seismic deployment from 1996 to 1997 reveals 1 5 earthquakes per day whereas 60 events day were seen during 5 days using one seismometer in 2003 however differences in analysis methods and magnitudes of completeness do not allow direct comparison of these seismicity rates the rate of seismic activity at uturuncu is higher than at other well monitored inflating volcanoes during periods of repose the frequent swarms and triggered earthquakes suggest the hydrothermal system is metastable