extensive liquid meltwater storage in firn within the greenland ice sheet surface melt water from the greenland ice sheet can become trapped in firn delaying its journey to the sea radar and ice core observations provide direct evidence of a perennial aquifer in the firn layer in southern greenland that represents a potentially significant contribution to the greenland mass budget

wetting front advance and freezing of meltwater within a snow cover 1 observations in the canadian arctic in a naturally stratified snow cover the movement of meltwater into dry snow is complicated by the interaction of the wetting front with stratigraphic horizons field observations showed that when the wetting front reached premelt stratigraphic horizons water ponded at the interface and then flow fingers developed and penetrated the lower stratum the flux in these fingers which was increased to about twice that of the surface flux was used to feed water to the impeding horizons where it froze to form ice layers these ice layers were the major source of latent heat released within the snow cover and they were responsible for the warming of the snow and the underlying soil these continuous ice layers grew only at stratigraphic boundaries because of this ice layer growth the wetting front advance was retarded and the arrival of meltwater at the snow cover base was significantly delayed owing to a cold substrate the strong heat flux from the snow into the soil delays the warming of the snow cover and limits runoff after the snow is isothermal at 0 c by the refreezing of soil infiltration and the development of a basal ice layer

recent advances in modeling swash zone dynamics influence of surf swash interaction on nearshore hydrodynamics and morphodynamics the role of the swash zone in influencing the whole nearshore dynamics is reviewed with a focus on the interaction between surf and swash zone processes local and global hydromorphodynamic phenomena are discussed in detail and a description of the overall swash zone operation is given the effects of swash zone boundary conditions are highlighted together with the importance of surf zone boundary conditions major emphasis is placed on illustrating the interactions of various hydrodynamic modes which in turn control the swash and surf zone morphology finally methods to account for swash zone processes in coastal models with different temporal and spatial resolutions are proposed

the influence of swash based reflection on surf zone hydrodynamics a wave by wave approach abstract a detailed understanding of the behaviour of waves in the nearshore is essential for coastal engineers as these waves cause beach erosion coastal flooding and damage to coastal structures significantly the influence of reflected waves is often neglected in surf zone studies although they are known to influence wave properties and circulation in the nearshore in this paper a phase resolving model is rigorously applied to model conditions from the prototype scale bardexii experiment in order to examine and assess the influence of swash based reflection on surf zone hydrodynamics at both the individual wave and time averaged timescales surface elevation is separated into incoming and outgoing signals using the radon transform and a crest tracking algorithm is used to extract incident and reflected wave properties it is found that on steep beaches tan 1 9 the swash based reflection the reflection generated in the swash during the backwash contributes significantly to the intrawave variability of individual wave properties such as the wave height to water depth ratio through the generation of quasi nodes antinodes system for expressed with individual wave heights variations up to 25 and 40 are obtained for the modelled regular and irregular wave tests whereas it reaches 15 when it is based on the significant wave height the outgoing wave field induced hydrodynamics is also found to affect time averaged parameters undertow and horizontal velocity skewness the undertow is mainly strengthened particularly in the shoaling region where the outgoing component dominates over the contribution from the incoming wave field offshore of the bar an onshore directed flow streaming close to the bed is also generated under the outgoing wave field and is suspected to help in stabilising the bar position this along with the influence of the outgoing wave field on the horizontal velocity skewness and the presence of quasi standing waves suggests a complex contribution of the hydrodynamics induced by swash based reflection into sediment transport rates and nearshore bar generation migration

observations and predictions of fluid velocities in the surf and swash zones 1 surfzone and swashzone water level fluctuations and cross shore fluid velocities observed at seven cross shore locations for 139 51 min periods on a low sloped fine grained sandy beach are shown to be modeled well by the depth averaged nonlinear shallow water equations the model initialized with observations about 150 m from the shoreline in 300 cm water depth accurately predicts the observed decrease in sea swell wave heights and orbital velocities with decreasing water depth across the surf zone and the observed maxima in infragravity orbital velocities and offshore directed mean flows near the outer edge of the swash zone in the surf zone both the observed and predicted velocity fluctuations are dominated by sawtoothed shaped pitched forward asymmetrical sea swell waves in the swash zone infragravity waves contribute substantially to the total velocity variance and asymmetry shoreward of the outer swash zone observed and predicted total and infragravity velocity asymmetry decrease onshore changing sign from positive to negative in the upper swash

wave setup and runup an integrated approach abstract by extending and sharpening the usual definitions of water depth and mean water level throughout the swash zone it is shown that the elevation of the shore line on an impermeable beach is logically at the runup limit not at the average water line level thus the runup limit becomes a point on the setup profile and this leads to a significant expansion of the experimental information on setup which can be extracted from the existing literature from the experimental data base thus extended a first approximation to a universal description of setup profiles is derived i e the most appropriate vertical and horizontal scaling parameters are suggested for the shape of the mean water surface in the inner surf zone existing setup data show that traditional simplistic setup models are inadequate for modelling the shape of the most landward part of the mean water surface and hence for predicting the shoreline setup instead it is suggested to base estimates of the shoreline setup on the relation between this quantity and the maximum runup height together with well proven runup formulae the validity of this approach needs to be established quantitatively however through simultaneous measurements of runup and mean water surface on in natural beaches

radiative heat transfer in lithospheric geodynamics implications for asthenospheric partial melting and basin modelling concepts abstract in geodynamic models describing basin formation by stretching the base of the lithosphere is commonly defined by a fixed temperature an alternative definition is proposed where the base of the lithosphere is determined by the solidus of mantle rocks some consequences of this definition are illustrated in a one dimensional model the model applies the finite difference method and approximates the temperature regime in the upper 400 km of the earth the model uses temperature dependent thermal conductivity and includes effects from heat storage by partial melting in the mantle low velocity layer mlv radiative heat transfer contributes significantly at elevated temperatures giving rise to a steepening of the thermal gradient below the lithosphere this change in gradient allows the perturbed geotherm to exceed the mantle solidus in a limited depth interval which accounts for the mlv and thus defines the base of the lithosphere in the model variations in thickness of and degree of partial melting in the mlv results from thinning of the overlying lithospheric plate thus the thickness of the mlv is roughly inversely proportional to the thickness of the lithosphere whereas the base of the mlv is almost flat temperature dependent variations in the carrying capacity of the mlv where the main isostatic compensation occurs is proposed as an epeirogenetic agent this effect may be significant and is expressed as an enhancement of the initial subsidence if no preexisting mlv is available a secondary effect occurs as solidification of the mlv ceases a long time 300 ma after stretching and causes slow uplift

thermal tracking of meltwater retention in greenland s accumulation area 1 poorly understood processes controlling retention of meltwater in snow and firn have important implications for greenland ice sheet s mass balance and flow dynamics here we present results from a 3 year 2007 2009 field campaign studying firn thermal profiles and density structure along an 85 km transect of the percolation zone of west greenland we installed one or two thermistor strings at 14 study sites each string having 32 sensors spaced between 0 and 10 m depth data from our network of over 500 sensors were collected at 15 60 min intervals for 1 2 years thereby recording the thermal signature of meltwater infiltration and refreezing during annual melt cycles we document three types of heating of firn related to different mechanisms of meltwater motion and freezing including heterogeneous breakthrough events wetting front advance and year round heating from freezing of residual deep pore water vertically infiltrating meltwater commonly penetrates through cold firn accumulated over decades even where ice layers are present at the previous summer surface and where ice layer thickness exceeds several decimeters the offset between the mean annual air temperature and the 10 m firn temperature reveals the elevation dependency of meltwater retention along our transect the firn is 10 c warmer than the mean annual air temperature at the region where meltwater runoff initiates during 2007 2009 runoff was limited to elevations lower than 1500 m with no sharp runoff limit rather the ratio of retention to runoff transitioned from all retention to all runoff across a 20 km wide zone

divergent behavior of the swash zone in response to different foreshore slopes and nearshore states abstract ultrasonic distance sensors were used to measure changes in bed elevation to a resolution of 0 1 mm following individual swash excursions in the high tide swash zone of an intermediate foreshore changes in bed elevation are related to depth averaged eularian swash velocities estimated by assuming mass balance of the swash discharge also measured using the ultrasonic distance sensors early in the study may 21 25 2008 the alongshore uniform foreshore had a cross shore slope of 0 18 and was fronted by a nearshore bar that forced wave breaking at low tide towards the end of the study june 5 9 2008 the foreshore had a slope of 0 12 and was fronted by a nearshore terrace that developed as the innermost bar migrated landward despite similar inshore wave heights and periods during these times the steeper foreshore eroded by 0 69 m 3 m 1 while the more gently sloped foreshore accreted by 0 21 m 3 m 1 the divergent behavior of the swash zone is attributed to the welding of the innermost bar to the foreshore creating a shallow nearshore terrace that saturated the inshore wave field this in turn led to a faster uprush velocity that promoted the landward advection of sediment picked up over the shallow terrace it is argued that accretion in the upper swash zone was the result of a larger supply of sediment at the base of the foreshore and a transformation of the wave field by the shallower depths of the welded feature in this respect swash hydrodynamics and morphological change can only be made if the swash zone is considered within the context of the broader nearshore state

beach face and berm morphodynamics fronting a coastal lagoon this study documents two different modes of berm development i vertical growth at spring tides or following significant beach cut due to substantial swash overtopping and 2 horizontal progradation at neap tides through the formation of a proto berm located lower and further seaward of the principal berm concurrent high frequency measurements of bed elevation and the associated wave runup distribution reveal the details of each of these berm growth modes in mode i sediment is eroded from the inner surf and lower swash zone where swash interactions are prevalent the net transport of this sediment is landward only resulting in accretion onto the upper beach face and over the berm crest the final outcome is a steepening of the beach face gradient a change in the profile shape towards concave and rapid vertical and horizontal growth of the berm in mode 2 sediment is eroded from the lower two thirds of the active swash zone during the rising tide and is transported both landward and seaward on the falling tide sediment is eroded from the inner surf and transported landward to backfill the zone eroded on the rising tide the net result is relatively slow steepening of the beach face a change of the profile shape towards convex and horizontal progradation through the formation of a neap berm the primary factor determining which mode of berm growth occurs is the presence or absence of swash overtopping at the time of sediment accumulation on the beach face this depends on the current phase of the spring neap tide cycle the wave runup height and indirectly offshore wave conditions and the height of the pre existing berm a conceptual model for berm morphodynamics is presented based on sediment transport shape functions measured during the two modes of berm growth c 2006 elsevier b v all rights reserved

field observations of wave driven setdown and setup wave driven setdown and setup observed for 3 months on a cross shore transect between the shoreline and 5 m water depth on a barred beach are compared with a theoretical balance between cross shore gradients of the mean water level and the wave radiation stress the observed setdown the depression of the mean water level seaward of the surf zone is predicted well when radiation stress gradients are estimated from the observations using linear theory at each location along the transect the observed setdown also agrees with analytical predictions based on offshore wave observations and the assumption of linear dissipationless normally incident waves shoaling on alongshore homogeneous bathymetry the observed setup the superelevation of the mean water level owing to wave breaking is predicted accurately in the outer and middle surf zone but is increasingly underpredicted as the shoreline is approached similar to previous field studies setup at a fixed cross shore location increases with increasing offshore wave height and is sensitive to tidal fluctuations in the local water depth and to bathymetric changes numerical simulations and the observations suggest that setup near the shoreline depends on the bathymetry of the entire surf zone and increases with decreasing surf zone beach slope defined as the ratio of the surf zone averaged water depth to the surf zone width a new empirical formula for shoreline setup on nonplanar beaches incorporates this dependence

large runup controls on a gently sloping dissipative beach observations on a mildly sloping beach suggest that the largest runup events are related to bore bore capture bbc a numerical model based on the reynolds averaged navier stokes equation is implemented to evaluate the effects that bbc have on runup from simulations with realistic sea states bbc is found to be a necessary but not sufficient condition for large runup generation the dominant dynamics leading to bbc are amplitude dispersion and interactions with infragravity waves in the outer and inner surf zone respectively when the effects of bbc are isolated it is found that the runup associated with the merging of two bores is at least 50 larger than that associated with the larger of the two waves in a monochromatic wave train the phase difference of the incident waves with the infragravity wave can generate up to 30 variability of the runup maxima the majority of the shoreward directed momentum flux prior to runup initiation is related to the interaction between the bores and the infragravity wave followed by that of the incident infragravity waves alone

fluid acceleration effects on suspended sediment transport in the swash zone 1 suspended sediment concentrations and fluid velocities measured in the swash zone of a high energy steep beach were used to investigate the importance of fluid accelerations to suspended sediment transport swash flow acceleration was nearly constant at about one half downslope gravitational acceleration with two important exceptions we observed strong short lived periods of accelerating uprush at the beginning of the swash cycle and decelerating backwash at the end of the swash cycle magnitudes of both approximately twice that of the expected downslope gravitational acceleration interestingly spikes in suspended load followed the anomalies in acceleration in a way that was not apparent from the nearly symmetric in magnitude ensemble averaged velocity time series suspended load values were largest during accelerating uprush associated with the shoreward propagating turbulent bore or swash front during backwash suspended loads were generally not as large correspondingly suspended sediment transport rates obtained from the sediment concentration and velocity measurements showed best comparisons with a modified sediment transport model that includes a physical mechanism for enhancing transport rates due to flow acceleration the modified sediment transport model reduced the overall root mean square prediction error by up to 35 and shifted the predicted peak in uprush sediment transport rate earlier in the swash cycle resulting in a better fit to the observations these findings suggest that the inclusion of the acceleration term may account for physical mechanisms that include bore turbulence and horizontal pressure gradients typically associated with the accelerating portion of uprush

eulerian flow velocities in the swash zone field data and model predictions measurements of eulerian flow velocity obtained within the swash zone on a relatively steep beach face gradient 1 23 are compared with an extended ballistic swash model the model only requires a friction factor beach slope and terminal bore velocity as input the following model predictions matched well with observations 1 the maximum eulerian flow velocity is the shoreline velocity when it arrives at the fixed point of interest on the beach face 2 at any location the time of flow reversal occurs prior to the shoreline reaching its maximum landward excursion 3 the maximum flow velocity in the backwash is the velocity recorded as the shoreline recedes past the fixed point of interest and this is less than the maximum uprush velocity and 4 the duration of the uprush flow is shorter than the duration of the backwash flow previous studies have already confirmed that the ballistic swash model including friction can accurately predict shoreline motion and maximum run up on steep beaches this study shows that it is similarly successful in predicting eulerian flow velocities during individual swash events the model does not presently account for interacting swash however and so may be less appropriate on gently sloping beaches

on the use of the radon transform in studying nearshore wave dynamics in the nearshore describing the complex individual wave dynamics remains a key challenge in this paper we test the ability of the radon transform to produce estimates of individual wave celerities and to separate incoming and outgoing waves conserving the temporal characteristics the radon transform is a projection of a two dimensional field into polar space oblique features such as propagating crests in a spatio temporal space are identified with density peaks in the polar space in this paper the radon transform is applied to synthetic test cases including a wide range of beach slopes and wave conditions the radon transform shows good skills at estimating individual celerity and separating incoming and outgoing components with a relative rms error lower than 10 even a standing wave node the accuracy is fairly insensitive to wave characteristics whereas the main limitations rise from the sampling scheme and are the number and density of wave gauges the distance between gauges should be less than one third of the shortest wavelength while the set of gauges should cover more than one third of the longest wavelength

three dimensional models of mantle flow across a low viscosity zone implications for hotspot dynamics the influence of a shallow low viscosity zone on the planform of infinite prandtl number rayleigh benard convection has been investigated at rayleigh numbers ra ranging from 1 5 103 to 3 105 a viscosity contrast of130 has been set between the low and high viscosity layers for ra exceeding about 3 104 and no slip conditions at the top and bottom boundaries a hexagonal convective planform is systematically observed free slip boundary conditions lead to the development of a square convective planform the flow is ascending along the center of the cells and is strongly narrowed in the low viscosity layer a moving top boundary does not destabilize significantly these plumes as long as the drift velocity is lower than the upwelling velocity in the low viscosity layer a higher drift velocity of the upper boundary rapidly induces the formation of convective rolls at ra of 3 105 unsteady flow occurs along the descending currents when the convective cells have the horizontal size predicted by the linear theory this time dependence is prevented when the cells are smaller suggesting that there is a competition between the aspect ratio and the steadiness of the cells such a reduction of the horizontal dimensions of the cells with increasing ra has been observed in large experiments these results show that a moderate viscosity drop at shallow depth in the mantle compatible with the geoid and bathymetric data at hotspot swells induces a marked asymmetry between the upper and lower convective boundary layers this promotes the development of ascending plumes large at depth and very narrow a few tens of kilometers in section below the lithosphere these plumes are very stable in time and space it is inferred that the channelling of the ascending plumes induced by a viscosity drop precludes the development of secondary convection in a top low viscosity layer

nearshore wave height variation in unsaturated surf the nearshore evolution of wave height is presented from field observations during unsaturated surf conditions from 10 different beaches characterized by microtidal conditions and predominantly swell dominated wave climates wave evolution is presented in terms of wave height to water depth ratio gamma for comparison with previous data from saturated surf both conventional time averaged gamma rms and a new wave by wave analysis gamma w are performed values of gamma increase with increasing offshore wave height indicating unsaturated surf the observations show a variation in gamma values from near constant values in the mid surf zone to rapidly and asymptotically increasing gamma values in the inner surf zone in contrast to previous data from saturated surf gamma shows no dependence on either the absolute beach slope or the relative beach slope beta k h over bar the skewness of the distributions of gamma w is consistent with waves that are not depth limited the inner surf zone wave heights are approximately equally dependent on the water depth and offshore wave height the previous observations of gamma from saturated surf are shown to be consistent with a terminal bore height at the shoreline which is in excellent agreement with a previously derived value for the miche parameter in contrast for the present unsaturated surf conditions the terminal bore height at the shoreline can be approximated by h b approximate to 0 12h o which is consistent with recent laboratory data sets

analysis and modeling of melt water refreezing in dry snow a dynamic zone of thermal disequilibrium is described which separates wet snow at 0 c from dry sub freezing snow the dynamic zone tends to be eliminated by thermal equilibrium through freezing but is sustained and propagated into the sub freezing snow by water flow from the wet snow the width of the dynamic zone is controlled by the rate of water inflow and by the rate of freezing of water on to sub freezing ice grains which is in turn controlled by the ice water geometry two ice water geometries are investigated isolated ice spheres and capillary tubes of ice into which water is pulled by capillary suction the rate of freezing of water is calculated for the two models for various initial dimensions and temperatures equilibrium times are short typically about 0 5 s but depend on the assumed geometry which is poorly constrained by existing data equilibration times and freeze on mass fluxes are calculated for a variety of general conditions these results can be used in numerical models of wetting front propagation into cold snow

swash zone boundary conditions derived from optical remote sensing of swash zone flow patterns 1 optical remote sensing is used to measure flow patterns in the swash zone timestack images are analyzed to measure the asymmetry and the relative duration of the inflow into the swash zone this varies significantly between individual swashes contrary to the classical analytical swash model for runup induced by bores which predicts a similar flow pattern for all events for swash forced by breaking bores the gradient of the x t locus of flow reversal varies over a wide range and flow reversal can occur simultaneously across the whole swash zone this variation of the gradient of the locus of flow reversal in x t space can be parameterized in terms of a single free variable in recent solutions to the nonlinear shallow water equations which fully defines the swash boundary inflow condition consistent with the theory the horizontal runup the swash period and the swash similarity parameter were observed to be independent of the swash inflow conditions but the flow asymmetry is not only a weak correlation was observed between the swash boundary condition and the iribarren number and beach slope conversely the analysis suggests that the degree of swash swash interaction does influence the swash boundary condition and the resulting internal flow kinematics the variation in inflow conditions is expected to influence the magnitudes of the velocity moments within the swash zone and therefore sediment transport rates

a novel method for tracking individual waves in the surf zone a new method to obtain information on individual waves as they travel shoreward is described the wave tracking method involves analysis of synchronous water level records obtained from a shore normal transect of pressure transducers pts the method is illustrated here using data from field deployments inside the surf zone although the method could equally be applied outside the surf zone as a wave travels shoreward along the instrument transect it appears in the pt records at a progressively later time serial cross correlation techniques are used to shift each record in time so that each wave in the first record aligns approximately with the corresponding wave if it exists in subsequent records from along the transect a local minima analysis power et al 2010 is used to delimit individual waves in each record and then parameters of interest can be determined for each wave at each pt location the wave tracking method provides the evolution of these parameters for a single wave as it travels to shore and they can be analysed as a function of distance or local water depth the utility of the method is illustrated with case studies including the evolution of wave height and the evolution of the ratio of wave height to water depth for surf zone waves

variations of near surface firn density in the lower accumulation area of the greenland ice sheet pâkitsoq west greenland firn density variations have been studied in the lower accumulation area of the greenland ice sheet 1440 1620 m a s l near p kitsoq west greenland the main control on density in the near surface firn layer of 5 10 m thickness is the formation of ice layers by the refreezing of meltwater that reaches depths of 2 4 m below the surface the density variations are described by the ratio of annual surface melt m to the annual accumulation c the ratio m c is about 0 6 at the run off limit at about 1400 m a s l in the study area where refreezing of meltwater transforms snow into impermeable ice the mean density of near surface firn decreases with elevation reflecting a decrease in melt with elevation there is a surprising decrease in firn density at depths of more than about 4 m below the 1991 summer surface which reflects lower melt rates and or higher accumulation in the early 1980s and late 1970s when this firn was passing through the surface layer the formation of such low density firn may have partially contributed to the 1978 85 thickening of the ice sheet observed by satellite radar altimetry near surface firn density is therefore very sensitive to climate change and might be an attractive target for climate monitoring

cryosphere greenland s lurking aquifer runoff estimates from the greenland ice sheet carry uncertainty because the fate of surface melt in permanently snow covered regions is unconstrained in situ and airborne observations reveal large scale liquid water storage in buried layers of aged and compacted snow

a novel machine learning algorithm for tracking remotely sensed waves in the surf zone abstract this paper describes a novel image processing technique that detects wave breaking and tracks waves in the surf zone using machine learning procedures using time space images timestacks the algorithm detects white pixel intensity peaks generated by breaking waves confirms these peaks as true wave breaking events by learning from the data s true colour representation clusters individual waves and obtains optimal wave paths the method was developed and tested using data from four sandy australian beaches under different incident wave and light conditions results are a representation of the position of the wave front through time i e space time data which when shown overlaid on the original timestack shows the high degree of accuracy of the method developed here the utility of the method is demonstrated in two ways 1 through a comparison between the instantaneous wave speed calculated from the wave paths with the theoretical shallow water wave speed and 2 by obtaining optical intensities that could be translated into wave roller lengths the algorithm developed here has the potential to improve understanding numerous nearshore process such as bore propagation and capture in the surf zone surf zone energy dissipation surf beat and infragravity waves and as a direct speed input for depth inversion methods

variability of depth limited waves in coral reef surf zones wave breaking and transformation on coral reef flats is an important process protecting tropical coastlines and regulating the energy regimes of coral reefs however the high hydraulic roughness shallow water and steep bathymetries of coral reefs may confound common surf zone assumptions such as a depth limited and saturated surf zone with a constant wave height to water depth ratio here we examine wave transformation across a coral reef flat during three separate swell events on both a time averaged and a wave by wave basis we use the relationship between significant wave height and water depth to examine the change in surf saturation across the reef flat and compare the measured wave height decay to results of modelled wave energy dissipation in the surf zone our results show that was not cross reef constant and varied according to location on the reef flat and local water depth on average was greatest at the outer reef flat near the reef crest and progressively reduced towards the inner reef flat near the reef lagoon this was most pronounced in shallow water with large values 0 85 at the outer reef flat and small values 0 1 at the inner reef flat this indicates that there is an increase in wave energy dissipation in shallow water most likely due to increased breaker and bed frictional dissipation the measured wave energy dissipation across the entire reef flat could on average be modelled accurately however this required location specific calibration of the free parameters the wave friction factor f and and further suggests that there is no value for either parameter that is universally applicable to coral reef flats despite model calibration inaccuracies were still observed primarily at the outer reef flat these inaccuracies reflected the observed cross reef variation of on the reef flat and potentially the limitations of random wave breaker dissipation models in complex surf zones our results have implications for the use of wave energy dissipation models in predicting breaker dissipation and subsequent benthic community change on coral reef flats and suggest that careful consideration of the free parameters in such models such as f and is required