wave induced advective transport below a rippled water sediment interface the exchange between the water column and the sediment bed and the transport inside the permeable sediment layer are important processes in the cycles of chemical elements in this paper we examine quantitatively the effects of modulations in the profile of the water sediment interface on this exchange and on the advective transport inside the sediment bed the flow field inside a sediment layer bounded between spatially periodic ripples on top and an impermeable bottom surface is modeled using darcy s law the forcing is due to progressive gravity waves in the water above the results of two different models for the pressure variation imposed on the upper boundary are compared the two pressure profiles are derived from potential flow theory and from a numerical solution to the navier stokes equations for the oscillatory flow over a rippled bed from an analytic solution to the two dimensional model the trajectories of pore water particles immediately below the ripple profile are found to be quite different from the simple elliptical pattern found below a flat bed the shapes of these trajectories can be quite complicated and vary considerably both along the length of the ripple and over the depth of the sediment layer close to its surface the total exchange across the water sediment interface averaged over one wave period is significantly higher across a rippled interface than across a flat bed this difference increases with increasing ripple slope and the strength of the wave motion and it decreases with increasing thickness of the sediment layer relative to the length of the gravity wave since rippled bed forms are commonly found in coastal waters the increase in the total exchange across a rippled water sediment boundary can enhance the exchange of solutes due to wave pumping immediately below the water sediment interface circulation cells with net advective transport over a wave period are found such net advection patterns can lead to spatial in the horizontal direction inhomogeneities of the vertical concentration or temperature profiles if the overlying water column and or the sediment bed act s as source or sink this gives a plausible physical mechanism to explain the spatial variations in vertical concentration profiles found in field measurements

solute transport in rivers with multiple storage zones the stir model solute transport in rivers is controlled by surface hydrodynamics and by mass exchanges between the surface stream and distinct retention zones this paper presents a residence time model for strea

hyporheic transport and biogeochemical reactions in pool riffle systems under varying ambient groundwater flow conditions at the interface between stream water groundwater and the hyporheic zone hz important biogeochemical processes that play a crucial role in fluvial ecology occur solutes that infiltrate into the hz can react with each other and possibly also with upwelling solutes from the groundwater in this study we systematically evaluate how variations of gaining and losing conditions stream discharge and pool riffle morphology affect aerobic respiration ar and denitrification dn in the hz for this purpose a computational fluid dynamics model of stream water flow is coupled to a reactive transport model scenarios of variations of the solute concentration in the upwelling groundwater were conducted our results show that solute influx residence time and the size of reactive zones strongly depend on presence magnitude and direction of ambient groundwater flow high magnitudes of ambient groundwater flow lower ar efficiency by up to 4 times and dn by up to 3 orders of magnitude compared to neutral conditions the influence of stream discharge and morphology on the efficiency of ar and dn are minor in comparison to that of ambient groundwater flow different scenarios of o2 and no3 concentrations in the upwelling groundwater reveal that dn efficiency of the hz is highest under low upwelling magnitudes accompanied with low concentrations of o2 and no3 our results demonstrate how ambient groundwater flow influences solute transport ar and dn in the hz neglecting groundwater flow in stream groundwater interactions would lead to a significant overestimation of the efficiency of biogeochemical reactions in fluvial systems

synoptic features associated with temporally coherent modes of variability of the north pacific jet stream abstracttime extended eof te eof analysis is employed to examine the synoptic scale evolution of the two leading modes of the north pacific jet stream variability namely its zonal extension retraction te eof 1 and the north south shift of its exit region te eof 2 use of the te eof analysis enables a temporally coherent examination of the synoptic scale evolution preceding and following peaks in each of the two leading modes that provides insight into the preferred evolutions of the north pacific jet composite analyses are constructed based upon selecting peaks in the principal component time series of both phases of each te eof whose magnitude exceeded 1 5 standard deviations jet extension events are associated with an anomalous cyclonic circulation over the gulf of alaska that induces a low level warm anomaly over western north america jet retractions are associated with a nearly opposite configuration characterized by an anomalous anticyclonic circulation over the aleutians and anomalous low

sorting out abrasion in a gypsum dune field 1 grain size distributions in eolian settings are the result of both sorting and abrasion of grains by saltation the two are tightly coupled because mobility of particles determines abrasion rate while abrasion affects the mobility of particles by changing their mass and shape few field studies have examined this quantitatively we measured grain size and shape over a 9 km transect downwind of a line sediment source at white sands national monument a gypsum dune field the sediment source is composed of rodlike elongate coarse particles whose shapes appear to reflect the crystalline structure of gypsum dispersion in grain size decreases rapidly from the source coarse particles gradually become less elongate while an enrichment of smaller more elongate grains is observed along the transect transport calculations confirm that white sands is a threshold sand sea in which 1 the predominant particle diameter reflects grains transported in saltation under the dune forming wind velocity and 2 smaller elongate grains move in suspension under this dominant wind size selective transport explains first order trends in grain size however abrasion changes the shape of saltating grains and produces elongate smaller grains that are spallation and breaking products of larger particles both shape and size changes saturate 5 6 km downwind of the source as large particles become more equant abrasion rates slow down because protruding regions have been removed such asymptotic behavior of shape and abrasion rate has been observed in theory and experiment and is likely a generic result of the abrasion process in any environment

three dimensional model of modern channel bend deposits 1 we present a three dimensional model of heterogeneous modern channel bend deposits developed through purely structure imitating interpolation kriging of hydraulic properties this model augmented with ground penetrating radar data and directional variograms agrees with detailed observations in similar modern environments and leads to a process based interpretation of the presented hydraulic conductivity structure integration of all available information permitted delineation and characterization of the modern streambed as a distinct hydrostratigraphic unit without coring or outcrop studies our results imply that the modern streambed is commonly oversimplified in available analytical and numerical models of groundwater surface water interactions where it is assumed to be homogeneous and isotropic and characterized by a constant width and thickness this three dimensional approach that integrates concepts and principles developed in sedimentology hydrogeology geophysics and geostatistics has potential implications on model development of stream aquifer systems

dynamics of rossby wave packets in a quantitative potential vorticity potential temperature framework abstractrossby wave packets rwps have been associated with increased atmospheric predictability but also with the growth and propagation of forecast uncertainty to address the important question of under which conditions rwps imply high and low predictability a potential vorticity potential temperature pv framework is introduced to diagnose rwp dynamics finite amplitude rwps along the midlatitude waveguide are considered and are represented by the synoptic scale wavelike undulations of the tropopause the evolution of rwps is examined by the amplitude evolution of the individual troughs and ridges troughs and ridges are identified as pv anomalies on levels intersecting the midlatitude tropopause by partitioning the pv tendency equation individual contributions to the amplitude evolution are identified a novel aspect is that the important role of the divergent flow and the diabatic pv modification is quantified explicitly arguably prominent upper tropospheric divergent flow is associated

effects of velocity gradients and secondary flow on the dispersion of solutes in a meandering channel two contrasting mechanisms created by channel curvature which strongly affect longitudinal dispersion of solutes in rivers are examined in natural channels the large cross sectional variability of the primary velocity component tends to increase longitudinal dispersion by providing a large difference between adjacent fast and slow moving zones of fluid by contrast secondary circulation tends to decrease longitudinal dispersion by enhancing transverse mixing a series of tests have been carried out in a very large flume containing a meandering water formed sand bed channel to measure the longitudinal dispersion coefficient at various locations around a meander these experimental observations are compared with experimental data obtained from meandering channels with smooth fixed sides and regular cross sectional shapes all the data has been compared against predictions from two current modeling approaches finally the significance of the two competing mechanisms in curved channels is discussed with regard to their relative influence on longitudinal mixing

transport by wind of granular materials of different grain shapes and densities aeolian transport rates were measured for three sands a quartz sand relative density 2 68 sphericity 0 73 a shelly sand relative density 2 64 sphericity 0 59 carbonate content 67 and granular aluminium oxide relative density 3 95 sphericity 0 67 low sphericity depresses the transport rate particularly at high wind speeds high density also does so but the effect is more marked at low transporting wind speeds the sand of low sphericity undergoes longer saltations than the other materials but is dislodged less frequently than they are and is transported less freely in a given wind of the more spherical materials the denser was the least effective at extracting energy from the wind for grain transport in the range of our experiments both of the more spherical materials showed evidence of a fairly sudden transition of transport behaviour at modest wind speed it is inferred that this marks the transfer of the function of grain dislodgement from direct wind action to inter granular collision the transition did not occur in experiments on the material of lower sphericity for a given transport rate wind speed near the bed is highest for the grains of low sphericity by a considerable margin and lowest for the more compact quartz grains propensity for transport by wind is greatest for the quartz sand less so for the heavy material and least for the material of low sphericity

zonal jet structure and the leading mode of variability an observational analysis of the northern hemisphere circulation during winter reveals that the leading mode of variability depends on longitude in particular the first eof of the zonal mean circulation differs over the atlantic and pacific sectors these results provide motivation for a series of model runs where a simple gcm is used to investigate the effects of the zonal jet structure on the leading mode of variability in the northern hemisphere model results indicate that the leading mode of variability depends on the distance between the eddy driven and subtropical jets when the jets are well separated the leading mode of variability describes latitudinal shifting of the eddy driven jet however when the two jets are nearly collocated pulsing of the combined jet dominates the variability this change coincides with a weakening of the positive feedback between the eddies and zonal flow anomalies these results provide a possible explanation for the reduced amplitude of the northern annular mode in the pacific sector relative to that in the atlantic sector during northern hemisphere winter

sedimentology of cold climate aeolian sandsheet deposits in the askja region of northeast iceland active aeolian sandsheets cover an area of 270 km2 in the askja region of ne iceland the sandsheet is divided into i a deflationary upwind part that is characterised by deflation lags ventifacted boulder fields and restricted aeolian sand patches in the lee of obstacles ii an accumulating central part that is characterised by continuous aeolian sand cover up to 10 m thick where three scales of aeolian ripples are evident along with larger scale forms including longitudinal zibars and iii a downwind part that is currently subject to aeolian bypass with only localised accumulation occurring in topographic hollows between basaltic lava fields trains of aeolian granule megaripples are a characteristic landform of the downwind part of the sandsheet these sinuous crested oblique bedforms attain heights of 0 35 m and downwind spacings of 6 m they are composed principally of low density pebble grade pumice clasts and other reworked volcaniclastic detritus derived from the nearby askja volcanic centre the distribution of landforms across the sandsheet reflects a regional aeolian sediment budget that is controlled by dynamic interactions between glacial ice margin fluvial aeolian and volcaniclastic processes under the influence of a distinctive climatic regime sediment supply at the upwind sandsheet margin adjacent to the dyngjujokull glacier increases during summer periods when rates of glacial ablation are at a maximum longer term controls on sediment supply rate are determined by episodes of glacier surging and retreat and by periodic eruptions of volcaniclastic debris from the askja volcanic centre the availability of sediment for aeolian transport is controlled by the level of the groundwater table the depth and continuity of the permafrost layer in the proglacial area the degree of development of armoured pebble lags and freezing of the sandsheet surface during winter months the transport capacity of the wind varies on a seasonal basis with mean wind velocities and wind gustiness reaching a maximum during winter periods both winter and summer mean wind velocities have been steadily increasing since formal records began in 1969 the suite of aeolian landforms developed within the askja region as defined by sediment texture and composition and bedform scale morphology and spatial distribution reflects a special set of controlling circumstances which mean that the askja sandsheet deposits differ significantly from those described from elsewhere

aerodynamic dislodgement of multiple size sand grains over time an experimental study was undertaken in a large scale wind tunnel to investigate sand particle dislodgement by wind over time in the absence of grain bed collisions aerodynamic dislodgement was measured for six groups of sand particles under two known wind velocity profiles the results show that the dislodgement rate for both fine and coarse particles decreases rapidly during the transition of the particle surface from a non wind worked condition to a wind worked condition and that the dislodgement rate continues to decay under a wind worked condition even though the mean grain size of surface particles remains nearly the same a previously developed theoretical method for calculating the number of particles left on the bed by wind was developed further the derived method was used to calculate the time decay of the dislodgement rate and the length of time required for the dislodgement rate to reach an equilibrium the length of time for dislodgement rate to reach an equilibrium in this study is of the order of 10 15 min this not only provides further observation of the second long stage of aeolian sediment transport system development reported previously but also indicates a potentially large variation in the time decay of transport rate under different conditions the results indicate that the time decay of the particle dislodgement rate is related to sorting processes because of the artificial method of preparation of the grain surface and the wind velocity profiles the results of this study should be applied with caution to natural conditions

the ohio valley wave merger cyclogenesis event of 25 26 january 1978 part ii diagnosis using quasigeostrophic potential vorticity inversion abstract the dynamical interactions between precursor disturbances during the wave merger cyclogenesis event of 25 26 january 1978 over eastern north america are diagnosed using quasigeostrophic potential vorticity qgpv inversion this case is characterized by two prominent preexisting upper level disturbances that induce rapid surface cyclogenesis as they come into close proximity static qgpv inversion is used to attribute a particular geopotential height field to the qgpv associated with each precursor disturbance the full flow is partitioned into the following components the northern upper precursor the southern upper precursor and the background flow prognostic qgpv inversion is used to quantify the instantaneous geopotential height tendencies attributable to each of these flow components the static inversion results for the upper precursors exhibit the structure of baroclinic vortices with maximum amplitude near the tropopause during the 48 h period spanning the period of study of this even

the impact of bedform migration on benthic oxygen fluxes permeable sediments are found wide spread in river beds and on continental shelves the transport of these sediments is forced by bottom water currents and leads to the formation of bedforms such as ripples and dunes the bottom water flow across the bedforms results in pressure gradients that drive pore water flow within the permeable sediment and enhance the supply of reactive substrates for biogeochemical processes this transport reaction system has been extensively studied for the case of stationary bedforms whereas bedform migration the most ubiquitous form of sediment transport has been often ignored to study the impact of sediment transport on pore water flow we incorporated an empirical model of bedform migration into a numerical transport reaction model for porous media using oxygen as reactive solute the modeled oxygen flux changes significantly as soon as the sediment divides into an upper mobile layer migrating bedform and a stationary layer underneath the bedform is increasingly flushed with oxic bottom water whereas pressure gradients and pore water flow reverse at increasing rate underneath the bedform this suppresses net pore water displacement and reduces the oxygen penetration depth up to 90 in effect the overall oxygen uptake decreases significantly with bedform migration although bottom water velocities increase this counterintuitive effect is systematically described for a range of different sediment types current velocities and respiration rates and should be considered in future studies

teleconnections in the geopotential height field during the northern hemisphere winter abstract contemporaneous correlations between geopotential heights on a given pressure surface at widely separated points on earth referred to as teleconnections in this paper are studied in an attempt to identify and document recurrent spatial patterns which might be indicative of standing oscillations in the planetary waves during the northern hemisphere winter with time scales on the order of a month or longer a review of existing literature on the subject reveals the existence of at least four such patterns the north atlantic and north pacific oscillations identified by walker and bliss 1932 a zonally symmetric seesaw between sea level pressures in polar and temperature latitudes first noted by lorenz 1951 and what we will refer to as the pacific north american pattern which has been known to operational long range forecasters in this country since the 1950 s a data set consisting of nmc monthly mean sea level pressure and 500 mb height analyses for a 15 year period is used as a basis fo

synoptic analysis of the pacific north american teleconnection pattern in this study we use various diagnostic techniques to investigate the synoptic evolution of the pacific north american teleconnection pattern pna national center for environment prediction national center for atmospheric research reanalysis data are used these data cover the years 1948 2008 for the months of november march it is found that the positive pna is initiated by enhanced convection over the western tropical pacific and weakened convection over the tropical indian ocean the excitation of the negative pna exhibits opposite features for both phases the response to tropical convection excites a small amplitude pna about 8 12 days prior to the pattern attaining its maximum amplitude this is followed by slow steady growth for about 5 days after which driving by synoptic scale waves via their eddy vorticity flux together with stationary eddy advection lead to much more rapid growth and the establishment of the full pna for the positive pna the synoptic scale waves propagate eastward into the midlatitude northeastern pacific where they are observed to undergo cyclonic wave breaking for the negative pna the synoptic scale waves first amplify over the midlatitude northeastern pacific and then propagate equatorward into the subtropics where they undergo anticyclonic wave breaking once established for both phases the pna appears to be maintained through a positive feedback that involves a succession of wave breakings these results suggest that preconditioning may play an important role in the formation of the pna for the positive pna in its early development the strengthening and eastward extension of the subtropical jet result in an increase in the cyclonic shear and a decrease in the meridional potential vorticity gradient features that are known to favour cyclonic wave breaking for the negative pna opposite changes were observed for the background flow which favour equatorward wave propagation and anticyclonic wave breaking the role of optimal growth is also discussed our results also suggest that the pna is potentially predictable 1 2 weeks in advance copyright 2011 royal meteorological society

characteristics of aeolian grain transport over a fluvio glacial lacustrine braid delta lake tekapo new zealand this paper presents results from one of the few scientific studies to examine the physical characteristics of aeolian sediment transport in an alpine area where topographically reinforced foehn winds initiate dust storm events the major objective of this study is to improve knowledge of aeolian processes in mid latitude alpine regions experiencing extreme wind speeds of particular interest is the role of surface characteristics in contributing to the unusually deep saltation layer which is seen to form over fluvio glacial deposits in the southern alps of new zealand sediment was collected at several heights 0 5 1 2 and 4 m and locations over a large alpine braided river delta and standard laboratory techniques used to examine grain size characteristics an image processing technique was also used to evaluate grain roundness grains filtered from the airstream at 0 5 m and 1 m above such surfaces were found to display a mean grain size of approximately 300 to 435 m resembling grain size characteristics of saltation clouds previously observed in high latitude cold climate locations in contrast to desert and prairie environments samples collected at 2 and 4 m above the surface were found to consist of 60 to 65 per cent sand sized material with some grains exceeding 1 1 5 mm in diameter grain shape analysis conducted on silt and clay sized grains filtered from the airstream above mixed sand and gravel surfaces showed such grains to display an increase in grain roundness with height this characteristic is thought to reflect the airstream s shape sorting ability and has important implications with respect to the often observed increase in grain roundness in aeolian deposits with increasing distance from source areas namely if more rounded grains are preferentially carried higher into the airstream and therefore into regions of higher wind speed they should theoretically be transported further from the entrainment zone before being deposited the high wind speeds observed often exceeding 30 m s 1 are seen to transport significantly larger sediment than reported in the literature for desert and prairie environments in addition the mixture of grain sizes and especially the pebble and cobble sized clasts that dominate the fluvio glacial deposits associated with the braided rivers in this mountain region also appear to increase significantly the trajectory height of saltating sand grains as a result of these two factors the depth of the saltation cloud often exceeds 1 m observations made in this study therefore highlight the need for field and laboratory aeolian process studies to be extended to examine grain transport over coarse grained beds during much higher wind velocities than typically reported in the literature such studies would provide a valuable insight into aeolian processes in high latitude altitude environments such as loess genesis 1997 by john wiley sons ltd

classification seasonality and persistence of low frequency atmospheric circulation patterns abstract orthogonally rotated principle component analysis rpca of northern hemisphere 1 month mean 700 mb heights is used to identify and describe the seasonality and persistence of the major modes of interannual variability the analysis is detailed and comprehensive in that 1 a high resolution approximately equal area 358 point grid is used for the virtually maximum possible 35 year period of record 2 a positive bias in the nmc data base in the early 1950s in the subtropics is largely eliminated for the first time and 3 homogeneous separate analyses of each month of the year are carried out detailing the mouth to month changes in the dominant circulation patterns winter results are similar to those of other recent rpca and teleconnection studies except that some less obvious patterns are identified and further detail of the better known patterns is provided two north south dipole patterns are found over the pacific ocean west pacific oscillation and east pacific pattern and over the atla

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

the effect of streambed topography on surface subsurface water exchange in mountain catchments a numerical hydrological simulation suggested that water exchange between stream channels and adjacent aquifers is enhanced by convexities and concavities in streambed topography at st kevin gulch an effluent stream in the rocky mountains of colorado subsurface hydraulic gradients and movement of ionic tracers indicated that stream water was locally recharged into well defined flow paths through the alluvium stream water filled flow paths in the alluvium referred to as substream flow paths returned to the stream a short distance downstream 1 to 10 m recharge to the substream flow paths occurred where stream water slope increased at the transition from pools 1 to steeper channel units 5 20 return of substream flow paths to the stream occurred where stream water slope decreased at the transition from steeper channel units to pools a net water flux calculation is typically used to characterize water and solute fluxes between surface and subsurface zones of catchments along our study reach at st kevin gulch the net inflow of water from subsurface to stream 1 6 ml s 1 m 1 underestimated the gross inflow 2 7 ml s 1 m 1 by 40 the influence of streambed topography is to enhance hydrological fluxes between stream water and subsurface zones and to prolong water sediment contact times these effects could have important consequences for solute transport retention and transformation in catchments

particle dislodgement from a flat sand bed by wind the rate with respect to area and time at which grains are dislodged from a sand bed for given wind conditions is an important factor in determining the grain transport rate and the intensity of grain activity in each of the transport modes the literature of the subject contains little direct information about particle dislodgement the paper describes a series of experiments in which dyed sand grains spread on the surface of quartz dune sand in a wind tunnel were photographed at five second intervals while the sand was exposed to wind the data on rate of loss of coloured grains was used for two of three chosen size fractions to deduce the dislodgement rate for each size fraction the variation of this dislodgement rate with shear velocity is shown graphically for values of u between 24 cm s 1 and 50 cm s 1 because of the artificial method of distribution of the coloured sand grains the results should be applied with caution to natural conditions the interpretation of the observations of dyed grain loss involved the numerical simulation of the process which comprises removal of coloured grains slightly offset by replenishment as upwind coloured grains settle briefly in the observed zone an estimation of grain excursion length has to be incorporated in the simulation this estimation was made by trial but general corroboration was found from earlier work comparisons are made between dislodgement rates obtained thus and rates estimated by anderson 1986 and by jensen and sorensen 1986 reasonable agreement with the latter is found in the u range 30 cm s 1

grains in motion a review abstract this review paper aims to present a practical integrated procedure for the prediction of sediment transport rates in both the continental and marine environment based on the settling velocity of particles such studies have important applications in diverse fields such as civil and coastal engineering sedimentology and environmental geology where they can assist in flood control and the prevention of contaminant dispersal by wind and water as well as the siltation of dams and harbours the formation of placer deposits including gold diamonds and other heavy minerals is also controlled by sediment transport processes so that these studies can form the basis for more efficient exploration programs the first part of the paper discusses some basic principles important in sediment transport followed by an overview of published methods to determine the settling velocity of differently shaped particles including natural grains the application of settling velocity to predict the entrainment threshold of sediments on plane horizontal and inclined beds by unidirectional currents and oscillatory waves is then discussed which finally leads to the determination of sediment transport rates over plane and rippled beds the validity of this approach to natural conditions is tested against published field data where possible other widely used methods are also critically discussed pointing out severe problems in current sampling technology and the calculation of bedload transport in natural environments

the initiation of particle movement by wind when air blows across the surface of dry loose sand a critical shear velocity fluid threshold ut must be achieved to initiate motion however since most natural sediments consist of a range of grain sizes fluid threshold for any sediment cannot be defined by a finite value but should be viewed as a threshold range which is a function of the size shape sorting and packing of the surface sediment in order to investigate the initiation of particle movement by wind a series of wind tunnel tests was carried out on a range of pre screened fluvial sands and commercially available glass beads with differing mean sizes and sorting characteristics a sensitive laser monitoring system was used in conjunction with a high speed counter to detect initial grain motion and to count individual grain movements test results indicate that when velocity is slowly increased over the sediment surface the smaller or more exposed grains are first entrained by the fluid drag and lift forces either in surface creep rolling or in saltation bouncing or hopping downwind as velocity continues to rise larger or less exposed grains may also be moved by fluid drag on striking the surface saltating grains impart momentum to stationary grains this impact may result in the rebound of the original grain as well as the ejection of one or more stationary grains into the air stream at shear velocities lower than that required to entrain a stationary particle by direct fluid pressure as a result there is a cascade effect with a few grains of varying size initially moving over a range of shear velocities the fluid threshold range and setting in motion a rapidly increasing number of grains results of the tests showed that the progression from fluid to dynamic threshold based on grain movement can be characterized by a power function the coefficients of which are directly related to the mean size and sorting characteristics of the sediment

the effect of river bend morphology on flow and timescales of surface water groundwater exchange across pointbars summary head gradients along meandering rivers induce surface water groundwater exchange across pointbars we analyze this exchange process by finite element modeling of groundwater flow and transport through model generated river planforms taken from previous studies the river is considered as a constant head boundary that decreases linearly along the direction of the channel this leads to an irregular groundwater flowfield in the pointbar that is controlled by channel sinuosity simulations under homogeneous aquifer conditions shows that the variable flowpath lengths coupled with the heterogeneous flow velocities lead to non fickian transport characterized by power law residence time distributions this anomalous transport behavior is enhanced by increased aquifer dispersivity the broad transit time distribution implies that time dependent chemical transformation processes in near stream zones may be equally variable as the flowfield even in the absence of heterogeneity in aquifer hydraulic properties

rates of aerodynamic entrainment in a developing boundary layer despite its significance for inception of grain transport by wind the initial dislodgement of grains from a static surface by aerodynamic forces of drag and lift in the absence of grain collision has received little attention this paper describes a series of wind tunnel experiments in which the erosion of narrow strips of loose grains from the roughened surface of a flat plate exposed to a range of wind speeds was examined the progressive downwind development of the boundary layer over the plate provided a range of airflow conditions which permitted systematic evaluation of grain entrainment rates arising from purely aerodynamic forces use of closely graded size fractions in flat single grain layers resting on identical fixed grain support eliminated the effects of surface irregularities and impacts from saltation results show that erosion of strips of loose grains develops with time according to an inverse exponential function in which the entrainment rate time constant relates to shields dimensionless shear stress function an empirical expression defining aerodynamic entrainment rate in terms of rate of strip erosion is derived and comparisons are made between present and published data the need for additional data to resolve several questions raised by the present investigation is stressed in addition a simple objective technique for accurate determination of the aerodynamic entrainment threshold of any loose granular sediment is proposed

impact of heterogeneity bed forms and stream curvature on subchannel hyporheic exchange 1 advection through hyporheic zones hz consisting of heterogeneous channel bend streambed deposits and their equivalent homogenous medium was investigated using finite difference groundwater flow and transport simulations and forward particle tracking the top prescribed head boundary was varied in order to mimic various stream channel head distributions resulting from the presence of bed forms and channel curvature flux calculations show that heterogeneity causes significant additional hz flux compared to an equivalent homogenous medium however the major cause of hz flux is a spatially periodic sinusoidal head distribution along the boundary representing the effect of bed forms the additional influence of heterogeneity on the total channel bed exchange and the overall hz geometry are increased when boundary head sinusoidal fluctuation is more subdued we present dimensionless numbers that summarize these relationships heterogeneity s influence is further magnified by considering the effect of channel curvature on boundary heads the simulations illustrate the dynamic influence of heterogeneity on the hyporheic zone since the various head boundaries employed in our modeling efforts are a proxy for different surface water conditions and bed form states that may occur during a single flood furthermore we show that residence times total tracking times of particles originating from the streambed follow a lognormal distribution in the presence of heterogeneity residence times can decrease or they can increase compared to residence times for homogeneous conditions depending on the relative positions of the heterogeneities and the bed forms hence streambed heterogeneity and stream curvature factors often neglected in previous modeling efforts combine with bed form configuration to dynamically determine hz geometry fluxes and residence time distributions

advective transport affecting metal and nutrient distributions and interfacial fluxes in permeable sediments abstract our laboratory flume experiments demonstrate that advective porewater flows produce biogeochemical reaction zones in permeable sediments leading to specific and reproducible complex patterns of fe mn and nutrients oxygenated water forced into the sediment when boundary flows were deflected by protruding sediment structures generated distinct zones of nitrification and ferric iron precipitation this inflow was balanced by ammonium rich porewater ascending from deeper sediment layers thereby creating an anoxic channel where dissolved fe2 and mn2 could reach the surface between the zones of ferric iron precipitation and fe2 upwelling a layer with increased manganese oxide and solid phase fe ii concentrations formed indicating redox reaction between these components the establishment of topography on the previously smooth sediment surface reversed the net interfacial flux of solutes while the smooth control core was found to be a sink for metals and nutrients the sediment with mounds acted as a source for these substances our experiments show that in sandy sediment with an oxidised surface layer reduced metal species can be released to the water column by flow topography interactions we conclude that advective transport processes constitute an important process controlling biogeochemical zonations and fluxes in permeable sea beds

the negligible effect of bed form migration on denitrification in hyporheic zones of permeable sediments bed form celerity the migration rate of ripples along a sediment bed has previously been shown to have dramatic effects on oxygen distribution and transport within the hyporheic zone of permeable sediments this has the potential to influence denitrification rates in particular by increasing the coupling of nitrification and denitrification to further understand this we numerically modeled nitrogen cycling under migrating ripples while the simulated oxygen profiles match with expected behavior almost no effect on denitrification or coupled nitrification denitrification was observed with increasing celerity instead denitrification rates were dominantly controlled by the flow velocity of water overlying the sediment

patterns of wintertime jet stream variability and their relation to the storm tracks abstract a new approach is put forward for defining extratropical teleconnection patterns the zonal wind field at 250 hpa is analyzed separately in the north atlantic and north pacific ocean sectors during the winter season december march teleconnectivity of this field is found to be particularly strong eof analysis of the zonal wind field yields patterns that i are robust with respect to the range of frequencies included in the data ii relate clearly to the position of the climatological mean jets and iii are broadly consistent with their traditionally defined counterparts in terms of climatic impacts the patterns are characterized by a north south shifting or an extension retraction of the eddy driven jet in its exit region and similar changes at the entrance region of the subtropical jet the patterns also reflect the degree of separation between the subtropical and eddy driven jets atlantic eofs 1 and 2 are counterparts of the north atlantic oscillation nao and eastern atlantic patter

changes in the saltation flux following a step change in macro roughness author s gillies john a etyemezian vicken nikolich george nickling william g kok jasper f

the effect of wind speed and bed slope on sand transport this paper reports on a wind tunnel study of the effects of bed slope and wind speed on aeolian mass transport the use of a sloping wind tunnel has enabled estimation of the friction angle to be about 40 for saltating particles in the range 170 540 m a formula relating dimensionless mass transport to friction speed and bed slope is proposed and mass transport data for five uniform sand samples and one non uniform sand sample are shown to fit the equation well in particular the relationship reveals an overshoot in mass transport slightly above threshold collisions a feature also evident when previous experimental data is re examined as the number of mid air collisions between the saltating particles increases greatly with wind speed the overshoot may occur as a result of increasing energy losses resulting from the collisions finally it is demonstrated that data for saltating snow shows a similar overshoot in the dimensionless transport rate

entrainment threshold of natural grains in liquids determined empirically from dimensionless settling velocities and other measures of grain size abstract grain size plays a crucial role in sediment transport but different methods of size analysis do not yield comparable results particle diameters obtained by settling tube for example cannot be employed in equations based on sieve sizes although the majority of experiments on the initiation of sediment motion has been based on sieve data settling tube analysis is probably superior as both the settling rate and the entrainment threshold of grains are determined by the same factors of size shape and density in this paper empirical equations are provided which allow the determination of a non dimensional critical shear stress directly from the equivalent sedimentation diameter the equations apply to the entrainment of well sorted grains with a high sphericity on flat sediment beds and are valid for particles and liquids of different densities

a synoptic climatology of episodic subseasonal retractions of the pacific jet abstracttwenty eight years of ncep ncar reanalysis data are employed in a composite analysis of the structure and evolution of the large scale circulation associated with rapid subseasonal westward retractions of the northern hemisphere pacific jet nineteen pacific jet retractions are identified in the dataset the salient characteristics of these transitions are presented emphasizing the structure and evolution of anomalies in the zonal wind upper tropospheric geopotential height sea level pressure slp and tropopause potential vorticity pv the composite analysis demonstrates that as the jet retracts a transition from a dominant negative anomaly to a dominant positive anomaly across the north pacific in both the 500 hpa geopotential height and slp fields occurs in 10 days the resulting anticyclonic anomalies are sprawling and intense with slp anomalies of nearly 20 hpa and 500 hpa geopotential height anomalies of more than 200 m the vertical superposition of these upper and lower level an

ash mists and brown snow remobilization of volcanic ash from recent icelandic eruptions recent eruptions in iceland and chile have demonstrated that volcanic ash problems persist long after an eruption for this reason ash dispersion models are being extended to include ash remobilization critical to these models is knowledge of the ash source and the particle sizes that can be mobilized under different wind and moisture conditions here we characterize the physical and chemical characteristics of ash deposited on new snow in reykjavik iceland following a blizzard on 6 march 2013 morphological textural and compositional analyses indicate resuspension from multiple eruptive deposits including both grimsvotn 2011 and eyjafjallajokull 2010 eruptions grain size measurements show a mode of 32 63 m with particles as large as 177 m there is little mass in the very fine fraction 10 m pm10 we compare our observations to predictions using the lagrangian particle dispersion model name uk met office the model output is consistent with observations in that it forecasts resuspension from both eyjafjallajokull and grimsvotn source regions and shows ash deposition coincident with the timing of observed deposition in reykjavik the modeled deposit in reykjavik predicts however a substantially lower proportion of grimsvotn ash than observed this discrepancy has highlighted the need to reassess the assumptions used in the simulations particularly regarding the source area and precipitation thresholds furthermore we suggest that modification of ash deposits in the form of erosion redeposition compaction or cementation may influence the dynamics of resuspension over time thus influencing the ability of model simulations to accurately forecast remobilization events

using tropopause maps to diagnose midlatitude weather systems the use of potential vorticity pv allows the efficient description of the dynamics of nearly balanced atmospheric flow phenomena but the distribution of pv must be simply represented for ease in interpretation representations of pv on isentropic or isobaric surfaces can be cumbersome as analyses of several surfaces spanning the troposphere must be constructed to fully apprehend the complete pv distribution following a brief review of the relationship between pv and nearly balanced flows it is demonstrated that the tropospheric pv has a simple distribution and as a consequence an analysis of potential temperature along the dynamic tropopause here defined as a surface of constant pv allows for a simple representation of the upper tropospheric and lower stratospheric pv the construction and interpretation of these tropopause maps which may be termed isertelic analyses of potential temperature are described in addition techniques to construct dynamical representations of the lower tropospheric pv and near surface potential temperature which complement these isertelic analyses are also suggested case studies are presented to illustrate the utility of these techniques in diagnosing phenomena such as cyclogenesis tropopause folds the formation of an upper trough and the effects of latent heat release on the upper and lower troposphere

transfer of nonsorbing solutes to a streambed with bed forms theory a theoretical analysis of the exchange of nonsorbing solutes between a permeable streambed and the overlying water of a stream or river is presented in this paper in a companion paper elliott and brooks this issue the results of experimental studies of such exchange are presented the analysis focuses on water movements into within and out of the bed which result from the presence of bed forms ripples and dunes and highlights the mechanisms of pumping and turnover pumping is the movement of fluid through the bed induced by steady dynamic pressure variations over bed forms turnover occurs as moving bed forms trap and release interstitial fluid the detailed analysis was used to generate the distribution of the residence time of solute within the bed which can be used to calculate the net mass exchange due to pumping and turnover

migration modification and merging in aeolian systems and the significance of the depositional mechanisms in permian and triassic dune sands of europe and north america abstract the main depositional mechanisms acting in aeolian systems can be assessed in two different and independent ways first by the internal anatomical approach with analysis of the cross stratification fabric and second by the external morphological approach with study of shape and surface features of dunes anatomical investigations of various ancient aeolian systems in the permian and triassic of europe and north america and an extensive literature review concerning both internal anatomy and external morphology of ancient and modern dunes reveal three main depositional mechanisms operating in aeolian systems migration modification and merging migration comprises downwind movement of isolated dunes or trains of aeolian bedforms under predominantly constant wind direction the orientation of the interset surfaces partially depends on the angular relationship between the path of bedform climb and the depositional surface and also on subsidence simple systems are characterized by successive dunes of similar size whereas compound systems originate by superimposition of smaller aeolian bedforms on larger dunes or draas compound migration gives rise to the hierarchy of diastems modification comprises overprinting of existing dunes by winds coming from directions other than those that govern migration of the original aeolian bedforms the anatomical elements representing modification include both intrasets and extensive intersets depending on duration and on the scale of the changes of the winds simple modification comprises occasional origin of either only cross dipping erosional boundaries or both diastems and intraset laminae oriented in different directions a special case of modification is the creation of intrasets dipping in the same direction as the foreset inclination comprising either reactivation or relaxation surfaces depending on whether the foreset slope is steepened or lowered in angle complex modification takes place under repeatedly alternating winds of changing effectiveness small scale modification comprises minor alterations of existing dune types and medium scale modification is characterized by conversion of primary aeolian bedforms into secondary dune types small scale and medium scale modification take place during short term to smaller scale long term changes of effective wind direction large scale modification originates by large scale long term changes accompanied by a certain subsidence which result in burial of the older dunes by a younger train of aeolian bedforms within the same sand sea migrating obliquely to perpendicularly to the former wind direction very large scale modification is induced by very large scale long term changes which result int deflationary erosion of older dune fields and subsequent superimposition of a new sand sea on top of the eroded remnants of the former erg merging comprises coalescing of bedforms with only negligible erosion during burial of the first bedform by subsequent dunes merging is induced mainly when migration of dunes is stopped by a morphological barrier or ceases as a consequence of weakening winds merging further occurs during coalescence of numerous isolated barchans in rows to parallel wavy barchanoid ridges as a consequence of increasing availability of sand the main depositional mechanism in compound systems is migration of hierarchically superimposed bedforms with subordinate modification under predominantly unidirectional winds in complex systems migration modification and merging take place in different amounts under unidirectional bidirectional and multidirectional winds depending on direction and strength of both earlier and subsequent winds complex systems are either charcterized by mainly migration with subordinate modification or by considerable modification with either conversion or dissection of older dunes the effects of migration modification and merging are discussed in the light of both internal anatomy of ancient cross stratification fabrics and external morphology of recent dunes a classification of dune types based on these mechanisms divides the aeolian bedforms into primary secondary and tertiary types primary dunes originate by simple or compound migration with minor modification and comprise mainly transverse dunes secondary dunes originate by complex interaction of migration and modification or even predominantly by modification with the best examples being longitudinal dunes tertiary dunes form by merging of transverse and longitudinal elements with varying amounts of migration and modification and are mainly represented by star dunes the results of various anatomical case studies of ancient aeolianites indicate that simple migration was the dominant mechanism for construction of ergs in the northern hemisphere in permian and triassic times

piecewise tendency diagnosis of dynamical processes governing the development of an upper tropospheric mobile trough abstract the intensification and evolution of midlatitude upper tropospheric mobile troughs may be viewed in terms of the isentropic advection and deformation of the tropopause potential vorticity gradient the potential vorticity viewpoint allows one to qualitatively assess observed events in the context of existing theories of mobile trough genesis such as baroclinic instability or downstream development in order to quantitatively determine the role of distinct dynamical process the method of piecewise tendency diagnosis or ptd is developed ptd is an extension of piecewise potential vorticity inversion applied to height tendencies with the forcing terms in the quasigeostrophic height tendency equation partitioned into potential vorticity advection associated with distinct dynamical processes a particular case of mobile trough genesis which occurred during 1 4 december 1980 over north america is diagnosed using ptd although about 20 of the intensification of the trough was due to superposition

reduction of the hyporheic zone volume due to the stream aquifer interaction 1 pore water in stream sediments is continuously exchanged with the surface water from the overlying stream this exchange of water and solutes that occurs across the stream sediment interface plays an important role for fluvial ecology because of the unique biochemical conditions rich biodiversity and high rates of metabolism while many studies have observed the extent of the hyporheic zone to be modified by changes in the level of the groundwater table the actual importance of this interaction is still difficult to quantify here we focus on the case of bedform induced hyporheic exchange to show how the the volume of hyporheic sediments that receive water from the stream is significantly reduced by the upwelling of subsurface water a simple scaling relationship for the assessment of maximum depth of the hyporheic zone is proposed by relating hyporheic flow to the groundwater discharge in an aquifer with given hydraulic properties and head difference between the stream and the aquifer

aeolian sand transport an optimized model consideration of the available equations for prediction of aeolian sand transport led to the development of a model which relies on optimization techniques for the production of improved performance the model has been applied to the characterization of dunesand movement in coastal environments in eastern australia

enhancement of denitrification in permeable carbonate sediment due to intra granular porosity a multi scale modelling analysis abstract it has recently been hypothesized that bulk denitrification rates in carbonate sands may be enhanced by reactions occurring in the intra granular pores cracks and crevices we tested this hypothesis using a series of flow and reactive transport models spanning from the pore scale mm to the continuum scales 10 cm bedforms pore scale simulations solved the coupled navier stokes and brinkman equations and represented flow through reactor experiments previously performed on coral reef sands the results revealed that intra granular transport and reactions can explain over all denitrification enhancement a sensitivity study with a single grain diffusive transport model showed that in the majority of cases the resultant increase in denitrification was not coupled to nitrification within a single grain only for large grain diameters of 2 and 4 mm was coupled nitrification denitrification important in most cases coupled nitrification denitrification instead arose as conditions became more reducing along a flow path as is the case in quartz sands without intragranular pores an intra granular reaction rate based on a single grain model was incorporated into a continuum scale darcy flow and reactive transport model for a rippled sand bed where porewater flow is driven by the turbulent current over the ripple the results of the darcy scale model suggest that intra granular pores increase the amount of slow flowing areas within which coupled nitrification denitrification can occur we conclude that the complex advective flow field does not strongly inhibit denitrification enhancement by carbonate sand grains as it does in silica sands thus intra granular reactions may enhance bulk denitrification in carbonate sediment with porous grains under natural advective conditions

aeolian system sediment state theory and mojave desert kelso dune field example the sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply sediment availability and the transport capacity of the wind the sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system numerous factors impact the susceptibility of grains on a surface to transport but these are cumulatively manifested by the actual transport rate which serves as a proxy for sediment availability transport capacity is the potential sediment transport rate of the wind because the three aspects of sediment state can be given as a volumetric rate they are directly comparable plotted simultaneously against time the generated curves define nine possible classes of sediment state sediment supply that is stored occurs because it is transport or availability limited or generated at a rate greater than the potential or actual transport rates respectively contemporaneous or lagged influx to an aeolian system may be limited by sediment availability but cannot exceed the transport capacity of the wind for the kelso dune field in the mojave desert of california a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system the sediment supply was generated during the latest pleistocene and earliest holocene during humid periods of enhanced discharge by the mojave river to form the lake mojave fan delta or terminal fan and has been calculated over time from the sedimentation rate and the frequency of floods estimation of transport capacity over time was based upon modern wind data with an allowance for greater winds during the pleistocene based upon climatic models sediment availability was approximated by calculation of a modern dune mobility index with variation over time based upon climatic inferences while quantifying the kelso or any natural system is subject to numerous uncertainties the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at kelso as well as illuminating questions for future research

a simple method to predict the threshold of particle transport under oscillatory waves abstract various methods of predicting the incipient motion of sediments under oscillatory waves have been proposed in the past but most require that at least the wave orbital diameter d o be known methods based on the oscillatory shields parameter ws need the wave friction factor f w in order to calculate the critical threshold velocity for which d o is also required other existing methods not requiring d o are only applicable to quartz grains and yield highly inaccurate results in this paper a new dimensionless threshold orbital velocity based on all the important variables playing a role in wave transport is plotted against the dimensionless settling velocity this yields a relationship that allows the prediction of the threshold velocity from the wave period and grain size alone the method has the advantages that it is valid for grains of varying size and density in laminar or turbulent fluids of different density and viscosity does not require the wave orbital diameter and is more accurate than previous equations

single discharge events increase reactive efficiency of the hyporheic zone in this study we investigate the impact of single stream discharge events on water exchange solute transport and reactions in the hyporheic zone below a natural in stream gravel bar we set up a reactive transport groundwater model with streamflow scenarios that vary by event duration and peak discharge a steady ambient groundwater flow field is assumed that results in losing neutral or gaining stream conditions depending on the stream stage across the streambed dissolved oxygen organic carbon and nitrate are transported into the subsurface additional nitrate is received from upwelling groundwater aerobic respiration and denitrification are simulated for scenarios with different stream solute concentrations results show that hyporheic exchange flux solute transport and consumption increase during events however their intensities depend highly on the interplay between event characteristics and ambient groundwater conditions during events where reversals in the hydraulic gradient occur stream water and solutes infiltrate deeper into the aquifer where they have more time to react for those events the reactive efficiency of the hyporheic zone solute consumption as fraction of influx for aerobic respiration and denitrification is up to 2 7 and 10 times higher compared to base flow conditions the fraction of stream nitrate load consumed in the hyporheic zone increases with stream discharge up to 150 mg m2 h but remains below the value under base flow conditions for weak events events also increase denitrification of groundwater borne nitrate but groundwater nitrate flux to the stream decreases by up to 33 due to temporary gradient reversals

pore water advection and solute fluxes in permeable marine sediments ii benthic respiration at three sandy sites with different permeabilities german bight north sea this contribution presents total oxygen uptake tou rates and nutrient fluxes of organically poor permeable shelf sands of the german bight measurements have been made in situ with the novel autonomous benthic chamber system sandy under controlled conditions of advective pore water exchange average oxygen consumption rates of 31 3 18 2 mmol m 2 d 1 measured in this study were relatively high as compared with rates reported from shelf sediments with much higher organic contents tou of highly permeable medium and coarse grained sands was substantially enhanced in the presence of advection this indicates that advective oxygen supply contributed significantly to respiration in these sediments and that advection has to be considered when assessing oxygen consumption and organic matter mineralization in shelf areas in fine grained less permeable sands no effect of advection could be measured a lower advective oxygen supply in these sediments is in agreement with a release of ammonium instead of nitrate and a shallower oxygen penetration depth scaled up to the entire german bight the results imply that in 40 of the area an effect of advection on benthic oxygen uptake and other advectionrelated processes can be largely excluded while in the remaining 60 significant pore water advection potentially takes place however because permeabilities of the sediments investigated in this study were widely spaced a significant effect on oxygen supply was only verified for highly permeable sands that are likely to cover approximately 3 of the area

the concept of transport capacity in geomorphology the notion of sediment transport capacity has been engrained in geomorphological and related literature for over 50 years although its earliest roots date back explicitly to gilbert in fluvial geomorphology in the 1870s and implicitly to eighteenth to nineteenth century developments in engineering despite cross fertilization between different process domains there seem to have been independent inventions of the idea in aeolian geomorphology by bagnold in the 1930s and in hillslope studies by ellison in the 1940s here we review the invention and development of the idea of transport capacity in the fluvial aeolian coastal hillslope debris flow and glacial process domains as these various developments have occurred different definitions have been used which makes it both a difficult concept to test and one that may lead to poor communications between those working in different domains of geomorphology we argue that the original relation between the power of a flow and its ability to transport sediment can be challenged for three reasons first as sediment becomes entrained in a flow the nature of the flow changes and so it is unreasonable to link the capacity of the water or wind only to the ability of the fluid to move sediment secondly environmental sediment transport is complicated and the range of processes involved in most movements means that simple relationships are unlikely to hold not least because the movement of sediment often changes the substrate which in turn affects the flow conditions thirdly the inherently stochastic nature of sediment transport means that any capacity relationships do not scale either in time or in space consequently new theories of sediment transport are needed to improve understanding and prediction and to guide measurement and management of all geomorphic systems

breaking waves at the tropopause in the wintertime northern hemisphere climatological analyses of the orientation and the theoretical lc1 2 classification abstract breaking waves on the tropopause are viewed as potential vorticity pv streamers on middle world isentropic levels a northern hemisphere winter climatology of the streamers spatial distribution and meridional orientation is derived from the 40 yr ecmwf re analysis era 40 dataset and used to assess the nature and frequency of occurrence of breaking synoptic scale waves the streamers are grouped into two classes related to the so called cyclonic lc2 and anticyclonic lc1 patterns and the ambient wind strength and wind shear is also noted it is shown that the occurrence of cyclonic and anticyclonic pv streamers exhibits a distinct spatial variability in the horizontal and the vertical the majority of cyclonic pv streamers are found on lower isentropic levels that intersect the tropopause at more poleward latitudes whereas anticyclonic streamers predominate at higher elevations in the subtropics an analysis of the streamer patterns for the two phases of the north atlantic oscillation

rates of aeolian transport on a beach in a temperate humid climate short term field measurements of aeolian transport using a saltiphone indicate that actual rates of transport deviate considerably from the potential rates predicted by transport equations when examined in detail however potential rates are approached during special conditions mostly when the wind blows offshore parallel or slightly oblique to the beach and relative humidity is below 85 also the potential rates of transport may be approached when wind speeds considerably exceed threshold speed even during showers the main cause of the large deviation of actual from potential rates is in the absence of transport during very wet conditions prolonged rainfall regardless of wind speed and the variation of threshold velocity with time the threshold velocity increases during wet conditions with onshore winds or because of the presence of algae therefore potential transport is defined as the transport optimum to predict the actual rates of transport accurately insight into the variation of the threshold velocity is essential aeolian transport of sand seems to be related to moderate events and not to extreme events conforming to the principle of magnitude and frequency

quantifying denitrification in rippled permeable sands through combined flume experiments and modeling we measured denitrification in permeable sediments in a sealed flume tank with environmentally representative fluid flow and solute transport behavior using novel measurements numerical flow and reactive transport models representing the flume experiments were implemented to provide mechanistic insight into the coupled hydrodynamic and biogeochemical processes there was broad agreement between the model results and experimental data the model showed that the coupling between nitrification and denitrification was relatively weak in comparison to that in cohesive sediments this was due to the direct advective transport between anoxic pore water and the overlying water column and little interaction between the mostly oxic advective region and the underlying anoxic region denitrification was therefore mainly fueled by nitrate supplied from the water column this suggests that the capacity of permeable sediments with nonmigratory ripples to remove bioavailable nitrogen from coastal ecosystems is lower than that of cohesive sediments we conclude that while experimental measurements provide a good starting point for constraining key parameters reactive transport models with realistic kinetic and transport parameters provide critical insight into biogeochemical processes in permeable sediment that are difficult to experimentally evaluate

piecewise tendency diagnosis of weather regime transitions abstract piecewise tendency diagnosis ptd is extended and employed to study the dynamics of weather regime transitions originally developed for adiabatic and inviscid quasigeostrophic flow on a beta plane ptd partitions local geopotential tendencies into a linear combination of dynamically meaningful source terms within a potential vorticity pv framework here ptd is amended to account for spherical geometry diabatic heating and ageostrophic processes and is then used to identify the primary mechanisms responsible for northern hemisphere weather regime transitions height tendency patterns obtained by summing the contributions of individual ptd forcing terms correspond very well to actual height tendencies composite ptd analyses reveal that linear pv advections provide the largest dynamical forcing for the weather regime development over the north pacific specifically linear baroclinic growth provides the primary forcing while barotropic deformation of pv anomalies provides a secondary contrib

hyporheic exchange with heterogeneous streambeds laboratory experiments and modeling 1 hyporheic exchange is generally analyzed with the assumption of a homogeneous hyporheic zone in reality streambed sediments have a heterogeneous structure and this natural heterogeneity produces spatially variable interfacial fluxes and complex hyporheic exchange patterns to assess the basic effects of sediment structure on hyporheic exchange we performed salt and dye injection experiments in a recirculating laboratory flume with two heterogeneous sediment beds characterized by negative exponential correlated random hydraulic conductivity fields dye injections showed that the hyporheic flow structure was controlled by the spatial relationship of bed forms to high and low permeability regions of the streambed as no existing model could represent these effects we developed a new finite element model to calculate the pore water flow field resulting from the interaction of the bed form induced boundary head distribution and the heterogeneous sediment structure a numerical particle tracking approach was then used to assess the resulting hyporheic exchange the combined flow transport model did an excellent job of predicting the complex hyporheic flow pathways in the heterogeneous bed and the net hyporheic exchange up to t 30 hours the heterogeneous hydraulic conductivity field caused both greater spatial variability in the water flux through the bed surface and a greater average interfacial flux than would have occurred with a homogeneous bed the layered correlation structure of the streambed produced an effective anisotropy that favored longitudinal pore water flow and caused a relatively rapid decrease of the mean pore water velocity with depth as a result solute penetration into the bed was confined to a more shallow region than would have occurred with a homogeneous bed the combination of faster near surface transport and shallower solute penetration produced a shorter mean hyporheic residence time on the basis of the combination of experimental results and model simulations we conclude that the structural heterogeneity of streambed sediments produces more spatially limited hyporheic exchange that occurs with greater spatial variability and at a higher overall rate

physical abrasion of mafic minerals and basalt grains application to martian aeolian deposits abstract sediment maturity or the mineralogical and physical characterization of sedimentary deposits has been used to identify sediment sources transport medium and distance weathering processes and paleoenvironments on earth mature terrestrial sands are dominated by quartz which is abundant in source lithologies on earth and is physically and chemically stable under a wide range of conditions immature sands such as those rich in feldspars or mafic minerals are composed of grains that are easily physically weathered and highly susceptible to chemical weathering on mars which is predominately mafic in composition terrestrial standards of sediment maturity are not applicable in addition the martian climate today is cold and dry and sediments are likely to be heavily influenced by physical weathering rather than chemical weathering due to these large differences in weathering processes and composition martian sediments require an alternate maturity index this paper reports the results of abrasion tests conducted on a variety of mafic materials and results suggest that mature martian sediments may be composed of well sorted well rounded spherical polycrystalline materials such as basalt volcanic glass is also likely to persist in a mechanical weathering environment while more fragile and chemically altered products are likely to be winnowed away a modified sediment maturity index is proposed that can be used in future studies to constrain sediment source paleoclimate mechanisms for sediment production and surface evolution this maturity index may also provide insights into erosional and sediment transport systems and preservation processes of layered deposits

hyporheic zone hydrologic science a historical account of its emergence and a prospectus the hyporheic zone defined by shallow subsurface pathways through river beds and banks beginning and ending at the river is an integral and unique component of fluvial systems it hosts myriad hydrologically controlled processes that are potentially coupled in complex ways understanding these processes and the connections between them is critical since these processes are not only important locally but integrate to impact increasingly larger scale biogeochemical functioning of the river corridor up to the river network scale thus the hyporheic zone continues to be a growing research focus for many hydrologists for more than half the history of water resources research this manuscript partly summarizes the historical development of hyporheic zone hydrologic science as gleaned from papers published in water resources research from the birth of the concept of the hyporheic zone as a hydrologic black box sometimes referred to as transient storage zone to its adolescent years of being torn between occasionally competing research perspectives of interrogating the hyporheic zone from a surface or subsurface view to its mature emergence as an interdisciplinary research field that employs the wide array of state of the art tools available to the modern hydrologist the field is vibrant and moving in the right direction of addressing critical fundamental and applied questions with no clear end in sight in its growth there are exciting opportunities for scientists that are able to tightly link the allied fields of geology geomorphology hydrology geochemistry and ecology to tackle the many open problems in hyporheic zone science

subsurface flow in lowland river gravel bars geomorphic and hydraulic processes which form gravel bars in large lowland rivers have distinctive characteristics that control the magnitude and spatial patterns of infiltration and exfiltration between rivers and their immediate subsurface environments we present a bedform infiltration relation together with a set of field measurements along two reaches of the san joaquin river ca to illustrate the conditions required for infiltration and exfiltration of flow between a stream and its undulating bed and a numerical model to investigate the factors that affect paths and residence times of flow through barforms at different discharges it is shown that asymmetry of bar morphology is a first order control on the extent and location of infiltration which would otherwise produce equal areas of infiltration and exfiltration under the assumption of sinusoidal bedforms hydraulic conductivity varies by orders of magnitude due to fine sediment accumulation and downstream coarsening related to the process of bar evolution this systematic variability not only controls the magnitude of infiltration but also the residence time of flow through the bed the lowest hydraulic conductivity along the reach occurred where the difference between the topographic gradient and the water surface gradient is at a maximum and thus where infiltration would be greatest into a homogeneous bar indicating the importance of managing sand supply to maintain the ventilation and flow through salmon spawning riffles numerical simulations corroborate our interpretation that infiltration patterns and rates are controlled by distinctive features of bar morphology

on the efficiency of vertical array aeolian field traps the efficiency of three vertical array field traps for measuring the distribution of aeolian mass transport with height was compared with measurements with an isokinetic trap the vertical traps seem to work adequately efficiency 80 at heights greater than 15 mm above the bed closer to the bed however the open array trap catches about 70 the two continuous arrays only about 50 because most of the transport takes place close to the surface the overall efficiency of the field traps ranges from 50 to 70 based on these wind tunnel tests we have developed a low single compartment wedge shaped trap its shape in combination with flow straighteners at the entrance minimizes stagnation effects so that the efficiency is 80 our data indicate that with the wedge trap used in combination with the vertical array aberdeen trap we can sample mass transport in the field during periods with unidirectional winds with an overall efficiency of the order of 80

estimation of some aeolian saltation transport parameters a re analysis of williams data a new method for analysing observed aeolian sand transport rate profiles of the kind obtained by williams is presented the method involves a mathematical model of aeolian saltation detailed information about the saltation process can be calculated from the transport rate profile by means of this model the method is used to perform a re analysis of williams trap data among the main findings of this analysis is that the grain borne shear stress appears to be a smaller fraction of the total shear stress than assumed by bagnold owen in their theories of aeolian saltation other findings are that the probability distribution of the jump height of the grains does not depend much on the wind speed once the saltation is established and that the vertical component of the mean launch velocity decreases with the grain size it is approximately inversely proportional to the grain diameter our estimates of the landing angles indicate that estimates of the impact angles obtained from photographically recorded trajectories are too small due to biased sampling the influence of grain shape on the transport characteristics is mainly due to changes in the grains ability to jump when hitting the bed it is found that angular grains have a lower mean jump height than spherical grains

small scale permeability heterogeneity has negligible effects on nutrient cycling in streambeds 1 aquatic sediment hosts coupled pore water flow and biogeochemical reactions that mediate water quality across the fluvial corridor including adjacent alluvial aquifers however the effect of small scale variations in sediment hydraulic properties on nutrient transformation occurring within the sediment is poorly understood we show through numerical flow and transport simulations for two realistic heterogeneous permeability cases typical of lowland rivers and two idealized homogeneous ones that there is little difference in the reactive transport fields the bulk reaction rates and nutrient sink source function despite stark differences in flow fields this is because the reactions are ultimately controlled by characteristic or bulk residence times that are similar for the heterogeneous and homogeneous cases this is a promising result for predictive models based solely on relative ratios of residence times and reactive time scales

the large scale modulation of subtropical cyclogenesis in the central and eastern pacific ocean abstract an analysis of the composite large scale circulation associated with periods of enhanced active or diminished inactive cyclogenesis in the subtropical central and eastern pacific ocean is presented composites were constructed using surface and tropospheric analyses from the ecmwf tropical ocean global atmosphere toga dataset for 10 northern hemisphere cool seasons 1986 96 active periods of subtropical cyclogenesis were defined to be periods in which two or more cyclones developed in close succession to each other while inactive periods were defined to be periods of at least 10 days duration during which no cyclones with a subtropical origin were present in the pacific basin the analysis revealed that the occurrence of subtropical cyclones in the central and eastern pacific ocean is strongly linked to the strength and location of the asian jet with active periods characterized by a weaker zonally retracted asian jet while inactive periods are characterized by a stronger zonally elon

a two species model of aeolian sand transport the transport of sand by wind results from the equilibrium between the erosion of grains dragged by the flow and the resulting slow down of the wind velocity the dynamical mechanisms governing the saturation of the sand flux are investigated theoretically we first demonstrate that previous models based on the assumption that all the grains have the same trajectory are either not self consistent or lead to unstable solutions a model based on a discrete number of states is derived which solves these problems two well defined species of grain appear which correspond to saltons high energy grains and reptons grains ejected from the sand bed by the impact of saltons they play specific roles the negative feedback of the transport on the wind is limited to the reptation layer while most of the transport is due to saltation the model is further simplified benefiting from the existence of these two species and the dependencies of the threshold velocity the saturated flux the aerodynamic roughness and the saturation length are derived and compared to experimental measurements

shape effects in aeolian grain transport wind tunnel experiments are reported the results of which show that grain transport rate is responsive to characteristic grain shape it is relatively less sensitive to changes of sorting unless these involve the fraction smaller than 50 m a cubic relationship between transport rate and velocity gradient appears to be valid only for well worked natural grains having high sphericity

the importance and challenge of hyporheic mixing the hyporheic zone is the interface beneath and adjacent to streams and rivers where surface water and groundwater interact the hyporheic zone presents unique conditions for reaction of solutes from both surface water and groundwater including reactions which depend upon mixing of source waters some models assume that hyporheic zones are well mixed and conceptualize the hyporheic zone as a surface water groundwater mixing zone but what are the controls on and effects of hyporheic mixing what specific mechanisms cause the relatively large 1m mixing zones suggested by subsurface solute measurements in this commentary we explore the various processes that might enhance mixing in the hyporheic zone relative to deeper groundwater and pose the question whether the substantial mixing suggested by field studies may be due to the combination of fluctuating boundary conditions and multi scale physical and chemical spatial heterogeneity we encourage investigation of hyporheic mixing using numerical modeling and laboratory experiments to ultimately inform field investigations

combined role of advective pumping and mechanical dispersion on time scales of bed form induced hyporheic exchange this study analyzes the effect of advective pumping and pore scale dispersion on bed form induced hyporheic exchange advection and dispersion play a competitive role in the exchange dynamics betwe