modeling study of turbulent mixing over the continental shelf comparison of turbulent closure schemes limitation is imposed during upwelling favorable winds the majority of turbulent mixing occurs in the top and the bottom boundary layers and in the vicinity of the vertically and horizontally sheared coastal jet turbulent mixing in the coastal jet is primarily driven by shear production the near surface flow on the inner shelf becomes convectively unstable as wind stress forces the upwelled water to flow offshore in the surface layer during downwelling favorable winds the strongest mixing occurs in the vicinity of the downwelling front the largest turbulent kinetic energy and dissipation are found near the bottom of the front turbulence in the bottom boundary layer offshore of the front is concentrated between recirculation cells which are generated as a result of symmetric instabilities in the boundary layer flow index terms 4219 oceanography general continental shelf processes 4568 oceanography physical turbulence diffusion and mixing processes 4255 oceanography general numerical modeling 4279 oceanography general upwelling and convergences

field measurements of the flux and speed of wind blown sand a field experiment was conducted to measure the flux and speed of wind blown sand under known conditions in a natural setting the experiment run at pismo beach california involved a tract 100 m long parallel with the wind by 20 m wide the site was instrumented with four arrays of anemometers to obtain wind velocity profiles through the lower atmospheric boundary layer temperature probes to determine atmospheric stability and wind vanes to determine wind direction from these measurements wind friction speeds were derived for each experimental run in order to measure sand saltation flux a trench 3 m long by 10 m wide transverse to the wind direction by 0 5 m deep was placed at the downwind end of the tract and lined with 168 collector bins forming an egg box pattern the mass of particles collected in each bin was determined for four experimental runs in order to assess various sand trap systems used in previous experiments 12 leatherman traps one fryberger trap and one array of ames traps were deployed to collect particles concurrently with the trench collection particle velocities were determined from analysis of high speed 3000 and 5000 frames per second motion pictures and from a particle velocimeter sand samples were collected from the trench bins and the various sand traps and grain size distributions were determined fluxes for each run were calculated using various previously published expressions and then compared with the flux derived from the trench collection results show that bagnold s 1941 model and white s 1979 equation most closely agree with values derived from the trench comparison of the various collector systems shows that the leatherman and ames traps most closely agree with the flux derived from the trench although these systems tended to under collect particles particle speeds were measured from analysis of motion pictures for saltating particles in ascending and descending parts of their trajectories results show that particle velocities from the velocimeter are in the range 0 5 7 0 m s 1 compared to a wind friction velocity of 0 32 0 43 m s 1 and a wind velocity of 2 7 3 9 m s 1 at the height of the particle measurements descending particles tended to exceed the speeds of ascending particles by 0 5 m s 1

on the genesis and evolution of barchan dunes morphodynamics barchan dunes are crescent shaped formations of sand that can dominate both desert and subaqueous landscapes when the supply of sand is scarce because of the complexity and scale of the underlying phenomena the mechanisms governing the entire process from the genesis to the long term evolution of barchan fields still remain to be better understood herein we attempt to present a description of this process in a subaqueous environment by employing a large eddy simulation approach that couples turbulent flow and sand bed morphodynamics we show that the seeds of the emergent structure in barchan fields are random turbulent flow motions near the initially flat bed we also provide high resolution insights into phenomena such as barchan migration and merging and show how transverse sand waves are formed and migrate over the barchan horns furthermore the transverse sand waves over the barchan horns are shown to be the seeds of the newly born barchans at the end points of the two horns of a barchan through the process known as calving to show this we examine the celerity wavelength and amplitude of the transverse sand waves over the barchan as they approach the end of its horn the celerity and wavelength of these transverse sand waves are shown to be the defining factors in determining the frequency of the calving process the amplitude of the newly born barchans through calving is also shown to be associated with the amplitude of the transverse waves near the end of the horn the simulation data also show that the wavelength of the newly born barchans the distance between individual dunes is closely related to that of the transverse sand waves over their maternal barchan finally we use the simulation results to discuss past conclusions derived from theory conventional models and field observations

retracted the influence of sand bed temperature on lift off and falling parameters in windblown sand flux in deserts the temperature of the sand bed can reach up to more than 50 degrees c however existing knowledge on the lift off and falling velocities and angles of sand particles in windblown sand flux is based mainly on experimental results over a temperature range of 20 degrees c to 30 degrees c consequently existing experimental results cannot reflect the actual interaction between saltating sands and the bed in this study the influence of the sand bed temperature on the probability distribution of lift off and falling velocities and angles of sand particles is investigated through an improved particle image velocimetry piv system results demonstrate that the distribution shape of lift off and falling velocities and angles of sand particles is not greatly influenced by the temperature of sand bed however it has a certain influence on the center of the probability distributions of horizontal velocities as well as the average decay constant and amplitude of the distributions of vertical velocities and angles we present formulas to describe the probability distributions of lift off and falling velocities and angles with regard to the influence of sand bed temperature c 2013 elsevier b v all rights reserved

saltation and wind flow interaction in a variable slope wind tunnel abstract in a 15 m long horizontal wind tunnel we studied the internal boundary layer over fixed arrays with roughness z0 similar to the roughness of saltation of a sand bed utilizing turbulence spires the logarithmic overlap layer is at least 75 mm thick some 5 m downwind of the entry this permits a precise determination of u from wind profile data over a sand bed in a variable slope wind tunnel with a 6 m long working section we studied how slope grain size and friction speed influence bed roughness owen 1964 proposed that z 0 c u 2 2 g where c is a constant but we found that c depends in a complex manner on grain size and friction speed c increases from very low values near the threshold to an apparently limiting value at large friction speeds for an almost uniform 125 m sand the limiting value of c is 0 06 the limiting value decreases however with particle size for a uniform 544 m sand c only approaches 0 03 for friction speeds as high as 1 m s with the measured values of friction speed corrected for the effects of slope our results appear to show that the limiting values of c do not depend on the slope angle even though the slope affects the individual shapes of particle trajectory c is perhaps independent of slope angle because the roughness height and therefore c is more directly related to the amount of momentum extracted from the air by the particles i e the shear stress rather than to the manner of extraction particle trajectory shapes

gravitational circulation and its response to the variation in river discharge in the seto inland sea japan 1 in this study gravitational circulation and its response to the variation in river discharge have been investigated in the seto inland sea japan monthly hydrographic surveys have been conducted from 2002 to 2006 in hiuchi nada a basin located between two straits horizontal salinity gradients along the channel varied as the 9 10 power of the river discharge into one end of the basin significant volume fluxes which were proportional to the horizontal salinity gradients were found the relationship between river discharge r m3 s 1 and the volume flux q m3 s 1 was represented as q 13 9r0 88 when river discharge increased vertical density gradients were intensified and then along channel horizontal density gradients were gradually enhanced due to vertical mixing at the western strait these gradients drove gravitational circulations along the channel the gravitational circulations were not uniform across the channel and accompanied by horizontal circulations in the northern part of the basin low salinity water at the eastern end of the basin was directed toward the west and high salinity water was directed toward the east the eastward flows arising from the intrusion of relatively heavy water at the western strait extended from the bottom to the surface in the southern part low salinity water was directed toward the west the westward flows arising from the intrusion of relatively light water at the eastern strait concentrated in the middle layer and formed a plume like structure accompanied by the eastward flow under the plume these situations were responsible for the development of the bulk exchange flux

the vertical structure of turbulent dissipation in shelf seas abstract the free fall fly profiler has been used to determine the variation in energy dissipation in the water column over a tidal cycle at mixed and stratified sites in the irish sea it was found that exhibits a strong m4 variation with a pronounced phase lag that increases with height above the bed in mixed conditions this m4 signal which extends throughout the water column is reasonably well reproduced by turbulent closure models of the vertical exchange in the summer stratified situation the m4 signal in is confined to about 40 m above the seabed with phase delays of more than 4 h relative to the seabed the lowest levels of dissipation 10 5 w m 3 measured in the pycnocline are significantly above the system noise level and much higher than predicted by a model using the mellor yamada level 2 closure scheme my2 0 however when allowance is made for the diffusion of tke the model my2 2 simulates the depth time distribution of dissipation in the stratified case satisfactorily if

residual currents in a semi enclosed bay of the seto inland sea japan 1 on the basis of the princeton ocean model a robust diagnostic model is developed to calculate the residual currents in the hiuchi nada a semi enclosed bay of the seto inland sea the input water temperature and salinity data are from six hydrographic surveys in 2002 and the model results in the summer show a clockwise eddy in the western part of the bay and an anticlockwise eddy in the eastern part the magnitude of two eddies is 10 cm s the modeled flow pattern is consistent with early observations derived from moored arrays the two eddies are persistent throughout the summer because winds are usually weak and cannot influence their existence the tide induced residual currents are appreciable only in the close vicinity of the kurushima strait and bisan strait the two straits connecting the bay to the other bays and have little affect on the two eddies that are inside the bay thus the two density driven eddies in the hiuchi nada are the basic pattern of circulation in the summer using the calculated residual currents we examined the intrusion route of waters from the kurushima strait and bisan strait with passive tracer experiments the results show that the water in the kurushima strait intrudes into the bay mainly through the middle and bottom layers while the water in the bisan strait intrudes mainly through the surface layer furthermore the tracer cannot reach the center of the anticlockwise eddy in the eastern part of the bay this indicates that the eddy prevents water exchange between its center and outer edges and provides favorable conditions for the formation of oxygen deficient waters in the central part of the eddy

a wind tunnel study of aeolian sediment transport response to unsteady winds abstract although moderate attention has been paid to the response of the aeolian mass transport rate to wind gusts it is still unclear how the particle size and volumetric concentration affect this relation very little is known about the response time of the particle speed and specifically how the sensor scale and elevation affect measurements of this variable the present study addresses this knowledge gap through a series of wind tunnel experiments in which a gusty wind was generated by programming the fan motor to adjust to a randomly selected rpm every 10 s beds consisting of either medium or coarse sand were investigated through synchronous co located measurements of the local wind speed and particle speed count rate obtained via a customized laser doppler anemometry lda system the vertically integrated sand transport rate q and the wind speed in the freestream were quantified using a passive sand trap and pitot tube respectively the results of the experiments indicate that the response of the aeolian transport system to wind gusts is generally faster in terms of the particle speed than the mass transport rate while the degree of correlation is found to vary with the sensor elevation as well as with the particle size and volumetric concentration in essence the coupling within the transport system is demonstrated to be strongly scale dependent

measurements of the aeolian sand transport saturation length the wavelength at which a dune pattern emerges from a flat sand bed is controlled by the sediment transport saturation length which is the length needed for the sand flux to adapt to a change in wind strength the influence of the wind shear velocity on this saturation length and on the subsequent dune initial wavelength has remained controversial in this paper we present direct measurements of the saturation length performed in a wind tunnel experiment complementary to this initial dune wavelengths are measured under different wind conditions in particular after storms using the linear stability analysis of dune formation it is then possible to deduce the saturation length from field data both direct and indirect measurements agree that the saturation length is almost independent of the wind strength this result supports the idea that grain inertia is the dominant dynamical mechanism limiting sediment transport saturation on dunes

craters as sand traps dynamics history and morphology of modern sand transport in an active martian dune field abstract aeolian transport of sand is abundant on modern day mars as revealed by remote sensing measurements of the motion of dunes and of the meter scale ripples that mantle them we study a large scale natural sand trap within the meroe patera dune field a 1 8 km diameter crater which features a dune free shadow in its lee we compare the volume of sand trapped within this crater to the sand volume that would be expected to cover the area of the crater and its dune free shadow behind it if the crater were not present we find that the crater holds less sand than this missing volume would predict implying that sand escapes from the crater over time modern day imagery shows an apparent lack of sand escaping from the meroe crater however suggesting that changes in the wind regime at the site may have allowed sand to escape in the past the persistence of an altered dune morphology all the way to the far downwind edge of the dune field suggests consistent wind conditions over the time of the crater dune field interaction

steady state saltation in air coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain surface impact process experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced including a diminution of the sub saltation layer mean wind speed as the friction speed increases in this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free stream shear stresses and the grain mass flux increases with friction speed uf less rapidly than uf3

response of a multi domain continental margin to compression study from seismic reflection refraction and numerical modelling in the tagus abyssal plain abstract the effects of the miocene through present compression in the tagus abyssal plain are mapped using the most up to date available to scientific community multi channel seismic reflection and refraction data correlation of the rift basin fault pattern with the deep crustal structure is presented along seismic line iam 5 four structural domains were recognized in the oceanic realm mild deformation concentrates in domain 1 adjacent to the tore madeira rise domain 2 is characterized by the absence of shortening structures except near the ocean continent transition oct implying that miocene deformation did not propagate into the abyssal plain in domain 3 we distinguish three sub domains sub domain 3a which coincides with the oct sub domain 3b which is a highly deformed adjacent continental segment and sub domain 3c the miocene tectonic inversion is mainly accommodated in domain 3 by oceanwards directed thrusting at the ocean continent transition and continentwards on the continental slope domain 4 corresponds to the non rifted continental margin where only minor extensional and shortening deformation structures are observed finite element numerical models address the response of the various domains to the miocene compression emphasizing the long wavelength differential vertical movements and the role of possible rheologic contrasts the concentration of the miocene deformation in the transitional zone tc which is the addition of sub domain 3a and part of 3b is a result of two main factors 1 focusing of compression in an already stressed region due to plate curvature and sediment loading and 2 rheological weakening we estimate that the frictional strength in the tc is reduced in 30 relative to the surrounding regions a model of compressive deformation propagation by means of horizontal impingement of the middle continental crust rift wedge and horizontal shearing on serpentinized mantle in the oceanic realm is presented this model is consistent with both the geological interpretation of seismic data and the results of numerical modelling

numerical simulation of flow around a tall isolated seamount part i problem formulation and model accuracy abstract a sigma coordinate ocean circulation model is employed to study flow trapped to a tall seamount in a periodic f plane channel in part i errors arising from the pressure gradient formulation in the steep topography strong stratification limit are examined to illustrate the error properties a linearized adiabatic version of the model is considered both with and without forcing and starting from a resting state with level isopycnals the systematic discretization errors from the horizontal pressure gradient terms are shown analytically to increase with steeper topography relative to a fixed horizontal grid and for stronger stratification as measured by the burger number for an initially quiescent unforced ocean the pressure gradient errors produce a spurious oscillating current that at the end of 10 days is approximately 1 cm s 1 in amplitude the period of the spurious oscillation about 0 5 days is shown to be a consequence of the particular form of the pressure gradient terms in th

saltating particles over flat beds measurements of ejection and impact velocities trajectory lengths and maximum rise heights of sand grains median diameters 118 and 188 m in saltation over a flat sand bed in a wind tunnel have been obtained from the digitization of multiple image photo graphs the mean angle of ejection is found to be about 30 o from the horizontal rather than 90 o with mean vertical ejection velocity of about 2u where u is the friction velocity trajectories of saltating grains have been computed using the measurements of the initial ejection velocities and the mean velocity profile of the air flow

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

what processes control the net currents through shallow straits a review with application to the bohai strait china we review the key processes controlling the net water circulation in shallow straits the circulation is mainly barotropic driven by the difference in mean sea level and tides on either side of the strait the wind the meandering of the currents in the adjoining seas the bathymetry including the shape and width of the mouths and the presence of narrows shoals sills islands and lateral embayments as the oceanography forcing can be non synchronised on either side of the strait the resulting net currents through the strait may appear chaotic we apply these findings to the net circulation through the bohai strait in china using local wind remote sensing data of currents and sea level and a hydrodynamic model the net currents through the bohai strait were episodic they were largest during winter storms when the wind generated a cyclonic coastal current in the northern yellow sea that flowed across the width of the bohai strait like the luzon strait this current meandered and a branch of the current intruded into the northern bohai strait along the deeper channel the wind set up in the bohai sea and the wind driven longshore current in the southern bohai sea generated a seaward flow through the southern bohai strait somewhat like in irbe strait as with the straits of the seto inland sea stagnation occurred in some embayments as in the torres strait the islands in the southern bohai strait retarded the net flow and increased the width of the outflow zone these flows were also modulated at very long periods 60 days by long waves in the yellow sea possibly generated by meandering of the kuroshio current that created an additional sea level slope through the strait the transit time in the bohai strait was 60 and 10 days respectively during calm weather and winter storms the residence time in the bohai sea was estimated to be about 1 68 years by the loicz model and 1 56 years by the hydrodynamic model and the influence of the river runoff including the yellow river was negligible

the summer gyre in the western irish sea shelf sea paradigms and management implications a cyclonic near surface gyre has been discovered above the deep western irish sea basin the gyre present in spring and summer each year is formed when stratification isolates a dome of cold bottom water which then drives a baroclinic circulation simultaneous observations using ship mounted broad band acoustic doppler current profiler towed undulating ctd conductivity temperature depth and satellite tracked drifting buoys reveal the detailed horizontal and vertical structure of the gyre gyre dynamics have been investigated using a three dimensional steady diagnostic circulation model with realistic bathymetry the role of the gyre within the existing bowden simpson paradigm of shelf circulation is discussed knowledge of seasonal gyre systems is important when formulating management strategies for example there is evidence that the irish sea gyre acts as a retention system for the planktonic larvae of the commercially valuable norway lobster nephrops norvegicus linnaeus 1758 which inhabits the geographically isolated mud patch beneath the gyre moreover in the event of an accidental summer time spill the gyre could also act to retain contaminants suggesting that this system is at disproportionate risk from environmental damage

the effect of surface slope on saltation threshold a new wind tunnel has been constructed to study the mass transport properties of wind blown sand this report on the first set of experiments in the new wind tunnel concerns the effect of slope on threshold friction speed the results of this series of static threshold experiments and one dynamic threshold experiment for a range of particle diameters and bed slope angles show that provided the effects of reynolds number variation and interparticle cohesive force are accounted for the static friction angle is independent of slope and close in value to the measured static angle of response

fortnightly variation in the bottom thermal front and associated circulation in a semienclosed sea abstractsummer hydrographic data from 1971 to 2000 demonstrate the presence of bottom cold water bcw and a bottom thermal front around the bcw in a semienclosed sea in the seto inland sea of japan the horizontal gradient of water temperature across the bottom front was larger in neap tide than in spring tide which is the opposite the pattern observed in the fortnightly variations in other areas e g the irish sea a numerical model for the inland sea reproduced the presence of bcw and a bottom front as well as the same fortnightly variation in the horizontal gradient of water temperature across the bottom front as in observational data being the same bcw as those in other areas the presence of bcw in the inland sea is also caused by spatial variation in tidal mixing the intensification of the bottom front in neap tide in the inland sea results from a combination of tidal mixing and horizontal advection of warm water to the bcw the presence of a bottom residual current in the direction across th

observations of turbulence in a partially stratified estuary the authors present a field study of estuarine turbulence in which profiles of reynolds stresses were directly measured using an adcp throughout a 25 h tidal day the dataset that is discussed quantifies turbulent mixing for a water column in northern san francisco bay that experiences a sequence of states that includes a weak ebb and flood that are stratified followed by a strong and eventually unstratified ebb and flood these measurements show that energetic turbulence is confined to a bottom mixed layer by the overlying stratification examination of individual reynolds stress profiles along with profiles of richardson number and turbulent froude number shows that the water column can be divided into regions based on the relative importance of buoyancy effects using the measured turbulence production rate p the dissipation rate e is estimated the observed turbulence had values of e nn 2 20 all of the time and e nn 2 200 most of the time suggesting that the observed motions were buoyancy affected turbulence rather than internal waves however at times turbulent froude numbers in much of the upper water column were less than one indicating important stratification effects taken as a whole the data show that stratification affects the turbulent velocity variance q2 most severely that is observed reductions in u9w9 are largely associated with small values of q2 rather than with a dramatic reduction in the efficiency with which turbulent motions produce momentum fluxes finally the dataset is compared to predictions made using the popular mellor yamada level 2 5 closure these comparisons show that the model tends to underestimate the turbulent kinetic energy in regions of strong stratification where the turbulence is strongly inhomogeneous and to overestimate the turbulent kinetic energy in weakly stratified regions the length scale does not appear to compensate for these errors and as a result similar errors are seen in the eddy viscosity predictions it is hypothesized that the underestimation of q2 is due to an inaccurate parameterization of turbulence self transport from the near bed region to the overlying stratification

a steady state model of wind blown sand transport a model for steady state eolian saltation is presented in which computation of sand grain trajectories in the wind and of the wind velocity profile modified by saltating grain drag forces is combined with experimental data on grain bed collisions in an iterative simulation scheme which resembles the time development of natural saltation qualitative features of eolian saltation are reproduced in the simulations including a height in the saltating layer at which the wind velocity remains roughly constant with changing free stream friction velocity and an approximate separation of the saltating population into successively hopping grains and reptating or creeping grains the numerical solutions give rise to more general arguments in support of the conclusions that fluid entrainment of grains is unimportant in steady state saltation that the mass flux profile obeys roughly an inverse power law dependence on height and that the vertically integrated mass flux dependence on the free stream friction velocity

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 grain bed collision in sand transport by wind in the last decade much progress has been made toward the development of a comprehensive model of aeolian sediment transport in which the grain bed collision has been identified as having a significant role the grain bed collision has been studied by both physical experiments and numerical simulation a principal objective of these studies has been to gather sufficient data to characterize the collision in order that it may be represented as an empirical function in numerical models of the sand transport system thus the study of the exact physical mechanism of the collision has to some extent been neglected the transport of larger particles in saltation over a loose surface is known to promote the release of finer particles from that surface into suspension thus the precise physics of the grain bed collision become highly significant with regard to the physics of dust release from a surface this paper re examines previous collision data and compares them with a simple collision model this model proposed that the impinging grain strikes a single surface grain and rebounds an alternative mechanism might be that the impinging grain ploughs through the surface striking a number of bed grains before rebounding the collision data are shown to support the model first because the duration of the collisions observed on the high speed film supports a short contact time with the surface and second because the data fit well with the model