flow and sediment transport induced by tide propagation 2 the wavy bottom case 1 a model is formulated to describe the flow field locally induced by the propagation of a tidal wave and to evaluate the sediment transport the three dimensional reynolds averaged momentum equations are considered and coriolis terms are retained since they affect the tide propagation turbulent reynolds stresses are described by means of the boussinesq assumption and the eddy viscosity is assumed to be an assigned function of the distance from the sea bed the effects of wind waves on sediment transport are taken into account and both the bed load and the suspended load are computed a comparison of the obtained results with laboratory data and field measurements shows that the model provides a reliable and accurate description of both the velocity field and sediment transport and does not require high computational costs

application of hydrochemical signatures to delineating portable groundwater resources in ordos basin china the ordos basin of china encompasses shaanxi gansu and shanxi provinces ningxia and inner mongolia autonomous regions it lacks significant surface water resources among the water bearing formations the luohe formation with an area of 1 32 105 km2 is the most prospective aquifer groundwater quality data collected at 211 boreholes drilled into the luhe formation indicate a complex distribution of groundwater chemistry the hydrochemical properties were used to study the recharge runoff and discharge conditions of the groundwater in ordos basin and to evaluate sustainable groundwater resources in the northern part of the basin the hydrochemistry types and the total dissolved solids tds show a clear lateral transition from see to nww indicating that the groundwater gets recharge in the northwest region and discharges in the southeast region in the southern part of the basin maximum tds occurs at the center of the malian river valley from which the tds decreases radially therefore the groundwater in the southern basin gets recharge from the southeast and southwest regions and the malian river valley is the discharge zone as a result of this research the areas with portable groundwater were delineated they include most of the southeast region of the sishili ridge east of the ziwu mountain and the southwest corner of the basin the tds of the groundwater in these regions is less than 1 g l and the hydrochemistry type is either hco3 or hco3 so4

the morphodynamics of tidal sand waves a model overview this paper reviews recent theoretical studies of sand waves which are rhythmic large scale bedforms observed in the continental shelf far from the near shore region emphasis is given to the investigations carried out in the framework of the eu research project humor first the results of linear morphodynamic stability analyses are described which allow to understand the initial behavior of the sand waves hence indications on the physical processes controlling the appearance and development of sand waves are obtained along with quantitative predictions of the wavelength of sand waves and of their migration speed then nonlinear models are described which are used to predict the equilibrium profile of sand waves and their interaction with human interventions like sand extraction or the construction of pipelines finally we discuss an analytical model which describes how the sand wave instability behaves when it is triggered locally this leads to the generation growth and expansion of a so called sand wave packet

effects of melting on frontogenesis abstract frontal precipitation systems are simulated with a 2d cloud model including ice phase microphysics despite the use of idealized frontogenetic forcing in the simulations some observed characteristics of frontal zones and their associated cloud and precipitation fields are reproduced in the simulations the effects of melting snow on surface frontogenesis is investigated it is found that the cooling effects of melting snow significantly accelerate surface frontogenesis in winter storm environments especially when the melting layer is close to the surface however the steady state surface frontal strength in the model is not sensitive to the melting effects finescale thermal and kinematic perturbations inside the frontal zone near the melting level quite similar to those recently reported in the literature are evident in the model results analysis of the model results suggests that cooling from melting snow may induce these thermal and kinematic perturbations and may enhance baroclinicity

research aircraft observations and numerical simulations of a warm front approaching vancouver island abstract the mesoscale structure of the low level flow and the character of the turbulence are investigated for a warm front as it approached the prominent terrain of the pacific northwest in the vicinity of vancouver island flight level and airborne doppler radar measurements collected from a national oceanic and atmospheric administration p 3 research aircraft on 9 december 1995 during the coastal observation and simulation with topography experiment and high resolution numerical simulations performed with the naval research laboratory s nonhydrostatic coupled ocean atmosphere mesoscale prediction system coamps model have been used to document the warm front as it approached the coast and the structure of cool air trapped along vancouver island this air mass was capped by a flat thin transition zone of enhanced static stability and vertical wind shear the flow within the trapped air mass was oriented parallel to the terrain of vancouver island but appears to have been more an outflow from the s

searching for the elusive cold type occluded front abstractaccording to the norwegian cyclone model whether a warm type or cold type occluded front forms depends upon which cold air mass is colder the prewarm frontal air mass or the postcold frontal air mass for example a cold type occlusion is said to occur when the occluded front slopes rearward with height because the prewarm frontal air mass is warmer than the postcold frontal air mass this temperature difference and the resulting occluded frontal structure in the norwegian cyclone model is part of what is called the temperature rule paradoxically no clear example of a rearward sloping cold type occluded front has been found in the literature even though the required temperature difference has been documented in several cases this article presents the first documented rearward sloping cold type occluded front this occluded front forms in a cyclone over the north atlantic ocean on 3 5 january 2003 and is documented in model output from the european centre for medium range weather forecasts

vertical structures of precipitation in cyclones crossing the oregon cascades abstract the vertical structure of radar echoes in extratropical cyclones moving over the oregon cascade mountains from the pacific ocean indicates characteristic precipitation processes in three basic storm sectors in the early sector of a cyclone a leading edge echo lee appears aloft and descends toward the surface updraft cells inferred from the vertically pointing doppler radial velocity are often absent or weak in the middle sector the radar echo consists of a thick vertically continuous layer extending from the mountainside up to a height of approximately 5 6 km that lasts for several hours when the middle sector passes over the windward slope of the cascades the vertical structure of the precipitation exhibits a double maximum echo dme one maximum is associated with the radar reflectivity bright band the second reflectivity maximum is located approximately 1 2 5 km above the bright band the secondary reflectivity maximum aloft does not appear until the middle sector passes over the wi

hydrodynamics and sediment dynamics of north sea sand waves and sand banks and discussion seabed drag coefficients have been measured at a site within the norfolk banks and at a site within the sand wave field in the southern north sea using pressure sensors and moored current meters at the sand banks site a seabed tripod measuring turbulent flows within 1 m of the bed was also used the results are generally in agreement with values used in numerical models at the sand banks site the drag coefficient increases with wind conditions but at the sand waves site there is a reduction during the highest wave conditions attributed to the drag reduction caused by sand resuspension from the bed this result suggests that sediment effects must be considered if wave current interaction is included in numerical models of the region the drag coefficient for reversing tidal flows over the asymmetric sand waves is found to be larger for flow towards the steeper face suggesting a small form drag component studies of sand movement used bedform mapping fluorescent sand tracing and photography of migrating ripples there is evidence for the early stages of formation of a new bank between broken bank and well bank at the sand waves site ripple migration is found to be a useful estimator of bedload transport under conditions of negligible suspension over the sand banks a definite correlation of surface water properties notably the turbidity with the topography of the banks was observed the mechanism for this surface effect is not known but it is consistent with the observation of features seen in satellite visible light images which outline the shapes of the banks

spatially varying environmental properties controlling observed sand wave morphology sand wave morphology and dynamics on continental shelves vary substantially and we hypothesize that these spatial variations depend on local bed properties and hydrodynamic characteristics to date process based modeling studies have not been able to simulate realistic equilibrium sand wave heights and empirical studies are mostly limited to case studies in order to explain the spatial variation in the morphology of equilibrium sand waves on continental shelves with processes and local bed conditions a large scale investigation is required in this paper we use high resolution multibeam echo soundings hydrodynamic models and databases and sedimentary data for the analysis of respectively sand wave shape characteristics and the comparison to hydrodynamic and sedimentary characteristics for the netherlands continental shelf the results are quantified lengths heights and asymmetry of all sand waves in the dutch part of the north sea furthermore we show that the mode of sediment transport bed load or suspended is a dominant factor in explaining sand wavelength height and asymmetry full results of shape characteristics of all sand waves on the netherlands continental shelf together with the tidal velocity water depth surface wave height and median grain size are provided in a repository with this paper http doi org 10 4121 uuid 0d7e016d 2182 46ea bc19 cdfda5c20308 these results are highly valuable for applied offshore engineering projects and to modelers for validating their morphodynamic model results

revisiting the linear theory of sand dune formation a linear stability theory for dune and antidune formation is presented that implements a rotational two dimensional flow model as in previous linear theories the phase lag between sediment transport and bed elevation remains the main mechanism driving instability however it is shown that this phase lag varies significantly in a neighbourhood of the bed moreover since the layer in which sediments are moving has a finite though small thickness it is assumed that the perturbations of the fluid stress driving bedload transport should be evaluated at the top of the layer itself it is shown that such an apparently minor modification of the classical approach alters remarkably the balance between stabilizing and destabilizing effects that drives the instability process this also allows a clarification of the debated role of bedslope on the formation of dunes and antidunes following the above ideas antidunes are shown to form in the absence of suspended sediment load and without any effect associated with sediment inertia the present analysis ultimately allows a successful unification of the theories of dune and antidune formation

ice lollies an ice particle generated in supercooled conveyor belts on 21 january 2009 a maturing low pressure weather system approached the uk along with several associated frontal systems as a part of the aerosol properties processes and influences on the earth s climate clouds project an observational research flight took place in southern england sampling the leading warm front of this system during the flight a distinctive hydrometeor type was repeatedly observed which has not been widely reported in previous studies we refer to the hydrometeors as drizzle rimed columnar ice or ice lollies for short due to their characteristic shape we discuss the processes that led to their formation using in situ and remote sensing data

the role of suspended load transport in the occurrence of tidal sand waves tidal sand waves are dynamic bed patterns which are formed by the complex interaction between hydrodynamics sediment transport and geomorphology field data from the southern north sea reveal that sand waves are absent where suspended load transport is the dominant transport mode in order to understand the mechanisms responsible for the absence of sand waves we study the influence of suspended load transport on the formation of tidal sand waves with a numerical process based geomorphological model delft3d model simulations are presented in which the vertical eddy viscosity and sediment diffusivity are both spatially and temporally variable k e turbulence model first it is shown that the preferred wavelength of sand waves for a relatively large grain size increases by the inclusion of suspended sediment while for a relatively small grain size the flat bed is stable and no sand waves evolve second it is shown that suspended load transport causes the suppression of long sand waves resulting in a finite range of wavelengths that experience growth finally by varying flow velocity amplitude and grain size critical conditions for sand wave formation are found i e conditions for which sand waves are marginally generated

tidal sand wave formation influence of graded suspended sediment transport 1 the influence of graded suspended sediment transport on the formation of tidal sand waves is investigated hereto in the framework of a linear stability analysis we examine the time development of small amplitude perturbations superimposed to a flat heterogeneous seabed in a shallow tidal sea dominated by one tidal constituent the heterogeneous bottom material is modeled by a bimodal sediment mixture and the transport of sediment close to the bottom as well as in suspension is considered the active layer concept is employed and hiding exposure effects are taken into account the mechanisms giving rise to the sediment sorting process along the bed forms are found to be unaffected by the fractional transport of sediment in suspension on the other hand the formation of sand waves is quantitatively substantially affected by the graded suspended sediment transport and the wavelength of the most amplified mode is found to depend strongly on the characteristics of the sediment mixture in particular the graded suspended sediment transport largely affects the contribution to the amplification rate related to the sorting process this leads to the growth of bed forms characterized by a smaller wavelength compared with that predicted for a uniform sediment when the mean grain size of the sediment mixture is relatively coarse on the other hand longer bed forms appear when the sediment mixture is characterized by a relatively fine mean grain size moreover a destabilizing effect is identified in the former case while the flat bed configuration is stabilized in the latter case a comparison of the theoretical predictions with field observations seems to support the model even though it cannot be considered conclusive

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones xiii structure of a warm front abstract mesoscale measurement from radars aircraft and rawinsondes and synoptic and satellite data are used to provide a detailed description of a warm front as it approached the washington coast in many respects the warm front was consistent with the classical model temperature rises were concentrated within a forward sloping frontal zone winds veered with height and lapse rates were more stable within the frontal zone clouds and precipitation were produced by upglide over the warm frontal surface and as the warm front approached clouds lowered and precipitation generally increased however in several important respects the warm front differed from the classical picture air flowed through the warm front and the warm frontal zone also the warm frontal zone had a staircase profile with some segments nearly horizontal and other segments with steep slopes finally precipitation was by no means uniformly distributed instead it occurred in both irregular and banded shaped mesoscale features

modeling sand wave characteristics on the belgian continental shelf and in the calais dover strait 1 the capability of the model of besio et al 2006 to predict the main geometrical characteristics crest orientation wavelength of tidal sand waves is tested by comparing the theoretical predictions with field data in particular the field observations carried out by mouchet 1990 and van lancker et al 2005 along the continental shelf of belgium are used additional comparisons are carried out against the field measurements described by le bot 2001 and le bot and trenteseaux 2004 which were carried out in an adjacent region attention is focused on the prediction of the wavelength of the bottom forms indeed the capability of a linear stability analysis to predict the occurrence of sand waves has been already tested by hulscher and van den brink 2001 and more recently by van der veen et al 2006 the obtained results show that the theoretical predictions fairly agree with field observations even though some of the comparisons suggest that the accuracy of the predictions depends on the accurate evaluation of the local current and sediment characteristics

mesoscale precipitation patterns in extratropical cyclones and implications for cyclone development analysis of the surface distribution of precipitation associated with intense extratropical cyclones reveals elongated maxima aligned with the steering flow rapidly intensifying cyclones create precipitation maxima which are better defined and contain more moisture than weaker cyclones the distributions are associated with the formation of a precipitation generating zone pgz aligned with the midtropospheric jet ahead of and parallel to the surface cold front extending southward from the surface low these zones tend to move along their axes and are host to the most frequent and intense convective activity in the extratropical cyclone hourly rain gage measurements at sites in their path indicate an average of three precipitation peaks in 16 storms studied although radar echoes disclose that many mesoscale structures with high reflectivity are simultaneously active it is hypothesized that formation of such zones is most pronounced when momentum transport out of the jet increases the advection of warm moist air at low levels thereby sustaining or increasing convective instability in this context the pgz becomes the dominant feature of the extratropical cyclone and is crucial to intensification

a wide cold frontal rainband and its relationship to frontal topography a wide cold frontal rainband associated with a vigorous surface cold front was investigated using conventional meteorological data and the velocity and reflectivity data from high resolution doppler radar it was found that the movement of the rainband was closely related to the velocity of the frontal surface and that the substructural features of the rainband corresponded closely with the topographical features of that surface the evolution of the topography of the frontal surface is described and some processes that might affect frontal topography and therefore the structure and movement of rainbands are examined

tidal induced large scale regular bed form patterns in a three dimensional shallow water model the three dimensional model presented in this paper is used to study how tidal currents form wave like bottom patterns inclusion of vertical flow structure turns out to be necessary to describe the formation or absence of all known large scale regular bottom features the tide and topography are treated as a coupled system and the potential free instabilities in this system are investigated a linear stability analysis is used to study the dynamics of bed forms this yields information on the role of physical mechanisms such information cannot be obtained by direct numerical nonlinear simulation the basic state represents a horizontal elliptical flow veering in vertical direction over a flat bottom it is shown that in such a system modes can grow which have characteristics of both tidal sand banks and sand waves the model predictions are compared with observations and show good agreement

the influence of storms on sand wave evolution a nonlinear idealized modeling approach we present a new 2dv nonlinear process based morphodynamic model to investigate the effects of storms specifically wind driven flow and wind waves on finite amplitude tidal sand wave evolution simulations are performed on periodic domains of two lengths i on a 350 m domain comparable to the wavelength of observed sand waves we study the evolution toward equilibrium shapes and ii on a 4 km domain we study the evolution from a randomly perturbed seabed our model results demonstrate that both wind driven flow and wind waves reduce sand wave height and tend to increase wavelength wind driven flow breaks the tidal symmetry resulting in horizontal sand wave asymmetry and migration waves alone do not induce migration but can enhance migration induced by for example tidal asymmetry and wind driven flow on the 350 m domain we further find that migration rates decrease with increasing sand wave height however in an irregular sand wave field large sand waves tend to overtake the smaller ones suggesting a complicated interaction among neighboring bed forms the above results concern steady state storm conditions however since storms occur on an intermittent basis we also simulated a synthetic storm climate consisting of alternating short periods of storm conditions and long periods of fair weather conditions simulations reveal a dynamic equilibrium with sand wave heights significantly below those obtained for tide only conditions also for relatively short storm duration our work identifies mechanisms that explain why sand wave heights are generally overpredicted by numerical models that do not include storm processes

horizontal and vertical sediment sorting in tidal sand waves modeling the finite amplitude stage the bed of coastal seas displays a large number of rhythmic bed features of which sand waves are relevant to study from an engineering perspective sediments tend to be well sorted over these bed forms which is so far poorly understood in terms of modeling of finite amplitude sand waves using delft3d we employ bed stratigraphy and consider four different grain size classes which are normally distributed on the phi scale the standard deviation sortedness is then varied whereas the geometric mean grain size is kept constant the results show that typically the crests of sand waves are coarser than the troughs residual flow causes net sedimentation on the lee side of the crest and consequently the general sorting pattern is distorted since larger grains experience a larger settling velocity they are deposited on the upper lee slope whereas the smaller grains are found on the lower lee slope due to sand wave migration also the internal structure of the sand wave is revealed which follows the same sedimentation pattern as the lee slope surface these results qualitatively agree with sorting patterns observed offshore the sorting processes lead to longer wavelengths and lower wave heights as a function of standard deviation this relates to the dampening effect of suspended sediment transport for fine grains finally it appears that the modeled wave heights fall in the same range as observations in the north sea these results are valuable for e g predicting the morphological response after engineering activities and determining suitable aggregates for sand extraction

modelling the effect of suspended load transport and tidal asymmetry on the equilibrium tidal sand wave height abstract tidal sand waves are rhythmic bed forms found in shallow sandy coastal seas reaching heights up to ten meters and migration rates of several meters per year because of their dynamic behaviour unravelling the physical processes behind the growth of these bed forms is of particular interest to science and offshore industries various modelling efforts have given a good description of the initial stages of sand wave formation by adopting a linear stability analysis on the coupled system of water movement and the sandy seabed however the physical processes causing sand waves to grow towards equilibrium are far from understood we adopt a numerical shallow water model delft3d to study the growth of sand waves towards a stable equilibrium it is shown that both suspended load transport and tidal asymmetry reduce the equilibrium sand wave height a residual current results in asymmetrical bed forms that migrate in the direction of the residual current the combination of suspended load transport and tidal asymmetry results in predicted equilibrium wave heights comparable to wave heights found in the field

the fluid dynamics of river dunes a review and some future research directions 1 dunes are present in nearly all fluvial channels and are vital in predicting flow resistance sediment transport and deposition within many rivers progress in understanding the fluid dynamics associated with alluvial dunes has been significant in the last 15 years and has witnessed huge advances in field laboratory and numerical investigations progress has been made in detailing the principal features of mean and turbulent flow over asymmetric dunes that possess flow separation in their leesides and how these forms affect both downstream boundary layer structure and stress partitioning over the dune additionally the links between sediment transport over dunes and instantaneous coherent flow structures are being increasingly understood with the feedback of dune three dimensionality upon flow and sediment dynamics over these bed forms beginning to be recognized as vital such research now provides an outstanding background for beginning to address areas of greater complexity that will enable a fuller understanding of these important natural features this review paper summarizes the principal features of mean and turbulent flow over alluvial sand dunes five areas are then highlighted and discussed as a possible focus for future research 1 the influence of dune leeside angle upon flow processes in the dune wake and downstream flow field 2 the influence of three dimensionality in dune shape upon the generation of turbulence and distribution of bed shear stress 3 flow field modification resulting from bed form superimposition and amalgamation 4 the scale and topology of dune related turbulence and its interactions with sediment transport and the flow surface and 5 the influence of suspended sediment on the dune flow field and dune morphology

a field study of turbulence and sediment dynamics over subaqueous dunes with flow separation this study examines flow turbulence and sand suspension over large dunes in canoe pass a distributary channel of the fraser river delta canada dune morphology is characterized by a symmetrical shape and steep leeside slopes over 30 velocity was measured with an electromagnetic current meter and suspended sand concentration with four optical backscatter obs probes the general patterns of time averaged velocity and sand suspension are consistent with previous studies including an increase in mean velocity and decrease in turbulence intensity and sand concentration with height above the bed reversed flow with high turbulence intensity and high sand concentrations in the leeside flow separation zone and an increase in near bed velocity and sand concentration along the stoss side of the dune frequency spectra of near bed velocity and obs records from leeside separation zones are composed of two distinct frequencies providing field confirmation of scale relations based on flume experiments the low frequency spectral signal probably results from wake flapping and the high frequency signal from vortex shedding the wake flapping frequency predominates outside the separation zone and is linked to turbulent structures that suspend sand predictions from a depth scale strouhal law show good agreement with measured wake flapping frequencies cross correlations of obs records reveal that turbulent sand suspension structures advect downstream at 23 25 above the horizontal these advection angles are similar to coherent flow structures measured in flumes and to sand suspension structures visualized over large dunes in the field

on the formation of sand waves and sand banks a fully three dimensional model is proposed for the generation of tidal sand waves and sand banks from small bottom perturbations of a flat seabed subject to tidal currents the model predicts the conditions leading to the appearance of both tidal sand waves and sand banks and determines their main geometrical characteristics a finite wavelength of both sand waves and sand banks is found around the critical conditions thus opening the possibility of performing a weakly nonlinear stability analysis able to predict the equilibrium amplitude of the bottom forms as shown by previous works on the subject the sand wave crests turn out to be orthogonal to the direction of the main tidal current the present results also show that in the northern hemisphere sand bank crests are clockwise or counter clockwise rotated with respect to the main tidal current depending on the counter clockwise or clockwise rotation of the velocity vector induced by the tide only for unidirectional or quasi unidirectional tidal currents are sand banks always counter clockwise rotated the predictions of the model are supported by comparisons with field data finally the mechanisms leading to the appearance of sand waves and sand banks are discussed in the light of the model findings in particular it is shown that the growth of sand banks is not only induced by the depth averaged residual circulation which is present around the bedforms and is parallel to the crests of the bottom forms a steady drift of the sediment from the troughs towards the crests is also driven by the steady velocity component orthogonal to the crests which is present close to the bottom and can be quantified only by a three dimensional model while the former mechanism appears to trigger the formation of counter clockwise sand banks only the latter mechanism can give rise to both counter clockwise and clockwise rotated sand banks

notes and correspondence polarimetric radar observation of multiple freezing levels abstract an unusual multiple freezing level event observed with polarimetric radar during the second phase of the improvement of microphysical parameterization through observational verification experiments improve 2 field program is described the event occurred on 28 november 2001 when a warm front moved over the oregon cascade mountains as the front approached an elevated melting layer formed above a preexisting melting layer near ground continued warming of the lower atmosphere eventually dissipated the lower melting layer the polarimetric measurements are used to estimate the height of the freezing levels document their evolution and deduce hydrometeor habits the measurements indicate that when the two freezing levels were first observed melting was incomplete in the upper melting layer and characteristics of particles that passed through the two melting layers were similar as warming progressed the character of particles entering the lower melting layer changed possibly becoming ice pell

process based modeling of tsunami inundation and sediment transport 1 the infrequent and unpredictable nature of tsunamis precludes the use of field experiments to measure the hydrodynamic and sediment transport processes that occur instead these processes are often approximated from laboratory numerical and theoretical studies or inferred from observations of the resultant sediment deposits here delft3d a three dimensional numerical model is used to simulate the inundation and sediment transport of a tsunami similar in magnitude to the 26 december 2004 indian ocean tsunami over one measured and three idealized morphologies the model is first shown to match well the observations taken at kuala meurisi sumatra and then used to examine in detail the processes that occur during the tsunami the model predicts that at a given cross shore location the onshore flow accelerates rapidly to a maximum as the wavefront passes and then gradually decelerates before reversing direction and flowing offshore the onshore flow does not tend to zero everywhere at maximum inundation but instead flow reversal occurs near the shoreline even as the wavefront continues to inundate landward while some sediment is eroded by the passing wavefront the suspension of sandy sediment is dominated by the long duration high velocity backwash that occurs along the beach face and offshore of the shoreline some of the sediment suspended during backwash is advected shoreward by the subsequent wave creating large spatial gradients in the suspended sediment concentrations which may not be in equilibrium with the local hydrodynamics the inundation and transport of sediment during a tsunami can be affected by complexities in the morphological profile and interactions between multiple waves and many of the hydrodynamic and sediment transport processes predicted here are similar to analogous processes previously observed in the swash zone

orographic effects on precipitating clouds 1 precipitation over and near mountains is not caused by topography but rather occurs when storms of a type that can occur anywhere deep convection fronts tropical cyclones form near or move over complex terrain deep convective systems occurring near mountains are affected by channeling of airflow near mountains capping of moist boundary layers by flow subsiding from higher terrain and triggering to break the cap when low level flow encounters hills near the bases of major mountain ranges mesoscale convective systems are triggered by nocturnal downslope flows and by diurnally triggered disturbances propagating away from mountain ranges the stratiform regions of mesoscale convective systems are enhanced by upslope flow when they move over mountains in frontal cloud systems the poleward flow of warm sector air ahead of the system may rise easily over terrain and a maximum of precipitating cloud occurs over the first rise of terrain and rainfall is maximum on ridges and minimum in valleys if the low level air ahead of the system is stable blocking or damming occurs shear between a blocked layer and unblocked moist air above favors turbulent overturning which can accelerate precipitation fallout in tropical cyclones the tangential winds encountering a mountain range produce a gravity wave response and greatly enhanced upslope flow depending on the height of the mountain the maximum rain may occur on either the windward or leeward side when the capped boundary layer of the eye of a tropical cyclone passes over a mountain the cap may be broken with intense convection resulting

structure of a warm front helsinki testbed observations and model simulation abstractthe purpose of this study is to document the structure of a warm front in northeast europe identify the effects that the finnish coastline has on the evolution of the front and investigate factors that influence the speed that the warm front moves at within and above the boundary layer the warm front formed over estonia traveled northward across the gulf of finland and then crossed the southern coastline of finland surface based measurements from the helsinki testbed are analyzed together with output from a high resolution numerical weather prediction model application of research to operations at mesoscale arome during the early stages of development the warm front interacted with a stationary baroclinic zone and consequently evolved into an s shape as the front approached the southern coast of finland the temperature gradient at 1000 hpa increased as it merged with a diabatically generated temperature gradient at 1000 hpa the front stalled at the coastline due to friction enh

development of subaqueous barchanoid shaped dunes due to lateral grain size variability in a tidal inlet channel of the danish wadden sea 1 high resolution bathymetry at centimeter scale accuracy acquired with a multibeam echo sounder system revealed the existence of barchanoid shaped large i e length 10 100 m to very large i e length 100 m dunes in the gradyb tidal inlet channel in the danish wadden sea the development of these dunes is due to an increase in dune celerity from 12 m yr in the center of the channel to around 30 m yr at the sides this increase in dune celerity can be explained by the fact that dune heights decrease from 3 1 m in the center of the channel to 1 4 m at the sides as a smaller sediment volume has to be moved per unit time for equal dune celerity water depth is uniform across the channel likewise high resolution acoustic doppler current profiler measurements across the channel showed a uniform distribution of both ebb and flood flow thus no correlation between dune dimensions and water depth or flow velocity was established instead high accuracy bed sampling along the crests of the dunes showed a decrease in mean grain size from 0 63 mm in the center to 0 36 mm at the sides of the channel the decrease in dune height is ascribed to this decrease in grain size given that flow depth and flow velocity are uniform across the channel the lateral decrease in grain size is suggested to result from sorting effects by secondary currents directed from the center toward the sides of the channel in the trough lee side region of the barchanoid shaped dunes

development of subaqueous barchanoid shaped dunes due to lateral grain size variability in a tidal inlet channel of the danish wadden sea development of barchanoid shaped dunes

introduction to special section on marine sand wave and river dune dynamics marine sand waves and river dunes are rhythmic patterns formed at the interface between a sandy bed and turbulent flows driven by tidal and river currents their origin development and dynamics are far from being fully understood to enhance the discussion on this topic we organized the international conference marine sand wave and river dune dynamics marid in april 2004 in enschede netherlands scientific papers that originated from the marid conference are presented in this special section this introduction places these papers in the context of recent progress

airborne radar observations of a cold front during fastex1 abstract a detailed analysis using airborne doppler radars of an oceanic cold front associated with precipitation core and gap regions is presented the precipitation cores appear to form as a result of the combined effects of horizontal shearing instability and the advection of hydrometeors by the core relative winds in contrast to previous schematic models it is shown that a strong surface discontinuity does not exist along the entire length of the precipitation core the southern section of the core can be accompanied by an abrupt discontinuity and lighter precipitation while the northern section can be associated with more slowly changing variables but heavier precipitation the peak updrafts at the leading edge of the front appeared to be primarily driven by frictional convergence and the acceleration of the vertical vorticity in the boundary layer the overall motion of the cold front was not well predicted using density current theory even though the kinematic structure of the front resembled cla

comparison between predicted and observed sand waves and sand banks in the north sea for the first time a prediction model of regular morphological patterns on the seabed was tested against observations of sand wave and sand bank occurrence in the entire north sea the model which originates from first physical principles predicts this occurrence via two dimensionless parameters on the basis of the water depth the tidal velocity amplitude the level of zero intercept above the seabed z0 and a viscosity variation parameter alt epsilon the latter two quantities were varied in a number of predictions for the entire north sea whereas for the first two local values were used the range of realistic values of alt epsilon and z0 was large enough to let these two parameters distinguish between the possible combinations of bed forms as is shown in the shallower southern bight of the north sea the results were more sensitive to variations in z0 than in alt epsilon a slightly more detailed approach focused on sand waves only in the southern north sea and estimated local values for z0 using depth information quantification of the results showed that the model was able to predict the contours of the sand wave patches but it could not account for the absence of the bed features within this area the type of bed deposit partly explains the smaller scale variation the work confirms the validity of the theoretical bed form prediction model and verifies the hypothesis that the large scale seabed features are formed as free instabilities of tide topography interactions