field observations on the morphodynamic evolution of a low energy rip current system abstract a field experiment was conducted at palm beach nsw australia to investigate morphodynamic relationships operating within a rip current system during a near complete sequence of beach state evolution under decreasing energy conditions observations and measurements were obtained from nearshore surveys multiple dye releases oblique photographs and the deployment of various combinations of nine ducted impellor flow meters and five pressure sensors over a seven day period in rip and feeder channels the dataset provides a unique opportunity to assess and quantify aspects of the accretionary model of wright and short mar geol 56 1984 93 118 for intermediate beaches the morphology of the main rip channel exhibited distinct sequential adjustments with a narrowing of channel width an increase in channel depth a reduction in the cross sectional area available for rip flow and an increase in morphological relief rip flow velocity increased during the observed evolution and was also modulated by the tide experiencing maximums at low tide and minimums at high tide morphologic and kinematic adjustments of the rip system were most pronounced during the transition to a transverse bar and rip state the existence of a dominant feeder in a twin feeder and rip system contributed to the migration and establishment of a rip head over the seaward slope of the opposite longshore bar net erosion in the rip channel was balanced by net deposition in the feeder channels and bar crests and qualitative observations suggest that this rip head bar both receives transported sediments from the rip channel and contributes sediments to the ongoing accretion of the beach system a direct relationship was found to exist between rip morphology and flow with rip velocity increasing as cross sectional rip channel area decreases and these results are incorporated in a conceptual modification of the model of wright and short the results of this study are restricted to a subset of commonly occurring beaches but show that accretionary beach state evolution on intermediate beaches is indeed characterised by an increase in rip current velocity the nature of which can be linked to morphologic control in addition the rip head bar is identified as a potentially key component of low energy rip system evolution

global change oceanic action at a distance glacial intervals are characterized by low levels of atmospheric co2 a new explanation for that connection invokes nutrient export from the southern ocean to warmer waters at such times

mechanism of heating and the boundary layer over the tibetan plateau abstract the structure of the boundary layer and the mechanism of heating over the tibetan plateau are examined using the data obtained from the first garp global atmospheric research program global experiment and the chinese qinghai xizang tibet plateau meteorological experiment from may to august 1979 the meteorological elements near the plateau surface exhibit pronounced diurnal variations there is a large ground air temperature difference during the daytime generating a thin layer of superadiabatic lapse rates near the surface the horizontal wind speed attains the minimum in the morning and the maximum in the evening a deep and well mixed layer of potential temperature is observed over the western and central plateau in the evening 1200 utc however moisture is not well mixed vertically and water vapor mixing ratio is larger in the morning 0000 utc than in the evening the mixed layer becomes shallower from the western to the central plateau and it disappears over the eastern plateau the

numerical simulations of a gravity wave event over ccope part iii the role of a mountain plains solenoid in the generation of the second wave episode abstract mesoscale model simulations have been performed of the second episode of gravity waves observed in great detail in previous studies on 11 12 july 1981 during the cooperative convective precipitation experiment the dominant wave simulated by the model was mechanically forced by the strong updraft associated with a mountain plains solenoid mps as this updraft impinged upon a stratified shear layer above the deep well mixed boundary layer that developed due to strong sensible heating over the absaroka mountains the gravity wave was created this wave rapidly weakened as it propagated eastward however explosive convection developed directly over the remnant gravity wave as an eastward propagating density current produced by a rainband generated within the mps leeside convergence zone merged with a westward propagating density current in eastern montana the greatly strengthened cool pool resulting from this new convection then generated a bore wave that appeared to be continuous with the move

the predictability of cross shore bed evolution of sandy beaches at the time scale of storms and seasons using process based profile models deterministic and probabilistic profile models have been compared with hydrodynamic and morphodynamic data of laboratory and field experiments on the time scale of storms and seasons the large scale laboratory experiment is a pure 2d case and offers and ideal test case for cross shore profile models as disturbing alongshore non uniformities are absent the field experiments are performed at the egmond site the netherlands during the eu coast3d project and represent storm time scale oct nov 1998 as well as seasonal time scale conditions may 1998 sep 1999 the objective of the paper is to present information of coastal processes on these time scales and to assess the predictive capabilities coastal process based profile models with respect to hydrodynamics and morphodynamics at sandy beaches on the time scales of storms and seasons profile models can quite accurately errors smaller than 10 represent the cross shore significant wave height distribution in the surf zone if the wave breaking model is properly calibrated the wave breaking coefficient should be a function of local wave steepness and bottom slope for most accurate results profile models can reasonably represent the cross shore and longshore currents undertow in a pure 2d case and in 3d field conditions profile models including cross shore mixing effects and breaker delay effects do not produce better predictions of the longshore and cross shore current velocities profile models using default settings can quite reasonably simulate the behaviour of the outer and inner bars on the storm time scale the behaviour of the beach cannot be modelled with sufficient accuracy on the storm time scale profile models can reasonably simulate the post storm onshore bar migration provided that the near bed orbital velocities and wave asymmetry related sand transport are represented in a sufficiently accurate way using non linear wave theories profile models cannot simulate the beach recovery processes on the post storm time scale because these essentially 3d processes are not sufficiently known to be included in the models profile models using default settings cannot simulate the behaviour of the outer and inner bars and the beach on the seasonal time scale the behaviour of the outer bar on the seasonal time scale can only be represented properly after tuning using measured bed profiles the simulation of the inner bar and beach morphology on the seasonal time scale could not be improved by tuning

numerical simulations of a gravity wave event over ccope part ii waves generated by an orographic density current a mesoscale numerical model and detailed observations are used to investigate the generation and maintenance of a mesoscale gravity wave event observed in eastern montana on 11 july 1981 during the cooperative convective precipitation experiment ccope it is shown that the interaction between an orographic density current and a mountain barrier leads to the generation of the gravity waves the simulation results suggest the following four stage conceptual model during stage i shortly after sunset the remnant up branch of a thermally driven upslope flow east of the rockies was driven back toward the mountain by the pressure gradient force associated with a cool pool over north dakota the nocturnal stable layer over eastern montana was strengthened during passage of this density current during the 1 2 h transition period of stage ii the advancing density current became blocked as it encountered the higher terrain an isentropic ridge developed above the original warm lee trough due to strong adiabatic cooling caused by the sustained upward motion in the presence of orographic blocking during stage iii an even stronger upward motion center formed to the east of the density current head updraft in response to an eastward horizontal pressure gradient force produced by the isentropic ridge in stage iv as the density current head collapsed and downward motion developed to the west of the original updraft in quadrature phase with the isentropic perturbation a gravity wave was generated this wave propagated eastward with the mean wind opposite to the motion of the earlier density current and was maintained by the strong wave duct established earlier by the density current thus the mountain plains circulation may at times generate mesoscale gravity waves and deep convection hours after diurnal heating has ended

deterministic profile modelling of nearshore processes part 2 sediment transport and beach profile development abstract this is the second of two companion papers concerned with the modelling of sediment transport and beach profile response to wave and tidal action part i was concerned with the wave and current modelling aspects whilst the present paper considers the sediment transport and profile development predictions particular adaptations of well known theoretical developments that distinguish the present modelling work are described both cross shore and alongshore sediment transport processes the latter including the influence of tides are represented the alongshore transport description is included to allow this quasi 2dv model to act as a module to a quasi 3dh morphodynamic model the q3dh model is not described here the beach profile evolution phase of the model is capable of considering a fixed boundary such as a seawall the fixed boundary may also be erodible as in the case of a sandy beach overlying a cohesive till substratum in both parts i and ii the validity of each individual predictive phase of the model has been assessed where the accuracy was found to be insufficient additional processes were considered until the level of accuracy could be considered suitable for input to the next predictive phase of the model whilst free variables exist within the formulation of the model these have been considered to be fixed in almost every application of the model the consideration of additional processes is the approach used to achieve improved model prediction not the arbitrary tuning of free variables some of the validation tests for the sediment transport phases alongshore and cross shore are presented in this paper along with evaluations of the integrated model for profile change the foremost value of the model rests with the ability to identify links between observed beach changes and the underlying coastal processes

diurnal variations of summer precipitation over contiguous china 1 diurnal variations of summer precipitation over contiguous china are studied using hourly rain gauge data from 588 stations during 1991 2004 it is found that summer precipitation over contiguous china has large diurnal variations with considerable regional features over southern inland china and northeastern china summer precipitation peaks in the late afternoon while over most of the tibetan plateau and its east periphery it peaks around midnight the diurnal phase changes eastward along the yangtze river valley with a midnight maximum in the upper valley an early morning peak in the middle valley and a late afternoon maximum in the lower valley summer precipitation over the region between the yangtze and yellow rivers has two diurnal peaks one in the early morning and another in the late afternoon

oblique flow over dredged channels ii sediment transport and morphology a three dimensional 3d investigation of sediment transport processes across long straight channels exposed to an oblique incoming current is presented for this purpose a detailed mathematical flow and sediment transport model is applied the governing flow and sediment transport equations are solved in a curvilinear coordinate system the sediment transport model is composed of bed and suspended load applying a convection diffusion equation for the latter as the flow crosses a channel obliquely it is refracted toward the channel alignment the effect of the current refraction on the sediment transport is studied using both 2d depth averaged and 3d models shortcomings of models based on depth averaged descriptions are underlined through comparison with the 3d model a reliable description of the backfilling process requires a flow and sediment transport model capable of capturing the following aspects the direction and magnitude of the bed shear stress and the adaptation of suspended sediment to local sediment transport capacities calculations of the morphological development of small scale excavations such as pipeline trenches exposed to an oblique incoming current are presented

numerical solutions of the sediment conservation law a review and improved formulation for coastal morphological modelling numerical solutions of the sediment conservation law are reviewed in terms of their application to bed update schemes in coastal morphological models it is demonstrated that inadequately formulated numerical techniques lead to the introduction of diffusion dispersion and the bed elevation oscillations previously reported in the literature four different bed update schemes are then reviewed and tested against benchmark analytical solutions these include a first order upwind scheme two lax wendroff schemes and a non oscillating centred scheme nocs recently applied to morphological modelling by saint cast saint cast f 2002 modelisation de la morphodynamique des corps sableux en milieu littoral modelling of coastal sand banks morphodynamics university bordeaux 1 bordeaux 245 pp it is shown that nocs limits and controls numerical errors while including all the sediment flux gradients that control morphological change further no post solution filtering is required which avoids difficulties with selecting filter strength finally nocs is compared to a recent lax wendroff scheme with post solution filtering for a longer term simulation of the morphological evolution around a trained river entrance c 2006 elsevier b v all rights reserved

diurnal variations of warm season precipitation over northern china abstract this study examines the diurnal variations of the warm season precipitation over northern china using the high resolution precipitation products obtained from the climate prediction center s morphing technique cmorph during may august of 2003 09 the areas of focus are the yanshan taihangshan mountain ranges along the east peripheries of the loess and inner mongolian plateaus and the adjacent north china plains it is found that the averaged peak in local precipitation begins early in the afternoon near the top of the mountain ranges and propagates downslope and southeastward at a speed of 13 m s 1 the peak reaches the central north china plains around midnight and the early morning hours resulting in a broad area of nocturnal precipitation maxima over the plains the diurnal precipitation peak minimum is closely collocated with the upward downward branch of a mountain plains solenoid mps circulation both the mps and a low level southwesterly nocturnal jet are likely to be jointly resp

bar generating cross shore flow mechanisms on a beach random waves normally incident on a dissipative beach induce a variety of cross shore flows such as asymmetric oscillatory flow wave grouping induced long wave flow breaking induced turbulent flow and momentum decay induced undertow these flows are identified analyzed and hindcasted in a set of laboratory experiments with the aim of revealing the role of each of the flow mechanisms in the two dimensional case of bar generation on a beach

the behaviour of nearshore bars on the time scale of years a conceptual model abstract soundings of the sandy dutch coast have demonstrated the presence of a multiple bar system that exhibits cyclic offshore directed migration on the time scale of years medium term a cycle comprises bar generation interannual offshore migration and decay in this paper we propose a conceptual model in which we qualitatively outline the relationships between short term hourly process knowledge waves currents sediment transport based on a 15 week data set of near bed hydrodynamics obtained in the multiple bar system of terschelling the netherlands and medium term bar behaviour the short term process knowledge is aggregated to the medium term by combining probabilistic characterisations of medium term wave conditions and sediment transports estimated from the short term field measurements essential to the conceptual model is the observed cross shore change in short term conditions that dominate medium term onshore and offshore sediment transport q on and q off respectively in the cross shore direction the conditions contributing most to q on change from predominantly breaking situations outside the bar zone to mainly non breaking conditions on the beach in contrast q off remains restricted to breaking conditions over the entire profile in the model we qualitatively describe how this cross shore change in the forcing conditions results in interannual offshore bar migration bar decay and the onset of the next cycle as such our model may serve as a first step towards a more coherent quantitative model on medium term periodic bar behaviour

initiation of precipitation episodes relative to elevated terrain abstract the western and central united states experience a pronounced diurnal cycle in rainfall during the warm season over the higher terrain west of 105 w most precipitation occurs in the afternoon whereas the central united states experiences more nocturnal events this coherent phase transition between the rocky mountains and the u s great plains is well defined for all warm seasons between 1996 and 2003 provided that the rainfall observations are remapped relative to the elevated terrain in the western united states prior to north south averaging due to the westward shift of the continental divide north of 42 n and its intersection with the warm season storm track for 2002 the diurnal coherence greatly improves after remapping the 2002 rainfall observations this speaks to the long range influence of orography on precipitation frequency and suggests that the primary east west corridor of precipitation for an individual warm season intersects the cordillera over a fairly narrow latitude range

role of morphologic feedback in surf zone sandbar response several aspects of feedback mechanisms associated with surf zone sandbar response have been characterized using bathymetric surveys sampled approximately monthly over a 16 year period at the army corps of engineers field research facility north carolina the measured bathymetry was alongshore averaged and modeled by the superposition of two gaussian shaped sandbars on an underlying planar slope a third half gaussian shaped bar represented steepening at the shoreline the rms error between the measured bathymetry and the profile model was 0 10 m estimated over 322 different surveys the model explained 99 of the profile variance that remained after first removing the linear cross shore trend from each observed profile bar response which was extracted from the modeled profiles was compared to a local hydrodynamic forcing variable was defined as the ratio of the wave height to water depth evaluated at bar crest locations at low values of i e nonbreaking conditions bars migrated onshore and their amplitude tended to decay at high values of i e breaking conditions bars migrated offshore with relatively little change in amplitude the transition between onshore and offshore migration occurred at a value of that was consistent with the onset of wave breaking bar migration was associated with a stabilizing feedback mechanism which drove bar crests toward an equilibrium position at the wave breakpoint however we observed that the rate of bar response showed no reduction for any nonzero choice of indicating that bars never reached equilibrium systematic bar amplitude decay was observed under nonbreaking conditions bar amplitude decay could drive farther away from breaking conditions allowing further bar amplitude decay this is a destabilizing feedback mechanism potentially leading to bar destruction

variability of warm season cloud episodes over east asia based on gms infrared brightness temperature observations abstract the present study has used the geostationary meteorological satellite gms ir brightness temperature observations to investigate the regional and intraseasonal variability of east asian warm season cloud precipitation episodes in distance time space due to land sea contrast and latitudinal effects the data period was may august 1998 2001 and harmonic analysis was employed as the major tool for analysis the full domain of study 20 40 n 95 145 e was divided into northern and southern zones and into eastern and western sectors and statistics of episodes in each subregion were derived and compared for latitudinal effects episodes were found to be significantly larger in span and duration in northern 30 40 n than in southern 20 30 n zones in the northern zone the propagation characteristics were also stronger and remain evident even in midsummer while episodes south of 30 n reversed in direction and traveled westward in july and august for land sea contrast the may august tra

a climatology of warm season cloud patterns over east asia based on gms infrared brightness temperature observations abstract in the present study hourly infrared ir brightness temperatures observed by the geostationary meteorological satellite gms over the region 20 40 n 95 145 e in may august 1998 2001 are used to compile a climatology of warm season cloud precipitation episodes over east asia with a goal to better understand the characteristics of warm season convection and the behavior of these episodes results are compared with those obtained in north america using radar derived data the convection in east asia similar to that in north america is shown to also exhibit coherent patterns and characteristics of propagating events in the longitude time hovmoller space with a preferred phase speed of 10 25 m s 1 considerably faster than warm season synoptic scale waves near the eastern edge of the tibetan plateau convection was most active with a strong diurnal signal peaking in late afternoon or early evening then propagating eastward the zonal span and duration of episodes could reach 3000 km and

the role of diurnal solenoidal circulation on propagating rainfall episodes near the eastern tibetan plateau abstract in this case study numerical simulations using the weather research and forecasting model wrf are performed for 3 4 may 2002 in which two propagating rain episodes occurred in successive days with close ties to the terrain of the tibetan plateau tp in east asia through sensitivity tests it is found that the eastern tp not only facilitated convective development in the afternoon but that the solenoidal circulation between this region and its leeside lowlands near the sichuan basin also contributed to the longevity and farther downstream propagation of the episodes under prevailing westerly winds reversed every 12 h the thermally driven circulation induced ascending motion near the eastern edge of the tp during daytime but over the leeside at night the episode propagation in this case as often observed was in phase with the ascent from eastern tp in the afternoon to the lee at night indicating both enhancing and modulating effects of the solenoidal circulation

rainy season precipitation over the sichuan basin and adjacent regions in southwestern china abstractthe spatial variability and diurnal propagation of mean precipitation in the summer rainy seasons from 2003 to 2010 over the sichuan basin scb and adjacent mountainous regions are examined using high spatiotemporal resolution satellite derived precipitation estimates the scb is located just east of the tibetan plateau tp and is prone to heavy precipitation that often peaks over nighttime and early morning the large scale environment over the scb during the rainy season is characterized by weak low to midtropospheric convergence in the lee of the tp and by the upper tropospheric jet stream to the north under this flow configuration the study links the unique diurnal variations in the precipitation pattern and propagation to the unique topography in this region it is found that during the rainy season the local diurnal precipitation maximum moves primarily downslope and southeastward from over the tp in the daytime to scb at night a secondary maximum moves northeastward downslope of t

test of self organization in beach cusp formation 1 field observations of swash flow patterns and morphology change are consistent with the hypothesis that beach cusps form by self organization wherein positive feedback between swash flow and developing morphology causes initial development of the pattern and negative feedback owing to circulation of flow within beach cusp bays causes pattern stabilization the self organization hypothesis is tested using measurements from three experiments on a barrier island beach in north carolina beach cusps developed after the beach was smoothed by a storm and after existing beach cusps were smoothed by a bulldozer swash front motions were recorded on video during daylight hours and morphology was measured by surveying at 3 4 hour intervals three signatures of self organization were observed in all experiments first time lags between swash front motions in beach cusp bays and horns increase with increasing relief representing the effect of morphology on flow second differential erosion between bays and horns initially increases with increasing time lag representing the effect of flow on morphology change because positive feedback causes growth of beach cusps third after initial growth differential erosion decreases with increasing time lag representing the onset of negative feedback that stabilizes beach cusps a numerical model based on self organization initialized with measured morphology and alongshore uniform distributions of initial velocities and positions of the swash front at the beginning of a swash cycle reproduces the measurements except for parts of one experiment where limited surveys and a significant low frequency component to swash motions might have caused errors in model initialization

data analysis of bed concentration of suspended sediment an empirical relation for the bed concentration of suspended sediment cb defined at a few grain diameters above the bed is developed its derivation is based on the experimental data sets of guy et al in 1966 the values of the near bed concentration were obtained by first isolating the suspended load transport from the observed rates of total load transport by use of a bed load formula the suspended load was then equated to einstein s formulation which allowed the determination of cb the influence of large concentrations of suspended sediment and of the reference level for cb on the calculated values of bed concentration has been analyzed the proposed relation compares satisfactorily to values of near bed concentration obtained from independent data sets the procedure used to derive cb provides a simple method for the calculation of the total transport rate of noncohesive sediment