reservoir sedimentation i delta and density current deposits in china where rivers draining loess soils have some of the highest sediment concentrations in the world reservoir sedimentation is a serious problem that has been the subject of intensive investigation since 1957 this paper describes patterns of sediment deposition and erosion drawing on field measurements in china at the sanmenxia guanting shanyiujiang and liujiaxia reservoirs as well as the shanshenggorig barrage and xijin hydropower station the longitudinal and lateral geometry of reservoir deltas is described it is shown that a marked change in grain size can occur at the downstream limit of the delta even when this limit is not apparent from geometry density current data for guanting reservoir are presented problems caused by reservoir sedimentation in china are described including aggredation in the backwater region causing increased flood levels elevated ground water levels and navigation impairment deposition from or into a tributary entry of sediments into hydropower turbines a

composite vortex2 supercell environments from near storm soundings abstractthree dimensional composite analyses using 134 soundings from the second verification of the origins of rotation in tornadoes experiment vortex2 reveal the nature of near storm variability in the environments of supercell thunderstorms based upon the full analysis it appears that vertical wind shear increases as one approaches a supercell within the inflow sector providing favorable conditions for supercell maintenance and possibly tornado formation despite small amounts of low level cooling near the storm the seven analyzed tornadic supercells have a composite environment that is clearly more impressive in terms of widely used metrics than that of the five analyzed nontornadic supercells including more convective available potential energy cape more vertical wind shear higher boundary layer relative humidity and lower tropospheric horizontal vorticity that is more streamwise in the near storm inflow the widely used supercell composite parameter scp and significant tornado param

reservoir turbidity current modeling with a two dimensional layer averaged model abstracta two dimensional layer averaged model is developed and verified to simulate turbidity current characteristics and its sluicing in reservoirs the governing equations consist of mass and momentum conservation laws for the turbidity current mixture equations for the sediment transport and bed dynamics and auxiliary relations for the interactions among clear water turbidity current and bed a finite volume unstructured polygonal mesh method is adopted so that reservoirs with complex terrains may be simulated special algorithms are developed to capture the turbidity current front movement through a clear water bed and to simulate turbidity current sluicing through reservoir outlets the developed model has been tested and verified with both conservative and nonconservative turbidity currents ranging from simple to complex reservoir terrains case studies presented include a lock exchange turbidity current with large eddy simulation and direct numerical simulation results a laboratory test of

comments on a comparison of tropical and midlatitude thunderstorm evolution in response to wind shear in a recent paper wissmeier and goler 2009 hereafter wg09 presented the results of a numerical experiment meant to examine the different potential for convective storm splitting in the tropics versus the midlatitudes wg09 found that simulated storms in tropical environments were less likely to undergo classical supercellular splitting because the storms outflow quickly spread out and cut the storms off from their source of inflow before the splitting process could occur the main conclusion reached by wg09 is that higher vertical wind shear values are necessary for storm splitting to occur in the tropics than in the midlatitudes because the comparatively large vertical wind shear weakens and delays the convective outflow produced by storms these are thought provoking results and conclusions but they hinge on some questionable assumptions and choices in the experimental design the purpose of the present comment is to document and discuss several aspects of the study that bias wg09 s results and diminish the generality of their conclusions

the life cycle of thunderstorm gust fronts as viewed with doppler radar and rawinsonde data abstract this paper presents the time dependent analysis of the thunderstorm gust front with the use of project nimrod data rhi cross sections of reflectivity and doppler velocity are constructed to determine the entire vertical structure the life cycle of the gust front is divided into four stages 1 the formative stage 2 the early mature stage 3 the late mature stage and 4 the dissipation stage a new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground while carrying some of the smaller precipitation ahead of the main echo core of the squall line this feature is called a precipitation roll as determined from rawinsonde data the cold air behind the gust front accounts for the observed surface pressure rise calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow the equation governing the propagation speed of a density current accurately predicts t

role of ponded turbidity currents in reservoir trap efficiency the capacity to store water in a reservoir declines as it traps sediment a river entering a reservoir forms a prograding delta coarse sediment e g sand deposits in the fluvial topset and avalanching foreset of the delta and is typically trapped with an efficiency near 100 the trap efficiency of fine sediment e g mud on the other hand may be below 100 because some of this sediment may pass out of the reservoir without settling out here a model of trap efficiency of mud is developed in terms of the mechanics of a turbidity current that plunges on the foreset the dam causes a sustained turbidity current to reflect and form a muddy pond bounded upstream by a hydraulic jump if the interface of this muddy pond rises above any vent or overflow point at the dam the trap efficiency of mud drops below 100 a model of the coevolution of topset foreset and bottomset in a reservoir that captures the dynamics of the internal muddy pond is presented numerical implementation comparison against an experiment and application to a field scale case provide the basis for a physical understanding of the processes that determine reservoir trap efficiency

the influence of horizontal environmental variability on numerically simulated convective storms part i variations in vertical shear abstract severe convective storms are typically simulated using either an idealized horizontally homogeneous environment i e single sounding or an inhomogeneous environment constructed using numerous types of observations representing opposite ends of the spectrum the former allows for the study of storm dynamics without the complicating effects of either land surface or atmospheric variability though arguably at the expense of physical realism while the latter is especially useful for prediction and data sensitivity studies though because of its physical completeness determination of cause can be extremely difficult in this study the gap between these two extremes is bridged by specifying horizontal variations in environmental vertical shear in an idealized controlled manner so that their influence on storm morphology can be readily diagnosed simulations are performed using the advanced regional prediction system arps though with significant modification to accommodate the analytically

a theory for strong long lived squall lines abstract we study herein the mechanics of long lived line oriented precipitating cumulus convection squall lines using two and three dimensional numerical models of moist convection these models used in juxtaposition enable us to address the important theoretical issue of whether a squall line is a system of special long lived cells or whether it is a long lived system of ordinary short lived cells our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but on occasion a squall line may be composed of essentially steady supercell thunderstorms the numerical experiments presented herein show that either type of squall line may develop from an initial line like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line with shallow shear oriented perpendicular to the line a long lived line evolves containing individually short lived cells our analysis of this type o

the relationship of surface pressure features to the precipitation and airflow structure of an intense midlatitude squall line abstract observations from the oklahoma kansas preliminary regional experiment for storm central ok pre storm have been used to document the surface pressure features accompanying an intense midlatitude squall line with trailing stratiform precipitation three well known features are observed a pre squall mesolow a squall mesohigh and a wake low particular attention is given to the wake low its life cycle and association with the trailing stratiform portion of the squall line during the formative stage the pressure field to the rear of the squall line mesohigh is relatively flat with only weak stratiform precipitation present as the squall line enters the developing to mature stages a pronounced wake low appears at the back edge of the surface stratiform precipitation area the squall line at this time is characterized by a strong rear inflow jet descending from the upper troposphere as far as 500 km behind the leading convective line to the lower troposphere just behind the line the trailin

density currents in two layer shear flows an earlier two fluid model of an idealized densitycurrent in low level shear is extended to include variable upper level shear far field solutions are determined based on the conservation of mass momentum and vorticity and the conservation of bernoulli function energy along streamlines for inviscid flows it is found that the upper level and low level shears play similar roles in controlling the depth of steady state density currents in most cases large positive upper level shear supports a deeper density current and steeper front and therefore a stronger updraught it is also found that when the low level shear is weak and upper level shear occupies about half the domain depth larger positive shear can result in a shallower rather than a deeper density current this behaviour was not found for either constant shear flow or flows with only low level shear the behaviour is understood by examining the flow structure and flow force components as a function of the upper level shear furthermore by allowing the upper level shear to vary an overturning flow is permitted ahead of the density current this was not possible in the earlier model in which the upper level flow was assumed to be constant the present extension allows us to draw closer analogues between the model solutions and the circulation patterns found in typical squalllines in sheared environments time dependent numerical experiments are conducted for a range of upper and low level shears the depth and the propagation speed of simulated density currents are found to agree very well with predictions by the idealized theoretical model this verifies the validity of the theoretical model in addition numerical experiments with identically zero low level shear but differing upper level shears suggest that the deeper shear is as important as the low level shear in determining the uprightness of the upward branch of inflow in fact the presence of positive low level inflow shear may not be essential such results are also supported by the theoretical model and may have important implications for our understanding of squall line dynamics

processes controlling the structure and longevity of two quasi linear convective systems crossing the southern new england coast abstractthis paper explores the structural evolution and physical processes that explain the modification of two quasi linear convective systems qlcss that encountered the densely populated new york city atlantic coastal region one qlcs on 31 may 2002 traversed the atlantic coastal boundary with little change in its intensity producing widespread severe wind damage across new york city and long island during this event warm air advection at 925 hpa helped destabilize the layer above this level over the coastal zone while the marine boundary layer deepened below this level the 0 3 km line perpendicular vertical wind shear was relatively strong which supported ascent along the leading edge of the diabatically generated cold pool the surface based convective system became slightly elevated as it moved over the marine waters in contrast the 23 july 2002 qlcs decayed upon encountering the atlantic coastline despite its coincidence with a surface cold front the most unstable cape values during thi

a field study on turbidity currents initiated from spring runoffs the hydrodynamics of turbidity currents from spring runoffs of an influent river is examined by measuring velocity water temperature and concentration of suspended sediment in a reservoir crosssectional measurement of turbidity currents made from a bridge shows the effect of centrifugal force on the downstream movement this is due to considerable meandering of the old river channel the turbidity currents initiated at high sediment concentration of 1 700 mg l were bifurcated into an underflow and some interflows in the downstream reach because of the relatively dense bottom water of the reservoir the lower layer of turbidity currents exhibits a log linear relationship between dimensionless velocity and height under a hydraulically smooth condition the velocity distribution in the upper and lower layers could be explained by overall richardson number two drag coefficients related with shear stresses at bottom bed and interface generally decrease with increasing densimetric froude number but are independent of reynolds number the conditions of sediment deposition and erosion by turbidity currents can be explained by using the extended shields diagram

the structure and classification of numerically simulated convective storms in directionally varying wind shears abstract using a three dimensional numerical cloud model we investigate the effects of directionally varying wind shear on convective storm structure and evolution over a wide range of shear magnitudes as with a previous series of experiments using unidirectional wind shear profiles weisman and klemp the current results evince a spectrum of storm types ranging from short lived single cells at low shears multicells at intermediate shears to supercells at high shears with a clockwise curved hodograph the supercellular growth is confined to the right flank of the storm system while multicellular growth is favored on the left flank an analysis of the dynamic structure of the various cells reveals that the quasi steady supercell updrafts are strongly enhanced by dynamically induced pressure gradients on the right flank of the storm system we use this feature along with other related storm characteristics such as updraft rotation to propose a dynamically based storm classification scheme following

the interaction of simulated squall lines with idealized mountain ridges abstract numerical simulations of squall lines traversing sinusoidal mountain ridges are performed using the advanced regional prediction system cloud resolving model precipitation and updraft strength are enhanced through orographic ascent as a squall line approaches a ridge the simulated squall line then weakens as it descends the ridge because some of the cold pool is blocked by the terrain resulting in less lift along the gust front and weaker convective cells the flow within the cold pool accelerates slightly and the depth of the cold air decreases owing to upstream blocking transitioning the flow in the cold pool head from subcritical to supercritical then back to subcritical at the bottom of the ridge a hydraulic jump forms when the flow transitions the second time enabling the development of a new convective line downwind of the mountain these new updrafts grow and eventually replace the older updrafts that weakened during descent this process results in the discrete propagation of a squ

simulation and analysis of tornado development and decay within a three dimensional supercell thunderstorm abstract a three dimensional numerical simulation using a two way interactive nested grid is to study tornado genesis within a supercell during a 40 minute period two tornadoes grow and decay within the storm s mesocyclone the tornadoes have life spans of approximately 10 minutes maximum ground relative surface wind speeds exceed 60 m s 1 during both tornadoes and horizontal pressure gradients reach 18 hpa km 1 during the second tornado comparison of the simulated storm evolution with doppler and field observations of supercells and tornadoes shows many similar features vertical vorticity in the mesocyclone and the tornado vortex at low levels is initially created by the tilting of the environmental vorticity and baroclinically generated vorticity along the forward gland gust front of the storm tornadogenesis is initiated when mesocyclone rotation increase above cloud base the increased rotation generates lower pressure in the mesocyclone increasing the upward pressure gradient forces the upwar

the effect of mesoscale heterogeneity on the genesis and structure of mesovortices within quasi linear convective systems abstract this study examines the structure and evolution of quasi linear convective systems qlcss within complex mesoscale environments convective outflows and other mesoscale features appear to affect the rotational characteristics and associated dynamics of these systems thus real data numerical simulations of two qlcs events have been performed to i identify and characterize the various ambient mesoscale features that modify the structure and evolution of simulated qlcss and then to ii determine the nature of interaction of such features with the systems with an emphasis on the genesis and evolution of low level mesovortices significant low level mesovortices develop in both simulated qlcss as a consequence of mechanisms internal to the system consistent with idealized numerical simulations of mesovortex bearing qlcss and not as an effect of system interaction with external heterogeneity however meso scale order of 10 km heterogeneity in the form of a convective outflow boundary is su

a numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines abstract a set of 13 two dimensional numerical simulations based on the 22 and 23 june soundings from the convectionprofonde tropicale in 1981 copt81 experiment in west africa is used to study the organization and interactionof the convective and stratiform regions of squall line type convective systems the initial wind profiles arecharacterized by the african easterly jet aej and the tropical easterly jet tej located at about 3 5 km and14 km respectively the physical processes that generate and maintain the mesoscale inflow at the rear of squall line type mesoscaleconvective systems are thereby examined horizontal potential temperature gradients generated by a combinationof latent heat release in the convective region and unsaturated mesoscale descent both modulated by evaporation cause a horizontal pressure gradient and generate horizontal line parallel vorticity the rear inflow is a consequenceof these processes the convective activity induces a significant upscale influence ahead of the

human impact and the historical transformation of saltmarshes in the marano and grado lagoon northern adriatic sea abstract historical transformations of the saltmarshes in the six sub basins of the marano and grado lagoon were analyzed using aerial photographs 1954 1990 2006 and the support of historical maps and topographic surveys analysis of the 2006 set of aerial photographs enabled the definition of the present extent and distribution of the saltmarshes inside the lagoon 760 ha with a total reduction in saltmarsh area of 16 144 ha compared to 1954 direct human actions played a significant role in the budget since total loss due to land reclamation and dredging during this period amounted to 126 ha after excluding the total loss due to direct human interventions different erosional and depositional marsh types were recognized and associated with different forcing factors based on morphological and geographical evidence over the 52 year period marshes were lost due to a drowning the combined effects of eustatism regional subsidence and autocompaction 102 ha b edge retreat by wind wave attack 34 ha c erosion by vessel generated waves 37 ha and d coastal dynamics and inlet migration 5 7 ha conversely marshes gained in area due to a fluvial input 63 ha b tidal input 27 ha c paralagoonal deposition 45 ha d the re opening of abandoned fish farms 18 ha and e the dumping of dredged material 8 ha our analysis demonstrates that local and short term forcing factors can obliterate or compensate the long term ones especially the relative sea level rise a test of the integrated sediment budget carried out on one third of the total lagoon through a bathymetric comparison between datasets from 1964 to 2009 pointed out that conservation or slight expansion of the marshes inside these basins were linked to an overall positive sediment budget of 61 000 m 3 y nevertheless significant morphological changes occurred in the submerged basin which is affected by sustained deposition along the inner margins due to sediment supplies by an overall erosion of tidal and sub tidal flats far from the tributaries and by an important infilling of the channels the analyzed data along with information available for the venice lagoon highlights how the fate of open water lagoons is to flatten whilst submerging because of the strong influence of wind waves which tend to transform the lagoon into a marine embayment this transgressive condition reduces if not negates the compensative effect of the sedimentation rate on wind wave induced shear stress excess since supplies seem to contribute primarily to the morphological accommodation

numerical simulation of a midlatitude squall line in two dimensions abstract a two dimensional anelastic cloud model was used in attempts to numerically replicate the observed structure of a midlatitude squall line initial conditions were adapted from observations of the 22 may 1976 oklahoma line model simulations were made with and without considering the ice phase of water these model storms have within the constraints of the model s geometry replicated the basic multicellular character and general line normal airflow typical of these lines in addition the structure and intensity of the subcloud cold air pool and the propagation speeds developed by the storms appear to be reasonable further it was found that the initial conditions chosen resulted in model storms which were not only long lasting but also essentially repetitive indicating that a state of quasi equilibrium had been attained the storms did not decay because the environmental conditions ahead of the storms were favorable and essentially unchanged during the course of the simulations the inclusio

hydrologic parameters of salt marsh peat because of their profound influence on water movement and nutrient cycling in salt marshes the two key physical properties of hydraulic conductivity and compressibility were studied in the great sippewissett marsh and in the ebben creek marsh in massachusetts hydraulic conductivity was the most variable property most frequently observed conductivities were of the order of 10 3 cm s 1 in both marshes but extremes ranged from about 10 1 to 10 5 cm s 1 compressibility was much less variable and contributed of the order of 10 3 cm 1 to the specific storativity of marsh sediment making compression a major mechanism for changes in water storage in the sediment surface sediments frequently exhibited below average conductivity in contrast to freshwater bog peats which are usually most conductive at the surface these measured properties may be applied to estimate the importance of many critical processes such as the extent of infiltration occurring on the marsh surface the hydrologic influence of the tidally varying creeks and the hydrologic response to spring neap tidal cycles

reservoir infill by hyperpycnal deltas over bedrock 1 we propose a simple morphodynamic theory of reservoir deltas prograding over bedrock under the influence of turbid underflows the theory models the geomorphic actions of fluvial and hyperpycnal flows as diffusion processes and treats bedrock alluvial and river lake transitions as moving internal boundaries it yields self similar analytical solutions for the co evolving river and lake bed profiles under different slope and influx conditions these include a special case in which delta deposits are swept underwater to prograde into a subaqueous turbid pool the theoretical predictions are in good agreement with microscale experiments and should help interpret and anticipate longitudinal sedimentation patterns in mountain reservoirs

structure and evolution of numerically simulated squall lines abstract using a three dimensional numerical cloud model we investigate the effects of vertical wind shear on squall line structure and evolution over a wide range of shear magnitudes depths and orientations relative to the line we find that the simulated squall lines are most sensitive to the magnitude of the component of shear perpendicular to the line and that we may reproduce much of the range of observed structures by varying this single parameter for weak shear a line of initially upright to downshear tilted short lived cells quickly tilts upshear producing a wide band of weaker cells extending behind the surface outflow boundary for moderate to strong shear the circulation remains upright to downshear tilted for longer periods of time with vigorous short lived cells confined to a relatively narrow band along the system s leading edge at later times however these systems may also weaken as the circulation tilts upshear for strong deep shears oriented obliquely to the line the squal

formation of mesoscale lines of pirecipitation severe squall lines in oklahoma during the spring abstract four distinct kinds of severe mesoscale convective line development are identified in oklahoma during the spring based on the analysis of an 11 year period of reflectivity data from the national severe storms laboratory s 10 cm radar in norman oklahoma the primary classes of fine formation are broken line back building broken areal and embedded areal each is described in detail along with illustrative examples comparisons are made with other observations and with numerical model simulations the former two classes of line formation have been previously documented while the latter two have not only the broken areal squall line has been realistically simulated numerically the environment for each of the types of line development was determined from data from the standard national weather service surface and upper air networks and from special rawinsonde launches it was found that broken line formation tends to occur along cold fronts in a multicell environment while back building occur

high precision measurements of wetland sediment elevation i recent improvements to the sedimentation erosion table abstract the sedimentation erosion table set developed by boumans and day 1993 is herein renamed the surface elevation table set to better reflect the conceptual view of the processes being measured the set was designed for making high resolution measurements of small scale changes in elevation of loose unconsolidated sediments in shallow water and mudflat habitats the set has undergone three major improvements to increase precision and so that it can be used to measure sediment elevation in vegetated wetlands as well as shallow water habitats the remote release sliding plate mechanism has been replaced with a single plate collars first 2 5 cm then 7 5 cm in length have been attached to the plate to reduce play in the placement of the measuring pins and the brass measuring pins have been replaced with fiberglass pins to reduce bending and consequent loss of precision under ideal laboratory conditions the 95 confidence limit for individual pin measurements averaged about 1 4 mm range 0 7 to 1 9 mm these modifications have resulted in a reduction of error by about 50

freshwater megaflood sedimentation what can we learn about generic processes abstract there is growing recognition that sedimentary deposits related to high energy large scale freshwater floods are widespread across the continents and in the main can be related to quaternary outbreak flows from ice dammed lakes such deposits may also occur in the neogene and earlier recognition of these megaflood deposits is hindered by the lack of well conditioned stratigraphic models of depositional successions however descriptions of successions from many locations often exhibit commonalities when compared carefully thus the primary purpose of this paper is to examine and condense the published stratigraphic and sedimentological evidence so as to identify the key signatures of megaflood successions the deposits often are the only record of a flood and so the secondary purpose is to interpret the sedimentary sequences in order to recreate the behaviour of floods through single and multiple events finally some pointers are provided as to those areas of sedimentological research that might be profitably explored in more depth to improve understanding of megaflood dynamics a key characteristic of megaflood deposits is the apparent massive appearance of single units which on closer inspection usually are amalgamated layers of similar grain size distribution individual beds and sets of beds are commonly near horizontal planar and laterally extensive successions evidently are not channelized and extensivity is indicative of simultaneous and coherent deposition over large areas localised large scale clinoforms or smaller scale cross beds are common associated with bar front or dune progradation respectively but otherwise extensivity implies that cross cutting erosional contacts notably steeply inclined examples and small scale scours are not common except immediately around large isolated clasts such as dropstones and ice blocks a common vertical stack of sequences in any one succession consists of 1 basal thick coarse parallel bedded units 2 large scale clinoforms 3 horizontally bedded thin laminated units 4 ripple and dune cross beds 5 silt beds and 6 succession capping debris flow deposits in some cases such a succession usually is indicative of a single cycle of waxing and waning flood flow generally dominated by high concentration suspension deposition onto plane beds the high power allows the transport of fine gravel and coarse sand in suspension leading upon deposition to relatively coarse beds from suspension fall out clay and silt is generally sparse or absent being transported further down the system at the scale of the hydrograph the successions indicate that flows initially accelerate and then decelerate with significant shorter period flow pulses also evident in the sedimentary signatures in parallel bedded sequences primarily during waxing flows bedload transport often was ineffective in sorting sediment due to the high rates of deposition from the highly sediment charged waters resulting in diffuse accretion surfaces at each location similar sequence stacks are common leading to the impression of rhythmic deposition from pulsing flow including from repetitive series of floods although often composed of coarse grains many sequence stacks excluding any debris flows are not dissimilar to turbidity flow sequences the bouma sequence in particular although there is limited experimental evidence for turbidity currents associated with high energy freshwater flows theory does not preclude the deposition of similar sedimentary motifs from high concentration flows such that an analogy between some megaflood sedimentary beds and turbidites is not unreasonable despite considerable variation in megaflood sedimentary successions commonalities occur which are indicative of similar processes of deposition from high energy floods at geographically different locations the changes in style of deposition throughout a succession provide clues as to the nature of the flood flows and this information could be used to improve estimates of flood hydrograph behaviour however hypotheses linking deposit characteristics to flow dynamics are currently ill conditioned and often poorly tested the majority of published studies describe the coarser scale stratigraphy with only passing reference to the fine scale structure of individual beds closer attention to lamination styles particle orientation and syn depositional deformation structures for example should provide information on the flow mechanics at the time of bed deposition thus better consideration of the sedimentology of megaflood deposit beds will lead to better understanding of the controlling hydraulics and should result in better models of flood deposition

low level mesovortices within squall lines and bow echoes part i overview and dependence on environmental shear this two part study proposes fundamental explanations of the genesis structure and implications of lowlevel meso g scale vortices within quasi linear convective systems qlcss such as squall lines and bow echoes such mesovortices are observed frequently at times in association with tornadoes idealized simulations are used herein to study the structure and evolution of meso g scale surface vortices within qlcss and their dependence on the environmental vertical wind shear within such simulations significant cyclonic surface vortices are readily produced when the unidirectional shear magnitude is 20 m s 21 or greater over a 0 2 5 or 0 5 km agl layer as similarly found in observations of qlcss these surface vortices form primarily north of the apex of the individual embedded bowing segments as well as north of the apex of the larger scale bow shaped system they generally develop first near the surface but can build upward to 6 8 km agl vortex longevity can be several hours far longer than individual convective cells within the qlcs during this time vortex merger and upscale growth is common it is also noted that such mesoscale vortices may be responsible for the production of extensive areas of extreme straight line wind damage as has also been observed with some qlcss surface vortices are also produced for weaker shears but remain shallow weak and short lived although similar in size and strength to mesocyclones associated with supercell storms and also sometimes producing similar hooklike structures in the rain field it is also shown that the present vortices are quite distinct structurally and dynamically most critically such vortices are not associated with long lived rotating updrafts at midlevels and the associated strong dynamically forced vertical accelerations as occur within supercell mesocyclones

estimating shallow subsidence in microtidal salt marshes of the southeastern united states kaye and barghoorn revisited abstract simultaneous measurements of vertical accretion and change in surface elevation relative to a shallow 3 5 m subsurface datum were made in selected coastal salt marshes of louisiana florida and north carolina to quantitatively test kaye and barghoorn s contention that vertical accretion is not a good surrogate for surface elevation change because of autocompaction of the substrate rates of subsidence of the upper 3 5 m of marsh substrate were calculated for each marsh as the difference between vertical accretion and elevation change measured with feldspar marker horizons and a sedimentation erosion table surface elevation change was significantly lower than vertical accretion at each site after 2 years indicating a significant amount of shallow subsidence had occurred ranging from 0 45 to 4 90 cm the highest rate of shallow subsidence occurred in the mississippi delta results confirm kaye and barghoorn s contention that vertical accretion is not generally a good surrogate for elevation change because of processes occurring in the upper few meters of the substrate including not only compaction but also apparently shrink swell from water storage and or plant production decomposition at some sites indeed surface elevation change was completely decoupled from vertical accretion at the florida site the assumption of a 1 1 relationship between vertical accretion and surface elevation change is too simplistic a generalization of the complex interactions between accretionary and substrate processes consequently the potential for coastal marsh submergence should be expressed as an elevation deficit based on direct measures of surface elevation change rather than accretion deficits these findings also indicate the need for greater understanding of the influence of subsurface and small scale hydrologic processes on marsh surface elevation

convection initiation by density currents role of convergence shear and dynamical organization abstract steady state analytic models establish two key points concerning the impact of vertical shear on density currents and the implication for convection initiation first shear decreases the horizontal convergence and therefore the mean ascent associated with downshear propagating currents second shear has a basic effect on the dynamical organization if the downshear current travels at the speed of the ambient flow at a critical steering level an overturning circulation provides deep lifting although mean ascent is increased by shear in the case of upshear propagating currents the lifting is comparatively shallow because jumplike ascent occurs rather than deep overturning the convection initiation mechanism involving the downshear current is therefore very different from the upshear case these basic principles are borne out in two dimensional numerical simulations density currents generated by a stationary cold source imposed on an initially horizontally homogeneous sheared and neutra

mesoscale air transport at a midlatitude squall line in europe a numerical analysis this study examines a fast propagating squall line over western and central europe a high resolution regional weather prediction model is used to analyze the three dimensional wind field and the corresponding air mass transport induced by the mesoscale convective system the squall line can be divided into a bow shaped strong part with a continuous line of heavy convective precipitation and a weaker part exhibiting several isolated multi cell storms in the strong part a deep and intense rear inflow jet occurs with relative wind speeds of 12 18 m s 1 this part is also characterized by moderate to strong wind shear in contrast to the weaker part of the squall line where a shallow cold pool surges under the potentially unstable presquall air to investigate the air mass transport along the squall line in the model simulations passive fluid tracers are initialized in several vertical layers with different thermodynamic conditions at the strong part of the squall line a very efficient vertical tracer transport occurs potentially warm low level air is lifted from the lowermost 2 km to the upper troposphere where an organized horizontal outflow is observed the potentially coldest presquall air descends just in front of the updraught region and partly penetrates the convective precipitation area in the weaker part of the squall line the potential instability is not significantly reduced because the vertical air mass transport is less effective there additionally passive tracers are used to analyze the composition of the cold pool which turns out to be a very heterogeneous mixture of descending post frontal cold air and presquall air originating at different heights

views on applying rkw theory an illustration using the 8 may 2009 derecho producing convective system abstractthis work presents an analysis of the vertical wind shear during the early stages of the remarkable 8 may 2009 central u s derecho producing convective system comments on applying rotunno klemp weisman rkw theory to mesoscale convective systems mcss of this type also are provided during the formative stages of the mcs the near surface based shear vectors ahead of the leading convective line varied with time location and depth but the line normal component of the shear in any layer below 3 km ahead of where the strong bow echo developed was relatively small 6 9 m s 1 concurrently the midlevel 3 6 km line normal shear component had magnitudes mostly 10 m s 1 throughout in a previous companion paper it was hypothesized that an unusually strong and expansive low level jet led to dramatic changes in instability shear and forced ascent over mesoscale areas these mesoscale effects may have overwhelmed the interactions between the cold pool and low level shear that modulate system st

environmental distinctions between cellular and slabular convective lines abstract the organizational mode of quasi linear convective systems often falls within a spectrum of modes described by a line of discrete cells on one end cellular and an unbroken two dimensional swath of ascent on the other slabular convective events exhibiting distinctly cellular or slabular characteristics over the continental united states were compiled and composite soundings of the respective inflow environments were constructed the most notable difference between the environments of slabs and cells occurred in the wind profiles lines organized as slabs existed in much stronger low level line relative inflow and stronger low level shear a compressible model with high resolution x 500 m was used to investigate the effects of varying environmental conditions on the nature of the convective overturning the numerical results show that highly cellular convective lines are favored when the environmental conditions and initiation procedure allow the convectively generated cold pools to

multiscale convective overturning in mesoscale convective systems reconciling observations simulations and theory abstract an analysis of how parameterized convection interacts with hydrostatic explicitly resolved precipitation processes to represent multiscale convective overturning in a mesoscale resolution numerical simulation is presented critically important ingredients of the successful simulation are identified and the degree to which simulations are consistent with observations and theoretical considerations is examined of particular concern is how convective parameterization routines reconcile the deep moist absolutely unstable structures that form in mesoscale convective systems it is found that these structures arise primarily from resolvable scale vertical motion and that the model responds to these structures not only by maintaining parameterized convection but also by developing a hydrostatic manifestation of convective overturning on its smallest resolvable horizontal scales the strongest and most distinctive mesoscale perturbations develop in regions where the mesoscale contribution to convectiv

hydraulics of shallow and stratified mixing channel the properties of buoyant and dense discharges in a horizontal mixing channel are evaluated analytically and experimentally the mixing channel is shallow of specified length and roughness and connected to a reservoir or stream the flow in the channel consists of a mixing zone and a gradually varied counterflow mixing modes associated with four possible interactions are described a hydraulic theory is formulated and used to develop synthesized dilution curves experiments conducted in a laboratory flume confirmed the existence of four mixing modes and dilution measurements are consistent with predicted trends in practice the theory would be useful in the design and operation of such a facility

the simulated structure and evolution of a quasi idealized warm season convective system with a training convective line abstractthis study details the development and use of an idealized modeling framework to simulate a quasi stationary heavy rain producing mesoscale convective system mcs a 36 h composite progression of atmospheric fields computed from 26 observed warm season heavy rain producing training line adjoining stratiform tl as mcss was used as initial and lateral boundary conditions for a numerical simulation of this mcs archetype a realistic tl as mcs initiated and evolved within a simulated mesoscale environment that featured a low level jet terminus maximized low level warm air advection and an elevated maximum in convective available potential energy the first stage of mcs evolution featured an eastward moving trailing stratiform type mcs that generated a surface cold pool the initial system was followed by rearward off boundary development where a new line of convective cells simultaneously redeveloped north of the surface cold pool boundary backbuilding persisted on the western end of the new lin

abstract geologic framework processes and rates of subsidence in the mississippi river delta plain abstract since the 1930 s louisiana has lost an estimated 3 950 km2 of coastal wetlands and barrier islands the loss of these coastal lands resulted primarily from subsidence and erosion rather than the draining and filling of wetlands more than 40 of the coastal wetlands in the u s are found in louisiana and 80 of our nation s total wetland loss occurs here at alarming rates beach erosion rates exceed 10 m yr and the rate of wetland loss is currently measured at 75 km2 yr the causes of louisiana s coastal land loss include delta switching storm impacts man s impacts on this deltaic system and high rates of subsidence recent studies indicate that the highest subsidence and coastal land loss rates occur where the underlying holocene sediments are the thickest this relationship suggests that subsidence through the consolidation and settlement of these young deposits is of primary importance to the coastal land loss problem found in louisiana to understand the role of subsidence in louisiana s coastal land loss problem requires a knowledge of the late quaternary history of the lower mississippi river of the infilling of its incised valley during the holocene transgression and of the processes of delta switching end of record last page 644

comments on a numerical investigation of the organization and interaction of the convective and stratiform regions of tropical squall lines ce commentaire de l article de lafore et montcreiff 1989 sur l organisation des lignes de grains tropicales est une comparaison avec l article de rottuno et al 1988 afin de mettre en evidence des resultats et des points de vue communs

the development of organized convection in a simplified squall line model squall lines stand out from ordinary cumulonimbus convection because of a special structure that dramatically extends their duration precipitation output and area of influence time dependent models have identified surface outflow boundaries and low level environmental wind shear as the crucial elements in squall line formation and or maintenance steady state models have shown how properties of the downdraught reservoir or cold pool must be matched to the far field wind and thermodynamic profiles to sustain the convection however the dynamical role of the environmental shear and the nature of the interaction between the positively and negatively buoyant air have remained uncertain for help with these issues a time dependent numerical model is developed with all moist processes either parametrized or based on ad hoc assumptions the simplifications make it possible to distinguish more clearly than in the past between the formation and maintenance of squall lines and between the initial and disturbed far field environments the convective updraught forms within an expanding region of disturbed flow separated from the undisturbed environment by storm fronts forced or neutral ascent occurs at a surface gust front on the upwind side of the cold pool we distinguish squall lines from ordinary convection by looking at the coherence between the forced and convective updraughts it is found that subsidence over the cold pool disorganizes the updraughts whereas a deep overturning circulation downshear from the gust front has an organizing effect hence factors which allow the initial subsidence to occur ahead of the gust front namely contrasting lower and upper level winds and large potential buoyancy favour squall line development it is argued that in cases with large convective potential energy the deep mesoscale circulation is more important in the formation and maintenance of the line than either the vertical shear of the air reaching the cold pool or the strength of the forced updraught

effects of upper level shear on the structure and maintenance of strong quasi linear mesoscale convective systems recent observational studies have shown that strong midlatitude mesoscale convective systems mcss tend to decay as they move into environments with less instability and smaller deep layer vertical wind shear these observed shear profiles that contain significant upper level shear are often different from the shear profiles considered to be the most favorable for the maintenance of strong long lived convective systems in some past idealized simulations thus to explore the role of upper level shear in strong mcs environments a set of two dimensional 2d simulations of density currents within a dry statically neutral environment is used to quantify the dependence of lifting along an idealized cold pool on the upper level shear a set of three dimensional 3d simulations of mcss is produced to gauge the effects of the upper level shear in a more realistic framework results from the 2d experiments show that the addition of upper level shear to a wind profile with weak to moderate low level shear increases the vertical displacement of parcels despite a decrease in the vertical velocity along the cold pool interface parcels that are elevated above the surface 1 2 km overturn and are responsible for the deep lifting in the deep shear environments while the surface based parcels typically are lifted through the cold pool region in a rearward sloping path this deep overturning helps to maintain the leading convection and greatly increases the size and total precipitation output of the convective systems in more complex 3d simulations even in the presence of 3d structures these results show that the shear profile throughout the entire troposphere must be considered to gain a more complete understanding of the structure and maintenance of strong midlatitude mcss

climatology of summer midtropospheric perturbations in the u s northern plains part i influence on northwest flow severe weather outbreaks northwest flow severe weather outbreaks nwf outbreaks describe a type of summer convective storm that occurs in areas of mid level nwf in the central united states convective storms associated with nwf outbreaks often travel a long distance systematically along a northwest southeast oriented track across the northern plains previous studies have observed that these migrating convective storms are frequently coupled with subsynoptic scale midtropospheric perturbations mps initiated over the rocky mountains this study traces mps for the decade of 1997 2006 using the north american regional reanalysis to examine their climatology and possible influence on nwf outbreaks mps are characterized by a well organized divergent circulation with persistent ascending motion at the leading edge promoting convection the divergent circulation is further enhanced by low level convergence along the northern terminus of the great plains low level jet the downstream propagation of mps assists in forming the progressive feature of the associated convective storms mps have a maximum frequency in july consistent with nwf outbreaks in july and august the fully developed north american anticyclone produces prevailing nwf over the northern plains where up to 60 of rainfall and storm reports are linked to mps the movement timing and rainfall distribution of mps remarkably resemble those of nwf outbreaks when encountering strong low level jets ascending motion and convergence of water vapor flux associated with mps intensify considerably and precipitation is greatly enhanced it is likely that nwf outbreaks are generated whenever mps occur in association with strong low level jets

response of simulated squall lines to low level cooling abstract organized convection has long been recognized to have a nocturnal maximum over the central united states the present study uses idealized numerical simulations to investigate the mechanisms for the maintenance propagation and evolution of nocturnal like convective systems as a litmus test for the basic governing dynamics the experiments use horizontally homogeneous initial conditions i e they include neither fronts nor low level jet streams the simulated storms are allowed to mature as surface based convective systems before the boundary layer is cooled in this case it is then surprisingly difficult to cut the mature convective systems off from their source of near surface inflow parcels even when 10 k of the low level cooling has been applied the preexisting system cold pool is sufficient to lift boundary layer parcels to their levels of free convection the present results suggest that many of the nocturnal convective systems that were previously thought to be elevated may actually

the dependence of storm longevity on the pattern of deep convection initiation in a low shear environment the sensitivity of storm longevity to the pattern of deep convection initiation e g multiple quasi linearly arranged initial deep convective cells versus an isolated deep convective cell is examined using idealized cloud resolving simulations conducted with a low shear initial environment when multiple deep convective cells are initialized in close proximity to one another using either a line of thermals or a shallow airmass boundary long lived storms are produced however when isolated deep convection is initiated the resultant storm steadily decays following initiation these results illustrate that a quasi linear mechanism such as a preexisting airmass boundary that initiates multiple deep convective cells in close proximity can lead to longer lived storms than a mechanism that initiates isolated deep convection the essential difference between the experiments conducted is that an isolated initial storm produces a shallower cold pool than when a quasi linear initiation is used it is argued that the deep cold pools promote deep forced ascent systematic convective cell redevelopment and thus long lived storms even in environments with small values of vertical shear the difference in cold pool depth between the simulations is attributed to differences in the horizontal flux of cold air to the gust front with a single initial storm the few convective cells that subsequently form provide only a limited source of cold air leading to a cold pool that is shallow and incapable of fostering continued updraft redevelopment

the role of convectively generated rear inflow jets in the evolution of long lived mesoconvective systems abstract in this study the structure of convectively generated rear inflow jets and their role in the evolution of long lived mesoconvective systems are investigated through an analysis of idealized three dimensional simulations using a nonhydrostatic cloud model rear inflow jets are generated within these systems in response to the upshear tilting of the convective circulation as the horizontal buoyancy gradients along the back edge of the expanding system create a circulation that draws midlevel air in from the rear within this framework a wide range of rear inflow strengths and structures are produced depending on the magnitude of the ambient convective available potential energy cape and the vertical wind shear in general for environments characterized by weak to moderate vertical wind shear and weak to moderate cape the rear inflow jet descends and spreads along the surface well behind the leading edge of the gust front and the subsequent convective activity becomes weaker however for e

note on the analysis of plunging of density flows this note is devoted to the correction of an error in a calculation contained in a classical published paper on the plunging of river flows as they enter lakes or reservoirs the correction of the error provides a felicitous result according to which not only the ratio of underflow thickness just after plunging to depth just before plunging but also the densimetric froude numbers both just before and just after plunging are all specified as a function of a single parameter this single parameter characterizes the tendency of plunging to entrain ambient water into the density underflow

a high plains squall line associated with severe surface winds abstract the characteristics of a severe squall line are examined using data acquired from the 1981 cooperative convective precipitation experiment ccope the case is unusual in that the squall line was decoupled from an immediate surface based inflow source due to a meso scale 20 200 km outflow pool produced by a separate mesoscale convective system both systems participated in the development of a mesoscale convective complex which subsequently produced sustained severe surface winds throughout its entire life cycle despite the absolutely stable presquall atmospheric boundary layer the squall line produced radar reflectivity values of 70 dbz and storm induced outflow of 30 m s 1 aircraft soundings in the presquall environment suggest the storm was sustained by an elevated layer of high valued c air overriding the cold dome produced by the developing mcc the strongest surface winds were located upshear from the convective line and consisted of a northerly alongline component because the

large scale meteorological conditions associated with midlatitude mesoscale convective complexes abstract objective analyses of composited meteorological conditions attending ten mesoscale couvective complexes mcc reveal a number of distinctive characteristics and important interactions with the large scale environment the systems appear to be linked to eastward progression of a weak middle tropospheric short wave trough initial thunderstorms develop within a region of mesoscale convergence and lifting that is apparently forced primarily by low level warm advection the mcc system acquires mesoscale organization while it moves eastward ahead of the short wave trough diabatic heating eventually produces a system that is warm core in the middle troposphere and cold core in upper levels the mature mcc exhibits many similarities to tropical convective systems although it occurs within a considerably different large scale setting inflow within the lower half of the troposphere feeds convection within a region characterized by significant net upward mass flux and widespread precipitation thickne

linking river flood dynamics to hyperpycnal plume deposits experiments theory and geological implications turbid river plumes entering ocean or lake water of lesser density i e hyperpycnal plumes can plunge to form turbidity currents providing an important link between terrestrial sediment sources and marine depositional sinks a leading hypothesis suggests that hyperpycnal plume deposits accurately record the rising and falling discharge of a flooding river in terms of sediment size grading bedform sequence and deposit thickness which if correct has significant implications for unraveling river dynamics reservoir potential and earth history from marine event beds herein we present one of the first experimental flume studies aimed at testing this hypothesis results indicate that depth averaged hyperpycnal plume velocities can be uncorrelated or even anti correlated with river discharge at certain seabed locations because of translation of the plunge point resulting from temporal variations in discharge and sediment concentration through the duration of a river flood an advection length scale of settling sediment is found to be an important control on hyperpycnal plume deposits where coarse sediment sand is most likely to record multiple flow accelerations and decelerations related to plunge point translation even for a river flood with a single peaked hydrograph in contrast fine sediment mud is relatively insensitive to local plunge point dynamics and is most likely to preserve directly rising and falling river discharge finally it was found that the necessary fluvial sediment concentration to form a plunging plume can be much larger than the concentration typically used assuming density equivalence because of deposition upstream of the plunge point

simulation of a progressive derecho using composite initial conditions abstract this study focuses on the progressive derecho a widespread convectively induced windstorm produced by a mesoscale convective system that often occurs within a relatively benign synoptic scale environment sounding data from 12 progressive derechos which occurred in weakly forced large scale environments are composited in order to examine important large scale features in the preconvective environment this analysis captures many features that are common in warm season derecho environments such as an upper level wind maximum a relatively dry midtroposphere and low level warm advection initial and boundary conditions for the pennsylvania state university national center for atmospheric research fifth generation mesoscale model mm5 are created using this analysis a three dimensional horizontally nonhomogeneous explicitly resolved simulation of a progressive derecho is produced and compared to previous more idealized simulations of similar convective systems that have been used to expla

hydraulic jumps in sediment driven bottom currents laboratory experiments were conducted to observe the behavior of turbidity currents in the vicinity of a slope transition both sediment laden and saline hydraulic jumps were produced the vertical structure of the currents was found to depend on flow regime the saline and turbid hydraulic jumps showed similar characteristics the amount of water entrained by the flows through a jump was small the change in flow regime caused a marked reduction of the bed shear stress downstream of the jump in nature a turbidity current experiencing a hydraulic jump will drop most of its bedload immediately downstream from the jump while the suspended load will respond more gradually to the change in flow regime and will deposit sediment over a distance far exceeding 1 000 times the jump height

origin of mud in turbidites and hybrid event beds insight from ponded mudstone caps of the castagnola turbidite system north west italy the partitioning of different grain size classes in gravity flow deposits is one of the key characteristics used to infer depositional processes turbidites have relatively clean sandstones with most of their clay deposited as part of a mudstone cap or as a distal mudstone layer whereas sand bearing debrites commonly comprise mixtures of sand grains and interstitial clay hybrid event beds develop alternations of clean and dirty clay rich sandstones in varying proportions analysis of co genetic mudstone caps in terms of thickness and composition is a novel approach that can provide new insight into gravity flow depositional processes bed thickness data from the ponded castagnola system show that turbidites contain more clay overall than do hybrid event beds the castagnola system is characterized by deposits of two very different petrographic types thanks to this duality analyses of sandstone and mudstones composition allow inference of which proportion of the clay in each of the deposit types was acquired en route in combination with standard sedimentological observations the new data allow insight into the likely characteristics of their parent flows clean turbidites were deposited by lower concentration long duration erosive muddy turbidity currents which were more efficient at fractionating clay particles away from their basal layer hybrid event beds were deposited by shorter duration higher concentration less erosive sandier flows which were less efficient at clay fractionation the results are consistent with data from other turbidite systems for example marnoso arenacea the approach represents a new method to infer the controls on the degree of clay partitioning in gravity flow deposits

tornadoes and downbursts in the context of generalized planetary scales abstract in order to cover a wide range of horizontal dimensions of airflow the author proposes a series of five scales maso meso miso to be read as my so moso and muso arranged in the order of the vowels a e 1 o u the dimensions decrease by two orders of magnitude per scale beginning with the planet s equator length chosen to be the maximum dimension of masoscale for each planet mesoscale highs and lows were described on the basis of mesoanalyses while sub mesoscale disturbances were depicted by cataloging over 20 000 photographs of wind effects taken from low flying aircraft during the past 15 years various motion thus classified into these scales led to a conclusion that extreme winds induced by thunderstorms are associated with misoscale and mososcale airflow spawned by the parent mesoscale disturbances

two dimensional convection in non constant shear a model of mid latitude squall lines numerical simulations of two dimensional deep convection are analysed using analytical models extended to include shallow downdraughts and non constant shear the cumulonimbus are initiated by low level convergence created by a finite amplitude downdraught these experiments have constant low level shear and differ only in the profile of mid and upper level winds quasi steady convenction is produced if the mid and upper level flow has small shear and the low level shear is large the surface precipitation ismaximized for no intial relative relative flow aloft if stationary this storm p o can give prodigious locilized rainfall p o is the two dimentisonal equivalent of the supercell these results are placed in context with previous two dimensional simulations attention is drawn to the similiarity with previous two dimensional simulations attention is drawn to the similarity with squall lines in central and eastern u s a storm p o is analysed by construction of time averaged fields of streamfunction vorticity teperature and height deviation the smoothness of these fields suggests a conceptual model of the storm dynamics which involves cooperation between distinct charcteristic flows an overturning updraught a jump type updraught a shallow downdraught a low level rotor and a boundary layer an idealized analytical model is described by solution of the equations for steady convection these solutions for the remote flow are derived from energy conversation mass continuity and a momentum budget and they give relationships between the non dimensional parameteres of the problem it is apparent that the convection is a high froude or low richardson number flow demanding the existence of a cross storm pressure gradient inherent in this idealized model is a vortex sheet between updraught and down draught and it is considered that the dynamical instability of this sheet is related to complexities in the numerical simulation furthermore these results show that in two dimensions both non constant shear and a shallow downdraught are necessary to maintain steady convection

a theory for strong long lived squall lines revisited based on the analysis of idealized two and three dimensional cloud model simulations rotunno et al hereafter rkw and weisman et al hereafter wkr put forth a theory that squall line strength and longevity was most sensitive to the strength of the component of low level 0 3 km agl ambient vertical wind shear perpendicular to squall line orientation an optimal state was proposed by rkw based on the relative strength of the circulation associated with the storm generated cold pool and the circulation associated with the ambient shear whereby the deepest leading edge lifting and most effective convective retriggering occurred when these circulations were in near balance since this work subsequent studies have brought into question the basic validity of the proposed optimal state based on concerns as to the appropriate distribution of shear relative to the cold pool for optimal lifting as well as the relevance of such concepts to fully complex squall lines especially considering the potential role of deeper layer shears in promoting system strength and longevity in the following the basic interpretations of the rkw theory are reconfirmed and clarified through both the analysis of a simplified two dimensional vorticity streamfunction model that allows for a more direct interpretation of the role of the shear in controlling the circulation around the cold pool and through an analysis of an extensive set of 3d squall line simulations run at higher resolution and covering a larger range of environmental shear conditions than presented by wkr

coherent regeneration and the role of water vapor and shear in a long lived convective episode abstract in a recent radar based climatological study of warm season precipitation over the continental united states carbone et al found a high frequency of long lived coherent rainfall episodes many of the events were of longer duration than normally associated with mesoscale convective complexes and exhibited phase speeds 10 m s 1 in excess of the phase speed associated with synoptic systems the observations led to the speculation that cold pool dynamics and wavelike propagation mechanisms were responsible for the longevity of the systems one of the long lived episodes included in the statistics of the carbone et al study is described here occurring on 14 15 july 1998 the system lasted 50 h and traveled over 2800 km at its peak intensity the system was a bow echo producing damaging wind large hail and local flash flooding an interesting aspect of the event was an abrupt 90 turn in the storm s orientation and propagation vector midway through its life the environmental factors that led t

rear inflow in squall lines with trailing stratiform precipitation abstract the relative airflow and accompanying precipitation structure of squall lines trailed by mesoscale regions of stratiform rain are examined with emphasis on the occurrence of rear inflow i e the intrusion of environmental air into these storms across the trailing precipitation boundary three cases from kansas and oklahoma provide examples of strong rear inflow which crosses the back edge of the stratiform precipitation area at relative speeds exceeding 10 m s 1 the vertical profiles of relative flow at the trailing precipitation boundary of these three systems were remarkably similar with the rear inflow confined to a jet like layer centered at about 550 mb doppler radar data for two of these cases showed that the rear inflow jet occupied a continuous channel extending from middle levels at the back edge of the stratiform region to lower levels of the leading convective region where it merged with outflow from convective downdrafts to bolster the leading gust front strong front to rea

mesoscale surface pressure and temperature features associated with bow echoes abstract this study examines observed mesoscale surface pressure temperature and wind features of bow echoes bow echo events in the area of the oklahoma mesonet are selected for study to take advantage of high resolution surface data thirty six cases are identified using 2 km resolution radar reflectivity data over a 4 yr period 2002 05 their surface features are interrogated using the mesonet data distinct surface features usually associated with squall lines the mesohigh and cold pool are found to also accompany bow echoes a common surface pattern preceding bowing is identified prior to new bowing development the mesohigh surges ahead of the convective line while the cold pool remains centered behind it surface winds shift to a ground relative outflow pattern upon arrival of the mesohigh surge approximately 30 min later a new bowing segment forms with its apex slightly to the left with respect to the direction of system motion of the mesohigh surge the cold pool follows the convective

three dimensional evolution of simulated long lived squall lines abstract simulations of squall lines using nonhydrostatic convection resolving models have been limited to two dimensions or three dimensions with the assumption of along line periodicity the authors present 3d nonhydrostatic convection resolving simulations produced using an adaptive grid model where the lines are finite in length and the restriction to along line periodicity is removed the base state for the simulations is characterized by weak shallow shear and high convective available potential energy cape an environment in which longlived midlatitude mesoscale convective systems mcss are observed the simulated systems bear strong resemblance to many observed systems suggesting that large scale forcing absent in the horizontally homogeneous environment is not needed to produce many of the distinguishing features of midlatitude mcss in simulations without coriolis forcing the presence of line ends leads to mature symmetric systems characterized by a central region of strong convectio

mechanisms supporting long lived episodes of propagating nocturnal convection within a 7 day wrf model simulation abstract a large domain explicit convection simulation is used to investigate the life cycle of nocturnal convection for a one week period of successive zonally propagating heavy precipitation episodes occurring over the central united states similar to climatological studies of phase coherent warm season convection the longest lived precipitation episodes initiate during the late afternoon over the western great plains 105 100 w reach their greatest intensity at night over the central great plains 100 95 w and typically weaken around or slightly after sunrise over the midwest 95 85 w the longest lived episodes exhibit average zonal phase speeds of 20 m s 1 consistent with radar observations during the period composite analysis of the life cycle of five long lived nocturnal precipitation episodes indicates that convection both develops and then propagates eastward along an east west oriented lower tropospheric frontal zone an elevated 2 km deep layer of high e air helps sustain convect

simulation of a serial upstream propagating mesoscale convective system event over southeastern south america using composite initial conditions abstract serial upstream propagating mesoscale convective system mcs events over southeastern south america are important contributors to the local hydrologic cycle as they can provide roughly half of the total monthly summer precipitation however the mechanisms of upstream propagation for these events have not been explored to remedy this situation a numerical simulation of the composite environmental conditions from 10 observed serial mcs events is conducted results indicate that the 3 day simulation from the composite yields a reasonable evolution of the large scale environment and produces a large region of organized convection in the warm sector over an extended period as seen in observations upstream propagation of the convective region is produced and is tied initially to the development and evolution of untrapped internal gravity waves however as convective downdrafts develop and begin to merge and form a surface cold pool in the simulation the cold pool and its interaction with the env

the genesis of severe long lived bow echoes abstract a series of idealized simulations using a nonhydrostatic cloud model is used to investigate the genesis of bow echoes a bow shaped system of convective cells that is especially noted for producing long swaths of damaging surface winds it is hypothesized that severe long lived bow echoes represent a dynamically unique form of mesoconvective organization being produced for a restricted range of environmental conditions including a convective available potential energy cape of at least 2000 m2 s2 and vertical wind shears of at least 20 m s 1 over the lowest 2 5 5 km agl the key structural features include a 40 100 km long bow shaped segment of convective cells with a strong rear inflow jet extending to the leading edge of the bow at 2 3 km agl and cyclonic and anticyclonic eddies referred to as bookend vortices on the northern and southern flanks of the bowed segment respectively this structure characteristically develops three to four hours into the lifetime of a convective system a