a theoretical and numerical study of density currents in nonconstant shear flows the previous idealized two fluid model of a density current in constant shear is extended to the case where the inflow shear is confined to the low levels the analytical solution is determined by the conservation of mass momentum vorticity and energy it is found that a low level shear acts in a similar manner to a uniform vertical shear in controlling the depth of a steady state density current when the shear enhances the low level flow against the density current propagation the current is deeper than half of the domain depth time dependent numerical experiments are conducted for a variety of parameter settings including various depths and strengths of the shear layer the numerical results agree closely with the theoretical analyses numerical experiments are also performed for a case where the initial depth of the density current is set to be comparable to the low level shear which is much shallower than that given by the steady state solution the circulation at the density current head remains shallow and is nonsteady in this case whereas the time averaged flow still exhibits a deep jump updraft pattern that is close to the theoretical solution suggesting the applicability of the theoretical results to even more transient flows the simulated flow features are discussed in terms of balanced and unbalanced dynamics and in the context of forcing and uplifting at the frontal zone in long lived convective systems here the term balance refers to a flow configuration that satisfies the steady state solution of the idealized theoretical model

the bow and arrow mesoscale convective structure abstractthe accurate prediction of warm season convective systems and the heavy rainfall and severe weather associated with them remains a challenge for numerical weather prediction models this study looks at a circumstance in which quasi stationary convection forms perpendicular to and above the cold pool behind strong bow echoes the authors refer to this phenomenon as a bow and arrow because on radar imagery the two convective lines resemble an archer s bow and arrow the arrow can produce heavy rainfall and severe weather extending over hundreds of kilometers these events are challenging to forecast because they require an accurate forecast of earlier convection and the effects of that convection on the environment in this study basic characteristics of 14 events are documented and observations of 4 events are presented to identify common environmental conditions prior to the development of the back building convection simulations of three cases using the weather research and forecasting m

parameterized mesoscale forcing mechanisms for initiating numerically simulated isolated multicellular convection abstract two methods designed to parameterize mesoscale ascent in a three dimensional numerical cloud model via near surface momentum and heat fluxes are presented and compared to the commonly used technique of an initial perturbation placed within the model initial condition the flux techniques use a continuously reinforced thermal or convergent low level wind field to produce upward vertical motion on the order of 10 cm s 1 by which deep moist convection can be initiated the sensitivity of the convective response to the type strength and size of the forcing is evaluated using numerical simulations of a conditionally unstable environment with weak unidirectional shear precipitation free cloud processes are used to further simplify the model response to the forcing the three methods tested produce an initial convective response but only the momentum and heat flux methods are able to produce sustained deep convection that approximately resembles isolated multicellular convection cell regeneration

facies distribution and textural variation in faro drift contourites velocity fluctuation and drift growth abstract detailed study of seismic profiles piston cores and bottom photographs from the faro drift on the southern margin of portugal has led to a better understanding of drift development and its relationship to bottom current circulation data on the contourite facies characteristics and the surface microphysiography have been published elsewhere here we concentrate on sediment distribution and geometry longitudinal trends in facies types mean grain size sedimentary structures and composition can be interpreted in terms of relative intensity of currents over different parts of the drift these are generally more intense in the marginal channels and at the upstream or eastern end of the drift three different scales of vertical variation of facies can be identified at the large scale 300 500 m of sediment has accumulated over 4 6 ma in a regular vertical succession due to the northward progradation of the drift at the medium scale the upper 20 30 m of sediment shows alternating phases of active lateral progradation and uniform vertical accumulation that may correlate with episodes of more and less current activity related to high and low sea level stands respectively over the past 0 3 ma at the small scale the topmost 2 3 m of sediment deposited in approximately 0 03 ma shows three zones of coarser grained sediments separated by finer grained contourites this sequence can also be interpreted in terms of long term fluctuation in bottom current activity although the signal is clearly complex this kind of analysis of sedimentary drifts can lead to more accurate reconstruction of paleocirculation patterns

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

episodic metamorphism and deformation in low pressure high temperature terranes in many cases low pressure high temperature metamorphism must be the consequence of advective heat transfer e g the ascent of granitic magma whereas such heating mechanisms are necessarily short lived the lifetime of the underlying orogenic processes is likely to be much longer and it is conceivable that during the evolution of an orogen repeated transient heating at shallow crustal levels is caused by episodes of segregation and upward passage of melts generated at deep crustal levels the number and timing of these events would depend on such factors as critical melt segregation volume moho temperature and strain rate evolution we investigate some of the controls on this multiple event superposition using a simple thermomechanical model designed to predict simultaneously the strain rate and the isostatic and thermal evolution of convergent orogenesis subject to a constant driving force an appealing aspect of this formulation is that in contrast to models that have attempted to explain event cyclicity through episodic processes acting on the orogenic boundaries it accounts for the repeated occurrence of events through processes inherent to the orogenic system itself

intrusive gravity currents received 5 may 2006 and in revised form 1 august 2006 the speed of a fluid intrusion propagating along a sharp density interface is predicted using conservation of mass momentum and energy for the special case in which the intrusion density equals the depth weighted mean density of the upper and lower layers the theory of holyer huppert j fluid mech vol 100 1980 p 739 predicts that the intrusion occupies one half the total depth its speed is one half the interfacial long wave speed and the interface ahead of the intrusion remains undisturbed for all other intrusion densities the interface is deflected vertically by a long wave that travels ahead of the intrusion and thereby changes the local upstream conditions in these cases the conservation equations must be matched to an exact solution of the two layer shallow water equations which describe the spatial evolution of the nonlinear wave we obtain predictions for the intrusion speed that match closely with experiments and numerical simulations and with a global energy balance analysis by cheong keunen linden j fluid mech vol 552 2006 p 1 since the latter does not explicitly include the energetics of the upstream wave it is inferred that the energy carried by the wave is a small fraction of the intrusion energy however the new more detailed model also shows that the kinematic influence of the upstream wave in changing the level of the interface is a critical component of the flow that has previously been ignored

mechanisms for quasi stationary behavior in simulated heavy rain producing convective systems abstract in this study idealized numerical simulations are used to identify the processes responsible for initiating organizing and maintaining quasi stationary convective systems that produce locally extreme rainfall amounts of particular interest are those convective systems that have been observed to occur near mesoscale convective vortices mcvs and other midlevel circulations to simulate the lifting associated with such circulations a low level momentum forcing is applied to an initial state that is representative of observed extreme rain events the initial vertical wind profile includes a sharp reversal of the vertical wind shear with height indicative of observed low level jets deep moist convection initiates within the region of mesoscale lifting and the resulting convective system replicates many of the features of observed systems the low level thermodynamic environment is nearly saturated which is not conducive to the production of a strong surface cold pool yet the convection qui

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 response of simulated nocturnal convective systems to a developing low level jet some recent numerical experiments have examined the dynamics of initially surface based squall lines that encounter an increasingly stable boundary layer akin to what occurs with the onset of nocturnal cooling the present study builds on that work by investigating the added effect of a developing nocturnal low level jet llj on the convective scale dynamics of a simulated squall line the characteristics of the simulated llj atop a simulated stable boundary layer are based on past climatological studies of the llj in the central united states a variety of jet orientations are tested and sensitivities to jet height and the presence of lowlevel cooling are explored the primary impacts of adding the llj are that it alters the wind shear in the layers just above and below the jet and that it alters the magnitude of the storm relative inflow in the jet layer the changes to wind shear have an attendant impact on low level lifting in keeping with current theories for gust front lifting in squall lines the changes to the system relative inflow in turn impact total upward mass flux and precipitation output both are sensitive to the squall line relative orientation of the llj the variations in updraft intensity and system relative inflow are modulated by the progression of the lowlevel cooling which mimics the development of a nocturnal boundary layer while the system remains surfacebased the below jet shear has the largest impact on lifting whereas the above jet shear begins to play a larger role as the system becomes elevated similarly as the system becomes elevated larger changes to systemrelative inflow are observed because of the layer of potentially buoyant inflowing parcels becoming confined to the layer of the llj

possible thermal buffering by crustal anatexis in collisional orogens thermobarometric evidence from the nepalese himalaya one dimensional thermal models suggest that large thrust faults may exert first order control on the thermal structure of collisional orogenic belts and an increasing body of petrologic data may be interpreted as consistent with these models we present new thermobarometric data from the central nepalese himalaya which imply that large sections of the himalayan orogen may have been at roughly uniform temperatures during active thrusting a phenomenon that is not predicted by existing models we attribute this phenomenon to thermal buffering as a consequence of widespread in situ anatexis if this process is common in the intermediate and lower crust of active orogens and if it is localized spatially one dimensional thermal models may produce an extremely oversimplified picture of heat transfer processes during collisional orogenesis

back reaction between restite and melt implications for geothermobarometry and pressure temperature paths partial melting is an important process in high grade metamorphism and is responsible for the production of large volumes of melt in the earth9s crust unless melt extraction is complete in situ crystallizing melt back reacts with the restite upon cooling in the northampton block western australia spinel formed by high temperature 850 c partial melting of metapelites is separated from leucosome by garnet sillimanite or cordierite sillimanite coronas formation of the coronas by back reaction with a silica undersaturated melt is inferred because 1 textural evidence indicates that quartz was exhausted as a result of progressive dehydration melting before formation of spinel melt 2 the coronitic phases are never in direct contact with quartz of the leucosome 3 barometry on cordierite and garnet bearing equilibria with spinel gives conflicting pressures 8 0 0 5 and 3 3 0 2 kbar respectively at 850 c if reactions with quartz are assumed but pressure estimates converge on 6 5 0 5 kbar at lower silica activity a sio 2 0 85 0 02 these results indicate that the use of corona textures for the derivation of pressure temperature p t conditions and pressure temperature vectors in migmatites should be reconsidered such coronas may result from a heating cooling cycle without substantial change in pressure back reaction with residual melt is likely to be an important process in migmatites generally affecting both the solid residuum and the leucosome composition

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

variscan geodynamic evolution of the carnic alps austria italy the south alpine carnic alps are part of the southern flank of the european variscides and display a continuous sedimentary record from late ordovician to devonian times followed by carboniferous s directed nappe stacking and late carboniferous to early permian post collisional collapse the tectonometamorphic and sedimentary evolution of the carnic alps resembles a continuous process where pre and syn orogenic volcanism syn orogenic flysch sedimentation deformation including nappe stacking metamorphism and tectonic collapse shift in age from internal zones in the n towards external zones in the s new structural petrological and sedimentological data are presented concerning the tectonometamorphic history of the carnic alps we distinguish three thrust sheets or tectonic nappes differing in their stratigraphic sedimentological deformational and metamorphic histories which were thrust over each other in carboniferous times our data lead to a new geodynamic model showing an evolution from rifting or back arc spreading in the late ordovician to the establishment of a mature passive continental margin in the late devonian early carboniferous flysch sedimentation in an active continental margin setting during the visean namurian and finally collision during the late carboniferous between the northern margin of gondwana and a microcontinent to the n

new u pb zircon ages and the duration and division of devonian time newly determined u pb zircon ages of volcanic ashes closely tied to biostratigraphic zones are used to revise the devonian time scale they are 1 417 6 1 0 ma for an ash within the conodont zone of icriodus woschmidti i w hesperius early lochkovian 2 408 3 1 9 ma for an ash of early emsian age correlated with the conodont zones of po dehiscens lower po inversus 3 391 4 1 8 ma for an ash within the po c costatus zone and probably within the upper half of the zone eifelian and 4 381 1 1 3 ma for an ash within the range of the frasnian conodont palmatolepis punctata pa punctata zone to upper pa hassi zone u pb zircon ages for two rhyolites bracketing a palyniferous bed of the pusillites lepidophyta spore zone are dated at 363 8 2 2 ma and 363 4 1 8 ma respectively suggesting an age of363 ma for a level within the late famennian pa g expansa zone these data together with other published zircon ages suggest that the base and top of the devonian lie close to 418 ma and 362 ma respectively thus lengthening the period by20 over current estimates we suggest that the duration of the middle devonian eifelian and givetian is rather brief perhaps no longer than 11 5 myr 394 ma 382 5 ma and that the emsian and famennian are the longest stages in the period with estimated durations of15 5 myr and 14 5 myr respectively 1998 elsevier science b v all rights reserved

intrusive gravity currents and internal gravity wave generation in stratified fluid the excitation of internal gravity waves by fluid intrusions that propagate along the interface between a uniform upper layer and a uniformly stratified lower layer is examined by way of laboratory experiments intrusions are generated using a simple lock release apparatus experiments are conducted in which the density gradient of the uniformly stratified layer the density jump across the interface and the density difference between the lock fluid and the uniform upper layer are varied in all cases the fluid intrusions travelled at a constant speed the forcing imparted by the generated internal gravity waves did not deform the intrusion head or significantly retard the intrusion s rate of forward advance for a limited range of density parameters good agreement was obtained between the experimental data and the two layer analytical theory of j y holyer h e huppert j fluid mech vol 100 1980 pp 739 767 which provides estimates for the intrusion speed and depths of penetration into the upper and lower layers internal gravity wave excitation is due to the initial collapse of the lock fluid and the forcing imparted by the head of the intrusion waves are visualized and their amplitudes measured using synthetic schlieren the vertical flux of horizontal momentum due to internal gravity wave excitation is related to measurable properties of the fluid intrusion this analysis suggests that outflows produced by tall convective storms that travel along the tropopause may excite non hydrostatic internal gravity waves in the stratosphere whose momentum flux at least during the transient generation time is comparable to that of waves generated by topographic forcing

organization and environmental properties of extreme rain producing mesoscale convective systems this study examines the radar indicated structures and other features of extreme rain events in the united states over a 3 yr period a rainfall event is defined as extreme when the 24 h precipitation total at one or more stations surpasses the 50 yr recurrence interval amount for that location this definition yields 116 such cases from 1999 to 2001 in the area east of the rocky mountains excluding florida two kilometer national composite radar reflectivity data are then used to examine the structure and evolution of each extreme rain event sixty five percent of the total number of events are associated with mesoscale convective systems mcss while a wide variety of organizational structures as indicated by radar reflectivity data are seen among the mcs cases two patterns of organization are observed most frequently the first type has a line often oriented east west with training convective elements it also has a region of adjoining stratiform rain that is displaced to the north of the line the second type has a back building or quasi stationary area of convection that produces a region of stratiform rain downstream surface observations and composite analysis of rapid update cycle version 2 ruc 2 model data reveal that training line adjoining stratiform tl as systems typically form in a very moist unstable environment on the cool side of a preexisting slow moving surface boundary on the other hand back building quasistationary bb mcss are more dependent on mesoscale and storm scale processes particularly lifting provided by storm generated cold pools than on preexisting synoptic boundaries

some simple simulations of thunderstorm outflows three idealized simulations of thunderstorm outflows are presented each outflow is a response to an instantaneous low level cooling the vertical structures of the coolings differ as do the environments in which the outflows form and consequently the dynamics of the outflows differ one outflow is a gravity current another is a gravity wave and the third comprises both a gravity current and a gravity wave the horizontal transport of mass is important for the advance of the gravity current outflow but not for the gravity wave outflow and it is suggested that this is the defining dynamical distinction between the two outflows the simulations are compared to observations and it is suggested that some outflows previously characterized as gravity currents may better fit the gravity wave or gravity current wave archetypes it is also noted that the gravity wave component of an outflow may be generated directly by low level cooling in addition to the commonly suggested mechanism of the interaction of a gravity current with a stable layer

objective categorization of heavy rain producing mcs synoptic types by rotated principal component analysis abstractin this research rotated principal component analysis was applied to the atmospheric fields associated with a large sample of heavy rain producing mesoscale convective systems mcss cluster analysis in the subspace defined by the leading two resulting principal components revealed two subtypes with distinct synoptic and mesoscale characteristics which are referred to as warm season type and synoptic type events respectively subsequent composite analysis showed that both subtypes typically occurred on the cool side of a quasi stationary low level frontal boundary within a region of locally maximized low level convergence and warm advection synoptic type events which tended to exhibit greater horizontal extent than warm season type events typically occurred downstream of a progressive upper level trough along a low level potential temperature gradient with the warmest air to the south and southeast warm season type events on the other hand occurred within the right entrance region of

impacts of increasing low level shear on supercells during the early evening transition abstractthe dynamical response of simulated supercells to temporally increasing lower tropospheric vertical wind shear is investigated using idealized simulations these simulations are based upon observed soundings from two cases that underwent an early evening transition during the second verification of the origins of rotation in tornadoes experiment vortex2 mature supercells were simulated in observed afternoon environments with moderate vertical wind shear and then compared to simulated supercells experiencing observed evening increases in lower tropospheric shear the primary effect of the increase in low level shear is to establish larger values of vertical vorticity at lower altitudes in the storm s updraft in turn this leads to a nonlinear increase in the updraft strength due to the enhanced dynamic pressure minimum associated with larger vorticity in the storm s mesocyclone this is particularly important at low levels where it increases the storm s ability to lift cool surface air includ

the sources of energy for crustal melting and the geochemistry of heat producing elements crustal melting to produce granite magmas requires a tremendous amount of energy in principle there are three main mechanisms of heating that can extensively melt a fertile crust radiogenic heat caused by the decay of 40k 230th 235u and 238u increased subcrustal heat flux caused by the upwelling of deeper therefore hotter mantle materials advection of heat caused by the emplacement and crystallization of hot mantle magmas two dimensional finite elements modeling reveals that a fertile crust thickened to 65 70 km would produce copious granite magmatism after 30 40 m y if its average heat production is a 1 2 w m 3 but it would scarcely melt if a 0 65 w m 3 increasing the subcrustal heat flow from the normal value of qm 0 025 w m 2 to qm 0 04 w m 2 may also lead to extensive crustal melting especially if the crust does not thin to less than 30 35 km very high qm 0 06 w m 2 affecting the continental crust is unlikely but the combination of moderately high qm 0 04 w m 2 and a thick fertile crust with a 1 2 w m 3 such as often happens in the volcanic and back arc areas of subduction zones is ideal to produce copious granite magmatism lastly the emplacement of hot mantle magmas in a fertile crust can produce crustal melts in just a few thousand years but the volume of these is equal to or less than the volume of the intruding magma a clue for understanding the relative importance of each of these three mechanisms comes from the radiogenic heat production of granite rocks calculated from the concentration of 40k 230th 235u and 238u at the time of their formation this parameter estimated on more than 3400 granites samples of different ages and provenance reveals a strongly asymmetric distribution peaking around 2 4 w m 3 a value much higher than the average continental crust about 1 1 2 w m 3 and certainly much higher than the average lower continental crust about 0 4 0 8 w m 3 only those granite rock types that are clearly connected with mantle heat sources such as the archean ttg post archean subduction related trondhjemites and recent adakites have a heat production equal to or smaller than the lower continental crust since the bulk melt solid partition coefficient of the heat producing elements hpe k th and u is k 1 the elevated hpe contents of granites indicates that most of them have been derived from hpe rich sources we conclude that radiogenic heating is often essential and always advantageous for generating large volumes of granite magmas and that granite magmatism is the main cause of the accumulation of hpe in the upper crust

bores observed during ihop_2002 the relationship of bores to the nocturnal environment abstractthis study documents atmospheric bores and other convergent boundaries in the southern great plains nocturnal environment during the ihop 2002 summer campaign observational evidence demonstrates that convective outflows routinely generate bores statistically resampled flow regimes derived from an adaptation of hydraulic theory agree well with observations specifically convective outflows within the observed environments are likely to produce a partially blocked flow regime which is a favorable condition for generating a bore once a bore develops the direction of movement generally follows the orientation of the bulk shear vector between the nose of the nocturnal low level jet and a height of 1 5 or 2 5 km agl this relationship is believed to be a consequence of wave trapping through the curvature of the horizontal wind with respect to height this conclusion comes after analyzing the profile of the scorer parameter overall these findings provide an impetus for future investigations ai

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

shear parallel mesoscale convective systems in a moist low inhibition mei yu front environment abstractnumerical simulations are performed to investigate organized convection observed in the asian summer monsoon and documented as a category of mesoscale convective systems mcss over the u s continent during the warm season in an idealized low inhibition and unidirectional shear environment of the mei yu moisture front the structure of the simulated organized convection is distinct from that occurring in the classical quasi two dimensional shear perpendicular and trailing stratiform ts mcs consisting of four airflow branches a three dimensional eastward propagating downshear tilted shear parallel mcs builds upshear by initiating new convection at its upstream end the weak cold pool in the low inhibition environment negligibly affects convection initiation whereas convectively generated gravity waves are vital upstream propagating gravity waves form a saturated or near saturated moist tongue and downstream propagating waves control the initiation and growth of convection within a pre

fossil contourites a critical review abstract despite three decades of study there is still great controversy over the recognition and interpretation of fossil contourites exposed in ancient series on land in order to best examine this problem we briefly review the evidence from modern systems including the many examples of cenozoic contourites that have been recovered from dsdp odp drilling on major drifts in the present day oceans the range of contourite facies described from both deep water 2000 m and mid water 300 2000 m drifts are mostly fine grained bioturbated and homogeneous often with a distinct bedding cyclicity and with some coarser grained sandy contourites developed under higher energy bottom currents there are also a number of current controlled sediment bodies that have formed in outer shelf upper slope settings 50 300 m under the influence of counter currents underflows and major surface currents these are not considered contourites sensu stricto but may be mistaken as such in ancient examples the most commonly described fossil contourites in the literature have been interpreted by the authors concerned as bottom current reworked turbidites however a critical review suggests that these are the facies most subject to misinterpretation and many of the sediments claimed as fossil contourites are almost certainly fine grained turbidites whereas others were more likely formed under outer shelf upper slope current systems there remain very few ancient examples that are more closely comparable to modern contourites these include the cretaceous talme yafe formation in israel the ordovician jiuxi drift in china and parts of the paleogene lefkara formation cyprus and the neogene misaki formation in japan we present a set of possible criteria for the recognition of fossil contourites and bottom current reworked turbidites

a composite perspective on bore passages during the pecan campaign abstractatmospheric bores have been shown to have a role in the initiation and maintenance of elevated convection previous observational studies of bores have been case studies of more notable eve

dynamics governing a simulated mesoscale convective system with a training convective line abstractthis research investigates the dynamics of a simulated training line adjoining stratiform tl as mesoscale convective system mcs with composite atmospheric fields used as initial and lateral boundary conditions for the simulation an initial forward propagating mcs developed within a region of elevated convective instability and low level lifting associated with warm air advection along the terminus of the low level jet the environmental conditions external to the mcs continued to provide lift moisture and instability to the western side of the forward propagating mcs and these conditions were initially responsible for backbuilding on the system s western side most parcels that encountered the southwestern outflow boundary were lifted insufficiently far to reach their levels of free convection lfcs and their lfc heights were increased by latent heating above them these parcels continued northeastward beyond the surface outflow boundary ofb were gradually lifted and initiated convec

intrusive gravity currents in a stratified ambient shallow water theory and numerical results the intrusion of a fixed volume of fluid which is released from rest and then propagates horizontally at the neutral buoyancy level in a vertically stratified ambient fluid is investigated the density change is linear in a restricted layer or over the full depth of the container and locks of both rectangular and cylindrical shapes are considered a closed one layer shallow water inviscid formulation is used to obtain solutions of the initial value problem similarity solutions for the large time developed motion and an approximate box model are also presented the results are corroborated by numerical solutions of the full two dimensional navier stokes equations and comparisons with previously published experiments

near isothermal conditions in the middle and lower crust induced by melt migration simple one dimensional thermal models that include the effects of melt migration have been developed the models show that long lived plutonism results in a quasi steady state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust the thermal structure of the crust strongly influences deformation metamorphism and plutonism1 2 3 models for the geothermal gradient in stable crust predict a steady increase of temperature with depth this thermal structure however is incompatible with observations from high temperature metamorphic terranes exhumed in orogens1 4 5 6 global compilations7 of peak conditions in high temperature metamorphic terranes define relatively narrow ranges of peak temperatures over a wide range in pressure for both isothermal decompression and isobaric cooling paths here we develop simple one dimensional thermal models that include the effects of melt migration these models show that long lived plutonism results in a quasi steady state geotherm with a rapid temperature increase in the upper crust and nearly isothermal conditions in the middle and lower crust the models also predict that the upward advection of heat by melt generates granulite facies metamorphism and widespread andalusite sillimanite metamorphism in the upper crust once the quasi steady state thermal profile is reached the middle and lower crust are greatly weakened due to high temperatures and anatectic conditions thus setting the stage for gravitational collapse8 exhumation and isothermal decompression after the onset of plutonism near isothermal conditions in the middle and lower crust result from the thermal buffering effect of dehydration melting reactions that in part control the shape of the geotherm

evidence for major mass transfer and volume strain during regional metamorphism of pelites systematic examination of published sedimentary and metamorphosed pelite analyses has revealed evidence of significant mass transfer and volume strain during regional metamorphism statistical analysis of the data shows that barrovian zone metamorphism of pelitic schist generally causes increases in the whole rock concentrations of the low solubility elements ti and all the observed increases in ti and al contents as functions of metamorphic grade are almost certainly due to residual enrichment caused by the removal of other more soluble species application of mass balance principles to the petite compositional trends indicates that the average pelite may lose as much as 30 of its mass and volume during progressive metamorphism from subgreenschist to amphibolite facies conditions the bulk of the lost mass is silica not volatiles in addition other elements particularly ca na and k appear to be highly mobile in deep crustal metamorphism contrary to conventional interpretations it is concluded that the regional metamorphism of pelites is not an isochemical process

the phosphorus cycle phosphogenesis and marine phosphate rich deposits abstract phosphorus in the form of phosphate is an essential nutrient and energy carrier on many different levels of life and a key element in mediating between living and lifeless parts of the biosphere one of the most important aspects of the phosphorus cycle is its vital role in governing productivity thereby interacting with the exogenic part of the carbon cycle which in turn is important in regulating earth s climate phosphorus is a prime element to be traced in earth s history because it allows for the reconstruction of long term feedback mechanisms between climate environment and ecology and of global change as such marine sedimentary phosphate deposits are particularly suited to study aspects of the phosphorus cycle because in the case of ubiquity their origin may result from a general acceleration of the global phosphorus cycle sources of sedimentary phosphate are microbial breakdown of buried organic matter and redox driven phosphate desorption from iron and manganese oxyhydroxides dissolved sea water phosphate represents an additional source which may become important in the formation of phosphatic hardgrounds the main locus of phosphogenesis is near the sediment water interface but phosphogenesis also occurs at greater sediment depths current induced winnowing and transport processes along the sea floor concentrate phosphate precipitates into deposits which exhibit internal stratification patterns typical for the prevailing hydraulic energy regime in a sequence stratigraphic context phosphate deposits preferentially occur along marine or maximum flooding surfaces consequent sedimentary reworking may result in the transfer of phosphates to highstand or lowstand deposits bio chemical weathering on continents represents the most significant source of bioavailable phosphorus this implies that long term changes in marine phosphorus levels and with these changes in marine ecology productivity rates and ratios of exported carbonate carbon and organic carbon are a response to changes in continental weathering rates a compilation of marine sedimentary phosphorus burial rates for the last 160 myr suggests that natural variations have occurred that span one order of magnitude for the late jurassic cretaceous and most of the paleogene the phosphorus cycle appears to have been accelerated in times of climate warming which was most likely due to the spreading of zones of humid climate and more intense continental weathering in the neogene the phosphorus cycle appears to have responded to changes in glacially induced weathering this suggests that uniform interpretations with respect to the emplacement of major phosphorite deposits should be treated with caution integrated analyses of the sedimentary and biogeochemical context of phosphorite occurrences may help to identify paleoenvironmental conditions as well as to improve our understanding of periods of enhanced phosphate accumulation periods which were usually characterized by steep gradients in the development of climate and environment with regard to the complexity of feedback mechanisms between the phosphorus cycle and the biosphere the present day input rates of phosphate into the world s oceans should be of great concern they are more than doubled by anthropogenic means and affect ecological systems on a rapidly increasing scale

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

response of a stably stratified flow to cooling abstract the spreading of the low level cold pool produced by evaporation of precipitation is generally acknowledged to be an important mechanism for the regeneration of moist convection we show that cooling a stably stratified nocturnal boundary layer produces very different results from the corresponding daytime case in which the boundary layer is neutral in particular dynamic behavior is sometimes closer to that of a gravity wave than to a density current the gravity wave speed defines a minimum propagation speed for a self regenerating convective system

low p high t metamorphism in the okiep copper district western namaqualand south africa the okiep copper district part of the 1 2 1 0 ga high grade terrane in western namaqualand is composed of a mid proterozoic supracrustal sequence and several pre to post orogenic intrusive suites affected by two high grade events m2a m2b m3 of kibaran and one low grade event m4 of pan african age peak assemblages in quartz bearing pelites are characterized either by garnet cordierite coexisting with sillimanite biotite or by biotite sillimanite garnet a difference controlled by bulk composition and variation in water activities 0 1 0 7 during dehydration melting maximum p t conditions were reached during m2a coevally with the major deformational event d2a and are estimated at 750 820 c and 5 6 kbar a counterclockwise p t path is indicated by regionally occurring pseudomorphs of sillimanite after andalusite and by prograde reaction textures preserved as relics in m2a porphyroblasts two stages of retrograde metamorphism are distinguished m2a garnet cordierite bearing assemblages were retrogressed to biotite sillimanite quartz m2b along discontinuous foliation planes and shear zones d2b retrograde m3 corona assemblages formed at similar p t conditions 580 660 c and 5 8 0 5 kbar to the m2b assemblages but m3 crystallization postdates penetrative d2 deformation intrusion of 1 06 ga granitoids and formation of associated w mo deposits it is concluded that a kibaran high grade metamorphism in the okiep copper district is thermally punctuated and b reaction textures documenting apparent isobaric cooling of this low p high t terrane must be interpreted with caution

erosion and deposition from bottom currents during the givetian and frasnian response to intensified oceanic circulation between gondwana and laurussia abstract calcareous bottom current deposits in otherwise pelagic and hemipelagic successions of the harz mountains in germany herzyn limestone formation the eastern moroccan central massif ziar mrirt nappe and the carnic alps in austria italy valentin and pal limestone formation show strong evidence of bottom current reworking of carbonate ooze during givetian and early frasnian associated phosphates confirm that higher hydraulic energy temporarily prevailed the bottom current deposited facies builds up strongly condensed and reduced successions with stratigraphic gaps which also occur with similar features in other areas of central europe and northwest africa at the same stratigraphic interval the widespread current induced reworking of calcareous sediments and phosphate formation during the givetian and early frasnian as well as the associated erosional processes marked by pronounced hiatuses all signal a major circulation event the temporal and spatial positions of these fossil calcareous contourites in the devonian are discussed research concluded that thermohaline currents were intensified by the acceleration of flows constricted in the narrow oceanic passages between the approaching continental plates laurussia and gondwana areas affected were the southeastern rhenish sea shelf the disintegrated northern continental margin of gondwana and deep marginal plateaus of the noric terrane in the western part of the prototethys the disposition of the fossil calcareous contourites and faunal data corroborate palaeogeographic reconstructions that show an advanced convergence between gondwana and laurussia and the smaller continental plates such as the armorican and noric terranes during givetian and frasnian times

ocean currents and unconformities the north gondwana middle devonian middle devonian sedimentary features in many areas of the north gondwana margin today situated between spain and iran indicate influence of currents these features are deposits of contourites condensations and surfaces of nondeposition or erosion with paraconformity or erosional disconformity all common occurrences beneath modern ocean currents such as the gulf stream a paleogeographic reconstruction for the middle devonian is used to show how the interdependence of continental arrangement latitudinal position and ocean current activity on the margin can reconcile apparently conflicting stratigraphic and sedimentological features

contourite drift types and their distribution in the north and south atlantic ocean basins abstract patterns of sediment accumulation under the control of deep bottom currents depend on interacting factors intensity of the regional deep circulation morphology of the sea floor coriolis effects abundance of terrigenous and biogenic supplies subsequent turbidity of deep nepheloid layers and depth of the ccd based on many examples of bottom current deposits from the atlantic and from other oceans we recognise three major types of contourite accumulation 1 giant elongate drifts 2 contourite sheets and 3 channel related drifts they generally occur in distinct topographic settings and show different geometries facies and rates of deposition in the north atlantic strong bottom circulation an open basin system and high sediment supply have led to the construction of large elongate contourite drifts which have striking relief 200 1000 m above the surrounding sea floor and are composed of muddy contourites rich in calcareous pelagic material in the south atlantic strong bottom currents are often trapped in gyre like circulation patterns around topographically closed basins or are accelerated through deep narrow connecting passages the resulting deposits are mainly extensive contourite sheets covered by fields of large sediment waves and channel related drifts including contourite fans constructed at the downstream exit of deep passages

the potential role of atmospheric bores and gravity waves in the initiation and maintenance of nocturnal convection over the southern great plains abstractthis investigation explores the relationship among bores gravity waves and convection within the nocturnal environment through the utilization of measurements taken during the internation

multi sensor observations of a wave beneath an impacting rear inflow jet in an elevated mesoscale convective system observations of a wave generated below and propagating with an elevated mesoscale convective system mcs are presented the mcs and the wave tracked directly across the 90 km extent of the observational network deployed for the convective storm initiation project csip and maintained a coherent structure and velocity during this 1 5 hour period the rear inflow jet of the mcs was observed to impact upon but not penetrate the stable undercurrent that was located beneath the mcs apparently constricting the depth of the undercurrent which did not participate in the main convective updraught the wave crest was located approximately 15 km ahead of this constriction the overall wave led to between 200 and 1000 m of ascent and approximately 1500 m of descent however undulations were superimposed on the overall wave with a wavelength of approximately 7 km and a crest totrough amplitude of approximately 1000 m these undulations were associated with double maxima in surface pressure and the elevation of the observed low level wind anomaly where low level winds were backed by more than 90 t hese winds were never at rest relative to the system and the wave was therefore a gravity wave without stagnation the hydrostatic effect expected from the observed lifting in the wave was broadly consistent with the observed surface pressure perturbation of 2 hpa no cold pool outflow was ever observed instead the observed surface cooling of approximately 4 c was shown to be consistent with reduced solar insolation the generation of the observed wave structure and its implications for the maintenance of the mcs and numerical modelling of the system are discussed copyright c

the structure evolution and dynamics of a nocturnal convective system simulated using the wrf arw model abstractprevious studies have documented a nocturnal maximum in thunderstorm frequency during the summer across the central united states forecast skill for these systems remains relatively low and the explanation for this nocturnal maximum is still an area of active debate this study utilized the wrf arw model to simulate a nocturnal mesoscale convective system that occurred over the southern great plains on 3 4 june 2013 a low level jet transported a narrow corridor of air above the nocturnal boundary layer with convective instability that exceeded what was observed in the daytime boundary layer the storm was elevated and associated with bores that assisted in the maintenance of the system three dimensional variations in the system s structure were found along the cold pool which were examined using convective system dynamics and wave theory shallow lifting occurred on the southern flank of the storm conversely the southeastern flank had deep lifting with favorable integrated vertical shear ov

ultrahigh metamorphic temperatures over 1050 c recorded by fe ti oxides and implications for paleoproterozoic magma induced crustal thermal perturbation in jining area north china craton abstract a pseudobrookite approximately mg0 04fe2 0 85ca0 01al0 05fe3 0 16ti1 89o5 was discovered in fe ti oxide intergrowths in a spinel garnet bearing granulite from the xuwujia paleoproterozoic ultrahigh temperature uht metamorphic region within the khondalite belt of the north china craton the presence of this pseudobrookite indicates a peak temperature possibly over 1120 c when compared with experimental data ferric pseudobrookite grains fe3 up to 0 58 per formula unit from the same granulite outcrop record retrograde temperatures as low as 940 c titanomagnetite and ilmenite intergrowths that resulted from reduction exsolution which was buffered by the decomposition of high ti pseudobrookite grains recorded a post peak temperature up to 1050 c the thermal gradient 100 c kbar required to generate the uht metamorphic bodies in an area over 10 000 km2 around the jining area was modeled by the finite element method the thermal modeling results show that 1 lithospheric thermal erosion induced by a mantle plume with potential temperature of 1550 c could not have provided enough thermal perturbation to allow the observed uht metamorphism 2 underplating at the scale of even the largest known horizontal magma intrusion on earth would not be able to elevate the geotherm to the gradients required for the observed uht metamorphism and 3 intraplating of dykes and intrusions at the depth of this uht metamorphism was able to increase the local temperature field to the required gradients these modeling results suggest that a modern back arc may not be able to provide sufficient heat at the middle crust level for uht metamorphism the uht metamorphism in the study area was probably accompanied by an orogeny plume interaction event which simultaneously supplied heat through thermal erosion magma underplating and magma intraplating

a comparison of the finescale structures of a prefrontal wind shift line and a strong cold front in the southern plains of the united states abstractthe objectives of this study are to determine the finescale characteristics of the wind and temperature fields associated with a prefrontal wind shift line and to contrast them with those associated with a strong cold front data from a mobile polarimetric x band doppler radar and from a surveillance s band radar temperature profiles retrieved from a thermodynamic sounder and surface observations from the oklahoma mesonet are used to analyze a prefrontal wind shift line in oklahoma on 11 november 2013 data from the same mobile radar and the oklahoma mesonet are used to identify the finescale characteristics of the wind field associated with a strong surface cold front in oklahoma on 9 april 2013 it is shown that the prefrontal wind shift line has a kinematic and thermodynamic structure similar to that of an intrusion elevated density current while the cold front has a kinematic structure similar to that of a classic density current other characteristics of the prefrontal wind shift line

observations and simulation of elevated nocturnal convection initiation on 24 june 2015 during pecan abstractthe environment of elevated nocturnal deep convection initiation ci on 24 june 2015 is investigated using radiosonde data from the plains elevated convection at night pecan field experi

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

thermal buffering effects at the solidus implications for the equilibration of partially melted metamorphic rocks abstract a simple thermal model is used to investigate to what extent buffering effects of latent heat of fusion may prolong the thermal evolution of a one dimensional statically cooling partially or completely melted heat source for a melting model appropriate for crustal rocks the time of cooling of rocks through the solidus may be about tripled by this process depending on the melting model cooling may halt completely near the solidus for time spans comparable to the thermal time constant of the heat source in order to test the influence of this buffering effect on the equilibration of partially melted metamorphic rocks these thermal model results are coupled with a simple diffusion model that relates the degree of equilibration of a simple assemblage to temperature and cooling rate it is shown that for a broad range of initial heat source temperatures freezing of selected mineral equilibria may occur in a comparably narrow temperature range at and around the solidus the calculations may have some relevance to low p high t metamorphic terranes in as much as they may provide an explanation for the widely observed equilibration and partial re equilibration of rocks near the crystallisation temperatures of partial melts

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

partial melting and the formation of granulite facies assemblages in namaqualand south africa abstract dehydration melting reactions in which water from a hydrous phase enters the melt leaving an anhydrous solid assemblage are the dominant mechanism of partial melting of high grade rocks in the absence of externally derived vapour equilibria involving melt and solid phases are effective buffers of ah2 o the element partitioning observed in natural rocks suggests that dehydration melting occurs over a temperature interval during which for most cases ah2o is driven to lower values the mass balance of dehydration melting in typical biotite gneiss and metapelite shows that the proportion of melt in the product assemblage at t 850 c is relatively small 10 20 and probably insufficient to mobilize a partially melted rock body granulite facies metapelite biotite gneiss and metabasic gneiss in namaqualand contain coarse grained discordant unfoliated anhydrous segregations surrounded by a finer grained foliated matrix that commonly includes hydrous minerals the segregations have modes consistent with the hypothesis that they are the solid and liquid products of the dehydration melting reactions bt sil qtz pi grt crd kfs l metapelite bt qtz pl opx kfs l biotite gneiss and hbl qtz opx cpx pl l metabasic gneiss the size shape distribution and modes of segregations suggest only limited migration and extraction of melt growth of anhydrous poikiloblasts in matrix regions development of anhydrous haloes around segregations and formation of dehydrated margins on metabasic layers enclosed in migmatitic metapelites all imply local gradients in water activity also they suggest that individual segregations and bodies of partially melted rock acted as sinks for soluble volatiles the preservation of anhydrous assemblages and the restricted distribution of late hydrous minerals suggest that retrograde reaction between hydrous melt and solids did not occur and that h2o in the melt was released as vapour on crystallization this model combined with the natural observations suggests that it is possible to form granulite facies assemblages without participation of external fluid and without major extraction of silicate melt

the metamorphic evolution of granulites at fyfe hills implications for archaean crustal thickness in enderby land antarctica granulites at fyfe hills in enderby land antarctica crystallized at temperatures in excess of 850 c and possibly as high as 1000 c and at pressures of 8 10kbar during the mid to late archaean a number of features including repeated retrograde metamorphism at 5 5 8kbar retrograde reaction textures and rimward zoning in pressure sensitive systems suggest that following peak metamorphism the granulites stabilized at a depth of 18 26 km after stabilization the granulites cooled near isobarically to temperatures of 600 700 c assuming a total crustal thickness of 35 40 km during this late archaean interval of isobaric cooling the peak metamorphic crustal thickness is estimated at 35 56 km this estimate is significantly less than the 60 70 km obtained by summing the depths of the present levels of exposure 26 34 km and the thickness of the crust presently beneath fyfe hills approxi mately 35km and is therefore consistent with independent evidence for extensive post archaean thickening of the enderby land crust

interactions between upper and lower tropospheric gravity waves on squall line structure and maintenance abstract using a simplified thermodynamic sounding and variable vertical wind shear we investigate the role of gravity waves on the structure and propagation of a simulated two dimensional squall line based on an observed squall line environment the modeled troposphere has been divided into three distinct thermodynamic layers these consist of an absolutely stable atmospheric boundary layer abl an elevated well mixed layer and an upper tropospheric layer of intermediate stability we find the mixed layer to have a dual role it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts and it provides an ideal layer to trap meso scale 20 200 km wave energy generated in the stable layers the generated waves thus have a significant and lasting impact on the simulation we also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long lived squall lines experiments that vary the vertical wind shear profile demonstrate

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

the relation between nocturnal mcs evolution and its outflow boundaries in the stable boundary layer an observational study of the 15 july 2015 mcs in pecan abstractthe vertical structures of a leading outflow boundary ahead of a continental nocturnal mcs and of the upstream environment are examined in order to answer the question of whether this verti

the formation of internal bores in the atmosphere a laboratory model summary recently several atmospheric observations have been interpreted as internal undular bores or internal solitary waves evolving from internal bores that propagate along low level temperature inversions it has been speculated that such disturbances are generated by some type of gravity current such as cold fronts sea breeze fronts and thunderstorm outflows interacting with an existing temperature inversion in this paper we describe a systematic laboratory study in a water channel of internal bores and their generation by the movement of gravity currents through a two layer model of the atmosphere we compare the results of our laboratory experiments with previous theories and numerical simulations and with several detailed atmospheric observations of internal bores at different stages of development