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

new evidence from the western canadian arctic archipelago for the resubmergence of bering strait widespread molluscan samples were collected from raised marine sediments to date the last retreat of the nw laurentide ice sheet from the western canadian arctic archipelago at the head of mercy bay northern banks island deglacial mud at the modern coast contains hiatella arctica and portlandia arctica bivalves as well as cyrtodaria kurriana previously unreported for this area multiple h arctica and c kurriana valves from this site yield a mean age of 11 5 14c ka bp with 740 yr marine reservoir correction the occurrence of c kurriana a low arctic taxon raises questions concerning its origin because evidence is currently lacking for a molluscan refugium in the arctic ocean during the last glacial maximum elsewhere the oldest late glacial age available on c kurriana comes from the laptev sea where it is 10 3 14c ka bp and attributed to a north atlantic source this is 2000 cal yr younger than the mercy bay samples reported here making the laptev sea 3000 km to the west an unlikely source an alternate route from the north atlantic into the canadian arctic archipelago was precluded by coalescent laurentide innuitian and greenland ice east of banks island until 10 14c ka bp we conclude that the presence of c kurriana on northern banks island records migration from the north pacific this requires the resubmergence of bering strait by 11 5 14c ka bp extending previous age determinations on the reconnection of the pacific and arctic oceans by up to 1000 yr this renewed ingress of pacific water likely played an important role in re establishing arctic ocean surface currents including the evacuation of thick multi year sea ice into the north atlantic prior to the younger dryas geochron

fault reactivation control on normal fault growth an experimental study field studies frequently emphasize how fault reactivation is involved in the deformation of the upper crust however this phenomenon is generally neglected except in inversion models in analogue and numerical models performed to study fault network growth using sand silicon analogue models we show how pre existing discontinuities can control the geometry and evolution of a younger fault network the models show that the reactivation of pre existing discontinuities and their orientation control i the evolution of the main fault orientation distribution through time ii the geometry of relay fault zones iii the geometry of small scale faulting and iv the geometry and location of fault controlled basins and depocenters these results are in good agreement with natural fault networks observed in both the gulf of suez and lake tanganyika they demonstrate that heterogeneities such as pre existing faults should be included in models designed to understand the behavior and the tectonic evolution of sedimentary basins

the pretornadic phase of the goshen county wyoming supercell of 5 june 2009 intercepted by vortex2 part i evolution of kinematic and surface thermodynamic fields abstractthe authors analyze the pretornadic phase 2100 2148 utc tornadogenesis began at 2152 utc of the goshen county wyoming supercell of 5 june 2009 intercepted by the second verification of the origins of rotation in tornadoes experiment vortex2 the analysis relies on radar data from the weather surveillance radar 1988 doppler wsr 88d in cheyenne wyoming kcys and a pair of doppler on wheels dow radars mobile mesonet observations and mobile sounding observations the storm resembles supercells that have been observed in the past for example it develops a couplet of counter rotating vortices that straddle the hook echo within the rear flank outflow and are joined by arching vortex lines with the cyclonic vortex becoming increasingly dominant in the time leading up to tornadogenesis the outflow in the hook echo region where sampled has relatively small virtual potential temperature deficits during this stage of evolution a few kilometers upstream north of the location of maxim

an observational and modeling study of mesoscale air masses with high theta e abstracttypically the cool side of an airmass boundary is stable to vertical motions due to its associated negative buoyancy however under certain conditions the air on the cool side of the bou

rear flank outflow dynamics and thermodynamics in the 10 june 2010 last chance colorado supercell abstracton 10 june 2010 the second verification of the origins of tornadoes experiment vortex2 armada observed a supercell thunderstorm near last chance colorado tempest unmanned aircraft system uas data collected in the rear flank outflow revealed what appeared to be an elevated outflow head turbulent wake and a cold rear flank internal surge rfis surface thermodynamic and kinematic data collected by the sticknet and mobile mesonet indicated that the outflow wake may have extended to or very near the surface perhaps modifying or outright replacing the leading edge of the outflow at times single doppler data collected by the noaa x pol mobile polarimetric doppler radar noxp were supportive of the possibility of a downdraft in the outflow wake associated with low level divergence a conceptual model of the hypothesized rear flank outflow structure in the nontornadic phase of the last chance supercell is presented the observed turbulent wake is consistent with mixing associated with the rel

the cause of internal outflow surges in a high resolution simulation of the 8 may 2003 oklahoma city tornadic supercell abstracta high resolution simulation of the 8 may 2003 oklahoma tornadic supercell is analyzed to determine the origin of internal outflow surges within the low level cold pool the analyzed simulation has 50 m horizontal grid spacing and is quadruply nested within larger lower resolution domains that were initialized via three dimensional variational data assimilation 3dvar of radar and other observations the high resolution simulation produces two tornadoes that track in close proximity to the observed tornado on 8 may 2003 the authors previous study determined that an internal outflow surge instigated tornadogenesis for the first tornado in this simulation but the cause of this internal outflow surge was unclear in this study the vertical momentum equation is analyzed along backward trajectories that are initialized within the tornado triggering internal outflow surge the analysis reveals that the internal outflow surge is forced by the dynamic part of the vertical pressure gradient further ex

vortex2 observations of a low level mesocyclone with multiple internal rear flank downdraft momentum surges in the 18 may 2010 dumas texas supercell abstractobservations collected in the second verification of the origins of rotation in tornadoes experiment during a 15 min period of a supercell occurring on 18 may 2010 near dumas texas are presented the primary data collection platforms include two ka band mobile doppler radars which collected a near surface short baseline dual doppler dataset within the rear flank outflow of the dumas supercell an x band phased array mobile doppler radar which collected volumetric single doppler data with high temporal resolution and in situ thermodynamic and wind observations of a six probe mobile mesonet rapid evolution of the dumas supercell was observed including the development and decay of a low level mesocyclone and four internal rear flank downdraft rfd momentum surges intensification and upward growth of the low level mesocyclone were observed during periods when the midlevel mesocyclone was minimally displaced from the low level circulation suggesting an upward directed perturbation pressure g

a dual polarization radar based assessment of the 8 may 2003 oklahoma city area tornadic supercell abstract on 8 may 2003 a tornadic supercell tracked through portions of the oklahoma city oklahoma metropolitan area and produced violent damage along portions of its path this storm passed through the dense in situ radar network in central oklahoma and provided close range operational prototype polarimetric and terminal doppler weather radar observations of the storm as it made the transition into the tornadic phase the time evolving polarimetric features were scrutinized with regard to storm morphology particularly as related to the development of a rear flank downdraft pulse within the storm immediately preceding the long track tornado event two new polarimetric terms are introduced the zdr shield and kdp foot along with a discussion of the orientation of the zdr and kdp columns relative to midlevel rotation signatures storm downdraft and gust front characteristics are discussed relative to polarimetric fields and background environment characteristics highlighted for this event are a warm

influence of rift obliquity on fault population systematics results of experimental clay models abstract we use clay models to simulate how fault population systematics vary as a function of rift obliquity rift obliquity is related to the acute angle between the rift trend and the displacement direction so that the value of is inverse to the degree of obliquity the range of azimuths in a fault population increases as rift obliquity increases i e as decreases the length of the longest faults the sum of fault lengths and the width of the deformed zone all increase as rift obliquity decreases i e as increases the majority of faults in our models are segmented and have highly tortuous traces tortuosity is maximum when 30 as segments of widely varying azimuth link during fault growth significant changes occur in the fault patterns between 30 and 45 at 30 two fault populations of equal importance develop in the center of the rift zone one approximately rift parallel and the other displacement normal between 30 and 45 the number of faults more than doubles and between 45 and 60 summed fault length more than doubles fault patterns for all models are fractal with fractal dimensions that increase with increasing and that are comparable to those found in the field fault populations are not multi fractal because the cumulative frequency distributions of fault lengths do not generally follow a power law relationship an exponential distribution best describes the data for whole faults with the characteristic length increasing with increasing segment lengths also follow an exponential distribution with characteristic length varying very little with the fault patterns in our models resemble the spatial pattern of brittle deformation observed at oblique mid ocean ridge segments and are similar in geometry to those in oblique continental rift basins at the rift margins tensional stresses are modulated and reoriented by a secondary stress field related to a change in boundary conditions resulting in the formation of two distinct sub populations of faults during oblique rifting

pore network characteristics of lacustrine shales in the dongpu depression bohai bay basin china with implications for oil retention abstract pore development and the location of retained hydrocarbons are key topics to understand the shale oil system in an attempt to investigate pore network characteristics and oil retention in lacustrine shales 24 samples from the es3 shales in the dongpu depression bohai bay basin were subjected to mineralogical petrological geochemistry and pore structure investigations using x ray diffraction xrd thin section field emission scanning electronic microscope fe sem n2 adsorption desorption after removing retained oil by solvent extraction 12 selected samples were again analyzed by rock eval pyrolysis and n2 adsorption desorption experiment according to the mineral compositions and petrological characteristics six types of lithofacies are identified in the es3 shales in the dongpu depression laminated calcareous shales lenticular calcareous shales laminated argillaceous shales massive argillaceous shales laminated siliceous shales and massive siliceous shales geochemical characteristics of these shales are quite different as calcareous laminated shales have higher toc contents than massive argillaceous and siliceous shales interparticle pores intraparticle pores organic matter pores and fracture pores are observed in the studied samples while bitumens were found retaining in well connected pore network the n2 adsorption desorption experiments on samples before and after solvent extraction reveal the different pore structure characteristics in six lithofacies and the occurrence of oil in lacustrine shales in general pore volumes within pore diameter range of 1 7 10nm and 10 200nm of samples show increase after solvent extraction indicating that large and small pores in shales both play important roles in oil retention oil retained in shales can be divided as free and heavy fractions as from the pyrolysis results in lacustrine shales larger pores are main storage space for hydrocarbon larger pores in different lithologies have similar capacity for free oil retention while those in calcareous and argillaceous shales are more efficient for heavy oil retention for both free and heavy oil small pores in argillaceous shales are efficient for their storage

comment on multi proxy record of postglacial environmental change south central melville island northwest territories canada by peros et al 2010 quaternary research 73 247 258

mechanics of extensional wedges the extensional wedges that overlie low angle basal detachment faults are characterized by a tapered cross section similar to that exhibited by a fold and thrust belt but with an opposite sense of shear on the basal fault such wedges are common in zones of crustal extension such as the basin and range province of the north american cordillera the north sea and the u s gulf coast this paper examines the mechanics of these upper crustal extensional wedges using a modification of the compressional coulomb critical taper model developed for fold and thrust belts and accretionary wedges if a compressional wedge is mechanically analogous to a wedge of soil being pushed up an incline by a moving bulldozer then an extensional wedge is analogous to the same wedge with the bulldozer moving in reverse gear down the incline a critically tapered extensional wedge is one that is on the verge of coulomb failure everywhere a wedge whose taper is narrower may be slid stably down the same incline without internal deformation a wedge whose taper is greater will fail by normal faulting and reduce its taper until it is critical after testing the quantitative predictions of this extensional critical taper theory in a series of laboratory experiments on dry sand wedges we apply the theory to a pair of active extensional wedges in the brazos area offshore texas both wedges are underlain by well imaged basal detachment faults dipping at 17 o down to depths of 10 km one wedge exhibits relatively little internal deformation whereas the other is pervasively faulted along a series of moderately dipping synthetic normal faults direct and indirect estimates of the pore fluid pressure distribution within the two wedges suggest that the undeformed wedge is stable whereas the pervasively faulted wedge is unstable but nearly critical regional values for the coefficients of basal and internal friction in excess of 0 3 together with vertical cohesion gradients in excess of 1 kpa m are consistent with the observed pore pressures and the geometry of the wedges as well as with in situ stress data acquired during drilling and the dips of active synthetic normal faults such values are comparable to values inferred from mechanical and thermal models of compressional wedges and they are consistent with laboratory rock mechanics data

reactivation tendency analysis a theory for predicting the temporal evolution of preexisting weakness under uniform stress state abstract understanding the mechanical controls on reactivation of preexisting weakness is a fundamental problem in tectonic studies in this study we develop a theoretical framework for evaluating the likelihood of seismicity on active faults and the sequence of reactivation of multiple sets of preexisting weakness our analysis overcomes the restrictions of the early work that assumes uniform coefficient of friction across all preexisting planes with negligible cohesive strength and vertical or horizontal orientations of principal stresses using a coordinate system transformation we developed a new graphical technique that can express the reactivation problem in the mohr space with the principal stresses oriented neither vertically nor horizontally we verified the predicted sequence of reactivation on preexisting weakness and initiation of new fractures according to the byerlee law and the coulomb fracture criterion by conducting a simple sandbox experiment our proposed reactivation tendency analysis can be expanded to evaluate the likelihood of seismicity on a single fault with curvi planar geometry e g listric normal faults ramp flat thrusts and strike slip faults with restraining and releasing bend geometry our analysis also provides a new insight into the mechanical cause of temporal evolution of pre existing weakness in geologic records finally our work implies that the sequence of faulting reactivated from preexisting weakness provides critical information on the mechanical properties of the deforming lithosphere

4 d evolution of rift systems insights from scaled physical models the four dimensional 4 d evolution of brittle fault systems in orthogonal oblique and offset rift systems has been simulated by scaled sandbox models using dry cohesionless fine grained quartz sand extensional deformation in the models was controlled by the orientation and geometry of a zone of stretching at the base of the model the results of these analog model studies are compared with natural examples of rift fault systems rift basins produced by orthogonal and oblique rifting are defined by segmented border fault systems parallel to the rift axes and by intrarift fault systems that are subperpendicular to the extension direction segmentation of the rift margin increases with increase in obliquity of the rift axis resulting in a consequent increase in displacement on intrarift fault systems offset rift models are characterized by highly segmented border faults and offset subbasins in the rift zone along strike displacement transfer in the model rifts occurred as a result of formation of two types of accommodation zones high relief extension parallel accommodation zones typically are found in 60 degrees rifts and above left steps in offset rift systems changes in fault polarities in these accommodation zones were achieved by interlocking arrays of conjugate extensional faults the second type of accommodation zone was generally oblique to the extension direction and consisted of conjugate fault arrays having rotated tips that bounded a low relief oblique slip zone or grabens these typically are found in highly oblique rift systems 45 degrees and above right steps in offset rift models

cenozoic structural deformation and dynamic processes of the bohai bay basin province china abstract cenozoic structures in the bohai bay basin province can be subdivided into eleven extensional systems and three strike slip systems the extensional systems consist of normal faults and transfer faults the normal faults predominantly trend nne and ne and their attitudes vary in different tectonic settings paleogene rifting sub basins were developed in the hanging walls of the normal faults that were most likely growth faults neogene quaternary sequences were deposited in both the rifting sub basins and horsts to form a unified basin province the extensional systems were overprinted by three nne trending right lateral strike slip systems fault zones although the principal displacement zones pdz of the strike slip fault zones are developed only in the basement and lower basin sequences in some cross sections the structural deformation characteristics of the upper basin sequences also indicate that they are basement involved right lateral strike slip fault zones according to the relationships between faults and sedimentary sequences the extensional systems were mainly developed from the middle paleocene to the late oligocene whereas the strike slip systems were mainly developed from the oligocene to the miocene strike slip deformation was intensified as extensional deformation was weakened extensional deformation was derived from horizontal tension induced by upwelling of hot mantle material whereas strike slip deformation was probably related to a regional stress field induced by plate movement

vortex lines within low level mesocyclones obtained from pseudo dual doppler radar observations abstract vortex lines passing through the low level mesocyclone regions of six supercell thunderstorms three nontornadic three tornadic are computed from pseudo dual doppler airborne radar observations obtained during the verification of the origins of rotation in tornadoes experiment vortex in every case at least some of the vortex lines emanating from the low level mesocyclones form arches that is they extend vertically from the cyclonic vorticity maximum then turn horizontally usually toward the south or southwest and descend into a broad region of anticyclonic vertical vorticity this region of anticyclonic vorticity is the same one that has been observed almost invariably to accompany the cyclonic vorticity maximum associated with the low level mesocyclone the vorticity couplet straddles the hook echo of the supercell thunderstorm the arching of the vortex lines and the orientation of the vorticity vector along the vortex line arches compared to the orientation of the ambient barotrop

finescale single and dual doppler analysis of tornado intensification maintenance and dissipation in the orleans nebraska supercell abstract finescale single and dual doppler observations are used to diagnose the three dimensional structure of the wind field surrounding a tornado that occurred near the town of orleans nebraska on 22 may 2004 the evolution of the vorticity and divergence fields and other structures near the tornado are documented in the lowest kilometer changes in tornado intensity are compared to the position of the tornado relative to primary and secondary gust fronts circulation on scales of a few kilometers surrounding the tornado remains relatively constant during the analysis period which spans the intensifying and mature periods of the tornado s life cycle stretching of vertical vorticity and tilting of horizontal vorticity are diagnosed but the latter is near or below the threshold of detectability in this analysis during the observation period in the analyzed domain low level circulation within 500 m of the tornado increased several minutes before vortex relative and ground relative near surface wind

a study of the tornadic region within a supercell thunderstorm abstract the transition of a supercell thunderstorm into its tornadic phase is investigated through high resolution numerical cloud model simulations initiated within the interior portion of a previously simulated mature supercell storm with the enhanced grid resolution the low level cyclonic vorticity increases dramatically and the gust front rapidly occludes as small scale downdrafts develop in the vicinity of the low level center of circulation as the occlusion progresses a ring of high vorticity air surrounds the circulation center and could be conducive to multiple vortex tornado formation numerous features of the simulated transition bear resemblance to those observed in tornadic storms in the model simulation the large low level vorticity is generated through the tilting and intense stretching of air from the inflow side of the storm this vertical vorticity is derived from the horizontal vorticity of the environmental shear and also from horizontal vorticity generated solenoidally as low l

revision of the nw laurentide ice sheet implications for paleoclimate the northeast extremity of beringia and arctic ocean sedimentation abstract for the past half century reconstructions of north american ice cover during the last glacial maximum have shown ice free land distal to the laurentide ice sheet primarily on melville and banks islands in the western canadian arctic archipelago both islands reputedly preserve at the surface multiple laurentide till sheets together with associated marine and lacustrine deposits recording as many as three pre late wisconsinan glaciations the northwest corner of banks island was purportedly never glaciated and is trimmed by the oldest and most extensive glaciation banks glaciation considered to be of matuyama age 780 ka bp inside the limit of banks glaciation younger till sheets are ascribed to the thomsen glaciation pre sangamonian and the amundsen glaciation early wisconsinan stade the view that the western canadian arctic archipelago remained largely ice free during the late wisconsinan is reinforced by a recent report of two woolly mammoth fragments collected on banks and melville islands both dated to 22 ka bp these dates imply that these islands constitute the northeast extremity of beringia a fundamental revision of this model is now warranted based on widespread fieldwork across the adjacent coastlines of banks and melville islands including new dating of glacial and marine landforms and sediments on dundas peninsula southern melville island ams 14 c dates on ice transported marine molluscs within the most extensive laurentide till yield ages of 25 49 ka bp these dates require that late wisconsinan ice advanced northwestward from visount melville sound excavating fauna spanning marine isotope stage 3 laurentide ice that crossed dundas peninsula 300 m asl coalesced with melville island ice occupying liddon gulf coalescent laurentide and melville ice continued to advance westward through m clure strait depositing granite erratics at 235 m asl that require grounded ice in m clure strait as do streamlined bedforms on the channel floor deglaciation is recorded by widespread meltwater channels that show both the initial separation of laurentide and melvile ice and the successive retreat of laurentide ice southward across dundas peninsula into viscount melville sound sedimentation from these channels deposited deltas marking deglacial marine limit forty dates on shells collected from associated glaciomarine rhythmites record near synchronous ice retreat from m clure strait and dundas peninsula to north central victoria island 11 5 ka bp along the adjacent coast of banks island deglacial shorelines also record the retreat of laurentide ice both eastward through m clure strait and southward into the island s interior the elevation and age 11 5 ka bp of deglacial marine limit there is fully compatible with the record of ice retreat on melville island the last retreat of ice from mercy bay northern banks island previously assigned to northward retreat into m clure strait during the early wisconsinan is contradicted by geomorphic evidence for southward retreat into the island s interior during the late wisconsinan this revision of the pattern and age of ice retreat across northern banks island results in a significant simplification of the previous quaternary model our observations support the amalgamation of multiple till sheets previously assigned to at least three pre late wisconsinan glaciations into the late wisconsinan this revision also removes their formally named marine transgressions and proglacial lakes for which evidence is lacking erratics were also widely observed armouring meltwater channels originating on the previously proposed never glaciated landscape an extensive late wisconsinan laurentide ice sheet across the western canadian arctic is compatible with similar evidence for extensive laurentide ice entering the richardson mountains yukon farther south and with the innuitian ice sheet to the north widespread late wisconsinan ice in a region previously thought to be too arid to sustain it has important implications for paleoclimate ice sheet modelling arctic ocean ice and sediment delivery and clarifying the northeast limit of beringia

uncertainties in trajectory calculations within near surface mesocyclones of simulated supercells abstractthis study addresses the sensitivity of backward trajectories within simulated near surface mesocyclones to the spatiotemporal resolution of the velocity field these backward trajectories are compared to forward trajectories computed during run time within the numerical model it is found that the population of backward trajectories becomes increasingly contaminated with inflow trajectories that owe their existence to spatiotemporal interpolation errors in time varying and strongly curved confluent flow these erroneous inflow parcels may mistakenly be interpreted as a possible source of air for the near surface vortex it is hypothesized that unlike forward trajectories backward trajectories are especially susceptible to errors near the strongly confluent intensifying vortex although the results are based on model output dual doppler analysis fields may be equally affected by such errors

finescale vertical structure of a cold front as revealed by an airborne doppler radar abstract in the afternoon of 24 may 2002 a well defined and frontogenetic cold front moved through the texas panhandle detailed observations from a series of platforms were collected near the triple point between this cold front and a dryline boundary this paper primarily uses reflectivity and doppler velocity data from an airborne 95 ghz radar as well as flight level thermodynamic data to describe the vertical structure of the cold front as it intersected with the dryline the prefrontal convective boundary layer was weakly capped weakly sheared and about 2 5 times deeper than the cold frontal density current the radar data depict the cold front as a fine example of an atmospheric density current at unprecedented detail 40 m the echo structure and dual doppler inferred airflow in the vertical plane reveal typical features such as a nose a head a rear inflow current and a broad current of rising prefrontal air that feeds the accelerating front to rear current over the head the 2d cross fro

an assessment of low level baroclinity and vorticity within a simulated supercell abstractidealized supercell modeling has provided a wealth of information regarding the evolution and dynamics within supercell thunderstorms however discrepancies in conceptual models exist including uncertainty regarding the existence placement and forcing of low level boundaries in these storms as well as their importance in low level vorticity development this study offers analysis of the origins of low level boundaries and vertical vorticity within the low level mesocyclone of a simulated supercell low level boundary location shares similarities with previous modeling studies however the development and evolution of these boundaries differ from previous conceptual models the rear flank gust front develops first whereas the formation of a boundary extending north of the mesocyclone undergoes numerous iterations caused by competing outflow and inflow before a steady state boundary is produced a third boundary extending northeast of the mesocyclone is produced through evaporative cooling of

the bowdle south dakota cyclic tornadic supercell of 22 may 2010 surface analysis of rear flank downdraft evolution and multiple internal surges abstractmobile mesonet sampling in the hook echo rear flank downdraft rfd region of a tornadic supercell near bowdle south dakota provided the opportunity to examine rfd thermodynamic and kinematic attributes and evolution focused analysis of the fifth low level mesocyclone cycle that produced two significant tornadoes including a violent tornado revealed four rfd internal surge rfdis events rfdiss appeared to influence tornado development intensity and demise by altering the thermodynamic and kinematic character of the rfd region bounding the pretornadic and tornadic circulations significant tornadoes developed and matured when the rfd modulated by internal surges was kinematically strong only weakly negatively buoyant and very potentially buoyant in contrast the demise of the bowdle tornado was concurrent with a much cooler rfdis that replaced more buoyant and far more potentially buoyant rfd air near the tornado this surge also likely contributed to a displacement of the tornado from

simulated density currents in idealized stratified environments abstract the effects of three distinct stratifications on density current dynamics are investigated using a nonhydrostatic numerical model i a stably stratified layer underneath a deep neutrally stratified flow representing a nocturnal boundary layer over land ii a neutrally stratified layer underlying a deep stably stratified flow representing a daytime boundary layer and iii a continuously stratified atmosphere in the first case a weak or intermediate stratification decreases the height of density currents and increases the propagation speed the same result holds in strongly stratified situations as long as the generated disturbances in the neighborhood of the head do not propagate away classical density currents occur in weak stratification multiheaded density currents in intermediate stratification and multiheaded density currents with solitary wave like or borelike disturbances propagating ahead of the current in strong stratification in the second case the upper layer stratificati

predictive modelling of structure evolution in sandbox experiments in this paper a computational approach is presented that is able to forward model complex structural evolution with multiple intersecting faults that exhibit large relative movement the approach adopts the lagrangian method complemented by robust and efficient automated adaptive meshing techniques a constitutive model based on critical state concepts and global energy dissipation regularized by inclusion of fracture energy in the equations governing state variable evolution the efficacy of the approach is benchmarked by forward simulation of two extensional analogue experiments that exhibit the development of a roll over anticline with a series of superimposed crestal collapse graben systems these sandbox experiments are excellent benchmarks for computational models as both intersecting localisations with different rates of slip and large relative movement on localisations must be represented while other complex phenomena associated with structural evolution over geological timeframes are not present

single and dual doppler analysis of a tornadic vortex and surrounding storm scale flow in the crowell texas supercell of 30 april 2000 abstract fine resolution single and dual doppler data were collected in the tornadic region of a supercell storm intercepted by two doppler on wheels radars on 30 april 2000 near crowell texas eleven dual doppler analyses characterize the 2d and 3d near surface wind fields associated with a tornado during a 13 min period an interesting evolution of the low level rotation is observed initially concentric tornado 500 m wide and tornado cyclone 2 km wide radar velocity couplets make a transition into a solitary intermediate sized 750 m wide circulation that widens and makes a further transition into a two celled multiple vortex structure with an asymmetric distribution of vertical vorticity the asymmetry and eventual disruption of the multiple vortex structure may have been partially controlled by locally strong outflow winds that affect the convergence fields in its vicinity a smaller 500 m wide tornado embedded in a broad area of rotation is subsequently observed the dual doppler wi

an experimental study of the secondary deformation produced by oblique slip normal faulting we have used scaled clay models to define the secondary deformation produced by oblique slip normal faulting in the models the master fault beneath the clay layer dips 45 degrees and strikes at a 45 degrees angle relative to the heave direction i e the horizontal component of the displacement direction thus the master fault has both normal dip slip and strike slip components of displacement the modeling results show the following 1 the fault patterns produced by oblique slip normal faulting vary significantly with depth secondary faults that strike obliquely to the master fault trend are more abundant near the top of the clay layer whereas secondary faults that are subparallel to the master fault trend are more abundant at depth 2 a single episode of oblique slip normal faulting produces two populations of secondary faults that have different trends and ages secondary faults that strike obliquely to the master fault trend are more abundant during the early stages of the experiments whereas secondary faults that strike subparallel to the master fault trend are more abundant during the later stages of the experiments 3 relay ramps between overlapping secondary synthetic normal faults are wide and temporally persistent in oblique slip models the ramps are cut by numerous small scale normal faults that are subparallel to the ramp bounding faults cross faults are uncommon and begin to develop only during the final stages of the experiments 4 both map and cross section data are necessary to distinguish among the deformation patterns produced by strike slip oblique slip and dip slip faulting the map views of oblique slip models closely resemble those of strike slip models in both models many of the secondary faults strike obliquely to the master fault trend these map views however differ considerably from those of dip slip models in which most of the secondary faults strike subparallel to the master fault trend alternatively the cross sectional views of oblique slip models are similar to those of dip slip models in both models a highly faulted extensional forced fold develops these cross sectional views are dissimilar to those of strike slip models which show no appreciable folding and no change of regional level

the innuitian ice sheet configuration dynamics and chronology abstract portrayal of north american ice cover during the last glacial maximum is dominated by the laurentide ice sheet leaving little detail for the adjacent innuitian ice sheet iis four decades of geological fieldwork across the queen elizabeth islands now warrant specific treatment of the iis including its chronology configuration dynamics and retreat this reconstruction is relevant to the sedimentary history of the arctic ocean and to high latitude climate forcing the iis was composed of both an alpine and lowland sector the advance of the alpine sector occurred as recently as 19 14 c ka bp geological evidence configures outflow from alpine and lowland divides that produced several palaeo ice streams one extending northwestward across the canadian arctic archipelago to the polar continental shelf retreat of the iis commenced along its southwest margin 11 6 14 c ka bp however most of the ice sheet remained on the continental shelf during the younger dryas by 10 14 c ka bp marine based ice experienced widespread calving through the western and central archipelago in response to holocene warming and ongoing eustatic sea level rise the sea penetrated the eastern archipelago by 8 5 14 c ka bp gutting the alpine sector of the iis regional isobases record the glacioisostatic signature of the ice sheet and are congruent with the primary geological evidence the delayed buildup of the iis was out of phase with the growth of the laurentide ice sheet that occasioned climatic and glacio eustatic forcing in the innuitian region recent modelling experiments reinforce the hypothesis that growth of the laurentide ice sheet culminated in a split jet stream that temporarily favoured augmented precipitation and growth of the iis

tornadogenesis in the 12 may 2010 supercell thunderstorm intercepted by vortex2 near clinton oklahoma abstractthe 12 may 2010 supercell thunderstorm intercepted by the second verification of the origins of rotation in tornadoes experiment vortex2 is analyzed during a time period of strong low lev

linear and nonlinear propagation of supercell storms abstract a nonlinear formula for updraft motion in supercell storms is derived from petterssen s formula for the motion of systems and the vertical equation of motion and tested on form preserving disturbances at each level continuous propagation of an updraft maximum is determined largely by the horizontal gradient of the nonhydrostatic vertical pressure gradient force nhvpgf at the updraft center the nhvpgf is deduced from the formal solution of the poisson equation for nonhydrostatic pressure in anelastic flow subject to homogeneous neumann boundary conditions at the ground and top boundary recourse also is made to published fields of partitioned vertical pressure gradient force updraft motion is partitioned into parts forced by horizontal gradients of hydrostatic pressure linear interaction between the environmental shear and updraft and nonlinear dynamical effects the dynamics of supercell storms for nearly straight and highly curved hodographs are found to be different nonlinear rotation

effect of block rotation on fault sealing an example in dongpu sag bohai bay basin china abstract pre existing faults or early formed faults may be reactivated or locked in subsequent evolution in a rift basin the reactivation or sealing of a fault affects sedimentation in the related fault block and in turn controls the accumulation of oil and gas based on fault mechanics of reactivation this paper established equations denoting the angle range of the reactivated faults or frictional sliding faults by using mohr stress circles the results were applied to explain the sealing property of pre existing faults or early formed faults in dongpu sag in the dongpu sag reactivation or sealing of the faults in fault blocks is supposed due to the rotation of the fault block which was caused by the activity of high order faults for quantitative calculation of dip range for activated faults the sum of the maximum and minimum principal stresses were determined by acoustic emission experiment based on the kaiser effect of rocks and the cohesion and inner frictional angle of rocks were measured by rock physical experiment the calculations show when the dips of early formed faults are from 87 to 45 the faults will remain activation in the subsequent deformation otherwise the faults will be locked as sealing faults the results of petroleum exploration show that the peak dips of sealing faults are from 36 to 40 and the most of sealing faults are those with dip angles less than 46 50 these results indicate that a shallower fault is of better sealing property in oil and gas reservoirs than a steeper fault in dongpu sag

talc bearing serpentinite and the creeping section of the san andreas fault the section of the san andreas fault located between cholame valley and san juan bautista in central california creeps at a rate as high as 28 mm yr 1 ref 1 and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust serpentinized ultramafic rocks have been associated with creeping faults in central and northern california and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the san andreas fault however the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength and these minerals also have the potential for unstable slip under some conditions here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the san andreas fault that was intersected during drilling of the san andreas fault observatory at depth safod main hole in 2005 we infer that the talc is forming as a result of the reaction of serpentine minerals with silica saturated hydrothermal fluids that migrate up the fault zone and the talc commonly occurs in sheared serpentinite this discovery is significant as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault and its inherently stable sliding behaviour is consistent with fault creep talc may therefore provide the connection between serpentinite and creep in the san andreas fault if shear at depth can become localized along a talc rich principal slip surface within serpentinite entrained in the fault zone

origin of rollover rollover is the folding of hanging wall fault blocks by bending or collapse in response to slip on nonplanar commonly listric normal faults the shapes of rollover folds are controlled by a number of variables including 1 the shape of the fault 2 the total fault slip after a bed is deposited 3 the direction of relative particle motion in hanging wall collapse 4 the history of sedimentation rate relative to fault slip rate and 5 compaction the importance and role of each of these variables is illustrated by a two dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments in this modeling an active axial surface oriented parallel to the direction of relative p rticle motion in hanging wall collapse emanates from each fault bend and is the instantaneous locus of folding the quantitative correctness of this theory of rollover is tested by modeling natural structures from the gulf of mexico for which both fault shape and fold shape are known from high quality seismic and well sections the direction of hanging wall collapse commonly is in the antithetic or synthetic normal fault or coulomb shear orientations although sliding along weak bedding planes also is an important collapse mechanism in some regions collapse is in the antithetic shear direction for concave fault bends and in the synthetic shear direction for convex bends these collapse directions can be observed directly in some high quality seismic images as axial surfaces emanating from fault bends the shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate as well as the total slip after a bed is deposited the crests of classic gulf coast rollovers are growth axial surfaces along which are abrupt changes in sedimentation rate within the growth st atigraphic interval these changes are produced by deformation of the sediment water interface along active axial surfaces compaction can substantially modify the relationship between fault shape and rollover shape however under certain common conditions the history of compaction can be neglected if the folding is modeled in the compacted state

an examination of in situ rock stress estimation using the kaiser effect abstract when rock cores are loaded in uniaxial compression acoustic emission occurs when the stress reaches a level greater than that which the rock has previously experienced this phenomenon known as the kaiser effect has been used as an indirect method of estimating the in situ stress field in rock masses if the procedure is valid then the kaiser effect method has the advantage that the stress field can be estimated through laboratory tests on rock cores rather than the field drillhole methods of overcoring and hydraulic fracturing however the kaiser effect method is an enigma on the one hand there are good reasons why the method cannot be valid on the other hand there are reports in the literature of it being successfully applied in this paper we explore the variety of geological and mechanical factors involved and report on a case example for the olkiluoto crystalline rock site in western finland we conclude that the kaiser effect method for stress measurement is only likely to be successful if it is supported by key geological and other stress measurement information and if certain procedures are followed as explained in the paper

numerical simulation of thunderstorm outflow dynamics part i outflow sensitivity experiments and turbulence dynamics abstract in this first paper of a two part series a two dimensional numerical model is developed and used to investigate the dynamics of thunderstorm outflows by focusing only on the outflow and using essentially inviscid equations and high spatial resolution we are able to explicitly represent important physical processes such as turbulent mixing to simplify interpretation of the results the model atmosphere used in all experiments is calm and dry adiabatic this approach allows us to establish basic characteristics of modeled outflows in simple physical settings and provides a foundation for future studies using more realistic environments all simulated outflows are initialized by prescribing a controlled horizontal flux of cold air into the model domain through a lateral boundary in a series of sensitivity tests we examine three parameters of the cold air source region 1 the vertical temperature deficit profile 2 the magnitude of the temperature deficit and 3 the cold air depth by hol

on the role of isostasy in the evolution of normal fault systems the footwalls of west dipping normal faults that separate the west central colorado plateau from the basin and range province record at least 5 7 km and perhaps as much as 15 20 km of west side up neogene uplift with an axis just 10 20 km west of undeformed plateau strata the uplift is expressed as folding and steep faulting in pre tertiary cratonic and disconformably overlying neogene strata forming a basement cored anticline and coincident topographic high on the western margin of the plateau the authors interpret the uplift as a nonelastic response of the crust to buoyancy forces accompanying the tectonic denudation of the plateau margin profound isostatically driven deformation of the footwalls of major normal faults may be common in extensional terrains calling into question several assumptions fundamental to existing models of the evolution of normal fault systems

high resolution dual doppler analyses of the 29 may 2001 kress texas cyclic supercell abstract on 29 may 2001 doppler on wheels radars collected data on a supercell near kress texas the supercellular storm cyclic in nature produced multiple mesocyclones throughout its lifetime dual doppler syntheses were conducted using a grid spacing of 100 m resulting in the highest resolution observational analysis of a cyclic supercell to date in addition collection of data from ground based radar allowed for the analysis of near ground features irresolvable with airborne radar providing another advantage over previous studies the syntheses revealed a number of evolving low level mesocyclones over the observation period of 900 s while nontornadic during the synthesis period the supercell exhibited evidence of strong vertical vorticity greater than 10 2 s 1 low level circulation with classic cyclic structure and multiple tornadoes beginning 3600 s later a comparison between the current results conceptual models and previous lower resolution analyses is presented a striking similarity

genesis of the goshen county wyoming tornado on 5 june 2009 during vortex2 abstractthe genesis of a strong and long lived tornado observed during the second verification of the origins of rotation in tornadoes experiment vortex2 in goshen county wyoming on 5 june 2009 is studied mobile radar mobile mesonet rawinsonde and photographic data are used to produce an integrated analysis of the evolution of the wind precipitation and thermodynamic fields in the parent supercell to deduce the processes that resulted in tornadogenesis several minutes prior to tornadogenesis the rear flank downdraft intensifies and a secondary rear flank downdraft forms and cyclonically wraps around the developing tornado kinematic and thermodynamic analyses suggest that horizontal vorticity created in the forward flank and hook echo is tilted and then stretched near the developing tornado tilting and stretching are enhanced in the developing low level circulation as the secondary rear flank downdraft develops intensifies and wraps around the circulation center shortly thereafter the to

a synoptic and photographic climatology of low precipitation severe thunderstorms in the southern plains abstract the occurrence of an unusual type of severe thunderstorm difficult to identify by radar has been documented in the southern plains all of the three storms previously studied occurred near the dryline although tornadoes and large hail were produced little rain was observed at the ground and there was no evidence of a strong surface downdraft two were described as bell shaped ten more cases of this rare type of storm have been recently documented visually and on radar a photographic and synoptic climatology of this class of storm the low precipitation severe storm is suggested on the basis of an analysis of the comparison among 11 of these cases it is found that these storms form in a supercell like environment which is drier in the moist layer and more weakly sheared than that of well documented supercells in oklahoma implications of our findings and additional storm characteristics are discussed in light of the larger sample size

tornadogenesis in a high resolution simulation of the 8 may 2003 oklahoma city supercell abstracta 50 m grid spacing advanced regional prediction system arps simulation of the 8 may 2003 oklahoma city tornadic supercell is examined a 40 min forecast run on the 50 m grid produces two f3 intensity tornadoes that track within 10 km of the location of the observed long track f4 intensity tornado the development of both simulated tornadoes is analyzed to determine the processes responsible for tornadogenesis trajectory based analyses of vorticity components and their time evolution reveal that tilting of low level frictionally generated horizontal vorticity plays a dominant role in the development of vertical vorticity near the ground this result represents the first time that such a mechanism has been shown to be important for generating near surface vertical vorticity leading to tornadogenesis a sensitivity simulation run with surface drag turned off was found to be considerably different from the simulation with drag included a tornado still developed in the no drag simulation but it was

is buoyancy a relative quantity basic concepts of buoyancy are reviewed and considered first in light of simple parcel theory and then in a more complete form it is shown that parcel theory is generally developed in terms of the density temperature difference between an ascending parcel and an environment surrounding that parcel that is buoyancy is often understood as a relative quantity that apparently depends on the choice of a base state environmental profile however parcel theory is most appropriately understood as a probe of the static stability of a sounding to finite vertical displacements of hypothetical parcels within the sounding rather than as a useful model of deep convection the thermal buoyancy force as measured by the temperature difference between a parcel and the base state and vertical perturbation pressure gradient force together must remain independent of the base state the vertical perturbation pressure gradient force can be decomposed to include a term due to thermal buoyancy and another due to the properties of motion in the flow some thought experiments are presented to illustrate the ambiguous relevance of the base state it is concluded that buoyancy is not a relative quantity in that it cannot be dependent on the choice of an essentially arbitrary reference state buoyancy is the static part of an unbalanced vertical pressure gradient force and as such is determined locally not relative to some arbitrary base state outside of a parcel this has direct application to the diagnosis of buoyancy from numerical simulations done properly such a diagnosis must include not only the thermal buoyancy term but also the perturbation pressure gradient force due to buoyancy

processes preventing the development of a significant tornado in a colorado supercell on 26 may 2010 abstracta supercell produced a nearly tornadic vortex during an intercept by the second verification of the origins of rotation in tornadoes experiment on 26 may 2010 using observations from two m

the behavior of squall lines in horizontally heterogeneous coastal environments abstractinland squall lines respond to the stable marine atmospheric boundary layer mabl as they move toward a coastline and offshore as a storm s cold pool collides with the marine layer characteristics of both determine the resulting convective forcing mechanism over the stable layer and storm characteristics idealized numerical experiments exploring a parameter space of mabl characteristics show that the postcollision forcing mechanism is determined by the buoyancy of the cold pool relative to the mabl when the outflow is less buoyant storms are forced by a cold pool within the marine environment when the buoyancies are equivalent a hybrid cold pool internal gravity wave develops after the collision the collision between a cold pool and less buoyant mabl initiates internal waves along the stable layer regardless of mabl depth these waves are inefficient at lifting air into the storm and ascent from the trailing cold pool is needed to support deep convection storm intensity decreases with

dual doppler analysis of winds and vorticity budget terms near a tornado abstract three dimensional dual doppler observations with unprecedented finescale spatial and temporal resolution are used to characterize the vector wind field and vorticity generation terms in and near a weak short lived tornado the beam widths of the two doppler on wheels dow mobile radars at the range of the tornado are 250 m with gate lengths of 75 m resulting in a resolution of less than 107 m3 one of the dows collected data during the 240 s prior to the formation of the tornado enabling examination of the genesis process a single set of volumetric scans suitable for dual doppler analyses were completed by both dows permitting the calculation of vertical and horizontal vorticity divergence and stretching and tilting terms in the vorticity budget of the large but weak tornado and its surroundings but no local tendency terms analyses of the dual doppler vector wind fields document for the first time in a supercellular tornado revealed several structures expected to be associated with

the impact of spatial variations of low level stability on the life cycle of a simulated supercell storm abstract this study reports on the dynamical evolution of simulated long lived right moving supercell storms in a high cape strongly sheared mesoscale environment which initiate in a weakly capped region and subsequently move into a cold boundary layer bl and inversion region before dissipating the storm simulations realistically approximate the main morphological features and evolution of the 22 may 1981 binger oklahoma supercell storm by employing time varying inflow lateral boundary conditions for the storm relative moving grid which in turn are prescribed from a parent fixed steady state mesoscale analysis to approximate the observed inversion region to the east of the dryline on that day a series of full life cycle storm simulations have been performed in which the magnitude of boundary layer coldness and the convective inhibition are varied to examine the ability of the storm to regenerate and sustain its main updraft as it moves into environments with increasing convective stability the

what is the intrinsic predictability of tornadic supercell thunderstorms abstracta 25 member ensemble of relatively high resolution 75 m horizontal grid spacing numerical simulations of tornadic supercell storms is used to obtain insight on their intrinsic predictabil

the relationship between overshooting tops in a tornadic supercell and its radar observed evolution abstractthis is a study of a tornadic supercell in kansas on 14 may 2018 in which data of relatively high spatiotemporal resolution from a mobile polarimetric x band doppler radar were integrate

fault and bed rotation during continental extension block rotation or vertical shear abstract for almost a century the view has existed that the tilting of blocks between closely spaced planar normal faults is rigid body rotation this interpretation requires only simple geometry and has consequently found widespread application however it is not consistent with the deformation expected around normal faults given the present knowledge of stress fields and rheology in basement in the brittle upper crust which is better regarded instead as distributed vertical simple shear rigid body rotation and vertical shear involve different relations between fault and bed tilting and thus predict different initial fault dips for particular present day dips of faults and beds these two schemes also predict different amounts of extension and it is consequently important to establish which is correct with this aim in mind we examine normal faults associated with neogene extension in western turkey and the western united states and with mesozoic extension in the north sea except where extension and the associated tilting are minimal rigid body rotation predicts unrealistically steep initial fault dips some extensional basins also exhibit reversals of normal fault polarity and tilt polarity of beds which are incompatible with rigid body rotation we therefore conclude that the general cause of the tilting is vertical shear not rigid body rotation this has three main observational consequences first the heave on any normal fault equals the amount of extension across it second no feature near a normal fault can rotate through the vertical a normal fault thus cannot rotate through the vertical and appear as a reverse fault third any initially vertical feature near a normal fault will remain vertical a vertical dyke in the tilted surroundings of a normal fault is thus not necessarily younger than the extension that caused the tilting

kelvin helmholtz instability around the tropical tropopause observed with the equatorial atmosphere radar 1 in november 2001 the equatorial atmosphere radar 0 20 s 100 32 e observed a continuous strong eastward wind shear 10 50 m s 1 km 1 westward wind 2 27 m s 1 and the radar echo layer tilted downward to the west in the region 0 1 km above the tropopause during the same period the richardson number calculated with hourly averaged horizontal wind and radiosonde temperature data was almost continuously 0 5 and sometimes 0 25 which seems to indicate that the kelvin helmholtz instability khi frequently occurs in that region the existence of the tilted radar echo layer can be explained by khi billows a spurious updraft caused by the khi induced tilted echo layer and by the strong westward wind was also observed in the region

tornado resolving ensemble and probabilistic predictions of the 20 may 2013 newcastle moore ef5 tornado abstractan ensemble of 10 forecasts is produced for the 20 may 2013 newcastle moore ef5 tornado and its parent supercell using a horizontal grid spacing of 50 m nested within ensemble forecasts wi

hook echoes and rear flank downdrafts a review abstract nearly 50 years of observations of hook echoes and their associated rear flank downdrafts rfds are reviewed relevant theoretical and numerical simulation results also are discussed for over 20 years the hook echo and rfd have been hypothesized to be critical in the tornadogenesis process yet direct observations within hook echoes and rfds have been relatively scarce furthermore the role of the hook echo and rfd in tornadogenesis remains poorly understood despite many strong similarities between simulated and observed storms some possibly important observations within hook echoes and rfds have not been reproduced in three dimensional numerical models

numerical simulation of tornadogenesis in an outer rainband minisupercell of typhoon shanshan on 17 september 2006 abstract on 17 september 2006 three tornadoes occurred along the east coast of kyusyu island in western japan during the passage of an outer rainband in the right front quadrant of typhoon shanshan to clarify the structure of the tornado producing storms and the mechanism of tornadogenesis quadruply nested numerical simulations were performed using a nonhydrostatic model with an innermost horizontal grid spacing of 50 m several simulated convective storms in the outermost rainband exhibited characteristics of a minisupercell one storm had a strong rotating updraft of more than 30 m s 1 and a large vertical vorticity exceeding 0 06 s 1 this storm spawned a tornado when the low level mesocyclone intensified the tornado was generated on the rear flank gust front near the mesocyclone center when a secondary rear flank downdraft rfd surge advanced cyclonically around the low level mesocyclone and overtook the rear flank gust front at its left front edge backward trajectories and vorticity budget anal

the mechanical feasibility of low angle normal faulting abstract low angle normal faulting is precluded by elementary rock mechanics arguments however the existence of many low angle normal faults in regions such as western north america has long suggested that this familiar theory is incomplete initiation of a low angle normal fault requires the stress tensor within the brittle upper crust to have inclined principal axes which necessitates a substantial shear stress within the vertical plane nonetheless no one has previously identified any specific extensional stress field which is suitably oriented to allow shear failure of previously unfractured rock at a low angle dip of 30 in preference to the steeper dip of 50 60 predicted by standard theory despite previous claims to the contrary this study shows for the first time that appropriately oriented stress fields can exist however they require the shear stress to locally reach 100 mpa near the base of a 10 km thick brittle layer this is only possible under extreme conditions as it requires dramatic lateral variations in the state of stress across the extending region it is suggested that a shear stress of this order can develop due to the combined effects of lower crustal flow which imparts a horizontal shear traction at the base of the brittle layer and loading which imparts shear traction in the vertical plane associated with the isostatic response to changes in heat flow caused by changes to the geometry of subducting slabs beneath an extending region regional patterns of low angle normal faulting in western north america are thus interpreted in terms of changes to the geometry of subduction of the farallon plate

direct surface thermodynamic observations within the rear flank downdrafts of nontornadic and tornadic supercells abstract despite the long surmised importance of the hook echo and rear flank downdraft rfd in tornadogenesis only a paucity of direct observations have been obtained at the surface within hook echoes and rfds in this paper in situ surface observations within hook echoes and rfds are analyzed these mobile mesonet data have unprecedented horizontal spatial resolution and were obtained from the verifications of the origins of rotation in tornadoes experiment vortex and additional field experiments conducted since the conclusion of vortex the surface thermodynamic characteristics of hook echoes and rfds associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and rfds are favorable or unfavorable for tornadogenesis tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy potential buoyancy as measured by the convective available potential energy and equivalent potential temperature in the r

shale oil occurring between salt intervals in the dongpu depression bohai bay basin china abstract the saline mudstone sequence of the paleogene shahejie formation in the dongpu depression of bohai bay basin is a major source rock for conventional hydrocarbons however shale oil having a density of 0 9 g cm 3 and a viscosity of 21 mpa s has also been discovered and is being produced from these mudstones to better understand the occurrence of such shale oil geochemical petrological and mineralogical investigations were carried out on a suite of saline mudstone samples by using rock eval x ray diffraction and sem the shahejie formation samples analyzed in the current study are characterized by type ii kerogen with lesser amounts of types i and iii such that petroleum generation ranges from fair to excellent in general terms with the exception of es1 uppermost member which is immature to marginally mature for oil generation the sequence studied ranges from mature for oil generation to mature for wet gas generation on the basis of vitrinite reflectance data the most prospective depths for shale oil condensate generation are between 2300 and 4000 m depth in area studied the saline mudstones comprise mainly clay minerals followed by carbonates quartz and feldspar based solely on the brittle mineral index the amounts of carbonate and quartz in many of the samples lead to a similar or greater suitability for hydraulic fracturing than for many haynesville woodford and upper bakken shales sem studies show that abundant micro fractures commonly occur in the mudstones and these include tectonic fractures bedding plane fractures shrinkage fractures and over pressured fractures for the es3 m middle third member unit shale oil prospectivity is likely to be enhanced around the towns of wl wenliu lt liutun and hbz hubuzhai due to the increased frequency of tectonic fractures which would contribute to oil storage and flow mineral interparticle mineral intraparticle and organic matter pores are well developed in the rocks studied mineral dissolution has created secondary pores in some areas such that reservoir capacity is increased the increased concentration of organic acids between 2800 and 3500 m is likely to lead to enhanced shale oil prospectivity the micro fractures and dissolution pores are probably the main reservoirs for shale oil accumulation the association of these features with saline facies potentially offers an exploration tool for further shale oil occurrences in the studied area as well as in other areas containing similar lithologies

the role of surface drag in mesocyclone intensification leading to tornadogenesis within an idealized supercell simulation abstractthe idealized supercell simulations from our previous study are further analyzed to clarify the physical mechanisms leading to differences in mesocyclone intensification between an experiment with surface friction applied to the full wind fwfric and an experiment with friction applied to the environmental wind only envfric the low level mesocyclone intensifies rapidly during the 3 minutes preceding tornadogenesis in fwfric while the intensification during the same period is much weaker in envfric which fails to produce a tornado to quantify the mechanisms responsible for this discrepancy in mesocyclone evolution material circuits enclosing the low level mesocyclone are initialized and traced back in time and circulation budgets for these circuits are analyzed the results show that in fwfric surface drag directly generates a substantial proportion of the final circulation around the mesocyclone especially below 1 km agl in envfric circulation budgets indicate the mesocyclone circulat

observations of near surface vertical wind profiles and vertical momentum fluxes from vortex se 2017 comparisons to monin obukhov similarity theory abstractobservations of near surface vertical wind profiles and vertical momentum fluxes obtained from a doppler lidar and instrumented towers deployed during vortex se in the spring of 2017 are an

tornado maintenance investigated with high resolution dual doppler and enkf analysis abstractdual doppler wind synthesis and ensemble kalman filter analyses produced by assimilating doppler on wheels velocity data collected in four tornadic supercells are examined in order to further understand the maintenance of tornadoes although tornado scale features are not resolved in these analyses larger scale processes involved with tornado maintenance are well represented the longest lived tornado is maintained underneath the midlevel updraft within a zone of low level horizontal convergence along a rear flank gust front for a considerable time and dissipates when horizontally displaced from the midlevel updraft the shortest lived tornado resides in a similar zone of low level convergence briefly but dissipates underneath the location of the midlevel updraft when the updraft becomes tilted and low level convergence is displaced several kilometers from the tornado this suggests that a location beneath the midlevel updraft is not always a sufficient condition for tornado maintenance particu

an investigation of the goshen county wyoming tornadic supercell of 5 june 2009 using enkf assimilation of mobile mesonet and radar observations collected during vortex2 part ii mesocyclone scale processes affecting tornado formation maintenance and decay abstractstorm scale and mesocyclone scale processes occurring contemporaneously with a tornado in the goshen county wyoming supercell observed on 5 june 2009 during the second verification of the origins of rotation in tornadoes experiment vortex2 are examined using ensemble analyses produced by assimilating mobile radar and in situ observations into a high resolution convection resolving model this paper focuses on understanding the evolution of the vertical structure of the storm the outflow buoyancy and processes affecting the vertical vorticity and circulation within the mesocyclone that correspond to changes in observed tornado intensity tornadogenesis occurs when the low level mesocyclone is least negatively buoyant relative to the environment possesses its largest circulation and is collocated with the largest azimuthally averaged convergence during the analysis period the average buoyancy circulation and convergence within the near surface mesocyclone on spatial scales resolved by the

surface analysis near and within the tipton kansas tornado on 29 may 2008 abstract data collected by a mesonet within the near tornado environment and in the tipton tornado on 29 may 2008 provided a rare opportunity to analyze rear flank downdraft rfd outflow properties closely bounding a tornado and to characterize parcel thermodynamics being ingested into a tornado from the rear flank downdraft parcels moving into the tornado on its right flank had very small negative buoyancy and considerable potential buoyancy measurements within and very near the tornado showed similar buoyancy characteristics to the storm inflow analyzed surface divergence and videographic evidence indicated that the rfd outflow just to the right and wrapping in front of the tornado was supported by parcels moving out of a narrow downdraft bordering the right flank of the tornado surface flow field analysis showed that parcels moved out of the downdraft associated divergence region and into the right side of as well as in front of the tornado an internal rfd surge boundary was positioned roughly

a numerical study of the 6 may 2012 tsukuba city supercell tornado part ii mechanisms of tornadogenesis abstractin part i the vorticity sources of midlevel and low level mesocyclones in the 6 may 2012 tsukuba city japan tornadic supercell were investigated by using high resolution simulation results with a 50 m horizontal grid spacing in part ii the analyses are extended to the mechanisms of tornadogenesis the tornado was generated at the leading edge of a rear flank downdraft rfd outflow surge backward trajectory and vortex line analyses revealed that the rfd outflow surge was a triggering factor for tornadogenesis and that horizontal vorticity around the strong rfd outflow region was ingested into the tornado to identify the vorticity source of the tornado the evolution of circulation along a material circuit surrounding the tornado was investigated owing to baroclinity at the tip of a hook shaped distribution of hydrometeors hereafter hook echo the circulation increased rapidly from a negative value when the core of the hydrometeors was descending about 10 min prior to tornadogenesis ana

multi proxy record of postglacial environmental change south central melville island northwest territories canada abstract a sediment core from lake bc01 75 10 945 n 111 55 181 w 225 m asl on south central melville island nwt canada provides the first continuous postglacial environmental record for the region fossil pollen results indicate that the postglacial landscape was dominated by poaceae and salix typical of a high arctic plant community whereas the arctic herb oxyria underwent a gradual increase during the late holocene pollen based climate reconstructions suggests the presence of a cold and dry period 12 000 cal yr bp possibly representing the younger dryas followed by warmer and wetter conditions from 11 000 to 5000 cal yr bp likely reflective of the holocene thermal maximum the climate then underwent a gradual cooling and drying from 5000 cal yr bp to the present suggesting a late holocene neoglacial cooling diatom preservation was poor prior to 5000 cal yr bp when conditions were warmest suggesting that diatom dissolution may in part be climatically controlled diatom concentrations were highest 4500 cal yr bp but then decreased substantially by 3500 cal yr bp and remained low before recovering slightly in the 20th century an abrupt warming occurred during the past 70 yr at the site although the magnitude of this warming did not exceed that of the early holocene

dual doppler and single doppler analysis of a tornadic storm undergoing mergers and repeated tornadogenesis abstract dual doppler observations with unprecedented finescale spatial and temporal resolution are used to characterize the vector wind field in and near a tornado occurring near kiefer oklahoma on 26 may 1997 analyses of the dual doppler vector wind fields document in detail for the first time several structures associated with the tornado a proximate updraft region a rear flank downdraft wrapping around the tornado a double gust front structure occluding near the tornado and a region of enhanced vorticity separated from the tornado that may have been associated with cyclic tornadogenesis the analyses are compared to conceptual and computer models of tornadic storms a subsequent tornadogenesis was observed with radar every 18 s providing a finescale temporal view of the genesis process the genesis process was complex and the evolution of tornado intensity parameters was not monotonic in time low level rotation contracted and intensified then broadened then contracted and intensified a se

les of laminar flow in the pbl a potential problem for convective storm simulations abstractin idealized simulations of convective storms which are almost always run as large eddy simulations les the planetary boundary layers pbls are typically laminar i e they lack turbulent eddies when compared with turbulent simulations theory or simulations with pbl schemes the typically laminar les used in the severe storms community produce unrealistic near surface vertical wind profiles containing excessive vertical wind shear when the lower boundary condition is nonfree slip such simulations are potentially problematic given the recent interest within the severe storms community in the influence of friction on vorticity generation within tornadic storms simulations run as les that include surface friction but lack well resolved turbulent eddies thus probably overestimate friction s effects on storms

observations of the surface boundary structure within the 23 may 2007 perryton texas supercell abstractin situ data collected within a weakly tornadic high precipitation supercell occurring on 23 may 2007 near perryton texas are presented data were collected using a recently developed fleet of 22 durable rapidly deployable probes dubbed sticknet as well as four mobile mesonet probes kinematic and thermodynamic observations of boundaries within the supercell are described in tandem with an analysis of data from the shared mobile atmospheric research and teaching radar observations within the rear flank downdraft of the storm exhibit large deficits of both virtual potential temperature and equivalent potential temperature with a secondary rear flank downdraft gust front trailing the mesocyclone a primarily thermodynamic boundary resided across the forward flank reflectivity gradient of the supercell this boundary is characterized by small deficits in virtual potential temperature coupled with positive perturbations of equivalent potential temperature the opposing thermodynamic perturbatio

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

the influence of environmental low level shear and cold pools on tornadogenesis insights from idealized simulations abstractidealized dry simulations are used to investigate the roles of environmental vertical wind shear and baroclinic vorticity generation in the development of near surface vortices in supercell like pseudostorms a cyclonically rotating updraft is produced by a stationary cylindrical heat source imposed within a horizontally homogeneous environment containing streamwise vorticity once a nearly steady state is achieved a heat sink which emulates the effects of latent cooling associated with precipitation is activated on the northeastern flank of the updraft at low levels cool outflow emanating from the heat sink spreads beneath the updraft and leads to the development of near surface vertical vorticity via the baroclinic mechanism as has been diagnosed or inferred in actual supercells that have been simulated and observed an intense cyclonic vortex forms in the simulations in which the environmental low level wind shear is strong and the heat sink is of intermediate strength relative to the

on the mechanisms of rain formation in an idealized supercell storm abstractdeep convective storms produce raindrops through three mechanisms condensation and coalescence growth of cloud liquid droplets i e warm processes melting of ice hydrometeors and shedding from wet hailstones to investigate the relative importance of these mechanisms and their contributions to exotic drop size distributions dsds observed near the surface in supercell storms an idealized simulation of a supercell is performed using a modified version of the morrison two moment microphysics scheme the modified scheme includes separate categories for warm shed and melted rain rain originating from melting ice dominates the rain mass at low levels especially along the right forward flank precipitation shield whereas shed rain drops dominate a region within the left forward flank warm rain is only dominant in the upshear portion of the rear flank of the storm at low levels though it dominates the total rain mass within the main updraft aloft the warm rain mass at low levels is associat

a method to control the environmental wind profile in idealized simulations of deep convection with surface friction abstractin idealized horizontally homogeneous cloud model simulations of convective storms the action of surface friction can substantially modify the near ground environmental wind profile over

can a descending rain curtain in a supercell instigate tornadogenesis barotropically abstract this paper investigates whether the descending rain curtain associated with the hook echo of a supercell can instigate a tornado through a purely barotropic mechanism a simple numerical model of a mesocyclone is constructed in order to rule out other tornadogenesis mechanisms in the simulations the flow is axisymmetric and boussinesq with constant eddy viscosity in a neutrally stratified environment the domain is closed to avoid artificial decoupling of a vortex from the storm scale circulation in the principal simulation the initial condition is a balanced slowly decaying beltrami flow describing an updraft that is rotating cyclonically at midlevels around a low pressure center surrounded by a concentric downdraft that revolves cyclonically but has anticyclonic vorticity the boundary conditions are no slip on the tangential wind and free slip on the radial or vertical wind to accommodate this initial condition and to allow strong interaction of a vortex with the ground precipitation is