core mantle boundary shape constraints inferred from the pressure torque acting between the core and the mantle summary in the last few years models of the flow at the top of the earth s core and of the related pressure field have been calculated from the secular variation of the geomagnetic field and core mantle topographies have been computed by seismologists a pressure torque results from the action of the pressure field on the core topography which can theoretically be computed from models of both the pressure field and the core mantle interface small scale features of the flow and of the topography are shown to be capable of contributing strongly to the pressure torque it is thus impossible to calculate the exact value of the torque from the knowledge of only the long wavelength components of the models but the interaction between the large scale components generates by itself torques two orders of magnitude larger than the torques inferred from the irregularities of the length of the day it is nevertheless possible to reconcile the topographic coupling mechanism with the length of the day observations keeping the amplitude of the core topography proposed by seismologists if an orthogonality relation between the geometry of the fluid upwellings and downwellings at the top of the core and the topography is satisfied it is shown how to compute such a topography for a given flow and close to an original topography provided by seismic tomography some consequences of the so inferred link between the fluid flow at the top of the core and the core mantle boundary topography are discussed

the topographic torque on a bounding surface of a rotating gravitating fluid and the excitation by core motions of decadal fluctuations in the earth s rotation general expressions with potential applications in several areas of geophysical fluid dynamics are derived for all three components of the contribution made by the geostrophic part of the pressure field associated with flow in a rotating gravitating fluid to the topographic torque exerted by the fluid on a rigid impermeable bounding surface of any shape when applied to the earth s liquid metallic core which is bounded by nearly spherical surfaces and can be divided into two main regions the torosphere and polosphere the expressions reduce to formulae given previously by the author thereby providing further support for his work and that of others on the role of topographic coupling at the core mantle boundary in the excitation by core motions of earth rotation fluctuations on decadal time scales they also show that recent criticisms of that work are vitiated by mathematical and physical errors contrary to these criticisms the author s scheme for exploiting earth rotation and other geophysical data either real or simulated in computer models in quantitative studies of the topography of the core mantle boundary cmb by intercomparing various models of a motions in the core based on geomagnetic secular variation data and b cmb topography based on seismological and gravity data has a sound theoretical basis the practical scope of the scheme is of course limited by the accuracy of real data but this is a matter for investigation not a priori assessment

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

observed and numerically simulated structure of a mature supercell thunderstorm abstract through the interactive use of doppler radar analyses and a three dimensional numerical storm simulation the detailed structure of a supercell tornadic storm is analyzed this storm named the del city storm occurred in central oklahoma on 20 may 1977 the storm exhibits certain important features which are essential to maintaining its longevity and which promote the storm s transition to its tornadic phase these features are strongly influenced by the rotational character of the storm separates the precipitation from the updraft and which orients the resulting downdrafts to which reinforce low level convergence along the gust front and sustain the storm analyses of air parcel and rain trajectories within the storm provide a detailed visualization of this internal structure these trajectories reveal that air parcels rising through the cyclonically rotating updraft actually turn anticyclonically with height owing to the influence of the storm relative environmental wind field downdraft traj

variations in cloud to ground lightning characteristics among three adjacent tornadic supercell storms over the tennessee valley region the contrasting behavior of cloud to ground cg lightning associated with three adjacent supercell thunderstorms observed on 18 may 1995 is examined thunderstorm characteristics and anvil interactions are related to north south variations in cg lightning properties while tornadic activity was not consistently related to variations in cg properties radar reflectivity factor area greater than 65 db z was generally inversely related to cg frequency it is hypothesized that suppression of cg activity was produced by reduction of large number concentrations of precipitation sized particles i e presence of large hail in the particle interaction mixed phase region it is further hypothesized that seeding from upstream storm anvil ice was associated with nearly coincident enhancement of cg activity in downstream storms likewise it is hypothesized that the reduction in 65 dbz echo area inferred suppression of hail is related to this inferred anvil seeding process

doppler radar analysis of a low precipitation severe storm abstract this is a detailed study of the structure and evolution of a convective storm that formed along the dryline in oklahoma on 26 april 1984 and developed into a supercell dual doppler wind analyses analyses of surface mesonetwork and instrumented tower data storm intercept photographs analyses of raingage data from a dense network and storm reports are presented and discussed the storm exhibited the following low precipitation lp characteristics the absence of any strong evaporatively cooled storm scale downdraft near the surface a paucity of rain reaching the ground large hail long lived rotating updrafts and a strong relatively small in areal extent unicellular radar echo the storm split and its right moving member developed into an isolated high precipitation supercell it is suggested that the region under cloud base was associated with strong radar echo caused by widely spaced large water droplets and hailstones separate cyclonically and anticyclonically rotating updrafts

three dimensional kinematic and microphysical evolution of florida cumulonimbus part i spatial distribution of updrafts downdrafts and precipitation abstract this paper is the first in a three part study that examines the kinematic and microphysical evolution of florida cumulonimbus and focuses on the convective to stratiform transition of the storm this first paper lays the groundwork for the subsequent papers by defining the problem under study delineating the setting for the storm and describing the spatial distribution of updrafts downdrafts and precipitation high resolution radar data of a typical line of storms associated with the florida sea breeze is the centerpiece of this study the high resolution data reveal details of the internal structure of the squall line that were beyond the resolution of previous squall line studies radar reflectivity filled in between cells at upper levels as the storm evolved reflectivity values were only weakly associated with updraft and downdraft magnitude the updrafts and downdrafts in the storm tended to be irregular in their three dimensional shape and less than 5 km in horizontal extent at any giv

theory and observations of controls on lightning flash size spectra previous analyses of very high frequency vhf lightning mapping array lma observations relative to the location of deep convective updrafts have noted a systematic pattern in flash characteristics in and near strong updrafts flashes tend to be smaller and more frequent while flashes far from strong vertical drafts exhibit the opposite tendency this study quantitatively tests these past anecdotal observations using lma data for two supercell storms that occurred in oklahoma in 2004 the data support a prediction from electrostatics that frequent breakdown and large flash extents are opposed an energetic scaling that combines flash rate and flash area exhibits a 5 3 power law scaling regime on scales of a few kilometers and a maximum inflashenergyatabout10km thespectralshapeis surprisinglyconsistentacrossarangeofmoderatetolarge flash rates the shape of this lightning flash energy spectrum is similar to that expected of turbulent kinetic energy spectra in thunderstorms in line with the hypothesized role of convective motions as the generator of thunderstorm electrical energy the correspondence between kinematic and electrical energy spectra suggests that advection of charge bearing precipitation by the storm s flow including in turbulent eddies couples the electrical and kinematic properties of a thunderstorm

effects of orographic precipitation variations on the concavity of steady state river profiles the concavity or curvature of river profiles has long been taken to be a fundamental indicator of the underlying processes governing fluvial erosion and thereby of landscape evolution however erosion laws have generally been derived without accounting for the strong orographically driven gradients in precipitation typically found in mountainous regions in addition field measurements have found discrepancies between the form of measured stream profiles and theoretically derived values introducing a simple physically based feedback we find that orographically induced variations in precipitation strongly affect the curvature of steady state river profiles this feedback complicates efforts to infer the form of erosion laws from observed profile concavities but could help explain discrepancies between observations and theory our results demonstrate a strong feedback through which climate influences the form of river profiles and show how such climatic effects act to limit the relief of unglaciated mountain ranges

creating a more realistic idealized supercell thunderstorm evolution via incorporation of base state environmental variability abstractconvective environments are known to be heterogeneous in both time and space yet idealized models use fixed base state environments to simulate storm evolution recently the base state su

the effect of spatial heterogeneities on geomorphic expression in a model of basin evolution spatial variability is a basic component of all natural systems the system being studied here is the topography of an evolving drainage basin spatially heterogeneous erosivity within the drainage basin influences the structure of the drainage network the roughness of small scale features and the overall appearance of the simulated topography the presence of heterogeneities does not simply blur and smear analytical relationships the heterogeneities themselves introduce new structure to the organization of the drainage basin three measures the slope area relationship the cumulative area relationship and the hypsometric distribution are all systematically affected by spatial heterogeneity we will demonstrate how and why these relationships are affected and also how different structures in the heterogeneities themselves are manifested in the resulting simulated topography

the role of forcing in cell morphology and evolution within midlatitude squall lines abstract this study assesses the role of mesoscale forcing on cell morphology and early evolution of midlatitude squall lines the forcing chosen was a cold front simulated to frontal collapse to produce a specific set of thermodynamic profiles at the leading edge of the front use of a realistic balanced and persistent forced state allowed a unique evaluation of its importance in thunderstorm evolution compared with a traditional homogeneous environment without forcing three dimensional squall lines were modeled with and without the front present in low and high bulk richardson number environments the forced convection evolved in significantly different ways than their isolated unforced counterparts in low shear conditions the line of isolated convective cells split with the adjacent split cells interfering destructively with neighboring cells in the line with forcing present differences in anticyclonic cell intensity and propagation prevented this interaction from occurring leading to longe

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

on the decay of supercells through a downscale transition visual documentation abstract photographic and mobile radar documentation of the dissipation of a supercell and a severe convective storm that had not yet developed into a mature supercell are discussed it is hypothesized based on these cases and on others that when a low precipitation or classic supercell and or a developing supercell moves into an environment of cooler surface temperatures and a strong capping inversion it eventually dissipates through a process of downscale transition in which vertical shear tilts the updraft more in the downshear direction as the cape decreases and the updraft becomes narrower as the storm dissipates during the downscale transition it is possible that a cold pool or lack thereof may play a role but the documentation in the cases detailed herein is not adequate to address this issue

blueprint for a coupled model of sedimentology hydrology and hydrogeology in streambeds the streambed constitutes the physical interface between the surface and the subsurface of a stream across all spatial scales surface water groundwater interactions are controlled by the physical properties of the streambed streambed properties such as topography or hydraulic conductivity are continuously altered through erosion and sedimentation processes recent studies from the fields of ecology hydrogeology and sedimentology provide field evidence that sedimentological processes themselves can be heavily influenced by surface water groundwater interactions giving rise to complex feedback mechanisms between sedimentology hydrology and hydrogeology more explicitly surface water groundwater exchanges play a significant role in the deposition of fine sediments which in turn modify the hydraulic properties of the streambed we explore these feedback mechanisms and critically review the extent of current interaction between the different disciplines we identify opportunities to improve current modeling practices for example hydrogeological models treat the streambed as a static rather than a dynamic entity while sedimentological models do not account for critical catchment processes such as surface water groundwater exchange a blueprint for a new modeling framework is proposed that bridges the conceptual gaps between sedimentology hydrogeology and hydrology specifically this blueprint 1 fully integrates surface subsurface flows with erosion transport and deposition of sediments and 2 accounts for the dynamic changes in surface elevation and hydraulic conductivity of the streambed the opportunities for new research within the coupled framework are discussed

earth s variable rotation recent improvements in geodetic data and practical meteorology have advanced research on fluctuations in the earth9s rotation theinterpretation of these fluctuations is inextricably linked with studies of the dynamics of the earth moon system and dynamical processes in the liquid metallic core of the earth where the geomagnetic field originates other parts of the earth9s interior and the hydrosphere and atmosphere fluctuations in the length of the day occurring on decadal time scales have implications for the topography of the core mantle boundary and the electrical magnetic and other properties of the core and lower mantle investigations of more rapid fluctuations bear on meteorological studies of interannual seasonal and intraseasonal variations in the general circulation of the atmosphere and the response of the oceans to such variations

dupuit ghyben herzberg analysis of strip island lenses dupuit ghyben herzberg analysis follows from combination of the continuity equation and darcy s law with the ghyben herzberg principle and the dupuit assumptions of horizontal flow the analysis is used to derive the position of the water table and salt water interface in island lenses in terms of island geometry distribution of hydraulic conductivity k and distribution of recharge r for small islands and cases for which the salt water head is zero application of dupuit ghyben herzberg analysis gives good results because the low r k ratios of natural lenses assure that height width ratios of natural lenses are extremely low 1 30 to 1 100 in earlier publications the dupuit ghyben herzberg differential equation has been integrated with the boundary condition that the water table and interface meet at sea level at the shoreline that is no outflow face the dimensions of an outflow face however are known from results of potential theory analysis integration of the dupuit ghyben herzberg equation with a boundary condition consistent with the potential theory outflow face a places the interface indistinguishably close to its position given by potential theory and b allows calculation of ghyben herzberg ratios interface depth to water table elevation which in this case depart from their usual value of 40 because equipotentials are curved close to the outflow face for natural sized lenses however such analysis is necessary only within a narrow strip within 1 to 5 of the island width of the shoreline outside this strip the regular analysis that ignores the presence of the outflow face positions the interface indistinguishably close to that of the potential theory solution analytical solutions are developed for a number of infinite strip islands it is shown by analysis of the homogeneous rectangular island case that an island can be considered an infinite strip to 0 1 accuracy if its length width ratio is larger than 4 4 asymmetric lenses occur if the island is composed of strips of different k or different r with the greater asymmetry occurring with differences in k a high k basement compresses the root of the lens and thereby decreases the water table in the island a lens perched on impermeable basement has a higher water table than would otherwise occur in the island but the volume of the lens is less

organizational modes of midlatitude mesoscale convective systems this paper discusses common modes of mesoscale convective organization using 2 km national composite reflectivity data the authors investigated linear mesoscale convective systems mcss that occurred in the central united states during may 1996 and may 1997 based upon the radar observed characteristics of 88 linear mcss the authors propose a new taxonomy comprising convective lines with trailing ts leading ls and parallel ps stratiform precipitation while the ts archetype was found to be the dominant mode of linear mcs organization the ls and ps archetypes composed nearly 40 of the studied population in this paper the authors document the characteristics of each linear mcs class and use operational surface and upper air data to describe their different environments in particular wind profiler data reveal that the stratiform precipitation arrangement associated with each class was roughly consistent with the advection of hydrometeors implied by the mean middle and upper tropospheric storm relative winds which were significantly different among the three mcs modes case study examples are presented for both the ls and ps classes which have received relatively little attention to this point as well the authors give a general overview of the synoptic scale meteorology accompanying linear mcss in this study which was generally similar to that observed by previous investigators

landscape response to tectonic forcing digital elevation model analysis of stream profiles in the mendocino triple junction region northern california the topographic evolution of orogens is fundamentally dictated by rates and patterns of bedrock channel incision quantitative field assessments of process based laws are needed to accurately describe landscape uplift and denudation in response to tectonics and climate we evaluate and calibrate the shear stress or similar unit stream power bedrock incision model by studying stream profiles in a tectonically active mountain range previous work on emergent marine terraces in the mendocino triple junction region of northern california provides spatial and temporal control on rock uplift rates digital elevation models and field data are used to quantify differences in landscape morphology associated with along strike northwest to southeast changes in tectonic and climatic conditions analysis of longitudinal profiles supports the hypothesis that the study area channels are in equilibrium with current uplift and climatic conditions consistent with theoretical calculations of system response time based on the shear stress model within uncertainty the profile concavity of the trunk streams is constant throughout the study area 0 43 as predicted by the model channel steepness correlates with uplift rate these data help constrain the two key unknown model parameters the coefficient of erosion k and the exponent associated with channel gradient n this analysis shows that k cannot be treated as a constant throughout the study area despite generally homogeneous substrate properties for a reasonable range of slope exponent values n best fit values of k are positively correlated with uplift rate this correlation has important implications for landscape evolution models and likely reflects dynamic adjustment of k to tectonic changes due to variations in orographic precipitation and perhaps channel width sediment load and frequency of debris flows the apparent variation in k makes a unique value of n impossible to constrain with present data

the dependence of numerically simulated convective storms on vertical wind shear and buoyancy abstract the effects of vertical wind shear and buoyancy on convective storm structure and evolution are investigated with the use of a three dimensional numerical cloud model by varying the magnitude of buoyant energy and one directional vertical shear over a wide range of environmental conditions associated with severe storms the model is able to produce a spectrum of storm types qualitatively similar to those observed in nature these include short lived single cells certain types of multicells and rotating supercells the relationship between wind shear and buoyancy is expressed in terms of a nondimensional convective parameter which delineates various regimes of storm structure and in particular suggests optimal conditions for the development of supercell type storms applications of this parameter to well documented severe storm cases agree favorably with the model results suggesting both the value of the model in studying these modes of convection as well as the value of this representation i

the influence of vertical wind shear on moist thermals abstractalthough it is well established that vertical wind shear helps to organize and maintain convective systems there is a longstanding colloquial notion that it inhibits the development of dee

airflow and hail growth in a severe northern high plains supercell abstract air and particle trajectory calculations using internal motions from doppler radar observations are used to identify kinematic feature and hail growth processes operating in a supercell storm that occurred on 2 august 1981 in southeastern montana as the rock springs storm moved rapidly east southeastward across the cooperative convective precipitation experiment ccope observational network it produced a significant hailfall composed mostly of 2 to 3 cm diameter hailstones some diameters were as large as 6 to 10 cm at least one funnel cloud was sighted and there was extensive crop and property damage in the hail growth model 1 to 5 cm hailstones were readily produced from frozen drops in the size range 50 m to about 1 mm most of the hail apparently grew from frozen droplets that originated within either the upwind stagnation zone southwest of the main updraft core or the overhanging radar echo ahead of the updraft potential hailstone embryos entered the stagnation zone from a flanking c

tangentially geostrophic flow at the core mantle boundary compatible with the observed geomagnetic secular variation the large scale component of the flow abstract we present a method for determining the large scale component of a tangentially geostrophic flow beneath the core mantle boundary compatible with magnetic secular variation observations we use a tangentially geostrophic basis to ensure the geostrophy of the motion the fit of the secular variation sv generated by the motion to the observed sv in fact sv models is adequate taking into account the existing error level as in any horizontal geostrophic motion the flow is expressed as a sum of two independent tangentially geostrophic flows a zonal component which is toroidal and a non zonal component which is directly linked with the motions deeper in core the flow derived for the recent epoch 1970 1985 presents interesting symmetry properties the non zonal velocities are the same at two antipodal points while the zonal velocities are the same at two points symmetrical about the equator the non zonal component of the flow is more vigorous than the zonal one the consoidal ingredient though weaker than the toroidal ingredient is essential and indicates strong vertical motion at depth in low latitude areas the sv is actually compatible with a geostrophic motion at the core mantle boundary and appears to be mainly due to the action of the non zonal component of the flow

drainage basin responses to climate change recent investigations have shown that the extent of the channel network in some drainage basins is controlled by a threshold for overland flow erosion the sensitivity of such basins to climate change is analyzed using a physically based model of drainage basin evolution the golem model simulates basin evolution under the action of weathering processes hillslope transport and fluvial bedrock erosion and sediment transport results from perturbation analyses reveal that the nature and timescale of basin response depends on the direction of change an increase in runoff intensity or a decrease in vegetation cover will lead to a rapid expansion of the channel network with the resulting increase in sediment supply initially generating aggradation along the main network followed by downcutting as the sediment supply tapers off by contrast a decrease in runoff intensity or an increase in the erosion threshold will lead to a retraction of the active channel network and a much more gradual geomorphic response cyclic changes in runoff intensity are shown to produce aggradational degradational cycles that resemble those observed in the field cyclic variations in runoff also lead to highly punctuated denudation rates with denudation concentrated during periods of increasing runoff intensity and or decreasing vegetation cover the sediment yield from threshold dominated basins may therefore exhibit significant variability in response to relatively subtle environmental changes a finding which underscores the need for caution in interpreting modern sediment yield data

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

fluid flow near the surface of earth s outer core maps of the fluid flow at the core surface are important for a number of reasons foremost they may provide some insight into the workings of the geodynamo and may place useful constraints on geodynamo models from the flow the force balance at the top of the core can at least in part be deduced the flow can provide short term predictions of the secular variation the flow is important in understanding changes in the length of day and constraints on lateral temperature variations and topography at the core mantle boundary may be derived unlike the case of mantle convection only very small lateral variations in core density are required to drive the flow these density variations are too small by several orders of magnitude to be imaged seismically so instead we use the geomagnetic secular variation to infer the flow despite considerable recent progress in mapping the core flow substantial differences exist between maps produced by different researchers here we examine the possible underlying reasons for these differences paying particular attention to the inherent problems of nonuniqueness we focus on the aspects of the flow which do seem to be well determined and discuss their geophysical implications

how do streamflow generation mechanisms affect watershed hypsometry several studies have shown that the dominant streamflow generation mechanism in a river basin can leave distinct geomorphological signatures in basin topography in particular it has been suggested previously that basins generated by groundwater discharge tend to have a larger hypsometric integral than surface runoff basins because fluvial erosion is more focused in the valleys where groundwater discharge tends to occur in this analysis we aim to clarify this relationship by developing an alternative method to quantify the effects of streamflow generation mechanisms on basin hypsometry and by using a numerical model that can generate streamflow by different processes to evaluate the sensitivity of the results to the hydrological and geomorphological properties of the basin the model results suggest that the hypsometric characteristics that are usually associated with groundwater discharge basins such as a larger hypsometric integral occur primarily when drainage networks are still advancing in the watershed during later stages of development an additional factor such as lithological controls or a distinct geomorphological process would be needed to preserve these features the model results also show that the hypsometric effects are stronger when the parameters of the fluvial erosion process promote the influence of small discharge rates copyright 2007 john wiley sons ltd

the role of midtropospheric winds in the evolution and maintenance of low level mesocyclones abstract using a three dimensional numerical model supercell simulations initialized in environments characterized by hodographs with large curvature in the lowest 3 km and a range of linear midlevel shears are investigated for low values of the midlevel shear 0 005 s 1 the storm develops a mesocyclone at the lowest model level within the first hour of the simulation the gust front starts to move ahead of the main updraft and cuts off the inflow to the storm by approximately 2 h resulting in decay of the initial storm and growth of a new rotating storm on the outflow as the midlevel shear increases to approximately 0 010 s 1 the initial development of the low level mesocyclone is delayed but the mesocyclone that develops is more persistent lasting for over 2 h further increases of the shear to 0 015 s 1 result in the suppression of any low level mesocyclone despite the presence of intense rotation at midlevels of the storm we hypothesize that differences in the distribution of precipitation

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

inverting topography for landscape evolution model process representation 3 determining parameter ranges for select mature geomorphic transport laws and connecting changes in fluvial erodibility to changes in climate

the influence of the shear induced pressure gradient on thunderstorm motion abstract in the present investigation we propose a simple theory to explain how a veering environmental wind shear vector can cause an initially symmetric updraft to grow preferentially to the right of the shear vector and acquire cyclonic rotation the explanation offered is based on linear theory which predicts that interaction of the mean shear with the updraft produces favorable vertical pressure gradients along its right flank to asses the validity of linear theory for large amplitude updrafts the three dimensional shallow anelastic equations are numerically integrated using a simple parameterization for latent heating within a cloud and the linear and nonlinear forcing terms are separately analyzed these results suggest that although the nonlinear effects strongly promote splitting of the updraft the linear forcing remains the dominant factor in preferentially enhancing updraft growth on the right flank we believe this differential forcing is a major contributor to the observed predominance o

influences of the local environment on supercell cloud to ground lightning radar characteristics and severe weather on 2 june 1995 abstract radar cloud to ground cg lightning characteristics and storm reports were documented for 20 long lived supercell thunderstorms that occurred during a 6 h period in the west texas panhandle on 2 3 june 1995 these thunderstorms occurred in proximity to a preexisting mesoscale outflow boundary storms that remained on the warm side of the mesoscale outflow boundary and storms that formed directly on the boundary tended to produce weaker low level rotation lower maximum heights for the 40 dbz echo top and had the largest negative cg flash rates the largest negative flash rate was produced as each storm was gradually weakening in contrast out of 11 boundary crossing storms several important radar based measurands increased unambiguously after storms crossed the boundary 40 dbz echo top height in 5 cases radar reflectivity above the environmental freezing level in 6 cases and low level mesocyclone strength in 9 cases trends of the first two measurands were ambiguous for 4 of 11 cases aff

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

an observational study of the dryline abstract this study presents analyses of data collected in the vicinity of a cloud free dryline that occurred in western oklahoma on 24 may 1989 observations reveal sharp contrasts across the quasi stationary north south dryline during midafternoon of greatest significance is a pronounced gradient of virtual potential temperature although horizontal convergence and vorticity also maximize at the dryline the environment of the 24 may dryline is dominated by vertical mixing that maintains a convective boundary layer cbl on both sides of the dryline the dryline resembles a mixing zone containing varying proportions of hot dry air to the west side and warm moist air from the lowest 200 m within 10 km to the east of the dryline the mixing zone slopes eastward from the surface dryline location then becomes a quasi horizontal elevated moist layer above the cbl east of the dryline saturation point analysis indicates that the mixing zone is characterized by a single mixing line structure defined by

precipitation production in a large montana hailstorm airflow and particle growth trajectories abstract computations of air motion and precipitation growth using winds derived from doppler radar measurements were analyzed to reveal important flow features that influenced the production of precipitation during the nearly steady phase of a well observed severe storm in montana that produced hail as large as 5 cm in diameter the storm had many features commonly associated with supercells though it exhibited a gently sloping overhang on its low level inflow side rather than the more classical vaulted structure formed initially as the right member of a splitting storm pair it moved slowly eastward while embedded in moderately sheared environmental winds characteristic hail growth trajectories and precipitation fallout positions are considered in conjunction with the deduced embryo sources and formation regions based on particle growth calculations measurements by radar and research aircraft cloud photography and direct hailstone examination four general sources of hail embryos were apparent 1

precipitation growth trajectories in a ccope storm abstract the growth trajectories of precipitation particles that attain diameters from 0 5 to 2 0 cm are modeled within the wind field of a small relatively steady state southeastern montana thunderstorm the trajectories are calculated backwards from systematic arrays of particles of specified sizes at a level near cloud base using a simple set of criteria for rejecting the obviously impossible trajectories the patterns of accepted trajectory end points are compared with the radar echo patterns good agreement lends credence to the qualitative aspects of the trajectories for a given size of precipitation particle the method helps one to assign different trajectory types to specific regions within the horizontal plant on which the calculations were started the relative importance of the different types of trajectories can thus be estimated particle origin mechanisms are discussed in terms of the regions in which the trajectories are found to start the variety of successful trajectories leading t

revisiting the hypsometric curve as an indicator of form and process in transport limited catchment hypsometry has historically been used as an indicator of geomorphic form of catchments and landforms yet there has been little work aimed at relating hypsometry to landform process and scale this paper uses the siberia catchment evolution model to explore linkages between catchment process and hypsometry siberia generates results that are qualitatively and quantitatively similar to observed hypsometric curves for physically realistic parameters however we show that not only does the hypsometry reflect landscape runoff and erosion process but it is strongly dependent on channel network and catchment geometry we show that the width to length ratio of the catchment has a significant influence on the shape of the hypsometric curve though little on the hypsometric integral for landforms dominated by fluvial sediment transport the classic strahler mature hypsometric curve is only generated for catchments with roughly equal width and length narrow catchments show a hypsometric curve more similar to strahler s monadnock form for landscapes dominated by diffusive transport the simulated hypsometric curve is concave down everywhere this being consistent with curves reported for some example catchments in france because the transition between diffusive dominance to fluvial is scale dependent with larger catchments exhibiting greater fluvial dominance then the hypsometric curve is a scale dependent descriptor of landforms experimental results for simulated landforms from a small scale rainfall erosion simulator are reported it is shown that siberia yields satisfactory fits to the data confirming its ability to predict the form of the hypsometric curve from a simple model of geomorphic processes 1998 john wiley sons ltd

impact of variations in upper level shear on simulated supercells abstractit has previously been suggested based on limited observations that vertical wind shear in the upper troposphere is a key control on supercell morphology with the low precipitation high precipitation and classic archetypes favored under strong weak and moderate shear respectively the idea is that with increasing upper level shear uls hydrometeors are transported farther from the updraft by stronger storm relative anvil level winds limiting their growth and thereby reducing precipitation intensity the present study represents the first attempt to test this hypothesis using idealized simulations of supercells performed across a range of 6 12 km shear profiles contrary to expectations there is a significant increase in surface precipitation and an associated strengthening of outflow winds as uls magnitude is increased from 0 to 20 m s 1 these changes result from an increase in storm motion which drives stronger low level inflow a wider updraft and enhanced condensation a further

a physical explanation of an observed link area slope relationship an observed log log linear relationship between channel slope and contributing area is explained by the erosional physics that lead to catchment form it is postulated that tectonic uplift is in balance with the fluvial erosion down wasting that dominates catchment erosion and it is shown that this relationship results in the observed log log linear relationship at dynamic equilibrium in addition it has been observed that there are deviations from this log log linear relationship near the catchment divide with observed slopes being lower than those predicted from the relationship this is explained by noting that for small areas fluvial erosion effects are dominated by soil creep and rain splash modeled by diffusive physics the area at which this deviation from log log linearity occurs is that point on the hillslope at which diffusive physics like soil creep and rain splash begin to dominate fluvial erosion these predictions are confirmed by numerical simulations using a catchment evolution model

on the reliability of hook echoes as tornado indicators abstract a study of radar echoes associated with the tornadoes of the 3 april 1974 outbreak was performed to evaluate the usefulness of echo shape as an indicator of tornadic thunderstorms the hook shape was usually successful in characterizing an echo as tornadic with a false alarm rate of 169 because hook echoes were relatively rare however a less restrictive shape called distinctive was more successful at detecting tornadic thunderstorms identifying 65 of the tornadic echoes an echo had a distinctive shape if it possessed a marked appendage on its right rear flank or was in the shape of a spiral comma or line echo wave pattern lewp characteristics of the distinctive echo are given

variations in supercell morphology part i observations of the role of upper level storm relative flow abstract it is hypothesized that the precipitation intensity beneath a supercell updraft is strongly influenced by the amount of hydrometeors that are reingested into the updraft after being transported away in the divergent upper level flow of the anvil this paper presents the results of a climatological analysis of soundings associated with three types of isolated supercells having distinctive precipitation distributions the so called classic low precipitation lp and high precipitation hp storms it is shown that storm relative flow at 9 10 km above the ground is strongest in the environments of lp storms and relatively weak in the environments of hp storms with classic storms occurring in environments with intermediate magnitudes of upper storm relative flow it is plausible that comparatively strong flow in the anvil bearing levels of lp storms transports hydrometeors far enough from the updraft that they are relatively unlikely to be reingested into the updraft leading to greatly diminishe

a geomagnetic jerk for the end of the 20th century abstract the series of magnetic measurements at some european observatories give some hint of a new geomagnetic jerk around 1999 the geomagnetic impulses would present the remarkable and intriguing property to occur with a frequency of one per decade in the last third of the 20th century the geomagnetic jerks have been proposed to be indicators which anticipate the changes in the earth s rotation rate if this statement is of general validity the 1992 jerk should be followed by an acceleration of the earth rotation which would take place now and along the same lines we might predict a new deceleration in less than 10 yr

hodograph variability within analytically modeled synoptic scale baroclinic systems although the relationship between the behavior of convective storms and their environmental vertical wind shear has been examined using proximity soundings and idealized numerical modeling experiments the manner in which the vertical shear profiles as visualized by hodographs is regulated by the larger scale baroclinic wave structure has not been considered in detail to examine this synoptic scale dependence a relatively simple analytic model for baroclinic systems in midlatitudes having exact solutions for a frictionless quasigeostrophic atmosphere is employed the analytical model consists of a checkerboard of high and low pressure areas at 1000 mb hydrostatically modulated above by a mean meridional temperature gradient and a checkerboard of warm and cold centers at 1000 mb aloft the model atmosphere consists of a zonally oriented wave train this approach allows a systematic examination of the dependence of hodographs on the following five synoptic scale parameters included in the model 1 mean meridional temperature gradient 2 system wavelength 3 phase lag between the height and temperature fields at 1000 mb 4 magnitude of the temperature perturbation associated with the checkerboard of warm and cold centers at 1000 mb and 5 magnitude of the 1000 mb height perturbation it is seen that the phase lag between the height and temperature fields and the system wavelength have the greatest quantitative influence on the relative contribution of the ageostrophic wind component to the total wind these two parameters are associated with significant clockwise curvature with height in the hodograph of the total wind particularly if the deep layer ageostrophic wind shear is oriented perpendicular and to the right of the geostrophic shear hodograph curvature however is not ubiquitous in the model and despite the model s simplicity likely speaks to the importance of features departing from the model mesoscale variability and boundary layer friction in enhancing hodograph curvature

the sensitivity of simulated supercell structure and intensity to variations in the shapes of environmental buoyancy and shear profiles abstract convective storm simulations are conducted using varying thermal and wind profile shapes subject to the constraints of strict conservation of convective available potential energy cape and hodograph trace small and large cape regimes and straight and curved hodographs are studied each with a matrix of systematically varying thermal and wind profile shapes having identical levels of free convection and bulk richardson numbers favorable to supercell development differences in storm intensity and morphology resulting from changes in the profile shapes can be profound especially in the small cape regime where for the moderate shears studied here storms are generally weak except when the buoyancy is concentrated at low levels in stronger cape regimes less dramatic relative enhancements of storm updraft intensity are found when both the buoyancy and shear are concentrated at low levels peak midlevel vertical vorticity correlates roughly with peak updraft speed in the small cape regime but

implications of sediment flux dependent river incision models for landscape evolution 1 developing a quantitative understanding of the factors that control the rate of river incision into bedrock is critical to studies of landscape evolution and the linkages between climate erosion and tectonics current models of long term river network incision differ significantly in their treatment of the role of sediment flux we analyze the implications of various sediment flux dependent incision models for large scale topography in an attempt 1 to identify quantifiable and diagnostic differences between models that could be detected from topographic data or from the transient responses of perturbed systems and 2 to explain the apparent ubiquity of mixed bedrock alluvial channels in active orogens although certain forms of the various models can be discarded as inconsistent with morphological data we find that the relative intrinsic concavity indices of detachment and transport limited systems defined herein largely dictate whether the various models can be tied to distinctive steady state morphologies preliminary data suggest that no such diagnostic differences may exist and other methods must be developed to test models accordingly we develop and explore differences in the scaling behavior of topographic relief and the extent of detachment versus transport limited channels as a function of rock uplift rate that may allow discrimination among various models further we explore potentially diagnostic differences in the rates and patterns of transient channel response to changes in rock uplift rate in addition to general differences between detachment and transport limited systems our analysis identifies an interesting hysteresis in landscape evolution hybrid channels at the threshold between detachment and transport limited conditions are expected to act as detachment limited systems in response to an increase in rock uplift rate or base level fall and as transport limited systems in response to a decrease in rock uplift rate especially during postorogenic topographic decline the analyses presented set the stage for field studies designed to test quantitatively the various river incision models that have been proposed

evidence for nonlinear diffusive sediment transport on hillslopes and implications for landscape morphology steep soil mantled hillslopes evolve through the downslope movement of soil driven largely by slope dependent transport processes most landscape evolution models represent hillslope transport by linear diffusion in which rates of sediment transport are proportional to slope such that equilibrium hillslopes should have constant curvature between divides and channels on many soil mantled hillslopes however curvature appears to vary systematically such that slopes are typically convex near the divide and become increasingly planar downslope this suggests that linear diffusion is not an adequate model to describe the entire morphology of soil mantled hillslopes here we show that the interaction between local disturbances such as rainsplash and biogenic activity and frictional and gravitational forces results in a diffusive transport law that depends nonlinearly on hillslope gradient our proposed transport law 1 approximates linear diffusion at low gradients and 2 indicates that sediment flux increases rapidly as gradient approaches a critical value we calibrated and tested this transport law using high resolution topographic data from the oregon coast range these data obtained by airborne laser altimetry allow us to characterize hillslope morphology at 2 m scale at five small basins in our study area hillslope curvature approaches zero with increasing gradient consistent with our proposed nonlinear diffusive transport law hillslope gradients tend to cluster near values for which sediment flux increases rapidly with slope such that large changes in erosion rate will correspond to small changes in gradient therefore average hillslope gradient is unlikely to be a reliable indicator of rates of tectonic forcing or baselevel lowering where hillslope erosion is dominated by nonlinear diffusion rates of tectonic forcing will be more reliably reflected in hillslope curvature near the divide rather than average hillslope gradient