grain growth kinetics of cairo3 perovskite and post perovskite with implications for rheology of d layer abstract grain growth kinetics of cairo 3 perovskite and post perovskite aggregates was investigated by time series experiments at pressures of 2 and 3 gpa and temperatures ranging from 1373 to 1773 k in a piston cylinder apparatus the experiments were conducted in the stability field of both perovskite and post perovskite the increase of grain size g with time t follows a growth law g n g 0 n k t k k 0 exp h rt the growth exponents n for perovskite and post perovskite are 2 6 0 5 and 4 5 0 9 respectively the activation enthalpy h of post perovskite grain growth is 251 35 kj mol grain growth of post perovskite is distinctly slower than that of perovskite sluggish grain growth of the post perovskite is caused by its strong anisotropic shape the phase transition from the perovskite to the post perovskite at the d discontinuity would lead to maintenance of small grain size in the d layer if diffusion creep is the dominant deformation mechanism of post perovskite the phase transformation would induce the significant softening of the d layer due to slow growth rate of post perovskite compared with a situation without the phase transition

the influence of post perovskite strength on the earth s mantle thermal and chemical evolution abstract we have investigated the influence of post perovskite ppv viscosity on mantle convective dynamics and stirring efficiency using numerical modeling and simple analytical theory our results show that the strength of ppv has a dramatic influence on convective dynamics the presence of a weak ppv enhances heat transfer across the bottom thermal boundary layer resulting in higher temperatures lower mantle viscosities and considerably larger convective velocities this leads to a significant increase in stirring efficiencies with decreasing the ppv strength by at least one order of magnitude in addition using a simple parameterized convection evolution that includes the influence of ppv coupled to a mixing model we show that during the long term history of the earth s mantle the presence of ppv yields systematically hotter thermal evolution and more efficient convective stirring such a strong effect of ppv strength on mantle stirring efficiency suggests that the influence of ppv phase must be considered when interpreting both geochemical and geophysical observations

is the long wavelength geoid sensitive to the presence of postperovskite above the core mantle boundary 1 the analysis of seismic data represents today the primary tool in the search for the presence of postperovskite in the lowermost mantle d 00 this work aims at testing whether the inversion of gravitational data can also contribute to the detection of postperovskite in d 00 w e assume that the transition from perovskite to postperovskite is accompanied by a reduction in viscosity and test the effects of such viscosity change on the prediction of the dynamic geoid with a numerical model of subducted lithosphere our results show that the long wavelength component of the geoid is very sensitive to the presence of postperovskite areas in d 00 especially if their viscosity is significantly lower than the viscosity of the surrounding perovskite and if these areas are located close to density

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

an atmospheric solitary gust observed with a doppler radar a tall tower and a surface network abstract doppler radar and a 444 m tall instrumented tower provide a detailed view of the kinematic and thermodynamic structure of a solitary gust a study of the data fields and comparison with theoretical and laboratory work leads to the conclusion that the gust is an internal solitary wave of permanent form launched by a thunderstorm outflow onto an inversion layer formed by the passage of an earlier storm comparisons with results from fluid experiments and numerical models are made and the similarities are striking both observations show turbulent breakdown at the rear of the wave the ease with which solitary waves can be generated in experiments gives reason to believe that these nonlinear waves might be of considerable interest in the context of geophysical fluid dynamics

observational analysis of a gust front to bore to solitary wave transition within an evolving nocturnal boundary layer abstract the evolution of a gust front to bore to solitary wave transition and comprehensive information on the evolving nocturnal boundary layer nbl associated with this change are described with analysis of radar and profiler measurements the observations were obtained on 21 june 2002 in the oklahoma panhandle during the international h2o project the evolution of this system from a strong bore initiated by a vigorous gust front to a solitary wave was observed over a 4 h period with doppler radar and surface measurements detailed information on the mature bore structure was obtained by a cluster of profiling instruments including two boundary layer wind profilers a lidar ceilometer and a microwave profiling radiometer a strong bore was initiated by an extensive gust front that perturbed an incipient nbl whose development prior to sunset was enhanced by shading from the parent mesoscale convective system at the time of bore formation the nbl was about 300 m deep and exhibited a surface t

central australian cold fronts abstract this paper presents an observational study of the structure and behavior of cold fronts over central australia during the late dry season a time of year when the prefrontal convectively well mixed layer is particularly deep the study is based on the results of the central australian fronts experiments cafe held in 1991 three fronts were documented in unprecedented detail for the australian region using a greatly enhanced surface observing network and a boundary layer wind profiler as well as serial upper air soundings data on the surface energy balance were obtained also a common feature of the fronts observed during cafe was that they were dry shallow 1 km deep and moved into a deep 4 km convectively well mixed boundary layer one of them initiated major dust storms across central australia a prominent feature of the fronts was the marked diurnal variation of their surface signature as they moved through the network noteworthy was the tendency during the night for the initiatio

the stability and structure of primordial reservoirs in the lower mantle insights from models of thermochemical convection in three dimensional spherical geometry large scale chemical lateral heterogeneities are inferred in the earth s lowermost mantle by seismological studies we explore the model space of thermochemical convection that can maintain reservoirs of dense material for a long period of time by using similar analysis in 3 d spherical geometry in this study we focus on the parameters thought to be important in controlling the stability and structure of primordial dense reservoirs in the lower mantle including the chemical density contrast between the primordial dense material and the regular mantle material buoyancy ratio thermal and chemical viscosity contrasts volume fraction of primordial dense material and the clapeyron slope of the phase transition at 660 km depth we find that most of the findings from the 3 d cartesian study still apply to 3 d spherical cases after slight modifications varying buoyancy ratio leads to different flow patterns from rapid upwelling to stable layering and large thermal viscosity contrasts are required to generate long wavelength chemical structures in the lower mantle chemical viscosity contrasts in a reasonable range have a second order role in modifying the stability of the dense anomalies the volume fraction of the initial primordial dense material does not effect the results with large thermal viscosity contrasts but has significant effects on calculations with intermediate and small thermal viscosity contrasts the volume fraction of dense material at which the flow pattern changes from unstable to stable depends on buoyancy ratio and thermal viscosity contrast an endothermic phase transition at 660 km depth acts as a filter allowing cold slabs to penetrate while blocking most of the dense material from penetrating to the upper mantle

deformation in the lowermost mantle from polycrystal plasticity to seismic anisotropy in the deep earth deformation occurs at many scales large scale convection produces subduction of slabs and upwelling of plumes in the mantle at the high temperature high pressure conditions strain is accommodated through crystal plasticity either by diffusion or the movement of dislocations slip causes crystal rotations and thus produces a characteristic pattern of crystal preferred orientation and corresponding anisotropy of physical properties at the macroscopic scale in this study we use polycrystal plasticity with experimentally derived deformation mechanisms for perovskite post perovskite and magnesiowuestite to predict texture development along streamlines in a 2d geodynamic convection model of the lowermost mantle strong preferred orientation develops during subduction and upwelling while during spreading along the core mantle boundary the orientation pattern is relatively stable from preferred crystal orientation and single crystal elastic properties bulk elastic properties can be calculated and compared with seismic observations post perovskite with predominant 001 slip and magnesiowuestite with 110 and 111 slip produce anisotropy patterns which are consistent with observed anisotropy i e fast s waves polarized parallel to the core mantle boundary and anti correlation between p and s wave anisotropies in contrast perovskite with dominant 001 slip and two post perovskite models with dominant slip on 010 and 100 produce anisotropy patterns which are inconsistent with seismic observations

a doubling of the post perovskite phase boundary and structure of the earth s lowermost mantle a new model of the deep earth allows geophysicists to measure variations in earth s temperature near the boundary between the rocky mantle and core using a recently discovered phase change in the lowest part of the earth s mantle where atoms rearrange themselves in crystals under extreme pressure and temperature the model also explains odd features of this part of the mantle such as patchy regions that reflect seismic waves generated by large earthquakes and inferences of dense partial melting combined with observations the results imply very large variations in temperature providing further evidence for circulation of material throughout the earth s mantle the thermal structure of the earth s lowermost mantle the d layer spanning depths of 2 600 2 900 kilometres1 is key to understanding the dynamical state and history of our planet earth s temperature profile the geotherm is mostly constrained by phase transitions such as freezing at the inner core boundary or changes in crystal structure within the solid mantle that are detected as discontinuities in seismic wave speed and for which the pressure and temperature conditions can be constrained by experiment and theory a recently discovered phase transition at pressures of the d layer2 3 4 is ideally situated to reveal the thermal structure of the lowermost mantle where no phase transitions were previously known to exist here we show that a pair of seismic discontinuities observed in some regions of d can be explained by the same phase transition as the result of a double crossing of the phase boundary by the geotherm at two different depths this simple model can also explain why a seismic discontinuity is not observed in some other regions and provides new constraints for the magnitude of temperature variations within d

simulation of a north american monsoon gulf surge event and comparison to observations abstractgulf surges are transient disturbances that propagate along the gulf of california goc from south to north transporting cool moist air toward the deserts of northwest mexico and the southwest united states during the north american monsoon they have been shown to modulate precipitation and have been linked to severe weather and flooding in northern mexico and the southwest united states the general features and progression of surge events are well studied but their detailed evolution is still unclear to address this several convection permitting simulations are performed over the core monsoon region for the 12 14 july 2004 gulf surge event this surge event occurred during the north american monsoon experiment which allows for extensive comparison to field observations a 60 h reference simulation is able to reproduce the surge event capturing its main characteristics speed and direction of motion thermodynamic changes during its passage and strong northward moisture flux while the ti

size distribution and shape properties of relatively small sea ice floes in the antarctic marginal ice zone in late winter in the marginal sea ice zone miz where relatively small ice floes are dominant the floe size distribution is an important parameter affecting melt processes given the larger cumulative perimeter of multiple small floes compared with a single ice floe of the same area smaller ice floes are therefore subject to increased lateral melt however the available data have been very limited so far analysis of sea ice in the sea of okhotsk revealed that while floe size distribution is basically scale invariant a regime shift occurs at a size of about 40 m in order to extend this preliminary result to the antarctic miz and further examine the controlling factors the first concurrent ice floe size and ice thickness measurements were conducted in the northwestern weddell sea and off wilkes land around 64 s 117 e with a heli borne digital video camera in the late winter of 2006 and 2007 respectively the floe sizes ranged from 2 to 100 m our analysis shows 1 the scale invariance and regime shift are confirmed in both regions 2 the floe size at which regime shift occurs slightly increases from 20 to 40 m with ice thickness consistent with the theory of the flexural failure of sea ice and 3 the aspect ratio is 1 6 1 9 on average close to the previous results based on these results the processes affecting the floe size distribution and the subsequent implications on melt processes are discussed by applying a renormalization group method to interpret the scale invariance in floe size distribution the fractal dimension is related to the fragility of sea ice these results indicate the importance of wave ice interaction in determining the floe size distribution

the generation and propagation of a nocturnal squall line part i observations and implications for mesoscale predictability abstract the initiation and forcing of a squall line on 26 27 may 1985 near wichita kansas is examined the squall line is secondary convection in the sense that it is preceded in the diurnal cycle by other mesoscale convective systems the squall fine develops in synoptic conditions frequently associated with convective weather in the central united states this includes moderate shear moderate to high convective available potential energy a quasi stationary dryline a low level southerly jet and an 85 kpa moist tongue the initiation of free convection is specifically attributed to a gust front that propagates into a low level jet with attendant horizontal vorticity convergence and moisture the gust front initially propagates as a gravity current and subsequently as an internal undular bore observations of gravity currents gravity waves and the collision of radar echo boundaries provide valuable clues for short period forecasting they establish the presence of potential triggering mechanism

analysis of a lower tropospheric gravity wave train using direct and remote sensing measurement systems abstracton 10 august 2014 a gravity wave complex generated by convective outflow propagated across much of oklahoma the four dimensional evolution of the wave complex was analyzed using a synthesis of near surface and vertical observations from the oklahoma mesonet and atmospheric radiation measurement arm southern great plains networks two atmospheric emitted radiance interferometers aeri one located at the arm sgp central facility in lamont oklahoma and the other in norman oklahoma were used in concert with a doppler wind lidar dwl in norman to determine the vertical characteristics of the wave complex hydraulic theory was applied to the aeri derived observations to corroborate the observationally derived wave characteristics it was determined that a bore soliton wave packet initially formed when a density current interacted with a nocturnal surface based inversion and eventually propagated independently as the density current became diffuse the soliton propagated within an elevated invers

dynamics of a simulated north american monsoon gulf surge event abstractgulf surges are transient disturbances that propagate along the gulf of california goc from south to north transporting cool moist air toward the deserts of northwest mexico and the southwest united states during the north american monsoon they have been shown to modulate precipitation and have been linked to severe weather and flooding in northern mexico and the southwest united states the general features and progression of surge events are well documented but their detailed dynamical evolution is still unclear in this study a convection permitting simulation is performed over the core monsoon region for the 12 14 july 2004 gulf surge event and the dynamics of the simulated surge are examined initially convection associated with the tropical easterly wave precursor to tropical cyclone blas creates a disturbance in the southern goc on early 12 july this disturbance is a precursor to the gulf surge on 13 july and is a kelvin shock internal bore under the influence of rotation that diss

characteristics of sea ice floe size distribution in the seasonal ice zone 1 the size distribution of sea ice floes was observed by coordinated landsat imagery and video monitoring conducted from an icebreaker and a helicopter for an area 38 km 26 km in seasonal sea ice in the southern sea of okhotsk in february 2003 the combination of imagery on several scales allowed measurements of ice floes over three orders of magnitude from 1 m to 1 5 km two different regimes were observed floes larger than about 40 m have a power law number density with an exponent of 1 87 in the lower range of earlier results below 40 m the power law exponent is 1 15 the cause of these two different regimes is hypothesized to lie in the effects of swell on floes of different sizes and thicknesses the importance of the floe size distribution for lateral melting is elucidated

the impact of variability in the rheological activation parameters on lower mantle viscosity stratification and its dynamics the recently discovered spin state crossover in iron in major mantle minerals at high pressures should exert dramatic influences on transport properties such as the activation creep parameters in the deep mantle wentzcovitch et al 2009 have computed the elastic parameter of ferropericlase that is affected by this crossover the bulk modulus at high realistic conditions using first principles density functional theory plus hubbard u as a consequence of the spin crossover there is a significant softening of the bulk modulus and an attendant reduction of the activation energy in the creep law using a parameterized model capturing the basic physics we have studied the dynamical consequences within the framework of a 2 d cartesian convection model our models reveal a series of asthenospheres and are characterized first by a low viscosity channel below the 670 km boundary second asthenosphere caused by an increase of temperature and decrease in grain size reduction superplascity with the post spinel transition which results in partially layered convection further the variability of the rheological parameters leads to the prevalent formation of viscosity minimum at a depth of about 1600 km third asthenosphere caused by spin crossovers followed by a viscosity hill in the bottom half of the lower mantle the latter due to the increase of the activation energy in some low spin irons at the bottom of the mantle our numerical simulations reveal a tendency to the formation of small scale convection below the 670 km boundary and bigger stabilized plumes and or plume clustering in the deep lower mantle other material properties at the bottom of the mantle decrease of thermal expansivity radiative thermal conductivity also exert significant influence on the multi scale lower mantle plume dynamics

morning glory wave clouds in oklahoma a case study abstract early in the morning on 9 june 1982 a system of traveling wave cloud lines passed over 0klahoma and in particular over the relatively dense mesonetwork of surface stations including the instrumented 444 m ktvy television tower operated by the u s national severe storms laboratory an analysis of the network and other data presented herein shows that in structure the associated disturbance was an internal undular bore propagating on a low level stable layer similar to morning glory type disturbances which are common at certain times of the year in parts of northern australia moreover the speed of propagation of the component bore waves is in broad agreement with theoretical calculations there is evidence that the disturbance emanated late the previous evening from an area of rapidly growing intense thunderstorms located more than 400 km north northwest of the surface mesonetwork possible mechanisms for generation are discussed but the data pertaining to genesis are insufficient to

effects of lateral viscosity variations on the geoid 1 we investigate the effects of lateral viscosity variations lvvs on mantle circulation model predictions of the global geoid the present study is motivated in part by earlier findings that lvvs due to stiff slabs in the lower mantle have a strong influence on the geoid and that slabs in the lower mantle are perhaps no stronger than the ambient mantle however more recently it has been argued based on global seismic tomography models that lvvs are a minor effect on the geoid in the light of these seemingly contradictory results we re visit the problem of slab strength in the lower mantle we substantiate that the geoid calculated from tomography is hardly affected by the presence of lvvs whereas the geoid computed from global slab models yields a poor fit to the observed geoid when lvvs are considered however this degradation of fit only occurs for very long wavelengths of flow indicating inherent differences between the slab and tomography models we also investigate the effects on the geoid due to weak plate boundaries strong cratonic keels and a low viscosity region in the d layer due to post perovskite in addition to the geoid we attempt to fit plate motions with a circulation model that has prescribed weak zones at plate boundaries motions are matched well and taking into account lvvs the geoid with appropriate surface velocity boundary conditions agrees with the observed geoid as well as for free slip cases

structure of an atmospheric undular bore generated from colliding boundaries during cape abstract a case study of a sea breeze front originating from the east coast of florida colliding with a gust front moving toward the southeast is presented single and multi doppler radar analyses combined with serial balloon ascents suggest that the denser sea breeze flow undercut the cold pool behind the gust front and may have generated a westward propagating undular bore in addition another undular bore was generated by the collision and propagated eastward through the cape network the latter bore propagated upstream against the southeasterly flow behind the sea breeze front and was apparent as a narrow band 5 6 km wide of near zero radial velocities embedded within an overall flow that was positive as seen by the cp 4 doppler radar as the eastern bore propagated through the dual doppler lobe defined by the ncar cp 3 and cp 4 radars a detailed and unique wind synthesis revealed its three dimensional kinematic structure significant along bore variability was shown in the convergence and verti

in situ measurement of texture development rate in cairo3 post perovskite the rate of crystallographic preferred orientation cpo development during deformation of post perovskite is crucial in interpreting seismic anisotropy in the lowermost mantle but the stability field of mgsio3 post perovskite prevents high strain deformation experiments being performed on it therefore to constrain the rate of cpo development in post perovskite we deformed cairo3 a low pressure analogue of mgsio3 post perovskite in simple shear at 3 2 gpa and 400 c to a shear strain of 0 81 from x ray diffraction patterns acquired during deformation we invert for cpo as a function of strain by comparing the cpo that develops with visco plastic self consistent vpsc models we constrain the critical resolved shear stresses crss of the non primary slip systems in cairo3 to be of order 6 times stronger than the primary 100 010 slip system this value is significantly less than has been assumed by previous studies and if applicable to mgsio3 implies that seismic anisotropy in the d layer develops slower than has previously been assumed

an observational study of atmospheric bore formation from colliding density currents abstract observations from east central florida during the convection and precipitation electrification cape experiment are used to investigate the factors that influence atmospheric bore formation from colliding density currents ten cases involving the collision of a gust front with a sea breeze front are analyzed with doppler radar and sounding and surface mesonet data the gust fronts in these collisions were generally deeper denser and faster propagating than their sea breeze front counterparts seven of the 10 cases produced dual boundaries that moved away from each other in a relative sense after the collision post collision boundaries moving in the same direction and oriented similarly to the pre collision gust front occurred in all 10 cases they transported mass in the manner of a density current in six cases while the others behaved more like bores or bore density current hybrids as they were characterized by little or no mass transport similarity of gust front and sea breeze front prop

evolution and vertical structure of an undular bore observed on 20 june 2015 during pecan abstractthis study documents the evolution of an impressive largely undular bore triggered by an mcs generated density current on 20 june 2015 observed as part of the plains elevated convection at night pecan experiment the university of wyoming king air with profiling nadir and zenith viewing lidars sampled the south bound bore from the time the first bore wave emerged from the nocturnal convective cold pool and where updrafts over 10 m s 1 and turbulence in the wave s wake were encountered through the early dissipative stage in which the leading wave began to lose amplitude and speed through most of the bore s life cycle its second wave had a higher or equal amplitude relative to the leading wave striking roll clouds formed in wave crests and wave energy was detected to about 5 km agl the upstream environment indicates a negative scorer parameter region due to flow reversal at midlevels providing a wave trapping mechanism the observed bore strength of 2 4 2 9 and speed of 15 16 m s 1 agree

observations of the summer breakup of an arctic sea ice cover the arctic sea ice cover evolves dramatically through the summer melt season floe size distribution fsd is a critical parameter used to examine this change as the ice cover transitions from large rectilinear plates in spring to an ensemble of discrete rounded floes by midsummer the fsd at a given time impacts the dynamic and thermodynamic behavior of the ice cover focusing on the seasonal marginal ice zone in the beaufort and chukchi seas from may to september 2014 we present qualitative and quantitative results derived from national technical means high resolution imagery and supported by ice mass balance buoy data results indicate that as melt accelerates floe breaking pattern and therefore fsd is heavily influenced by the distribution of melt ponds discrete element model results using morphological conditions derived from analyzed satellite imagery confirmed that breaking occurs along ponds and perpendicular to applied stress

multiyear sea ice floe distribution in the canadian arctic ocean a series of aerial photographic flights made over the polar pack ice during april 1982 from m clure strait in the south to ellef ringnes island in the north shows the changes in ice floe type size and area as the pack moves southwest under the effect of the polar gyre the area has some of the most heavily ridged and dynamically active ice in the arctic ocean floe size distributions were found to fit a negative power relationship y a xb where b 3 7 rather than the expected negative exponential function a prony analysis suggests that a single physical parameter controls the distribution most likely splitting rather than bending shear or crushing the effect of proximity to the coast was investigated and showed that nearshore floes were about 20 smaller than their offshore counterparts the ratio of maximum to minimum floe diameter was consistently 1 5 1 6 the mean floe diameter was found to be 700 m the mean area was 0 38 km 2 heavily hummocked and ridged floes were found to be a fairly constant fraction 1 of the multiyear pack

analysis of the 13 14 july gulf surge event during the 2004 north american monsoon experiment abstract gulf surges are disturbances that move northward along the gulf of california goc frequently advecting cool moist air from the goc or eastern tropical pacific ocean into the deserts of the southwest united states and northwest mexico during the north american monsoon nam little attention has been given to the dynamics of these disturbances because of the lack of reliable high resolution data across the nam region high temporal and spatial observations collected during the 2004 north american monsoon experiment are used to investigate the structure and dynamical mechanisms of a significant gulf surge on 13 14 july 2004 integrated sounding systems deployed along the east coast of the goc and an enhanced network of rawinsonde sites across the nam region are used in this study observations show that the 13 14 july gulf surge occurred in two primary stages the first stage was preceded by anomalous low level warming along the northern goc on 13 july sharp cooling moistening and increased

initiation of a solitary wave family in the demise of a nocturnal thunderstorm density current abstract this paper describes the characteristics and evolving nature of a vigorous thunderstorm density current very early in the morning of 9 may 1981 in oklahoma because the ambient lower atmosphere was stratified interesting interactions between the outflow current and the ambient environment resulted the leading portion of the current was modulated by at least three gravity wavelike perturbations of horizontal spacing 12 km which initially coexisted with it however as the current evolved it initiated an undular borelike disturbance which propagated ahead into the stable boundary layer carrying cold outflow air in large amplitude rolls eventually the wave family left the decelerating outflow air in its wake this borelike disturbance resembles the australian morning glory phenomenon and appears to represent an early stage in the development of a solitary wave family the observations resemble other reported morning glories and solitary waves as well their laboratory and numerically simulated

the cold front of 15 april 1994 over the central united states part i observations abstract detailed observations of the interactions of a cold front and a dryline over the central united states that led to dramatic undulations in the boundary layer including an undular bore are investigated using high resolution water vapor mixing ratio profiles measured by raman lidars the lidar derived water vapor mixing ratio profiles revealed the complex interaction between a dryline and a cold frontal system an elevated well mixed and deep midtropospheric layer as well as a sharp transition between 5 and 6 km altitude to a drier region aloft was observed the moisture oscillations due to the undular bore and the mixing of the prefrontal air mass with the cold air at the frontal surface are all well depicted the enhanced precipitable water vapor and roll clouds the undulations associated with the bore the strong vertical circulation and mixing that led to the increase in the depth of the low level moist layer and the subsequent lifting of this moist layer by the cold frontal surface

wave disturbances and their role in the maintenance structure and evolution of a mesoscale convection system abstractthis study investigates a nocturnal mesoscale convective system mcs observed during the plains elevated convection at night pecan field campaign a series of wavelike features were obse

the lower atmosphere over the gulf of california automated meteorological observations at coastal and island stations instrumented aircraft flights and coastal soundings were made between july 1983 and june 1985 to define the variations of the lower atmosphere over the northern half of the gulf of california during the two important climatic seasons the mid latitude winter and the subtropical summer a marine layer is well defined over water during both seasons but dissipates within a few kilometers inland the winter large scale pressure field is dominated by the great basin high over the southwestern united states modulated by upper level synoptic activity it causes 3 to 6 days events of northwesterly winds 8 12 m s 1 directed along the gulf s axis which are coherent over basin scales and bring cool dry desert air over the gulf the vertical scale of these winds is 700 1000 m over land on the western side of the gulf close to the height of the baja california mountains but only 100 400 m over water defined by the wintertime inversion cross gulf winds related to topographic effects during upper level trough passages over baja california are particularly intense in the northwestern portion of the gulf the winter marine layer is defined by a 1 to 4 5 c temperature inversion and a dew point temperature of 6 11 c a moisture content of 6 8 g kg 1 weak subsidence reduces dew point values to 4 c above the inversion winter winds above the marine layer are coherent 0 8 across the width of the gulf the summer large scale pressure field is dominated by a thermal low over the southwestern united states and drives weak 2 5 m s 1 southeasterly winds also directed primarily along the gulf which are less spatially uniform than winter winds air temperature and moisture content are rather constant and topographic effects are absent the summer marine layer is on the order of 200 300 m thick with dew point temperatures of 26 28 c 21 24 g kg 1 and capped by a weak temperature inversion 1 2 c over water aloft the dew point temperature is 17 21 c and winds are weak and uncorrelated across the width of the gulf during june and early july 1984 four week long pulses of southeasterly winds 10 m s 1 support the existence reported previously of moisture surges from the gulf into the sonoran desert a dynamical analysis of subdiurnal motions shows that the cross gulf momentum balance is geostrophic the along gulf balance is ageostrophic with the along gulf pressure gradient opposed by friction at the surface these results are consistent with a scale analysis of the equations of motion in the marine layer over the gulf of california

an observational and modeling study of an atmospheric internal bore during name 2004 abstract observations from the 2004 north american monsoon experiment name have been used to identify an atmospheric internal bore that occurred over the gulf of california goc on 31 july 2004 this bore disturbance was identified at bahia kino along the northwest coast of mexico during the late evening of 31 july it was hypothesized to have originated from the interaction of a gravity current from a large mesoscale convective system mcs which formed along the western slopes of the sierra madre occidental during the afternoon and a surface stable layer that developed from a sea breeze circulation it is suggested that the bore s energy was trapped at low levels by an elevated stable layer the vertical structure and undular nature of the bore was initially identified from 915 mhz wind profiler data at bahia kino results show a series of waves along the bore s leading edge and turbulent mixing of air from above the stable layer to the surface on the downstream face of the leading undulation the

the influence of the rocky mountains on the 13 14 april 1986 severe weather outbreak part ii evolution of a prefrontal bore and its role in triggering a squall line abstract in this paper part ii of a series the evolution of a prefrontal bore on the leeside of the rockies and its subsequent propagation and initiation of convection farther downstream over eastern colorado and western nebraska are investigated the observational evidence for this sequence of events was obtained from combined analyses of high resolution goes satellite imagery and program for regional observing and forecasting services mesonetwork data over the colorado region for the severe weather event that occurred during 13 14 april 1986 a 2d nonhydrostatic numerical model is used to further understand the initiation of the bore and its ability to propagate farther downstream and trigger convection analysis of satellite imagery and mesonet data indicated that an internal bore ahead of a cold front a moderate downslope windstorm and a quasi stationary hydraulic jump were generated within a few hours along the iceslope as a pacific cold front and its attendant upper level jet streak advanced o

multiscale variability of the flow during the north american monsoon experiment abstract the 2004 north american monsoon experiment name provided an unprecedented observing network for studying the structure and evolution of the north american monsoon this paper focuses on multiscale characteristics of the flow during name from the large scale to the mesoscale using atmospheric sounding data from the enhanced observing network the onset of the 2004 summer monsoon over the name region accompanied the typical northward shift of the upper level anticyclone or monsoon high over northern mexico into the southwestern united states but in 2004 this shift occurred slightly later than normal and the monsoon high did not extend as far north as usual consequently precipitation over the southwestern united states was slightly below normal although increased troughiness over the great plains contributed to increased rainfall over eastern new mexico and western texas the first major pulse of moisture into the southwest occurred around 13 july in association with a strong gulf of californi

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

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

structure of an internal bore and dissipating gravity current as revealed by raman lidar abstract detailed moisture observations from a ground based raman lidar and special radiosonde data of two disturbances associated with a dissipating gust front are presented a synthesis of the lidar data with conventional meteorological data in conjunction with theoretical calculations and comparison to laboratory studies leads to the conclusion that the disturbances seen in both the lidar and accompanying barograph data represent a weak gravity current and an associated undular bore the disturbances display excellent coherence over hundreds of kilometers upstream of the lidar site bore formation occurs at the leading edge of the gust front coincidentally with the rapid weakening of the gravity current analysis suggests that the bore was generated by the collapse of the gravity current into a stable nocturnal inversion layer and subsequently propagated along this wave guide at nearly twice the speed of the gravity current the raman lidar provided detailed measurements of the vertical structure o

a nonclassical cold front observed during cops 91 frontal structure and the process of severe storm initiation abstract this case study addresses the issue of gravity current and bore development at surface cold fronts and the role of these phenomena in the generation of severe frontal convection the event investigated occurred on 27 april 1991 during the cooperative oklahoma profiler studies 1991 field project the development of a bore from a gravity current like structure along a cold front the subsequent propagation of the bore ahead of the front on a low level inversion and the process of severe thunderstorm development along the front are revealed by a dense network of remote sensing and other special observations evidence for the gravity current and bore is strengthened by comparisons made between the synthesized observations and theory the bore developed after a nocturnal inversion which acted as a waveguide had become established the bore and gravity current were both evident as fine lines in the radar reflectivity displays a microscale envelope of enhanced water vapor with an embedded roll cl

the morning glory of the gulf of carpentaria an atmospheric undular bore abstract this paper presents the results of a field expedition mounted in late september early october 1979 to investigate the structure and origin of the morning glory of the gulf of carpentaria in northern australia the morning glory is a line wind squall accompanied by a pressure jump and often by a long roll cloud or series of such clouds it frequently occurs in the early morning especially in october in the gulf area a light aircraft fitted with a temperature and humidity probe was flown in two glories to determine their thermodynamic structure and wind fields wore obtained principally by tracking pilot balloons using the double theodolite method data also were obtained from a network of surface stations recording wind velocity and pressure installed at locations across cape york peninsula which is believed to be the area of genesis the morning glory is identified as an internal undular bore propagating on the nocturnal and or maritime inversion its origin appears to lie frequently

weakening of calcium iridate during its transformation from perovskite to post perovskite magnesium silicate perovskite the dominant mineral in the lower mantle is thought to transform into a post perovskite phase in the mantle s lowermost region laboratory experiments suggest substantial weakening could occur during the transformation from perovskite to post perovskite which could explain the anomalous physical properties of the lowermost mantle

efficacy of the post perovskite phase as an explanation for lowermost mantle seismic properties constraining the chemical rheological and electromagnetic properties of the lowermost mantle d is important to understand the formation and dynamics of the earth s mantle and core to explain the origin of the variety of characteristics of this layer observed with seismology a number of theories have been proposed including core mantle interaction the presence of remnants of subducted material and that d is the site of a mineral phase transformation this final possibility has been rejuvenated by recent evidence for a phase change in mgsio3 perovskite thought to be the most prevalent phase in the lower mantle at near core mantle boundary temperature and pressure conditions here we explore the efficacy of this post perovskite phase to explain the seismic properties of the lowermost mantle through coupled ab initio and seismic modelling of perovskite and post perovskite polymorphs of mgsio3 performed at lowermost mantle temperatures and pressures we show that a post perovskite model can explain the topography and location of the d discontinuity apparent differences in compressional and shear wave models and the observation of a deeper weaker discontinuity furthermore our calculations show that the regional variations in lower mantle shear wave anisotropy are consistent with the proposed phase change in mgsio3 perovskite

the seasonal evolution of sea ice floe size distribution the arctic sea ice cover undergoes large changes over an annual cycle in winter and spring the ice cover consists of large snow covered plate like ice floes with very little open water by the end of summer the snow cover is gone and the large floes have broken into a complex mosaic of smaller rounded floes surrounded by a lace of open water this evolution strongly affects the distribution and fate of the solar radiation deposited in the ice ocean system and consequently the heat budget of the ice cover in particular increased floe perimeter can result in enhanced lateral melting we attempt to quantify the floe evolution process through the concept of a floe size distribution that is modified by lateral melting and floe breaking a time series of aerial photographic surveys made during the sheba field experiment is analyzed to determine evolution of the floe size distribution from spring through summer based on earlier studies we assume the floe size cumulative distribution could be represented by a power law d where d is the floe diameter the exponent as well as the number density of floes ntot are estimated from measurements of total ice area and perimeter as summer progressed there was an increase in as the size distribution shifted toward smaller floes and the number of floes increased lateral melting causes the distribution to deviate from a power law for small floe sizes

observations of pressure jump lines in the midwest 10 12 august 1976 abstract strong thunderstorm activity over iowa on two successive afternoons was the apparent source of pressure jump lines pjl s which moved sse at 50 km h 1 through the nocturnal boundary layer and were detected by national weather service nws stations as far away as paducah kentucky rainshowers and thunder were reported at many nws stations as the pjl s passed the regional air monitoring system rams network at st louis provided detailed information on the pjl s arrival there was indicated by an abrupt pressure rise of 1 5 mb a near reversal of the surface flow and a vertical displacement of 750 m extending through the lower 4 km of the atmosphere the passage of each pjl was coincident with the turbulent collapse of the nocturnal jet the observations of the pjl events seem indicative of an internal bore and are similar to those of the morning glory seen in northern australia we speculate that the bore originates from a late afternoon convergence produced by thunderstorm outflow and oppos

transformation textures in post perovskite understanding mantle flow in the d layer of the earth 1 deformation and texture formation in mg fe sio3 post perovskite ppv is a potential explanation for the strong seismic anisotropy that is found in the d layer of the earth however different experimental approaches have resulted in different lattice preferred orientations lpo in deformed ppv that have led to ambiguity in the interpretation of deformation in the lowermost mantle here we show that deformation of the analogue substance cairo3 during a phase transformation from perovskite to ppv leads to a transformation texture that differs from the cairo3 ppv deformation texture but resembles the results from ppv deformation experiments in diamond anvil cells assuming material spreading parallel to the core mantle boundary our results predict a widespread shear wave splitting with fast horizontal s waves which is compatible with seismic studies downwelling material that undergoes a phase transformation may develop a transformation texture that would locally result in vertically polarized fast s waves

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