seismic detection of folded subducted lithosphere at the core mantle boundary seismic tomography of the d region beneath the caribbean reveals a 100 km vertical step in an otherwise flat d shear velocity discontinuity and localized low velocities at the edge of the slab material which may involve upwellings caused by the slab laterally displacing a thin hot thermal boundary layer seismic tomography has been used to infer that some descending slabs of oceanic lithosphere plunge deep into the earth s lower mantle1 2 the fate of these slabs has remained unresolved but it has been postulated that their ultimate destination is the lowermost few hundred kilometres of the mantle known as the d region relatively cold slab material may account for high seismic velocities imaged in d beneath areas of long lived plate subduction and for reflections from a seismic velocity discontinuity just above the anomalously high wave speed regions3 4 the d discontinuity itself is probably the result of a phase change in relatively low temperature magnesium silicate perovskite5 6 here we present images of the d region beneath the cocos plate using kirchhoff migration of horizontally polarized shear waves and find a 100 km vertical step occurring over less than 100 km laterally in an otherwise flat d shear velocity discontinuity folding and piling of a cold slab that has reached the core mantle boundary as observed in numerical and experimental models can account for the step by a 100 km elevation of the post perovskite phase boundary due to a 700 c lateral temperature reduction in the folded slab we detect localized low velocities at the edge of the slab material which may result from upwellings caused by the slab laterally displacing a thin hot thermal boundary layer

lateral variation of the d discontinuity beneath the cocos plate 1 broadband shear wave signals from 15 deep south american earthquakes reveal small scale variations of the d shear velocity discontinuity beneath the cocos plate deconvolution by average event source wavelets allows multiple event and california station combinations to be double array stacked in 4 geographically separate reflection point bins with lateral dimensions of about 200 km reflections from the top of d are apparent in all 4 bins with significant variations in timing and strength relative to scs the data are compatible with a discontinuous shear velocity increase 264 km above the core mantle boundary that varies laterally in strength from 0 9 to 2 6 with corresponding lateral velocity variations within d

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

three dimensional effects in high drag state flows over long ridges abstract numerical simulations of nonrotating flow with uniform basic wind and stability past long three dimensional 3d ridges are compared to the corresponding two dimensional 2d limit to reveal the importance of 3d effects for mountain heights smaller than the threshold for breaking waves the low level flow over the interior of the ridge is well described by 2d theory when the horizontal aspect ratio is roughly 10 or greater by contrast in flows with wave breaking significant discrepancies between 2d and 3d results remain apparent even for 12 it is found that the onset of wave breaking and the transition to the high drag state is accompanied in 3d by an abrupt increase in deflection of the low level flow around the ridge the increased flow deflection is produced at least in part by upstream propagating columnar disturbances forced by the transition to the high drag state the deflection of the incident flow reduces the amplitude of the mountain wave aloft relative to 2d and acts as a neg

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

the origin of d reflections a systematic study of seismic array data sets regional seismic studies of the earth s deep mantle indicate the presence of p and s wave reflectors at the top of the d layer such reflectors appear to be laterally intermittent occur at a range of depths and produce waveforms whose amplitude and polarity differ at different locations a number of possible explanations for d reflections have been proposed including a phase change from perovskite to post perovskite thermal or thermochemical discontinuities generated by subducted slabs alignment of anisotropic minerals and global or small scale chemical heterogeneity arising perhaps from core mantle interactions or primitive material however some of the observed heterogeneity in d structure may be related to lack of consistency between the style quality and number of seismic observations in different regions as well as complexity in wave propagation due to 3 d structure in this study we present a compilation of observations of d reflections for regions in which 1 a reflector is seen in both p and s wave arrivals and 2 consistent array seismology techniques have been applied to infer the properties of the reflectors using a recent monte carlo thermodynamic modelling method to convert changes in chemistry and mineralogy into changes in seismic velocity we compare the fit of different isotropic thermochemical structures to the observed seismic properties of d reflections namely the polarity and amplitude of the waveforms our results indicate that it may be possible to discriminate between different possible origins for d reflections on the basis of polarity observations alone amplitude measurements may provide further constraints but must be treated with caution due to a large number of factors which can influence waveform amplitudes in regions such as the caribbean which are characterized by high amplitude negative polarity p waveforms and positive polarity s waveforms a phase change from perovskite to post perovskite provides the most likely explanation for the discontinuity however in other regions notably siberia and the central pacific p and s waveforms both display a positive polarity and multiple explanations for the discontinuity are possible including subduction related chemical heterogeneity and anisotropy

thermally forced convection over a mountainous tropical island abstractobservations from the dominica experiment domex and cloud resolving numerical simulations are used to study a thermally forced convection event over the caribbean island of dominica on 18 april 2011 a clear diurnal cycle of island thermal forcing and cumulus convection was observed with cumuli initiating over the southwestern flank of the ridge and deepening as they drifted eastward apart from errors in cloud fraction and notably precipitation the simulations verified well against the observations provided horizontal grid spacings of 500 m or less were used the simulated flows developed an island scale solenoidal circulation with an organized and intense updraft over the ridge that focused convective initiation sensitivity tests investigated the impacts of topographic forcing subcloud winds and cloud radiative feedbacks on the island scale horizontal inflow and cloud vertical mass flux these experiments confirmed that thermal forcing drove the island convection and that the inflow an

phase transitions in pyrolite and morb at lowermost mantle conditions implications for a morb rich pile above the core mantle boundary abstract subduction of mid oceanic ridge basalt morb gives rise to strong chemical heterogeneities in the earth s mantle possibly extending down to the core mantle boundary phase relations in both pyrolite and morb compositions are precisely determined at high pressures and temperatures corresponding to lowermost mantle conditions the results demonstrate that the post perovskite phase transition occurs in pyrolite between 116 and 121 gpa at 2500 k while post perovskite and sio2 phase transitions occur in morb at 4 gpa lower pressure at the same temperature theory predicts that these phase changes in pyrolite and morb cause shear wave velocity increase and decrease respectively near the northern margin of the large low shear velocity province in the lowermost mantle beneath the pacific reflections from a negative shear velocity jump near 2520 km depth are followed by reflections from a positive velocity jump 135 to 155 km deeper these negative and positive velocity changes are consistent with the expected phase transitions in a dense pile containing a mixture of morb and pyrolitic material this may be a direct demonstration of the presence of accumulations of subducted morb crust in the deep mantle

upstream effects of mesoscale mountains abstract the alpine experiment alpex has revealed that low level air is typically diverted around the alps without reaching the mountaintop in pursuit of an understanding of the physical basis of this phenomenon and of its generality we have explored the characteristics of orographic blocking of a rotating continuously stratified quid as revealed in a simple model problem retaining full nonlinear and transient effects hydrostatic dynamics is assumed and the obstacle is taken to be an infinitely long ridge with height h x the key questions treated are the strength of the upstream deceleration of cross mountain flow and the length scale over which the decelerated region extends by means of scale analysis the controlling parameters are found to be the rossby number ro u fl and the froude number fr nhm u where u is the speed of the oncoming flow f is the coriolis parameter l the mountain half width n the brunt vaisala frequency and hm is the maximum mountain height the scale analysis also

geological origins of seismic bright spot reflections in the ordovician strata in the halahatang area of the tarim basin western china anomalously high amplitude seismic reflections are commonly observed in deeply buried ordovician carbonate strata in the halahatang area of the northern tarim basin these bright spots have been demonstrated to be generally related to effective oil and gas reservoirs these bright spot reflections have complex geological origins because they are deeply buried and have been altered by multi phase tectonic movement and karstification currently there is no effective geological model for these bright spots to guide hydrocarbon exploration and development using core well logs and seismic data the geological origins of bright spot are classified into three types controlled by karstification faulting and volcanic hydrothermal activity bright spots differing by geological origin exhibit large differences in seismic reflection character such as reflection amplitude curvature degree of distortion and the number of vertically stacked bright spots in the seismic section by categorizing the bright spot

influence of the wind profile on the initiation of convection in mountainous terrain a number of days with small precipitating convective cells were investigated using weather radars during the cops convective and orographically induced precipitation study field campaign in the region of the vosges and the rhine valley in central europe depending on the weather situation two distinct mechanisms could be identified for the initiation of convection on some days cells were initiated over the ridge of the vosges whereas on other days cells were initiated in the lee of the vosges the initiation of convection appeared to be concentrated in a few favourable locations using the froude number it was possible to describe the two distinct mechanisms when the froude number was low the flow was diverted around the vosges and thermally driven convergence at the ridge initiated convection whereas when the froude number was high the flow passed through mountain gaps and then converged on the lee side with the flow in the rhine valley the convergence on the lee side was enhanced at locations where the outflows through valleys converged low froude numbers were accompanied by weak winds varying with height whereas high froude numbers were observed during situations with stronger southwesterly winds increasing with height

the addition of heat to a stratified airstream with application to the dynamics of orographic rain summary the response of a stratified airstream to combined thermal and orographic forcing is investigated theoretically using the linearized hydrostatic equations of motion the magnitude of the heating aloft is computed from observed rainfall rates the elevated heating is shown to produce vertically propagating waves whose amplitude relative to the mountain waves is determined by the parameter gqb cptuanh symbols appendix i for typical wind speeds and rainfall rates the thermally generated waves will equal or exceed the orographically generated waves these waves produce large vertical variations in the wave momentum flux at the levels where heat is being added and alter or even reverse the mountain drag in hydrostatic flow the phase relationship between the heating rate and the induced vertical displacements is found unexpectedly to be negative in the vicinity of the heating air parcels are displaced downwards this result can be explained by wave propagation ideas but not by parcel arguments this helps to explain why mountain wave amplitudes are sometimes reduced in moist atmospheres the broad heat induced descent may act to limit the amount of condensation precipitation occurring in a stable middle level orographic cloud other types of orographic precipitation involving embedded convection in stratus layers low level feeder clouds or deep cumulus convection may not be adversely effected by the heat induced descent

on thermally forced circulations over heated terrain abstracta combination of analytical and numerical models is used to gain insight into the dynamics of thermally forced circulations over diurnally heated terrain solutions are obtained for two layer flows representing the boundary layer and the overlying free troposphere over an isolated mountainlike heat source a scaling based on the linearized boussinesq system of equations is developed to quantify the strength of thermally forced updrafts and to identify three flow regimes each with distinct dynamics and parameter sensitivities this scaling closely matches corresponding numerical simulations in two of these regimes the first characterized by a weakly stable boundary layer and significant background winds and the second by a strongly stable boundary layer in the third regime characterized by weak winds and weak boundary layer stability this scaling is outperformed by a fundamentally different scaling based on thermodynamic heat engines within this regime the inability of wind ventilation or

seismic evidence links the subducted mongol okhotsk slab to deformation in d near the northeastern margin of the perm anomaly abstract numerical simulations have indicated that the large low shear velocity provinces llsvps dynamically interact with subducted slabs in d and slabs in turn play an important role in influencing the shape and location of the llsvps in this study we find evidence for the existence of the d discontinuity near the northeastern margin of the perm anomaly a smaller llsvp beneath eurasia by analyzing the scd the lower mantle triplication phases generated from two deep events occurring in the northwestern pacific subduction zone and recorded at stations in europe and turkey the d discontinuity in the north is characterized with a 2 velocity increase occurring at a height of 300 km above the core mantle boundary cmb whereas the discontinuity in the south is characterized with a 3 velocity increase occurring at a height of 260 km above the cmb the d discontinuity is most likely attributed to a phase transition from perovskite to post perovskite in conjunction with the presence of relic slab materials likely related to the subducted mongol okhotsk slab our observations along with previous seismic anisotropic detections provide a clue to dynamically link the subducted slab to deformation taking place near the edges of the llsvp

temperature profile in the lowermost mantle from seismological and mineral physics joint modeling the internal structure of the core mantle boundary cmb region of the earth plays a crucial role in controlling the dynamics and evolution of our planet we have developed a comprehensive model based on the radial variations of shear velocity in the d layer the base of the lower mantle and the high p t elastic properties of major candidate minerals including the effects of post perovskite phase changes this modeling shows a temperature profile in the lowermost mantle with a cmb temperature of 3 800 200 k which suggests that lateral temperature variations of 200 300 k can explain much of the large velocity heterogeneity observed in d a single crossing phase transition model was found to be more favorable in reproducing the observed seismic wave velocity structure than a double crossing phase transition model

cloud trails past the lesser antilles abstractobservations and cloud resolving simulations of elongated cloud plumes or cloud trails past the lesser antilles islands in the caribbean sea are presented analysis of one year of visible satellite images reveals that each island forms cloud trails on 30 40 of days typically in the afternoon in response to diurnal island heating on around 10 of days the cloud bands are very well organized with lengths of and durations of min radiosonde analysis suggests that the well organized events are favored by moderate to strong trade winds 6 10 m s 1 and stronger trade inversions the simulated cloud trails which are consistent with observations in their morphology and diurnal cycle are organized by quasi linear bands of thermally forced convergence within the heated island wake they are sensitive to overland surface fluxes inversion strength and height terrain height and trade wind speed while surface fluxes control the strength of the wake thermal circulations the inversion controls pr

lateral heterogeneity imaged by small aperture scs retrieval from the ambient seismic field interpreting core related body wave travel times is challenging for seismologists because earth s heterogeneities are averaged over thousands of kilometers and the sparsity of earthquake measurements makes these heterogeneities difficult to localize we show how the ambient seismic wave field can be used to overcome these limitations we present a regional scale analysis of core mantle boundary reflections scs under mexico we show that body wave arrivals i e p and scs are retrieved from higher order cross correlations c3 a technique that provides a more uniform and controlled source distribution using the scattered waves of the coda of classical ambient field cross correlations c1 then we extract scs travel times along a dense linear array in mexico and find that lithospheric lateral heterogeneity such as the subducting cocos slab beneath mexico may have a strong impact on scs travel times in parallel we show that lateral heterogeneity such as a possible ultra low velocity zone ulvz near the core mantle boundary might also affect although to a lesser extent the travel time anomalies our results and interpretation are supported through numerical simulations that accounts for slab and ulvz properties

low froude number flow past three dimensional obstacles part i baroclinically generated lee vortices abstract we study the flow of a density stratified fluid past a three dimensional obstacle using a numerical model our special concern is the response of the fluid when the froude number is near or less than unity linear theory is inapplicable in this range of froude number and the present numerical solutions show the rich variety of phenomena that emerge in this essentially nonlinear flow regime two such phenomena which occupy parts i and ii of this study are the formation of a pair of vertically oriented vortices on the lee side and a zone of flow reversal on the windward side of the obstacle the ice vortices have been explained as a consequence of the separation of the viscous boundary layer from the obstacle however this boundary layer is absent by design in the present experiments and lee vortices still occur we argue that a vertical component of vorticity develops on the lee side owing to the tilting of horizontally oriented vorticity produced baroclinically as the isentropes deform in r

deep mantle seismic modeling and imaging detailed seismic modeling and imaging of earth s deep interior is providing key information about lower mantle structures and processes including heat flow across the core mantle boundary the configuration of mantle upwellings and downwellings phase equilibria and transport properties of deep mantle materials and mechanisms of core mantle coupling multichannel seismic wave analysis methods that provide the highest resolution deep mantle structural information include network waveform modeling and stacking array processing and 3d migrations of p and s wave seismograms these methods detect and identify weak signals from structures that cannot be resolved by global seismic tomography some methods are adapted from oil exploration seismology but all are constrained by the source and receiver distributions long travel paths and strong attenuation experienced by seismic waves that penetrate to the deep mantle large and small scale structures with velocity variations ranging from a fraction of a percent to tens of percent have been detected and are guiding geophysicists to new perspectives of thermochemical mantle convection and evolution

the evolution of convective storms initiated by an isolated mountain ridge abstractthe evolution of convective storms over the black hills an isolated mountain ridge in south dakota and wyoming and a regional convection hotspot is investigated using a 10 yr observational climatology and quasi idealized numerical simulations radar observed diurnally forced mountain convection events are classified according to their maximum cell track length and duration which are quantified using an automated cell tracking algorithm environmental conditions during these events are obtained from operational radiosonde and model analysis data these data suggest that mountain forced convective cells generally struggle to survive in the convectively inhibited flow downwind of the black hills those cells that do survive downwind prefer environments with strong bulk vertical shear over the 0 6 km layer which favors organized multicellular or supercellular convection under slightly weaker shear the cells tend to dissipate rapidly as they propagate downwind relatively weak winds aloft when c

a lower mantle s wave triplication and the shear velocity structure of d summary a lower mantle s wave triplication detected with short and longperiod wwssn and csn recordings indicates a substantial shear velocity discontinuity near 280 km above the core mantle boundary the triplication can be observed in rotated sh seismograms from intermediate and deep focus events throughout the distance range from 70 to 95 three distinct source region receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity modelling of the observations with synthetic seismograms produced with the cagniard de hoop and reflectivity methods constrains the shear velocity increase to be 2 75 k 0 25 per cent comparable to upper mantle discontinuities short period observations indicate that the velocity increase may be a sharp first order discontinuity or may extend over a transition zone no more than 50 km thick the shear velocity gradient below the discontinuity within the d layer is not well constrained by the sh data but slightly positive or near zero velocity gradients are consistent with the long period amplitude ratios of scsh sh

mineralogical effects on the detectability of the postperovskite boundary the discovery of a phase transition in mg silicate perovskite pv to postperovskite ppv at lowermost mantle pressure temperature p t conditions may provide an explanation for the discontinuous increase in shear wave velocity found in some regions at a depth range of 200 to 400 km above the core mantle boundary hereafter the d discontinuity however recent studies on binary and ternary systems showed that reasonable contents of fe2 and al for pyrolite increase the thickness width of the mixed phase region of the pv ppv boundary 400 600 km to much larger than the d discontinuity 70 km these results challenge the assignment of the d discontinuity to the pv ppv boundary in pyrolite homogenized mantle composition furthermore the mineralogy and composition of rocks that can host a detectable pv ppv boundary are still unknown here we report in situ measurements of the depths and thicknesses of the pv ppv transition in multiphase systems san carlos olivine pyrolitic and midocean ridge basaltic compositions at the p t conditions of the lowermost mantle searching for candidate rocks with a sharp pv ppv discontinuity whereas the pyrolitic mantle may not have a seismologically detectable pv ppv transition due to the effect of al harzburgitic compositions have detectable transitions due to low al content in contrast al rich basaltic compositions may have a detectable pv ppv boundary due to their distinct mineralogy therefore the observation of the d discontinuity may be related to the pv ppv transition in the differentiated oceanic lithosphere materials transported to the lowermost mantle by subducting slabs

convective circulations induced by surface heterogeneities a simple theory for convective circulations induced by surface heterogeneities is proposed the theory is based on the thermodynamics of heat engines and provides a simple physical explanation for the general characteristics of circulations forced by surface inhomogeneities in sloping terrains in particular the theory is applied to a mesoscale circulation induced by deforestation it predicts that the intensity of the mesoscale convective circulation forced by deforestation depends on the difference of the near surface nonadiabatic temperature and humidity between the forest and cleared regions and on the depth of the convective boundary layer the theory was successfully tested against observations made during a field experiment in the amazon forest and a nearby clearing

topographically forced convergence in western washington state abstract several times a year when the low level winds from off the pacific ocean are within a narrow range of speed and direction air passes both north and south of the olympic mountains of washington state and is forced to converge in puget sound by the north south oriented cascade range this phenomenon termed the puget sound convergence zone often results in a band of cloudiness and precipitation in northern and central puget sound with clear subsiding air to the north and south this paper presents the results of a series of case studies in which the structure of the zone its meteorological manifestations and the environmental conditions necessary for its formation are explored it is shown that the convergence zone is skillfully forecast by surface coastal winds and undergoes a strong annual and diurnal cycle being most frequent during the late spring and early summer months and during the afternoon and early evening it is also found that the zone is structurally similar to a shallow cold fr

stability of fe bearing hydrous phases and element partitioning in the system mgo al2o3 fe2o3 sio2 h2o in earth s lowermost mantle abstract we performed high pressure temperature p t experiments on a model composition of hydrous subducted slabs in the mgo al2o3 fe2o3 sio2 h2o system using laser heated diamond anvil cells the phase assemblages were characterized combining in situ synchrotron x ray diffraction and ex situ transmission electron microscope techniques the hydrous phase alooh feooh mgsio2 oh 2 sio2 coexists with bridgmanite bdg post perovskite ppv or both in a broad p t range of 104 126 gpa and 1750 2500 k the hydrous pyrite type feoohx phase was observed in situ over a p t range of 112 123 gpa and 1750 2300 k coexisting with the ppv phase chemical analysis on recovered samples showed that considerable amount of fe2o2 oh 2 8 13 mol and sio2 9 13 mol in the phase does not reduce its thermal stability compared to the al endmember indicating that the fe bearing phase can transport water to the lowermost mantle along the mantle geotherm in this hydrous system we observed that al depletion in both the bdg and ppv phases can significantly reduce the width of the bdg to ppv transition in contrast to a wide two phase coexistence region in a dry al rich system meanwhile the fe enrichment in the ppv phase relative to the coexisting bdg phase lowers the transition pressure to the depth of the d discontinuity accordingly the depth and thickness of the bdg to ppv transition in subducted basaltic crustal materials can explain the seismically detected d discontinuity partial melting could be triggered by dehydration of the fe bearing hydrous phases due to a steep temperature gradient at the core mantle boundary cmb and therefore the ultralow velocity zones ulvzs might be the regions where partial melting occurs at the lowermost mantle

entrainment rayleigh friction and boundary layer winds over the tropical pacific abstract winds over the tropical pacific are interpreted using mixed layer theory the theory which posits that the surface winds can be derived in terms of a force balance among surface drag pressure gradients coriolis forces and the vertical mixing of momentum into the boundary layer entrainment is very successful in predicting the seasonal climatology of the surface winds the model is also used as a basis for interpreting previous results in particular the model illustrates why studies that model the momentum flux divergence as a rayleigh damping find optimal damping coefficients that are anisotropic a linear variant of the model which also incorporates entrainment but neglects the quadratic relation between the wind speed and the surface stress is also found to predict the surface winds skillfully in addition to improving the representation of the winds it leads to realistic representations of the divergence of the vector wind if the key parameters of the model the entrainment rate and t

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

high resolution imaging of lowermost mantle structure under the cocos plate 1 broadband seismic shear waves are analyzed to investigate the fine velocity structure in the lowermost mantle beneath the cocos plate a region where previous studies have indicated the presence of a shear velocity increase about 200 300 km above the core mantle boundary data from 14 south american earthquakes recorded at california broadband networks provide dense ray path sampling of the lowermost mantle in an approximately 700 km long north south corridor roughly 150 km wide application of a simplified seismic migration method that uses a homogeneous background velocity model suggests topography of the previously imaged positive impedance jump varying in depth from north to south by as much as 150 km with a weakly reflecting transition zone in between the migration approach enables examination of small scale spatial variations and out of plane scattering effects topography of the discontinuity may account for observed variations in the amplitude of reflected arrivals or there may be lateral variations in the velocity contrast across the boundary lateral variations of the shear velocity structure within the d layer may produce some apparent topography in the discontinuity image but any such volumetric structure is not yet well enough determined to incorporate in the migration a localized negative impedance contrast reflector or scatterer is imaged at depths about 100 km below the positive reflector in the northern portion of the study area several scenarios can explain these results including 1 a slab that reaches the lowermost mantle 2 the birth of an upwelling beneath a recumbent slab or 3 chemical layering in this region

postperovskite phase transition and its geophysical implications 1 the stability of mg fe sio3 perovskite in the deep lower mantle has long been uncertain recently a phase transition from perovskite to postperovskite was discovered through a significant change in the x ray diffraction pattern at high pressure and high temperature conditions corresponding to the core mantle boundary region this phase transition was also confirmed by first principles calculations these suggest that mg fe sio3 postperovskite is the predominant mineral in the lowermost mantle called the d layer and phase transition from perovskite to postperovskite has significant geophysical implications for its nature and dynamics the postperovskite phase may account for the large seismic anomalies observed in the d region such as the d discontinuity s wave anisotropy and anticorrelation between the anomalies in s wave and bulk sound velocities in addition this phase transition is a strongly exothermic reaction it destabilizes the thermal boundary layer at the base of the mantle and should promote the formation of high temperature upwelling plumes

seismic evidence for the depression of the d discontinuity beneath the caribbean implication for slab heating from the earth s core the lowermost 100 300 km of the earth s mantle commonly regarded as the thermal boundary layer tbl of mantle circulation is characterized by its complex physical properties beneath the caribbean this so called d layer features relatively high velocities and abrupt impedance increase at the top designated as the d discontinuity these seismic characteristics have been attributed to the accumulation of ancient subducted slab material and the phase transition in the major lower mantle mineral of pervoskite geodynamic models predict that the blanketing cold slabs may trap enough heat from core to be buoyantly destabilized and eventually broken apart and entrained into the bottom of the convection cell here we explore the d structure with unprecedented resolution through modeling traveltimes amplitudes and waveform shapes from the usarray we find an east to west asymmetrical undulation of the d discontinuity with a v shaped depression of 70 160 km over a lateral distance of 600 km beneath northern south america the shear velocity perturbations vary in the same trend showing the most pronounced reduction of 3 4 below the thinnest d layer in close proximity to an intermittently undetected discontinuity the strong correlation between the d topography and velocity variations indicates the phase transition boundary has been perturbed or even disrupted by the large lateral temperature gradient of slab material which has been reheated from the core over extended periods of time

thermally forced low froude number flow past three dimensional obstacles abstract the present study extends the discussion of the flow of a density stratified fluid past three dimensional obstacles for froude number o 1 to flows past an isolated obstacle with heated cooled surface the study focuses on a response of thermally forced stratified flows representative of mesoscale flows past mountainous islands such as hawaii or taiwan in order to minimize the span of the parameter space uniform ambient wind and stratification axially symmetric bell shaped hills with moderate slopes representative of mesoscale mountains and a simple thermal forcing function that mimics natural effects over mountainous islands are assumed the earth s rotation surface friction viscosity dissipation and moisture are neglected with these simplifications flows can be characterized with two parameters the froude number and a characteristic scale of thermal forcing defined later in this paper the principal question addressed in this study is under what circumstances will a transition occur

diurnal variation of surface airflow and rainfall frequencies on the island of hawaii abstract the high resolution portable automated mesonet data from the hawaiian rainband project are used to document the circulation over the entire island of hawaii for the first time it is shown that the surface airflow and rainfall occurrences over the island are strongly modulated by the diurnal heating cycle most areas over the island show daytime upslope and nighttime downslope components separated by morning and evening transitions on the windward slope the onset of the downslope upslope wind during the evening morning transition starts on the slopes and progresses downward the effects of island blocking are also evident on the windward slopes of mauna kea and mauna loa the mean winds are weak 1 m s 1 due to flow deceleration flow splitting occurs in the hilo bay area as the trade winds are forced to move around the island on the lee side the trade winds are absent for stronger trade wind days the island blocking is more significant with a higher surface pressure 0 2 0 5 hpa on

boundary layer updrafts driven by airflow over heated terrain abstractthis study presents linear and nonlinear scalings for boundary layer ascent forced by airflow over heated terrain and compares them to results from corresponding high resolution numerical simulations close agreement between theory and simulation is found over most of the parameter space considered including variations in background winds boundary layer stability mountain height and diabatic heating rate as expected the linear and nonlinear scalings perform best for linear and nonlinear flows respectively for a convective boundary layer the scalings accurately predict vertical motion for all flows considered including those that extend well into the nonlinear regime thus these scalings may ultimately help to improve the parameterization of subgrid orographic ascent in large scale models the vertical velocity scalings are less accurate for mechanically blocked flows in stable boundary layers for which a simple vertical displacement scaling is superior although the scalings do not tre

cloud resolving simulations of deep convection over a heated mountain abstract cloud resolving numerical simulations of airflow over a diurnally heated mountain ridge are conducted to explore the mechanisms and sensitivities of convective initiation under high pressure conditions the simulations are based on a well observed convection event from the convective and orographically induced precipitation study cops during summer 2007 where an isolated afternoon thunderstorm developed over the black forest mountains of central europe but they are idealized to facilitate understanding and reduce computational expense in the conditionally unstable but strongly inhibited flow under consideration sharp horizontal convergence over the mountain acts to locally weaken the inhibition and moisten the dry midtroposphere through shallow cumulus detrainment the onset of deep convection occurs not through the deep ascent of a single updraft but rather through a rapid succession of thermals that are vented through the mountain convergence zone into the deepening cloud mass emerging t

a modeling study and scaling analysis of orographic effects on boundary layer shallow convection effects of orography on boundary layer shallow convection under various background winds are studied using a two dimensional model together with scaling analysis under a motionless background state the flow response over a heated hill is a vortex pair with one horizontal vortex over each slope additional lifting of the inversion by this vortex pair is estimated by wt n with wt being the intensity of the terrain induced thermal circulation and n the brunt vaisalafrequency for which a simple expression is given based on heat engine framework modification of the cbl top by terrain induced waves is quantified using linear gravity wave theory in a two layer atmosphere configuration these simple estimates of the cbl top perturbation are found to be consistent with the model results when waves and convection dominate the flow pattern a convective core is found to occur over high hills when the winds are light developing in the late afternoon when the cbl depth is high a scaling is proposed to determine the existence of the convective core in terms of hill slope background winds boundary layer depth and tropospheric stability the downstream modification of convection by terrain related processes is also examined under light wind conditions the reversed thermal circulation in the lee and corresponding downward motions may suppress thermal eddies in the vicinity of the lee of a hill

sharpness of the d discontinuity beneath the cocos plate implications for the perovskite to post perovskite phase transition 1 the depth extent or sharpness of the d seismic velocity discontinuity is determined by modeling distance dependence of reflected s wave signals sampling the lowermost mantle beneath the cocos plate the data spanning distances from 64 to 83 and grouped in two small turning point bins from 5 to 10 n and 10 to 15 n are compatible with a sharp 1 to 2 shear velocity increase near 300 km above the core mantle boundary but not with reflection from a transition zone extending more than 30 km in depth interpretation of the velocity structure as the result of magnesium silicate perovskite undergoing a phase transition to post perovskite requires limited presence of al and fe which will broaden the phase transition current experiments indicate that the phase transition may occur over 5 gpa 90 km in pyrolitic material requiring either distinct chemistry or dynamical mechanisms to sharpen the phase transition for this high velocity sub circum pacific region

seismic imaging the d region beneath the central atlantic abstract the lowermost mantle beneath the central atlantic is bounded by the western edge of african large low shear wave province llsvp and a high velocity region beneath the eastern americas which is attributed to the accumulated subducted slab material at the core mantle boundary cmb data recorded at broadband seismic arrays in western europe and morocco allow us to investigate the detailed structure beneath the central atlantic which provides an important constraint on how remnant slab material and llsvp interact and reshape the d layer here we exploit lower mantle triplicated s waveform data from deep earthquakes beneath south america our modelling results show that the d discontinuity is constantly observed from north to south beneath the central atlantic but at varied depths and with varied shear velocities in the lowermost mantle beneath east of caribbean our results indicate a sharp 2 1 shear velocity increase at 340 km above the cmb representing a possible bridgmanite dominant normal mantle d layer whereas the models of northern and southern portions of central atlantic indicate a gradual velocity increase of 4 2 across 200 km starting from 360 km above the cmb downwards we also find a distinct low velocity zone on top of the velocity jump which can be interpreted as an upwelling produced by the dynamic response of ancient slab material reaching the cmb

anisotropy as cause for polarity reversals of d reflections recordings of seismic events that sample the deep mantle can test different hypotheses of mantle processes and composition seismic reflections from structures in the d region the bottom 200 400 km of the earth s mantle can provide information on the velocity contrasts in this region by studying the waveforms and polarities of the d reflections in p and s waves we can potentially distinguish between different explanations for the observed structures such as phase transitions mineral texture or thermal anomalies here we use source receiver combinations that contain reflections from d in two different regions that are both characterised by fast seismic velocities in tomographic models beneath the caribbean a positive s velocity contrast but negative p wave velocity contrast across the d reflector has been reported previously consistent with a model of a phase change in mgsio3 in the second fast velocity region eurasia we detect positive p and s wave velocity contrasts in two orthogonal paths crossing in the lowermost mantle indicating a different scenario for d a path that crosses this region in 45 to the other two great circle paths shows evidence for a negative p velocity contrast one explanation to reconcile observations in both regions is a phase transition from perovskite to post perovskite with a fraction of 12 preferred crystal alignment in the post perovskite phase depending on the travel direction of the waves with respect to the flow direction in the lower mantle positive or negative velocity jumps can be expected other anisotropic models are considered but cannot fully explain the range of observations we find in the data c 2011 elsevier b v all rights reserved

the mississippi valley convection minimum on summer afternoons observations and numerical simulations abstractanalysis of 15 years of composite radar images over the continental united states reveals a distinct minimum of deep convection occurrence over the interior lower mississippi valley on summer afternoons relative to surrounding areas to understand the mechanisms behind this convection signature quasi idealized numerical simulations with the weather research and forecasting wrf model are performed the simulations which broadly reproduce the valley convection minimum suggest that convective inhibition is maximized and low level ascent minimized over the flat valley terrain by contrast weaker inhibition and stronger mechanically forced ascent over the hills flanking the valley combine to initiate convection more readily although the orography of the region is unremarkable it has a stronger influence on the regional convection pattern than do variations in land use

seismic velocity structures and detailed features of the d discontinuity near the core mantle boundary beneath eastern eurasia abstract shear and compressional velocity structures and features of the d discontinuity near the core mantle boundary cmb beneath eastern eurasia are studied based on forward waveform modeling and differential travel time analysis of scsh sh sscs ss pcp p and ppcp pp phases high quality differential travel time analysis reveals two high velocity patches spanning a width of 3000 km and separated by a 300 km wide normal velocity gap the maximum shear and compressional velocity perturbations reach 3 8 and 2 7 respectively seismic data indicate that the d discontinuity emerges in the high velocity patches and disappears in the normal velocity gap furthermore the features of the d discontinuity although constrained by rather sparse seismic data vary from diffusive discontinuities in the eastern patch to sharp discontinuities in the western patch the d discontinuities occur at about 270 km above the cmb in the eastern patch with a shear velocity jump of 3 0 5 0 and a transitional depth of at least 100 km and at 250 km above the cmb in the western patch with an abrupt shear velocity jump of 5 0 a diffusive p wave discontinuity is also observed in the eastern patch with the similar features of the s wave discontinuity the inferred features of the d discontinuity can be explained by the post perovskite transformation in the lower mantle the observations that the d discontinuities occur only in the high velocity patches may be due to the effect of the positive clapeyron slope of the post perovskite transition and the varying transitional depths of the d discontinuity in the high velocity regions may be caused by the compositional effects on the post perovskite transition in the presence of strong variations of composition within the high velocity anomalies the high velocity patches geographically correlate with the subduction of the pacific and indian plates in the past 30 60 ma and the normal velocity gap falls into the subduction gap between the pacific and indian plates these geographical correlations raise the possibility that the existence of the d discontinuity is intimately related to the presence of the past subduction material at the cmb and varying features of the discontinuity further suggest existence of strong variations of composition within the subduction materials at the cmb

the role of small scale orographic features in the spatial distribution of precipitation radar images from the cevennes experiment which took place during the autumns of 1986 88 on the southeastern flank of the massif central in france reveal a characteristic and repetitive structure of the precipitation distribution in southerly mediterranean flows which are frequent there in autumn precipitation is often organized into narrow bands or plumes oriented south north the period 14 and 15 november 1986 has been selected as an example on which to perform numerical simulations and analyse the mechanism yielding this specific precipitation organization the numerical model correctly reproduces the bands especially at high horizontal resolution 1 km these rainfall bands are focused on the small scale topography oriented east west composed of a succession of ridges and penetrating valleys the bands are generated upstream of these ridges and enhanced on the lee side by convergence created by deflection around the obstacle and penetration of the flow into the valleys idealized simulations and sensitivity experiments have been carried out to verify these hypotheses these simulations suggest that small scale orography plays a decisive role in forcing the precipitation into bands while upstream moisture conditions mainly affect the precipitation intensity and its south north extent copyright 2002 royal meteorological society