the 3d numerical modeling of subduction dynamics plate stagnation and segmentation and crustal advection in the wet mantle transition zone 1 water content in the mantle transition zone mtz has been broadly debated in the earth science community as a key issue for plate dynamics in this study a systematic series of three dimensional 3d numerical simulation are performed in an attempt to verify a hypothesis regarding the behavior of subducted oceanic plate and the role of crust in the mtz under wet conditions this hypothesis is based on the seismic observations of structural variations associated with large scale flattened high velocity anomalies i e stagnant slabs in the mtz in our model weak low viscosity fault zones wfzs which presumably correspond to the fault boundaries of large subduction earthquakes are imposed on the top part of subducting plates the phase transitions of olivine to wadsleyite and ringwoodite to perovskite magnesiowustite under both dry and wet conditions are considered based on recent high pressure experiments the effect of viscosity reduction of wet garnet is also considered for oceanic crust in the mtz results show that there is a substantial difference in the behaviors of subducting plate and the trace of crustal materials between the models under dry and wet conditions under wet conditions the subducting plate tends to stagnate with a maximum lateral extent of over 1000 km the weaker and denser crustal material garnet rich is fed into the wfzs and is advected faster than the plate main body of peridotite olivine rich thus the viscosity and density contrast between the crustal materials and the peridotite permits the plate segmentation under a wet mtz condition

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones viii a model for the seeder feeder process in warm frontal rainbands abstract previous field studies have indicated that warm frontal rainbands form when ice particles from a seeder cloud grow as they fall through a lower level feeder cloud in this paper we present results from a parameterized numerical model of the growth processes that can lead to the enhancement of precipitation in a seeder feeder type situation the model is applied to two types of warm frontal rainbands in the first type 1 situation the vertical air motions are typical of those associated with slow widespread lifting in the vicinity of warm fronts in the second type 2 situation the vertical air motions are stronger and more characteristic of the mesoscale the model simulations show that in the type 1 situations the growth of the seed ice crystals within the feeder zone is due to vapor deposition the feeder zone in this case is slightly sub saturated with respect to water due to the presence of the seed crystals in regions where the feeder zone is not seeded from aloft snow cryst

evolution structure cloud microphysical and surface rainfall processes of monsoon convection during the south china sea monsoon experiment abstract a two dimensional cloud resolving simulation is combined with dual doppler and polarimetric radar analysis to study the evolution dynamic structure cloud microphysics and rainfall processes of monsoon convection observed during the south china sea scs summer monsoon onset overall the model simulations show many similarities to the radar observations the rainband associated with the convection remains at a very stable position throughout its life cycle in the northern scs the reflectivity pattern exhibits a straight upward structure with little tilt the positions of the convective transition and stratiform regions produced by the model are consistent with the observations the major difference from the observations is that the model tends to overestimate the magnitude of updraft as a result the maximum reflectivity generated by the model appears at an elevated altitude the surface rainfall processes and associated thermodynamic dynamic and cloud microphysical processes are examine

slab avalanche induced tectonics in self consistent dynamic models subducted slabs often temporarily stagnate within the transition zone and then periodically penetrate into the lower mantle in the form of a slab avalanche we investigate the influence of such slab avalanches on tectonics with dynamically consistent numerical models we suggest that slab avalanches can induce substantial changes in surface tectonics including synchronous broad scale basin inversion and dynamic subsidence in regions above the avalanched slab and reducing or even reversing trench retreat however basins distal from the trench and avalanched slab continue extension following avalanche onset we suggest that a slab avalanche event beneath the sundaland continental promontory began in the early miocene to trigger a wide range of tectonic responses within southeast asia including 1 a reduction of the sunda java trench retreat velocity that may have transitioned to advance regionally since the early miocene 2 synchronous basin compression and subsidence within southern sundaland since the early miocene and 3 continuous basin extension with some developing to ultra deep rifted basins in the northern part of sundaland we also discuss the implication of the slab avalanche model in the context of the history of the taranaki basin offshore new zealand

organization of oceanic convection during the onset of the 1998 east asian summer monsoon the organizational modes of convection over the northern south china sea scs during the onset of the summer monsoon are documented using radar and sounding data from the may june 1998 south china sea monsoon experiment scsmex the onset occurred in mid may with a rapid increase in deep convection over a 10 day period accompanied by a major shift in the circulation over the east asian region analysis of bureau of meteorology research centre bmrc radar data from dongsha island reveals a wide range of organizational modes of convection over the northern scs proximity sounding data indicate that lower and middle level vertical wind shears exerted a dominant control over the orientation of convective lines within mesoscale convective systems in this region as has been found in the australian monsoon region and the equatorial western pacific the results are consistent with the conceptual model of lemone et al based on the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare except two new organizational modes have been identified shear parallel bands for strong low level shear and weak midlevel shear when there is weak instability and the air is dry aloft and shear parallel bands for strong shears in both layers when the shear vectors are in the same direction midlatitude influences namely the passage of troughs over southern china likely contributed to these two additional modes the stratiform rain fraction from the convective systems during the monsoon onset period was relatively small 26 compared to the estimated average of about 40 for the entire tropics this small fraction is attributed to the weak instability during the onset period and relatively dry air in the upper troposphere

whole mantle convection and the thermal evolution of the earth abstract three recent results in the fields of geophysics and geochemistry have combined to rejuvenate the study of the earth s thermal history the first and perhaps most important of these is the new understanding of the planetary accretion process which has been realized as a consequence of work by v s safronov and others this work shows that the terrestrial planets accrete on a time scale of 10 8 y by the accumulation of planetesimal sized objects and that they probably form sufficiently hot to induce immediate differentiation of iron from silicates the second important recent contribution has been from geochemistry and concerns the net complement of radioactivity of the earth as a whole although the net complement of radioactive elements has usually been inferred on the basis of the assumption of equilibrium with the surface heat flow a priori analysis based upon elemental condensation temperatures in the primitive solar nebula yields a model in which the net radioactivity is at least a factor of two less than would be required to explain the mean surface heat flow there are good reasons to believe that this model is valid for the earth the third result has been the successful development of a technique for incorporating the influence of convective heat transfer in a thermal evolution model which does not require explicit solution of the dynamical equations this paper begins with an analysis of the mantle convective circulation which is required to understand plate tectonics and continental drift an analysis which presents the arguments for and against the crucial notion that the main circulation has a depth scale equal to the mantle thickness using the idea of parameterized convection the radial heat transfer by the mantle convective circulation is included in a thermal history model of the earth this model turns out to contain within it an interesting explanation of the fact that no crustal rocks with ages in excess of 3 8 ga have ever been found an observation that has heretofore proven enigmatic

simulation of active tectonic processes for a convecting mantle with moving continents summary numerical models are presented that simulate several active tectonic processes these models include a continent that is thermally and mechanically coupled with viscous mantle flow the assumption of rigid continents allows use of solid body equations to describe the continents motion and to calculate their velocities the starting point is a quasi steady state model of mantle convection with temperature pressure dependent viscosity after placing a continent on top of the mantle the convection pattern changes the mantle flow subsequently passes through several stages eventually resembling the mantle structure under present day continents a extension tectonics and marginal basins form on boundary of a continent approaching to subduction zone roll back of subduction takes place in front of moving continent b the continent reaches the subduction zone the extension regime at the continental edge is replaced by strong compression the roll back of the subduction zone still continues after closure of the marginal basin and the continent moves towards the upwelling as a result the ocean becomes non symmetric and c the continent overrides the upwelling and subduction in its classical form stops the third stage appears only in the upper mantle model with localized upwellings

topographic asymmetry of the south atlantic from global models of mantle flow and lithospheric stretching the relief of the south atlantic is characterized by elevated passive continental margins along southern africa and eastern brazil and by the bathymetric asymmetry of the southern oceanic basin where the western flank is much deeper than the eastern flank we investigate the origin of these topographic features in the present and over time since the jurassic with a model of global mantle flow and lithospheric deformation the model progressively assimilates plate kinematics plate boundaries and lithospheric age derived from global tectonic reconstructions with deforming plates and predicts the evolution of mantle temperature continental crustal thickness long wavelength dynamic topography and isostatic topography mantle viscosity and the kinematics of the opening of the south atlantic are adjustable parameters in thirteen model cases model predictions are compared to observables both for the present day and in the past present day predictions are compared to topography mantle tomography and an estimate of residual topography predictions for the past are compared to tectonic subsidence from backstripped borehole data along the south american passive margin and to dynamic uplift as constrained by thermochronology in southern africa comparison between model predictions and observations suggests that the first order features of the topography of the south atlantic are due to long wavelength dynamic topography rather than to asthenospheric processes the uplift of southern africa is best reproduced with a lower mantle that is at least 40 times more viscous than the upper mantle

finite amplitude convection through a phase boundary summary a finite amplitude numerical model of convective motions through a phase boundary is studied the phase change is assumed to occur over a finite vertical transition zone in the fluid and thus a continuously stratified model is considered the motions are driven by horizontal temperature gradients on the boundaries or super critical vertical stratification in the case of rayleigh instability the numerical results reproduce the neutral curve previously obtained in the analytic solution of the linearized problem when the parameters of the problem are chosen to simulate the olivine spinel transition in the upper mantle the following results are obtained 1 the vertical scale of motion is the entire depth of the fluid 2 the horizontal scale is not significantly changed from the case of a single phase fluid 3 the amplitude of the motion is not significantly changed due to buoyancy changes at the phase boundary being offset by corresponding sources and sinks of latent heat and 4 the phase boundary varies in depth by as much as 30 km when the vertical velocities are of the order of 10 1 cm yr 1

phase transition modulated mixing in the mantle of the earth and discussion recently published a priori analyses of the mantle convection process demonstrate that the influence of the endothermic phase transformation at 660 km depth could be profound and such as to enforce a strongly intermittent style of circulation in this phase transition modulated regime mass transfer across the 660 km horizon is inhibited leading to the formation of an internal thermal boundary layer that straddles the phase change interface this thermal boundary layer is itself episodically disrupted by convective instability leading to the development of intense avalanches of cold material from the transition zone into the lower mantle the consequences of such a circulation regime to the understanding of trace element geochemistry and the supercontinent cycle may be extremely important equally important are the implications for mantle viscosity since a circulation of this type would appear to allow reconciliation of a wide range of geodynamic data with a single viscosity model the relevant data include convection related observables such as the aspherical geoid and postglacial rebound related observables such as relative sea level histories and certain anomalies in earth rotation including the so called non tidal acceleration of rotation and the ongoing wander of the rotation pole towards greenland at a rate near 0 95 degrees per million years

role of korteweg stresses in geodynamics 1 it has come to our attention that the constitutive relationship used in the modeling of geodynamical flow problems with strongly variable physical properties should have additional terms in the stress tensor known in the literature as korteweg stresses k stresses these stresses arising at diffuse interfaces which can best be explained in terms of density gradients have already been mentioned in the literature for more than one hundred years but have not received attention recently until a combination of experimental and numerical evidence have confirmed their existence we will discuss the important potential role these new terms have for geophysics it has many ramifications in geodynamics ranging from mantle convection to earthquakes and magma fragmentation

structure and sharpness of phase transitions and mantle discontinuities the structure of phase transitions expected from equilibrium thermodynamics is examined we show that in binary systems the shape of the coexistence region phase loop is controlled primarily by the partition coefficient k we derive a general and simple one parameter expression for the pressure dependence of the yield of the high pressure phase and show that this function can be highly nonlinear most of the transition occurs in a narrow interval near the boundary of the phase loop estimates of the effective width of binary phase transitions are less than half the total width of the coexistence region even for relatively mild partitioning k 1 4 we generalize these results to multiphase and multicomponent transitions we show that the presence of nontransforming phases can affect the width of the transition substantially we predict that the width of the olivine to wadsleyite transition in the presence of pyroxene and garnet is approximately half that of the binary phase loop at typical transition zone temperatures the estimated effective width of this transition in the mantle 4 8 km is marginally consistent with observations of high frequency 0 5 1 0 hz p wave reflections from the 410 km discontinuity we show that the effective width of the garnet to perovskite transition is sufficiently narrow to reflect s wave energy in the frequency range of scs reverberations 10 40 mhz and that this transition can account for the observed properties of the 710 km discontinuity

the development and structure of an oceanic squall line system during the south china sea monsoon experiment abstract a primary goal of the south china sea monsoon experiment scsmex a major field campaign of the tropical rainfall measuring mission trmm is to define the initiation structure evolution and dynamics of precipitation processes associated with the onset of the south china sea scs summer monsoon in this study dual doppler and dual polarimetric radar analysis techniques are used to investigate the development and structure of a squall line system observed on 24 may 1998 the focus is the linkage between the airflow and the microphysical fields through the system the squall line system including three distinct lines persisted from 1200 utc 24 may to the following day a detailed study was performed on the structure of the second and most intense line lasting for over 10 h compared to tropical squall lines observed in other regions this narrow squall line system had some interesting features including 1 maximum reflectivity as high as 55 dbz 2 relatively little stratiform rainfall th

radar observations of the kinematic microphysical and precipitation characteristics of two mcss in trmm lba 1 dual doppler and polarimetric radar observations are used to analyze two mesoscale convective systems mcss that occurred during the tropical rainfall measuring mission large scale biosphere atmosphere field campaign the mcss formed in different meteorological regimes based on profiles of atmospheric wind and thermodynamic data the first mcs event 26 january 1999 was a squall line that formed in low level easterly flow and had an intense leading line of convection in contrast the 25 february 1999 mcs formed in low level westerly flow and was best characterized by stratiform precipitation with embedded convective elements the radar analyses suggest that the mcss were distinct in terms of overall vertical structure characteristics in particular polarimetric radar cross sections indicated the presence of an active mixed phase zone in the easterly mcs that was largely absent in the westerly case the easterly mcs had considerably more precipitation ice in the middle to upper troposphere compared to the westerly mcs composite analyses showed that the easterly mcs had higher peak reflectivities and a smaller reflectivity gradient above the 08c level in convective regions of the storm compared to the westerly mcs event moreover mean profiles of both vertical air motion and vertical mass transport in the convective portion of the easterly mcs were larger over a factor of 2 at some heights below the 08 c level than those in the westerly event these observations suggest that the easterly and westerly wind regimes in the southwest amazon region produce convection with different vertical structure characteristics similar to regimes elsewhere in the global tropics e g maritime continent index terms 3314 meteorology and atmospheric dynamics convective processes 3329 meteorology and atmospheric dynamics mesoscale meteorology 3354 meteorology and atmospheric dynamics precipitation 1854 3374 meteorology and atmospheric dynamics tropical meteorology keywords microphysics tropical precipitation polarimetric radar mesoscale convective system

dynamic origins of seismic wavespeed variation in d the d discontinuity is defined by a seismic velocity increase of 1 3 about 250 km above the core mantle boundary cmb and is mainly detected beneath locations of inferred paleosubduction a phase change origin for the interface can explain a triplicated arrival observed in seismic waveform data and is supported by the recent discovery of a post perovskite phase transition we investigate the interaction of slabs plumes and the phase change within d in 2 d compressible convection calculations and predict waveform complexity in synthetic seismic data the dynamic models produce significant thermal and phase heterogeneity in d over small distances and reveal a variety of behaviors including 1 distinct ppv blocks separated by upwellings 2 notches at the top of a ppv layer caused by plume heads 3 regions of pv embedded within a ppv layer due to upwellings advected isotherms produce complicated thermal structure that enable multiple crossings of the phase boundary perturbations to s sds and scs arrivals distances 80 degrees

implications of slab mineralogy for subduction dynamics phase relations among mantle minerals are perturbed by the thermal environment of subducting slabs both under equilibrium and disequilibrium metastable conditions such perturbations yield anomalies not only in seismic velocities but also in density the buoyancy forces arising from these density anomalies may exert several important effects they contribute to the stress field within the slab in a fashion consistent with observed patterns of seismicity they may affect subduction rates both by inducing time dependent velocity changes under equilibrium conditions and by imposing velocity limits through a thermal feedback loop under disequilibrium conditions they may affect slab morphology possibly inhibiting penetration of slabs into the lower mantle and allowing temporary stagnation of deflected or detached slabs latent heat release from phase transitions under disequilibrium conditions in slabs can yield isobaric superheating which may generate adiabatic shear instabilities capable of triggering deep seismicity 2001 elsevier science b v all rights reserved

velocity structure in the izu bonin seismic zone and the depth of the olivine spinel phase transition in the slab in this study a method is presented for determining the in situ elastic wave velocities of slabs simultaneously with the relative relocations of earthquakes the method is an extended arrival time difference atd scheme that incorporates a realistic model several assumptions are made about the characteristics of ray paths in and around the slab in order to take advantage of the realism of the model without completely sacrificing the simplicity inherent in the atd technique the errors introduced by these approximate rays are estimated by comparing them with more exact rays calculated with a three dimensional ray tracing routine in general the comparisons suggest that these errors are usually much less than 0 1 s the inverse method is tested on the same set of hypothetical data the results of these tests show that the parameters that influence the arrival time differences the most are the amplitude of the slowness anomaly within the slab and the azimuths and takeoff angles for rays leaving the master event the arrival time differences are relatively insensitive to the other parameters describing the slab strike dip half width and the position of the master event which implies that the scheme is fairly insensitive to the particular choice of analytic function to describe the slowness variation within the slab convergence of the inverse routine is rapid in the tests usually less than five iterations are required before a stable solution is reached it appears that coupling between solutions for slab parameters and ray directions is insignificant unlike the standard atd approach solutions for hypocenters and velocities are not directly coupled in this scheme although well located events are required to insure that the hypocenters are not unduly biased by variations in structure once the velocity structure of the slab is determined that information can be used routinely by this scheme to relocate less well recorded events this method was applied to arrival time data from earthquakes located between 200 and 600 km depth in the izu bonin subduction zone structure in the 200 300 km depth range could not be resolved and results in the 500 600 km depth range proved to be too sensitive to noise data from events between 27 n and 31 n show that p wave velocities within the slab from 400 to 500 km depth are 4 6 higher than those of the ambient mantle between 300 and 400 km depth the slab structure varies regionally north of about 32 5 n the p wave velocity within the slab is 3 4 higher than ambient from 180 to 375 km depth in contrast the velocities south of 32 5 n increase from 6 7 higher than ambient in the 375 410 km depth range to 8 10 higher in the 325 375 km depth range and appear to be 9 higher than ambient in the 270 325 km depth range the determination of the high velocities in this region between 325 and 375 km depth proved to be independent of the selection of data and of the choice of starting model above 325 km depth however the velocity determinations are somewhat dependent on the choice of starting model as a result of this dependence no upper bound on the depth of the high velocities could be resolved a plausible explanation for the exceptionally high p wave velocities within the slab south of 32 5 n is that the olivine spinel phase change boundary is elevated in this part of the slab while north of 32 5 n it is not significantly elevated it is not known if the phase change is elevated south of 31 n but the velocities between 400 and 500 km depth in this region imply that it is not depressed the high velocity zone south of 32 5 n lies in an area that marks the beginning of a seismic gap increasing in breadth to the south between deep and shallow seismicity the dip of the slab also becomes gradually steeper to the south of this region in contrast the lower velocities north of 32 5 n occur in a region where the seismicity is fairly continuous and is aligned along a shallow dipping plane if the seismic gap south of 32 5 n represents a region of high velocity then the increasingly vertical dip of the slab in the south could be due to a substantial gravitational force provided by the excess mass extending above the ambient olivine spinel phase boundary the decreasing width of the gap to the north suggests that the evolution of the izu bonin seismic zone has been governed by a propagating instability from the south

evolution and structure of the mesoscale convection and its environment a case study during the early onset of the southeast asian summer monsoon abstract the evolution and structure of mesoscale convection in the south china sea scs region are documented for the first time mainly using the dual doppler radar dataset collected during the south china sea monsoon experiment scsmex in 1998 in particular this study focuses on the convection associated with a subtropical frontal passage during the early onset of the southeast asian monsoon seam for the case of 15 may 1998 interaction between the tropical monsoon flow and frontal circulation played an important role in the evolution and structure of mesoscale convection in the prefrontal region the southwesterly monsoon flow converged with the southwesterly frontal flow to generate northeast to southwest oriented convection in the postfrontal region the southwesterly monsoon flow converged with the northerly frontal flow to produce a wide convective line with an east to west orientation in addition the convergence between the southerly monsoon flow and the northerly postfrontal flow gener

dynamical and thermal structure of a subduction zone influence of slab geometry on the convective state of the earth s upper mantle preliminary results received 29 march 1996 accepted 3 july 1996 abstract the aim of this study is to investigate the influences of plate kinematics and of the downgoing slab s geometry on internal dynamics and thermal structure in the vicinity of a subduction zone a systematic study depending on these parameters has been carried out based on a two dimensional finite differences code in a convective domain representing the upper mantle plate displacements are prescribed by kinematic boundary conditions in particular the subducting plate moves and dips at a constant velocity under the slower overriding one typical subduction dips 30 45 and 70 have been studied for a realistic rayleigh number of about l05 and for plate velocities around a coupling velocity of 2 3 cm year 1 this limit value was inferred from the balance between surface forces and buoyancy forces for any subduction dip and plate velocities lower than this coupling velocity plate kinematics variations induce significant effects on thermal and mineralogical patterns at greater plate velocities these patterns stabilize for plate velocities of the order of the coupling velocity a subduction dip decrease of 40 involves a heating of the downgoing plate of 100degrees and a depth increase of 20 km of the olivine spinel phase transition in this plate in the particular case of a non moving overriding plate the balance between this plate and underlying internal dynamics requires a moving plate velocity lower than 2 3 cm year j for a lower dip moreover a shallow subduction enhances a lateral large scale convection under the upper plate even if this plate is motionless in contrast to the case of a steep subduction this is consistent with observed back arc extension at the surface which is often associated with steep subductions our model suggests a spreading zone of between 500 and 1000km in the upper plate for a subduction velocity lower than 2 3 cm year i and a rayleigh number of about l05 1 introduction the earth s mantle undergoes a rayleigh benard thermal convection coupled to oceanic lithospheric plate movements rigid and moving lithospheric corresponding author plates form the thermal and mechanical limit bound aries of the earth s mantle convection cells one of the concerns of numerical modelling is to account for a real dynamic convection model coupled to plate tectonics to force the movement of rigid plates one may either model a dynamic plate by prescribing a lower viscosity at plate margins or impose the horizontal surface velocity davies 1989 0031 9201 97 17 00 copyright 1997 elsevier science b v all rights reserved ph s0031 9201 96 03209 8

effects of depth dependent properties on the thermal anomalies produced in flush instabilities from phase transitions abstract the effects of depth dependent thermal expansivity viscosity and thermal conductivity on mantle convection with phase transitions have been examined with two dimensional finite element simulations in an aspect ratio four the model includes a core mantle thermal coupling boundary condition which takes into play the secular cooling of the core by the overlying mantle flow initial surface rayleigh numbers between 2 10 7 and 10 8 have been considered with time the surface rayleigh number decreases to a value upon which a transition takes place from layered to single cell convection this tumultuous period is marked by large scale coherent breakthrough of cold material trapped in the transition zone all the way to the base of the mantle and a violent reaction of hot plume in the upper mantle both hot and cold anomalies have large magnitudes cold anomalies with temperatures exceeding 1000 k are found at the base of the mantle the magnitude of the cold anomalies is largest with all three depth dependent properties the timescales of this catastrophic event are between 20 and 50 myr with the longest being produced by the model with depth dependent expansivity and viscosity results from these simple two dimensional cartesian models represent a lower bound to the larger cold thermal anomalies potentially capable of being generated in spherical shell convection models

the relationship between precipitation and lightning in tropical island convection a c band polarimetric radar study one of the primary scientific objectives of the maritime continent thunderstorm experiment was to study cloud electrification processes in tropical island convection in particular the coupling between ice phase precipitation and lightning production to accomplish this goal a c band polarimetric radar was deployed in the tropics 11 68s 130 88e for the first time accompanied by a suite of lightning measurements using observations of the propagation corrected horizontal reflectivity and differential reflectivity along with specific differential phase rain and ice masses were estimated during the entire life cycle of an electrically active tropical convective complex known locally as hector over the tiwi islands on 28 november 1995 hector s precipitation structure as inferred from these raw and derived radar fields was then compared in time and space to the measured surface electric field cloud to ground cg and total lightning flash rates and ground strike locations during hector s developing stage precipitating convective cells along island sea breezes were dominated by warm rain processes no significant electric fields or lightning were associated with this stage of hector despite substantial rainfall rates aided by gust front forcing a cumulus merger process resulted in larger taller and more intense convective complexes that were dominated by mixed phase precipitation processes during the mature phase of hector lightning and the surface electric field were strongly correlated to the mixed phase ice mass and rainfall merged convective complexes produced 97 of the rainfall and mixed phase ice mass and 100 of the cg lightning as hector dissipated lightning activity rapidly ceased as evidenced from the multiparameter radar observations the multicell nature of hector resulted in the continuous lofting of supercooled drops to temperatures between 2108 and 2208c in discrete updraft cores during both the early and mature phases the freezing of these drops provided instantaneous precipitation sized ice particles that may have subsequently rimed and participated in thunderstorm electrification via the noninductive charging mechanism

the effects of a deep mantle endothermic phase change on the structure of thermal convection in silicate planets previous studies of the effects of silicate phase transitions on mantle convection have tended to use numerical models in which the phase transitions have the same depths as found in the earth s mantle or in which a single transition is placed at a depth equal to half the layer thickness this study presents a systematic investigation designed to examine the effects of an endothermic phase transition near the core mantle boundary of terrestrial type planets the results indicate that a deep mantle endothermic phase change may trap the lower thermal boundary layer allowing only one upwelling to penetrate the phase transition in these situations the deep endothermic phase change excites a dipolar convection pattern this mechanism may have a role in the degree one power observed in the distribution of the earth s hotspots and the development of the martian crustal dichotomy

geodynamic models of deep subduction abstract numerical and laboratory models that highlight the mechanisms leading to a complex morphology of subducted lithospheric slabs in the mantle transition zone are reviewed an increase of intrinsic density with depth an increase of viscosity or phase transitions with negative clapeyron slope have an inhibiting influence on deep subduction the impingement of slabs on a viscosity and density interface has been studied in laboratory tanks using corn syrup slab interaction with equilibrium and non equilibrium phase transitions has been modelled numerically in two dimensions both the laboratory and the numerical experiments can reproduce the variety of slab behaviour that is found in tomographic images of subduction zones including cases of straight penetration into the lower mantle flattening at the 660 km discontinuity folding and thickening of slabs and sinking of slabs into the lower mantle at the endpoint of a flat lying segment aside from the material and phase transition properties the tectonic conditions play an important role in particular the retrograde motion of the point of subduction trench rollback has an influence on slab penetration into the lower mantle a question that still needs to be clarified is the mutual interaction between plate kinematics and the subduction process through the transition zone

structure and evolution of the 22 february 1993 toga coare squall line aircraft observations of precipitation circulation and surface energy fluxes abstract this study documents the precipitation and kinematic structure of a mature eastward propagating oceanic squall line system observed by instrumented aircraft during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare doppler radar and low level in situ observations are used to show the evolution of the convection from an initially linear nnw sse oriented convective line to a highly bow shaped structure with an embedded low to midlevel counterclockwise rotating vortex on its northern flank in addition to previously documented features of squall lines such as highly upshear tilted convection on its leading edge a channel of strong front to rear flow that ascended with height over a rear inflow that descended toward the convective line and a pronounced low level cold pool apparently fed from convective and mesoscale downdrafts from the convective line rearward the observations of this system showed distinct multiple maxima in updraft strength with

the interaction of a subducting lithospheric slab with a chemical or phase boundary we have used a finite element model of time dependent convection to determine the conditions for penetration of the subducted plate into the lower mantle a temperature dependent and non newtonian rheology is applied to achieve platelike behavior of the upper and sinking thermal boundary layer of convection the 650 km discontinuity is taken as either a chemical or phase boundary or as a combination of both it is represented by a marker chain which effects additional buoyancy when distorted out of its equilibrium position when the compositional density contrast is greater than about 5 the descending slab is deflected sidewards at the boundary and two layer convection prevails a resulting depression of the boundary in the range of 50 200 km should be detectable with seismic methods below 5 density difference the slab plunges several hundred kilometers into the lower mantle and below 2 it will probably not stop before reaching the core mantle boundary and extensive mixing would be expected with a pure phase change a negative clapeyron slope of about 6 mpa k 60 bar k is required to establish a type of leaky double layer convection a more moderate slope can aid a small compositional density difference to prevent slab penetration into the lower mantle with the present uncertainties about the physical nature of the 650 km discontinuity a variety of convective styles appears possible on dynamical grounds

cloud resolving modeling of tropical cloud systems during phase iii of gate part i two dimensional experiments abstract a formal framework is established for the way in which cloud resolving numerical models are used to investigate the role of precipitating cloud systems in climate and weather forecasting models emphasis is on models with periodic lateral boundary conditions that eliminate unrealistic numerically generated circulations caused by open boundary conditions in long term simulations defined in this formalism is the concept of large scale forcing and the cloud environment interactions that are consistent with the periodic boundary conditions two dimensional numerical simulations of the evolution of cloud systems during 1 7 september 1974 in phase iii of the global atmospheric research program atlantic tropical experiment gate are conducted based on the above formalism a simple technique is used to force an anelastic cloud resolving model with evolving large scale horizontal wind field and large scale forcing for the temperature and moisture obtained from the gate data the 7 day period selected i

temporal evolution of the stress state in a supercontinent during mantle reorganization summary previous numerical simulation models of isoviscous mantle convection in both 2 d and 3 d geometries have revealed that the presence of a supercontinental lid produces large scale horizontal mantle flow thereby reorganizing the thermal structure of the mantle interior large scale upwelling plumes arising from the core mantle boundary cmb beneath the supercontinent are observed in some models these upwelling plumes have been hypothesized to produce tensional stresses in the supercontinent and to be responsible for the subsequent continental lifting and breakup in this study numerical simulations using a 3 d spherical shell model are conducted in order to elucidate temporal changes in stress state in an idealized supercontinent during mantle reorganization the model supercontinent is assumed to be an undeformable highly viscous supercontinental lid hvsl with respect to the mantle the mantle is heated from the base and from within the hvsl is imposed abruptly on well developed convection systems and remains fixed during the simulations i first consider the scenario in which the hvsl is imposed on isoviscous or weakly temperature dependent viscosity convection exhibiting short wavelength thermal structures the simulation results reveal that a tensional stress regime rapidly predominates throughout the hvsl within 50 100 myr in model time in response to the horizontal mantle flow beneath the hvsl the subsequent large scale plumes upwelling from the cmb also induce tensional stresses in the supercontinent but the effects of these stresses on the overall stress state appear to be secondary for most of the models investigated herein the maximum deviatoric tensional stress generated in the hvsl once convection is re established is 30 90 mpa which is probably comparable to the minimum stress magnitude required to break up the supercontinent to investigate the effects of the hvsl on other convection regimes i next consider the scenario in which an hvsl having a viscosity contrast of 10 is imposed on much longer wavelength i e degree one or degree two dominant convection systems with moderately temperaturedependent viscosity the patterns of such low degree dominant convection are affected little by the imposition of the hvsl and notable mantle reorganization does not occur on the other hand when a reasonable yielding stress is imposed at the continental margins newly formed weak upwelling plumes develop due to enhanced downwelling at the margins rather than large scale mantle reorganization however the newly upwelling plumes do not seem to produce a visible tensional stress regime in the hvsl interior and the spatial distributions of the stress regime and stress magnitude in the hvsl depend on the spatial relationship between the hvsl and the convection pattern since the series l models of the present study do not include the yield stress in the oceanic part of the lithosphere plate like behaviour and subducting plates are not actualized the results for series s models in which the oceanic lithosphere freely sinks and the convection pattern is easily organized imply that the broadly tensional regime prevailing in the supercontinent may support the penetration of mantle plumes into the supercontinent the emergence of hotspots and subsequent continental uplift and break up

schmidt hammer exposure age dating shd of rock glaciers in the schoderkogel eisenhut area schladminger tauern range austria schmidt hammer rebound values r values enable relative age dating of landforms with r values relating to degree of weathering and therefore length of exposure this method recently termed as schmidt hammer exposure age dating shd was applied to date five rock glaciers size range 0 01 0 12 km2 and one recent rockfall deposit at the study area schoderkogel eisenhut in the schladminger tauern range 14 03 e 47 15 n austria the rock glaciers consist of gneiss or high metamorphic series of mica schist that are comparable in their r values four of them are relict permafrost absent and one is intact containing patches of permafrost on each of the five rock glaciers shd was carried out at 4 6 sites 50 measurements per site along a longitudinal transect from the frontal ridge to the root zone results at all five rock glaciers are generally consistent with each other sharing statistically significant r values along transects the range between the highest and the lowest mean r value at ea

on the effects of the lithosphere on mantle convection and evolution abstract an idealized model is discussed for plate motion and mantle convection it consists of a numerical square box filled with a newtonian boussinesq fluid heated from below at the surface the fluid has a high viscosity i e it contains a stiff lid which may or may not be laterally decoupled by weakness zones lid stiffness and coupling were varied systematically only if the lid is effectively decoupled laterally does it move as a quasi rigid plate it is then driven by the lateral pressure differential from the rising and descending plumes in the fluid and it moves against viscous shear at its bottom gravitational sliding and sinking may also drive the plate if allowed for in the model a decoupled freely moving stiff plate effectively removes heat from the interior a strongly coupled stiff lid on the other hand or one with a very large aspect ratio inhibits heat transfer through the convecting layer if heating occurs by a given flux through the bottom or internal heat generation the internal cell temperatures become very high because the stagnant surface lid can only conduct the heat by assuming the required temperature gradient throughout its thickness since the earth is partly covered by conducting continental lids which in addition contain much of the heat sources the thermal history may be affected caution must be exercised in parameterized convection modelling in this respect the overall effect cannot easily be predicted because mantle flow and continental plates will interact the importance of mechanical decoupling for plate motion strongly calls for thorough studies of the decoupling mechanisms particularly at convergent plate boundaries bending stick slip processes and the role of volatiles

initiation maintenance and properties of convection in an extreme rainfall event during scmrex observational analysis a long lived mesoscale convective system mcs with extreme rainfall over the western coastal region of guangdong on 10 may 2013 during the southern china monsoon rainfall experiment scmrex is studied the environmental conditions are characterized by little convective inhibition low lifting condensation level moderate convective available potential energy and precipitable water and lack of low level jets from the tropical ocean repeated convective back building and subsequent northeastward echo training of convective cells are found during the mcs s development stages however the initiation maintenance factors and organization of convection differ significantly during the earlier and later stages from midnight to early morning convection is continuously initiated as southeasterly flows near the surface impinge on the east side of mesoscale mountains near the coastal lines and then moves northeastward leading to formation of two quasi stationary rainbands from early morning to early afternoon new convection is repeatedly triggered along a mesoscale boundary between precipitation induced cold outflows and warm air from south china sea and gulf of tokin resulting in the formation of band training of several parallel rainbands that move eastward in a later time i e two scales of training of convective elements are found as the mcs dissipates a stronger squall line moves into the region from the west and passes over within about 3 5 h contributing about 10 15 to the total rainfall amount it is concluded that terrain near surface winds warm advection from the upstream ocean in the boundary layer and precipitation generated cold outflows play important roles in initiating and maintaining the extreme rain producing mcs

phase transitions and the three dimensional planform of thermal convection in the martian mantle previous convection models for earth have shown that the endothermic phase transition from spinel to perovskite plus magnesiowustite at 660 km depth has a strong effect on mantle convection for mars the depth of the main phase transitions is estimated using a set of 32 000 density models with randomly perturbed input parameters it is found that on average the depth of the spinel to perovskite transformation is 1910 km thus a perovskite layer exists in the martian mantle only if the core radius is less than 0 45 of the planetary radius recent results on the density of fes at pressure and temperature conditions relevant to the martiam core indicate that such a small core size is consistent with the cosmochemical estimate of around 14 sulphur content of the core numerical simulations of three dimensional spherical convection demonstrate that the presence of the endothermic phase transition close to the core mantle boundary is the crucial factor determining the style of convection in addition to the endothermic phase boundary the simulations assume an isoviscous mantle which is heated both from below and within a rigid upper boundary and rayleigh numbers of the order of 106 under these conditions it is shown that the natural planform of mantle convection exhibits only one or two strong mantle plumes the number of plumes decreases with time during planetary evolution for high rayleigh numbers the plumes start to oscillate about a mean position and pulsate in time strongly localized mantle upwellings are obtained even when the proportion of basal heating is only 10 these results are robust in the sense that they do not critically depend on the parameters of the phase transition the rayleigh number or the rate of basal heating this planform of mantle convection is consistent with the volcanic history of mars where early volcanism was widespread but later concentrated in two provinces tharsis and elysium and finally in tharsis alone