making coulomb angle oriented shear bands in numerical tectonic models abstract localization of shear strain can be induced by strain weakening plasticity in continuum models and is often used to represent faults of various scales the orientation of the shear bands is thus required to be consistent with those observed from natural faults and with simple theories of brittle failure although the coulomb angle at which a shear band satisfies the coulomb failure criterion is widely used as an assumed fault orientation the currently available numerical techniques do not always produce shear bands oriented at this angle we demonstrate that under an associated plastic flow rule for which dilation and friction angles are equal the coulomb angle becomes a unique initial shear band orientation regardless of the numerical methods employed and model resolution the known problem of overly expanding shear bands in case of a constant dilation angle is preventable if dilation angle is reduced as shear strain along the shear band increases this treatment corresponds to natural processes reducing roughness of a fault plane

ice nucleation and overseeding of ice in volcanic clouds 1 water is the dominant component of volcanic gas emissions and water phase transformations including the formation of ice can be significant in the dynamics of volcanic clouds the effectiveness of volcanic ash particles as ice forming nuclei in is poorly understood and the sparse data that exist for volcanic ash in have been interpreted in the context of meteorological rather than volcanic clouds in this study single particle freezing experiments were carried out to investigate the effect of ash particle composition and surface area on water drop freezing temperature measured freezing temperatures show only weak correlations with ash in composition and surface area our measurements together with a review of previous volcanic ash in measurements suggest that fine ash particles equivalent diameters between approximately 1 and 1000 m from the majority of volcanoes will exhibit an onset of freezing between 250 260 k in the context of explosive eruptions where super micron particles are plentiful this result implies that volcanic clouds are in rich relative to meteorological clouds which typically are in limited and therefore should exhibit distinct microphysics we can expect that such overseeded volcanic clouds will exhibit enhanced ice crystal concentrations and smaller average ice crystal size relative to dynamically similar meteorological clouds and that glaciation will tend to occur over a relatively narrow altitude range

sage ii measurements of early pinatubo aerosols sage ii satellite measurements of the mt pinatubo eruption cloud in the stratosphere during june july and early august 1991 show that aerosols in the tropics reached as high as 29 km altitude with most of the cloud between 20 and 25 km the most optically thick portions of the cloud covered latitudes from 10 s to 30 n during the early part of this period by late july high stratospheric optical depths were observed to at least 70 n with the high values north of about 30 n from layers below 20 km high pressure systems in both hemispheres were observed to be correlated with the movement of volcanic material at 21 km into the westerly jet stream at high southern latitudes and similarly to high northern latitudes at 16 km by august the entire southern hemisphere had experienced a 10 fold increase in optical depth relative to early july due to layers above 20 km initial mass calculations using sage ii data place the aerosol produced from this eruption at 20 to 30 megatonnes well above the 12 megatonnes produced by el chichon

injection of gases into the stratosphere by explosive volcanic eruptions 1 explosive eruptions can inject large amounts of volcanic gases into the stratosphere these gases may be scavenged by hydrometeors within the eruption column and high uncertainties remain regarding the proportion of volcanic gases which eventually reach the stratosphere these are caused by the difficulties of directly sampling explosive volcanic eruption columns and by the lack of laboratory studies in the extreme parameter regime characterizing them using the nonhydrostatic nonsteady state plume model active tracer high resolution atmospheric model atham we simulated an explosive volcanic eruption we examined the scavenging efficiency for the climatically relevant gases within the eruption column the low concentration of water in the plume results in the formation of relatively dry aggregates more than 99 of these are frozen because of their fast ascent to low temperature regions consideration of the salinity effect increases the amount of liquid water by one order of magnitude but the ice phase is still highly dominant consequently the scavenging efficiency for hcl is very low and only 1 is dissolved in liquid water however scavenging by ice particles via direct gas incorporation during diffusional growth is a significant process the salinity effect increases the total scavenging efficiency for hcl from about 50 to about 90 the sulfur containing gases so2 and h2s are only slightly soluble in liquid water however these gases are incorporated into ice particles with an efficiency of 10 to 30 despite scavenging more than 25 of the hcl and 80 of the sulfur gases reach the stratosphere because most of the particles containing these species are lifted there sedimentation of the particles would remove the volcanic gases from the stratosphere hence the final quantity of volcanic gases injected in a particular eruption depends on the fate of the particles containing them which is in turn dependent on the volcanic and environmental conditions

new insights on entrainment and condensation in volcanic plumes constraints from independent observations of explosive eruptions and implications for assessing their impacts abstract the turbulent entrainment of atmosphere and the condensation of water vapor govern the heights of explosive volcanic plumes these processes thus determine the delivery and the lifetime of volcanic ash and aerosols into the atmosphere predictions of plume heights using one dimensional integral models of volcanic plumes however suffer from very large uncertainties related to parameterizations for entrainment and condensation in particular the wind entrainment coefficient which governs the contribution of crosswinds to turbulent entrainment is subject to uncertainties of one order of magnitude leading to relative uncertainties of the order of 50 on plume height in this study we use a database of 94 eruptive phases with independent estimates of mass eruption rate and plume height to constrain and evaluate four popular 1d models we employ re sampling methods to account for observational uncertainties we show that plume height predictions are significantly improved when i the contribution of water vapor condensation to the plume buoyancy flux is excluded and ii the wind entrainment coefficient is held constant between 0 1 and 0 4 we explore implications of these results for predicting the climate impacts of explosive eruptions and the likelihood that eruptions will form stable umbrella clouds or devastating pyroclastic flows last we discuss the sensitivity of our results to the definition of plume height in the model in light of a recent set of laboratory experiments and draw conclusions for improving future databases of eruption parameters

particle sedimentation and diffusive convection in volcanic ash clouds 1 understanding the longevity of volcanic ash clouds generated by powerful explosive eruptions is a long standing problem for assessing volcanic hazards and the nature and time scale of volcanic forcings on climate change it is well known that the lateral spreading and longevity of these clouds is influenced by stratospheric winds particle settling and turbulent diffusion observations of the recent 2010 eyjafjallajokull and 2011 grimsvotn umbrella clouds as well as the structure of atmospheric aerosol clouds from the 1991 mt pinatubo event suggest that an additional key process governing the cloud dynamics is the production of internal layering here we use analog experiments on turbulent particle laden umbrella clouds to show that this layering occurs where natural convection driven by particle sedimentation and the differential diffusion of primarily heat and fine particles give rise to a large scale instability where umbrella clouds are particularly enriched in fine ash this particle diffusive convection strongly influences the cloud longevity more generally cloud residence time will depend on fluxes due to both individual settling and diffusive convection we develop a new sedimentation model that includes both sedimentation processes and which is found to capture real time measurements of the rate of change of particle concentration in the 1982 el chichon 1991 mt pinatubo and 1992 mt spurr ash clouds a key result is that these combined sedimentation processes enhance the fallout of fine particles relative to expectations from individual settling suggesting that particle aggregation is not the only mechanism required to explain volcanic umbrella longevity

experimental and model constraints on degassing of magma during ascent and eruption surface volcanic gases may reflect volatile budgets of magma and forecast impending eruptions and their release to the atmosphere may affect climate the dynamics of magma degassing is complicated however by differences in the solubility partitioning and diffusion of the various volatiles all of which can vary with pressure temperature and melt composition to constrain possible gas outputs we carried out experiments to determine how cl partitions between water bubbles and silicate melt and decompression experiments to examine how cl behaves during closed and open system degassing we incorporate our findings and those from the literature for co2 and s into a steady isothermal and homogeneous flow model to estimate fluxes of gases at the vent from ascending water rich magma assuming different scenarios for the onset and development of permeability in bubbly magma we find that for given permeability scenarios total gas fluxes vary with magma flux but ratios of gas species do not change the s cl and so2 co2 ratios do change however depending on whether the magma is oxidized or reduced after magma fragments into a plinian eruption column gases continue to escape from cooling pumice in the plume but here the rate of gas release is controlled by diffusion which varies with temperature degassing of pumice and ash was modeled by linking a steady state plume model which gives the vertical variation of mean temperature and velocity of particles inside the plume to a conductive cooling model of pumices which controls diffusion of cl co2 and s in pumice we find that gas loss increases with column height mass flux and initial temperature because in both cases pumices cool over a longer time period allowing more gas to diffuse out of the matrix glass the amount of gas released also depends on the size distribution of particles in the erupting mixture with less being released for a finely skewed distribution

estimation of ash injection in the atmosphere by basaltic volcanic plumes the case of the eyjafjallajokull 2010 eruption 1 during explosive eruptions volcanic plumes inject ash into the atmosphere and may severely affect air traffic as illustrated by the 2010 eyjafjallajokull eruption quantitative estimates of ash injection can be deduced from the height reached by the volcanic plume on the basis of scaling laws inferred from models of powerful plinian plumes in less explosive basaltic eruptions there is a partitioning of the magma influx between the atmospheric plume and an effusive lava flow on the ground we link the height reached by the volcanic plume with the rate of ash injection in the atmosphere via a refined plume model that 1 includes a recently developed variable entrainment law and 2 accounts for mass partitioning between ground flow and plume we compute the time evolution of the rate of injection of ash into the atmosphere for the eyjafjallajokull eruption on the basis of satellite thermal images and plume heights and use the dispersion model of the volcanic ash advisory center of toulouse to translate these numbers into hazard maps the classical plinian model would have overestimated ash injection by about 20 relative to the refined estimate which does not jeopardize risk assessment this small error was linked to effective fragmentation by intense interactions of magma with water derived from melting of ice and hence strong mass partitioning into the plume for a less well fragmented basaltic dry eruption the error may reach 1 order of magnitude and hence undermine the prediction of ash dispersion which demonstrates the need to monitor both plume heights and ground flows during an explosive eruption

the 12 900 years bp laacher see eruption estimation of volatile yields and simulation of their fate in the plume we estimated the volatile emissions of the 12 900 years bp eruption of laacher see volcano germany using a modified petrological method glass inclusions in phenocrysts and matrix glasses sampled over the laacher see tephra profile were analysed by synchrotron x ray fluorescence microprobe and electron microprobe to obtain the emitted masses of halogens sulphur and water these data were used to initialize the numerical plume model atham in order to investigate the fate of volcanic gases in the plume and to estimate volatile masses injected into the stratosphere the scavenging efficiency of each volatile component depends on its interactions with both liquid water and ice we found a scavenging efficiency of c 5 for the sulphur species and of only c 30 for hydrogen halides despite their high water solubility our simulations showed that the greatest fraction of hydrometeors freeze to ice due to the fast plume rise and great height of the eruption column for the dry atmospheric conditions of the laacher see eruption the amount of liquid water was not sufficient to completely scavenge hcl and hbr so that a large proportion could reach the stratosphere

interaction between volcanic plumes and wind during the 2010 eyjafjallajokull eruption iceland 1 estimates of volcanic source mass flux currently deduced from observations of plume height are crucial for ash dispersion models for aviation and population hazard this study addresses the role of the atmospheric wind in determining the height at which volcanic plumes spread in the atmosphere and the relationship between source mass flux and plume height in a wind field we present a predictive model of volcanic plumes that describes the bending over of the plume trajectory in a crosswind and show that model predictions are in accord with a dataset of historic eruptions if the profile of atmospheric wind shear is described the wind restricts the rise height of volcanic plumes such that obtaining equivalent rise heights for a plume in a windy environment would require an order of magnitude increase in the source mass flux over a plume in a quiescent environment our model calculations are used to calibrate a semi empirical relationship between the plume height and the source mass flux that explicitly includes the atmospheric wind speed we demonstrate that the model can account for the variations in plume height observed during the first explosive phase of the 2010 eyjafjallajokull eruption using independently measured wind speeds and show that changes in the observed plume height are better explained by changing meteorology than abrupt changes in the source mass flux this study shows that unless the wind is properly accounted for estimates of the source mass flux during an explosive eruption are likely to be very significant underpredictions of the volcanic source conditions

experimental constraints on the distinct effects of ash lapilli and larger pyroclasts on entrainment and mixing in volcanic plumes the rate at which atmosphere is entrained and mixed into relatively dense explosive volcanic jets of gas and pyroclasts ash lapilli determines whether they rise into the atmosphere as buoyant plumes or collapse to form pyroclastic flows here we use analog experiments on grid stirred turbulence to isolate particle inertial effects related to the motions of ash and lapilli on the quantitative dynamical conditions for the onset of turbulent entrainment and mixing from energetic considerations we estimate the distinct effects of ash and lapilli on the entrainment rate we find that whereas large ash and lapilli sized pyroclasts contribute angular momentum to entraining eddies and may enhance entrainment into volcanic jets by 37 compared to expectations without inertial particles ash is dissipative and may reduce entrainment by 14 we also show that the internal turbulent mixing properties are largely insensitive to particle inertial effects our results predict that highly explosive events involving water or ice should for example feed plumes that rise higher than eruption columns that are enriched in larger pyroclasts although these events are also more likely to produce pyroclastic flows

transport of atmospheric water vapor by volcanic eruption columns contrary to assumptions often made in the literature explosive volcanic eruptions are capable of transporting significant amounts of water into the stratosphere in addition to the magmatic water component atmospheric water vapor is entrained by the column at lower levels a theoretical model for the conservation of mass momentum and thermal energy of four separate components dry air water vapor liquid condensates and solid particles is used to determine the extent of atmospheric water redistribution we examine the effects of water vapor condensation on dynamical characteristics and ambient water vapor transport a simple technique is presented for deriving canonical forms for the complex system of ordinary differential equations governing the column components solutions of this model are presented that show the influence of different volcanic boundary conditions and a range of ambient water vapor distributions on transport of the buoyant column we show that the water component vapor liquid of small eruption columns rising through a wet atmosphere is dominated by entrained water whereas larger columns are dominated by the magmatic water this is due in part to the proportionately smaller entrainment surface area in relation to the control volume for the larger columns we also show that a maintained column with an initial mass flux of 2 7 108 kg s 1 erupted into a wet atmosphere would inject 96 mt of water vapor into the stratosphere over 24 hours comparable to the annual input from methane oxidation or 100 midlatitude thunderstorms this increase may accelerate the conversion of simultaneously erupted volcanic so2 into sulfuric acid

volcanic eruption plume top topography and heights as determined from photoclinometric analysis of satellite data photoclinometry a shape from shading technique is applied to satellite data to determine the three dimensional height and morphology of the upper optical surface of a plume from the eruption of redoubt volcano alaska the technique can be applied to visible images of volcanic plumes that have uniform scattering properties and is most effective on images with moderate incident sun angles and plume transport perpendicular to the azimuth of the sun the two most significant sources of error are the finite resolution of the image data and the choice of image data number dn value for a flat plume element provided the plume spans at least 10 image pixels useful results can be obtained for the central portion of the plume the closest approximation to a flat plume element is found to be the visible dn value corresponding to the coldest pixel along a transect errors in the element altitude due to uncertainty in the dn value for a flat pixel amount to 10 20 m per pixel and are cumulative along the plume transect for redoubt this results in an uncertainty of approximately 300 m at the highest points the method indicates that the redoubt plume rose to an altitude of approximately 3 km while traveling a distance of 150 km from the vent and that the surface topography of the plume exhibits influences of the ambient wind through simple wave structures this technique will have a tremendous impact on studies of plume spreading dynamics and the time integrated evolution of plumes

volcanic plume heights on mars limits of validity for convective models 1 previous studies have overestimated volcanic plume heights on mars in this work we demonstrate that volcanic plume rise models as currently formulated have only limited validity in any environment these limits are easily violated in the current mars environment and may also be violated for terrestrial and early mars conditions we indicate some of the shortcomings of the model with emphasis on the limited applicability to current mars conditions specifically basic model assumptions are violated when 1 vertical velocities exceed the speed of sound 2 radial expansion rates exceed the speed of sound 3 radial expansion rates approach or exceed the vertical velocity or 4 plume radius grossly exceeds plume height all of these criteria are violated for the typical mars example given here solutions imply that the convective rise model is only valid to a height of 10 km the reason for the model breakdown is that the current mars atmosphere is not of sufficient density to satisfy the conservation equations it is likely that diffusion and other effects governed by higher order differential equations are important within the first few kilometers of rise when the same criteria are applied to eruptions into a higher density early mars atmosphere we find that eruption rates higher than 1 4 109 kg s also violate model assumptions this implies a maximum extent of 65 km for convective plumes on early mars the estimated plume heights for both current and early mars are significantly lower than those previously predicted in the literature therefore global scale distribution of ash seems implausible

explosive volcanic eruptions v observations of plume dynamics during the 1979 soufriere eruption st vincent summary during the 1979 eruption of the soufritre of st vincent west indies a 14min period of explosive activity occurred on april 22 the first three minutes of this activity were filmed five individual explosions formed distinct plumes which fed an eruption column which eventually ascended to a height of over 18 km the ascent velocities of the fronts of these plumes ranged from 8 5 to 61 7m s the half angle of spreading of the plume fronts ranged from 21 5 to 24 one of the plumes was observed to 8 km height and decelerated steadily from 5 1 5 to 23 m s l the main column fed by individual explosions was however observed to accelerate from 8 5 to 58 m s l in the same height interval a theoretical treatment of volcanic plume motion is presented measurements of dimensions velocities and spreading rates from the film are used to estimate plume parameters such as temperature particle content and volume discharge rate of magma from the theoretical relationships these calculations show that the individual plumes became hotter and richer in juvenile ash with time the acceleration of the main eruption column was the result of being fed by increasingly hotter and more ash rich explosions an average volume discharge rate of 12600m3s is estimated from the analysis of the plume motions this value agrees closely with an estimate of discharge rate based on the heat flux required to form an 18 km high column this agreement suggests that theories of convective motions in plumes can be successfully adapted to the volcanic case as long as the effect of hot entrained ash particles is considered

the 1980 eruptions of mount st helens physical and chemical processes in the stratospheric clouds we investigate the physical and chemical properties of the stratospheric clouds produced by the recent eruptions of the mount st helens volcano the large and diverse set of observational data collected in the high altitude plumes of the may 18 may 25 and june 13 1980 eruptions is organized and analyzed for evidence of the processes at work the data are used to guide and constrain detailed model simulations of the volcanic clouds for this purpose a comprehensive one dimensional model of stratospheric sulfate aerosols sulfur precursor gases and volcanic ash and dust is utilized the model accounts for gas phase and condensed phase heterogeneous chemistry in the clouds aerosol nucleation and growth and cloud expansion computational results are given for the time histories of the gaseous species concentrations aerosol size distributions and ash burdens of the eruption clouds the long term buildup of stratospheric aerosols in the northern hemisphere and the persistent effects of injected chlorine and water vapor on stratospheric ozone are also investigated we conclude that so2 water vapor and ash were probably the most important substances injected into the stratosphere by the mount st helens volcano both with respect to their widespread effects on composition and their impact on climate despite the seemingly large magnitude of the mt st helens eruption we also find that the volcano had little influence on the climate 0 05 k surface cooling in the northern hemisphere or on stratospheric ozone 0 2 maximum hemispherical depletion the remaining problems in the field of volcanic cloud physics and chemistry and the requirements for future experimental work are also discussed

assessing eruption column height in ancient flood basalt eruptions a buoyant plume model is used to explore the ability of flood basalt eruptions to inject climate relevant gases into the stratosphere an example from the 1986 izu oshima basaltic fissure eruption validates the model s ability to reproduce the observed maximum plume heights of 12 16 km above sea level sustained above fire fountains the model predicts maximum plume heights of 13 17 km for source widths of between 4 16 m when 32 by mass of the erupted magma is fragmented and involved in the buoyant plume effective volatile content of 6 wt assuming that the miocene age roza eruption part of the columbia river basalt group sustained fire fountains of similar height to izu oshima 1 6 km above the vent we show that the roza eruption could have sustained buoyant ash and gas plumes that extended into the stratosphere at 45 n assuming 5 km long active fissure segments and 9000 mt of so2 released during explosive phases over a 10 15 year duration the 180km of known roza fissure length could have supported 36 explosive events phases each with a duration of 3 4 days each 5 km fissure segment could have emitted 62 mt of so2 per day into the stratosphere while actively fountaining the equivalent of about three 1991 mount pinatubo eruptions per day each fissure segment could have had one to several vents which subsequently produced lava without significant fountaining for a longer period within the decades long eruption sensitivity of plume rise height to ancient atmospheric conditions is explored although eruptions in the deccan traps 66ma may have generated buoyant plumes that rose to altitudes in excess of 18 km they may not have reached the stratosphere because the tropopause was substantially higher in the late cretaceous our results indicate that some flood basalt eruptions such as roza were capable of repeatedly injecting large masses of so2 into the stratosphere thus sustained flood basalt eruptions could have influenced climate on time scales of decades to centuries but the location i e latitude of the province and relevant paleoclimate is important and must be considered

factors controlling large wood transport in a mountain river as with bedload transport wood transport in rivers is governed by several factors such as flow regime geomorphic configuration of the channel and floodplain or wood size and shape because large wood tends to be transported during floods safety and logistical constraints make field measurements difficult as a result direct observation and measurements of the conditions of wood transport are scarce this lack of direct observations and the complexity of the processes involved in wood transport may result in an incomplete understanding of wood transport processes numerical modelling provides an alternative approach to addressing some of the unknowns in the dynamics of large wood in rivers the aim of this study is to improve the understanding of controls governing wood transport in mountain rivers combining numerical modelling and direct field observations by defining different scenarios we illustrate relationships between the rate of wood transport and discharge wood size and river morphology we test these relationships for a wide multithread reach and a narrower partially channelized single thread reach of the czarny dunajec river in the polish carpathians results indicate that a wide range of quantitative information about wood transport can be obtained from a combination of numerical modelling and field observations and from document contrasting patterns of wood transport in single and multithread river reaches on the one hand log diameter seems to have a greater importance for wood transport in the multithread channel because of shallower flow lower flow velocity and lower stream power hydrodynamic conditions in the single thread channel allow transport of large wood pieces whereas in the multithread reach logs with diameters similar to water depth are not being moved on the other hand log length also exerts strong control on wood transport more so in the single thread than in the multithread reach in any case wood transport strongly decreases with increasing piece volume although this relation is not linear we also document a nonlinear relationship between wood transport and flood magnitude a threshold discharge was identified below which wood transport is negligible this threshold is higher in the multithread reach while in the single thread reach floods of lower magnitude are able to transport wood downstream wood transport ratio increases with discharge until it reaches an upper threshold or tipping point and then decreases or increases much more slowly this threshold is clearly related to bankfull discharge but it is much higher for the multithread reach than for the single thread one although modelling input and field observations were taken from a specific river our findings and conclusions are likely to be applicable to a much larger suite of mountain rivers

influence of particle aggregation on deposition of distal tephra from the may 18 1980 eruption of mount st helens volcano the may 18 1980 eruption of mount st helens msh produced an extensive ashfall deposit in washington idaho and montana with a minumum volume of 0 55 km3 tephra an unusual feature of the deposit is the occurrence of a second thickness maximum 325 km ene of msh near ritzville washington grain size and component abundance analysis of samples along the main dispersal axis indicates that ash in this region is very fine grained mean size 22 m poorly sorted polymodal and rich in glass shards and pumice fragments a computer simulation of ash fallout from an atmospherically dispersed eruption plume was developed to evaluate various hypotheses for the origin of the distal ash characteristics particularly the thickness versus distance relationship the model was constrained by observations of the eruption column height elevation of major ash transport lateral spreading of the eruption plume and atmospheric wind structure in the vicinity of msh results of different simulations indicate that the second thickness maximum cannot be attributed to either decreased wind velocities over central washington or injection of fine ash above the horizontal wind velocity maximum near the tropopause for the model to fit the observed characteristics of the deposit significant particle aggregation of ash finer than 63 m must be invoked the best fit occurs when ash less than 63 m is aggregated into particles several hundred microns in diameter with a settling velocity of 0 35 m s support for this process comes from the observation and collection of fragile ash clusters of similar size which fell at pullman washington during the may 18 eruption sorem 1982 the premature fallout of fine ash as particle aggregates is a fundamental process in the origin of the grain size characteristics variations in component abundances and thickness versus distance relationship of the may 18 msh ash fall deposit

moist convection and the injection of volcanic ash into the atmosphere if unsaturated water vapor is carried upward by a volcanic eruption column it may eventually become saturated owing to the decrease in temperature of the column as it expands through decompression and transfers heat to entrained air heat released as a result of the subsequent condensation of water vapor causes the air within the column to expand we show that this increases the buoyancy and therefore the total height of rise of the column the increase in height is significant in relatively small sub plinian and strombolian eruptions in which mass eruption rates lie in the range 103 to 106 kg s in such eruptions the latent heat released as the entrained water vapor condenses may provide the main source of heat which drives the ash and clasts upward the height of rise then becomes relatively insensitive to the mass flux erupted at the vent and depends primarily upon the vapor loading of the atmosphere in a moist atmosphere ash may rise several kilomtres higher than in an eruption of comparable strength in a dry environment moist convection leads to much wider ash dispersal particularly from very small eruptions subsequently rain flushing of ash from umbrella clouds may result from the water which forms through the condensation of entrained vapor the ash provides natural condensation nuclei for some of this entrained vapor whose mass may be much greater than that of the ash in small columns 107 kg s the latent heat released by condensation of vapor is relatively small in comparison with the thermal energy provided by the hot clasts and therefore moisture has no significant effect upon the eruption column dynamics furthermore the mass of water vapor originating from the erupted volatiles is usually comparable to or greater than that entrained from the ambient air our model also shows that if the erupting mixture becomes buoyant then the eruption columns associated with phreatomagmatic eruptions ascend nearly as high as plinian columns with the same mass eruption rate this is because the water which is vaporized by the hot ash at the source condenses higher in the column and thereby restores its latent heat to the ascending ash

the effect of total grain size distribution on the dynamics of turbulent volcanic plumes abstract the impact of explosive volcanic plumes on climate and on air traffic strongly depends on the concentration and grain size distribution gsd of pyroclastic fragments injected into the atmosphere accurate and robust modelling of the evolution of gsd during pyroclast transport from the vent to the ash cloud is therefore crucial for the assessment of major volcanic hazards analysis of field deposits from various recent plinian eruptions shows that their total gsd is well described by a power law as expected from the physics of magma fragmentation with an exponent d ranging from 3 0 to 3 9 by incorporating these measured gsd into the initial conditions of a steady state 1d model of explosive eruption columns we show that they have a first order impact on the dynamical behaviour of explosive eruption columns starting from an initial value of d the model tracks the evolution of gsd in the column and calculates the dynamical consequences of particle sedimentation the maximum height reached by the column one of the first order results relevant to aircraft safety changes by 30 for mass fluxes of 10 7 kg s 1 or larger and by 45 85 for mass fluxes between 105 and 10 7 kg s 1 depending on exponent d we compare our predictions to a specially assembled set of geologic field data and remote sensing observations from 10 plinian eruptions for which maximum column height and mass flux are known independently the incorporation of realistic power law gsd in the model greatly improves the predictions which opens new perspectives for estimation of ash load and gsd in volcanic clouds from near real time measurements available from satellite payloads our results also contribute to the improvement of volcanic source term characterization that is required input for meteorological dispersion models

failures in detecting volcanic ash from a satellite based technique abstract immediate and accurate detection of airborne volcanic ash is an operational imperative of the aviation industry especially jet aircraft ash encounters place passengers aboard these aircraft at severe risk and significantly impact via forced rerouting both the safety and profit margins of freight carriers due to their limited fuel supply moreover the airlines can suffer high economic costs for repair and replacement of equipment operational detection and tracking of volcanic ash by most national weather services has relied heavily on a split window differencing technique of thermal longwave infrared channels on currently operational satellites unfortunately prior work on volcanic ash detection has not emphasized the dynamical interaction between the erupting volcano and the effects of overlying atmospheric water vapor phreatic and phreatomagmatic water sources six volcanic ash eruptions from around the globe were chosen for study because they have wide variation in ambient atmospheric water vapor available ground and surface water and different magma types results show that the present differencing technique is not uniformly effective in properly classifying volcanic ash pixels in the satellite scene and often falsely interprets meteorological clouds as volcanic ash clouds and conversely moreover it is not always a robust early detector an operational aviation requirement seasonal variability in global integrated atmospheric water vapor coupled with the geographical distribution of currently active volcanoes suggests the concerns discussed herein with regard to six specific eruptions have applicability to the global aviation industry operational implications are discussed and a strategic proposal is presented on necessary steps to improve detection

constraints on cooling and degassing of pumice during plinian volcanic eruptions based on model calculations during explosive volcanic eruptions pumice clasts are transported into the atmosphere and their thermal and degassing histories determine how much volatiles are lost syneruptively this process is studied by combining a steady state eruption column model with a model for the cooling and degassing of pumice in the model we investigate the impact of various parameters e g mass eruption rate biot number eruption temperature and geometry on the cooling and degassing of pumice and find that the biot number and the eruption temperature are the most influential during typical risetimes of pumices inside eruption columns 200 300 s those smaller than 0 5 cm in diameter are found to lose little of their volatiles syneruptively and remain in thermal equilibrium with the plume in the case of larger pumices the ratio of the cooling timescale r p 2 where rp is the pumice radius and is the thermal diffusivity to the degassing timescale w 2 d 0 where w is the vesicle wall thickness and d 0 is the species diffusivity controls degassing if this ratio is larger than 50 degassing of the pumice will be nearly complete with a few percent of the original volatiles left in the outer rind of the pumice for a ratio of less than 0 1 no volatiles escape the pumice typical values for thermal and species diffusivity as well as vesicle wall thickness indicate that in the case of degassing cl and h 2 o the transition from 0 1 to 50 will be in the pumice size range of 1 10 cm here volatile concentration is a strong function of position inside the pumice suggesting that values for volatile contents measured on picked matrix glass from crushed samples may not be the best way to estimate the volatile content of matrix glass a number which is frequently used to estimate the total volatile input of volcanic eruptions this effect is even more pronounced when looking at hydrogen isotopic fractionation with larger pumices tending toward d 80

charge structure in volcanic plumes a comparison of plume properties predicted by an integral plume model to observations of volcanic lightning during the 2010 eruption of eyjafjallajokull iceland observations of volcanic lightning made using a lightning mapping array during the 2010 eruption of eyjafjallajokull allow the trajectory and growth of the volcanic plume to be determined the lightning observations are compared with predictions of an integral model of volcanic plumes that includes descriptions of the interaction with wind and the effects of moisture we show that the trajectory predicted by the integral model closely matches the observational data and the model well describes the growth of the plume downwind of the vent analysis of the lightning signals reveals information on the dominant charge structure within the volcanic plume during the eyjafjallajokull eruption both monopole and dipole charge structures were observed in the plume by using the integral plume model we propose the varying charge structure is connected to the availability of condensed water and low temperatures at high altitudes in the plume suggesting ice formation may have contributed to the generation of a dipole charge structure via thunderstorm style ice based charging mechanisms though overall this charging mechanism is believed to have had only a weak influence on the production of lightning

characterization of aerosols from eruptions of mount st helens measurements of mass concentration and size distribution of aerosols from eruptions of mount st helens as well as morphological and elemental analyses were obtained between 7 april and 7 august 1980 in situ measurements were made in early phreatic and later minor phreatomagmatic eruption clouds near the vent of the volcano and in plumes injected into the stratosphere from the major eruptions of 18 and 25 may the phreatic aerosol was characterized by an essentially monomodal size distribution dominated by silicate particles larger than 10 micrometers in diameter the phreatomagmatic eruption cloud was multimodal the large size mode consisted of silicate particles and the small size modes were made up of mixtures of sulfuric acid and silicate particles the stratospheric aerosol from the main eruption exhibited a characteristic narrow single mode with particles less than 1 micrometer in diameter and nearly all of the mass made up of sulfuric acid droplets

a prognostic turbulence scheme for the nonhydrostatic plume model atham the presented turbulence scheme was developed for the active tracer high resolution atmospheric model atham to parameterize the effect of subgrid scale turbulence in contrast to the commonly used assumption of local isotropy in high resolution atmospheric modeling this scheme differentiates between horizontal and vertical turbulent exchange to represent the strong influence of buoyancy forces and vertical transports its computational efficiency is similar to classical turbulent kinetic energy approaches while preserving one of the main feature of higher order schemes the present extensions to include anisotropic effects in a turbulent kinetic energy approach do not need any ad hoc assumptions and are equivalent to the classical formulation in the isotropic limit the presence of high tracer concentrations in a particle laden plume is taken into account as well as supersonic effects at low mach numbers the turbulent exchange coefficients used in the equations of motion are derived from a set of three coupled differential equations for the horizontal and vertical turbulent energy and the turbulent length scale no turbulent equilibrium is assumed all turbulent quantities are treated prognostically numerical simulations of convective plumes of a typical plinian volcanic eruption with the nonhydrostatic plume model atham reveal that a complex treatment of turbulent quantities is necessary in order to capture the bulk characteristics of the plume such as the plume height the horizontal extent and plume development in time anisotropic effects of turbulence have a significant impact on the stability and internal structure of the plume for the first time results from a fully three dimensional simulation of a volcanic plume are presented because of its general formulation the presented turbulence scheme is suitable for a wide range of atmospheric applications

sensitivity of the dynamics of volcanic eruption columns to their shape this paper presents a one dimensional steady state model to investigate the sensitivity of the dynamics of sustained eruption columns to radius variations with height due to thermal expansion of the entrained air and decreases in atmospheric pressure with height in contrast to a number of previous models using an equation known as the entrainment assumption the new model is based on similarity arguments to derive an equation set equivalent to the model proposed by woods bull volcanol 50 169 193 1988 this approach allows investigation of the effect of gas compressibility on the entrainment rate of ambient air which has been little examined for a system in which a decrease in pressure significantly affects the density stratification of a compressible fluid the new model provides results that include two end members one in which the volume change within the eruption columns affects only the radial expansion without changing the vertical motion and the other is the converse the woods bull volcanol 50 169 193 1988 model can be regarded as being between those two end members the range of uncertainty arises because the extremely high temperature of discharged materials from a volcanic vent and the exceptional terminal height of the eruption columns allow significant expansion of the gas component in the eruption columns making them behave differently from common turbulent plumes this study indicates that the maximum height of the eruption columns is affected considerably by this uncertainty particularly when the eruption columns extend above a height of 10 km at which the pressure is about one fourth the pressure at the ground surface column collapse may also be suppressed in wider parameter ranges than previously estimated however the uncertainty can be reduced by measuring column radii through a vertical profile during actual volcanic eruptions accordingly this paper suggests that appropriate observation of eruption column shapes is essential for improving our understanding of the dynamics of eruption columns

the possible effects of large 19th and 20th century volcanic eruptions on zonal and hemispheric surface temperatures abstract the contribution of volcanic material to the stratosphere from major eruptions within the last two centuries has been estimated using volcanological criteria including eruption type eruption column height volume and duration of eruption and composition and degree of fragmentation of magma the chronology of major explosive volcanic eruptions is compared with a record of mean surface temperature deviation t for the same interval constructed from all available temperature data the temperature records are divided into 6 latitudinal zones allowing analysis for individual zones where temperature changes induced by aerosol perturbation might be intensified we focus on the explosive volcanic events which by our estimates injected the most material into the stratosphere these include tambora 1815 krakatau 1883 santa maria 1902 katmai 1912 and quizapu 1932 such eruptions appear to have produced a consistent but small temperature decrease on the order of 0 2 to 0 5 c on a hemispheric scale for periods ranging from one to five years although these changes are similar to background temperature variations the maximum change in t after some of these explosions appears to lag by up to three years in going from equatorial to polar latitudes somewhat smaller eruptions e g agung 1963 and possibly cosiguina 1835 seem to have produced about the same perturbation in t as the larger eruptions this suggests either a limiting mechanism on loading of the aerosol layer after a volcanic eruption or that the composition of injected material i e the ratio of silicate dust to volatiles and composition of the volatiles may significantly effect stratospheric optical depth perturbation temperatures do not remain depressed for a longer period after a series of closely timed eruptions e g the 1881 1889 or the 1902 1903 sequences than after single events

results of the eruptive column model inter comparison study this study compares and evaluates one dimensional 1d and three dimensional 3d numerical models of volcanic eruption columns in a set of different inter comparison exercises the exercises were designed as a blind test in which a set of common input parameters was given for two reference eruptions representing a strong and a weak eruption column under different meteorological conditions comparing the results of the different models allows us to evaluate their capabilities and target areas for future improvement despite their different formulations the 1d and 3d models provide reasonably consistent predictions of some of the key global descriptors of the volcanic plumes variability in plume height estimated from the standard deviation of model predictions is within 20 for the weak plume and 10 for the strong plume predictions of neutral buoyancy level are also in reasonably good agreement among the different models with a standard deviation ranging from 9 to 19 the latter for the weak plume in a windy atmosphere overall these discrepancies are in the range of observational uncertainty of column height however there are important differences amongst models in terms of local properties along the plume axis particularly for the strong plume our analysis suggests that the simplified treatment of entrainment in 1d models is adequate to resolve the general behaviour of the weak plume however it is inadequate to capture complex features of the strong plume such as large vortices partial column collapse or gravitational fountaining that strongly enhance entrainment in the lower atmosphere we conclude that there is a need to more accurately quantify entrainment rates improve the representation of plume radius and incorporate the effects of column instability in future versions of 1d volcanic plume models

a global sensitivity analysis of the plumerise model of volcanic plumes abstract integral models of volcanic plumes allow predictions of plume dynamics to be made and the rapid estimation of volcanic source conditions from observations of the plume height by model inversion here we introduce plumerise an integral model of volcanic plumes that incorporates a description of the state of the atmosphere includes the effects of wind and the phase change of water and has been developed as a freely available web based tool the model can be used to estimate the height of a volcanic plume when the source conditions are specified or to infer the strength of the source from an observed plume height through a model inversion the predictions of the volcanic plume dynamics produced by the model are analysed in four case studies in which the atmospheric conditions and the strength of the source are varied a global sensitivity analysis of the model to a selection of model inputs is performed and the results are analysed using parallel coordinate plots for visualisation and variance based sensitivity indices to quantify the sensitivity of model outputs we find that if the atmospheric conditions do not vary widely then there is a small set of model inputs that strongly influence the model predictions when estimating the height of the plume the source mass flux has a controlling influence on the model prediction while variations in the plume height strongly effect the inferred value of the source mass flux when performing inversion studies the values taken for the entrainment coefficients have a particularly important effect on the quantitative predictions the dependencies of the model outputs to variations in the inputs are discussed and compared to simple algebraic expressions that relate source conditions to the height of the plume