the 1815 tambora ash fall implications for transport and deposition of distal ash on land and in the deep sea tambora volcano lies on the sanggar peninsula of sumbawa island in the indonesian archipelago during the great 1815 explosive eruption the majority of the erupted pyroclastic material was dispersed and subsequently deposited into the indian ocean and java sea this study focuses on the grain size distribution of distal 1815 tambora ash deposited in the deep sea compared to ash fallen on land grain size distribution is an important factor in assessing potential risks to aviation and human health and provides additional information about the ash transport mechanisms within volcanic umbrella clouds grain size analysis was performed using high precision laser diffraction for a particle range of 0 2 m 2 mm diameter the results indicate that the deep sea samples provide a smooth transition to the land samples in terms of grain size distributions despite the different depositional environments even the very fine ash fraction 10 m is deposited in the deep sea suggesting vertical density currents as a fast and effective means of transport to the seafloor the measured grain size distribution is consistent with an improved atmospheric gravity current sedimentation model that takes into account the finite duration of an eruption in this model the eruption time and particle fall velocity are the critical parameters for assessing the ash component depositing while the cloud advances versus the ash component depositing once the eruption terminates with the historical data on eruption duration maximum 24 h and volumetric flow rate of the umbrella cloud 1 5 2 5 1011 m3 s as input to the improved model and assuming a combination of 3 h plinian phase and 21 h co ignimbrite phase it reduces the mean deviation of the predicted versus observed grain size distribution by more than half 9 4 to 3 7 if both ash components are considered

climate volcanism feedback and the toba eruption of 74 000 years ago abstract a general feedback between volcanism and climate at times of transition in the quaternary climate record is suggested exemplified by events accompanying the toba eruption 74 000 yr ago the largest known late quaternary explosive volcanic eruption the toba paroxysm occurred during the 18o stage 5a 4 transition a period of rapid ice growth and falling global sea level which may have been a factor in creating stresses that triggered the volcanic event toba is estimated to have produced between 1015 and 1016 g of fine ash and sulfur gases lofted in co ignimbrite ash clouds to heights of at least 32 5 km which may have led to dense stratospheric dust and sulfuric acid aerosol clouds these conditions could have created a brief dramatic cooling or volcanic winter followed by estimated annual northern hemisphere surface temperature decreases of 3 to 5 c caused by the longer lived aerosols summer temperature decreases of 10 c at high northern latitudes adjacent to regions already covered by snow and ice might have increased snow cover and sea ice extent accelerating the global cooling already in progress evidence for such climate volcanic feedback following milankovitch periodicities is found at several climatic transitions

origin of the giant eruption cloud of pinatubo june 15 1991 the series of eruptions of june 15 1991 at mt pinatubo philippines were observed hourly by satellite a giant discshaped cloud covering an area of 60 000 km2 appeared in the satellite images at 14 40 philippine time the cloud expanded radially against wind of 20 m s and spread to an area of more than 120 000 km2 within an hour according to eyewitness accounts there was heavy fine ash fall after 14 00 intermittent lapilli fall started at about 14 20 and heavy and continuous lapilli fall widely started at about 15 00 the occurrence of the giant cloud roughly corresponded to the initiation of the intermittent lapilli fall the air fall deposits of the major eruption are widely distributed including upwind from the vent they are composed of 3 units a silt size fine ash layer layer b a lapilli layer commonly including pumice grains of 1 cm in diameter layer c a lapilli bearing volcanic sand layer layer d judging from its wide distribution and depletion of coarse grains most of the fine ash of layer b is not distal deposits of a small eruption but is originated from a large co ignimbrite cloud it is suggested that the major eruption started with the generation of a pyroclastic flow which was subsequently followed by a plinian eruption resulting in the formation of the giant cloud and the lapilli fall the results of calculations on the dynamics of eruption cloud indicate that the dimension and dynamics of the giant eruption cloud is accounted for by a plinian eruption with a magma discharge rate of the order of 109 kg s

mass partition during collapsing and transitional columns by using numerical simulations pyroclast dispersal and mass partition between convective and collapsing transport systems represent a major issue in the understanding of the dynamics of quasi steady explosive eruptions and the interpretation of their deposits the spatial and temporal dispersal of pyroclasts during the initial 15 min of collapsing and transitional columns was investigated by using a transient two dimensional and three phase one gas phase and two solid phases representative of pyroclasts of varying size and density flow model the different behaviors of the column were simulated by using different water contents of the eruptive mixture and assuming both pressure balanced and overpressured conditions at the vent for each simulation results allowed the quantification of the mass of pyroclasts of different size forming the pyroclastic density current the co ignimbrite column and the convective plumes rising from the proximal area and above the fountain for collapsing columns the total mass transported in the convective system composed of the plume above the fountain and the co ignimbrite column ranges from about 10 to 30 wt of the total erupted mass according to the specific water content of the mixture at the vent fine particles are dominant in this convective system with the increase of the water content the character of the column shows a collapsing buoyant transitional behavior such a regime is characterized by greater collapse height generation of more dilute density currents shorter flow runout less steady behavior of the column and intermittent feeding of the flows for these columns the total mass forming the convective portions reaches values of up to 50 wt of the erupted pyroclasts with a significant amount of coarse particles entrained in the convective plume above the fountain at higher water contents the column becomes fully buoyant with the whole mass feeding the convective column simulations assuming overpressured conditions at the vent are characterized by more unsteady dynamics of the dispersal process with significant variations in the collapse height of the column mass flow rate of the flow and amount of mass elutriated in the convective system simulation results suggest that plinian columns that undergo a transition from convective to collapsing regimes as a consequence in our case of a water content decrease in the magma but not necessarily limited to this case would be characterized by a gradual decrease of the carrying capacity of the convective column above the fountain and by the emplacement of pyroclastic density currents which evolve from dilute to dense gradually increasing their runout

a laboratory study of explosive volcanic eruptions this paper describes a series of laboratory experiments in which buoyant mixtures of methanol and ethylene glycol meg are injected as a downward propagating jet into a tank of fresh water as the meg mixes with water it becomes denser than the water if the meg mixes with sufficient water before its initial momentum is exhausted the jet fluid may become dense and continue downward into the tank as a convecting plume if the jet does not have sufficient initial momentum then a collapsing fountain develops and the material in the jet rises back toward the top of the tank and spreads laterally as a gravity current subsequent mixing can cause some of the material in the gravity current to become dense separate from the current and sink into the tank although the direction of gravity is reversed these experiments simulate many of the important dynamical features of eruption columns which can develop during explosive volcanic eruptions in the volcanic situation a hot dense and dusty mixture of gas ash and clasts is erupted from a vent at high speed if sufficient ambient air is entrained into the jet then the mixture may become buoyant through heating and expansion of the air it therefore continues rising high into the atmosphere otherwise the material collapses back to the ground these experiments allow one to investigate systematically the different styles of behavior of the erupted material as the eruption conditions change four different styles of behavior have been identified with transitions from one style to the next as the initial momentum flux of the jet is decreased 1 plinian style convecting columns 2 the periodic release of discrete convecting clouds originating close to the vent from a collapsed fountain 3 coignimbrite eruption columns centered some distance from the vent which are generated when a fraction of a pyroclastic flow becomes buoyant and 4 pyroclastic flows in which the majority of the material remains relatively dense and therefore spreads laterally from the vent using a simple theoretical model of the laboratory experiments an analytical expression describing the conditions necessary for collapse of the analogue laboratory columns has been derived this is successfully compared with the laboratory experiments the simple analysis predicts that column collapse may be induced by 1 increasing the density of the erupted material 2 decreasing the maximum buoyancy the mixture can attain on mixing with ambient 3 increasing the erupted mass flux for a given momentum flux or 4 decreasing the initial momentum flux for a given mass flux as may occur if the vent is eroded these results are consistent with earlier studies wilson et al 1980 wilson and walker 1987 bursik and woods 1991 although the analogue experimental system is somewhat simplified the observations of the periodic release of convecting thermals just after column collapse suggest a mechanism for the complex grading which is commonly found just below the collapse horizons in fall deposits for example the fogo a deposit walker and croasdale 1970 and the vesuvius a d 79 deposit carey and sigurdsson 1987 in conjunction with partial column collapse this periodicity also suggests a means by which flow deposits may become interspersed with fall deposits this feature has been observed in a number of cases including the taupo deposit wilson and walker 1985

new estimates of the 1815 tambora eruption volume abstract volume estimates of the 1815 tambora eruption were re analysed using new ash thickness data and new methods giving a range of possible volumes for the different eruption phases volume calculations include ash fall extrapolation and caldera size approximations as well as volume estimates from mass eruption rates and eruption duration to find the most likely range of volumes consistent with the different methods and possible eruption dynamics the results give a total volume of about 41 4 km3 dre made of 23 3 km3 dre ash fall and 18 6 km3 dre pyroclastic flows the ash fall volume can be further divided into 11 2 km3 dre plinian ash fall and 12 4 km3 dre co ignimbrite making the plinian ash fall larger than previously assumed and more intense due to a higher mass eruption rate of 2 109 kg s

effects of parameterized boundary layer structure on hurricane rapid intensification in shear abstractthis study investigates the role of the parameterized boundary layer structure in hurricane intensity change using two retrospective hwrf forecasts of hurricane earl 2010 in which the ver

asymmetric hurricane boundary layer structure from dropsonde composites in relation to the environmental vertical wind shear this study investigates the asymmetric structure of the hurricane boundary layer in relation to the environmentalverticalwind shearin theinner core region data from1878 gps dropsondesdeployed by research aircraft in 19 hurricanes are analyzed in a composite framework kinematic structure analyses based on doppler radar data from 75 flights are compared with the dropsonde composites shear relative quadrantmeancomposite analysesshow that boththekinematicandthermodynamic boundarylayerheight scalestend to decrease with decreasing radius consistent with previous axisymmetric analyses there is still a clear separation between the kinematic and thermodynamic boundary layer heights both the thermodynamic mixed layer and height of maximum tangential wind speed are within the inflow layer the inflow layer depth is found to be deeper in quadrants downshear with the downshear right dr quadrant being the deepest the mixed layer depth and height of maximum tangential wind speed are alike at the eyewall but are deeper outside in quadrants left of the shear the results also suggest that air parcels acquire equivalent potential temperature ue from surface fluxes as they rotate through the upshear right ur quadrant from the upshear left ul quadrant convection is triggered in the dr quadrant in the presence of asymmetric mesoscale lifting coincident with a maximum in ue energy is then released by latent heating in the downshear left dl quadrant convective downdrafts bring down cool and dry air to the surface and lower ue again in the dl and ul quadrants this cycling process may be directly tied to shear induced asymmetry of convection in hurricanes

on the formation of eruption columns following explosive mixing of magma and surface water when magma vents into the sea or a crater lake the ensuing magma water interaction can affect the style of eruption dramatically if the mass of surface water incorporated into the erupting material is small 108 kg s the height progressively decreases with surface water content this occurs when the magma and surface water begin to constitute a significant fraction of the mass at the top of the column so that an increasing fraction of the initial magmatic thermal energy is converted to the surface water rather than the entrained air the transitions in eruption style which result from changes in the mass of surface water mixing with the magma may account for observations of both buoyant plumes and wet surge during the eruptions of taal in 1965 and miyake jima in 1983 and for the changes in the eruptive activity at surtsey in 1963 1964 as the access of seawater to the vent became more restricted we also present calculations which suggest that the accretionary lapilli which are often found in wet flow deposits may result from condensation of vapor in both the cold wet collapsing fountains and in the flows themselves

rapid development of the tropical cyclone warm core this paper presents a simple theoretical argument to isolate the conditions under which a tropical cyclone can rapidly develop a warm core thermal structure and subsequently approach a steady state the theoretical argument is based on the balanced vortex model and in particular on the associated transverse circulation equation and the geopotential tendency equation these second order partial differential equations contain the diabatic forcing and three spatially varying coefficients the static stability a the baroclinity b and the inertial stability c thus the transverse circulation and the temperature tendency in a tropical vortex depend not only on the diabatic forcing but also on the spatial distributions of a b and c experience shows that the large radial variations of c are typically the most important effect under certain simplifying assumptions as to the vertical structure of the diabatic forcing and the spatial variability of a b and c the transverse circulation equation and the geopotential tendency equation can be solved via separation of variables the resultingradialstructureequationsretainthedynamicallyimportantradialvariationofcandcanbe solvedin terms of green s functions these analytical solutions show that the vortex response to a delta function in the diabatic heating depends critically on whether the heating occurs in the low inertial stability region outside the radius of maximum wind or in the high inertial stability region inside the radius of maximum wind this resultsuggeststhatrapidintensification isfavoredforstormsthathaveat leastsome oftheeyewallconvection inside the radius of maximum wind the development of an eye partially removes diabatic heating from the high inertial stability region of the storm center however rapid intensification may continue if the eyewall heating continues to become more efficient as the warm core matures and static stability increases over the inner core conditions there become less favorable for deep upright convection and the storm tends to approach a steady state

the heat and fluid transfer associated with the flanges on hydrothermal venting structures we present a simple model of the heat transfer and fluid flow associated with flanges which have been observed on some of the sulphide structures formed at sites of hydrothermal venting on the endeavour segment of the juan de fuca ridge 1 we show that the majority of the heat transferred by a flange is conducted across its upper surface except in flanges which are vigorously overflowing in which case it is through convective heat transfer associated with the overflowing we calculate that the flux of hydrothermal fluid through a typical non overflowing flange of area 1 m2 and with upper surface temperature in the range 10 80 c is of the order of 10 100 cm3 s with an associated heat flux of 104 105 w this heat flux is small compared to the largest black smoker plumes at endeavour which have a heat flux of the order 107 w in isolation in a relatively quiescent environment the hydrothermal plume associated with such a flange would rise about 20 40 m into the overlying stratified water column in comparison to the largest smokers in the endeavour vent field which have been observed to ascend about 180 m 2 flange derived plumes may therefore contribute to the production of the thermal and chemical anomalies in the water column below the neutral buoyancy height of the large smoker plumes our model of the temperature structure of a flange suggests that only the upper few centimetres of a flange have a temperature below 300 c this suggests that high temperature 110 c micro organisms may only thrive in the upper few centimetres of the flange

mesoscale and convective structure of a hurricane rainband abstract the mesoscale thermodynamic kinematic and radar structure of a hurricane floyd rainband observed on 7 september 1981 is presented data are from 26 aircraft passes through the rainband from 150 to 6400 m a composite technique which presents rainband structure as a function of distance from the storm circulation center reveals inflow from the outer edge of the band and a partial barrier to this flow below 3 km in the direction parallel to rainband orientation radar reveals cellular reflectivity structure on the upwind and central portions of the rainband the frequency of cellular precipitation decreases in favor of stratiform precipitation further downwind as the band spirals gradually towards the eyewall in the radial direction a decrease of 12 k in e is observed across the rainband in the subcloud layer convective scale up and downdrafts that are associated with cellular reflectivity structure are hypothesized to be responsible for the thermodynamic modification of the cloud and subc

liftoff of the 18 may 1980 surge of mount st helens usa and the deposits left behind the distance that ground hugging pyroclastic density currents travel is limited partly by when they reverse buoyancy and liftoff into the atmosphere it is not clear however what deposits are left behind by lofting flows one current that was seen to liftoff was the surge erupted from mount st helens on the morning of 18 may 1980 before lofting it had leveled a large area of thick forest the blowdown zone the outer edge of the devastated area where trees were scorched but left standing the scorched zone is where the surge is thought to have lifted off deposits in the outer parts of the blowdown and in the scorched zone were examined at 32 sites the important finding is that the laterally moving surge traveled through the scorched zone and hence the change in tree damage does not mark the runout distance of the surge buoyancy reversal and liftoff are thus not preserved in the deposits where the surge lofted upwards we propose based on interpretation of eyewitness accounts and the impacts of the surge on trees and vehicles that the surge consisted of a faster dilute overcurrent and a slower undercurrent where most of the mass and heat was retained reasonable estimates for flow density and velocity show that dynamic pressure of the surge i e its ability to topple trees peaked near the base of the overcurrent we propose that where the overcurrent began to liftoff the height of peak dynamic pressure rose above the trees and stopped toppling them the slower undercurrent continued forward however scorching trees but lacked the dynamic pressure needed to topple them grain size variations argue that it slowed from 30 m s 1 when it entered the scorched zone to 3 m s 1 at the far end

sources of unsteady column dynamics in pyroclastic flow eruptions during the first several minutes of an explosive volcanic eruption the flow in the column above the vent is unsteady even if the discharge from the vent is steady at a fixed location in the column parameters such as temperature and ash content change with time until steady flow conditions are established for conditions believed to be typical of fountain forming silicic eruptions of intermediate volume numerical simulations show unsteadiness in temperature plume diameter mass flux vertical velocity and particle concentration in addition to the steady mass flux from the vent we have identified three sources of mass and heat flux into the column 1 recirculation of pyroclastic flow material into the base of the column low elevation inward flow 2 ash entrained from the top of the pyroclastic flow by atmospheric inflow back toward the column higher elevation inward flow and 3 waves reflected within the pyroclastic flow if it encounters topographic obstacles the thermal and mass flux pulses cause variations in column diameter and fountain height changing column diameter would be expected to be manifested in changes in fallout deposits e g by shifts in clast isopleths these temporal variations if observed for example by satellite measurements of temperature pulses or pulses or inferred from field observations of deposit stratigraphy could be misinterpreted as indicating source variations such as source mass flux volatile content or vent diameter when in fact they result from fluid mechanical processes

historic eruptions of tambora 1815 krakatau 1883 and agung 1963 their stratospheric aerosols and climatic impact abstract decreases in surface temperatures after the eruptions of tambora 1815 krakatau 1883 and agung 1963 were of similar magnitude even though the amount of material dust and volatiles injected into the stratosphere by these three events differed greatly large amounts of fine ash and volatiles were dispersed into the upper atmosphere by krakatau and tambora the agung eruption in 1963 was a much smaller vulcanian type eruption which injected dust and volatiles into the stratospheric aerosol layer more directly analyses of magmatic volatiles indicate that the agung eruption was proportionately richer in so 2 and cl than either tambora or krakatau relative amounts of fine ash produced by the tambora krakatau and agung eruptions are estimated at about 150 20 1 whereas the masses of atmospheric sulfate aerosols produced were on the order of 7 5 3 1 decreases in surface temperature of a few tenths of a degree c for several years following volcanic eruptions are primarily a result of the sulfate aerosols rather than of the silicate dust the similarity in the atmospheric response after these three eruptions supports the idea of limiting mechanisms on volcanic stratospheric aerosol loading which is suggested by microphysical processes of aerosol particles fluctuations in stratospheric aerosol optical depth seem to be controlled to a large degree by high intensity sulfur rich eruptions e g agung 1963 which may however be relatively small in total ejecta volume such eruptions leave little geologic record but appear as acidity peaks in polar ice cores

the dynamics of intensification in a hurricane weather research and forecasting simulation of hurricane earl 2010 we use a high resolution numerical simulation of atlantic hurricane earl 2010 to increase our understanding of earl s intensification in relatively strong vertical shear in the context of a recent paradigm for tropical cyclone intensification the integrity of the simulation is judged by comparing analyses thereof with those of the unprecedented observational data gathered in earl consistent with the classical view of spin up the amplification of the tangential wind field above the boundary layer is found to occur as the absolute angular momentumsurfaces are drawn inwards by the aggregate heating of the rotating convective clouds in the interior of the vortex in addition to this classical pathway spin up occurs within the inner core boundary layer where the maximum tangential winds occur the latter is another element of the new paradigm despite the detrimental influence of the shear on the vortex alignment and on depressing the pseudo equivalent potential temperature outside the developing eyewall the combined eddy processes associated with the vortical plume structures in and around the developing eyewall region are shown to contribute to an enhanced overturning circulation and an intensifying storm these eddy processes are distinctly agradient effects that are not features of the classical spin up mechanism it remains to be understood how the rotating convective updraughts combine to produce the diagnosed structures of the eddy terms themselves and how vortex rossby waves and other eddies contribute to the alignment of the vortex during intensification

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

dimensions and dynamics of co ignimbrite eruption columns very powerful volcanic eruptions cannot always form classical plinian eruption columns1 4 instead collapsing fountains may develop above the vent and shed pyroclastic flows which spread laterally along the ground5 the upper part of these hot dense pyroclastic flows may become buoyant through the entrainment heating and expansion of ambient air coupled with the sedimentation of larger clasts suspended in the flow the buoyant material may rise in a co ignimbrite eruption column carrying massive quantities of fine dust and volatiles into the stratosphere6 7 here we present a model of this process and show that the co ignimbrite columns associated with the eruptions of toba8 9 75 000 years ago and tambora10 in 1815 may have ascended only about 32 and 23 km the latter is comparable with the less powerful 1982 plinian eruption column of el chichon11 this corroborates arguments that the mass of sulphuric acid aerosols injected into the stratosphere and not the eruptive power determines the climatic impact of an eruption12 13

a numerical study on the effects of environmental flow on tropical storm genesis abstract the role of the environmental wind in tropical storm genesis is studied using a numerical simulationmodel the model used is an 11 level primitive equation model covering a channel domain of 25 span withopen lateral boundaries at 5 5 and 30 5 n a number of experiments were integrated for 96 h in which theinitial zonal mean flow was specified differently the superposed initial wave disturbances were identicalin all experiments the dynamic coupling between the upper level winds and the low level movement of the disturbance wasfound to be an important factor in explaining the role of the environmental wind in storm genesis anotherimportant factor is the impact of the low level winds on the latent energy supply this supply is affectedby the relative inflow into a disturbance and by the transfer of momentum from aloft into the boundarylayer in a large area surrounding the disturbance according to the model results the storm genesis potential is definitely biased toward easterly verticalshear

environmental flow impacts on tropical cyclone structure diagnosed from airborne doppler radar composites abstractfollowing a recent demonstration of multicase compositing of axisymmetric tropical cyclone tc structure derived from airborne doppler radar measurements the authors extend the analysis to the asymmetric structure using an unprecedented database from 75 tc flights in particular they examine the precipitation and kinematic asymmetry forced by the tc s motion and interaction with vertical wind shear for the first time they quantify the average magnitude and phase of the three dimensional shear relative kinematic asymmetry of observed tcs through a composite approach the composite analysis confirms principal features of the shear relative tc asymmetry documented in prior numerical and observational studies e g downshear tilt downshear right convective initiation and a downshear left precipitation maximum the statistical significance of the composite shear relative structure is demonstrated through a stratification of cases by shear magnitude the impact of storm motion on eyewall convect

a laboratory study of ash flows hot ash flows propagate from volcanic vents during some explosive volcanic eruptions as a flow entrains and heats ambient air and simultaneously deposits pyroclasts part of the flow may become less dense than the air and ascend from the flow if entrainment of air is more important than sedimentation in controlling the flowbehaviour then a large buoyant ash cloud may form above the flow conversely if sedimentation of particles dominates then a much smaller fraction of the particles are able to ascend in the buoyant cloud we have carried out a series of analog laboratory experiments to simulate the motion and buoyancy generation in dilute ash flows propagating along the ground the first series of experiments investigated buoyancy generation through entrainment using mixtures of methanol ethylene glycol and water the second series examined the role of sedimentation upon the generation of buoyancy using particle laden fresh water currents propagating in a saline ambient fluid the experiments showed that the rate of entrainment increases while the sedimentation rate decreases as the downward slope of the terrain increases suggesting that massive ash laden clouds are more likely to rise from flows traveling on steeper slopes changes in slope angle also control the motion of ash flows the experiments suggest that flow separation and hydraulic jumps resulting from sudden changes in slope angle can enhance entrainment or sedimentation and increase buoyancy observations of the the interactions of ash flows with topography at redoubt volcano in april 1990 and at mount st helens on may 18 1980 are consistent with our laboratory observations

mesoscale aspects of the downshear reformation of a tropical cyclone the downshear reformation of tropical storm gabrielle 2001 was investigated using radar reflectivity and lightning data that were nearly continuous in time as well as frequent aircraft reconnaissance flights initially the storm was a marginal tropical storm in an environment with strong 850 200 hpa vertical wind shear of 12 13 m s 1 and an approaching upper tropospheric trough both the observed outflow and an adiabatic balance model calculation showed that the radial vertical circulation increased with time as the trough approached convection was highly asymmetric with almost all radar return located in one quadrant left of downshear in the storm reconnaissance data show that an intense mesovortex formed downshear of the original center this vortex was located just south of rather than within a strong downshear left lightning outbreak consistent with tilting of the horizontal vorticity associated with the vertical wind shear the downshear mesovortex contained a 972 hpa minimum central pressure 20 hpa lower than minimum pressure in the original vortex just 3 h earlier the mesovortex became the new center of the storm but weakened somewhat prior to landfall it is argued that dry air carried around the storm from the region of upshear subsidence as well as the direct effects of the shear prevented the reformed vortex from continuing to intensify despite the subsequent weakening of the reformed center it reached land with greater intensity than the original center it is argued that this intensification process was set into motion by the vertical wind shear in the presence of an environment with upward motion forced by the upper tropospheric trough in addition the new center formed much closer to the coast and made landfall much earlier than predicted such vertical shear induced intensity and track fluctuations are important to understand especially in storms approaching the coast

an exceptionally thick middle pleistocene tephra layer from epirus greece abstract a newly recognized 2 m thick trachytic volcanic ash deposit from northwestern greece is dated at 374 000 7000 yr and correlated with the middle pleistocene volcanic activity of central italy the deposit represents ash fallout from one of the largest volcanic eruptions in europe of the past 400 000 yr and should provide an important stratigraphic marker within the poorly dated middle pleistocene deposits of italy and greece

dynamics of coupled diffusive and decompressive bubble growth in magmatic systems bubble growth in an ascending parcel of magma is controlled both by diffusion of oversaturated volatiles and decompression as the magma rises we have developed a numerical model which explores the processes involved in water exsolution from basaltic and rhyolitic melts rising at a constant rate from magma chamber depths of 4 and 1 km while the model does not attempt to simulate natural eruptions it sheds light on the processes which control eruptive behavior under various conditions ascent rates are defined such that a constant rate of decompression dp dt is maintained a variety of initial ascent rates are considered in the model from 1 m s to 100 m s for basalts and from a few centimeters per second to 10 m s for rhyolite at the base of the conduit the model results indicate that for any reasonable ascent rate basaltic melt degasses at a rate sufficient to keep the dissolved volatile concentration at equilibrium with the decreasing ambient pressure rhyolitic melt reaches the surface at equilibrium if its ascent rate is less than 1 m s but it can erupt with high oversaturation at greater ascent rates the latter may lead to explosive eruptions if the ascent rate of rhyolite is 10 m s or more then melt barely degasses at all in the conduit and erupts with the highest oversaturation possible for the case of slow magma rise bubble growth is limited by decompression for the case of rapid magma rise bubble growth is limited by diffusion the results of our simple model do not accurately simulate natural volcanic eruptions but suggest that subsequent more complex models may be able to simulate eruptions using the insights regarding diffusive and decompressive bubble growth processes explored in this study numerical modeling of volcanic degassing may eventually lead to better prediction of eruption timing energetics and hazards of active volcanoes

precipitation processes and vortex alignment during the intensification of a weak tropical cyclone in moderate vertical shear abstractthe mechanisms underlying the development of a deep aligned vortex and the role of convection and vertical shear in this process are explored by examining airborne doppler radar and deep

the dynamics and thermodynamics of volcanic clouds theory and observations from the april 15 and april 21 1990 eruptions of redoubt volcano alaska on april 15 1990 and april 21 1990 two relatively small explosive eruptions occurred at redoubt volcano alaska lasting about 4 and 8 minutes respectively on both occasions the erupted material travelled as a pyroclastic flow down an ice canyon on the north flank of the volcano using slow scan television recordings of the eruption and the seismic record in both the near and far field we deduce that after a few minutes the upper parts of these pyroclastic flows became buoyant and a large and hot ash cloud ascended off the flow these thermals rose to a height of about 12 km at which point they began to spread laterally as umbrella clouds using a simple thermodynamic model we estimate that the clouds had a temperature in the approximate range 600 700 k as they rose buoyantly from the flow after entraining and heating air and melting and vaporizing ice we also estimate that in each eruption approximately 109 kg of fine ash was injected into the atmosphere during the april 15 eruption a slow scan television camera recorded the ascent of the cloud these observations are described analysed and compared with the predictions of a new model for the dynamics governing the ascent of such clouds in accord with the observations our model predicts that the cloud initially ascended rather sluggishly since it is only just buoyant on rising from the pyroclastic flow as it ascends it entrains and heats up more air and hence generates more buoyancy therefore it accelerates upwards only much higher in the cloud does the velocity decrease again as the thermal energy of the cloud becomes exhausted the model also predicts that the height of rise of such coignimbrite thermals is a function of the initial mass and temperature of the cloud but is almost independent of the initial velocity during the april 21 eruption a sequence of photographs recorded the lateral spreading of the umbrella cloud during an interval of about 10 minutes after the eruption these photographs are analysed and successfully compared with a simple model for the spreading of the umbrella cloud as a gravity current in a stratified environment using an avhrr satellite image of the air fall deposit from the april 15 eruption combined with a simple model we suggest that the primary mechanism of ash dispersal was transport by the ambient wind

the effect of vertical shear on tropical cyclone intensity change abstract the effect of vertical shear on tropical cyclone intensity change is usually explained in terms of ventilation where heat and moisture at upper levels are advected away from the low level circulation which inhibits development a simple two layer diagnostic balance model is used to provide an alternate explanation of the effect of shear when the upper layer wind in the vortex environment differs from that in the lower layer the potential vorticity pv pattern associated with the vortex circulation becomes tilted in the vertical the balanced mass field associated with the tilted pv pattern requires an increased midlevel temperature perturbation near the vortex center it is hypothesized that this midlevel warming reduces the convective activity and inhibits the storm development previous studies have shown that diabatic heating near the storm center acts to reduce the vertical tilt of the vortex circulation these studies have also shown that there is an adiabatic process that acts to redu

how does hurricane edouard 2014 evolve toward symmetry before rapid intensification a high resolution ensemble study abstracta 14 member high resolution ensemble of edouard 2014 a moderately sheared tropical storm that underwent rapid intensification ri is used to determine causes of vortex alignment and pr

plinian pumice fall deposit of the campanian ignimbrite eruption phlegraean fields italy abstract a plinian pumice fall deposit associated with the campanian ignimbrite eruption 36 ka phlegraean fields caldera italy occurs at the base of the distal grey ignimbrite in 15 localities spread over an area exceeding 1500 km2 between benevento and the sorrentina peninsula in the thickest stratigraphic section at voscone 130 cm 45 km east of the phlegraean caldera centre pozzuoli the deposit consists of two units the lower fall unit lfu is well sorted exhibits reverse size grading and is composed of equidimensional light grey pumice clasts with very subordinate accidental lithics the upper fall unit ufu is from well to poorly sorted crudely stratified richer in lithics and composed of both equidimensional and prolate pumice clasts the two fall units show slightly different dispersal axis n90 for the lfu and n95 for the ufu volumes calculated with the method of pyle 1989 are about 8 km3 for the lfu and 7 km3 for the ufu the maximum height of the eruptive columns are estimated using the model of the maximum lithic clasts dispersal at 44 km for the lfu and 40 km for the ufu classifying both fall units as ultraplinian in character reverse size grading within the lfu suggests an increase of the height of the column and magma discharge rate with time moderate sorting and crude stratification of the ufu are consistent with short period oscillation of the column possibly associated with repeated partial column collapses sharp increases in lithic content at the transition to ufu and within the ufu suggest that changes in the eruptive behaviour was produced by a dramatic increase in conduit vent erosion the phase of column instability preceded the emplacement of widely dispersed pyroclastic flow the ultraplinian nature of the fall fits well with the wide dispersal of the campanian ignimbrite with an estimated aspect ratio of 3 4 10 4 lari

simulations of moist nearly neutral flow over a ridge although a fairly common atmospheric condition in orographic rain scenarios there is relatively little known about moist neutral flows over a ridge from theory and modeling presented in this paper are numerical simulations of the orographic flow modification occurring for a two dimensional moist nearly neutral flow over a ridge in the regime where the coriolis force can be neglected if an initially saturated moist neutral flow were to remain everywhere saturated as it flows over an obstacle then the expected solution would be the linear solution because the condition for linearity hill height less than the ambient wind velocity static stability is always met however for higher mountains the solutions indicate the development of areas of unsaturated air with correspondingly larger values of local static stability this internal switching from small to large values of static stability is an inherent nonlinearity which has far reaching consequences for understanding the orographic flow modification in this regime the sensitivity of the solution to the mountain height and to the initial cloud water content is analyzed here the authors find that the solutions fall into three basic categories if the mountain height is small enough a saturated flow can be maintained everywhere given sufficient initial cloud water for tall mountains the atmosphere upwind of the mountain is maintained in a saturated state and transitions to an unsaturated downslope flow on the lee side which has characteristics associated with downslope windstorms for mountains of intermediate height the solutions show the existence of an upwind propagating disturbance that has the effect of desaturating the atmosphere above the mountain

ultra distal tephra deposits from super eruptions examples from toba indonesia and taupo volcanic zone new zealand abstract voluminous rhyolitic eruptions from toba indonesia and taupo volcanic zone tvz new zealand have dispersed volcanic ash over vast areas in the late quaternary the 74 ka youngest toba tuff ytt eruption deposited ash over the bay of bengal and the indian subcontinent to the west the 340 ka whakamaru eruption tvz deposited the widespread rangitawa tephra dominantly to the southeast in addition to occurrences northwest of vent extending across the landmass of new zealand and the south pacific ocean and tasman sea with distal terrestrial exposures on the chatham islands these super eruptions involved 2500 km 3 and 1500 km 3 of magma dense rock equivalent dre respectively ultra distal terrestrial exposures of ytt at two localities in india middle son valley madhya pradesh and jurreru river valley andhra pradesh at distances of 2000 km from the source caldera show a basal primary ashfall unit 4 cm thick although deposits containing reworked ash are up to 3 m in total thickness exposures of rangitawa tephra on the chatham islands 900 km from the source caldera are 15 30 cm thick at more proximal localities 200 km from source rangitawa tephra is 55 70 cm thick and characterized by a crystal rich basal layer and normal grading both distal tephra deposits are characterized by very fine ash with high pm 10 fractions and are crystal poor glass chemistry stratigraphy and grain size data for these distal tephra deposits are presented with comparisons of their correlation dispersal and preservation using field observations ash transport and deposition were modeled for both eruptions using a semi analytical model hazmap with assumptions concerning average wind direction and strength during eruption column shape and vent size model outputs provide new insights into eruption dynamics and better estimates of eruption volumes associated with tephra fallout modeling based on observed ytt distal tephra thicknesses indicate a relatively low

a laboratory simulation of pyroclastic flows down slopes abstract laboratory experiments are described which explore the dynamical consequences of buoyant convective upflow observed above hot pyroclastic flows in nature the convection is produced by the hot ash particles exchanging heat with air mixed into the front and top of the pyroclastic flow this effect on the buoyancy due to the mixing of air and ash has been modelled in the laboratory using mixtures of methanol and ethylene glycol meg which have a nonlinear density behaviour when mixed with water intermediate mixtures of these fluids can be denser than either initial component and so the laboratory experiments were inverted models of the natural situation we studied meg flowing up under a sloping roof in a tank filled with water the experiments were performed both in a narrow channel and on a laterally unconfined slope the flow patterns were also compared with those of conventional gravity currents formed using fresh and salt water the presence of the region of reversed buoyancy outside the layer flowing along the slope had two significant effects first it periodically protected the flow from direct mixing with the environment resulting in pulses of relatively undiluted fluid moving out intermittently ahead of the main flow second it produced a lateral inflow towards the axis of the current which kept the current confined to a narrow tongue even on a wide slope in pyroclastic flows the basal avalanche portion has a much larger density contrast with its surroundings than the laboratory flows calculations show that mixing of air into the dense part of a pyroclastic flow cannot generate a mixture that is buoyant in the atmosphere however the overlying dilute ash cloud can behave as a gravity current comparable in density contrast to the laboratory flows and can become buoyant depending on the temperature and ash content in the august 7th pyroclastic flow of mount st helens hoblitt 1986 describes pulsations in the flow front which are reminiscent of those observed in the experiments as proposed by hoblitt the pulsations are caused by the ash cloud accelerating away from the front of the dense avalanche as a density current the ash cloud then mixes with more air becomes buoyant and lifts off the ground allowing the avalanche to catch up with and move ahead of the cloud the pulsing behaviour at the fronts of pyroclastic flows could account for the occurrence of cross bedded layer 1 deposits which occur beneath layer 2 deposits in many sequences

sediment laden gravity currents with reversing buoyancy there are many natural occurrences of sediment laden gravity currents in which the density of the interstitial fluid is less than that of the ambient fluid although the bulk density of the current is greater such currents are driven by the excess density of suspended particles however after sufficient particles have sedimented the current will become buoyant cease its lateral motion and ascend to form a plume examples of such currents include brackish underflows in deltas turbidity currents and pyroclastic flows experimental studies are described which show that due to sedimentation sediment laden gravity currents decelerate more rapidly than saline currents of the same density there is little difference in the experiments between a sediment laden current with neutrally buoyant interstitial fluid and one with buoyant interstial fluid until sufficient sediment has been lost to cause the latter kind of current to lift off a marked deceleration is then observed and a plume is generated with lift off occurring along the length of the current the resulting buoyant plume then generates a gravity current below the upper surface of the fluid in the tank the deposit from a current with buoyant fluid shows a fairly abrupt decrease in thickness beyond the lift off distance and has a flatter profile than that from a simple sediment current a theoretical model is presented which is based on the two layer shallow water equations and incorporates a model of the sedimentation in which particles are assumed to be uniformly suspended by the turbulence of the current the model shows good agreement with the observed lengths of the experimental currents as a function of time and predicts the lift off distance reasonably well these processes have implications for the behaviour of turbidity currents the interpretation of turbidites mixing processes in the oceans and the lift off of pyroclastic flows

the relationship between spatial variations in the structure of convective bursts and tropical cyclone intensification as determined by airborne doppler radar abstractthe relationship between radial and azimuthal variations in the composite characteristics of convective bursts cbs that is regions of the most intense upward motion in tropical cyclones

the 1883 eruption of krakatau the 1883 eruption of krakatau was a modest ignimbrite forming event the deposits are primarily coarse grained dacitic non welded ignimbrite large explosions produced pyroclastic flows that entered the sea generating destructive tsunami grain size studies of the ignimbrite suggest that these explosions were not driven by magma seawater interaction the total bulk volume of pyroclastic deposits including co ignimbrite ash is estimated to be 18 21 km3

quantifying volcanic ash dispersal and impact of the campanian ignimbrite super eruption 1 we apply a novel computational approach to assess for the first time volcanic ash dispersal during the campanian ignimbrite italy super eruption providing insights into eruption dynamics and the impact of this gigantic event the method uses a 3d time dependent computational ash dispersion model a set of wind fields and more than 100 thickness measurements of the ci tephra deposit results reveal that the ci eruption dispersed 250 300 km 3 of ash over 3 7 million km 2 the injection of such a large quantity of ash and volatiles into the atmosphere would have caused a volcanic winter during the heinrich event 4 the coldest and driest climatic episode of the last glacial period fluorine bearing leachate from the volcanic ash and acid rain would have further affected food sources and severely impacted late middle early upper paleolithic groups in southern and eastern europe citation costa a a folch g macedonio b giaccio r isaia and v c smith 2012 quantifying volcanic ash dispersal and impact of the campanian ignimbrite super eruption geophys res lett 39 l10310 doi 10 1029 2012gl051605

generation and dispersal of fine ash and dust by volcanic eruptions abstract the volcanic eruptions which generate the greatest quantities of fine ash and dust are those of ignimbrite forming plinian vulcanian and phreatomagmatic types these are also the eruptions which produce the widest dispersal of this material attributed to the superior height attained by their eruptive columns however much of the fine ash and dust may be rapidly flushed out of the eruptive plume by water particularly in phreatomagmatic eruptions recent studies made on the dispersal and grain size of pyroclastic deposits produced by examples of plinian and phreatomagmatic types have yielded estimates of the quantities of material generated in each grain size class besides the extent of their dispersal not all of the fine volcanic particles are produced by fragmentation at the eruptive vent in ignimbrite eruptions there is good evidence for their large scale generation in and loss from the moving ash flows

satellite observations and interpretation of the 1991 mount pinatubo eruption plumes we demonstrate the use of hourly geostationary meteorological satellite gms and noaa polar orbiting advanced very high resolution radiometer avhrr satellite images of the june 1991 mount pinatubo volcanic eruption plumes in providing details of the timing changing eruptive style and umbrella plume movement and dynamics a chronology of explosive events which culminated in a 9 hour long climactic phase on june 15 has been determined from satellite data and favorably compared to ground based and other ancillary information maximum eruption column altitudes of 40 km and column top undercooling in excess of 55 c below the ambient atmosphere occurred during the climactic phase phases dominated by plinian versus coignimbrite eruptive plumes can be distinguished on the images analysis of the spreading umbrella plume during the climactic phase suggests that over the first 4 5 hours the plume spread laterally as a gravitational intrusion before becoming advected in the ambient winds we compare the rate of plume growth to a model of a radially symmetric continuously fed intrusion to describe the motion the rate of plume growth shows good agreement with this simple model for the first 4 5 hours from column top heights we determine eruption rates and obtain an independent estimate of the total volume of magma erupted from june 12 16 of 5 5 km3 dense rock equivalent dre in good agreement with an independent estimate of the total eruptive volume made from deposits on land and submarine ash fall of up to 5 3 km3 dre we also show that secondary explosions in the pinatubo ignimbrite deposits produced ash plumes reaching altitudes as high as 19 km in september 1991 this study indicates that realistic interpretation and monitoring of major eruption plumes can be accomplished by analysis of high temporal resolution satellite data

reversing buoyancy in turbidity currents developing a hypothesis for flow transformation and for deposit facies and architecture a turbidity current that contains fresher or otherwise less dense water than its surroundings may initially be denser than the ambient and propagate as a bottom hugging flow but later reverse in buoyancy as its bulk density decreases through sedimentation to become lower than that of the ambient seawater it is proposed that this reversal in buoyancy may be a significant mechanism controlling the structure and facies of turbiditic deposits buoyancy reversal followed by lofting may directly affect the relative distribution of fine and coarse material in the deposit while buoyancy reversal itself may mediate the transformation between dilute and highly concentrated suspension flows particularly in distal regions and thus lead to the formation of complex turbiditic beds in particular the generation of distal co genetic debrites may be expected similar transformations occur within dilute pyroclastic density currents where a mobile basal concentrated flow termed a surge derived pyroclastic flow develops through rapid sedimentation from the suspended load of the overlying surge the physical mechanisms involved in these processes are discussed leading to the proposal of some associated facies models these are compared with field data from the northern apennines with some striking similarities being noted as well as some differences on the basis of this discussion some directions are suggested for future experimental and modelling work on the topic

large magnitude middle ordovician volcanic ash falls in north america and europe dimensions emplacement and post emplacement characteristics abstract middle ordovician k bentonites represent some of the largest known fallout ash deposits in the phanerozoic era they cover minimally 2 2 106 km2 in eastern north america and 6 9 105 km2 in northwestern europe and represents the coeval accumulation of plinian and co ignimbrite ash on both laurentia and baltica during the closure of the iapetus ocean the three most widespread beds are the deicke and millbrig k bentonites in north america and the kinnekulle k bentonite in northwestern europe the vents were located near the laurentian margin of iapetus on an arc or microplate undergoing collision with laurentia the volume of ash preserved in the stratigraphie record converted to dense rock equivalent dre of silicic magma is minimally estimated to be 943 km3 for the deicke 1509 km3 for the millbrig and 972 km3 for the kinnekulle the millbrig and kinnekulle beds are coeval and possibly equivalent yielding a combined dre volume of nearly 2500 km3 some unknown but probably large amount of additional ash fell into oceanic regions of the iapetus but these areas became subducted and the ash is not preserved in the geologic record the symmetry of the thickness contours is suggestive that one or more ash clouds interacting with equatorial stratospheric and tropospheric wind patterns dispersed pyroclastic material to both the northwest and southeast in terms of ordovician paleogeography based on grain size measurements and thickness area 1 2 plots we conclude the three beds were each formed from co ignimbrite or possibly phreatoplinian eruption columns analyses of melt inclusions in primary quartz crystals indicate the parental magma contained approximately 4 dissolved water at the time of the eruption this water provided the explosive energy during the initial gas thrust phase the implied fragmentation pressure on the magma would have reduced much of the ejecta to small particles forming a deposit composed largely of single crystals and glassy dust conversion of the ash to k bentonite resulted in a mass loss of approximately 35 mostly in the form of si with lesser amounts of na and k

observations of the structure and evolution of hurricane edouard 2014 during intensity change part ii kinematic structure and the distribution of deep convection abstractthe structural evolution of the inner core and near environment throughout the life cycle of hurricane edouard 2014 is examined using a synthesis of airborne and satellite measurements this study specifically focuses on differences in the distribution of deep convection during two periods when edouard intensified toward hurricane status and when edouard peaked in intensity and began to weaken while both periods saw precipitation maximized in the downshear left and upshear left quadrants deep convection was only seen from the aircraft during the intensifying period deep convection was located farther inside the radius of maximum winds rmw during the intensifying period than the weakening period this convection is traced to strong updrafts inside the rmw in the downshear right quadrant tied to strong low level convergence and high convective available potential energy cape as the storm remained over warm water in a moist environment strong updrafts persisted upshear left and were collo

toba ash layers in the south china sea evidence of contrasting wind directions during eruption ca 74 ka two cores from the southern south china sea contain discrete ash layers that mainly consist of rhyolithic glass shards on the basis of the specmap time scale the ash layers were dated to ca 74 ka the age of the youngest toba eruption in northern sumatra this link is supported by the chemical composition of the glass which is distinct from volcanic glass supplied from the philippines and the northern south china sea but is almost identical with the chemistry of the toba ash the youngest toba ash layers in the south china sea expand the previously known ash fall zone over more than 1800 km to the east the dispersal of ashes from sumatra in both western and eastern directions indicates two contrasting wind directions and suggests that 1 the toba eruption probably happened during the southeast asian summer monsoon season and 2 the volume of erupted magma was larger than previously interpreted

co ignimbrite ash fall deposits of the 1991 eruptions of fugen dake unzen volcano japan abstract fugen dake the main peak of unzen volcano began a new eruption sequence on november 17 1990 on may 20 1991 a new lava dome appeared near the eastern edge of the fugen dake summit small scale 10 4 10 6 m 3 in volume merapi type block and ash flows were frequently generated from the growing lava dome during may june 1991 these pyroclastic flows were accompanied by co ignimbrite ash plumes that deposited ash fall deposits downwind of the volcano three examples of co ignimbrite ash fall deposits from unzen pyroclastic flows are described the volume of fall deposits was estimated to be about 30 by volume of the collapsed portions of the dome that formed pyroclastic flows this proportion is smaller than that described for other larger co ignimbrite ash fall deposits from other volcanoes grain size distributions of the unzen co ignimbrite ash fall deposits are bi modal or tri modal most ashes are finer than 4 phi and two modes were observed at around 4 7 phi and 9 phi they are composed mainly of groundmass fragments fractions of another mode at around 2 phi are rich in crystals derived from dome lava some of the fine ash component fell as accretionary lapilli from the co ignimbrite ash cloud indicating either moisture or electrostatic aggregation we believe that the co ignimbrite ash of unzen block and ash flows were formed by the mechanical fracturing of the cooling lava blocks as they collapsed and moved down the slope these ashes were entrained into the convective plumes generated off the tops of the moving flows

frontogenesis in the presence of small stability to slantwise convection abstract it is often observed that despite the existence of near neutrality to slantwise convection rainbands and snowbands can persist for long periods with narrow intense updrafts producing large quantities of precipitation in many cases this is probably due to the presence of active frontogenesis which as shown here maintains such structures in order to investigate this process we use two dimensional semigeostrophic theory to solve the deformation forced frontogenesis problem for a circumstance in which the stability to moist slantwise convection is small but positive in this case numerical simulation is necessary to determine the evolution of the front several previous studies have described the role of diabatic forcing in modifying the cross frontal circulation by use of the sawyer eliassen equation in particular analytic solutions of that equation for the case of small moist symmetric stability show that a narrow updraft should occur ahead of the maximum geostrophic frontogenesis numeri

dynamics of the major plinian eruption of samalas in 1257 a d lombok indonesia the 1257 a d caldera forming eruption of samalas lombok indonesia was recently associated with the largest sulphate spike of the last 2 ky recorded in polar ice cores it is suspected to have impacted climate both locally and at a global scale extensive fieldwork coupled with sedimentological geochemical and physical analyses of eruptive products enabled us to provide new constraints on the stratigraphy and eruptive dynamics this four phase continuous eruption produced a total of 33 40 km3 dense rock equivalent dre of deposits consisting of i 7 9 km3 dre of pumiceous plinian fall products ii 16 km3 dre of pyroclastic density current deposits pdc and iii 8 9 km3 dre of co pdc ash that settled over the surrounding islands and was identified as far as 660 km from the source on the flanks of merapi volcano central java widespread accretionary lapilli rich deposits provide evidence of the occurrence of a violent phreatomagmatic phase during the eruption with a peak mass eruption rate of 4 6 108 kg s a maximum plume height of 43 km and a dispersal index of 110 500 km2 the 1257 a d eruption stands as the most powerful eruption of the last millennium eruption dynamics are consistent with an efficient dispersal of sulphur rich aerosols across the globe remarkable reproducibility of trace element analysis on a few milligrammes of pumiceous tephra provides unequivocal evidence for the geochemical correlation of 1257 a d proximal reference products with distal tephra identified on surrounding islands hence we identify and characterise a new prominent inter regional chronostratigraphic tephra marker

airborne doppler observations of the inner core structural differences between intensifying and steady state tropical cyclones abstractdifferences in the inner core structure of intensifying in intensity increase of at least 20 kt 24 h 1 where 1 kt 0 51 m s 1 and steady state ss intensity remaining between 10 kt 24 h 1 tropical cyclones tcs are examined using composites of airborne doppler observations collected from noaa p 3 aircraft missions the in dataset contains 40 eyewall passes from 14 separate missions while the ss dataset contains 53 eyewall passes from 14 separate missions intensifying tcs have a ringlike vorticity structure inside the radius of maximum wind rmw lower vorticity in the outer core a deeper stronger inflow layer and stronger axisymmetric eyewall upward motion compared with steady state tcs there is little difference in the vortex tilt between 2 and 7 km and both in and ss tcs show an eyewall precipitation and updraft asymmetry whose maxima are located in the downshear and downshear left region the azimuthal coverage of eyewall and outer core precipitation is greater for in tcs

rapidly intensifying hurricane guillermo 1997 part i low wavenumber structure and evolution abstract the structure and evolution of rapidly intensifying hurricane guillermo 1997 is examined using airborne doppler radar observations in this first part the low azimuthal wavenumber component of the vortex is presented guillermo s intensification occurred in an environmental flow with 7 8 m s 1 of deep layer vertical shear as a consequence of the persistent vertical shear forcing of the vortex convection was observed primarily in the downshear left quadrant of the storm the greatest intensification during the 6 h doppler observation period coincided with the formation and cyclonic rotation of several particularly strong convective bursts through the left of shear semicircle of the eyewall some of the strongest convective bursts were triggered by azimuthally propagating low wavenumber vorticity asymmetries mesoscale budget analyses of axisymmetric angular momentum and relative vorticity within the eyewall are presented to elucidate the mechanisms contributing to guillermo s structural evol

distributions of shallow to very deep precipitation convection in rapidly intensifying tropical cyclones abstractshear relative distributions of four types of precipitation convection in tropical cyclones tcs are statistically analyzed using 14 years of tropical rainfall measuring mission trmm precipitation radar pr data the dataset of 1139 trmm pr overpasses of tropical storms through category 2 hurricanes over global tc prone basins is divided by future 24 h intensity change it is found that increased and widespread shallow precipitation defined as where the 20 dbz radar echo height 6 km around the storm center is a first sign of rapid intensification ri and could be used as a predictor of the onset of ri the contribution to total volumetric rain and latent heating from shallow and moderate precipitation 20 dbz echo height between 6 and 10 km in the inner core is greater in ri storms than in non ri storms while the opposite is true for moderately deep 20 dbz echo height between 10 and 14 km and very deep precipitation 20 dbz echo height 14 km the authors argue that ri is more likely

azimuthal distribution of deep convection environmental factors and tropical cyclone rapid intensification a perspective from hwrf ensemble forecasts of hurricane edouard 2014 abstractforecasts from the operational hurricane weather research and forecasting hwrf based ensemble prediction system for hurricane edouard 2014 are analyzed to study the differences in both the tropical cyclone inner core structure and large scale environment between rapidly intensifying ri and non intensifying ni ensemble members an analysis of the innercore structure reveals that as deep convection wraps around from the downshear side of the storm to the upshear left quadrant for ri members vortex tilt and asymmetry reduce rapidly and ri occurs for ni members deep convection stays trapped in the downshear downshear right quandrant and storms do not intensify the budget calculation of tangential wind tendency reveals that the positive radial eddy vorticity flux for ri members contributes significantly to spinning up the tangential wind in the middle and upper levels and reduces vortex tilt the negative eddy vorticity flux for ni members spins down the tangential wind in the middle and u

effects of vertical wind shear on the intensity and structure of numerically simulated hurricanes a series of numerical simulations of tropical cyclones in idealized large scale environments is performed to examine the effects of vertical wind shear on the structure and intensity of hurricanes the simulations are performed using the nonhydrostatic pennsylvania state university national center for atmospheric research fifth generation mesoscale model using a 5 km fine mesh and fully explicit representation of moist processes when large scale vertical shears are applied to mature tropical cyclones the storms quickly develop wavenumber one asymmetries with upward motion and rainfall concentrated on the left side of the shear vector looking downshear in agreement with earlier studies the asymmetries develop due to the storm s response to imbalances caused by the shear the storms in shear weaken with time and eventually reach an approximate steady state intensity that is well below their theoretical maximum potential intensity as expected the magnitude of the weakening increases with increasing shear all of the storms experience time lags between the imposition of the large scale shear and the resulting rise in the minimum central pressure while the lag is at most a few hours when the storm is placed in very strong 15 m s21 shear storms in weaker shears experience much longer lag times with th e5ms 21 shear case showing no signs of weakening until more than 36 h after the shear is applied these lags suggest that the storm intensity is to some degree predictable from observations of largescale shear changes in all cases both the development of the asymmetries in core structure and the subsequent weakening of the storm occur before any resolvable tilt of the storm s vertical axis occurs it is hypothesized that the weakening of the storm occurs via the following sequence of events first the shear causes the structure of the eyewall region to become highly asymmetric throughout the depth of the storm second the asymmetries in the upper troposphere where the storm circulation is weaker become sufficiently strong that air with high values of potential vorticity and equivalent potential temperature are mixed outward rather than into the eye this allows the shear to ventilate the eye resulting in a loss of the warm core at upper levels which causes the central pressure to rise weakening the entire storm the maximum potential vorticity becomes concentrated in saturated portions of the eyewall cloud aloft rather than in the eye third the asymmetric features at upper levels are advected by the shear causing the upper portions of the vortex to tilt approximately downshear the storm weakens from the top down reaching an approximate steady state intensity when the ventilated layer can descend no farther due to the increasing strength and stability of the vortex at lower levels

reconstructing the tephra dispersal pattern from the bronze age eruption of santorini using an advection diffusion model the advection diffusion model tephra2 has been used in conjunction with the downhill simplex method dsm and one at a time oat inversion methods to reconstruct the eruption conditions and seasonality consistent with the deposit patterns from the bronze age minoan eruption of santorini we investigated three datasets representing different depositional environments proximal terrestrial distal terrestrial and deep sea core in order to determine source conditions such as plume height erupted mass and grain size and recreate the tephra fall deposit from the plinian co ignimbrite and combined eruptive phases the results of the dsm and oat method agreed adequately well with each other for erupted mass plume height and grain size distribution both approaches were able to successfully recreate the plinian deposit but estimating conditions that created the co ignimbrite and deep sea core dataset were less successful the reduced agreement is the result of the low quantity 6 to 28 deposit points and quality inconsistent deposit depths at localities adjacent to each other of the datasets and the different dynamics between co ignimbrite and plinian columns with the former not well represented in the model different sampling methods between archaeological and volcanological disciplines and post depositional processes which have acted on the tephra deposits since the bronze age can explain the discrepancy between these computed and observed deposits the seasonality of the minoan eruption was investigated by using seasonal wind profiles for winter spring summer and autumn we find that the bronze age eruption of santorini is likely to have during the spring and summer months with a main dispersal axis aligned east crete would have received very little ash fall and the eruption would not have caused much disruption to the life of the inhabitants of the island

goes weather satellite observations and measurements of the may 18 1980 mount st helens eruption we demonstrate the use of geostationary operational environmental satellite goes images of the may 18 1980 mount st helens volcanic plume in providing details of the dynamics and changing character of this major explosive eruption visible and thermal infrared ir data from a sequence of images at 30 min intervals from 0850 to 1720 local time lt give information on dispersal and plume top temperature initial visible and ir images at 0850 show the top of a spreading co ignimbrite like umbrella plume and an overshooting column emerging from it both rising off the ground hugging pyroclastic gravity flow generated by the opening directed blast the overshooting column had a minimum temperature significantly colder than local ambient atmosphere indicating substantial undercooling and a maximum altitude of 31 2 km at 0920 this large plume system then formed a high velocity radially spreading gravitationally driven current before becoming advected in the wind field at an average downwind velocity of 29 m s within 30 to 60 min the undercooled plume adjusted to ambient temperatures after this initial event the eruption column fluctuated over the next 8 hours between stable plinian column rise and unstable conditions favoring column collapse leading to pyroclastic flows and further development of co ignimbrite ash columns over this period of observation profiles of plume top temperatures derived from ir data versus distance along the downwind axis have distinctive patterns with steep slopes associated with co ignimbrite plumes and shallower slopes associated with plinian plumes moreover reflectance values from visible goes data change from lower to higher during periods of transition from darker toned plinian to lighter toned coignimbrite plumes indicating that in this case satellite data resolved changes in eruptive style from plumes with a coarser to a finer dominant particle size

idealized tropical cyclone responses to the height and depth of environmental vertical wind shear abstractthree sets of idealized cloud resolving simulations are performed to investigate the sensitivity of tropical cyclone tc structure and intensity to the height and depth of environmental vertical wind shear in the first two sets of simulations shear height and depth are varied independently in the third set orthogonal polynomial expansions are used to facilitate a joint sensitivity analysis despite all simulations having the same westerly deep layer 200 850 hpa shear of 10 m s 1 different intensity and structural evolutions are observed suggesting the deep layer shear alone may not be sufficient for understanding or predicting the impact of vertical wind shear on tcs in general vertical wind shear that is shallower and lower in the troposphere is more destructive to model tcs because it tilts the tc vortex farther into the downshear left quadrant the vortices that tilt the most are unable to precess upshear and realign resulting in their failure to intensify shear height appears to

tephra from the minoan eruption of santorini in sediments of the black sea the explosive eruption of santorini volcano in the aegean sea about 3 300 years ago is of considerable archaeological and volcanological significance1 5 here we report the discovery of tephra from this minoan event in laminated sediments of the black sea this finding provides constraints on the distribution of debris from the eruption we estimate a minimum fallout area of 2 l06 km2 extending from the black sea in the north to the southeastern mediterranean sea the main dispersal axis trends through southern turkey in agreement with other studies of minoan tephra6 7 the tephra deposits should provide a useful reference horizon for assessing the chronology of black sea sediments which has been much debated8 15

internal gravity waves in a saturated moist neutral atmosphere abstractthis work is motivated by an unusual feature associated with the start up of a moist nearly neutral atmospheric flow over a mountain ridge that was previously observed in a full physics numerical model in that study the upstream propagation of a wave of subsidence precluded the establishment of upward displaced and saturated flow that might be expected upstream of the topography this phenomenon was hypothesized to be a consequence of the peculiar property of saturated moist neutral flow an upward air parcel displacement produces zero buoyancy while a downward displacement desaturates the air parcel and produces a positive buoyancy anomaly in the present study this hypothesis is confirmed within numerical solutions to a reduced system of equations that incorporates the saturated atmosphere property in a particularly simple manner the relatively uncomplicated nature of these solutions motivates the numerical solution of a further simplified initial value problem for both nonhydrostatic and h

ordovician k bentonites in the upper plate active margin of western gondwana famatina ranges stratigraphic and palaeogeographic significance abstract numerous thin bedded tabular pale yellowish clay bands are interlayered with black shales in a biostratigraphically constrained early ordovician volcano sedimentary succession at famatina western argentina this region was part of a fairly continuous upper plate convergent volcanic chain that fringed western gondwana mineralogy on both clay and non clay fractions whole rock chemistry and field observations on these distinctive event beds indicate that they originated as relatively coarse fallout tephras altered first into bentonites and later through burial metamorphism into k bentonites metabentonites these tephras were deposited as single crystals and glassy dust or pumiceous fragments in a restricted subtidal environment the region of famatina has previously been inferred as the source of abundant distal k bentonites recorded in the adjacent lower plate allochthonous precordillera terrane however these k bentonites within the proximal arc site were unknown and rather unexpected since they are generally better preserved like distal deposits associated either with central vent plinian ultraplinian eruptions or with accompanying co ignimbrite ash clouds their chemistry and comparison with those k bentonites in the precordillera allow tracing an evolution from volcanic arcs into continental crust k bentonites described in this paper are much older than those recorded in the adjacent precordillera terrane and seem to be associated with a first eruptive period along western gondwana that has no counterpart in the argentine precordillera suggesting a significant longitudinal separation between these two regions by the early ordovician

coupled conduit and atmospheric dispersal dynamics of the ad 79 plinian eruption of vesuvius abstract the ad 79 eruption of vesuvius is certainly one of the most investigated explosive eruptions in the world this makes it particularly suitable for the application of numerical models since we can be quite confident about input data and the model predictions can be compared with field based reconstruction of the eruption dynamics magma ascent along the volcanic conduit and the dispersal of pyroclasts in the atmosphere were simulated the conduit and atmospheric domain were coupled through the flow conditions computed at the conduit exit we simulated two different peak phases of the eruption which correspond to the emplacement of the white and gray magma types that produced plinian fallout deposits with interlayered pyroclastic flow units during the gray phase the input data independently constrained and representative of each of the two eruptive phases consist of liquid magma composition crystal and water content mass flow rate and pressure temperature depth of the magma at the conduit entrance a parametric study was performed on the less constrained variables such as microlite content of magma pressure at the conduit entrance and particle size representative of the eruptive mixture numerical results are substantially consistent with the reconstructed eruptive dynamics in particular the white eruption phase is found to lead to a fully buoyant eruption plume in all cases investigated whereas the gray phase shows a more transitional character i e the simultaneous production of a buoyant convective plume and pyroclastic surges with a significant influence of the microlite content of magma in determining the partition of pyroclast mass between convective plumes and pyroclastic flows

two new end members of pyroclastic density currents forced convection dominated and inertia dominated abstract a great debate on the flow density of pyroclastic density currents pdcs in explosive eruptions has pervaded volcanology over the last decades dade and huppert 1996 dade and huppert 1996 dade and huppert 1997 wilson 1997 pdcs consist of gas particle flows that move for kilometers down the volcano and over the surrounding terrain they are generated because of vertical collapse of an eruptive column woods 1988 valentine and wohletz 1989 or explosion of a lava dome overpressure jet blast esposti ongaro et al 2008 2011 and they flow downstream as radial or unidirectional currents respectively radial currents are also generated because of collapse of an overpressurized mushroom like expanded column dellino et al 2010a pyroclastic fountaining woods and wohletz 1991 clarke et al 2002 or in ignimbrite forming eruptions fisher et al 1980 cas et al 2011 in this paper a new subdivision of pdcs forced convection dominated and inertia dominated is proposed based on flow property a simple pdc simulation and field evidences reveal that forced convection dominated flows may transport own density if they are continuously fed at source favored by steep volcano or channelized inertia dominated flows may leak the density floating over topography

inner core vacillation cycles during the intensification of hurricane katrina a simulation of hurricane katrina 2005 using the australian bureau of meteorology s operational model for tropical cyclone prediction tclaps shows that the simulated vortex vacillates between almost symmetric and highly asymmetric phases during the symmetric phase the eyewall comprises elongated convective bands and both the low level potential vorticity pv and pseudo equivalent potential temperature e fields exhibit a ring structure with the maximum at some radius from the vortex centre during this phase the mean flow intensifies comparatively rapidly as the maximum acceleration of the mean tangential wind occurs near the radius of maximum mean tangential wind rmw in contrast during the asymmetric phase the eyewall is more polygonal with vortical hot towers vhts located at the vertices the low level pv and e fields have monopole structures with the maximum at the centre the intensification rate is lower than during the symmetric phase because the mean tangential wind accelerates most rapidly well within the rmw the symmetric to asymmetric transition is accompanied by the development of vhts within the eyewall the vhts are shown to be initiated by barotropic convective instability associat ed with the ring like structure of pv in the eyewall where the convective instability is large during the reverse asymmetric tosymmetric transition the vhts weaken as the local vertical wind shear increases and the convective available potential energ yi s consumed by convection the weakened vhts move outwards similar to vortex rossby waves and are stretched by the angular shear of the mean vortex simultaneously the rapid filamentation zone outside the rmw weakens becoming more favourable for the development of convection the next symmetric phase emerges as the convection reorganizes into a more symmetric eyewall it is proposed that vacillation cycles occur in young tropical cyclones and are distinct from the eyewall replacement cycles that tend to occur in strong and mature tropical cyclones copyright c 2011 royal meteorological society

sedimentology of the krakatau 1883 submarine pyroclastic deposits the majority of tephra generated during the paroxysmal 1883 eruption of krakatau volcano indonesia was deposited in the sea within a 15 km radius of the caldera two syneruptive pyroclastic facies have been recovered in scuba cores which sampled the 1883 subaqueous pyroclastic deposit the most commonly recovered facies is a massive textured poorly sorted mixture of pumice and lithic lapilli to block sized fragments set in a silty to sandy ash matrix this facies is indistinguishable from the 1883 subaerial pyroclastic flow deposits preserved on the krakatau islands on the basis of grain size and component abundances a less common facies consists of well sorted planarlaminated to low angle cross bedded vitric enriched silty ash entrance of subaerial pyroclastic flows into the sea resulted in subaqueous deposition of the massive facies primarily by deceleration and sinking of highly concentrated deflated components of pyroclastic flows as they traveled over water the basal component of the deposit suggests no mixing with seawater as inferred from retention of the fine ash fraction high temperature of emplacement and lack of traction structures and no significant hydraulic sorting of components the laminated facies was most likely deposited from low concentration pyroclastic density currents generated by shear along the boundary between the submarine pyroclastic flows and seawater the krakatau deposits are the first well documented example of true submarine pyroclastic flow deposition from a modern eruption and thus constitute an important analog for the interpretation of ancient sequences where subaqueous deposition has been inferred based on the facies characteristics of encapsulating sedimentary sequences

limited global change due to the largest known quaternary eruption toba 74 kyr bp abstract the 74 kyr bp super eruption of toba volcano in sumatra is the largest known quaternary eruption on the basis of preserved deposits the eruption magnitude has been estimated at 7 10 15 kg 2800 km 3 of dense magma the largest sulphate anomaly in the greenland ice sheet project 2 core has been identified as fallout from toba s stratospheric aerosol veil correlation of the sulphate and oxygen isotope stratigraphy of the ice core suggests that the toba eruption might have played a role in triggering a millennium of cool climate prior to dansgaard oeschger event 19 although a comparable stadial preceded event 20 a possible 6 yr duration volcanic winter immediately following the eruption has also been proposed as the cause of a putative bottleneck in human population supporting in a general way the garden of eden model for the origin of modern humans however along with counter arguments regarding the timing of any demographic crash there remain major gaps in our understanding of the 74 kyr bp toba eruption that hinder attempts to model its global atmospheric and climatic and hence human consequences the tephra record reveals basic aspects of the eruption style but calculations of the duration and hence intensity and plume height of the event are poorly constrained furthermore estimates of the sulphur yield of the erupting magma central to predictions of its atmospheric and climatic impacts vary by two orders of magnitude 3 5 330 10 10 kg previous estimates of globally averaged surface cooling of 3 5 c after the eruption are probably too high a figure closer to 1 c appears more realistic the volcanological uncertainties need to be appreciated before accepting arguments for catastrophic consequences of the toba super eruption

relating the physical properties of volcanic rocks to the characteristics of ash generated by experimental abrasion interactions between clasts in pyroclastic density currents pdcs generate volcanic ash that can be dispersed to the atmosphere in co pdc plumes and due to its small size is far travelled we designed a series of experiments to determine the effects of pyroclast vesicularity and crystal content on the efficiency and type of ash generated by abrasion two different pyroclastic materials were used 1 basaltic andesite pyroclasts from fuego volcano guatemala with 26 46 vesicularity and high groundmass crystallinity and 2 tephri phonolite avellino pumice vesuvius italy with 55 75 vesicularity and low groundmass crystallinity when milled both clast types produced bimodal grain size distributions with fine ash modes between 4 and 5 32 63 m although the vesicular avellino pumice typically generated more ash than the denser fuego pyroclasts the ash generating potential of a single pyroclast was independent of density and instead governed by heterogeneous crystal and vesicle textures one consequence of these heterogeneities was to cause the vesicular avellino clasts to split in addition to abrading which further enhanced abrasion efficiency the matrix characteristics also affected ash shape and componentry which will influence the elutriation and transport properties of ash in the atmosphere the experimental abrasion successfully replicated some of the characteristics of natural co pdc ash samples as shown by similarities in the adherence factor which measures the proportion of attached matrix on phenocrysts of both the experimentally generated ash and natural co pdc ash samples our results support previous studies which have shown that abrasion is an effective mechanism for generating fine ash that is similar in size 5 30 m to that found in co pdc deposits we further show that both the abundance and nature shape density components size distribution of those ash particles are strongly controlled by the matrix properties of the abraded pyroclasts

new insights into source and dispersal of mediterranean s1 tephra an early holocene marker horizon erupted at mt erciyes turkey abstract deposition of early holocene eastern mediterranean s1 tephra and a black sea cryptotephra coincides with cultural transitions in the fertile crescent termed the neolithic revolution as well as sapropel formation during climate variability of the african humid period classifying them as paramount regional marker horizons for archaeology as well as paleoclimatology their correlations with specific eruptions of the mt erciyes stratovolcanic complex central anatolia remained inconclusive though here we use zircon double dating by u th he and u th disequilibrium methods major and trace element tephra glass geochemistry and probabilistic modeling of tephra dispersal in an attempt to characterize all major late quaternary proximal tephras of mt erciyes and to correlate them with distal deposits furthermore we discuss contrasting proximal and distal tephra dispersal three nearly coeval rhyolitic satellite domes dikkartin perikartin and karagullu erupted at mt erciyes in the early holocene and their dome extrusions were all preceded by explosive phases producing pyroclastic material that formed tephra fall and pyroclastic flow deposits the new eruption age of 9 03 0 55 ka 1 uncertainty here and elsewhere for proximal dikkartin pumice is consistent with 14c based s1 tephra chronologies in distal locations averaging 8 92 0 03 cal ka bp perikartin pyroclastic flow deposits predate s1 tephra by ca 0 8 ka according to a pair of published 14c ages and stratigraphically overlie karagullu fall out here dated to 8 2 1 8 ka previously undated proximal tephras of mt erciyes erupted in the late 85 2 4 9 ka and middle pleistocene 154 5 5 3 ka s1 tephra glass is chemically similar to that of dikkartin fall out but also indistinguishable from that of perikartin fall out karagullu pumice is characterized by a distinct glass chemical composition which correlates with that of unnamed cryptotephra reported for the southeastern black sea instead where these results call for a re evaluation of existing age models maximum lithic clast size isopleths for proximal dikkartin fall out indicate eastward dispersal of a 20 5 km high eruption plume by stratospheric winds in agreement with results of probabilistic tephra dispersal modeling this azimuth contrasts with the known distribution of s1 tephra at distal locations that are all south of mt erciyes significant tephra occurrences at up to 1300 km distance and orthogonal to prevalent stratospheric wind directions either result from very atypical wind conditions probability 10 or are caused by tephra transport by prevailing low altitude winds two scenarios are proposed for low altitude transport eolian reworking of primary fall out more likely from the more widespread dikkartin deposits or co ignimbrite ash cloud dispersal more likely from the perikartin eruption which predominantly produced pyroclastic flows because s1 tephra is chemically indistinguishable from both dikkartin and perikartin by major and trace element glass compositions its exact source and dispersal mechanism remain ambiguous although existing 14c ages for perikartin predating those for s1 tephra favor dikkartin as its source

simulation of the downshear reformation of a tropical cyclone abstractthe downshear reformation of tropical storm gabrielle 2001 was simulated at 1 km horizontal resolution using the weather research and forecasting wrf model the environmental shear tilted the initial parent vortex downshear left and forced azimuthal wavenumber 1 kinematic thermodynamic and convective asymmetries the combination of surface enthalpy fluxes and a lack of penetrative downdrafts right of shear allowed boundary layer moist entropy to increase to a maximum downshear right this contributed to convective instability that fueled the downshear convection within this convection an intense mesovortex rapidly developed with maximum boundary layer relative vorticity reaching 2 2 10 2 s 1 extreme vortex stretching played a key role in the boundary layer spinup of the mesovortex cyclonic vorticity remained maximized in the boundary layer and intensified upward with the growth of the convective plume the circulation associated with the mesovortex and adjacent localized cyclonic vorti

thermal disequilibrium at the top of volcanic clouds and its effect on estimates of the column height satellite images of large volcanic explosions reveal that the tops of volcanic eruption columns are much colder than the surrounding atmosphere1 we propose that this effect occurs whenever a mixture of hot volcanic ash and entrained air ascends sufficiently high into a stably stratified atmosphere although the mixture is initially very hot it expands and cools as the ambient pressure decreases we show that cloud top undercoolings in excess of 20 c may develop in clouds that penetrate the stratosphere this is consistent with observations of the 4 april 1982 eruption of el chichon1 and the 18 may 1980 eruption of mount st helens2 furthermore from our model results we predict that for a given cloud top temperature variations in the initial temperature of 100 200 c may correspond to variations in the column height of 5 10 km we deduce that the present practice of converting satellite based measurements of the temperature at the top of volcanic eruption columns to estimates of the column height will produce rather inaccurate results and should therefore be discontinued

volcanic winter and accelerated glaciation following the toba super eruption the eruption of toba in sumatra 73 500 years ago was the largest known explosive volcanic event in the late quaternary1 it could have lofted about 1015 g each of fine ash and sulphur gases to heights of 27 37 km creating dense stratospheric dust and aerosol clouds here we present model calculations that investigate the possible climatic effects of the volcanic cloud the increase in atmospheric opacity might have produced a volcanic winter 2 a brief pronounced regional and perhaps hemispheric cooling caused by the volcanic dust followed by a few years with maximum estimated annual hemispheric surface temperature decreases of 3 5 c the eruption occurred during the stage 5a 4 transition of the oxygen isotope record a time of rapid ice growth and falling sea level3 we suggest that the toba eruption may have greatly accelerated the shift to glacial conditions that was already underway by inducing perennial snow cover and increased sea ice extent at sensitive northern latitudes as the onset of climate change may have helped to trigger the eruption itself4 we propose that the toba event may exemplify a more general climate volcano feedback mechanism

the initial giant umbrella cloud of the may 18th 1980 explosive eruption of mount st helens abstract the initial eruption column of may 18th 1980 reached nearly 30 km altitude and released 10 17 joules of thermal energy into the atmosphere in only a few minutes ascent of the cloud resulted in forced intrusion of a giant umbrella shaped cloud between altitudes of 10 and 20 km at radial horizontal velocities initially in excess of 50 m s the mushroom cloud expanded 15 km upwind forming a stagnation point where the radial expansion velocity and wind velocity were equal the cloud was initiated when the pyroclastic blast flow became buoyant the flow reduced its density as it moved away from the volcano by decompression by sedimentation and by mixing with and heating the surrounding air observations indicate that much of the flow covering an area of 600 km 2 became buoyant within 1 5 minutes and abruptly ascended to form the giant cloud calculations are presented for the amount of air that must have been entrained into the flow to make it buoyant assuming an initial temperature of 450 c and a magmatic origin for the explosion these calculations indicate that the flow became buoyant when its temperature was approximately 150 c and the flow consisted of a mixture of 3 25 10 11 kg of pyroclasts and 5 0 10 11 kg of air if sedimentation is considered these figures reduce to 1 1 10 11 kg of pyroclasts and 1 0 10 11 kg of air

insights into the formation and dynamics of coignimbrite plumes from one dimensional models coignimbrite plumes provide a common and effective mechanism by which large volumes of fine grained ash are injected into the atmosphere nevertheless controls on formation of these plumes as a function of eruptive conditions are still poorly constrained herein two 1 d axysymmetric steady state models were coupled the first describing the parent pyroclastic density current and the second describing plume rise global sensitivity analysis is applied to investigate controls on coignimbrite plume formation and describe coignimbrite source and the maximum plume height attained for a range of initial mass flow rates between 108 and 1010 kg s modeled liftoff distance the distance at which neutral buoyancy is attained assuming radial supercritical flow is controlled by the initial flow radius gas mass fraction flow thickness and temperature the predicted decrease in median grain size between flow initiation and plume liftoff is negligible calculated initial plume vertical velocities assuming uniform liftoff velocity over the pyroclastic density current invasion area are much greater several tens of m s than those previously used in modeling coignimbrite plumes 1 m s such velocities are inconsistent with the fine grain size of particles lofted into coignimbrite plumes highlighting an unavailability of large clasts possibly due to particle segregation within the flow prior to plume formation source radius and initial vertical velocity have the largest effect on maximum plume height closely followed by initial temperature modeled plume heights are between 25 and 47 km comparable with plinian eruption columns highlighting the potential of such events for distributing fine grained ash over significant areas

rapid intensification of a sheared tropical storm abstract a weak tropical storm gabrielle in 2001 experienced a 22 hpa pressure fall in less than 3 h in the presence of 13 m s 1 ambient vertical wind shear a convective cell developed downshear left of the center and moved cyclonically and inward to the 17 km radius during the period of rapid intensification this cell had one of the most intense 85 ghz scattering signatures ever observed by the tropical rainfall measuring mission trmm the cell developed at the downwind end of a band in the storm core maximum vorticity in the cell exceeded 2 5 10 2 s 1 the cell structure broadly resembled that of a vortical hot tower rather than a supercell at the time of minimum central pressure the storm consisted of a strong vortex adjacent to the cell with a radius of maximum winds of about 10 km that exhibited almost no tilt in the vertical this was surrounded by a broader vortex that tilted approximately left of the ambient shear vector in a similar direction as the broad precipitation shield this st

the nature origins and distribution of ash aggregates in a large scale wet eruption deposit oruanui new zealand abstract this study documents the processes and products of volcanic ash aggregation in phreatomagmatic phases of the 25 4 ka oruanui supereruption from taupo volcano new zealand detailed textural and stratigraphic relationships of aggregates are examined in six of the ten erupted units which range from relatively dry styles of eruption and deposition units 2 5 to mixed units 6 7 8 and dominantly wet unit 3 aggregate structures and grain size distributions shift abruptly over vertical scales of cm to dm providing diagnostic features to identify deposits emplaced primarily as vertical fallout or pyroclastic density currents pdcs the six categories of ash aggregates documented here are used to infer distinct volcanic and meteorological interactions in the eruption cloud related to dispersal characteristics and mode of emplacement our field observations support the notion of brown et al 2010 origin of accretionary lapilli within ground hugging density currents evidence from pyroclastic couplets on tenerife geol soc am bull 122 305 320 that deposits bearing matrix supported accretionary lapilli with concentric internal structure and abundant rim fragments are associated with emplacement of pdcs however on the basis of grain size distributions and field relationships it is inferred that these types of ash aggregates formed their ultrafine ash dominantly 100 km from vent it is inferred that ambient meteorological conditions play a more important role in ash fallout in these cases entrainment of moist air and distal subsidence and melting of ice carried by the plume are both likely to have contributed to the observed features of late stage aggregation in the drier phases of eruption these observations suggest that proximal column influenced aggregation processes which weaken with distance from source are overprinted by secondary later stage aggregation mechanisms farther downwind

the 1982 eruptions of el chichon volcano mexico 2 observations and numerical modelling of tephra fall distribution tephra fallout from the a 1 march 29 0532 ut b april 4 0135 ut and c april 4 1122 ut 1982 explosive eruptions of el chichon produced three tephra fall deposits over southeastern mexico bidirectional spreading of eruption plumes as documented by satellite images was due to a combination of tropospheric and stratospheric transport with heaviest deposition of tephra from the ene tropospheric lobes maximum column heights for the eruptions of 27 32 and 29 km respectively have been determined by comparing maximum lithic clast dispersal in the deposits with predicted lithic isopleths based on a theoretical model of pyroclast fallout from eruption columns these column heights suggest peak mass eruption rates of 1 1 108 1 9 108 and 1 3 108 kg s maximum column heights and mass eruption rates occured early in each event based on the normal size grading of the fall deposits sequential satellite images of plume transport and the production of a large stratospheric aerosol plume indicate that the eruption columns were sustained at stratospheric altitudes for a significant portion of their duration new estimates of tephra fall volume based on integration of isopach area and thickness yield a total volume of 2 19 km3 1 09 km3 dre dense rock equivalent or roughly twice the amount of the deposit mapped on the ground up to one half of the erupted mass was therefore deposited elsewhere as highly dispersed tephra

ascent dynamics of large phreatomagmatic eruption clouds the role of microphysics 1 we examine the impact of abundant surface water interaction on the development of volcanic clouds from large scale 108 kg s 1 magma phreatomagmatic eruptions presenting the first 2 d large eddy simulations of wet volcanic plumes that incorporate the effects of microphysics the active tracer high resolution atmospheric model was forced with field derived inputs from an exceptionally large phreatomagmatic eruption the 27 ka oruanui supereruption taupo volcano new zealand surface water contents were varied from 0 to 40 wt for eruptions with equivalent magma eruption rates of 1 3 108 and 1 1 109 kg s 1 our findings confirm that increased surface water has a pronounced impact on column stability leading to transitional column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents pdcs contrary to the suggestion of previous workers however abundant surface water does not systematically lower the spreading level or maximum height of volcanic clouds owing to vigorous microphysics assisted lofting of pdcs the simulated behavior and ash cloud dimensions provide a close match to field evidence from the oruanui case study cloud heights from the collapsing eruption columns also show a notable sensitivity to changes in the altitude of the tropopause while ambient humidity primarily impacts the abundance of airborne hydrometeors particularly ice associated with the volcanic clouds general relationships between eruption style meteorological conditions and the resulting vertical profiles of volcanic emissions outlined here could also be adapted for use in operational volcanic ash forecasting and deposit reconstruction techniques already in existence

transport of so2 by explosive volcanism on venus observations by the pioneer venus orbiter and many different types of analyses have suggested the possibility of contemporary explosive volcanism on venus the rise of volcanic eruption plumes on venus is reexamined using recent improvements in buoyant plume modeling the first order model applied to venus by previous authors features nonphysical discontinuous solutions for all of the model parameters and lacks internal consistency in the formulation of the governing equations this makes it difficult to assess the validity of the venus applications and conclusions derived from these models the model used here contains several improvements including two corrections to the formulation and a change in the criterion for the transition of the plume from the jet region to the buoyancy driven region the model used in earlier works assumed a discontinuous transition between these two regions resulting in an overestimate of the transition height as well as the maximum plume height the effect of the transition criterion is magnified on venus where the continuous solution appears to have very little dependence on initial vent size in contrast the discontinuous solution shows a very strong dependence on initial vent size the continuous solution used here indicates that plumes on venus become dominated by buoyancy effects almost immediately above the vent use of the discontinuous solution however suggests that jets up to 10 km above the vent are possible for the boundary conditions considered the combined effect of using the older model for conditions on venus is a 5 8 overestimate of the maximum plume height for vent radii ranging from 20 to 250 m the influence of latitude and elevation are also explored for large eruptions on venus plumes rising in the northern highlands would rise much higher than plumes with identical boundary conditions erupted in the equatorial lowlands this is due to the greater stability of the upper atmosphere at higher latitudes and the sharp decrease in atmospheric pressure as a function of altitude to examine the net effect of all the model assumptions and ambient influences eruptions are simulated for a range of conditions at maat mons and compared with results in the literature these simulations indicate that for small mass fluxes the new model predicts smaller plumes than the older model for larger mass fluxes however the new model predicts larger plumes than the older model because the maat mons summit elevation is already more than 9 km above the mean planetary radius the reduced atmospheric pressure results in a plume with enough buoyancy to more than compensate for all of the model effects these results continue to support the possibility that explosive eruptions on venus may be capable of producing plumes that rise buoyantly to heights detected by the pioneer venus orbiter

large scale environmental parameters associated with tropical cyclone formations in the western north pacific abstract the local environmental conditions associated with 405 tropical cyclone tc formations in the western north pacific during 1990 2001 are examined in this study six large scale parameters are obtained and computed from the ncep reanalyses with 2 5 latitude resolution these include the sea surface temperature 200 850 hpa vertical and meridional zonal shear 500 700 hpa average relative humidity convective available potential energy and 200 hpa divergence it is found that these parameters show quite consistent values in almost all the cases examined and near gaussian distributions are found for them the mean values of these distributions are 28 9 c 3 2 m s 1 2 3 m s 1 61 8 1184 1 j kg 1 and 5 2 10 6 s 1 for the above six domain averaged parameters respectively based on these distributions of values a formation potential area is defined as a location that possesses these environmental parameters all within the thresholds one standard deviation from the mean value found above

experimental studies of particle laden plumes experiments are described in which buoyant plumes of fresh water and solid particles are generated in a tank of salt water dependence of the behavior of the plumes on particle concentration and grain size was studied at low concentrations behavior was similar to single component plumes and the principal effect of the particles was to reduce the buoyancy flux measurements of particle concentration across the plumes demonstrated that there were substantial amounts of particle reentrainment from an outer sedimenting veil reentrainment caused the plume margins to become denser than the ambient fluid total particle flux in the plumes increased with height and can be several times greater than the flux at the plume source plumes with high particle concentration exhibit several kinds of convective instability which generated gravity currents and rapid sedimentation gravity currents were generated at the margins of concentrated plumes these flows are attributed to reentrainment causing the flow margins to have excess density when the plume density was close to but less than the density of the ambient fluid a fountain forms high in the plumes which collapsed asymmetrically to generate a gravity current on one side of the plume when the density of the particle laden flow was greater than the ambient fluid a symmetrical fountain formed and gravity currents moved across the tank floor depositing sediment at a well defined distance from the source buoyant fluid ascended from the flow fronts to form a ring of plumes around the central fountain these plumes coalesced to form a broad buoyant cloud the plumes are generated by the sedimentation from the gravity currents which resulted in the fresh water and some entrained particles being released all the experimental plumes ascended to the top of the tank and spread out as gravity currents beneath the upper surface particles often fall from this layer in discrete parcels of particle rich fluid with shapes similar to thermals the fall velocities were much greater than the terminal velocities of individual particles and the sedimenting region is inhomogeneous these observations indicate that sedimentation is enhanced by convective instabilities when particle rich fluid is emplaced above particle free fluid the experiments indicate that there may be several different mechanisms of generating pyroclastic flows and surges from the collapse of volcanic eruptions columns the experiments also demonstrate that the sedimentation of particles from a plume has an important influence on dynamical behavior

toba ash on the indian subcontinent and its implications for correlation of late pleistocene alluvium the toba ash occurs extensively in the indian subcontinent and marks a ca 74 000 yr old event in the bay of bengal and indian ocean it is about 10 cm thick whereas in several alluvial basins it is usually 1 3 m thick the latter occurs in a partly reworked state but as nearly chemically pure first cycle sediments the ash has a broad northwesterly dispersal pattern samples of ash from the indian subcontinent compare closely with the youngest 74 000 yr b p toba tuff and the deep sea toba ash in bulk chemical composition ree signature and bubble wall shard morphology however a more proximally located and thicker 2 5 m ash bed from the alluvial basins in the gneissic area and close to east coast has a lower magnitude negative eu anomaly possibly because of minor contamination by feldspathic silt quaternary sediments in the central narmada and middle son basins contain rich late and middle pleistocene mammalian and cultural records based on the presence of the ash layer marker and stratigraphic relations late pleistocene sediments within the subcontinent can be correlated with those from central india and the deep sea

the 73 ka toba super eruption and its impact history of a debate abstract toba volcano in northern sumatra is located at the intersection of two major tectonic lineaments in one of the most seismically active regions in the world and its caldera is considered the largest quaternary caldera on earth the most recent toba explosive eruption at 73 ka was an order of magnitude larger than that of tambora in 1815 and has a volcanic explosivity index of 8 the exceptional magnitude of this super eruption and the widespread distribution of the younger toba tuff ytt in marine cores in the indian ocean the arabian sea and the south china sea have prompted sustained debate as to its global and regional impact on climate ecosystems and prehistoric human populations however despite climate modelling and appeals to a variety of proxy climatic data including geochemistry pollen analysis archaeology and stable isotopes the extent of the impact remains unclear in order to achieve better progress in the future greater care is needed to obtain higher resolution and more precise chronologies from the marine and terrestrial proxy records used to test whether or not the 73 ka toba super eruption had a significant environmental impact records from speleothems and ice cores have much to offer in this regard