a scale discriminating vorticity budget for a mesoscale vortex in a midlatitude continental mesoscale convective system abstract the authors employ data from the noaa wind profiler network to present a scale discriminating vorticity budget for a mesoscale convective vortex mcv that was generated by a mesoscale convective system mcs in oklahoma and kansas on 1 august 1996 a spatial bandpass filter was used to divide observed wind into mesoscale and synoptic components then the authors sought sources and sinks of vorticity in those two components over 9 h of the mcv s lifetime the vorticity budget reveals that both the mesoscale and synoptic winds supplied significant vorticity to the mcv the vortex s origin could not be proved but data weakly suggest that tilting may have been mostly responsible convergence of absolute vorticity by the mesoscale wind was the reason the mcv grew deeper and stronger as the mcs weakened finally tilting of synoptic and mesoscale vorticity by gradients in mesoscale vertical motion was responsible for a secondary deepening of the mcv as the mcs dissipated the budget suggests that if

patterns of precipitation and mesolow evolution in midlatitude mesoscale convective vortices abstract surface pressure manifestations of mesoscale convective vortices mcvs that traversed oklahoma during the periods may august 2002 05 are studied using the weather surveillance radar 1988 doppler wsr 88d the oklahoma mesonet and the noaa profiler network data forty five mcvs that developed from mesoscale convective systems mcss have been investigated 28 62 of which exhibit mesolows detectable at the surface within this group three distinct patterns of precipitation organization and associated mesolow evolution have been identified the remaining 17 38 of the cases do not contain a surface mesolow two repeating patterns of precipitation organization are identified for the latter group the three categories of mcvs possessing a surface mesolow are as follows nineteen are classified as rear inflow jet mcvs and tend to form within large and intense asymmetric mcss rear inflow into the mcs enhanced by the development of an mcv on the left hand side relative to system motion pro

geostrophic and ageostrophic circulations in midlatitude squall lines the eliassen transverse circulation equation is derived from the set of f plane semigeostrophic slab symmetric equations with the inclusion of both diabatic heating and ambient shear effects numerical integrations of this model in geostrophic coordinates are performed in the context of a typical midlatitude squall line characterized by a leading convective line and a trailing stratiform rain region some dynamical insights are provided by these integrations in conjunction with the theoretical framework which seem to be unavailable by primitive equation model simulations a more complete dynamical picture of a squall line circulation system is revealed from this study a balanced midlevel horizontal vortex coupled with an ageostrophic transverse circulation in the normalline plane this configuration suggests an interaction between two distinct dynamical scales that is the scales with geostrophic and ageostrophic like motions both of which play important roles in mesoscale convective systems the internal and external physical processes that affect the ageostrophic transverse circulation are further discussed

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

a three dimensional eulerian model for transport and deposition of volcanic ashes abstract volcanic ash fallout represents a serious threat to people living near active volcanoes because it can produce several undesirable effects such as collapse of roofs by ash loading respiratory sickness air traffic disruption or damage to agriculture the assessment of such volcanic risk is therefore an issue of vital importance for public safety and its mitigation often requires to evaluate the temporal evolution of the phenomenon through reliable computational models we develop an eulerian model named fall3d for the transport and deposition of volcanic ashes the model is based on the advection diffusion sedimentation equation with a turbulent diffusion given by the gradient transport theory a wind field obtained from a meteorological limited area model lam and the source term derived from by buoyant plume theory it can be used to forecast either ash concentration in the atmosphere or ash loading on the ground model inputs are topography meteorological data given by a lam mass eruption rate and a particle settling velocity distribution a test application to the july 2001 etna eruption is presented

modeling tephra sedimentation from a ruapehu weak plume eruption 1 we present a two dimensional model for sedimentation of well mixed weak plumes accounting for lateral spreading of the cloud downwind advection increase of volumetric flux in the rising stage and particle transport during fallout the 17 june 1996 subplinian eruption of ruapehu produced a bent over plume that rose to a height of 8 5 km in a wind field with an average velocity of 24 m s 1 and generated a narrow deposit on land extending up to 200 km from vent the sedimentation from the ruapehu plume was dominated by coarse ash with all the blocks and most of the lapilli falling while the plume was still rising particles with diameter 125 m show three accumulation maxima one coincident with a secondary maximum in the total tephra deposit about 150 km from the vent numerical modeling shows that the plume started spreading horizontally beyond about 30 km from vent investigations also show that fickian diffusion can be a good approximation to the crosswind spreading of both vigorous and low energy weak plumes but a wide range of diffusion coefficients is to be expected with the largest values necessary to describe the most vigorous weak plumes such as ruapehu one dimensional and two dimensional simulations show that tephra sedimentation must have been subvertical this feature also supported by direct observations can be the result of the turbulence structure of the local wind field and convective instabilities which could also have played an important role in the generation of the pronounced sinuosity shown by the ruapehu deposit

detection and prediction of warm season midtropospheric vortices by the rapid update cycle the authors perform a statistical and dynamical analysis of midtropospheric mesoscale vortices captured by analyses from the rapid update cycle version 2 ruc 2 during the period 1 may to 31 august 1999 a total of 203 vortices meeting conditions of an automated algorithm were found of these 86 were observed to form within organized convection and were termed mesoscale convective vortices mcvs mcvs were favored over a broad area from eastern colorado and western nebraska to the mississippi river valley essentially collocated with the loci of organized convection the remaining 117 vortices termed dry vortices clustered in the immediate lee of the rocky mountains and over the southeastern united states vortices arising from convection had considerably greater intensity and longevity than dry vortices they were roughly five times more likely to be involved with the triggering of new convection a relationship was found between intensity and longevity such that there appears to be a maximum vortex lifetime that can be estimated from its maximum intensity vortices arising from convection had markedly greater humidity and water vapor mixing ratio underneath their centers compared to dry vortices consistent with many dry vortices having a topographic origin and mcvs arising from organized convection parameters such as horizontal scale background vertical wind shear and horizontal deformation were not systematically related to intensity or longevity prediction of mesoscale vortices by the ruc 2 was examined for a subsample of all cases in general the ruc 2 was able to predict the evolution of vortices once analyzed but had virtually no skill at predicting 12 h in advance the formation of the vortices however forecasts of organized convection should still benefit from accurate predictions of long lived vortex tracks

case study of an intense african easterly wave abstract the life cycle of an intense african easterly wave aew over the african continent is examined using european centre for medium range weather forecasts ecmwf operational analyses meteosat satellite images and synoptic observations this system the strongest aew of 2000 can be tracked from central north africa into the eastern atlantic ocean where it is associated with the genesis of hurricane alberto synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity pv allowing exploration at the role of multiple scales in the evolution of this aew the authors analysis promotes the division of the aew life cycle into three distinctive phases i initiation the aew development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in sudan this convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical north africa ii baroclini

potential vorticity anomalies associated with squall lines abstract this study involves observations and model simulations of potential vorticity anomalies in the wake of midlatitude squall lines using data from the oklahoma kansas pre storm experiment we analyze potential vorticity fields near two squall lines one with and one without a trailing stratiform region from this observational analysis we suggest that squall lines with trailing stratiform regions can leave large positive midtropospheric potential vorticity anomalies in their wake to further interpret these observations we consider a two dimensional version of semigeostrophic theory formulated in isentropic and geostrophic coordinates which results in a simple potential pseudodensity inverse potential vorticity equation using apparent heat source fields that model those computed diagnostically from pre storm data we find that theory does indeed predict large midtropospheric potential vorticity anomalies for model squall lines with a trailing stratiform region but not for model squall lines t

3 d numerical simulations of eruption column collapse effects of vent size on pressure balanced jet plumes abstract buoyant columns or pyroclastic flows form during explosive volcanic eruptions in the transition state these two eruption styles can develop simultaneously we investigated the critical condition that separates the two eruption styles referred to as the column collapse condition by performing a series of three dimensional numerical simulations in the simulation results we identify two types of flow regime a turbulent jet that efficiently entrains ambient air jet type and a fountain with a high concentration of the ejected material fountain type hence there are two types of column collapse jet type and fountain type which type of collapse occurs at the column collapse condition depends on whether the critical mass discharge rate for column collapse mdr cc is larger or smaller than that for the generation of a fountain mdr jf for a given exit velocity temperature controls the relative magnitude of mdr cc relative to mdr jf and hence the type of collapse for given magma properties e g temperature and water content the column collapse condition is expressed by a critical value of the richardson number ri cc g 0 l 0 w 0 2 where g 0 is the source buoyancy l 0 is the vent radius and w 0 is the exit velocity when the jet type collapse occurs at the column collapse condition ri cc is independent of the exit velocity when the fountain type collapse occurs at the column collapse condition on the other hand ri cc decreases as the exit velocity increases as the exit velocity exceeds the sound velocity a robust flow structure with a series of standing shock waves develops in the fountain which suppresses entrainment of ambient air and enhances column collapse

magma discharge variations during the 2011 eruptions of shinmoe dake volcano japan revealed by geodetic and satellite observations we present precise geodetic and satellite observation based estimations of the erupted volume and discharge rate of magma during the 2011 eruptions of kirishima shinmoe dake volcano japan during these events the type and intensity of eruption drastically changed within a week with three major sub plinian eruptions on january 26 and 27 and a continuous lava extrusion from january 29 to 31 in response to each eruptive event borehole type tiltmeters detected deflation of a magma chamber caused by migration of magma to the surface these measurements enabled us to estimate the geodetic volume change in the magma chamber caused by each eruptive event erupted volumes and discharge rates were constrained during lava extrusion using synthetic aperture radar satellite imaging of lava accumulation inside the summit crater combining the geodetic volume change and the volume of lava extrusion enabled the determination of the erupted volume and discharge rate during each sub plinian event these precise estimates provide important information about magma storage conditions in magma chambers and eruption column dynamics and indicate that the shinmoe dake eruptions occurred in a critical state between explosive and effusive eruption

influences of gravity waves on convectively induced turbulence cit a review thunderstorms are known to produce turbulence such turbulence is commonly referred to as convectively induced turbulence or cit and can be hazardous to aviation although this turbulence can occur both within and outside the convection out of cloud cit is particularly hazardous since it occurs in clear air and cannot be seen by eye or onboard radar furthermore due to its small scale and its ties to the underlying convection it is very difficult to forecast guidelines for out of cloud cit avoidance are available but they are oversimplified and can be misleading in the search for more appropriate and physically based avoidance guidelines considerable research has been conducted in recent years on the nature of the phenomenon and in particular its connection to gravity waves generated by the convection this paper reviews the advances in our understanding of out of cloud cit and its relation to convective gravity waves and provides several detailed examples of observed cases to elucidate some of the underlying dynamics

imbalance in a mesoscale vortex within a midlatitude continental mesoscale convective system abstract the authors examine the degree of hydrostatic and gradient balances in a mesoscale convective vortex mcv in the stratiform region of a mesoscale convective system mcs that crossed oklahoma on 1 august 1996 results indicate that the mcv was partially unbalanced because the cool layer at the base of its core was too cool and too shallow to balance the tangential winds about the mcv s axis the apparent imbalance may have been due to strong unsteady forcing on the vortex insufficient or unrepresentative data approximations used in the analysis or reasons that are unknown

volcanic jets plumes and collapsing fountains evidence from large scale experiments with particular emphasis on the entrainment rate the source conditions of volcanic plumes and collapsing fountains are investigated by means of large scale experiments in the experiments gas particle jets issuing from a cylindrical conduit are forced into the atmosphere at different mass flow rates dense jets high particle volumetric concentration e g c 0 0 01 generate collapsing fountains whose height scales with the squared exit velocity this is consistent with bernoulli s equation which is a good approximation if air entrainment is negligible in this case kinetic energy is transformed into potential energy without any significant loss by friction with the atmosphere the dense collapsing fountain on hitting the ground generates an intense shear flow similar to a pyroclastic density current dilute hot jets low particle volumetric concentration e g c 0 3

effect of wind on the rise height of volcanic plumes a theoretical model of a volcanic plume based on applying the equations of motion in a plume centered coordinate system suggests that the interaction between a volcanic plume and wind causes enhanced entrainment of air and horizontal momentum plume bending and a decrease in plume rise height at constant eruption rate because of rapid dilution in the high windspeeds of the polar jet plumes that vary over more than one order of magnitude in mass eruption rate 106 to 107 108 kg s if injected into the polar jet may all attain rise heights only slightly different from that of the core of the jet 10 km as opposed to 17 33 km in a still atmosphere

potential vorticity generation by west african squall lines abstractthe west african summer monsoon features multiple complex interactions between african easterly waves aews moist convection variable land surface properties dust aerosols and the diu

volcanic particle aggregation in explosive eruption columns part ii numerical experiments abstract the goal of this paper is to determine the parameters that control the aggregation efficiency and the growth rate of volcanic particles within the eruption column numerical experiments are performed with the plume model atham active tracer high resolution atmospheric model in this study we employ the parameterizations described in a companion paper this issue the presence of hydrometeors promotes the aggregation of ash particles which strongly increases their fall velocities and thus their environmental impact the tephra mass is about two orders of magnitude greater than that of hydrometeors during typical plinian eruptions without interaction of external water ice is highly dominant in comparison to liquid water 99 by mass this is caused by the fast column rise 100 m s 1 on average to very cold altitudes most particles occur in the form of frozen aggregates with low ice content the collection efficiency is governed by the availability of hydrometeors acting as adhesives at the particles surface in our study and wet ash particles have a higher sticking capacity than icy ones therefore aggregation is fastest during the eruption within the column when limited regions of liquid water exist and when particle concentrations are very high of the order of 10 5 cm 3 increased humidity in the background atmosphere generally leads to enhanced ice formation but shows only a weak influence on the aggregation process first sensitivity studies showed however a significant increase of the liquid water fraction when considering salinity effects the availability of water or ice at the particles surfaces is also governed by the surface properties the porosity and permeability of ash which are not well established to date particle growth is significantly enhanced for greater differences in the sizes and fall velocities among particles as gravitational capture becomes more efficient our experiments indicate a major influence of the erupted particle size distribution first sensitivity studies show that electrostatic forces result in a significant enhancement of aggregated particles the present exploratory study provides new insights into the sensitivity of the ash aggregation process to a number of key parameters our results indicate the need of further constraining particle composition size porosity permeability and surface properties at low temperatures by in situ observations in the laboratory and in the field in addition further research on electrostatic aggregation would be desirable

development and nondevelopment of binary mesoscale vortices into tropical cyclones in idealized numerical experiments abstractthe evolution of two symmetric midlevel mesoscale vortices situated above a warm ocean is examined with a basic cloud resolving model idealized numerical experiments provide insight into how the evolution may vary with the initial vortex separation distance d and other parameters that influence the time scale for an isolated vortex to begin rapid intensification the latter parameters include the ambient middle tropospheric relative humidity rh and the initial midlevel wind speed of each vortex at relatively low rh there exists an interval of d where binary midlevel vortex interaction prevents tropical cyclone formation while tropical cyclones generally develop at high rh similar values of d can delay the process if the vortices are initially weak prevention or inhibition of tropical cyclone formation occurs in association with the outward expulsion of lower tropospheric potential vorticity anomalies as the two vortices merge in the middle troposphere it is proposed that the primary mecha

assessing the applicability of the tropical convective stratiform paradigm in the extratropics using radar divergence profiles abstractlong term radar observations from a subtropical location in southeastern texas are used to examine the impact of storm systems with tropical or extratropical characteristics on the large scale circulation climatological vertical profiles of the horizontal wind divergence are analyzed for four distinct storm classifications cold frontal cf warm frontal wf deep convective upper level disturbance dc uld and nondeep convective upper level disturbances nc uld dc uld systems are characterized by weakly baroclinic or equivalent barotropic environments that are more tropical in nature while the remaining classifications are representative of common midlatitude systems with varying degrees of baroclinicity dc uld systems are shown to have the highest levels of nondivergence lnd and implied diabatic heating maxima near 6 km whereas the remaining baroclinic storm classifications have lnd altitudes that are about 0 5 1 km lower analyses of climatological mean divergence profiles are also s

improving on mass flow rate estimates of volcanic eruptions 1 we introduce a novel analytical expression that allows for fast assessment of mass flow rate of both vertically rising and bent over volcanic plumes as a function of their height while first order physical insight is maintained this relationship is compared with a one dimensional plume model to demonstrate its flexibility and then validated with observations of the 1980 mount st helens and of the 2010 eyjafjallajokull eruptions the influence of wind on the dynamics of volcanic plumes is quantified by a new dimensionless parameter and it is shown how even vertically rising plumes such as the one associated with the mount st helens 1980 eruption can be significantly affected by strong wind comparison between a one dimensional model and the analytical equation gives anr2 value of 0 88 while existing expressions give negativer2 values due to their inability to adapt to different source and atmospheric conditions therefore this new expression has important implications both for current strategies of real time forecasting of ash transport in the atmosphere and for the characterization of explosive eruptions based on the study of tephra deposits in addition this work provides a framework for the application of more complete three dimensional numerical models as it greatly reduces the parameter space that needs to be explored

three dimensional structure and dynamics of african easterly waves part iii genesis this paper promotes the view that african easterly waves aews are triggered by localized forcing most likely associated with latent heating upstream of the region of observed aew growth a primitive equation model is used to show that aews can be triggered by finite amplitude transient and localized latent heating on a zonally varying basic state that is linearly stable heating close to the entrance region of the african easterly jet aej is shown to initiate aews downstream the heating leads to an initial trough that reaches the west african coast about 5 7 days later depending on the nature of the heating profile after this a structure that projects strongly onto the leading linear normal mode of the basic state becomes established characterized by a number of westward propagating disturbances that strongly resemble aews the sensitivity of the forced aews to the nature and location of the heating profile is examined aews are most efficiently triggered by heating profiles that establish lower tropospheric circulations close to the entrance region of the aej in the present study this was best achieved by lower tropospheric heating from shallow convection or upper level heating and lower level cooling from a stratiform precipitation profile both profiles have significant heating gradients in the vertical in the mid to lower troposphere this triggering paradigm for the genesis of aews has consequences for the variability and predictability of aews at weather and climate time scales in addition to the nature of the aej often emphasized it is crucial to consider the nature and variability of upstream heating triggers

effects of wind on entrainment efficiency in volcanic plumes the entrainment of air by turbulent mixing plays a central role in the dynamics of volcanic eruption clouds as the amount of entrained air controls the height of the plume in one dimensional models of volcanic plumes the efficiency of entrainment under a wind field is parameterized using two empirical constants the first is the coefficient associated with the entrainment caused by the shear between the volcanic plume and the ambient air i e the radial entrainment coefficient and the other is associated with the entrainment caused by wind i e the wind entrainment coefficient in this study we used three dimensional numerical simulations of volcanic plumes to determine the effective values of these empirical constants from the relationship between eruption conditions and plume heights these simulations suggest that the value of the radial entrainment coefficient is 0 05 0 06 which is slightly smaller than that of pure jets or plumes seen in laboratory experiments 0 07 0 15 the value of the wind entrainment coefficient was estimated to be 0 1 0 3 which is significantly smaller than those estimated from laboratory experiments 0 3 1 0 the entrainment coefficients derived from these simulations successfully explain the observations made during the 2011 shinmoe dake eruptions in which the volcanic plumes were significantly distorted by the wind

on the rise of turbulent plumes quantitative effects of variable entrainment for submarine hydrothermal vents terrestrial and extra terrestrial explosive volcanism 1 the maximum height reached by a turbulent plume rising in a stratified environment is a key tool to estimate the flux released at its source particularly for large scale flows because flux can often be very hard to measure directly this height is known to be mainly controlled by the stratification of the ambient fluid source buoyancy flux and the efficiency of turbulent mixing between the plume and the external fluid the latter effect has been only superficially explored in spite of its fundamental control on the dynamics here we show that commonly used one dimensional models incorporating a constant entrainment coefficient do not provide satisfying predictions we propose a new model allowing for variable entrainment which gives excellent predictions of maximum heights reached by laboratory plumes in stratified environments we then apply our formalism to natural plumes produced by explosive volcanic eruptions under terrestrial paleo martian and venusian conditions and by submarine hydrothermal activity at mid ocean ridges source mass discharge rates deduced from maximum volcanic column heights for terrestrial eruptions are found to be greater than those estimated with the commonly used constant entrainment parameterization by a factor of 2 in the paleo martian atmosphere existing models overestimate plume heights by 14 27 in the current atmosphere of venus the maximum height reached by a volcanic plume is also found to be smaller than previously estimated for large eruption rates the source heat flux released by the tag field atlantic ocean deduced from several submarine hydrothermal plumes is found greater by a factor 3 with our model

climatology of summer midtropospheric perturbations in the u s northern plains part i influence on northwest flow severe weather outbreaks northwest flow severe weather outbreaks nwf outbreaks describe a type of summer convective storm that occurs in areas of mid level nwf in the central united states convective storms associated with nwf outbreaks often travel a long distance systematically along a northwest southeast oriented track across the northern plains previous studies have observed that these migrating convective storms are frequently coupled with subsynoptic scale midtropospheric perturbations mps initiated over the rocky mountains this study traces mps for the decade of 1997 2006 using the north american regional reanalysis to examine their climatology and possible influence on nwf outbreaks mps are characterized by a well organized divergent circulation with persistent ascending motion at the leading edge promoting convection the divergent circulation is further enhanced by low level convergence along the northern terminus of the great plains low level jet the downstream propagation of mps assists in forming the progressive feature of the associated convective storms mps have a maximum frequency in july consistent with nwf outbreaks in july and august the fully developed north american anticyclone produces prevailing nwf over the northern plains where up to 60 of rainfall and storm reports are linked to mps the movement timing and rainfall distribution of mps remarkably resemble those of nwf outbreaks when encountering strong low level jets ascending motion and convergence of water vapor flux associated with mps intensify considerably and precipitation is greatly enhanced it is likely that nwf outbreaks are generated whenever mps occur in association with strong low level jets

surface cyclogenesis from convectively driven amplification of midlevel mesoscale convective vortices mesoscale convective vortices mcvs are midtropospheric warm core cyclonic circulations that often develop in the stratiform region of mesoscale convective systems typically divergent anticyclonically circulating mesoscale cold anomalies appear both above and below the mcv the upper level cold anomaly is usually found near the tropopause while the low level anomaly is surface based and exhibits locally higher pressure one aspect of mcvs that has received much attention recently is the role that they may play in tropical cyclogenesis of special interest is how an mcv amplifies when deep convection redevelops within the borders of its midlevel cyclonic circulation and how the amplified mcv transforms the divergent surface based cold pool with anomalously high surface pressure into a convergent cyclonic circulation with anomalously low pressure the pennsylvania state university national center for atmospheric research fifth generation mesoscale model is used to simulate an mcv that was instrumental in initiating within the borders of the midlevel vortex s circulation several successive cycles of convective development and decay over a 2 day period after each cycle of convection both the horizontal size of the cyclonic circulation and the magnitude of the potential vorticity associated with the vortex were observed to increase the simulation reproduces the development and evolution of the mcv and associated convective cycles mesoscale features responsible for the initiation of convection within the circulation of the vortex and the impact of this convection on the structure and evolution of the vortex are investigated a conceptual model is presented to explain how convective redevelopment within the mcv causes low level heights to fall and cyclonic vorticity to grow downward to the surface applying this conceptual model to a tropical marine environment is also considered

dynamics of wind affected volcanic plumes the example of the 2011 cordon caulle eruption chile the 2011 cordon caulle eruption represents an ideal case study for the characterization of long lasting plumes that are strongly affected by wind the climactic phase lasted for about 1 day and was classified as subplinian with plumes between 9 and 12 km above the vent and mass flow rate mfr on the order of 107 kg s 1 eruption intensity fluctuated during the first 11 days with mfr values between 106 and 107 kg s 1 this activity was followed by several months of low intensity plumes with mfr 106 kg s 1 plume dynamics and rise were strongly affected by wind during the whole eruption with negligible upwind spreading and sedimentation the plumes that developed on 4 6 and 20 22 june can be described as transitional i e plumes showing transitional behavior between strong and weak dynamics while the wind clearly dominated the rise height on all the other days resulting in the formation of weak plumes individual phases of the eruption range between volcanic explosivity indices veis 3 and 4 while the cumulative deposit related to 4 7 june 2011 is associated with veis 4 and 5 crosswind cloud and deposit dispersal of the first few days are best described by a linear combination of gravitational spreading and turbulent diffusion with velocities between 1 and 10 m s 1 downwind cloud velocity for the same days is best described by a linear combination of gravitational spreading and wind advection with velocities between 17 and 45 m s 1 results show how gravitational spreading can be significant even for subplinian and small moderate eruptions strongly advected by wind and with low richardson number and low mfr

mesoscale observations of the genesis of hurricane dolly 1996 abstract recent numerical studies of tropical cyclone genesis suggest a new paradigm for how the surface vortex is established based on a highly nonaxisymmetric mechanism involving the interaction of low level cyclonic circulations generated by deep cumulonimbus convection a reexamination of mesoscale observations during the genesis of hurricane guillermo 1991 confirms the presence of multiple cyclonic circulations more recently airborne doppler radar wind observations during the genesis of atlantic hurricane dolly 1996 also reveal multiple lower to middle tropospheric mesoscale cyclonic circulations during sequential 15 20 min compositing periods a particularly well organized but initially weak mean tangential wind of 7 m s 1 low level cyclonic vortex embedded within the pre dolly tropical disturbance is observed coincident with deep vertically penetrating cumulonimbus convection the earliest observations of this vortex show the peak circulation near 2 km height with a mean diameter of 30 4

mesoscale circulation growth under conditions of weak inertial instability abstract the hypothesis that inertial instability plays a role in the upscale development of mesoscale convective systems mcss is explored by sampling environments that supported the growth of mcss in the preliminary regional experiment for storm stormscale operational and research meteorology pre storm network with high quality special soundings secondary circulations that occurred in the presence of inertial instabilities were analyzed and documented using rawinsonde data with high spatial and temporal resolution from the pre storm field program additional examples of mcs environments were examined using data from the mesoscale analysis and prediction system results show strong divergence and cross stream accelerations occurred at upper tropospheric levels where inertial instabilities were present these accelerations were not uniform over the domain but were focused in the regions of instability also the analyses of these data showed that regions of inertial instability may be more commonpla

the kinematics of a midlatitude continental mesoscale convective system and its mesoscale vortex the authors present a unique scale discriminating study of the environment relative circulations within a mesoscale convective system mcs and mesoscale convective vortex mcv the mcs a leading convective line and trailing stratiform region that became asymmetric passed through the national oceanic and atmospheric administration noaa profiler network npn in kansas and oklahoma on 1 august 1996 the mcv appeared in the mcs s stratiform region just prior to the system s mature stage and grew to a depth of over 12 km as the mcs dissipated the mcv did not apparently survive to the next day a spatial bandpass filter was used to divide observed wind into a component that was predominantly synoptic background wind and a component that was predominantly a mesoscale perturbation on that background wind a mesoscale updraft mesoscale downdraft and divergent outflows in the lower and upper troposphere were evident after the synoptic background wind was removed so these four circulations were internal and fundamental to the mcs the mesoscale perturbation in wind in the middle troposphere extended farther behind the mcs than ahead of it consistent with analytic studies and numerical simulations of gravity waves generated by heat sources characteristic of mcss with leading convective lines and trailing stratiform regions deepening of the mcv appeared to be reflected in the vertical wind shear at the vortex s center as the mcv strengthened the mesoscale shear through its lower part decreased perhaps as wind became more vortical at increasing altitudes mesoscale and synoptic vertical shears were of similar magnitude so an average of environmental soundings outside an mcs probably does not accurately represent the shear that affects an mcv this suggests the need to reevaluate how the kinematical settings of mcvs are diagnosed

testing the accuracy of a 1 d volcanic plume model in estimating mass eruption rate during volcanic eruptions empirical relationships are used to estimate mass eruption rate from plume height although simple such relationships can be inaccurate and can underestimate rates in windy conditions one dimensional plume models can incorporate atmospheric conditions and give potentially more accurate estimates here i present a 1 d model for plumes in crosswind and simulate 25 historical eruptions where plume height hobs was well observed and mass eruption rate mobs could be calculated from mapped deposit mass and observed duration the simulations considered wind temperature and phase changes of water atmospheric conditions were obtained from the national center for atmospheric research reanalysis 2 5 model simulations calculate the minimum maximum and average values mmin mmax and mavg that fit the plume height eruption rates were also estimated from the empirical formula mempir 140hobs4 14 mempir is in kilogram per second hobs is in kilometer for these eruptions the standard error of the residual in log space is about 0 53 for mavg and 0 50 for mempir thus for this data set the model is slightly less accurate at predicting mobs than the empirical curve the inability of this model to improve eruption rate estimates may lie in the limited accuracy of even well observed plume heights inaccurate model formulation or the fact that most eruptions examined were not highly influenced by wind for the low wind blown plume of 14 18 april 2010 at eyjafjallajokull where an accurate plume height time series is available modeled rates do agree better with mobs than mempir

mesoscale convective vortices observed during bamex part i kinematic and thermodynamic structure abstract five cases of mesoscale convective vortices mcvs are described from observations collected during the bow echo and mcv experiment bamex over the central united states during the period from 20 may to 6 july 2003 in the present paper the kinematic and thermodynamic structure of each vortex and its environment are emphasized data consist of bamex dropsondes the national oceanic and atmospheric administration profiler network and national weather service soundings in addition weather surveillance radar 1988 doppler observations documented the signatures of nascent mcvs within nocturnal convection systems as well as the spatial pattern of convection within mcvs during the following day the vertical structure of each vortex was highly dependent on the vertical shear and the presence of upper tropospheric cyclonic vorticity anomalies in strong shear a pronounced downshear tilt of the vortex was evident but with the presence of an upper tropospheric trough the tilt was upshear in the upp

a three dimensional numerical simulation of spreading umbrella clouds 1 during explosive volcanic eruptions an eruption column buoyantly rises as a turbulent plume and an umbrella cloud spreads laterally as a gravity current at the neutral buoyancy level the source conditions of explosive eruptions such as mass discharge rates of magma at vents have been estimated from the field observations e g satellite images on the height of the eruption columns and the spreading rate of umbrella clouds on the basis of the one dimensional 1 d plume model and the gravity current model however these simplified models contain empirical constants entrainment coefficient of turbulent plume k and froude number of gravity current that should be justified we developed a time dependent three dimensional 3 d numerical model of eruption clouds to independently determine the values of these constants the 3 d model is designed to calculate quantitative features of turbulent mixing between an eruption cloud and the ambient air without any a priori empirical constants by applying a sufficiently fine grid size with a third order accuracy scheme it has reproduced the fundamental features of eruption clouds including eruption columns pyroclastic flows coignimbrite ash clouds and umbrella clouds the altitudes of the spreading umbrella clouds in the 3 d simulations are consistent with those of the neutral buoyancy level of eruption columns estimated by the 1 d plume model with the entrainment coefficient k 0 1 the relationship between the volumetric flow rate and the spreading rate of the umbrella cloud in the 3 d simulations is explained by the axisymmetrical gravity current model with 0 2 for eruption clouds in the tropical atmosphere and 0 1 for those in the midlatitude atmosphere on the other hand the total column height in the 3 d simulations strongly oscillates even for a constant mass discharge rate and its time average tends to be substantially greater than the column height estimated by the 1 d plume model with k 0 1 particularly for large scale eruption clouds in the midlatitude atmosphere we recommend a method to estimate the mass discharge rate from the observation of the column height or the spreading rate of umbrella cloud the method and the accuracy of the 3 d simulations are tested on the basis of the field data e g the total height of the eruption column the altitude and the spreading rate of the umbrella cloud and the mass discharge rate of the pinatubo 1991 eruption

a numerical study of turbulent mixing in eruption clouds using a three dimensional fluid dynamics model 1 the dynamics of eruption clouds in explosive volcanic eruptions are governed by entrainment of ambient air into eruption clouds by turbulent mixing we develop a new numerical pseudo gas model of an eruption cloud by employing three dimensional coordinates a third order accuracy scheme and a fine grid size in order to investigate the behavior of entrainment due to turbulent mixing the quantitative features of entrainment are measured by a proportionality constant relating the inflow velocity at the edge of the flow to the average vertical velocity i e the entrainment coefficient our model has successfully reproduced the quantitative features of entrainment observed in the laboratory experiments as well as fundamental features of the dynamics of eruption clouds such as the generation of eruption columns and or pyroclastic flows the value of the entrainment coefficient for eruption clouds is estimated from the column height and critical condition for column collapse by comparing results of our model with those of previous one dimensional models it is suggested that the value of the entrainment coefficient for an eruption cloud is approximately constant although the value estimated from the critical condition for column collapse k 0 07 is slightly smaller than that based on the column height k 0 1 this difference reflects the vertical change of flow structure in the eruption cloud the eruption cloud in the upper region exhibits a meandering instability which leads to efficient mixing whereas the cloud near the vent maintains a concentric structure with an inner dense core surrounded by an outer shear region our model is consistent with previous one dimensional models for steady eruption clouds supported by the laboratory experiments and it is also applicable to unsteady and transient features of actual eruption clouds

influence of balanced motions on heavy precipitation within a long lived convectively generated vortex abstract the forcing of heavy precipitation within a long lived convectively generated mesoscale vortex mcv is investigated with the aid of diagnoses from rapid update cycle gridded analyses organized convection within the mcv followed a distinct diurnal cycle which featured organized mesoscale convective systems mcss that matured overnight near the mcv center on successive days the mcv was typically most intense in the middle troposphere but intensified within the lower troposphere during the episodes of organized nocturnal convection the lower tropospheric vertical shear was an important organizing factor in mcs development and sustenance in the sense that its interaction with the cold temperature anomaly beneath the mcv center determined where balanced lower tropospheric ascent occurred from trajectory analyses we estimate that balanced ascent accounted for approximately half of the total vertical displacement of the thermodynamically unstable air that eventually composed elevated saturated

impact of wind on the condition for column collapse of volcanic plumes the collapse of volcanic plumes has significant implications for eruption dynamics and associated hazards we show how eruptive columns can collapse and generate pyroclastic density currents as a result of not only the source conditions but also of the atmospheric environment the ratio of the potential energy and the kinetic energy at the source quantified by the richardson number and the entrainment efficiency quantified by the radial entrainment coefficient have already been identified as key parameters in controlling the transition between a buoyant and collapsing plume here we quantify how this transition is affected by wind using scaling arguments in combination with a one dimensional plume model air entrainment due to wind causes a volcanic plume to lower its density at a faster rate and therefore to favor buoyancy we identify the conditions when wind entrainment becomes dominant over radial entrainment and quantify the effect of wind on column collapse these findings are framed into a generalized regime diagram that also describes previous regime diagrams for the specific case of choked flows many observations confirm how bent over plumes typically do not generate significant collapses a quantitative comparison with the 1996 ruapehu and the 2010 eyjafjallajokull eruptions shows that the likelihood of collapse is reduced even at moderate wind speeds relative to the exit velocity at the vent

the two dimensional response of a tropical jet to propagating lines of convection this paper uses simple one and two dimensional models to investigate the influence of a propagating line of convective forcing on a tropical jet representative of the african easterly jet the results are used to infer changes in the environment of the forcing region which would in reality tend to influence the evolution of the storm through convective mechanisms which are not resolved here from linear analytical solutions with a rigid lid it is found that the influence of the propagation of the forcing region is to intensify the response on the upstream side of the forcing this result is confirmed in two dimensional nonlinear simulations when linear wave modes are computed in a basic state that includes the jet structure small sensitivities to the basic state jet are found the two dimensional nonlinear model has been used further to compute the change in the environmental structure as a result of the forcing principally it is found that the modes of response to the forcing may be associated with characteristic changes in the basic state shear convective available potential energy cape and convective inhibition cin which would be expected to have significant influence on the convective system itself

numerical determination of the efficiency of entrainment in volcanic eruption columns 1 entrainment of air by turbulent mixing plays a central role in the dynamics of eruption clouds the amount of entrained air controls eruption styles and heights of eruption columns the efficiency of entrainment is quantified by the entrainment coefficient the ratio between mean inward radial velocity at the edge and the mean vertical velocity we directly determined the entrainment coefficient of eruption columns as a function of height on the basis of three dimensional numerical simulations the value of entrainment coefficient is similar to that for pure jets 0 07 just above the vent and approaches that for pure plumes 0 10 0 15 far from the vent between these two regions we identify a new transitional zone with a significantly small entrainment coefficient 0 05 this spatial variation in the entrainment coefficient is correlated with the change in vortical structure accompanying the addition of off source buoyancy in the transitional zone

climatology of summer midtropospheric perturbations in the us northern plains part ii large scale effects of the rocky mountains on genesis propagating convective storms across the us northern plains are often coupled with preexisting midtropospheric perturbations mps initiated over the rocky mountains a companion study part i notes that such mps occur most commonly at 12 utc early morning and 00 utc late afternoon using a regional reanalysis and a general circulation model gcm this study investigates how such a bimodal distribution of the mp frequency is formed the results point to two possible mechanisms working together while each has a different timing in terms of maximum effect the diurnal evolutions between the midtropospheric flows over the rockies and over the great plains are nearly out of phase due to inertial oscillation during the nighttime the westerly flows at 700 500 mb over the rockies intensify while flows at the same level over the great plains turn easterly these two flows converge over the eastern rockies and induce cyclonic vorticity through vortex stretching after sunrise the convergence dissipates and the cyclonic vorticity is redistributed by horizontal vorticity advection moving it downstream this process creates a climatological zonally propagating vorticity signal which in turn facilitates the early morning mp genesis at 12 utc the analysis also reveals marked dynamic instability conducive to subsynoptic scale disturbances in the midtroposphere over the rockies strong meridional temperature gradients appear over the north facing slopes of the rockies due to terrain heating to the south and the presence of cooler air to the north this feature along with persistent vertical shear creates a charney stern type of instability i e sign changes of the meridional potential vorticity gradient meanwhile the development of terrain boundary layer reduces the rossby deformation radius which subsequently enhances the likelihood for baroclinic short waves such effects are most pronounced in the late afternoon and therefore are supportive to the mp genesis around 00 utc examination of gcm experiments with and without orography further supports the critical role of the rocky mountains and its associated boundary layer impacts on the formation of mps

scale interactions during the formation of typhoon irving the development of typhoon irving is investigated using a variety of data including special research aircraft data from the tropical cyclone motion tcm 92 experiment objective analyses satellite data and traditional surface and sounding data the development process is treated as a dry adiabatic vortex dynamics problem and it is found that environmental and mesoscale dynamics mutually enhance each other in a cooperative interaction during cyclone formation synoptic scale interactions result in the evolution of the hostile environment toward more favorable conditions for storm development mesoscale interactions with the low level large scale circulations and with other midlevel mesoscale features result in development of vorticity in the midlevels and enhancement of the low level vorticity associated with the developing surface cyclone multiple developments of mesoscale convective systems after the storm reaches tropical depression strength suggests both an increase in low level confluence and a tendency toward recurrent development of associated mesoscale convective vortices this is observed in both aircraft data and satellite imagery where subsequent interactions including mergers with the low level tropical depression vortex are observed a contour dynamics experiment suggests that the movement of mesoscale convective systems in satellite imagery corresponds well to the movement of their associated midlevel vortices results from a simple baroclinic experiment show that the midlevel vortices affect the large scale low level circulation in two ways 1 initially interactions between midlevel vortices produce a combined vortex of greater depth 2 interaction between midlevel vortices and the low level circulation produces a development downward of the midlevel vorticity this strengthens the surface vortex and develops a more cohesive vortex that extends from the surface through the midtroposphere

mesoscale convective vortex formation in a weakly sheared moist neutral environment abstract idealized simulations of a diabatic rossby vortex drv in an initially moist neutral baroclinic environment are performed using the fifth generation national center for atmospheric research pennsylvania state university ncar psu mesoscale model mm5 the primary objective is to test the hypothesis that the formation and maintenance of midlatitude warm season mesoscale convective vortices mcvs are largely influenced by balanced flow dynamics associated with a vortex that interacts with weak vertical shear as a part of this objective the simulated drv is placed within the context of the bow echo and mesoscale convective vortex experiment bamex field campaign by comparing its tangential velocity radius of maximum winds cape and shear with the mcvs observed in bamex the simulations reveal two distinct scales of development at the larger scale the most rapidly growing moist baroclinic mode is excited and exponential growth of this mode occurs during the simulation embedded within the