numerical simulations of radiative cooling beneath the anvils of supercell thunderstorms abstract numerical simulations of supercell thunderstorms that include parameterized radiative transfer and surface fluxes are performed using the advanced regional prediction system arps to investigate the effects of anvil shadows on the near storm environment if the simulated storm is nearly stationary the maximum low level air temperature deficits within the shadows are about 2 k which is roughly half the cooling found in some previous observations it is shown that the extinction of downwelling shortwave radiation by the anvil cloud creates a differential in the flux of downwelling shortwave radiation between the sun and the shade that is at least an order of magnitude greater than the differential of any other term in either the surface radiation or the surface energy budgets the loss of strong solar heating of the model surface within the shaded regions leads to a reduction of surface temperatures and stabilization of the model surface layer beneath the anvil the reduction in vertical mixing

tephra layers in the siple dome and taylor dome ice cores antarctica sources and correlations 1 volcanic ash or tephra layers are found in the taylor dome siple dome a and siple dome b ice cores significant shard concentrations are found at a number of depths in all three cores electron and ion microprobe analyses indicate that the geochemical composition of most layers is basaltic basanitic or trachytic and the geochemical signatures of the layers suggest derivation from the pleiades volcanic center mt melbourne volcano or small mafic centers probably in the royal society range area presence of tephra layers suggests an episode of previously unrecognized antarctic volcanic activity between 1776 and 1805 a d from at least two volcanic centers a strong geochemical correlation d 3 49 and 3 97 with a value of 4 considered identical is observed between tephra layers at depth of 79 2 m in the taylor dome ice core and layers between 97 2 and 97 7 m depth in the siple b core this correlation and the highly accurate depth age scale of the siple b core suggest that the age of this horizon in the taylor dome ice core presented by steig et al 1998a 2000 should be revised downward to the younger age of 675 25 years before 1995 this revised chronology is consistent with vertical strain measurements presented by hawley et al 2003

the pretornadic phase of the goshen county wyoming supercell of 5 june 2009 intercepted by vortex2 part i evolution of kinematic and surface thermodynamic fields abstractthe authors analyze the pretornadic phase 2100 2148 utc tornadogenesis began at 2152 utc of the goshen county wyoming supercell of 5 june 2009 intercepted by the second verification of the origins of rotation in tornadoes experiment vortex2 the analysis relies on radar data from the weather surveillance radar 1988 doppler wsr 88d in cheyenne wyoming kcys and a pair of doppler on wheels dow radars mobile mesonet observations and mobile sounding observations the storm resembles supercells that have been observed in the past for example it develops a couplet of counter rotating vortices that straddle the hook echo within the rear flank outflow and are joined by arching vortex lines with the cyclonic vortex becoming increasingly dominant in the time leading up to tornadogenesis the outflow in the hook echo region where sampled has relatively small virtual potential temperature deficits during this stage of evolution a few kilometers upstream north of the location of maxim

composite vortex2 supercell environments from near storm soundings abstractthree dimensional composite analyses using 134 soundings from the second verification of the origins of rotation in tornadoes experiment vortex2 reveal the nature of near storm variability in the environments of supercell thunderstorms based upon the full analysis it appears that vertical wind shear increases as one approaches a supercell within the inflow sector providing favorable conditions for supercell maintenance and possibly tornado formation despite small amounts of low level cooling near the storm the seven analyzed tornadic supercells have a composite environment that is clearly more impressive in terms of widely used metrics than that of the five analyzed nontornadic supercells including more convective available potential energy cape more vertical wind shear higher boundary layer relative humidity and lower tropospheric horizontal vorticity that is more streamwise in the near storm inflow the widely used supercell composite parameter scp and significant tornado param

full depth englacial vertical ice sheet velocities measured using phase sensitive radar we describe a geophysical technique to measure englacial vertical velocities through to the beds of ice sheets without the need for borehole drilling using a ground based phase sensitive radio echo sounder pres during seven antarctic field seasons we measure the temporal changes in the position of englacial reflectors within ice divides up to 900 m thick on berkner island roosevelt island fletcher promontory and adelaide island recorded changes in reflector positions yield full depth profiles of vertical ice velocity that we use to examine spatial variations in ice flow near the divides we interpret these variations by comparing them to the results of a full stokes simulation of ice divide flow qualitatively validating the model and demonstrating that we are directly detecting an ice dynamical phenomenon called the raymond effect using pres englacial vertical ice velocities can be measured in higher spatial resolution than is possible using instruments installed within the ice we discuss how these measurements could be used with inverse methods to measure ice rheology and to improve ice core dating by incorporating pres measured vertical velocities into age modeling

vortex2 observations of a low level mesocyclone with multiple internal rear flank downdraft momentum surges in the 18 may 2010 dumas texas supercell abstractobservations collected in the second verification of the origins of rotation in tornadoes experiment during a 15 min period of a supercell occurring on 18 may 2010 near dumas texas are presented the primary data collection platforms include two ka band mobile doppler radars which collected a near surface short baseline dual doppler dataset within the rear flank outflow of the dumas supercell an x band phased array mobile doppler radar which collected volumetric single doppler data with high temporal resolution and in situ thermodynamic and wind observations of a six probe mobile mesonet rapid evolution of the dumas supercell was observed including the development and decay of a low level mesocyclone and four internal rear flank downdraft rfd momentum surges intensification and upward growth of the low level mesocyclone were observed during periods when the midlevel mesocyclone was minimally displaced from the low level circulation suggesting an upward directed perturbation pressure g

analysis of a nontornadic storm during vortex 95 abstract analysis of a supercell storm that did not produce a tornado near hays kansas is presented a well defined midlevel mesocyclone was apparent throughout most of the storm s life cycle numerous shallow circulations were observed along the rear flank gust front during the data collection period six of these circulations strengthened into intense low level mesocyclones each of these mesocyclones failed to produce a tornado the strongest low level mesocyclone referred to as vortex 4 underwent a life cycle that was consistent with other tornadic mesocyclones documented in the literature these results illustrate that the presence of a long lived mesocyclone at low levels is not sufficient for tornadogenesis to occur the kinematic structure of the low level mesocyclone that did not produce a tornado is compared with a tornadic mesocyclone from another storm in order to understand the characteristic differences between these circulations the results lead to the conclusion that the presence of

ocean island densities and models of lithospheric flexure estimates of the effective elastic thickness te of the oceanic lithosphere based on gravity and bathymetric data from island loads are commonly significantly lower than those based on the wavelength of plate bending at subduction zones the anomalously low values for ocean islands have been attributed to the finite yield strength of the lithosphere to erosion of the mechanical boundary layer by mantle plumes to pre existing thermal stresses and to overprinting of old volcanic loads by younger ones a fifth possible contribution to the discrepancy is an incorrect assumption about the density of volcanic loads we suggest that load densities have been systematically overestimated in studies of lithospheric flexure potentially resulting in systematic underestimation of effective elastic thicknesses and overestimation of the effects of hotspot volcanism we illustrate the effect of underestimating load density with synthetic examples and an example from the marquesas islands this effect combined with the other effects listed above in many cases may obviate the need to invoke hotspot reheating to explain low apparent elastic thicknesses

the pretornadic phase of the goshen county wyoming supercell of 5 june 2009 intercepted by vortex2 part ii intensification of low level rotation abstractthe dynamical processes responsible for the intensification of low level rotation prior to tornadogenesis are investigated in the goshen county wyoming supercell of 5 june 2009 intercepted by the second verification of the origins of rotation in tornadoes experiment vortex2 the circulation of material circuits that converge upon the low level mesocyclone is principally acquired along the southern periphery of the forward flank precipitation region which is a corridor characterized by a horizontal buoyancy gradient thus much of the circulation appears to have been baroclinically generated the descending reflectivity core drc documented in part i of this paper has an important modulating influence on the circulation of the material circuits a circuit that converges upon the low level mesocyclone center prior to the drc s arrival at low levels approximately the arrival of the 55 dbz reflectivity isosurface in this case loses some of its previously acquired circulation during the final f

evolution of low level angular momentum in the 2 june 1995 dimmitt texas tornado cyclone abstract the life cycle of the 2 june 1995 dimmitt texas tornado cyclone observed during the verification of the origins of rotation in tornadoes experiment vortex is described the tornado cyclone here is defined as a significantly axisymmetric flow larger than the visible tornado and characterized by increasing angular momentum with increasing radius its life cycle included three phases with somewhat differing evolution of angular momentum herein called intensifying transition and weakening during the intensifying stage the funnel and debris cloud gradually increased in size the azimuthally averaged secondary circulation of the larger scale tornado cyclone as determined using high resolution single doppler data obtained by a mobile radar was primarily inward and upward consistent with the presence of a wall cloud outside the tornado the azimuthally averaged angular momentum increased monotonically away from the tornado so inward advection allowed the angular momentum to increase slowly

simulated supercells in nontornadic and tornadic vortex2 environments abstractthe composite near storm environments of nontornadic and tornadic supercells sampled during the second verification of the origins of rotation in tornadoes experiment vortex2 both appear to be generally favorable for supercells and tornadoes it has not been clear whether small differences between the two environments e g more streamwise horizontal vorticity in the lowest few hundred meters above the ground in the tornadic composite are actually determinative of storms tornadic potential from the vortex2 composite environments simulations of a nontornadic and a tornadic supercell are used to investigate storm scale differences that ultimately favor tornadogenesis or tornadogenesis failure both environments produce strong supercells with robust midlevel mesocyclones and hook echoes though the tornadic supercell has a more intense low level updraft and develops a tornado like vortex exceeding the ef3 wind speed threshold in contrast the nontornadic supercell only produces shallow vortic

supercell low level mesocyclones in simulations with a sheared convective boundary layer abstractsimulations of supercell thunderstorms in a sheared convective boundary layer cbl characterized by quasi two dimensional rolls are compared with simulations having horizontally homogeneous environments the effects of boundary layer convection on the general characteristics and the low level mesocyclones of the simulated supercells are investigated for rolls oriented either perpendicular or parallel to storm motion as well as with and without the effects of cloud shading bulk measures of storm strength are not greatly affected by the presence of rolls in the near storm environment though boundary layer convection diminishes with time under the anvil shadow of the supercells when cloud shading is allowed simulations without cloud shading suggest that rolls affect the morphology and evolution of supercell low level mesocyclones initially cbl vertical vorticity perturbations are enhanced along the supercell outflow boundary resulting in nonnegligible near ground vertical vorticity regardles

a method to reconstruct past accumulation rates in alpine firn regions a study on fiescherhorn swiss alps annual snow layers in the accumulation zone of glaciers become thinner by viscous deformation while moving to greater depth the reconstruction of the accumulation rate history requires the correction of the thickness of annual layers we apply a novel method to determine the vertical velocity by repeated survey with a caliper probe of grooves scratched into the wall of a borehole with this information and the assumptions of steady state velocity and density fields the correction of layer thicknesses can be determined without knowledge of the ice density profile and thus can be applied to boreholes without ice cores the method is applied to a borehole on fiescherhorn swiss alps resulting in a substantially larger correction than with the simpler method of nye 1963 copyright 2006 by the american geophysical union

a simulation of the development of successive cells along a cold outflow boundary abstract a three dimensional numerical simulation is presented in which five new cells clouds develop in succession over a 4 h period the cells that develop have common characteristics including a lifetime of 40 min they form at 30 min intervals in a convergent region along the cold outflow boundary that is established by previous cells the propagation of the cloud system is several meters per second faster than individual cell movement due to the discrete development of new cells on the right flank of previous ones analysis of the simulation indicates that the most pronounced changes near the surface are due to downdraft development in contrast there is only a weak signature of new updraft development near the surface in the convergence field each new updraft develops along the cold outflow boundary which moves slowly away from the previous cell the air participating in the generation of a new cell appears to have originated 1 1 5 km below cloud base 2 6 km as the cell grows and rain begins

vortex lines within low level mesocyclones obtained from pseudo dual doppler radar observations abstract vortex lines passing through the low level mesocyclone regions of six supercell thunderstorms three nontornadic three tornadic are computed from pseudo dual doppler airborne radar observations obtained during the verification of the origins of rotation in tornadoes experiment vortex in every case at least some of the vortex lines emanating from the low level mesocyclones form arches that is they extend vertically from the cyclonic vorticity maximum then turn horizontally usually toward the south or southwest and descend into a broad region of anticyclonic vertical vorticity this region of anticyclonic vorticity is the same one that has been observed almost invariably to accompany the cyclonic vorticity maximum associated with the low level mesocyclone the vorticity couplet straddles the hook echo of the supercell thunderstorm the arching of the vortex lines and the orientation of the vorticity vector along the vortex line arches compared to the orientation of the ambient barotrop

a study of the tornadic region within a supercell thunderstorm abstract the transition of a supercell thunderstorm into its tornadic phase is investigated through high resolution numerical cloud model simulations initiated within the interior portion of a previously simulated mature supercell storm with the enhanced grid resolution the low level cyclonic vorticity increases dramatically and the gust front rapidly occludes as small scale downdrafts develop in the vicinity of the low level center of circulation as the occlusion progresses a ring of high vorticity air surrounds the circulation center and could be conducive to multiple vortex tornado formation numerous features of the simulated transition bear resemblance to those observed in tornadic storms in the model simulation the large low level vorticity is generated through the tilting and intense stretching of air from the inflow side of the storm this vertical vorticity is derived from the horizontal vorticity of the environmental shear and also from horizontal vorticity generated solenoidally as low l

west antarctica s sensitivity to natural and human forced climate change over the holocene the location and intensity of the austral westerlies strongly influence southern hemisphere precipitation and heat transport with consequences for human society and ecosystems with future warming global climate models project increased aridity in southern mid latitudes related to continued poleward contraction of the austral westerlies we utilize antarctic ice cores to investigate past and to set the stage for the prediction of future behaviour of the westerlies we show that holocene west antarctic ice core reconstructions of atmospheric circulation sensitively record naturally forced progressive as well as abrupt changes we also show that recent poleward migration of the westerlies coincident with increased emission of greenhouse gases and the antarctic ozone hole has led to unprecedented penetration compared with 100 000 years ago of air masses bringing warmth extra antarctic source dust and anthropogenic pollutants into west antarctica copyright 2012 john wiley sons ltd

impacts of lapse rates on low level rotation in idealized storms abstractadiabatic lapse rates appear to be a common feature in the lower troposphere on tornado days this article reviews physical reasons why lapse rates may influence surface vortex intensification and reports on numerical simulations designed to study the key processes in the idealized numerical model an initial mesocyclone like vortex and nonvarying convection like heat source are used in different environmental stability profiles the scales of interest in these simulations typify those of a parent supercell and the developing circulations constitute direct responses to the imposed heating downward parcel displacements are needed for surface vortex development in environments with no preexisting surface vorticity in the simulations under neutral stratification there is strong heating induced subsidence anchored near the storm edge whereas under stable stratification there are instead gravity waves that propagate away to the far field in addition under weak or neutral low level stratification

a numerical simulation of cyclic mesocyclogenesis abstract a three dimensional nonhydrostatic numerical model the advanced regional prediction system is used to study the process of cyclic mesocyclogenesis in a classic supercell thunderstorm during the 4 h simulation the storm s mesocyclone undergoes two distinct occlusions with the beginning of a third indicated at the end of the simulation the occlusion process exhibits a period of approximately 60 min and is qualitatively similar in each case initial midlevel 3 7 km mesocyclogenesis proceeds according to the classic picture that is via tilting of streamwise environmental vorticity the development of an evaporatively driven rear flank downdraft rfd signals the beginning of the occlusion process the developing rfd wraps cyclonically around the mesocyclone causing the gust front to surge outward simultaneously the occluding mesocyclone rapidly intensifies near the surface trajectory analyses demonstrate that this intensification follows from the tilting and stretching of near ground

on the use of doppler radar derived wind fields to diagnose the secondary circulations of tornadoes abstracta number of studies in recent years have used wind fields derived from portable doppler radars in combination with the ground based velocity track display gbvtd technique to diagnose the primary tangential and secondary radial and vertical circulations in tornadoes these analyses indicate very strong vertical motions in the vortex core in some cases with updrafts and downdrafts exceeding 100 m s 1 in addition many of the analyses indicate strong radial outflow at low levels and in the vicinity of the low level tangential wind maximum this paper shows that strong outward motion at this location cannot be consistent with a tornado circulation that lasts more than a few minutes in addition using data from numerical simulations as truth it is shown that using observed radial velocities to diagnose vertical velocities greatly overestimates the intensity of downward motion in the core for two reasons neglect of the mass flux into the core through the swirling boundary layer and the likel

uncertainties in trajectory calculations within near surface mesocyclones of simulated supercells abstractthis study addresses the sensitivity of backward trajectories within simulated near surface mesocyclones to the spatiotemporal resolution of the velocity field these backward trajectories are compared to forward trajectories computed during run time within the numerical model it is found that the population of backward trajectories becomes increasingly contaminated with inflow trajectories that owe their existence to spatiotemporal interpolation errors in time varying and strongly curved confluent flow these erroneous inflow parcels may mistakenly be interpreted as a possible source of air for the near surface vortex it is hypothesized that unlike forward trajectories backward trajectories are especially susceptible to errors near the strongly confluent intensifying vortex although the results are based on model output dual doppler analysis fields may be equally affected by such errors

simulation and analysis of tornado development and decay within a three dimensional supercell thunderstorm abstract a three dimensional numerical simulation using a two way interactive nested grid is to study tornado genesis within a supercell during a 40 minute period two tornadoes grow and decay within the storm s mesocyclone the tornadoes have life spans of approximately 10 minutes maximum ground relative surface wind speeds exceed 60 m s 1 during both tornadoes and horizontal pressure gradients reach 18 hpa km 1 during the second tornado comparison of the simulated storm evolution with doppler and field observations of supercells and tornadoes shows many similar features vertical vorticity in the mesocyclone and the tornado vortex at low levels is initially created by the tilting of the environmental vorticity and baroclinically generated vorticity along the forward gland gust front of the storm tornadogenesis is initiated when mesocyclone rotation increase above cloud base the increased rotation generates lower pressure in the mesocyclone increasing the upward pressure gradient forces the upwar

airborne pseudo dual doppler analysis of a rear inflow jet and deep convergence zone within a supercell abstract on 22 may 1995 numerous thunderstorms several of which produced large hail and small tornadoes formed along a dryline in the central and northern texas panhandle the only long lasting daytime severe storms developed later south of the earlier storms by late afternoon new cells merged and evolved into two large supercells the northern supercell produced three weak tornadoes and hail the southern supercell did not produce a tornado but did produce very large hail for an extended period of time verification of the origins of rotation in tornadoes experiment crews collected data in the southern supercell from its early stage through much of its mature stage the national center for atmospheric research electra doppler radar collected pseudo dual doppler radar data on this storm continuously for well over an hour detailed analyses of this high resolution dataset revealed several unusual features most notably an intense elevated rear inflow jet at midlevels having maximum wind speeds app

an assessment of low level baroclinity and vorticity within a simulated supercell abstractidealized supercell modeling has provided a wealth of information regarding the evolution and dynamics within supercell thunderstorms however discrepancies in conceptual models exist including uncertainty regarding the existence placement and forcing of low level boundaries in these storms as well as their importance in low level vorticity development this study offers analysis of the origins of low level boundaries and vertical vorticity within the low level mesocyclone of a simulated supercell low level boundary location shares similarities with previous modeling studies however the development and evolution of these boundaries differ from previous conceptual models the rear flank gust front develops first whereas the formation of a boundary extending north of the mesocyclone undergoes numerous iterations caused by competing outflow and inflow before a steady state boundary is produced a third boundary extending northeast of the mesocyclone is produced through evaporative cooling of

ice age storm trajectories inferred from radar stratigraphy at taylor dome antarctica the taylor dome ice core east antarctica provides a climate record that extends through the previous interglacial we use ice dynamics modeling and geochemical records to infer the local accumulation rate history a pronounced accumulation reduction occurred during the last glacial maximum lgm the spatial gradient of accumulation across the surface of taylor dome indicates the modern local prevailing storm direction radar internal layering shows that this gradient was reversed during the lgm suggesting a reorganization of synoptic weather systems occurred possibly linked to the advance and retreat of the west antarctic ice sheet

impacts of increasing low level shear on supercells during the early evening transition abstractthe dynamical response of simulated supercells to temporally increasing lower tropospheric vertical wind shear is investigated using idealized simulations these simulations are based upon observed soundings from two cases that underwent an early evening transition during the second verification of the origins of rotation in tornadoes experiment vortex2 mature supercells were simulated in observed afternoon environments with moderate vertical wind shear and then compared to simulated supercells experiencing observed evening increases in lower tropospheric shear the primary effect of the increase in low level shear is to establish larger values of vertical vorticity at lower altitudes in the storm s updraft in turn this leads to a nonlinear increase in the updraft strength due to the enhanced dynamic pressure minimum associated with larger vorticity in the storm s mesocyclone this is particularly important at low levels where it increases the storm s ability to lift cool surface air includ

dynamic multi scale modes of severe storm structure detected in mobile doppler radar data by entropy field decomposition abstractthe detection of complex spatially and temporally varying coherent structures in data from highly nonlinear and non gaussian systems is a challenging problem in a wide range of scientific d

instruments and methods in situ measurements of firn compaction profiles using borehole optical stratigraphy we have developed a technique in which we use a borehole video camera and post processing software to make a record of the optical brightness as a function of depth in polar firn we call this method borehole optical stratigraphy to measure firn compaction we note the positions of optical features on the borehole wall detected by an initial baseline log we track the displacements of these features in subsequent logs the result provides a measurement of the relative vertical motion and thus compaction of the firn over the survey period we have successfully used this system at summit greenland to measure the depth distribution of firn column shortening experienced in a borehole over three 1 year periods the uppermost 30 m of the firn at summit is compacting as predicted by a simple steady state model implying that the firn density profile at summit is at or close to steady state over the past 70 years

the taylor dome antarctic 18o record and globally synchronous changes in climate abstract the 18 o 16 o profile of a 554 m long ice core through taylor dome antarctica shows the climate variability of the last glacial interglacial cycle in detail and extends at least another full cycle taylor dome shares the main features of the vostok record including the early climatic optimum with later cool phase of the last interglacial period in antarctica taylor dome 18 o fluctuations are more abrupt and larger than those at vostok and byrd station although still less pronounced than those of the greenland gisp2 and grip records the influence of the atlantic thermohaline circulation on regional ocean heat transport explains the partly north atlantic character of this antarctic record under full glacial climate marine isotope stage 4 late stage 3 and stage 2 this marine influence diminished and taylor dome became more like vostok varying degrees of marine influence produce climate heterogeneity within antarctica which may account for conflicting evidence regarding the relative phasing of northern and southern hemisphere climate change

high resolution dual doppler analyses of the 29 may 2001 kress texas cyclic supercell abstract on 29 may 2001 doppler on wheels radars collected data on a supercell near kress texas the supercellular storm cyclic in nature produced multiple mesocyclones throughout its lifetime dual doppler syntheses were conducted using a grid spacing of 100 m resulting in the highest resolution observational analysis of a cyclic supercell to date in addition collection of data from ground based radar allowed for the analysis of near ground features irresolvable with airborne radar providing another advantage over previous studies the syntheses revealed a number of evolving low level mesocyclones over the observation period of 900 s while nontornadic during the synthesis period the supercell exhibited evidence of strong vertical vorticity greater than 10 2 s 1 low level circulation with classic cyclic structure and multiple tornadoes beginning 3600 s later a comparison between the current results conceptual models and previous lower resolution analyses is presented a striking similarity

tornadogenesis in a high resolution simulation of the 8 may 2003 oklahoma city supercell abstracta 50 m grid spacing advanced regional prediction system arps simulation of the 8 may 2003 oklahoma city tornadic supercell is examined a 40 min forecast run on the 50 m grid produces two f3 intensity tornadoes that track within 10 km of the location of the observed long track f4 intensity tornado the development of both simulated tornadoes is analyzed to determine the processes responsible for tornadogenesis trajectory based analyses of vorticity components and their time evolution reveal that tilting of low level frictionally generated horizontal vorticity plays a dominant role in the development of vertical vorticity near the ground this result represents the first time that such a mechanism has been shown to be important for generating near surface vertical vorticity leading to tornadogenesis a sensitivity simulation run with surface drag turned off was found to be considerably different from the simulation with drag included a tornado still developed in the no drag simulation but it was

instruments and methods borehole logging with an eight arm caliper inclinometer probe a newly designed eight arm caliper inclinometer probe for borehole logging is presented the caliper sounding gives eight points of cross sections of the borehole at 1 cm intervals along its axis regression circles and ellipses are calculated and used as obvious hypotheses to be tested in order to obtain information on strain rates in the horizontal planes and isotropic or anisotropic borehole closure the scratching of identifiable marks into the wall of the borehole enables material displacement to be tracked in particular in the vertical direction thus providing information for determining vertical strain rates along the borehole measurements in two boreholes drilled on high altitude glaciers in the swiss alps are used to demonstrate both the potential and the limitations of the probe and of the mathematical methods presented

non climate influences on stable isotopes at taylor mouth antarctica the late holocene trends in 18 o differ significantly in two ice cores 30 km apart from the area of taylor dome antarctica it is unlikely that the trend in the core from taylor mouth the flank site is due to a standard 18 o surface temperature relationship assuming that the taylor dome near summit core records local climate variations common to both cores we assess two leading possible causes for the observed differences 1 relative to taylor dome taylor mouth may collect snow from more sources with distinct isotopic compositions 2 vapor motion during prolonged near surface exposure may cause post depositional isotope enrichment at taylor mouth where the accumulation rate is low our model of firn pore space vapor and sublimating ice grains suggests that post depositional processes can modify 18 o values by several o isotopic samples from areas with significantly different accumulation rates near taylor mouth could differentiate between possibilities 1 and 2

a preliminary polarimetric radar comparison of pretornadic and nontornadic supercell storms abstractsupercell thunderstorms produce a variety of hazards including tornadoes a supercell will often exist for some time prior to producing a tornado while other supercells never become torna

processes preventing the development of a significant tornado in a colorado supercell on 26 may 2010 abstracta supercell produced a nearly tornadic vortex during an intercept by the second verification of the origins of rotation in tornadoes experiment on 26 may 2010 using observations from two m

observations of nontornadic low level mesocyclones and attendant tornadogenesis failure during vortex abstract three storms intercepted during the verification of the origins of rotation in tornadoes experiment generated a moderate to strong mesocyclone within the lowest several hundred meters above the ground and qualitatively appeared capable of tornadogenesis yet did not produce a tornado such novel observations of what is considered tornadogenesis failure are documented and used to show the insufficiency of a low level mesocyclone for tornadogenesis possible modes of failure are discussed

doppler radar observations of anticyclonic tornadoes in cyclonically rotating right moving supercells abstractsupercells dominated by mesocyclones which tend to propagate to the right of the tropospheric pressure weighted mean wind on rare occasions produce anticyclonic tornadoes at the trailing end of the rear flank gust front more frequently mesoanticyclones are found at this location most of which do not spawn any tornadoes in this paper four cases are discussed in which the formation of anticyclonic tornadoes was documented in the plains by mobile or fixed site doppler radars these brief case studies include the analysis of doppler radar data for tornadoes at the following dates and locations 1 24 april 2006 near el reno oklahoma 2 23 may 2008 near ellis kansas 3 18 march 2012 near willow oklahoma and 4 31 may 2013 near el reno oklahoma three of these tornadoes were also documented photographically in all of these cases a strong mesocyclone i e vortex signature characterized by azimuthal shear in excess of 5 10 3 s 1 or a 20 m s 1 change in doppler velocity over 5 km

the influence of a local swirl ratio on tornado intensification near the surface abstract the results of high resolution fully three dimensional unsteady simulations of the interaction of a tornado with the surface are presented the goal is to explore some of the range of structures that should be expected to occur in nature within the tornadic corner flow that region where the central vortex meets the surface the most important physical variables considered are the tornado scale circulation and horizontal convergence the effective surface roughness the tornado translation speed the low level inflow structure and the upper core structure a key ingredient of the corner flow dynamics is the radial influx of fluid in the surface layer with low angular momentum relative to that of the fluid in the main vortex above it this low swirl fluid arises initially from outside or below the larger scale vortex or through frictional loss of angular momentum to the surface and forms much of the vortex core flow after it exits the corner flow region changes in the surface layer inflow or

an investigation of the goshen county wyoming tornadic supercell of 5 june 2009 using enkf assimilation of mobile mesonet and radar observations collected during vortex2 part i experiment design and verification of the enkf analyses abstracthigh resolution doppler radar velocities and in situ surface observations collected in a tornadic supercell on 5 june 2009 during the second verification of the origins of rotation in tornadoes experiment vortex2 are assimilated into a simulated convective storm using an ensemble kalman filter enkf a series of enkf experiments using a 1 km horizontal model grid spacing demonstrates the sensitivity of the cold pool and kinematic structure of the storm to the assimilation of these observations and to different model microphysics parameterizations an experiment is performed using a finer grid spacing 500 m and the most optimal data assimilation and model configurations from the sensitivity tests to produce a realistically evolving storm analyses from this experiment are verified against dual doppler and in situ observations and are evaluated for their potential to confidently evaluate mesocyclone scale processes in the storm using trajectory analysis and calculations of lagrangian vorticity

wind and temperature retrievals in the 17 may 1981 arcadia oklahoma supercell ensemble kalman filter experiments abstract the feasibility of using an ensemble kalman filter enkf to retrieve the wind and temperature fields in an isolated convective storm has been tested by applying the technique to observations of the 17 may 1981 arcadia oklahoma tornadic supercell radial velocity and reflectivity observations from a single radar were assimilated into a nonhydrostatic anelastic numerical model initialized with an idealized horizontally homogeneous base state the assimilation results were compared to observations from another doppler radar the results of dual doppler wind syntheses and in situ measurements from an instrumented tower observation errors make it more difficult to assess enkf performance than in previous storm scale enkf experiments that employed synthetic observations and a perfect model nevertheless the comparisons in this case indicate that the locations of the main updraft and mesocyclone in the arcadia storm were determined rather accurately especially at midlevels the magnitudes of v

reexamining the vertical development of tornadic vortex signatures in supercells abstractobservations from a hybrid phased array doppler radar the mobile weather radar 2005 x band phased array mwr 05xp were used to investigate the vertical development of tornadic vortex signatures tvss during supercell tornadogenesis data with volumetric update times of 10 s an order of magnitude better than that of most other mobile doppler radars were obtained up to storm midlevels during the formation of three tornadoes it is found that tvss formed upward with time during tornadogenesis for two cases in a third case missing low level data prevented a complete time height analysis of tvs development however tvs formation occurred first near the ground and then at storm midlevels several minutes later these results are consistent with the small number of volumetric mobile doppler radar tornadogenesis cases from the past 10 years but counter to studies prior to that in which a descending tvs was observed in roughly half of tornado cases utilizing weather surveillance radar 1988 do

hook echoes and rear flank downdrafts a review abstract nearly 50 years of observations of hook echoes and their associated rear flank downdrafts rfds are reviewed relevant theoretical and numerical simulation results also are discussed for over 20 years the hook echo and rfd have been hypothesized to be critical in the tornadogenesis process yet direct observations within hook echoes and rfds have been relatively scarce furthermore the role of the hook echo and rfd in tornadogenesis remains poorly understood despite many strong similarities between simulated and observed storms some possibly important observations within hook echoes and rfds have not been reproduced in three dimensional numerical models

a 12 000 year record of explosive volcanism in the siple dome ice core west antarctica 1 air mass trajectories in the southern hemisphere provide a mechanism for transport to and deposition of volcanic products on the antarctic ice sheet from local volcanoes and from tropical and subtropical volcanic centers this study extends the detailed record of antarctic south american and equatorial volcanism over the last 12 000 years using continuous glaciochemical series developed from the siple dome a sdma ice core west antarctica the largest volcanic sulfate spike 280 g l occurs at 5881 b c e other large signals with unknown sources are observed around 325 b c e 270 g l and 2818 b c e 191 g l ages of several large equatorial or southern hemisphere volcanic eruptions are synchronous with many sulfate peaks detected in the sdma volcanic ice chemistry record the microprobe fingerprinting of glass shards in the sdma core points to the following antarctic volcanic centers as sources of tephra found in the sdma core balenny island pleiades mount berlin mount takahe and mount melbourne as well as mount hudson and possibly mount burney volcanoes of south america identified volcanic sources provide an insight into the poorly resolved transport history of volcanic products from source volcanoes to the west antarctic ice sheet

vertical strain measurements in firn at siple dome antarctica we measured vertical strain in the firn at siple dome antarctica using two systems both of which measure relative displacements over time of metal markers placed in an air filled borehole one system uses a metal detecting tuned coil and the other uses a video camera to locate the markers we compare the merits of the two systems we combine steady state calculations and a measured density profile to estimate the true vertical velocity profile this allows us to calculate a depth age scale for the firn at siple dome our steady state depth age scale has ages 10 15 younger at any given depth when compared to depth age scales derived by layer counting in a core 40 m away the age of a visible ash layer at 97 m in the core is 665 30 years in agreement with a similar analysis conducted at taylor dome antarctica where the same ash is also seen providing an additional dated tie point between the two cores

geometry motion and mass balance of dyer plateau antarctica geodetic surveying and ground based radar profiling were used to determine geometry and surface motion of the ice sheet on the dyer plateau antarctica in the vicinity of an ice core site on a local dome vertical strain measurements in the core hole constrain the depth profile of vertical velocity these geophysical measurements are used to analyze the profiles of density and annual layer thickness measured on the ice core to estimate the current mass balance of the ice column and the past history of accumulation rate consideration of horizontal and vertical mass flow divergence shows that the profiles of density and vertical velocity are not fully consistent with steady state mean density of the firn layer appears to be increasing which leads to the deduction of a small rate of mass increase 0 02 m a 1 ice equivalent thickness over the last 200a there has been a gradual increase in accumulation rate from about 0 46 m a 1 to 0 54 m a 1 ice equivalent thickness in recent time

direct surface thermodynamic observations within the rear flank downdrafts of nontornadic and tornadic supercells abstract despite the long surmised importance of the hook echo and rear flank downdraft rfd in tornadogenesis only a paucity of direct observations have been obtained at the surface within hook echoes and rfds in this paper in situ surface observations within hook echoes and rfds are analyzed these mobile mesonet data have unprecedented horizontal spatial resolution and were obtained from the verifications of the origins of rotation in tornadoes experiment vortex and additional field experiments conducted since the conclusion of vortex the surface thermodynamic characteristics of hook echoes and rfds associated with tornadic and nontornadic supercells are investigated to address whether certain types of hook echoes and rfds are favorable or unfavorable for tornadogenesis tornadogenesis is more likely and tornado intensity and longevity increase as the surface buoyancy potential buoyancy as measured by the convective available potential energy and equivalent potential temperature in the r

evidence for substantial accumulation rate variability in antarctica during the holocene through synchronization of co 2 in the taylor dome dome c and dml ice cores high resolution records of atmospheric co2 concentration during the holocene are obtained from the dome concordia and dronning maud land antarctica ice cores these records confirm that the co2 concentration varied between 260 and 280 ppmv in the holocene as measured in the taylor dome ice core however there are differences in the co2 records most likely caused by mismatches in timescales matching the taylor dome timescale to the dome c timescale by synchronization of co2 indicates that the accumulation rate at taylor dome increased through the holocene by a factor two and bears little resemblance to the stable isotope record used as a proxy for temperature this result shows that different locations experienced substantially different accumulation changes and casts doubt on the often used assumption that accumulation rate scales with the saturation vapor pressure as a function of temperature at least for coastal locations d 2004 elsevier b v all rights reserved

tornado maintenance investigated with high resolution dual doppler and enkf analysis abstractdual doppler wind synthesis and ensemble kalman filter analyses produced by assimilating doppler on wheels velocity data collected in four tornadic supercells are examined in order to further understand the maintenance of tornadoes although tornado scale features are not resolved in these analyses larger scale processes involved with tornado maintenance are well represented the longest lived tornado is maintained underneath the midlevel updraft within a zone of low level horizontal convergence along a rear flank gust front for a considerable time and dissipates when horizontally displaced from the midlevel updraft the shortest lived tornado resides in a similar zone of low level convergence briefly but dissipates underneath the location of the midlevel updraft when the updraft becomes tilted and low level convergence is displaced several kilometers from the tornado this suggests that a location beneath the midlevel updraft is not always a sufficient condition for tornado maintenance particu

an investigation of the goshen county wyoming tornadic supercell of 5 june 2009 using enkf assimilation of mobile mesonet and radar observations collected during vortex2 part ii mesocyclone scale processes affecting tornado formation maintenance and decay abstractstorm scale and mesocyclone scale processes occurring contemporaneously with a tornado in the goshen county wyoming supercell observed on 5 june 2009 during the second verification of the origins of rotation in tornadoes experiment vortex2 are examined using ensemble analyses produced by assimilating mobile radar and in situ observations into a high resolution convection resolving model this paper focuses on understanding the evolution of the vertical structure of the storm the outflow buoyancy and processes affecting the vertical vorticity and circulation within the mesocyclone that correspond to changes in observed tornado intensity tornadogenesis occurs when the low level mesocyclone is least negatively buoyant relative to the environment possesses its largest circulation and is collocated with the largest azimuthally averaged convergence during the analysis period the average buoyancy circulation and convergence within the near surface mesocyclone on spatial scales resolved by the

observations of near surface vertical wind profiles and vertical momentum fluxes from vortex se 2017 comparisons to monin obukhov similarity theory abstractobservations of near surface vertical wind profiles and vertical momentum fluxes obtained from a doppler lidar and instrumented towers deployed during vortex se in the spring of 2017 are an

les of laminar flow in the pbl a potential problem for convective storm simulations abstractin idealized simulations of convective storms which are almost always run as large eddy simulations les the planetary boundary layers pbls are typically laminar i e they lack turbulent eddies when compared with turbulent simulations theory or simulations with pbl schemes the typically laminar les used in the severe storms community produce unrealistic near surface vertical wind profiles containing excessive vertical wind shear when the lower boundary condition is nonfree slip such simulations are potentially problematic given the recent interest within the severe storms community in the influence of friction on vorticity generation within tornadic storms simulations run as les that include surface friction but lack well resolved turbulent eddies thus probably overestimate friction s effects on storms

dating firn cores by vertical strain measurements we have developed a system for measuring a vertical strain rate profile in the firn on polar ice sheets using a readily available video camera to detect metal bands inserted in an air filled hole we used this system in 1995 and 1996 at taylor dome antarctica we use density measurements combined with our strain rates to infer vertical velocities from our velocities we calculate a steady state depth age scale for the firn at taylor dome the age of a visible ash layer from 79 1 m is 675 25 years this ash can be correlated with ash found at 97 2 m in a recent ice core at siple dome west antarctica

high resolution mobile doppler radar observations of cyclic mesocyclogenesis in a supercell abstract on 15 may 2003 two ground based mobile doppler radars scanned a supercell that moved through the texas panhandle and cyclically produced mesocyclones the two radars collected data from the storm during a rapid cyclic mesocyclogenesis stage and a more slowly evolving tornadic period a 3 cm wavelength radar scanned the supercell continuously for a short time after it was cyclic but close to the time of tornadogenesis a 5 cm wavelength radar scanned the supercell the entire time it exhibited cyclic behavior and for an additional 30 min after that the volumetric data obtained with the 5 cm wavelength radar allowed for the individual circulations to be analyzed at multiple levels in the supercell most of the circulations that eventually dissipated moved rearward with respect to storm motion and were located at distances progressively farther away from the region of rear flank outflow the circulations associated with a tornado did not move nearly as far rearward relative to the storm the mean

the influence of environmental low level shear and cold pools on tornadogenesis insights from idealized simulations abstractidealized dry simulations are used to investigate the roles of environmental vertical wind shear and baroclinic vorticity generation in the development of near surface vortices in supercell like pseudostorms a cyclonically rotating updraft is produced by a stationary cylindrical heat source imposed within a horizontally homogeneous environment containing streamwise vorticity once a nearly steady state is achieved a heat sink which emulates the effects of latent cooling associated with precipitation is activated on the northeastern flank of the updraft at low levels cool outflow emanating from the heat sink spreads beneath the updraft and leads to the development of near surface vertical vorticity via the baroclinic mechanism as has been diagnosed or inferred in actual supercells that have been simulated and observed an intense cyclonic vortex forms in the simulations in which the environmental low level wind shear is strong and the heat sink is of intermediate strength relative to the

annual layers in polar firn detected by borehole optical stratigraphy 1 we have developed a system called borehole optical stratigraphy which can detect annual layers in boreholes in polar firn borehole optical stratigraphy consists of a downhole camera package and post processing routines which allow us to measure the intensity of light emitted from an on board source and returned from the borehole wall we manually identify the annual layers in this optical signal we used this system at siple dome antarctica in the 2001 2002 austral summer season our results agree well with counts of annual layers detected with two different methods in an ice core recovered 50 meters from our borehole

a method to control the environmental wind profile in idealized simulations of deep convection with surface friction abstractin idealized horizontally homogeneous cloud model simulations of convective storms the action of surface friction can substantially modify the near ground environmental wind profile over

can a descending rain curtain in a supercell instigate tornadogenesis barotropically abstract this paper investigates whether the descending rain curtain associated with the hook echo of a supercell can instigate a tornado through a purely barotropic mechanism a simple numerical model of a mesocyclone is constructed in order to rule out other tornadogenesis mechanisms in the simulations the flow is axisymmetric and boussinesq with constant eddy viscosity in a neutrally stratified environment the domain is closed to avoid artificial decoupling of a vortex from the storm scale circulation in the principal simulation the initial condition is a balanced slowly decaying beltrami flow describing an updraft that is rotating cyclonically at midlevels around a low pressure center surrounded by a concentric downdraft that revolves cyclonically but has anticyclonic vorticity the boundary conditions are no slip on the tangential wind and free slip on the radial or vertical wind to accommodate this initial condition and to allow strong interaction of a vortex with the ground precipitation is