rapid filamentation zones in intense tropical cyclones intense tropical cyclones often possess relatively little convection around their cores in radar composites this surrounding region is usually echo free or contains light stratiform precipitation while subsidence is typically quite pronounced in this region it is not the only mechanism suppressing convection another possible mechanism leading to weak echo moats is presented in this paper the basic idea is that the strain dominated flow surrounding an intense vortex core creates an unfavorable environment for sustained deep moist convection strain dominated regions of a tropical cyclone can be distinguished from rotationdominated regions by the sign of s 2 s 2 2 where s1 ux y and s2 x uy are the rates of strain and x uy is the relative vorticity within the radius of maximum tangential wind the flow tends to be rotation dominated 2 s 2 s 2 so that coherent structures such as mesovortices can survive for long periods of time outside the radius of maximum tangential wind the flow tends to be strain dominated s 2 s 2 2 resulting in filaments of anomalous vorticity in the regions of strain dominated flow the filamentation time is defined as fil 2 s 2 s 2 2 1 2 in a tropical cyclone an approximately 30 kmwide annular region can exist just outside the radius of maximum tangential wind where fil is less than 30 min and even as small as 5 min this region is defined as the rapid filamentation zone since the time scale for deep moist convective overturning is approximately 30 min deep convection can be significantly distorted and even suppressed in the rapid filamentation zone a nondivergent barotropic model illustrates the effects of rapid filamentation zones in category 1 5 hurricanes and demonstrates the evolution of such zones during binary vortex interaction and mesovortex formation from a thin annular ring of enhanced vorticity

cape in tropical cyclones abstractconvective available potential energy cape and the vertical distribution of buoyancy were calculated for more than 2000 dropsonde soundings collected by the noaa gulfstream iv aircraft calculations were done with and without the effects of condensate loading entrainment and the latent heat of fusion cape showed larger values downshear than upshear within 400 km of the center consistent with the observed variation of convective intensity the larger downshear cape arose from i higher surface specific humidity ii lower midtropospheric temperature and for entraining cape iii larger free tropospheric relative humidity reversible cape had only one half the magnitude of pseudoadiabatic cape as shown previously reversible cape with fusion closely resembled pseudoadiabatic cape without fusion entrainment had the most dramatic impact entraining cape was consistent with the observed radial distribution of convective intensity displaying the largest values downshear at inner radii with

multiscale structure and evolution of hurricane earl 2010 during rapid intensification abstractthe structure and evolution of hurricane earl 2010 during its rapid intensification as sampled by aircraft is studied here rapid intensification occurs in two stages during the early stage covering 24 h earl was a tropical storm experiencing moderate northeasterly shear with an asymmetric distribution of convection and the symmetric structure was shallow broad and diffuse the upper level circulation center was significantly displaced from the lower level circulation at the beginning of this stage deep vigorous convection termed convective bursts was located on the east side of the storm and appeared to play a role in positioning the upper level cyclonic circulation center above the low level center by the end of this stage the vortex was aligned and extended over a deep layer and rapid intensification began during the late stage rapid intensification continued as earl intensified 20 m s 1 during the next 24 h the vortex remained aligned in the presence of weaker vertical shear

platelet ice the southern ocean s hidden ice a review basal melt of ice shelves is not only an important part of antarctica s ice sheet mass budget but it is also the origin of one of the most peculiar types of sea ice found in the polar oceans platelet ice in many regions around coastal antarctica tiny ice crystals form and grow in supercooled plumes of ice shelf water releasing heat into the surrounding ocean they usually rise towards the surface eventually becoming trapped under an ice shelf as marine ice frequently masses of those crystals are advected out of the ice shelf cavity and accumulate below a solid sea ice cover to form a semiconsolidated layer when the overlying sea ice grows into this so called sub ice platelet layer the loose crystals are consolidated adding additional thickness to the sea ice these phenomena are generally referred to as platelet ice although confusion about the terminology is widespread in the literature the presence of platelet ice has a profound impact on sea ice properties and processes in several regions of antarctica with numerous implications for the local polar marine biosphere most notably sub ice platelet layers provide a stable sheltered nutrient and food rich habitat which usually results in a highly productive and uniquely adapted ecosystem it has also been hypothesised that platelet ice may be an indicator of the state of an ice shelf although comprehensive time series are limited to the ross sea this paper clears up the terminology by providing exact definitions of the relevant terms we review platelet ice formation observational methods as well as geographical and seasonal occurrence the physical properties and ecological implications are merged in a way understandable for physicists and biologists alike to lay the foundation for the interdisciplinary research that is necessary to tackle the current knowledge gaps

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

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

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

sheared deep vortical convection in pre depression hagupit during tcs08 1 airborne doppler radar observations from the recent tropical cyclone structure 2008 field campaign in the western north pacific reveal the presence of deep buoyant and vortical convective features within a vertically sheared westward moving pre depression disturbance that later developed into typhoon hagupit on two consecutive days the observations document tilted vertically coherent precipitation vorticity and updraft structures in response to the complex shearing flows impinging on and occurring within the disturbance near 18 north latitude the observations and analyses herein suggest that the low level circulation of the pre depression disturbance was enhanced by the coupling of the low level vorticity and convergence in these deep convective structures on the meso gamma scale consistent with recent idealized studies using cloud representing numerical weather prediction models further examination of these new observations is needed to quantify the relative role of these vortical convection features in the tropical cyclone spin up process

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

an investigation of low stress ice rheology on the ward hunt ice shelf deformation of the ward hunt ice shelf northwest territories canada can be used to study the rheology of ice in the low stress low strain rate regime 10 12s 1 finite element simulations are separately displayed to demonstrate what the effects of ice salinity ice temperature and sea ice back pressure have on the spreading rates of this ice shelf we conclude that in general these effects tend to obscure any differences between the commonly used non newtonian flow law and the newtonian flow law recently reinterpreted from field measurements the numerical simulations indicate that there are specific locations where useful field measurements could be carried out on the ice shelf

quadrant distribution of tropical cyclone inner core kinematics in relation to environmental shear abstractairborne doppler radar data collected in tropical cyclones by national oceanic and atmospheric administration wp 3d aircraft over an 8 yr period 2003 10 are used to statistically analyze the vertical structure of tropical cyclone eyewalls with reference to the deep layer shear convective evolution within the inner core conforms to patterns shown by previous studies convection initiates downshear right intensifies downshear left and weakens upshear analysis of the vertical distribution of radar reflectivity and vertical air motion indicates the development of upper level downdrafts in conjunction with strong convection downshear left and a maximum in frequency upshear left intense updrafts and downdrafts both conform to the shear asymmetry pattern while strong updrafts occur in the eyewall intense downdrafts show far more radial variability particularly in the upshear left quadrant though they concentrate along the eyewall edges strong updrafts are collocated with low level inflow and

calving and rifting on the mcmurdo ice shelf antarctica on 2 march 2016 several small en echelon tabular icebergs calved from the seaward front of the mcmurdo ice shelf and a previously inactive rift widened and propagated by 3 km 25 of its previous length setting the stage for the future calving of a 14 km2 iceberg within 24 h of these events all remaining land fast sea ice that had been stabilizing the ice shelf broke up the events were witnessed by time lapse cameras at nearby scott base and put into context using nearby seismic and automatic weather station data satellite imagery and subsequent ground observation although the exact trigger of calving and rifting cannot be identified definitively seismic records reveal superimposed sets of both long period 10 s sea swell propagating into mcmurdo sound from storm sources beyond antarctica and high energy locally sourced short period 10 s sea swell in the 4 days before the fast ice break up and associated ice shelf calving and rifting this suggests that sea swell should be studied further as a proximal cause of ice shelf calving and rifting if proven it suggests that ice shelf stability is tele connected with far field storm conditions at lower latitudes adding a global dimension to the physics of ice shelf break up

convective contribution to the genesis of hurricane ophelia 2005 abstract the convection occurring in the tropical depression that became hurricane ophelia 2005 was investigated just prior to tropical storm formation doppler radar showed a deep wide intense convective cell of a type that has been previously thought to occur in intensifying tropical depressions but has not heretofore been documented in detail the updraft of the cell was 10 km wide 17 km deep had updrafts of 10 20 m s 1 throughout its mid to upper levels and contained a cyclonic vorticity maximum the massive convective updraft was maintained by strong positive buoyancy which was maximum at about the 10 km level probably aided by latent heat of freezing evaporative cooling and precipitation drag occurred in the rain shower of the cell but were insufficient to produce a strong downdraft or gust front outflow to force the updraft the convective updraft was fed by a layer of strong inflow that was several kilometers deep wind induced turbulence just above the ocean surface enriched the equi

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

rheology of glacier ice a new method for calculating the stress field in bounded ice shelves is used to compare strain rate and deviatoric stress on the ross ice shelf antarctica the analysis shows that strain rate per second increases as the third power of deviatoric stress in newtons per square meter with a constant of proportionality equal to 2 3 x 10 25

effects of vertical wind shear and storm motion on tropical cyclone rainfall asymmetries deduced from trmm abstract vertical wind shear and storm motion are two of the most important factors contributing to rainfall asymmetries in tropical cyclones tcs global tc rainfall structure in terms of azimuthal distribution and asymmetries relative to storm motion has been previously described using the tropical rainfall measuring mission microwave imager rainfall estimates the mean tc rainfall distribution and the wavenumber 1 asymmetry vary with storm intensity and geographical location among the six oceanic basins this study uses a similar approach to investigate the relationship between the structure of tc rainfall and the environmental flow by computing the rainfall asymmetry relative to the vertical wind shear the environmental vertical wind shear is defined as the difference between the mean wind vectors of the 200 and 850 hpa levels over an outer region extending from the radius of 200 800 km around the storm center the wavenumber 1 maximum rainfall asymmetry is downshear left right in the northern

a vortical hot tower route to tropical cyclogenesis abstract a nonhydrostatic cloud model is used to examine the thermomechanics of tropical cyclogenesis under realistic meteorological conditions observations motivate the focus on the problem of how a midtropospheric cyclonic vortex a frequent by product of mesoscale convective systems during summertime conditions over tropical oceans may be transformed into a surface concentrated warm core tropical depression as a first step the vortex transformation is studied in the absence of vertical wind shear or zonal flow within the cyclonic vorticity rich environment of the mesoscale convective vortex mcv embryo the simulations demonstrate that small scale cumulonimbus towers possessing intense cyclonic vorticity in their cores vortical hot towers vhts emerge as the preferred coherent structures the vhts acquire their vertical vorticity through a combination of tilting of mcv horizontal vorticity and stretching of mcv and vht generated vertical vorticity horizontally localized and exhibiting conve

experimental tests of middle palaeolithic spear points using a calibrated crossbow abstract controlled experiments using a calibrated crossbow to thrust stone tipped spears into animal carcasses reveal a relationship between levallois point morphology and their performance as spear points a comparative study of morphometric variation of levallois points from levantine mousterian contexts supports the hypothesis that the demand for stone spear points was an important factor in levallois point production

rapidly intensifying hurricane guillermo 1997 part ii resilience in shear this paper examines the structure and evolution of a mature tropical cyclone in vertical wind shear vws using airborne doppler radar observations of hurricane guillermo 1997 in part i the modulation of eyewall convection via the rotation of vorticity asymmetries through the downshear left quadrant was documented during rapid intensification here the focus is on the relationship between vws vortex tilt and associated asymmetry within the tropical cyclone core region during two separate observation periods a method for estimating local vws and vortex tilt from radar datasets is further developed and the resulting verticalstructureanditsevolutionaresubjectedtostatisticalconfidencetests guillermowasahighlyresilient vortex evidenced by its small tilt magnitude relative to the horizontal scale of the vortex core the deep layer tilt was statistically significant oriented on average 608 left of shear large scale vorticity and thermal asymmetries oriented along the tilt direction support a response of guillermo to shear forcing that is consistent with balanced dynamics the time averaged vertical motion asymmetry within the eyewall exhibited maximum ascent values 408 left of the deep layer shear or in this case right of the deep layer tilt the observation based analysis of guillermo s interaction with vws confirms findings of recent theoretical and numerical studies and serves as the basis for a more comprehensive investigation of vws and tropical cyclone intensity change using a recently constructed multistorm database of doppler radar analyses

genesis of pre hurricane felix 2007 part i the role of the easterly wave critical layer abstract the formation of pre hurricane felix 2007 in a tropical easterly wave is examined in a two part study using the weather research and forecasting wrf model with a high resolution nested grid configuration that permits the representation of cloud system processes the simulation commences during the wave stage of the precursor african easterly wave disturbance here the simulated and observed developments are compared while in part ii of the study various large scale analyses physical parameterizations and initialization times are explored to document model sensitivities in this first part the authors focus on the wave vortex morphology its interaction with the adjacent intertropical convergence zone complex and the vorticity balance in the neighborhood of the developing storm analysis of the model simulation points to a bottom up development process within the wave critical layer and supports the three new hypotheses of tropical cyclone formation proposed recently by dunkerton montgome

the effects of basal melting on the present flow of the ross ice shelf antarctica on utilise un modele mixte aux elements finis et differences finies de l ecoulement et du transfert de chaleur avec deux hypotheses fusion nulle partout et fusion equilibrant tout epaississement a grande echelle

tropical cyclogenesis via convectively forced vortex rossby waves in a shallow water primitive equation model abstract this work examines further the problem of tropical cyclogenesis by convective generation of vertical vorticity within a preexisting cyclonic circulation whose initial maximum tangential wind is approximately 5 m s 1 this paper validates and extends recent work examining the suggested upscale cascade mechanism in a three dimensional quasigeostrophic framework using a simple shallow water primitive equation swpe numerical model and helps clarify certain aspects of the rossby adjustment problem on a nonresting basic state for finite amplitude nonaxisymmetric disturbances the swpe approach serves as a meaningful intermediate step between the quasigeostrophic and full physics frameworks and allows a simple investigation of the effects of unbalanced dynamics contributions of gravity waves and rossby numbers of order unity the authors compare quantitative results of the two models on the storm spinup time and magnitude for asymmetric initial conditions whose mass and wind field are out of balanc

tropical convection the effects of ambient vertical and horizontal vorticity we present a series of idealized numerical model experiments to investigate aspects of deep convection in tropical depressions including the effects of boundary layer wind structure on storm structure especially on vertical vorticity production and updraught splitting and the combined effects of horizontal and vertical shear on vertical vorticity production with and without background rotation in warm cored disturbances such as tropical depressions the vertical shear and horizontal vorticity change sign at some level near the top of the boundary layer so that unlike in the typical middle latitude supercell storm the tilting of horizontal vorticity by a convective updraught leads not only to dipole patterns of vertical vorticity but to a reversal also in sign of the updraught rotation with height this finding has implications for understanding the merger of convectively induced vorticity anomalies during vortex evolution ambient cyclonic horizontal shear and or cyclonic vertical vorticity favour amplification of the cyclonically rotating gyre of the dipole consistent with an earlier study storm splitting occurs in environments with pure horizontal shear as well as pure vertical shear but the morphology of splitting is different in both situations splitting is found to require a relatively unstable sounding and relatively strong wind shear

numerical models of the filchner ronne ice shelf an assessment of reinterpreted ice thickness distributions recent radio echo soundings of the filchner ronne ice shelf by the german antarctic expedition 1983 1984 suggest that previous ice thickness measurements may have misinterpreted an internal radio echo reflecting horizon as the true ice shelf bottom this and the analysis of total ice thickness from surface altimetry suggests that a previously defined thin ice region comprising approximately 1 5 of the total ice shelf area may be underlain by a thick layer of possibly saline ice one possible way to verify the existence of such a layer is by measurement of its influence on the ice shelf flow regime here we evaluate this influence by conducting finite element simulations of two alternative ice thickness configurations we conclude that flow differences are sufficiently large to allow verification of the possible saline basal ice layer provided that surface strain rate measurements are conducted in certain key areas

initial development and genesis of hurricane dolly 2008 abstract based on a successful cloud resolving simulation with the weather research and forecasting model this study examines key processes that led to the early development of hurricane dolly 2008 the initial development of dolly consisted of three stages i an initial burst of convection ii stratiform development dry intrusion and thermodynamic recovery and iii reinvigoration of moist convection and rapid intensification advanced diagnosis of the simulation including the use of vorticity budget analysis contour frequency analysis diagrams and two dimensional spectral decomposition and filtering suggests that the genesis of dolly is essentially a bottom up process the enhancement of the low level vorticity is mainly ascribed to the stretching effect which converges the ambient vorticity through stretching enhanced by moist convection in the rapid intensification stage smaller scale positive vorticity anomalies resulting from moist convection are wrapped into the storm center area u

hurricane vortex dynamics during atlantic extratropical transition abstract simulations of six atlantic hurricanes are diagnosed to understand the behavior of realistic vortices in varying environments during the process of extratropical transition et the simulations were performed in real time using the advanced research weather research and forecasting wrf model arw using a moving storm centered nest of either 4 or 1 33 km grid spacing the six simulations ranging from 45 to 96 h in length provide realistic evolution of asymmetric precipitation structures implying control by the synoptic scale primarily through the vertical wind shear the authors find that as expected the magnitude of the vortex tilt increases with increasing shear but it is not until the shear approaches 20 m s 1 that the total vortex circulation decreases furthermore the total vertical mass flux is proportional to the shear for shears less than about 20 25 m s 1 and therefore maximizes not in the tropical phase but rather during et this has important implications for predictin

tropical cyclone formation in a sheared environment a case study abstract the development of hurricane danny 1997 from depression to hurricane was examined using cloud to ground lightning data reconnaissance aircraft data and satellite imagery vertical wind shear between 850 and 200 hpa of 5 11 m s 1 produced persistent downshear convective outbreaks that became progressively more intense and closer to the center during the development early in the period the storm intensified steadily in the presence of this downshear convection during the last and most intense outbreak a second vortex appeared to develop within the convection evidence is presented that the new downshear vortex became the dominant vortex and absorbed the original based on these events it is hypothesized that the presence of moderate vertical wind shear accelerated the early development process equivalent potential temperature fields within 500 m of the surface were examined only well after the period of vortex interaction did the characteristic mature tropical cyclone radial profile of eq

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

formulation of ice shelf dynamic boundary conditions in terms of a coulomb rheology coastal boundaries where fast flowing ice shelves shear past stagnant grounded ice are typically riven with surface crevasses seawater filled basal crevasses and tidal strand cracks here we formulate a boundary condition describing stress transmission through these fractured boundaries in terms of the coulomb law as a result of this formulation agreement between finite element simulations of the ross ice shelf flow and field observations is improved over agreement obtained with formulations which do not account for ice failure our results additionally suggest that shear stress transmitted through ice shelf boundaries is lower than previously thought

asphaltum hafting and projectile point durability an experimental comparison of three hafting methods abstract the design of a projectile delivery system often plays a critical role in the durability and breakage patterns associated with spent projectile points this paper presents the results of an experimental project designed to examine projectile point durability and breakage patterns between three different hafting methods specifically we compare two asphaltum hafting techniques drawn from archaeological and ethnohistoric accounts from the central valley of california with a more stereotypical cross hatched sinew hafting system our results suggest a small yet statistically significant increase in durability among asphaltum hafted points opening the door to future research on these ethnohistorically documented hafting techniques

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

evaluation of tropical cyclone center identification methods in numerical models abstractidentifying the center of a tropical cyclone in a high resolution model simulation has a number of operational and research applications including constructing a track calculating azimuthal means and perturbations and diagnosing vortex tilt this study evaluated several tropical cyclone center identification methods in a high resolution weather research and forecasting wrf model simulation of a sheared intensifying asymmetric tropical cyclone the simulated tropical cyclone tc contained downshear convective cells and a mesovortex embedded in a broader tc vortex complicating the identification of the tc vortex center it is shown that unlike other methods the pressure centroid method consistently 1 placed the tc center within the region of weak storm relative wind 2 produced a smooth track 3 yielded a vortex tilt that varied smoothly in magnitude and direction and 4 was insensitive to changes in horizontal grid resolution based on these results the authors recommend using the pr

instability of the antarctic ross sea embayment as climate warms collapse of the antarctic ice sheet since the last glacial maximum 18 000 years ago is most pronounced in the ross sea embayment which is partly ice free during antarctic summers thereby breaching the o ring of ice shelves and sea ice surrounding antarctica that stabilizes the ice sheet the o ring may have vanished during early holocene 5000 to 3000 b c roman 1 to 400 a d and medieval 900 to 1300 a d warm periods and reappeared during the little ice age 1300 to 1900 a d we postulate further collapse in the embayment during the post 1900 warming may be forestalled because east antarctic outlet glaciers nail the ross ice shelf to the transantarctic mountains so it can resist the push from west antarctic ice streams our hypothesis is examined for byrd glacier and a static ice shelf using three modeling experiments having plastic viscous and viscoplastic solutions as more data and improved modeling became available observed crevasse patterns were not reproduced a new research study is needed to model a dynamic ross ice shelf with all its feeder ice streams outlet glaciers and ice calving dynamics in three dimensions over time to fully test our hypothesis the required model must allow accelerated calving if further warming melts sea ice and discerps the ice shelf calving must then successively pull the outlet glacier nails so collapse of the marine west antarctic ice sheet proceeds to completion

wound ballistics the prey specific implications of penetrating trauma injuries from osseous flaked stone and composite inset microblade projectiles during the pleistocene holocene transition alaska u s a abstract research in the field of wound ballistics has identified three major types of penetrating trauma injuries that will affect wound severity of a projectile point into hard or soft tissues puncture incised and lacerated in this study we report on dual ballistics experiments conducted to better understand the wounding mechanisms of three prehistoric projectile point classes made respectfully of polished bone bifacially flaked stone and composite antler inset with microblades each class of projectiles was launched into ballistics gelatin and into the carcass of a reindeer to explore the relative performance characteristics of each class in terms of tool durability and wound infliction our methods of evaluation included a detailed measurement of projectile attributes before and after penetration of both gelatin and carcass that were then compared using tip metrics penetration depth and total interior wound area our results strongly suggest that the wounding potential differed significantly between projectile point classes and in turn strongly influenced wound severity we suggest that point mechanics may implicate a prey specific hunting strategy and propose that such analyses can help us better understand prehistoric hunter gather behavior and technological variability

flow law for ice in polar ice sheets theories of glacier flow are based commonly on the assumption that ice is not a newtonian fluid but has a non linear stress dependent viscosity1 5 here we re examine the spreading of antarctic ice shelves and suggest that the data cannot define a unique flow law tilt measurements in four boreholes in both the arctic and antarctic seem to show that a linear flow law e a1 where a1 10 15 s 1 pa 1 at 28 c where e is the effective strain rate and the effective shear stress may be just as appropriate for describing the flow of polar ice sheets

the link between climate warming and break up of ice shelves in the antarctic peninsula a review of in situ and remote sensing data covering the ice shelves of the antarctic peninsula provides a series of characteristics closely associated with rapid shelf retreat deeply embayed ice fronts calving of myriad small elongate bergs in punctuated events increasing flow speed and the presence of melt ponds on the ice shelf surface in the vicinity of the break ups as climate has warmed in the antarctic peninsula region melt season duration and the extent of ponding have increased most break up events have occurred during longer melt seasons suggesting that meltwater itself not just warming is responsible regions that show melting without pond formation are relatively unchanged melt ponds thus appear to be a robust harbinger of ice shelf retreat we use these observations to guide a model of ice shelf flow and the effects of meltwater crevasses present in a region of surface ponding will likely fill to the brim with water we hypothesize building on weertman 1973 hughes 1983 and van der veen 1998 that crevasse propagation by meltwater is the main mechanism by which ice shelves weaken and retreat a thermodynamic finite element model is used to evaluate ice flow and the strain field and simple extensions of this model are used to investigate crack propagation by meltwater the model results support the hypothesis