formation of magnesium smectite during lacustrine carbonates early diagenesis study case of the volcanic crater lake dziani dzaha mayotte indian ocean the volcanic crater lake of dziani dzaha in mayotte is studied to constrain the geochemical settings and the diagenetic processes at the origin of mg phyllosilicates associated with carbonate rocks the dziani dzaha is characterized by intense primary productivity volcanic gases bubbling in three locations and a volcanic catchment of phonolitic alkaline composition the lake water has an alkalinity of ca 0 2 mol l 1 and ph values of ca 9 3 cores of the lake sediments reaching up to one metre in length were collected and studied by means of carbon hydrogen nitrogen elemental analyzer x ray fluorescence spectrometry and x ray powder diffraction in surface sediments the content of total organic carbon reaches up to 20 weight the mineral content consists of aragonite and hydromagnesite with minor amounts of alkaline feldspar and clinopyroxene from the volcanic catchment below 30 cm depth x ray diffraction analyses of the 2 m clay fraction indicate the presence of a saponite like mineral a mg rich smectite the saponite like mineral accumulates at depth to reach up to ca 30 weight concurrent with a decrease of the contents of hydromagnesite and organic matter thermodynamic considerations and mineral assemblages suggest that the evolution of the sediment composition resulted from early diagenetic reactions the formation of the saponite like mineral instead of al free mg silicates resulted from high aluminum availability which is favoured in restricted lacustrine environments hosted in alkaline volcanic terrains commonly emplaced during early stages of continental rifting supersaturation of the lake water relative to saponite is especially due to high ph values themselves derived from high primary productivity this suggests that a genetic link may exist between saponite and the development of organic rich carbonate rocks which may be fuelled by the input of co2 rich volcanic gases this provides novel insights into the composition and formation of saponite rich deposits under a specific geodynamic context such as the cretaceous south atlantic carbonate reservoirs

revisiting azimuthally asymmetric moist instability in the outer core of sheared tropical cyclones abstractthis study revisits the characteristics and physical processes of the azimuthally asymmetric distribution of moist instability in the outer core of vertically sheared tropical cyclones tcs

observations of the distribution of ice in hurricanes abstract observations of the type and distribution 0 c isotherm in three atlantic hurricanes are presented supercooled drops graupel columns and aggregated snowflakes were observed the supercooled drops were found only in convective updrafts stronger than 5 m s 1 but not all updrafts 5 m s 1 contained appreciable liquid graupel was found in all updrafts at temperatures 2 c and small columns were sometimes found in downdrafts nonconvective rainbands contained 15 30 l 1 of snow composed of columns and what appeared to be large aggregates other stratiform regions contained 1 15 l 1 of medium and large aggregates columns were occasionally found there also but only within about 15 km of convection hurricane convection is almost completely glaciated at the 5 c level it is suggested that the ice particles observed at 6 0 km inside the convection result primarily from downward mixing on both sides of the eyewall updraft of ice formed in the convective areas at higher colder levels the ice in t

structure and evolution of hurricane claudette on 7 september 1991 from airborne doppler radar observations part ii thermodynamics abstract in part i the kinematic and precipitating fields of hurricane claudette have been analyzed using airborne doppler radar data collected on 7 september 1991 by the two national oceanic and atmospheric administration noaa wp 3d research aircraft evidence of an incipient eyewall replacement cycle and its influence on hurricane claudette circulation have been revealed through the evtd extended velocity track display method this study has been conducted for six successive analyses in a domain of 200 km 200 km 12 km domain from 1700 to 2200 utc a thermodynamic retrieval method is adapted here to the evtd geometry to deduce the temperature and pressure perturbation fields from the previously evtd derived wind fields the relation between the evolution of the circulation and the thermodynamic structure of hurricane claudette can now be studied the main feature deduced from this method is a positive temperature perturbation about 8 9 k warmer than the environment at the center of the storm

eastern pacific hurricanes jimena of 1991 and olivia of 1994 the effect of vertical shear on structure and intensity abstract shear is a key inhibitor of tropical cyclone intensification although its signature is readily recognized in satellite imagery and theoretical or modeling studies provide some insight detailed observations have been limited airborne radar and in situ observations in hurricanes jimena of 1991 and olivia of 1994 are a step toward better understanding each storm was observed on two consecutive days initially both had small eyes 16 18 km radius and maximum winds of 57 m s 1 over sea surface temperatures sst 28 c in easterly environmental shear jimena maintained constant intensity or weakened gradually for 2 days in 13 20 m s 1 easterly shear olivia intensified in 8 m s 1 shear on the first day overnight the shear diminished to reverse and became westerly on the second day olivia weakened as the shear increased to 15 m s 1 from the west the storm moved over cooler sst and became surrounded by dryer air as convection weakened and the outer rainbands ceased to be effective barriers

the role of vortical hot towers in the formation of tropical cyclone diana 1984 abstract a high resolution 3 km horizontal grid spacing near cloud resolving numerical simulation of the formation of hurricane diana 1984 is used to examine the contribution of deep convective processes to tropical cyclone formation this study is focused on the 3 km horizontal grid spacing simulation because this simulation was previously found to furnish an accurate forecast of the later stages of the observed storm life cycle the numerical simulation reveals the presence of vortical hot towers or cores of deep cumulonimbus convection possessing strong vertical vorticity that arise from buoyancy induced stretching of local absolute vertical vorticity in a vorticity rich prehurricane environment at near cloud resolving scales these vortical hot towers are the preferred mode of convection they are demonstrated to be the most important influence to the formation of the tropical storm via a two stage evolutionary process i preconditioning of the local environment via diabatic production of mul

the evolution of hurricane humberto 2001 abstractin 2001 the national oceanic and atmospheric administration and the national aeronautical and space administration marshaled their resources to sample hurricane humberto for 3 successive days during the fourth convection and moisture experiment camex 4 humberto developed from a tropical storm into a category 2 hurricane despite the deep layer vertical shear of the environmental horizontal wind vws increasing markedly on the second and third days of sampling as exhibited in earlier studies the eyewall convection developed an azimuthal wavenumber 1 n 1 asymmetry as the vws increased horizontal divergence and vertical stability within 100 km of the eye exhibited persistent relationships to the vws vector the warm core evolved in an unexpected way the warm anomaly was initially located in the lower troposphere and built upward as the storm intensified the maximum temperature anomaly remained in the lower troposphere on all 3 days while the development of the upper tropospheric warm anom

low wavenumber structure and evolution of the hurricane inner core observed by airborne dual doppler radar abstract the asymmetric dynamics of the hurricane inner core region is examined through a novel analysis of high temporal resolution three dimensional wind fields derived from airborne dual doppler radar seven consecutive composites of hurricane olivia s 1994 wind field with 30 min time resolution depict a weakening storm undergoing substantial structural changes the symmetric and asymmetric mechanisms involved in this transformation are considered separately to zeroth order the weakening of the primary circulation is consistent with the axisymmetric vortex spindown theory of eliassen and lystad for a neutrally stratified atmosphere vertical shear however increased dramatically during the observation period leading to a strong projection of the convection onto an azimuthal wavenumber 1 pattern oriented along the maximum vertical shear vector recent theoretical ideas elucidating the dynamics of vortices in vertical shear are used to help explain this asymmetry the role of asymmetric vorticity d

the effect of relative flow on the asymmetric structure in the interior of hurricanes abstract asymmetric structure of tropical cyclones simulated by the geophysical fluid dynamics laboratory high resolution triply nested movable mesh hurricane model was analyzed emphasis was placed on the quasi steady component of the asymmetric structure in the region of the eyewall it was found that the asymmetry was primarily caused by the relative wind that is the flow entering and leaving the storm region relative to the moving storm a set of idealized numerical experiments was first performed both with a constant and a variable coriolis parameter f and the addition of basic flows that were either constant or sheared with height analysis was then made for one case of hurricane gilbert 1988 to demonstrate that the quasi steady asymmetric structure analyzed in the idealized studies could be identified in this real data case vorticity analysis in the variable f experiment indicated that quasi steady asymmetries resulted in the eyewall region through the effect of vorticity advection due to

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

radiolarian palaeoecology and radiolarites is the present the key to the past abstract radiolarian productivity pulses and related radiolarite deposition are phenomena difficult to understand from an exclusively actualistic viewpoint evolutionary selection pressure among silica secreting marine plankton both radiolarians and diatoms has led toward more economic usage of rapidly shrinking nutrient resources including dissolved silica of the photic zone in the late cenozoic oceans and in particular a substantial modification of oceanic cycle by the diatom explosive radiation even if there is a proved link between biomineralization and dissolved silica loading among the phytoplankton only the relative independence of modern siliceous planktic biotas from the available silica pool reflects mainly their progressive physiological specialisation during evolutionary history oceanic chemistry and productivity as well as patterns of circulation upwelling have changed radically during the phanerozoic radiolarites apparently represent an anachronistic facies as exemplified by their long lived and ocean wide distribution in palaeo pacific and hitherto highlighted actualistic models of localized intra oceanic wind driven upwelling loci are of largely questionable applicability in addition to plate drift hypersiliceous domains and intervals are explainable mostly by a large scale volcano hydrothermal activity during major plate boundary reconfigurations which in many ways favoured siliceous biotas acme and their skeletal remains accumulation and preservation factors tied to rapid voluminous submarine eruptions such as thermal buoyant megaplumes and basin overturns offer a viable alternative for traditional climatic circulation scenarios in case of hypersiliceous high productivity events irrelevant to greenhouse to icehouse climatic change the evolving carbon and silica cycles were coupled through the greenhouse effect and enhanced chemical weathering volcano hydrothermal and tectonic uplift events related mostly to extensive rifting and or accelerated oceanic spreading were the endogenous driving force that created this perturbation of the exogenous system the present biogeochemical cycle is representative only for the overall silica depleted post eocene oceanic ecosystems which broadly correlates with a major expansion of diatoms groups extremely efficient in silica removal and closely linking the silica budget with phosphorus and nitrogen cycles thus an orthodox uniformitarian approach to biosiliceous sedimentation based on a silica starved vigorous ocean is of limited significance when applied to the pre neogene settings especially in the peculiar planktic habitats of epeiric seas as well as during biotic crises marked by strong geotectonic overprint the major turnovers in marine siliceous biota composition in particular after the end permian radiolarite gap may have been coupled with discernible changes in an increasing biological control on the long term oceanic silica cycling punctuated equilibrium the evolutionary turnovers have induced a stepdown decrease of dissolved silica levels through the phanerozoic contemporaneously with the general secular trend of upward scaling of nutrient related ecological processes and increased effectiveness of resource utilization

numerical simulations of an observed gravity current and gravity waves in an environment characterized by complex stratification and shear abstract numerical simulations of a gravity current in an environment characterized by complex stratification and vertical wind shear have been performed using a nonhydrostatic two dimensional dry primitive equation model data from one of the most complete documentations to date of gravity waves associated with a gravity current presented in an earlier study are used both to prescribe the gravity current s environment and for validation of the simulated gravity current and its associated gravity waves these comparisons indicate that the gravity current observed by a doppler wind profiler and sodars was well simulated in terms of depth density contrast and propagation speed and that the model produced a variety of gravity waves similar in many ways to these observed because uncertainties remained concerning the gravity wave generation mechanisms derived from the observations e g wavelengths were not observed the validated simulations are used to test these tentative hypotheses the simulatio

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

mesoscale and convective scale characteristics of mature hurricanes part i general observations by research aircraft abstract the analysis of instrumented aircraft data from four recent mature hurricanes reveals common mesoscale and convective scale features the structure and organization of the convection precipitation and wind flow are studied by combining the airborne observations using basic instrumentation with quantitative radar measurements three predominant precipitation features seen are 1 the eyewall 2 a stratiform rain region adjacent to and outside the eyewall and 3 rainbands that contain both convective and stratiform regions the eyewall is so named because it surrounds the rain free eye but several additional features distinguish it from other convective bands in the hurricane in particular the updraft tangential wind maximum and highest rainfall have preferred locations relative to each other with the updraft being radially inward from the rain and wind maxima the 10 dbz radar reflectivity contour indicative of the inner edge slopes substantially outward with height and there is evide

convective structure of hurricanes as revealed by lightning locations cloud to ground lightning flash locations were examined for nine atlantic basin hurricanes using data from the national lightning detection network a common radial distribution in ground flash density was evident a weak maximum in the eyewall region a clear minimum 80 100 km outside the eyewall and a strong maximum in the vicinity of outer rainbands 210 290 km radius these results are consistent with the authors previous study of hurricane andrew none of the storms showed this characteristic radial structure during prehurricane stages the results support the division of precipitation in the hurricane into three distinct regimes the eyewall is a unique phenomenon but shares some attributes with deep weakly electrified oceanic monsoonal convection the region outside the eyewall and under the central dense overcast has characteristics of the trailing stratiform region of mesoscale convective systems including a relatively high fraction of positive polarity flashes the outer bands with mean maximum flash density at the 250 km radius contain the vast majority of ground flashes in the storms eyewall lightning defined as that within 40 km of the center was examined for four moderate to strong hurricanes such lightning occurred episodically during hurricane stage with 93 of hourly intervals containing no detected flashes eyewall lightning outbreaks over water always occurred at the beginning of or during times of intensification but often were indicative of the imminent end of deepening it is proposed that the existence of such inner core lightning might reveal the presence of an eyewall cycle for the one storm with available aircraft reconnaissance data eyewall cycles were reliably identified by the occurrence of inner core lightning and inner core lightning appeared only during such cycles suggestions are made as to how eyewall flashes in existing hurricanes might be used to help predict hurricane intensity change

evolution of the structure of precipitation in hurricane allen 1980 abstract reflectivity data from the airborne radar systems on board the three noaa aircraft were gathered during six consecutive days in hurricane allen of 1980 the data have been used to specify the horizontal and vertical precipitation distribution within 111 km radius of the hurricane center the evolution of the structure and intensity of the precipitation in the storm is described from representative time composite radar maps for seven research flights made during the 6 day period the eyewall was characterized by a narrow ring 12 15 km wide of intense reflectivity 42 47 dbz surrounding the center of the storm at a radius that varied in time from 12 40 km the eyewall had steep radial gradients of reflectivity 4 5 db km 1 and tilted radially outward in height the rainbands were characterized by areas of enhanced reflectivity embedded in a region of stratiform rainfall that contained a distinct bright band at the height of the 0 c isotherm the most striking changes in structure during the 6 d

the relationship between storm motion vertical wind shear and convective asymmetries in tropical cyclones abstract the influence of the direction of storm motion on the azimuthal distribution of electrified convection in 35 atlantic basin tropical cyclones from 1985 to 1999 was examined using data from the national lightning detection network in the inner 100 km flashes most often occurred in the front half of storms with a preference for the right front quadrant in the outer rainbands r 100 300 km flashes occurred predominantly to the right of motion although the maximum remained in the right front quadrant the results are shown to be consistent with previous studies of asymmetries in rainfall radar reflectivity and vertical motion with respect to tropical cyclone motion the motion effect has been attributed to the influence of asymmetric friction in the tropical cyclone boundary layer the authors previously found a strong signature in the azimuthal distribution of lightning with respect to vertical wind shear because both effects show clearly vertical wind shear and storm motion must themse

surface wind fields of 1995 hurricanes erin opal luis marilyn and roxanne at landfall hurricanes erin opal luis marilyn and roxanne were the most destructive hurricanes of 1995 at landfall luis and marilyn contained maximum sustained winds marine exposure estimated at near 60 and 46 m s21 respectively the strongest landfalling storm of the 1995 season luis decreased in intensity from a category 4 to 3 on the saffir simpson scale shortly before the eyewall crossed the islands of antigua barbuda st kittsnevis st barthelemy st martin and anguilla hurricane marilyn strengthened as it approached the u s virgin islands with st thomas bearing the brunt of the north and south eyewall winds of 46 m s21 marine exposure and st croix being affected by the relatively weak western eyewall peak winds of 35 40 m s21 marine exposure for luis and marilyn only surface winds with marine exposures were analyzed because of unknown small scale interactions associated with complex island terrain with 500 1000 m elevations wind engineering studies suggest that wind acceleration over blunt ridges can increase or speed up winds by 20 80 topographic effects were evident in damage debris analyses and suggest that an operational method of assessing terrain induced wind gusts such as a scaled down mesoscale model is needed after landfall as a marginal hurricane over central florida hurricane erin regained strength over the gulf of mexico with a well defined radar reflectivity structure erin struck the florida panhandle near navarre beach with maximum sustained surface winds of 35 40 m s21 affecting the destin ft walton area hurricane opal made landfall in nearly the identical area as erin with maximum sustained surface winds of 40 45 m s21 having weakened from an intensity of nearly 60 m s21 only 10 h earlier opal was characterized by an asymmetric structure that was likely related to cold front interaction and an associated midlevel southwesterly jet roxanne struck cozumel mexico with sustained surface winds marine exposure of 46 s21 crossed the yucatan and meandered in the southwest gulf of mexico for several days while in the bay of campeche roxanne s large area of hurricane force winds disabled a vessel which lead to the drowning deaths of five oil industry workers high resolution wind records are critical to preserving an accurate extreme wind climatology required for assessment of realistic building code risks unfortunately power interruptions to automated surface observing stations on the u s virgin islands st croix st thomas and destin florida prevented complete wind records of the eyewall passages of marilyn and opal respectively

a numerical study of outer rainband formation in a sheared tropical cyclone abstractthe dynamical process of outer rainband formation in a sheared tropical cyclone tc is examined in this study using the fully compressible nonhydrostatic tc model after the easterly vertical wind shear of 10 m s 1 was imposed upon an intensifying strong tc an outer rainband characterized by a wavenumber 1 structure formed as a typical principal rainband downshear further analysis indicates that the outer rainband formation was closely connected to the activity of the inner rainband previously formed downshear moving radially outward the inner rainband tended to be filamented owing to the strong radial gradient of angular velocity as the inner rainband approached the outer boundary of the inner core convection in its middle and upwind segments reinvigorated and nascent convective cells formed upwind of the rainband caused mainly by the decreased filamentation and stabilization subsequently the rainband reorganized into a typical outer rainband three different scenarios are found to be

a climatology of hurricane eye formation abstractthis paper presents a climatology of the initial eye formations of a broad set of atlantic tropical cyclones tcs during 1989 2008 a new dataset of structure and intensity parameters is synthesized from the vortex data messages transmitted by routine aircraft reconnaissance using these data together with satellite imagery and other established datasets the times when each tc achieved various stages of eye development are tabulated to form the basis of the climatology about 60 of atlantic tcs form eyes most often aircraft observe the eye structure before it appears in ir satellite imagery eyes tend to form in high potential intensity environments characterized by high sea surface temperatures and low to moderate environmental vertical wind shear a notable discovery is that most 67 tcs that form eyes tend to do so within 48 h of the cyclone s reaching tropical storm strength this suggests the existence of an opportune time window during which a tc can readily form an eye from the leng

boundary layer structure and dynamics in outer hurricane rainbands part i mesoscale rainfall and kinematic structure abstract results of hurricane boundary layer experiments conducted in outer rainbands of hurricanes josephine 1984 and earl 1986 are presented comparisons of precipitation and kinematic structures in these storms and in hurricane floyd 1981 indicate that principal rainbands have common characteristic mesoscale and convective scale features in the boundary layer the two dimensional mesoscale structure suggests that these rainbands are made up of a linear aggregate of cellular reflectivity elements on the inner upshear side of the band and stratiform rain on the outer downshear side the bands are oriented perpendicular to the shear above the boundary layer and cells move downband at about 85 of the density weighted mean wind speed of the 0 2 6 km layer the boundary layer wind field is strongly influenced by the rainband with alongband and crossband wind maxima located on the outer side of the band axis and minima 4 8 km to the inner side maximum crossband convergence and cyclonic shear vor

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

observed inner core structural variability in hurricane dolly 2008 abstracthurricane dolly 2008 exhibited dramatic inner core structural variability during a 6 h rapid intensification and deepening event just prior to making landfall in southern texas at 1800 utc 23 july in particular the eyewall was highly asymmetric from 0634 1243 utc with azimuthal wavenumber m 4 7 patterns in the eyewall radar reflectivity and prominent mesovortex and polygonal eyewall signatures evidence is presented that the most likely cause of the high wavenumber asymmetries is a convectively modified form of barotropic instability of the thin eyewall potential vorticity ring the rapid intensification and deepening event occurred while dolly was in a favorable environment with weak deep layer vertical wind shear and warm sea surface temperatures however the environmental conditions were becoming less favorable during the period of rapid intensification therefore it is plausible that the internal vortex dynamics were dominant contributors to the rapid intensification and deepening

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

ferric saponite and serpentine in the nakhlite martian meteorites abstract transmission electron microscopy and fe k x ray absorption spectroscopy have been used to determine structure and ferric content of the secondary phase mineral assemblages in the nakhlite martian meteorites nwa 998 lafayette nakhla gv y 000593 y 000749 mil 03346 nwa 817 and nwa 5790 the secondary phases are a rapidly cooled metastable assemblage that has preserved mg and ca fractionation related to distance from the fluid source for most of the nakhlites though one nwa 5790 appears not to have experienced a fluid pathway all nine nakhlite samples have also been analysed with scanning electron microscopy electron probe micro analysis bright field high resolution transmission electron microscopy hrtem and selected area electron diffraction by measuring the energy position of the fe k xanes 1 s 3 d pre edge transition centroid we calculate the ferric content of the minerals within the nakhlite meteorites the crystalline phyllosilicates and amorphous silicate of the hydrothermal deposits filling the olivine fractures are found to have variable fe 3 fe values ranging from 0 4 to 0 9 in lafayette the central silicate gel parts of the veins are more ferric than the phyllosilicates around it showing that the fluid became increasingly oxidised the mesostasis of lafayette and nwa 817 also have phyllosilicate which have a higher ferric content than the olivine fracture deposits with fe 3 fe values of up to 1 0 further study via tem analyses reveal the lafayette and nwa 817 olivine phyllosilicates to have 2 1 t o t lattice structure with a the d 001 spacing of 0 96 nm whereas the lafayette mesostasis phyllosilicates have 1 1 t o structure with d 001 spacings of 0 7 nm based on our analyses the phyllosilicate found within the lafayette olivine fractures is trioctahedral ferric saponite ca 0 2 k 0 1 0 3 mg 2 6 fe 2 1 3 fe 3 1 7 mn 0 1 5 7 si 6 7 al iv 0 9 fe 3 0 4 8 0 o 20 oh 4 n h 2 o and that found in the mesostasis fractures is an fe serpentine ca 0 1 mg 0 7 fe 3 1 0 al vi 0 4 2 2 si 2 o 5 oh 4 with a ferric gel of similar composition in lafayette and found as fracture fills throughout the other nakhlites

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

buoyancy of convective vertical motions in the inner core of intense hurricanes part ii case studies abstract this is the second of two papers on the buoyancy of convective vertical motions in the inner core of intense hurricanes this paper uses extensive airborne radar dropwindsonde and flight level observations in hurricanes guillermo 1997 and georges 1998 to illustrate typical azimuthal distribution of buoyant convection and demonstrate that the low level eye can be an important source region for buoyant eyewall convection in both hurricanes eyewall vertical velocity and radar reflectivity are asymmetric and exhibit persistent relationships with the direction of the environmental vertical wind shear mesoscale vertical motions exhibit a wavenumber 1 structure with maximum ascent downshear and weak descent upshear the mesoscale reflectivity maxima are located left of shear buoyant eyewall updraft cores and transient convective scale reflectivity cells are predominantly downshear and left of shear most eyewall downdraft cores that transport significant mass downward are located upshear nega

on the initial development of asymmetric vertical motion and horizontal relative flow in a mature tropical cyclone embedded in environmental vertical shear in this paper the authors focus on the initial development of asymmetric vertical motion and horizontal relative flow in a mature tropical cyclone tc embedded in an environmental vertical shear the fully compressible nonhydrostatic tc model was used to perform a series of numerical experiments with a mature tc with different intensities embedded in shear with different magnitudes and different vertical profiles results show that the development of both the wavenumber 1 asymmetric vertical motion and horizontal relative flow for a tc embedded in vertical shear is quite sensitive to both the magnitude and the vertical profile of wind shear as well as the intensity of the tc itself diagnostic analysis based on the quasi balanced potential vorticity inversion indicates that the balanced dynamics can only explain a small portion of the asymmetric vertical motion and relative flow the unbalanced processes contribute predominantly to the developmentoftheasymmetricflowinthesimulations itisshownthattheeyewallofamaturetcplaysarole somewhat like a material cylinder embedded in an environmental flow with vertical shear the interaction between the environmental shear and the eyewall produces vertical gradient of convergence divergence of horizontal wind around the lateral edge of the eyewall this forces much stronger asymmetric vertical motion than the balanced processes do and drives significant horizontal relative divergent flow over the storm core which opposes vertical shear and reduces the vertical tilt of the storm axis in addition the budget analysis for the axisymmetric tangential wind demonstrates that the asymmetric flow plays a dominant role in weakening the storm top down

concentric eye walls secondary wind maxima and the evolution of the hurricane vortex abstract research aircraft observations in recent hurricanes support the model of shapiro and willoughby 1982 for the tropical cyclone s response to circularly symmetric convective heat sources convective rings in both nature and the numerical model the tangential wind commonly increases rapidly just inside the radius of maximum wind and decreases inside the eye near the central axis of the vortex thus both secondary outer wind maxima and eyewall wind maxima often contract as they intensify this response is independent of the horizontal spatial scale of the maximum an outer maximum is frequently observed to constrict about a pre existing eye and replace it this chain of events often coincides with a weakening or at least a pause in intensification of the vortex as a whole the concentric eye phenomenon is a common but by no means universal feature of tropical cyclones it is most frequently observed in intense highly symmetric systems

eye and eyewall traits as determined with the noaa wp 3d lower fuselage radar abstracteye and eyewall traits were ascertained for 209 images from 37 tropical cyclones tcs using the lower fuselage 5 6 cm radar aboard the two national oceanic and atmospheric administration wp 3ds these tcs were almost entirely from the atlantic basin and were sampled from 1997 to 2012 for the eye these traits included area maximum diameter and roundness for the eyewall the traits included area completeness maximum width maximum reflectivity value and location number of local reflectivity maxima and mean rain rate these variables were compared to tc intensity and motion characteristics from the best track dataset and environmental characteristics from the statistical hurricane intensity prediction scheme interrelationships between eyewall features revealed that eyewall reflectivity features became more homogeneous as eye and eyewall areas shrank and maximum reflectivity and rain rate increased as the eyewall became wider and more complete as the tc intensified the eye area decreased

the effects of vertical wind shear on the distribution of convection in tropical cyclones abstract the influence of vertical wind shear on the azimuthal distribution of cloud to ground lightning in tropical cyclones was examined using flash locations from the national lightning detection network the study covers 35 atlantic basin tropical cyclones from 1985 99 while they were over land and within 400 km of the coast over water a strong correlation was found between the azimuthal distribution of flashes and the direction of the vertical wind shear in the environment when the magnitude of the vertical shear exceeded 5 m s 1 more than 90 of flashes occurred downshear in both the storm core defined as the inner 100 km and the outer band region r 100 300 km a slight preference for downshear left occurred in the storm core and a strong preference for downshear right in the outer rainbands the results were valid both over land and water and for depression storm and hurricane stages it is argued that in convectively active tropical cyclones deep divergent circulations oppose the ver

buoyancy of convective vertical motions in the inner core of intense hurricanes part i general statistics abstract the buoyancy of hurricane convective vertical motions is studied using aircraft data from 175 radial legs collected in 14 intense hurricanes at four altitudes ranging from 1 5 to 5 5 km the data of each leg are initially filtered to separate convective scale features from background mesoscale structure convective vertical motion events called cores are identified using the criteria that the convective scale vertical velocity must exceed 1 0 m s 1 for at least 0 5 km a total of 620 updraft cores and 570 downdraft cores are included in the dataset total buoyancy is calculated from convective scale virtual potential temperature pressure and liquid water content using the mesoscale structure as the reference state core properties are summarized for the eyewall and rainband regions at each altitude characteristics of core average convective vertical velocity maximum convective vertical velocity and diameter are consistent with previous studies of hurricane convection most cores are superi

low level thermodynamic kinematic and reflectivity fields of hurricane guillermo 1997 during rapid intensification abstract from 0600 utc 2 august to 1200 utc 3 august hurricane guillermo 1997 deepened by 54 hpa over the eastern north pacific ocean easily exceeding the thresholds that define rapid intensification ri the noaa wp 3ds observed a portion of this ri with similar two aircraft missions on consecutive days the aircraft jettisoned 70 successful global positioning system gps dropwindsondes or gps sondes which reveal how conditions in the lower troposphere on the octant to quadrant scale evolved within 250 km of the eye reflectivity fields demonstrate that the deepening is correlated with a spiraling in of the northern eyewall that reduces the eye diameter by 10 km this behavior contrasts the more uniform contraction witnessed during eyewall replacement cycles mixing between the lower eye and eyewall as detailed by other investigators appears to have triggered the reduction in the eye diameter after ri the eyewall remains asymmetrical with the tallest echo tops and heaviest rain rates located o

dual aircraft investigation of the inner core of hurricane norbert part ii mesoscale distribution of ice particles abstract horizontal fields of cloud microphysical parameters vertical air motion and horizontal wind at the 6 km level in hurricane norbert 1984 were obtained by mapping and interpolating data collected on board a wp 3d aircraft along numerous flight tracks executed within the central region of the storm although the storm was characterized by a strong vortex of winds reaching peak values 50 m s 1 all around the storm the precipitation was concentrated on the southwest side of the storm a sloping eyewall was located within 20 to 30 km of the eye stratiform precipitation dominated the region outside the eyewall a 25 km wide band of maximum stratiform precipitation was centered 60 70 km southwest of the storm center the ice particles at flight level tended to be relatively large both in the eyewall and in the outer band of stratiform precipitation particles were smaller and more numerous 100 300 l 1 in the zone between the eyewall and outer stratiform band these particles occurred on the out

mesoscale and convective scale characteristics of mature hurricanes part ii inner core structure of hurricane allen 1980 abstract the eyewall structure of hurricane alien is examined from analyses of multiple aircraft data on two days 5 and 8 august 1980 these data sets are unique in that for the first time three instrumented aircraft executed coordinated radial penetrations of the eyewall at multiple levels the data collected on 5 august illustrate the persistence of various features on horizontal scales 10 km over several hours composite cross sections constructed from the 8 august data show similar structure although the eye diameter had decreased to less than half that of 5 august the convergence of air in the eyewall was highly two dimensional this convergence supported organized ascent that was along the inner edge of the high reflectivity region and displaced inward several kilometers from the radius of maximum wind rmw a mean eyewall updraft of 5 6 m s 1 is computed from integration of the two dimensional continuity equation embedded within the two dimensional eyewall were cores of high reflectivity t

an aem tem study of weathering and diagenesis abert lake oregon ii diagenetic modification of the sedimentary assemblage abstract this paper compares the mineralogy and chemistry of clay minerals in sediments from various depths and positions in abert lake and surrounding playa with those of the weathered materials entering the lake in order to reveal the nature and extent of post depositional mineralogical modification analytical electron microscope aem data from individual clay particles reveal that each sample is comprised of a highly inhomogeneous smectite assemblage the thin clay flakes commonly less than 10 nm wide display a complete range in octahedral sheet compositions from nearly dioctahedral to nearly trioctahedral the very abundant mg rich lake smectites with an estimated composition k 0 29 al 0 23 mg 2 16 fe 0 30 si 3 80 al 0 20 o 10 oh 2 are not formed by weathering this confirms the importance of diagenetic mg uptake lattice fringe imaging failed to reveal distinct brucite like or vermiculite like layers suggesting that interstratifications of this type are rare or absent siliceous coatings on clay particles identified by silica excess in smectite analyses seem to favor topotactic overgrowth of stevensite rather than addition of brucite like layers to the dioctahedral nuclei the growth of k stevensite dilutes the al content of the crystal and thus the increasing diagenetic modification reduces rather than supplements its illite component smectite compositions within individual samples were highly variable yet source related characteristics such as the abundance of fe rich smectite were apparent little evidence for systematic k or mg enrichment with depth was identified in samples from depths of down to 16 feet below the sediment water interface the most magnesian assemblages are associated both with weathering sources of mg rich smectite and playa environments subjected to repeated wetting and drying cycles thus the observations suggest that clay compositions primarily reflect changes in lake levels brine composition and source characteristics rather than time and depth compaction effects other diagenetic reactions in the sediment include recrystallization of na rich silica gel and diatom fragments abundant submicron sized untwinned euhedral crystals of k feldspar are interpreted to be authigenic in origin

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

temporal changes of the primary circulation in tropical cyclones abstract more than 900 radial profiles of in situ aircraft observations collected in 19 atlantic hurricanes and tropical storms over 13 years confirm that the usual mechanism of tropical cyclone intensification involves contracting maxima of the axisymmetric swirling wind radar shows that annuli of convective echoes accompany the wind maxima these features called convective rings exist and move inward because latent heat released in the rings leads to descent adiabatic warming and rapid isobaric height falls in the area they enclose the radial change in rate of isobaric height fall is concentrated at the inner edge of the wind maximum causing the gradient wind to increase there and the maximum to contract vigorous convection organized in rings invariably causes well defined inward moving wind maxima but when convection is weak the rings are also weak or even absent in this case the swirling wind may be nearly constant with radius and change slowly in time hurricanes that have a single vigor