dryline on 19 june 2002 during ihop part i airborne doppler and leandre ii analyses of the thin line structure and convection initiation the evolution and finescale structure of a dryline that initiated a line of thunderstorms is presented both the along line variability and mean vertical structure were examined using data collected by an airborne doppler radar and a water vapor differential absorption lidar dial the initiation of convection appeared to result from the diurnally induced easterly flow in the maritime air east of the dryline that typically develops late in the day this flow increased the low level convergence and allowed rising parcels of air to reach the level of free convection the along line variability was largely attributed to numerous misocyclones distorting the thin line of radar reflectivity by advecting dry moist air across the dryline south north of the misocyclone the misocyclones also influenced the location of the updrafts with most of the peak values positioned north of the circulations as a result these updrafts were fortuitously positioned in regions of high mixing ratio where the first convective cells initiated

brunhes matuyama paleomagnetism in three lava sections in iceland this paper reviews previous field research on the paleomagnetism of volcanics in iceland at localities whose age is presumed to span the brunhes matuyama chron boundary additional mapping and laboratory measurements are presented from three localities of late quaternary reverse to normal transitions at two tjornes in north iceland and eyjafjoll in south iceland the identity of the transition is confirmed by k ar dating at tjornes and at ingolfsfjall in southwest iceland short period variations of the paleomagnetic field are demonstrated to be useful in stratigraphic mapping

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

unusual seawater composition of the late cretaceous tethys imprinted in glauconite of narmada basin central indiaunusual seawater composition of the late cretaceous tethysu bansal et al a detailed investigation of a glauconite bed within the late cretaceous bryozoan limestone formation of the bagh group in central india as well as the study of existing records reveals the existence of a glauconitic sea along the margins of the palaeo tethys ocean during the late cretaceous epoch the authigenic green mineral formed abundantly on shallow seafloors unlike in its modern deep sea counterpart we present an integrated petrographical geochemical and mineralogical investigation of the glauconite within late cretaceous transgressive deposits to highlight its unique geochemistry with moderate fe 2 o 3 and high al 2 o 3 sio 2 mgo as well as k 2 o contents x ray diffractional parameters identify the evolved to high evolved nature of the glauconite while mossbauer spectroscopic study reveals the dominance of fe 3 compared to fe 2 in the atomic structure the rare earth elements ree pattern of glauconite reveals moderate light ree heavy ree lree hree fractionation and weak negative eu anomaly the ce anomaly of the glauconite indicates a sub oxic diagenetic condition we propose that late cretaceous glauconites formed within a shallow marine depositional setting across the tethyan belt because of enhanced supply of k si al fe mg cations through continental weathering under the extant greenhouse climate

the numerical simulation of non supercell tornadogenesis part i initiation and evolution of pretornadic misocyclone circulations along a dry outflow boundary abstract high resolution three dimensional simulations are used to study misocyclone initiation and development along the leading edge of an outflow boundary model conditions were designed such that this development could be simulated independent of moist processes the outflow boundary is allowed to propagate into a region of southerly low level flow which results in a vertical vortex sheet along the outflow s leading edge lobe and cleft instabilities follow and provide perturbations for the subsequent development of horizontal shearing instabilities these growing instabilities are the inaugural circulations of leading edge misocyclones with wavelengths ranging from 1 6 to 3 2 km the structure of these modeled misocyclones compares favorably to observed pretornadic misocyclones along outflow boundaries in northeast colorado vortex sheet dynamics are observed to exert substantial control over the structure of the evolving outflow leading edge the vertical vortex sheet passes through discrete evoluti

petrology chemistry and texture of modern river sands amazon river system key idea following the spirit of p d krynine the sands of the world s largest tropical river system are studied to reveal much about the general origin of sand and sandstone thin section and chemical study of 95 sand samples from the amazon river system in brazil peru ecuador and colombia a river system with great contrasts in climate and rock types reveals that sands derived from the andes are lithic arenites whereas sands derived from precambrian terrains are much richer in quartz as are sands in rivers draining tertiary molasse sand becomes more mature downstream along the main stem of the amazon solimoes maranon so that a sample analyzed at the mouth does not accurately reflect headwater composition dilution by quartz rich tributaries and quartz rich tertiary outcrops possible differential elimination of rock fragments and even possible weathering on the modern flood plain of the amazon contribute to enhanced maturity downstream we speculate that although data on modern large subarti

sawtooth variation in magnetic intensity profiles and delayed acquisition of magnetization in deep sea cores abstract recent records of normalised intensity of natural remanent magnetization nrm obtained from deep sea sediments document an asymmetrical saw tooth pattern associated with geomagnetic reversals this observation may reflect a fundamental aspect of the geodynamo time evolution or alternatively delay in acquisition of the nrm it is demonstrated here that a simple model in which a large fraction of the magnetic grains acquire nrm at deposition time while remaining grains reorientate or acquire magnetization after deposition is able to produce a saw tooth profile strikingly similar to that observed interestingly this mechanism of nrm acquisition does not damp out high frequency oscillations of the field such as those observed in the deep sea records exhibiting the saw tooth profile it provides an accurate record of field directions except for periods of very low field intensity

a database for the matuyama brunhes magnetic reversal abstract we present a paleomagnetic database designated mbd97 comprising sedimentary and lava data for the matuyama brunhes reversal transition field using criteria regarding suitability of the rock samples measurement methods and temporal resolution of the data we consider 62 different studies of the matuyama brunhes reversal of these only 11 satisfy our selection criteria four lava sites one loess site three shallow marine sedimentary sites and three deep ocean sedimentary sites we discuss our reasons for accepting or disguarding each candidate data set we investigate the distribution of transitional virtual geomagnetic poles vgps they fall along american and asian australian longitudes the spatial and temporal complexity of the data indicate that the transitional field was almost certainly nonaxisymmetric and nondipolar the directional transition of the matuyama brunhes reversal took 2300 5000 years during which time the surface intensity fell to 10 20 of its pre and post transitional value we do not find any systematic bias due to site distribution or inclination shallowing although the errors in the data do vary over the duration of the reversal mbd97 is a small database but it has a fairly good geographic distribution and is suitable for studies of the matuyama brunhes transition nonetheless it is clear that further analysis of transitions will benefit from a continued program of data collection and analysis the data in mbd97 are listed

geographical distribution of transitional vgps evidence for non zonal equatorial symmetry during the matuyama brunhes geomagnetic reversal abstract paleomagnetic records of the matuyama brunhes polarity reversal obtained recently from the northern equatorial and southern latitudes of the atlantic sector display a distinct symmetry with respect to the equator the virtual geomagnetic pole vgp paths from the mid latitude sites in the northern and southern hemispheres dsdp site 609 and core v16 58 are nearly coincident tracking northward through the americas the vgp path from the equatorial site odp site 664 tracks northward through eastern asia nearly antipodal to the other two paths modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records these results are consistent with the bulk of the data available from other records of this polarity transition suggesting that the transitional vgps during this reversal tend to fall along two distinct longitudinal bands the grouping of transitional vgps suggests that there is a strong geographical control over the reversal process an apparent correlation between the longitudinal bands of transitional vgps and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core mantle boundary 1 suggests that the processes which cause these flux concentrations may have influenced the geometry of the matuyama brunhes transitional field

a detailed chronology of the australasian impact event the brunhes matuyama geomagnetic polarity reversal and global climate change a mechanism had been recently proposed to show how an impact event can trigger a geomagnetic polarity reversal by means of rapid climate cooling we test the proposed mechanism by examining the record from two high sedimentation rate 8 11 cm kyr deep sea sediment cores odp sites 767 and 769 from marginal seas of the indonesian archipelago which record the australasian impact with well defined microtektite layers the brunhes matuyama polarity reversal with strong and stable remanent magnetizations and global climate with oxygen isotope variations in planktonic foraminifera both odp cores show the impact to have preceded the reversal of magnetic field directions by about 12 kyr both records indicate that the field intensity was increasing near the time of impact and that it continued to increase for about 4 kyr afterwards furthermore the oxygen isotope record available from sediments at odp site 769 shows no indication of discernible climate cooling following the impact the microtektite event occurred in the later part of glacial stage 20 and was followed by a smooth warming trend to interglacial stage 19 thus the detailed chronology does not support the previously proposed model which would predict that a decrease in geomagnetic field intensity resulted from a minor glaciation following the impact event we conclude that the evidence for a causal link between impacts and geomagnetic reversals remains insufficient to demonstrate a physical connection

arkose subarkose quartz sand and associated muds derived from felsic plutonic rocks in glacial to tropical humid climates abstract in the range of climates from high latitude glacial to equatorial tropical the mineral and chemical compositions of over 600 modern nonmarine first cycle sands and muds produced from felsic crystalline basement provenance in various tectonic settings were studied under glacial conditions sediments are developed by mostly mechanical weathering and thus are mineralogically and chemically similar to their source rocks humid tropical climates yield quartz rich sands kaolinite rich muds along with si mg ca na and k in solution between these extremes a wide range of detrital sediment compositions are developed in arid to wet climates in arid climate and semi arid climate sands feldspar is commonly more abundant than quartz and plagioclase k feldspar except in the cases of granite source rocks the muds are feldspathic with smectite dominant in the clays with increasing temperature and moisture in temperate to subtropical climates mafic silicates and feldspars are destroyed so that the sands range from arkoses with plagioclase as the major feldspar to subarkoses with dominant k feldspar through this range of climate muds become more kaolinitic and thus more aluminous with increasingly severe weathering typically feldspathic sands are associated with feldspathic muds with decreasing feldspar in sands there is less feldspar in muds with increasingly intense chemical weathering rare earth elements ree are more fractionated into muds typically sands contain 150 ppm ree with much of the ree in heavy minerals abundance differences are greatest between humid tropical sands and muds in many samples the ree are mostly in monazite allanite sphene and apatite more intensely weathered samples contain more xenotime and zircon evidenced by yb enrichment

the zonal harmonic model of polarity transitions a test using successive reversals a recently developed zonal model for the last geomagnetic field reversal which describes time and latitude dependent transitional behavior of intensity and inclination in terms of dominance of low order field harmonics was tested using a latitudinal and chronological succession of transition records the primary data were derived from a high resolution study of five matuyama to brunhes deep sea cores collected along a 40 meridional band in the north central pacific the transitions analyzed were the on sets and terminations of the olduvai and jaramillo events and the matuyama brunhes boundary supplementary data published by previous workers included a jaramillo onset record from the southern indian ocean and a transition recorded in the triassic chugwater formation of north america in a general sense the measured inclination and intensity records indicate the jaramillo transitions and the last reversal to be remarkably similar to each other and to differ from the olduvais crosscorrelation of the measured data sets with synthetic zonal harmonic records for the core latitudes indicates that the zonal harmonic model does predict the behavior of matuyama brunhes reversal in the northern hemisphere for the older reversals however the present model requires substantial modification this is particularly so in the case of the jaramillo onset although two inclination and intensity records of this transition from northern and southern hemisphere sediments strongly resemble each other the current model produces a synthetic record for one hemisphere that is incompatible with that hemisphere s measured data further to model zonally the olduvai reversals for which a wide latitudinal northern hemisphere sampling was available a substantially different ratio of low order multipole components from that of the standard model is required and the modeled solution was unsatisfactory at very low latitude a modified zonal harmonic approach which introduces a small sectoral component may offer a solution to the incompatibilities uncovered by the present study

longitudinally confined geomagnetic reversal paths from non dipolar transition fields it has long been thought that conditions at the boundary between the core and mantle influence the earth s magnetic field but the supporting evidence is rather indirect1 3 recent palaeomagnetic results suggesting that there are persistent preferred longitudinal paths for the virtual geomagnetic pole vgp during reversals4 would provide the first direct evidence of the solid mantle s influence on the core although their statistical significance has been disputed5 6 the results are potentially exciting because the preferred paths lie close to the pacific rim where the present geomagnetic secular variation changes character2 7 here we present a simple model based on an extension of a previous theory8 that produces reversals with vgp paths confined within relatively narrow longitude bands despite the transition field having a substantially non dipolar structure thus although longitude bias of the vgp paths is definitive evidence for core mantle interaction simple vgp paths are not evidence of near dipolar transition fields

dipolar reversal states of the geomagnetic field and core mantle dynamics palaeomagnetic records from lavas suggest that at least two specific inclined dipolar field configurations have dominated the reversal process for the past ten million years these long lived states provide a way to explain directional rebounds aborted reversals and the recording in sediments of what appear to be preferred longitudinal paths of the virtual geomagnetic pole the polar orientations correlate with near radial flux concentrations recognizable when today s field is stripped of its axial dipole and with lower mantle seismic anomalies suggesting a tie to deep earth dynamics

diagenetic evolution of a cretaceous quartz arenite narmada rift basin india abstract the nimar sandstone forms the basal unit of a siliciclastic carbonate sequence albian turonian cropping out in the narmada basin situated below the maastrichtian flood basalts deccan traps and above precambrian basement the sandstones are mostly moderately well sorted fine to medium grained quartzarenites of marginal marine origin the diagenetic history of the nimar sandstone comprises several events mechanical compaction due to depth of burial in the range 1000 3500 m reduced the porosity to an average 25 quartz overgrowths resulted from partial silica cementation uplift of the sediments preceding the outpouring of the flood basalts maastrichtian resulted in dissolution and leaching of feldspars by meteoric water formation of infiltrated clay and iron oxide that line the interparticle pore spaces and subsequent deposition of calcite cement in a freshwater phreatic zone diagenetic effects of grain dissolution have contributed to the highly quartzose nature of the sandstones during the maastrichtian thick basalts flows covered the sedimentary sequence giving rise to extensive silicification near the contact

the matuyama brunhes transition recorded from lake tecopa sediments california abstract the paleomagnetic record of the matuyama brunhes reversal in the sediment of ancient lake tecopa provided the first evidence that the geomagnetic field during transition is not dipolar we resampled the section to increase the time resolution of the record our new data show that the characteristic direction could not be completely isolated by af demagnetization the transitional directions obtained from thermal demagnetization are thus quite different from those of the previous study but the primary conclusion that the field was non dipolar remains the vgp paths lying close to the site longitude is consistent with an axisymmetric transition field

the effect of mesoscale heterogeneity on the genesis and structure of mesovortices within quasi linear convective systems abstract this study examines the structure and evolution of quasi linear convective systems qlcss within complex mesoscale environments convective outflows and other mesoscale features appear to affect the rotational characteristics and associated dynamics of these systems thus real data numerical simulations of two qlcs events have been performed to i identify and characterize the various ambient mesoscale features that modify the structure and evolution of simulated qlcss and then to ii determine the nature of interaction of such features with the systems with an emphasis on the genesis and evolution of low level mesovortices significant low level mesovortices develop in both simulated qlcss as a consequence of mechanisms internal to the system consistent with idealized numerical simulations of mesovortex bearing qlcss and not as an effect of system interaction with external heterogeneity however meso scale order of 10 km heterogeneity in the form of a convective outflow boundary is su

saw toothed pattern of relative paleointensity records and cumulative viscous remanence several studies of relative paleointensity data derived from marine sediments spanning the last 4 myr display an asymmetrical saw toothed pattern polarity reversals of the geomagnetic field are associated with low points in paleofield intensity preceded by a long term progressive decay of the field and followed by a rapid post transitional recovery 1 3 since similar behavior is observed in far flung sites whose rock magnetic records are different it is argued that the world wide saw toothing represents geomagnetic field behavior 3 we present an alternative explanation calling on the effect of hard viscous remanence

40ar 39ar dating of the brunhes matuyama geomagnetic field reversal magnetostratigraphic studies are widely used in conjunction with the geomagnetic polarity time scale gpts to date events in the range 0 to 5 million years ago a critical tie point on the gpts is the potassium argon age of the most recent brunhes matuyama geomagnetic field reversal astronomical values for the forcing frequencies observed in the oxygen isotope record in ocean drilling project site 677 suggest that the age of this last reversal is 780 ka thousand years ago whereas the potassium argon based estimate is 730 ka results from 4039 ar incremental heating studies on a series of lavas from maui that straddle the brunhes matuyama reversal give an age of 783 11 ka in agreement with the astronomically derived value the astronomically based technique appears to be a viable tool for dating young sedimentary sequences

a precursor to the matuyama brunhes transition field instability as recorded in pelagic sediments abstract the period some 20 25 kyr just prior to the most recent generally recognized geomagnetic field polarity transition the matuyama to brunhes reversal appears to be marked by significant geomagnetic variability manifested as pronounced oscillations in intensity we compare several previously published paleomagnetic records with new high resolution paleomagnetic data obtained from five pelagic marine sites north atlantic dsdp hole 609b equatorial atlantic odp hole 665a and western equatorial pacific odp holes 803b 804c and 805b using standard rock magnetic normalization for all of the samples as well as a thellier thellier method on the sediments of hole 804c we consistently find a decrease in paleointensity dip of 1 approximately 15 kyr prior to the matuyama to brunhes transition in the five new records as well as in the previously published records despite sedimentation accumulation rates sar that range from 11 cm kyr to 1 cm kyr these sequences yield paleointensity curves that are broadly similar in form even at the lowest sars the intensity of the pre reversal low dip1 appears to be of the same magnitude as that of the transition itself dip2 of 1 in some of the records a directional excursion to nearly full normal polarity accompanies dip1 and remains after alternating field af and or thermal demagnetization whereas in other records the directional changes vanish with demagnetization and appear to be caused by overprinting a viscous remanent magnetization vrm contribution to nrm was identified in two of the records and until removed by af or thermal demagnetization was found to blur the double dip nature of the paleointensity profiles into an apparent single dip and also resulted in an apparent but erroneous sawtooth like post transitional sudden increase in paleointensity after appropriate normalization the magnitude of the post transitional recovery was much reduced the magnetic directions of three of the new records after cleaning and adjusting the stable declinations to either 0 normal or 180 reverse map to vgp positions lying in the pacific the directional variations however are far less consistent than the intensity variations the confirmed global existence of this dip so closely preceding a major reversal invites questions about its relation to the reversal itself the apparent normal character of this interval can also present problems for magnetostratigraphical interpretations based on coarse or incomplete sampling by mimicking the brunhes matuyama reversal some 15 kyr earlier in the section than its true location

do virtual geomagnetic poles follow preferred paths during geomagnetic reversals virtual geomagnetic poles vgps recorded in sediments during reversals of the earth s magnetic field show an apparent preference for two antipodal sectors of longitude1 3 not only in records of the same reversal from different sites but also in records of different reversals if preferred bands really have persisted from one reversal to the next this would imply that the mantle exerts a significant control over the reversal process4 here we analyse the available database of reversal records from the past 12 myr using a statistical test specifically designed to test the hypothesis of two preferred antipodal longitudinal bands our analysis shows that the records taken as a group of independent observations do show an overall preference for two antipodal longitudinal bands however the site longitudes are also strongly grouped5 and a comparison of the transitional vgp longitudes with site longitudes shows an unlikely grouping under the hypothesis of a genuine geographical preference for transitional vgps we conclude that it is premature to accept the hypothesis of mantle control over the core during geomagnetic reversals

the 8 june 1995 mclean texas storm part ii cyclic tornado formation maintenance and dissipation abstract on 8 june 1995 scientists participating in the verification of the origins of rotation in tornadoes experiment vortex collected a unique dataset with the electra doppler radar eldora the eldora observations document the sequential life cycles of storm scale circulations associated with three large tornadoes in a supercell thunderstorm near mclean texas a qualitative description of the evolution of the storm was provided in part i of this paper during the first stage of development of each storm scale circulation interaction of the updraft with the environmental low level horizontal vorticity produced a vorticity column that increased in intensity with height as the vortex matured vorticity increased greatly at low levels i e below 2 km agl and exceeded that aloft each tornadic vortex was located near the rear side of the updraft where the surrounding low level horizontal vorticity was modified locally most likely by weak baroclinity within the storm tilting of low level horizo

single cycle cratonic quartzarenites produced by tropical weathering the nimar sandstone lower cretaceous narmada basin india abstract the world wide turonian marine transgression is represented by a thin a few tens to a few hundreds of metres thick mixed siliciclastic carbonate sequence deposited in the narmada basin presently located across the precambrian shield in central india the area was part of the interior of gondwana in the cretaceous when major changes were initiated leading to the break up of the supercontinent in the absence of any sedimentary record it is difficult to say what happened in the area between the precambrian and the early cretaceous the sandstone petrofacies of the cretaceous sequence of the narmada basin were studied with a view to understanding the nature of the basin and its tectonic setting during an important event which took place on a global scale the studied sandstones are generally quartzarenites composed mainly of several varieties of quartz and some mica rock fragments feldspar and heavy minerals derived from the underlying precambrian basement for the study of provenance of the sandstones in the light of plate tectonics the present study employed the qt f l and qm f lt triangular diagrams of dickinson on the qt f l plots 78 of the studied samples lie in the craton interior field and 22 of the samples lie in the recycled orogen field on the qm f lt plots most samples fall in the craton interior field 75 another 25 fall in the quartzose recycled field the texturally sub mature to immature locally derived quartz rich sandstones do no represent a typical stable platform succession they were derived from uplifted basement and deposited in the nearby narmada rift the ancient humid tropical climate and long residence time in the soil horizon produced quartz rich sands

sampling bias in vgp longitudes i derive probability densities for virtual geomag netic pole vgp longitudes for a general statistical model of local magnetic field variations i show that even for very simple statistically homogeneous models of secular varia tion the distribution of vgp longitudes is peaked 90 o away from the sampling longitude thus when sites are distri buted unevenly a non uniform overall distribution of vgp longitudes is to be expected analysis of the recent geomag netic field indicates that this bias can be significant particu laxly when random errors in paleomagnetic data are allowed for it is possible that some of the deviations from unifor mity seen in recent compilations of paleomagnetic reversal and secular variation data are a statistical artifact resulting from the distortion of the vgp transformation and the non uniformity of paleomagnetic sampling sites

correlation of paleointensity variation records in the brunhes matuyama polarity transition interval abstract paleomagnetic records of the 0 78 ma brunhes matuyama b m polarity reversal interval described from high resolution 8 11 cm kyr deep sea sediments from western equatorial pacific odp sites 767b and 769a show two successive and marked decreases in paleointensity of about 15 kyr wavelength each and a shift from full reversed to fuli normal polarity directions in only 2 kyr associated with the uppermost paleointensity decrease in contrast transition records from deep sea sediments with lower sedimentation rates tend to show simpler paleointensity variation with more complex patterns of directional change for example the b m transition record described from deep sea sediment core v16 58 from the southern indian ocean we suggest that the twin 10 be peaks reported in v16 58 correspond to increased production of this cosmogenic isotope associated with the double paleointensity dips recorded in odp 767b 769a the low resolution v16 58 paleomagnetic record can be accounted for by modeling of the geomagnetic field history in odp 767b at a nominal recording rate of 1 cm kyr and using an exponential lock in process filter with a half fixing depth of 16 cm the double paleointensity dips become a single broad magnetization low and the change in paleomagnetic directions is offset by about 16 cm below the actual level of the polarity transition similar to what we infer for v16 58 the low resultant magnetizations will be especially prone to overprinting which if in the present field will tend to give an americas vgp path the main features of the 10 be profile will be unaffected by the remanence lock in process and consequently retain their usefulness for correlation as a proxy of geomagnetic dipole intensity variation

age and duration of the matuyama brunhes geomagnetic polarity reversal from 40ar39ar incremental heating analyses of lavas abstract constraints on the timing of geomagnetic polarity reversals have come mainly from k ar or more recently 40 ar 39 ar age determinations of lavas or their k rich phenocrysts that erupted prior or subsequent to particular geomagnetic events we have obtained 40 ar 39 ar isochron ages using incremental heating techniques on groundmass separates phenocryst poor whole rock samples or plagioclase from eight basaltic to andesitic lavas that erupted during the matuyama brunhes m b polarity transition at four geographically dispersed sites these eight lavas range from 784 6 7 1 ka to 770 8 5 2 ka 1 errors the weighted mean 778 7 1 9 ka gives a high precision age that is remarkably consistent with revised astronomical age estimates for the m b polarity transition 6 12 13 despite uncertainties in absolute calibration of 40 ar 39 ar ages relative to mineral standards used as neutron fluence monitors our age determinations are consistent with five other 40 ar 39 ar studies focused on the m b transition these results confirm that the earlier k ar based geomagnetic polarity time scale underestimated the age of the m b reversal by about 6 none of the eight isochron ages are distinguishable from one another at the 95 confidence level however we are tantalizingly close to testing for the duration of this reversal one lava at the base of a sequence of transitionally magnetized flows in chile and the uppermost lava in a similar sequence on maui are only just indistinguishable in age at the 95 confidence level and preserve different magnetic orientations we suggest that the 12 kyr difference in age represents an upper limit for the duration of the reversal and is similar to the period of low magnetic field intensity associated with records of the m b reversal from deep sea sediment cores together with the short duration 2 kyr of the directional reversal observed in several different marine sediment sections our data suggest that reversal of the field s direction could have occurred at slightly different times depending on the position of the recording site

a case study of nonmesocyclone tornado development in northeast colorado similarities to waterspout formation abstract the evolution of the 26 july 1985 erie colorado tornado is described using data from ncar s cp 2 doppler radar this tornado develops within 20 km of the radar site under weakly forced synoptic conditions and weak tropospheric flow and is not accompanied by a mesocyclone the initial circulation forms near the surface at the intersection of two mesoscale boundaries and develops vertically intensifying into an fl tornado when it becomes collocated with the intense updrafts of a rapidly developing cumulonimbus this tornado appears to be the land equivalent of a waterspout and comparisons between the two vortices are made it is speculated that florida and portions of the western high plains may be prone to nonmesocyclone tornado development and that vortex intensification processes associated with nonmesoscyclone tornadoes may be important in mesocyclone tornadogenesis suggestions on how to better forecast these tornadoes are also presented

the evolution of low level rotation in the 29 may 1994 newcastle graham texas storm complex during vortex abstract this paper reports the results of an analysis of airflow evolution in the tornadic newcastle graham texas storm complex of 29 may 1994 a series of seven pseudo dual doppler analyses from 2242 to 2315 are performed from tail radar observations by the national oceanic and atmospheric administration p 3 aircraft subjective analyses of quasi horizontal single doppler radar observations provide a detailed look at structure and evolution of the hook echo and the low level newcastle mesocyclone special emphasis is placed on the evolution of low level i e below 1 km above ground level agl rotation of the parent mesoscale circulation of the newcastle tornado and the origins of mesoscale rotation preceding tornadogenesis the structure and evolution of the newcastle and graham mesocyclones are compared and contrasted the airborne doppler analyses reveal that the tornadic newcastle cell had supercell characteristics and that the newcastle storm circulation could be classified as a mesocyclone ba

absence of preferred longitude sectors for poles from volcanic records of geomagnetic reversals according to a recent compilation of sedimentary records from the past 12 myr the reversing geomagnetic field displays a marked long term longitudinal organization which seems to be correlated with the thermal structure of the lower mantle1 however the crucial palaeomagnetic observation that intermediate virtual geomagnetic poles are preferentially confined to longitudes over the americas or antipodal to this sector2 3 might be an artefact caused by distortion and smoothing of the geomagnetic signal by sediments4 6 here we present a compilation of 400 intermediate poles from 121 volcanic records of excursions and reversals less than 16 myr old sampled at various longitudes and latitudes which does not confirm previous inferences1 and reinforces doubts4 6 about the ability of sediments to properly record a rapidly varying field in contrast to virtual poles from excursions those from reversals are uniformly distributed in longitude which indicates that the reversing field is statistically axisymmetrical thus we conclude that there is no evidence for control of transitional fields by the temperature distribution in the lowermost mantle

the matuyama brunhes reversal at tecopa basin southeastern california revisited again abstract numerous records of the matuyama brunhes geomagnetic transition have been obtained from paleomagnetic studies because few of the reversal records are of acceptable reliability however the exact behavior of the field during the transition has remained enigmatic to provide confirmation of one of the more reliable records we have re examined the transition at two sites 150 m apart in lake sediments of tecopa basin southeastern california the two sites are geographically very close to that of valet et al 10 who previously obtained a record from that site indicating that the transitional field was non dipolar and axisymmetric the matuyama brunhes reversal is recorded differently at each of our two sites and at that of valet et al 10 zones of mixed polarities and or intermediate directions occur at all three sites but they differ greatly in polarity character thickness and stratigraphic position it appears that all three sites have provided mutually contradictory records of the transition it is unlikely therefore that any of the records is acceptable for establishing the nature of the transition at this locality obliteration of the transition is apparently the result of acquisition of a stable normal polarity overprint that appears to consist of two remanence components one acquired during post depositional compaction and dewatering and one during later sediment diagenesis

matuyama brunhes transition recorded in lava flows of the chilean andes evidence for dipolar fields during reversals the transitional behavior exhibited by earth9s magnetic field during polarity reversals holds the key to understanding the complexities of the field interpretation of the severely limited reversal data set however remains controversial discussion centers around the nature of pole paths during transitions and their possible relation to other geophysical phenomena paleomagnetic data from volcan tatara san pedro in the central chilean andes lat 36 s long 71 w provide the first record of the matuyama brunhes reversal from south america and only the third transitional record from the southern hemisphere a stratigraphic section of ten lava flows yielded intermediate pole positions that center in australia the mean pole is 16 8 s 133 0 e k ar analyses of two of these flows provided dates of 768 8 ka and 763 14 ka ages coincident with the established matuyama brunhes boundary the clustering of these transitional poles strengthens the hypothesis that earth9s magnetic field retains a strong dipole component during reversals

the matuyama brunhes and upper jaramillo transitions recorded in a loess section at weinan north central china abstract the paleomagnetic records of the matuyama brunhes mb and upper jaramillo uj polarity reversals have been obtained from a loess section at weinan shaanxi province in china an investigation of the rock magnetic properties using stepwise irm acquisition hysteresis loops thermomagnetic analysis and x ray diffraction on magnetic extracts identifies pseudo single domain magnetite as the carrier of the main fraction of the remanence associated with maghemite and hematite the total duration of the directional change based upon independent loess accumulation rates is about 5000 yr for the mb reversal and about 3200 yr for the uj reversal these values are consistent with previous estimates obtained from marine or lacustrine sediments apart from a few intermediate points observed on north america mb and in the pacific uj the transitional virtual geomagnetic pole vgp paths are situated in two longitudinal bands over eastern asia and australia mb and the americas uj the bulk of the uj record and the part of the mb record over the americas are not consistent with the hypothesis of a record arising from smoothing of pre and post transitional directions 1 a reappraisal of the statistical approach of mcfadden et al 2 shows that the longitudinal distribution of the records of reversals is consistent with the hypothesis of genuine geographically preferred sectors for the paths independent of site longitude with a probability higher than 0 45 when the two weinan records are considered thus these results obtained from wind blown loess sustain the hypothesis of two preferential longitudinal bands for reversing transitional vgps suggested from the study of marine and lacustrine sediments

palaeomagnetic location of the jaramillo subchron and the matuyama brunhes transition in the castlecliffian stratotype section wanganui basin new zealand abstract magnetostratigraphy is described for the lower part of the castlecliff section in wanganui basin new zealand based on analysis of multiple samples from 37 horizons the castlecliff section is the stratotype of the middle pleistocene castlecliffian stage and comprises eleven unconformity bound sequences which accumulated at inner shelf depths during interglacials a range of rock magnetic experiments undertaken on representative lithologies within the section show that the bulk of the remanence in all specimens is carried by ferrimagnetic mineral close to magnetite in composition in the stable single domain or pseudo single domain size range however at some sites post depositional chemical changes appear to have produced small amounts of another ferrimagnetic mineral chiefly within the superparamagnetic single domain size range resulting in hard secondary components of magnetization nevertheless at most sites the primary component of remanence was clearly identified by stepwise demagnetization four magnetozones are identified and these can be correlated to the magnetic polarity time scale using existing biostratigraphic constraints the section sampled spans the interval from below the jaramillo subchron to above the matuyama brunhes boundary the matuyama brunhes transition extends over 10 5 m of section within the upper westmere siltstone virtual geomagnetic pole positions describe an equatorial excursion in an otherwise far sided transition path these represent the first published data on the occurrence of the matuyama brunhes boundary on land in new zealand

the numerical simulation of nonsupercell tornadogenesis part ii evolution of a family of tornadoes along a weak outflow boundary abstract nonsupercell tornadogenesis along a weak outflow boundary has been simulated using a three dimensional moist convective cloud model thermodynamic conditions similar to those observed for nonsupercell tornado nst events of the high plains were utilized in the model initialization as the ensemble system of storm outflow boundary and leading edge vortices evolve six distinct life cycle stages for the development and decay of nsts are documented that span a period of about 35 min consistent with the results of part i of this numerical study vortex sheet dynamics exert considerable control over the outflow leading edge the progression of pretornadic life cycle stages serves to concentrate vertical vorticity effectively along the outflow boundary in discrete misocyclone circulations aligned in a 3 km wavelength pattern the organization of larger scale misocyclones and ultimate intensification to initial tornadic intensity occurs coincident with the rapid development of deep convection overhe

magnetostratigraphic lithostratigraphic and tephrostratigraphic constraints on lower and middle pleistocene sea level changes wanganui basin new zealand wanganui basin north island new zealand contains a complex sedimentary record of lower and middle pleistocene sea level changes palaeomagnetic results allow identification of the matuyama brunhes transition the jaramillo subchron and the cobb mountain subchron correlations of rhyolitic tuff horizons across the basin are consistent with magnetostratigraphic correlations between sections isothermal plateau fission track itpft ages of 1 05 0 05 ma and 1 63 0 15 ma on two tuffs potaka pumice and pakihikura pumice respectively are consistent with the interpreted magnetostratigraphy and also with the astronomically tuned timescale of odp site 677 magnetostratigraphy and itpft ages allow correlation of sedimentary cycles at wanganui with odd numbered oxygen isotope stages 17 31 in deep sea cores the stratotype section for the new zealand castlecliffian stage is shown to be incomplete relative to other studied sections in the basin below stage 31 the character of the cyclothems changes at wanganui from marine dominated to a greater representation of non marine and estuarine strata this change may be partly related to a change in amplitude and frequency of climatic cycles identified in oxygen isotope records but the primary cause is likely to be increased basinal uplift

detailed paleomagnetic records during the brunhes matuyama geomagnetic reversal and a direct determination of depth lag for magnetization in marine sediments abstract detailed paleomagnetic records were studied from homoclinal bathyal siltstone deposited during the brunhes matuyama geomagnetic reversal outcropping in the boso peninsula central japan the records are based on three continuous sediment cores from three sections within a distance of 14 km the stratigraphic horizons were correlated using a volcanic ash bed which occurs about 2 m above the brunhes matuyama reversal the sedimentation rate according to the oxygen isotope stratigraphy for the three sections decreases eastward and is 3 70 2 20 and 1 83 m ka 1 the depth lag of magnetization in the sediments was calculated directly as 42 cm based on the differences in the relative position of the key ash layer in the correlated virtual geomagnetic pole vgp paths for the three sections vgp movement during the geomagnetic reversal is characterized by short intervals of very rapid motion between periods of relative stability the last swing of the vgp from the southern hemisphere to the northern hemisphere in the brunhes matuyama geomagnetic reversal took only 38 yr intensities of the remanent magnetization normalized by anhysteretic remanent magnetization indicate that the intensity of the geomagnetic field decreased to one fifth for about 5 4 ka including the geomagnetic reversal the movement of the vgp longitude is 0 36 yr 1 westward and this westward drift can be related to the rotation of the non dipole field

inclination shallowing and preferred transitional vgp paths transitional vgp paths recorded in sediments cluster into two antipodal preferred longitude bands that tend to lie 90 away from their site longitudes the latter also being clustered vgp paths obtained from lava flow sequences though much fewer appear not to show these biases suggesting a rock magnetic influence on vgp paths recorded in sediments inclination shallowing of detrital magnetic remanence enhanced under low transitional field strengths is the most likely candidate we illustrate the effects of inclination shallowing by applying a simple shallowing model tan ir f tan ia where ia is the inclination of the magnetic remanence and ia is the inclination of the ambient field with field variation to hypothetical data sets shallowing induced clustering increases as f decreases and becomes extreme as f approaches 0 1 we have used the model to de shallow the available set of transitional vgp sediment records for various values of f the probability that the observations arise from inclination shallowing of a uniform random distribution of paths increases as f decreases when f drops to 0 13 there is a 50 chance of getting at least as much grouping as observed to decide if inclination shallowing is a dominant factor in the clustering we need to know whether such extreme shallowing is widespread in sedimentary records under transitional field conditions field and laboratory redeposition data are not yet adequate to resolve this question

sedimentary evolution and drowning of a passive margin shelf giumal group zanskar tethys himalaya india palaeoenvironmental changes during final break up of gondwanaland

the structure and evolution of a numerically simulated high precipitation supercell thunderstorm the structure and evolution of a high precipitation hp supercell thunderstorm is investigated using a threedimensional nonhydrostatic cloud scale numerical model tass the model is initialized with a sounding taken from a mesoscale modeling study of the environment that produced the 28 november 1988 raleigh tornadic thunderstorm tass produces a long lived convective system that compares favorably with the observed raleigh tornadic thunderstorm the simulated storm evolves from a multicell type storm to a multiple updraft supercell storm the storm complex resembles a hybrid multicell supercell thunderstorm and is consistent with the conceptual model of cool season strong dynamic hp supercells that are characterized by shallow mesocyclones the origin of rotation in this type of storm is often in the lowest levels interactions between various cells in the simulated convective system are responsible for the transition to a supercellular structure an intense low level updraft core forms on the southwest flank of the simulated storm and moves over a region that is rich in vertical vorticity the stretching of this preexisting vertical vorticity in the storm s lowest levels is the most important vertical vorticity production mechanism during the initial stages of the main updraft s development interactions with an extensive cold pool created by the storm complex are also important in producing vertical vorticity as the main updraft grows overall the development of vorticity associated with the main updraft appears similar to nonsupercellular tornadic storms however classic supercell signatures are seen early in the simulation associated with other updrafts e g formation of vortex couplet due to tilting of ambient horizontal vorticity storm splitting etc and are deemed important in the storm s supercell stage rotation is sustained in the lowest levels of the storm despite large amounts of precipitation located near and within the main mesocyclone pulsating downdrafts periodically invigorate the storm and the gust front never occludes thus allowing the main updraft to persist for a prolonged period of time the storm s intensity is also maintained by frequent updraft mergers

the numerical simulation of nonsupercell tornadogenesis part iii parameter tests investigating the role of cape vortex sheet strength and boundary layer vertical shear abstract nonsupercell tornadogenesis has been investigated in a three part numerical study building on the results of parts i and ii part iii addresses the sensitivity of nonsupercell tornadogenesis to variations in convective available potential energy cape outflow boundary vortex sheet strength and boundary layer vertical shear a three dimensional nonhydrostatic quasi compressible convective cloud model has been employed to examine nonsupercell tornado nst development in an environment typical of the colorado high plains a strong relationship was shown to exist between the magnitude of the environmental cape and the structure and intensity of the misocyclones and nonsupercell tornadoes that developed as cape was increased from 0 to 1700 j kg 1 the simulated vortices markedly contracted and intensified multiple cape thresholds were identified that yielded markedly different vortex intensity the highest cape runs produced nst families with peak ground relative surface winds of 47 m s 1 v

persistent patterns in the geomagnetic field over the past 2 5 myr historical geomagnetic measurements covering the past 400 years reveal a symmetrical pattern of four relatively stationary flux concentrations lobes at the surface of the liquid core and regions of rapid change extending from the atlantic to the indian ocean1 palaeomagnetic data define a time average over several thousand years2 which might reflect the stationary parts of the present field but unfortunately the historical record is too short to provide a satisfactory average3 here we model palaeomagnetic directions from the past 2 5 myr using the same methods as for modern data4 and find the two northern lobes in the same position as today over arctic canada and siberia in southern regions by contrast the field appears to have been smoothed out as might be expected from the current rapid secular variation1 we propose that the present geomagnetic field morphology and pattern of secular variation have persisted for several million years as would occur if the solid mantle controls flow at the top of the core

palaeomagnetic constraints on the geometry of the geomagnetic field during reversals palaeomagnetic records of the path of the pole during reversals of the earth s magnetic field provide a test of the hypothesis that dipolar or low order axisymmetric components of the field dominate during reversals multiple records of reversals during the past 12 myr show no simple or consistent geographical pattern although a more robust analysis of the transitional field awaits a greater number of well distributed sampling sites the present data are not inconsistent with the simplest models in which a field reminiscent of the non dipole component of the present day field becomes dominant

kinematic observations of misocyclones along boundaries during ihop during the international h2o project mobile radars collected high resolution data of several 0 5 2 kmwide vertically oriented vortices or misocyclones along at least five mesoscale airmass boundaries this study analyzes the properties of the misocyclones in three of these datasets 3 10 and 19 june 2002 to verify findings from finescale numerical models and other past observations of misocyclones and to further the understanding of the role that they play in the initiation of deep moist convection and nonsupercell tornadoes misocyclones inflect or disjoint the swath of low level convergence along each boundary to varying degrees depending on the size of their circulations when several relatively large misocyclones are next to each other the shape of low level convergence along each boundary is arranged into a staircase pattern mergers of misocyclones are an important process in the evolution of the vorticity field as a population of small vortices consolidates into a smaller number of larger ones additionally merging misocyclones may affect the mixing of thermodynamic fields in their vicinity when the merger axis is perpendicular to the boundary misocyclones interact with linear and cellular structures in the planetary boundary layers pbls of the air masses adjacent to each boundary cyclonic low level vertical vorticity generated by both types of structures makes contact with each boundary and sometimes is incorporated into preexisting misocyclones intersections of either type of pbl structure with the boundary result in strengthened pockets of low level convergence and typically strengthened misocyclones

the garden city kansas storm during vortex 95 part i overview of the storm s life cycle and mesocyclogenesis abstract analysis of a supercell storm that produced an f1 tornado near garden city kansas is presented this event provided one of the first opportunities to synthesize data collected by a new airborne radar platform called eldora electra doppler radar developed by the national center for atmospheric research the early stages of development of the midlevel mesocyclone and the entire evolution of the low level mesocyclone are captured over a 70 min period the low level mesocyclone began as an incipient shallow circulation along a synoptic scale trough the circulation intensified and grew in depth via vortex stretching under the influence of a strong updraft as this rotation built up from the boundary layer it initially remained separate and distinct from the midlevel mesocyclone subsequently the two mesocyclones merge to produce a single column of rotation 4 5 km in diameter an occlusion downdraft develops within the mesocyclone circulation during the last passes by the storm signaling the beg

the arcadia oklahoma storm of 17 may 1981 analysis of a supercell during tornadogenesis abstract on 17 may 1981 an extensive dataset was collected for a supercell thunderstorm that produced an f2 tornado near arcadia in central oklahoma coordinated dual doppler scans of the storm by 10 cm research radars were collected at approximately 5 min intervals from 30 min before the tornado touched down until 15 min after the tornado had dissipated the arcadia storm was also well sampled by a 444 m tall instrumented tower the low level inflow updraft mesocyclone and rear precipitation core of the supercell all passed across the tower a comparison of the instrumented tower measurements with a dual doppler synthesis reveals that the latter qualitatively resolved the low level flow however the magnitudes of the low level horizontal winds and updraft speed were underestimated in addition the vertical shear of the horizontal wind in the lowest kilometer was unresolved in the doppler winds in the storm environment horizontal vorticity was strong 1 5 10 2 s 1 and approximately streamwise

rock magnetism of lower middle pleistocene marine sediments wanganui basin new zealand rock magnetic studies were carried out in order to identify the remanence carrying minerals and the domain state of magnetic grains in lower middle pleistocene marine sediments from the wanganui basin southwestern north island new zealand isothermal remanent magnetization irm acquisition and stepwise thermal demagnetization of a three axis composite irm suggest that titanomagnetite or titanomaghaemite is the dominant remanence carrier in these sediments other rock magnetic data suggest that these grains occur in a range of sizes in the superparamagnetic to pseudo single domain range which helps account for the wide range of observed palaeomagnetic behaviour isolated occurrences of ferrimagnetic iron sulphides cannot be ruled out

multiple doppler radar observations of the evolution of vorticity extrema in a convective boundary layer abstract overdetermined dual doppler wind syntheses are used to document the evolution structure and dynamics of vertical vorticity extrema observed in a convective boundary layer during the 12 june 2002 international h2o project ihop mission discrete vertical vorticity extrema having horizontal scales of 1 2 km can be observed continuously for periods exceeding an hour the evolution of the vorticity field is characterized by complex interactions among vorticity extrema and between the vertical vorticity and vertical velocity fields the most prominent vorticity maxima have amplitudes of approximately 0 01 s 1 and are associated with retrieved pressure deficits of order 0 1 mb the vorticity extrema weaken with height and tilt in the presence of vertical wind shear advection and propagation both contribute substantially to the motion of the vorticity extrema amplifications of vertical vorticity are closely linked to the intensification of updrafts both stretching and tilting can contribute signi

a statistical model of the fluctuations in the geomagnetic field from paleosecular variation to reversal the statistical characteristics of the local magnetic field of earth during paleosecular variation excursions and reversals are described on the basis of a database that gathers the cleaned mean direction and average remanent intensity of 2741 lava flows that have erupted over the last 20 million years a model consisting of a normally distributed axial dipole component plus an independent isotropic set of vectors with a maxwellian distribution that simulates secular variation fits the range of geomagnetic fluctuations in terms of both direction and intensity this result suggests that the magnitude of secular variation vectors is independent of the magnitude of earth s axial dipole moment and that the amplitude of secular variation is unchanged during reversals

equatorial and mid latitude records of the last geomagnetic reversal from the atlantic ocean abstract three records of the matuyama brunhes reversal have been obtained from odp deep sea cores distributed along the west african margin in the atlantic ocean these studies in addition to the record from dsdp site 609b provide a latitudinal transect extending from the equator to 50 n simulations of natural smoothing by post depositional processes show that characteristic features of a transitional field geometry dominated by axisymmetry should be preserved in the records especially at the equator the results do not seem to favour the hypothesis that axisymmetrical terms would dominate during this transition

time dependent mapping of the magnetic field at the core mantle boundary we consider the problem of constructing a time dependent map of the magnetic field at the core mantle boundary we use almost all the available data from the last 300 years to produce two maps one for the period 1690 1840 and the other for 1840 1990 we represent the spatial dependency of the field using spherical harmonics the time dependency using a cubic b spline basis and seek the smoothest solutions compatible with the observations particular attention must be paid to the effects of the crustal field in the data we argue that for observations from permanent magnetic observatories the most efficient strategy is to use first differences of annual means for satellite data the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field the resulting model fits the observatory data better than any previous model yet has less power in the secular variation than previous models demonstrating that very simple models fit the data the resulting time dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies

a funnel cloud in a convective cloud line to the rear of a surface cold front abstract this brief case study describes the unusually benign environment in which a funnel cloud formed along a line of convective towers during the summer in kansas the parent cloud line was solitary and very narrow yet organized on the mesoscale the cloud line appeared to be best correlated with a zone of horizontal temperature gradient to the northwest of cool evaporatively produced outflow from an area of precipitation located just to the rear of a cold front implications for forecasting such an event are noted

longitudinal confinement of geomagnetic reversal paths as a possible sedimentary artefact the positions of the virtual geomagnetic pole vgp during a large number of reversals of the earth s magnetic field seem to show a remarkable confinement to longitudes over the americas or to antipodal longitudes1 2 although it has been argued that this confinement is statistically insufficiently constrained3 it has also been pointed out4 that the same bands of longitude appear in other geophysical observations such as the pattern of fluid motion in the outer core and regions of higher seismic velocities in the lower mantle suggesting a causal relationship here we show that longitudinal confinement of vgps can arise from the smoothing of non antipodal stable directions just before and after a geomagnetic reversal because of the filtering effect of the remanence acquisition process in sediments the origin of this non antipodality is still uncertain and must remain speculative until more reversal records become available that include sufficiently long pre and post transitional intervals