convectively forced internal gravity waves results from two dimensional numerical experiments two dimensional numerical simulations were performed to investigate the nature of tropospheric internal gravity waves of the type which are observed to occur above active thermal convection over an unstable boundary layer these gravity waves are believed to be excited by a combination of pure thermal forcing and by the boundary layer eddies and cumulus clouds acting as obstacles to the flow in the presence of mean environmental wind shear large amplitude internal gravity waves were obtained in the simulations with amplitudes and horizontal scales similar to the 12 june 1984 aircraft observations over western nebraska this was a day with strong wind shear in the lowest 3 km above the ground and with scattered cumulus clouds topping the boundary layer the simulations show that there is significant difference between the early time solutions as might be predicted by linear theory and late time solutions for the boundary layer eddy structure a layer interaction occurs in which gravity waves of the stable layer are excited by the boundary layer convection there is evidence to suggest that this layer interaction occurs both with and without shear but that it is stronger in the presence of low level shear results indicate that shear or the obstacle effect is a more efficient generator of gravity waves than is the pure thermal forcing the simulations show that the gravity waves initially forced by the boundary layer eddies lead to a feedback mechanism that acts to organize the boundary layer eddies and the cumulus convection the solutions suggest that the character of fair weather convection moist or dry is a non local problem involving at times the full depth of the troposphere the clouds produced in the simulations have very little influence on the wave field or boundary layer eddy structure as they are relatively small cumuli on the other hand the clouds are strongly influenced by the interactions between the wave and eddy fields upshear growth of cumulus clouds similar to that which is frequently observed in nature is reproduced in the simulations the development of feeder feeder is used here in a dynamical sense only clouds on typically the upshear side of the cloud is found to be a result of the interaction between the gravity wave field and the dry and moist convection the relative phase velocity between the gravity waves and the cloud plays a crucial role in determining the character of the cumulus cloud growth in the present simulations these simulations suggest that the dynamics both internal and external to the boundary of a cumulus cloud is a complicated mix between wave dynamics and the usually considered convection dynamics a brief discussion of the implications of the present results to cloud boundary baroclinic instability dynamics is also presented

combined wind profiler polarimetric radar studies of the vertical motion and microphysical characteristics of tropical sea breeze thunderstorms abstract an experiment combining wind profiler and polarimetric radar analyses of intense but shallow tropical thunderstorms has been performed these storms are important as they are very common along many tropical coasts and islands and are sometimes the precursors to large intense multicellular storms such as occur over the tiwi islands north of darwin australia all the storms sampled had a similar structure with intense updrafts on the periphery of the cells producing significant sized hail and downdrafts in the storm center the hail concentrations are relatively small but have a large effect on the radar reflectivity and polarimetric measurands because of the size 10 20 mm it is this hail melting that produces characteristic zdr columns in the polarimetric radar data

mean magnetic contrasts between oceans and continents abstract the magnetic contrast between continents and oceans is an important constraint for understanding the regional distribution of magnetic sources in the crust and upper mantle this magnetic difference was investigated using satellite 2 c scalar magnetic anomalies collected by nasa s magsat mission over the atlantic and eastern pacific oceans north and south america europe and africa reducing the magnetic anomalies differentially to vertical polarization yields mean anomaly values which correlate with the tectonic ages of the various continental and oceanic regions the range of magnetic anomaly values of tectonically older lithosphere is greater than that of younger lithosphere statistically at the 99 9 confidence level the mean radially polarized magnetic anomaly of the continents is greater than that of the oceans a careful inversion was performed for regional effective susceptibility contrasts which accurately fit the magsat data assuming a model in which the moho represents a boundary between crustal magnetic and predominantly non magnetic mantle rocks the root mean square of the magnetic properties of oceans compares favorably with magnetic properties predicted from models constrained by magnetic studies of the oceanic rocks and which incorporate viscoremanent components from rocks near the base of the crust the mean magnetic anomaly and property contrasts of the oceans are negative and show large variability indicating widespread deviations from the standard oceanic crust the effective susceptibility contrasts obtained facilitate quantitative testing of inferences concerning large scale petrologic variations crustal thickness and thermal perturbations and other geologic processes which control the regional distribution of magnetic properties within the lithosphere

suppression of eddy diffusivity across jets in the southern ocean abstract geostrophic eddies control the meridional mixing of heat carbon and other climatically important tracers in the southern ocean the rate of eddy mixing is typically quantified through an eddy diffusivity there is an ongoing debate as to whether eddy mixing in enhanced in the core of the antarctic circumpolar current or on its flanks a simple expression is derived that predicts the rate of eddy mixing that is the eddy diffusivity as a function of eddy and mean current statistics this novel expression predicts suppression of the cross jet eddy diffusivity in the core of the antarctic circumpolar current despite enhanced values of eddy kinetic energy the expression is qualitatively and quantitatively validated by independent estimates of eddy mixing from altimetry observations this work suggests that the meridional eddy diffusivity across the antarctic circumpolar current is weaker than presently assumed because of the suppression of eddy mixing by the strong zonal current

modelling the generation of gravity waves by a maritime continent thunderstorm observed wind and temperature profiles are used to initialize a model calculation of a maritime continent thunderstorm and the numerical solution is used to explore the effect of tropospheric shear on the gravity wave generation the resultant convective system is qualitatively similar to that observed the modelled gravity waves propagate away from the cloud with wave fronts that are approximately circular implying that the convective clouds do not generate waves which propagate in a preferred direction the gravity wave generation is related to the oscillation of the convective updraughts about their level of neutral buoyancy while the general features of the gravity waves are similar in most respects to a previous study which used an idealized wind profile the frequency of the gravity waves is doppler shifted by the tropospheric wind shear the result is a much broader power spectrum in comparison to the idealized cases

mjo related intraseasonal variation in the stratosphere gravity waves and zonal winds previous work has shown eastward migrating regions of enhanced temperature variance due to long vertical wavelength stratospheric gravity waves that are in sync with intraseasonal precipitation and tropopause wind anomalies associated with the madden julian oscillation mjo here the origin of these intraseasonal gravity wave variations is investigated with a set of idealized gravity wave resolving model experiments the experiments specifically test whether tropopause winds act to control gravity wave propagation into the stratosphere by a critical level filtering mechanism or play a role in gravity wave generation through an obstacle source effect all experiments use identical convective latent heating variability but the large scale horizontal wind profile is varied to investigate relationships between stratospheric gravity waves and zonal winds at different levels results show that the observed long vertical wavelength gravity waves are primarily sensitive to stratospheric zonal wind variations while tropopause wind variations have only a very small effect thus neither the critical level filter mechanism nor the obstacle source play much of a role in the observed intraseasonal gravity wave variations instead the results suggest that the stratospheric waves follow the mjo precipitation sources and tropopause wind anomalies follow the same sources we further find evidence of intraseasonal wave drag effects on the stratospheric circulation in reanalyzed winds the results suggest that waves drive intraseasonal stratospheric zonal wind anomalies that descend in altitude with increasing mjo phases 3 through 7 eastward anomalies descend further than westward suggesting that mjo related stratospheric waves cause larger eastward drag forces

eddy zonal flow feedback in the southern hemisphere abstract the variability of the zonal mean zonal wind in the southern hemisphere is studied using eof analysis and momentum budget diagnostics of ncep reanalysis data 1978 97 the leading eof of the zonal mean zonal wind is well separated from the remaining eofs and represents the north south movement of the midlatitude jet analysis of the momentum budget shows that a positive feedback between the zonal mean wind anomalies and the eddy momentum fluxes accounts for the unusual persistence of eof1 and plays an important role in the selection of the leading eof of midlatitude variability further analysis also shows a propagating feedback common to both eof1 and eof2 which is responsible for the poleward drift of wind anomalies with time the observations support the following feedback mechanism anomalous baroclinic wave activity is generated at the latitude of anomalous temperature gradient that by thermal wind coincides with the latitude of the anomalous zonal jet the net propagation of baroclinic

a model study of zonal forcing in the equatorial stratosphere by convectively induced gravity waves a two dimensional cloud resolving model is used to examine the possible role of gravity waves generated by a simulated tropical squall line in forcing the quasi biennial oscillation qbo of the zonal winds in the equatorial stratosphere a simulation with constant background stratospheric winds is compared to simulations with background winds characteristic of the westerly and easterly qbo phases respectively in all three cases a broad spectrum of both eastward and westward propagating gravity waves is excited in the constant background wind case the vertical momentum flux is nearly constant with height in the stratosphere after correction for waves leaving the model domain in the easterly and westerly shear cases however westward and eastward propagating waves respectively are strongly damped as they approach their critical levels owing to the strongly scale dependent vertical diffusion in the model the profiles of zonal forcing induced by this wave damping are similar to profiles given by critical level absorption but displaced slightly downward the magnitude of the zonal forcing is of order 5 m s21 day21 it is estimated that if 2 of the area of the tropics were occupied by storms of similar magnitude mesoscale gravity waves could provide nearly 1 4 of the zonal forcing required for the qbo

gravity waves generated by a tropical cyclone during the step tropical field program a case study overflights of a tropical cyclone during the australian winter monsoon field experiment of the stratosphere troposphere exchange project step show the presence of two mesoscale phenomena a vertically propagating gravity wave with a horizontal wavelength of about 110 km and a feature with a horizontal scale comparable to that of the cyclone s entire cloud shield wavelength of 250 km or greater the larger feature is fairly steady though its physical interpretation is ambiguous the 110 km gravity wave is transient having maximum amplitude early in the flight and decreasing in amplitude thereafter its scale is comparable to that of 100 to 150 km diameter cells of low satellite brightness temperatures within the overall cyclone cloud shield these cells have lifetimes of 4 5 to 6 hours aircraft flights through the anvil show that these cells correspond to regions of enhanced convection higher cloud altitude and upwardly displaced potential temperature surfaces a three dimensional transient linear gravity wave simulation shows that the temporal and spatial distribution of meteorological variables associated with the 110 km gravity wave can be simulated by a slowly moving transient forcing at the anvil top having an amplitude of 400 600 m a lifetime of 4 5 6 hours and a size comparable to the cells of low brightness temperature the forcing amplitudes indicate that the zonal drag due to breaking mesoscale transient convective gravity waves is definitely important to the westerly phase of the stratopause semiannual oscillation and possibly important to the easterly phase of the quasi biennial oscillation there is strong evidence that some of the mesoscale gravity waves break below 20 km as well the effect of this wave breaking on the diabatic circulation below 20 km may be comparable to that of above cloud diabatic cooling

contributions of cretaceous quiet zone natural remanent magnetization to magsat anomalies in the southwest indian ocean the magsat magnetic anomalies over the southwest indian ocean are modeled using a combination of induced plus viscous remanent magnetization im vrm and natural remanent magnetization nrm two broad roughly parallel sw to ne trending triple peaked positive anomalies dominate the region one lying south of africa and the other north of antarctica although these anomaly peaks generally correspond with the agulhas plateau maud rise mozambique plateau astrid ridge and madagascar ridge conrad rise conjugate pairs the im vrm contribution from structural characteristics i e crustal thickness accounts for only about 20 of the anomaly amplitudes a spatially variable but observationally constrained nrm contribution in cretaceous quiet zone kqz crust is required to account for the location shape and amplitude contrast of these anomalies many crustal features in the southwest indian ocean near antarctica have little geophysical data to constrain their structure but do have tectonic conjugates near africa for which much more geophysical data are generally available using geophysical and geological constraints from one member to model the magnetization structure of its conjugate reproduces the observed magsat reduced to pole anomalies over both structures very well this suggests that no significant alteration in their magnetization structure has occurred since the features split models of these conjugate structures show that im vrm reproduces the magsat anomalies associated with non kqz crust but that both im vrm and a dominant nrm component are required to explain the anomalies associated with kqz crust

source of marine magnetic anomalies some results from dsdp leg 83 leg 83 of the deep sea drilling project represents the deepest penetration of undisturbed marine crust yet achieved providing samples from the lower part of the extrusive layer and the upper 300 m of the sheeted dike complex we present here measurements of the magnetic properties of rocks from leg 83 and address the question of their suitability as sources for marine magnetic anomalies we find that due to extensive hydrothermal alteration the extrusive part of the leg 83 section is insufficiently magnetized to be part of the magnetic source layer the sheeted dike complex on the other hand does have sufficient magnetization and contrary to recent models of marine magnetization based on ophiolite studies may be a contributor to magnetic anomalies the strong dependence of magnetic properties on the alteration history of the rocks however makes extension of these results to other regions problematical

interpretation of satellite elevation magnetic anomalies the magnetic field satellite magsat obtained total field data over almost the whole of the earth we have studied these data in two different ways the first way was to use spherical harmonic coefficients it is often believed that these coefficients can be separated into those originating within the core of the earth and those from the crust at about degree 13 because of the change in slope of the power spectrum using coefficients above degree 13 in two different representations we have shown that there is little latitudinal variation of the source function and that the continent to ocean source function ratio is almost unity we suggest that these unlikely observations are in reality due to the possibility that much of the signal carried in coefficients above degree 13 may originate within the core of the earth if this is true it is then possible to keep the energy external to the core of the field generated within the core at a constant even though the dipole field has varied in strength by a factor of 3 over the past 104 years we have also produced models of crustal magnetization using the 2 2 average anomalous field and have shown how it is possible to obtain a physically meaningful susceptibility model in which magnetizations are always positive this is done by using an annihilator which is a magnetization distribution within a spherical shell which produces no external field the use of an annihilator is also appropriate for many problems connected with oceanic basins since present day oceanic crust is everywhere less than 180 ma the average magnetization seen at the altitude of a satellite is either zero for crust which is produced during times of rapid field reversal or positive for crust produced during times of constant normal polarity some results from continental and oceanic areas are discussed in the light of applying an annihilator

an experimental study of magnetic viscosity in synthetic monodomain titanomaghemites implications for the magnetization of the ocean crust a study of magnetic viscosity in well characterized synthetic monodomain titanomaghemites of composition fe2 4ti0 6o4 is described it is found that the samples are capable of acquiring a viscous remanent magnetization vrm only when they are highly oxidized the same assembly does not show significant magnetic viscosity for 0 1 z 0 6 where z is the oxidation degree this is a result of their being in the thermally stable monodomain state with high anisotropies the observed increase in vrm for the highly oxidized material may be entirely due to the decrease in magnetic anisotropy a decrease in the total number of fe2 as a functin of oxidation results in a decrease in coercive force i e magnetic anisotropy of the initial monodomain titanomaghemites it is thought that the dramatic decrease in magnetic anisotropy at room temperature may be the main cause of the observed increase in vrm for the highly oxidized material finally we conclude that the ability of the magnetic layer of the ocean crust to acquire a vrm increases as the maghemitization degree increases

the characterization of viscous remanent magnetization in large and small magnetite particles previous work on the properties of viscous remanent magnetization vrm have shown that it can play an important role in some paleomagnetic investigations the results of this study on the properties of vrm of very fine single domain sd and coarse multidomain md magnetite particles ranging in size from 0 02 to 60 m indicate that the md samples are very sensitive to the previous magnetic history of the samples the af demagnetization causes a dramatic decrease in the ability of md samples to acquire vrm compared to the same sample with an undemagnetized thermal remanent magnetization trm thermal demagnetization of the same md samples causes an increase in the ability to acquire vrm components submicron sd samples show vrm acquisition and decay curves that are independent of previous magnetic treatment both md and sd samples are extremely sensitive to the length of time of zero and weak field storage that occurs prior to the application of the external vrm field a model using the total energy of the system is used to describe the above results

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

an idealized nonlinear model of the northern hemisphere winter storm tracks abstract in this paper a nonlinear dry model forced by fixed radiative forcing alone has been constructed to simulate the northern hemisphere winter storm tracks a procedure has been devised to iterate the radiative equilibrium temperature profile such that at the end of the iterations the model climate closely resembles the desired target climate this iterative approach is applied to simulate the climatological storm tracks in january it is found that when the three dimensional temperature distribution in the model resembles the observed distribution the model storm tracks are much too weak it is hypothesized that this is due to the fact that eddy development is suppressed in a dry atmosphere owing to the lack of latent heat release in the ascending warm air to obtain storm tracks with realistic amplitudes the static stability of the target climate is reduced to simulate the enhancement in baroclinic energy conversion due to latent heat release with this modification the storm tracks in the

long wavelength magnetic anomalies in the caribbean plate boundaries and allochthonous continental blocks vector magnetic anomaly maps are derived at satellite altitude from magsat data over the caribbean and middle america to minimize smoothing of the anomaly field caused by averaging the data over the whole altitude range of magsat measurements two maps are derived in two nonoverlapping altitude ranges 350 420 and 420 550 km a composite map of the vertical component of the anomaly field is then computed at ground level this map displays large positive anomalies associated with fast subduction zones these anomalies define discontinuities in the downgoing slab associated with seismic gaps positive anomalies are also consistently observed above allochthonous blocks of continental origin in the caribbean area and suggest that the average magnetization is larger in the continental lithosphere elongated magnetic lows are observed above transform boundaries of the caribbean plate the cayman trough anomaly is consistent with magnetization of the order of 1 a m in the upper mantle demagnetization of the lithosphere along the bocono el pilar fault system which bounds the caribbean plate to the south may be accounted for by frictional heating along shear zones the pattern of anomalies along the southern margin of the united states is related to its accretionary tectonic history various estimates of the vertically integrated magnetization of both oceanic and continental lithospheres are computed using these anomalies and provide constraints on the vertical distribution of lithospheric sources of magsat anomalies

sensitivity of gravity wave fluxes to interannual variations in tropical convection and zonal wind using an idealized model framework with high frequency tropical latent heating variability derived from global satellite observations of precipitation and clouds the authors examine the properties and effects of gravity waves in the lower stratosphere contrasting conditions in an el nino year and a la nina year the model generates a broad spectrum of tropical waves including planetary scale waves through mesoscale gravity waves the authors compare modeled monthly mean regional variations in wind and temperature with reanalyses and validate the modeled gravity waves using satellite and balloon based estimates of gravity wave momentum flux some interesting changes in the gravity spectrum of momentum flux are found in the model which are discussed in terms of the interannual variations in clouds precipitation and large scale winds while regional variations in clouds precipitation and winds are dramatic the mean gravity wave zonal momentum fluxes entering the stratosphere differ by only 11 the modeled intermittency in gravity wave momentum flux is shown to be very realistic compared to observations and the largest amplitude waves are related to significant gravity wave drag forces in the lowermost stratosphere this strong intermittency is generally absent or weak in climate models because of deficiencies in parameterizations of gravity wave intermittency these results suggest a way forward to improve model representations of the lowermost stratospheric quasi biennial oscillation winds and teleconnections

a numerical study of three dimensional gravity waves triggered by deep tropical convection and their role in the dynamics of the qbo abstract a 3d mesoscale model is used to study the structure of convectively triggered gravity waves in the tropics and their role in the dynamics of the middle atmosphere simulations with three stratospheric background zonal wind cases are examined in the first case the background wind profile is constant the other two are representative of the easterly and westerly phases of the quasi biennial oscillation qbo spectral analysis is used to link the structure of the triggered gravity waves to the dominant vertical wavelength of the latent heating within the convection in the qbo wind shear cases upward propagating gravity waves are damped as they approach their critical layer the signature of critical layer absorption is clearly visible in the profiles of vertical momentum flux divergence in the simulations with open boundary conditions the response to vertical momentum flux divergence takes the form of large dynamic pressure differences between the east and west boundaries together with acceler

effects of tropospheric wind shear on the spectrum of convectively generated gravity waves abstract the authors examine the effects of tropospheric wind shear on the phase speed spectrum of gravity waves generated by tropical convection a two dimensional cloud resolving model is used to perform numerous squall line simulations with the vertical shear of the horizontal wind varied in three layers of the troposphere several simplified simulations using prescribed heating are also performed to elucidate the interactions of wind shear with thermal forcing it is found that the dominant phase speed range of convectively generated stratospheric gravity waves is primarily determined by the vertical scale of the tropospheric heating and is then modified by the tropospheric wind the gravity wave spectrum is especially sensitive to shear in the upper troposphere through a mechanism similar to critical level filtering such shear acts to reduce the momentum flux of waves propagating in the same direction as the storm relative mean wind through interaction with convective turrets shear in the upper t

magnetization of lower crust and interpretation of regional magnetic anomalies example from lofoten and vesteralen norway the interpretation of regional magnetic anomalies commonly observed over continental crust in both satellite and near surface magnetic data has remained enigmatic in its requirement for laterally extensive thick highly magnetic and deep seated sources presented here are the results of a detailed field analytical and experimental study of the magnetic mineralogy and magnetic properties of rocks from lofoten and vesteralen norway a province of deep seated origin with associated regional magnetic and gravity anomalies these rocks have a bulk magnetic character of surprising uniformity with susceptibilities hysteresis and curie temperatures defined solely by coarse grained magnetite stable remanence is observed in samples from sw lofoten but is not of significant magnitude the thermal enhancement of magnetic susceptibility the hopkinson effect is not important rocks from the deepest part of the section have the largest average susceptibilities 6 10 2 si and magnitudes of natural remanent magnetization nrm 5 10 4 a m2 kg these values of susceptibility together with the oft observed viscous character of the nrm suggest that these granulite facies lithologies particularly those of sw lofoten are perfect examples of lower crustal sources of typical regional magnetic anomalies furthermore these intermediate pressure granulites define a unique magnetic petrology especially for mafic compositions at higher grades of metamorphism high pressure granulites and eclogites or lower grades greenschist and amphibolite facies such magnetite bearing assemblages will be unstable and their high or low grade equivalents will be only weakly magnetic this evidence and these conclusions may be used to constrain models of lower crustal sources of regional magnetic anomalies over stable cratons it portrays the lower crust as a highly magnetic unit defined by metamorphic grade with a curie isotherm of 575 c magnetite and magnetization parallel to the ambient geomagnetic field induced and viscous components

a consistent poleward shift of the storm tracks in simulations of 21st century climate 1 a consistent poleward and upward shift and intensification of the storm tracks is found in an ensemble of 21st century climate simulations performed by 15 coupled climate models the shift of the storm tracks is accompanied by a poleward shift and upward expansion of the midlatitude baroclinic regions associated with enhanced warming in the tropical upper troposphere and increased tropopause height the poleward shift in baroclinicity is augmented in the southern hemisphere and partially offset in the northern hemisphere by changes in the surface meridional temperature gradient the poleward shift of the storm tracks also tends to be accompanied by poleward shifts in surface wind stress and precipitation and a shift towards the high index state of the annular modes these results highlight the integral role that the storm tracks play in the climate system and the importance of understanding how and why they will change in the future

a comparison of the magnetic properties of synthetic titanomaghemites and some oceanic basalts the variation of some magnetic properties of 1 synthetic titanomaghemites and 2 oceanic basalts from dsdp site 417 d as a function of grain size and oxidation is presented the synthesis of the initial titanomagnetite x 0 6 was carried out using controlled fugacity and self buffering techniques single phase titanomaghemites were produced by heating the titanomagnetite in air at low temperatures 350 c the magnetic properties studied on the synthetic titanomagnetite grain size 1 m show the following variations with increasing degrees of oxidation 0 z 0 6 1 curie temperature tc increases 2 saturation magnetization js at room temperature increases 3 js measured at 77 k decreases for z 0 3 and then increases for 0 3 z 0 6 4 bulk and remanent coercivities hc and hr increase slightly initially z 0 2 and then decrease at both room temperature and 77 k 5 susceptibility 0 decreases for z 0 3 and then increases 6 the magnetic viscosity acquisition and decay coefficients sa and sd increase and 7 the median destructive fields for an anhystretic remanent magnetization and a viscous remanent magnetization increase for z 0 3 and then decrease these magnetic data also suggest that the synthetic titanomaghemites are pseudo single domained psd and with increasing degrees of oxidation these psd grains become more sd like the basalt samples from site 417d are divided into fine grained pillow basalts and coarse grained massive flows most of the magnetic properties measured are distinctly different between these two groups the magnetic data indicate that the domain states of these basalts are probably psd although it is difficult to separate the effects of grain size and oxidation on the magnetic properties of these basalts it appears that grain size effects predominate a preliminary investigation of the magnetic viscosity of the synthetic and oceanic titanomaghemites suggests that a combination of thermally activated and diffusive magnetic viscosity is responsible for their viscous behavior finally we will discuss the roles of grain size and oxidation variation in oceanic basalts and their separate and combined effects on their magnetic behavior

characterization of viscous remanent magnetization in single and multi domain magnetite grains viscous remanent magnetization acquisition and decay coefficients are shown to be strongly affected by the previous magnetic history of the samples demagnetization of coarse grain multi domain magnetite samples using alternating magnetic fields causes a sharp reduction in the ability to acquire viscous magnetization vrm acquisition in single domain particles is not affected by af demagnetization but is sensitive to the amount of time the sample is kept in zero magnetic field storage prior to applying the vrm field vrm acquisition coefficients are strongly dependent on magnetic grain size with high values at both very small and very large grain diameters

some aspects of magnetic viscosity in subaerial and submarine volcanic rocks summary the magnetic viscosity of 334 upper tertiary and pre bruhnes quaternary volcanic rocks from the massif central france and the steens mountain oregon and of 40 basaltic cores from dsdp leg 37 has been investigated thellier s viscosity index follows a log normal distribution with mean values equal to 6 and 3 5 per cent for subaerial and submarine rocks respectively for subaerial rocks the average intensity of the viscous remanent magnetization vrm acquired in the earth s field since the beginning of the bruhnes polarity epoch t 0 7 myr is estimated to be equal to one quarter of the average intensity of the primary remanence alternating field demagnetization of vrms acquired in low fields for acquisition times t ranging from 2 day to 32 month indicates the resistance to alternating fields is quite different from sample to sample and increases linearly with log t neel s diagnostic parameter of domain structure of the grains involved in magnetic viscosity shows that hard vrm is carried by single domain grains and soft vrm carried by multidomain particles single domain particles carrying hard vrm in subaerial volcanic rocks are almost equant magnetite intergrowths with size near the superparamagnetism threshold resulting from high temperature oxidation of titanomagnetite soft vrm is carried by low curie point homogeneous titanomagnetite unlike his single domain theory neel s multidomain theory of magnetic viscosity does not account quantitatively for the resistance of vrm to alternating fields

satellite magnetic anomalies over subduction zones the aleutian arc anomaly positive magnetic anomalies seen in magsat average scalar anomaly data overlying some subduction zones can be explained in terms of the magnetization contrast between the cold subducted oceanic slab and the surrounding hotter nonmagnetic mantle three dimensional modeling studies show that peak anomaly amplitude and location depend on slab length and dip a model for the aleutian arc anomaly matches the general trend of the observed magsat anomaly if a slab thickness of 7 km and a relatively high induced plus viscous magnetization contrast of 4 a m are used a second source body along the present day continental margin is required to match the observed anomaly in detail and may be modeled as a relic slab from subduction prior to 60 m y ago

eddy activity sensitivity to changes in the vertical structure of baroclinicity abstractthe relation between the mean meridional temperature gradient and eddy fluxes has been addressed by several eddy flux closure theories however these theories give little information on the dependence of eddy fluxes on the vertical structure of the temperature gradient the response of eddies to changes in the vertical structure of the temperature gradient is especially interesting since global circulation models suggest that as a result of greenhouse warming the lower tropospheric temperature gradient will decrease whereas the upper tropospheric temperature gradient will increase the effects of the vertical structure of baroclinicity on atmospheric circulation particularly on the eddy activity are investigated an idealized global circulation model with a modified newtonian relaxation scheme is used the scheme allows the authors to obtain a heating profile that produces a predetermined mean temperature profile and to study the response of eddy activity to changes in the vertical structure o

a model of ocean basin crustal magnetization appropriate for satellite elevation anomalies a model of ocean basin crustal magnetization measured at satellite altitudes is developed which will serve both as background to which anomalous magnetizations can be contrasted and as a beginning point for studies of tectonic modification of normal ocean crust the model is based on published data concerned with the petrology and magnetization of the ocean crust and consists of viscous magnetization and induced magnetization estimated for individual crustal layers thermal remanent magnetization and chemical remanent magnetization are excluded from the model because seafloor spreading anomalies are too short in wavelength to be resolved at satellite altitudes the exception to this generalization is found at the oceanic magnetic quiet zones where thermal remanent magnetization and chemical remanent magnetization must be considered along with viscous magnetization and induced magnetization

magnetite rods in plagioclase as the primary carrier of stable nrm in ocean floor gabbros abstract the ubiquitous occurrence of magnetite rods in plagioclase may account for the stable intense magnetization of ocean floor gabbros these single domain grains are formed by the subsolidus exsolution of fe 2 and fe 3 incorporated into the plagioclase at higher temperatures we expect that this process will be particularly important in the upper portions of the oceanic gabbro unit because of the relative paucity of femic minerals at these levels the geometry of accretion prohibits a constructive contribution to marine magnetic anomalies by the lower olivine rich gabbros as a result of the presence of these rods ocean floor gabbros may constitute an important source for marine magnetic anomalies

multicell squall line structure as a manifestation of vertically trapped gravity waves abstract two dimensional and three dimensional simulations of a midlatitude squall line with a high resolution non hydrostatic model suggest that the multicellular structure of the storm may be associated with gravity waves generated by convection time lapse display of model output demonstrates that the commonly described cut off process is actually a gravity wave phenomenon the convective cells arise as gravity waves which are forced by continuous strong low level convergence at the storm s gust front the waves propagate to both sides of the gust front the stronger westward front to rear mode dominates at the mature stage of the squall line continuous low level updraft is generated at the nose of the cold pool which propagates at the speed of a density current updraft cells periodically break away from this persistent low level gust front updraft and move at phase speeds of their associated gravity waves not at the surrounding airflow speeds as implied by the traditional multicell model lin

global survey of concentric gravity waves in airs images and ecmwf analysis concentric gravity waves cgws are atmospheric phenomena with ring shape perturbations originating in the troposphere they can propagate up to the ionosphere and thermosphere and dynamically couple the lower and upper atmosphere in this study we developed a novel ring detection algorithm to extract cgws from the atmosphere infrared sounder airs radiance data and the european center for medium range weather forecasting ecmwf analysis temperature in the stratosphere to produce the first global maps of such phenomena the algorithm is capable of estimating wave amplitude wavelength propagation direction and source location both airs and ecmwf data show a significant diurnal variation in wave propagation direction and generation in addition to strong seasonal variations in wavelength and amplitude occurrence of these ring waves is associated not only with tropical deep convections but also with summertime midlatitude convection wintertime extratropical jets and topography such as islands the high resolution ecmwf analysis data capture most of the cgw features but the wave amplitude is significantly weaker than airs observations showing few convectively generated cgws

stationary gravity wave structure in flows with directional wind shear most theoretical studies of flow over mountains assume that the wind direction is constant for two dimensional orographic ridges this is not an issue since only the component of the wind across the ridge forces internal gravity waves a special case of rectilinear flow is when the wind component changes sign i e the wind direction changes discontinuously through 180 degrees such a point is a critical level for all stationary gravity waves for isolated three dimensional mountains the gravity wave response comprises waves of all azimuthal orientations and each of these may have a different critical level height depending on whether the wind backs or veers with height using a combination of ray tracing methods and stationary phase solutions the structure of the stationary hydrostatic gravity wave field generated by an isolated mountain is studied for a simple wind field which backs with height in contrast to the behaviour for unidirectional flow wave action does not accumulate indefinitely beneath the critical level but is continuously advected downwind by the component of the wind parallel to the phase lines instead of the wave energy becoming unbounded following a ray as in the 2d case the wave energy decays to zero along a trailing wake zone downwind of the mountain along phase lines this is just an extension of the asymptotic wake found in the linear solutions of smith for constant flow over an isolated mountain the work described here underlines the desirability of including the effects of selective critical level absorption in the gravity wave drag parametrization schemes of numerical weather prediction models

the role of the central asian mountains on the midwinter suppression of north pacific storminess abstractthe role of the central asian mountains on north pacific storminess is examined using an atmospheric general circulation model by varying the height and the areas of the mountains a series of model integrations show that the presence of the central asian mountains suppresses the north pacific storminess by 20 30 during boreal winter their impact on storminess is found to be small during other seasons the mountains amplify stationary waves and effectively weaken the high frequency transient eddy kinetic energy in boreal winter two main causes of the reduced storminess are diagnosed first the decrease in storminess appears to be associated with a weakening of downstream eddy development the mountains disorganize the zonal coherency of wave packets and refract them more equatorward as the zonal traveling distance of wave packets gets substantially shorter downstream eddy development gets weaker second the central asian mountains suppress the global baroclinic energy conversion the decre

midwinter suppression of baroclinic wave activity in the pacific abstract seasonal variations in baroclinic wave activity and jet stream structure in the northern hemisphere are investigated based upon over 20 years of daily data baroclinic wave activity at each grid point is represented for each day by an envelope function the lowpass filtered time series of the squared highpass filtered geopotential height baroclinic wave activity over the atlantic exhibits a single maximum in january whereas in the pacific it exhibits peaks in late autumn and in early spring and a significant weakening in midwinter which is more evident at the tropopause level than near the surface this suppression occurs despite the fact that the low level baroclinity and the intensity of the jet stream are strongest in midwinter based on the analysis of 31 day running mean fields for individual winters it is shown that over both the oceans baroclinic wave activity is positively correlated with the strength of the upper tropospheric jet for wind speeds up to 45 m s 1 when the strength of t

model study of waves generated by convection with direct validation via satellite abstract atmospheric gravity waves have a major effect on atmospheric circulation structure and stability on a global scale gravity waves can be generated by convection but in many cases it is difficult to link convection directly to a specific wave event in this research the authors examine an event on 12 january 2003 when convective waves were clearly generated by a period of extremely intense rainfall in the region of darwin australia during the early morning the waves were observed by the atmospheric infrared sounder airs instrument on board the aqua satellite and a dry version of a nonlinear three dimensional mesoscale cloud resolving model is used to generate a comparable wave field the model is forced by a spatially and temporally varying heating field obtained from a scanning radar located north of darwin at gunn point with typical cloud resolving model studies it is generally not possible to compare the model results feature for feature with observations since although the model pr

magnetic anomalies at satellite altitude over continent ocean boundaries abstract a pronounced difference in vertically integrated magnetization produces strong magnetic anomalies at satellite altitude over continent ocean boundaries however these anomalies are seldom seen in satellite anomaly maps our simulation of the technique normally used to extract anomaly fields from satellite observations of the geomagnetic field suggests that power leaks from short wavelength crustal anomalies into the longer wavelengths this phenomenon distorts the anomaly field and adds power to the lower degrees in the anomaly field power spectrum we will also show that some of the few magnetic anomalies observed in satellite anomaly maps over continent ocean boundaries in the atlantic ocean could be caused by thermal destruction of magnetic minerals rather than by a pronounced difference in vertically integrated magnetization the very low permeability of sediments deposited along continental margins basically precludes convective heat transfer in the sediments the low thermal conductivity of these sediments results in the underlying lithosphere having a high temperature due to the temperature gradient within the sediment blanket our calculations predict that thermal destruction should occur along continental margins where several kilometres of sediments have accumulated in order to produce magnetic anomalies similar to magsat anomalies we assume that the upper mantle is magnetized

crustal structure interpreted from magnetic anomalies this review discusses publications during the last quadrennium 1987 1990 that used aeromagnetic data marine magnetic data satellite magnetic data and rock magnetic and petrologic data to provide information on the sources of magnetic anomalies the publications reviewed reflect increased integration of rock magnetic property and petrologic studies with magnetic anomaly interpretation studies particularly in deep crustal magnetization exploration for hydrocarbons and inversion of marine magnetic anomalies interpretations of aeromagnetic data featuring image display techniques and using the horizontal gradient method for locating magnetization boundaries became standard

impact of a convectively forced gravity wave drag parameterization in ncar ccm3 a parameterization of gravity wave drag forced by subgrid scale cumulus convection gwdc proposed by chun and baik is implemented into the national center for atmospheric research community climate model ncar ccm3 and its effect on perpetual january and july climate is investigated the cloud top gravity wave stress is concentrated in the intertropical convergence zone where persistent deep cumulus clouds exist the resultant zonal wind acceleration due to the breaking of convectively forced gravity waves is predominantly found in the tropical lower stratosphere with westerly acceleration above cloud top and easterly acceleration just below it since the parameterized gravity waves are stationary relative to convective clouds wave breaking occurs mainly in the tropical lower stratosphere where the zonal wind is weak enough for wave saturation it is shown that the gwdc parameterization significantly alleviates the systematic model biases of zonal mean zonal wind and temperature in particular excessive easterlies in the tropical stratosphere and excessive cold temperatures in the tropical lower stratosphere are reduced by more than 50 by including the gwdc parameterization the horizontal wind divergence field in the tropical upper troposphere and lower stratosphere is also significantly improved with the gwdc parameterization the impact of the gwdc parameterization extends to mid to high latitudes through planetary wave activity in the winter hemisphere the increased amplitude of zonal wavenumber 3 in the january northern hemisphere and the increased amplitude of zonal wavenumber 2 in the july southern hemisphere lead to significant improvements in model performance the impact of the gwdc parameterization on eliassen palm ep flux divergence forcing by stationary waves is generally opposite to that by transient waves in the extratropics especially in the northern hemisphere wintertime hence the zonal mean zonal wind change by the gwdc parameterization occurs mainly in the tropics by direct gravity wave drag forcing

msx satellite observations of thunderstorm generated gravity waves in mid wave infrared images of the upper stratosphere data from the midcourse space experiment msx has provided the first observations of thunderstorm generated gravity waves imaged from space gravity wave theory predicts that isolated sufficiently convective thunderstorms can launch waves and create a unique intensity pattern of concentric circles on a radiating surface of constant altitude above such a storm among the msx constant nadir angle mid wave infrared mwir observations two instances of such patterns have been identified it was confirmed from meteorological satellite images that highly convective isolated thunderstorms occurred at the locations and times expected

the seafloor spreading rate dependence of the anomalous skewness of marine magnetic anomalies summary we have determined the anomalous skewness of marine magnetic anomalies other than anomaly 33 34 in slow spreading regimes in the arctic atlantic and indian oceans and found a significant dependence of the anomalous skewness on the spreading rate the anomalous skewness is highest for slow spreading and insignificant for spreading above 6 cm yr 1 also the anomalous skewness of anomaly 33 34 is significantly larger than that of anomalies 25 26 and 20 21 whether or not anomalies m2 m4 exhibit anomalous skewness is uncertain because of the large variability in their shapes to facilitate the study of anomalous skewness of marine magnetic anomalies we have developed an analytic method based on functions calculated directly from the magnetic profiles that does not require a priori knowledge about the exact locations of magnetization contrasts

sensitivity of the latitude of the surface westerlies to surface friction the sensitivity to surface friction of the latitude of the surface westerlies and the associated eddy driven midlatitude jet is studied in an idealized dry gcm the westerlies move poleward as the friction is reduced in strength an increase in the eastward phase speed of midlatitude eddies is implicated as playing a central role in this shift this shift in latitude is mainly determined by changes in the friction on the zonal mean flow rather than the friction on the eddies if the friction on the zonal mean is reduced instantaneously the response reveals two distinctive adjustment time scales in the fast adjustment over the first 10 20 days there is an increase in the barotropic component of zonal winds and a substantial decrease in the eddy kinetic energy the shift in the surface westerlies and jet latitude occurs in a slower adjustment the space time eddy momentum flux spectra suggest that the key to the shift is a poleward movement in the subtropical critical latitude associated with the faster eastward phase speeds in the dominant midlatitude eddies the view is supported by simulating the upper tropospheric dynamics in a stochastically stirred nonlinear shallow water model

the effect of the madden julian oscillation on the north atlantic oscillation using idealized numerical experiments abstractthe aim of the paper is to investigate the influence of the madden julian oscillation mjo on the north atlantic oscillation nao using a quasigeostrophic model on the sphere a simplifie

elevated temperature viscous remanent magnetization of natural and synthetic multidomain magnetite abstract the time temperature relationship of magnetization is a subject of much interest and debate by paleomagnetists rock magnetists and magnetic anomaly modellers we have investigated this relationship by studying the viscous remanent magnetization vrm of coarse grained multidomain md magnetite our experiments covered the temperature range from 22 to 400 c times from minutes to months and included both australian granulite samples and multidomain magnetite samples synthesized by the glass ceramic method vrm acquisition was found to generally increase with temperature but not always at the rate predicted from classical thermal fluctuation theories thermal cycling between room temperature at which the measurements were made and the vrm acquisition temperature sharply decreased the temperature dependence of the vrm room temperature vrm acquisition accelerates with time when plotted on a semilog plot whereas at elevated temperature the curves are quasilinear against log time for both the natural and synthetic samples this change in behavior may suggest a variation in the vrm acquisition mechanism as a function of temperature for md magnetite the granulites have a nearly linear increase in vrm acquisition rate with temperature whereas the glass ceramics display little change in the acquisition rate between 22 and 200 c but increase by nearly a factor of 3 by 400 c the increase in vrm of the glass ceramics between 200 and 400 c is in general qualitative agreement with thermal fluctuation theory there was no systematic change in the rate of vrm acquisition with grain size for the multidomain magnetites used in this study elevated temperature e g 400 c vrm acquisition by the deep crustal granulites if extrapolated over the brunhes chron would produce a magnetization of several a m which if true is of the order required by models for the source of long wavelength magnetic anomalies

magnetic viscosity of some doleritic basalts in relation to the interpretation of the oceanic magnetic anomalies the growth of induced magnetization vim with time t in a 5 oe acquisition field followed by subsequent decrease in zero field of the viscous remanent magnetization vrm with time t was investigated at room temperature for 5 oceanic doleritic basalts and at temperature up to 105 c for 1 dike sample both vim acquisition and vrm decay are enhanced by increasing temperature and time however the growth and decay do not uniformly obey a log t law on an experimental time of 2 hours the results show that far from the ridge axes at the bottom of oceanic pillow layer 2a and in the upper part of the dike layer 2b the induced magnetization might largely dominate the remanent one while only a small fraction of the primary trm corresponding to the highest blocking temperatures tb would be preserved the partial trm corresponding to the lowest tb having been replaced by a viscous magnetization consequently the doleritic basalts would rarely if ever contribute to oceanic magnetic anomalies

effect of upper and lower level baroclinicity on the persistence of the leading mode of midlatitude jet variability abstractthe sensitivity of the variability of an eddy driven jet to the upper and lower level baroclinicity of the mean state is analyzed using a three level quasigeostrophic model on the sphere

gravity waves generated by convection in the darwin area during the darwin area wave experiment 1 we describe a model study of gravity wave generation by convection near darwin during the darwin area wave experiment dawex we focus on an 7 hour period on 17 november 2001 for our study which included a major hector event over the tiwi islands followed by a continental convective outbreak that included a squall line to the southeast of darwin our model is a dry version of a three dimensional 3 d mesoscale cloud resolving model with horizontally uniform background wind and stability fields the model is forced with a spatially and temporally varying heating field representative of the convective latent heating in the area we derive this heating field from 3 d volumetric reflectivity from the precipitation radar located just north of darwin at gunn point during dawex the conversion from radar reflectivity to a 3 d latent heating field requires numerous assumptions that do not allow a quantitative heating estimate but which do provide a very realistic measure of the spatial morphology and temporal variations of the latent heating gravity waves generated by convective heating are known to be very sensitive to these characteristics of the heating so our resulting wave field is likely to be the most realistic possible description of the waves emitted from convection in the darwin area uncertainty in the wave amplitudes is large both because the input forcing heating is uncertain and because the use of heating as the sole proxy for wave forcing leads to additional wave amplitude errors we therefore compare the results of the model to other dawex measurements and previous modeling studies both to validate the waves in our model and to calibrate the wave amplitudes our study in turn aids in the interpretation of the other gravity wave measurements during dawex

vertical fluctuation energy in united states high vertical resolution radiosonde data as an indicator of convective gravity wave sources 1 convectively generated internal gravity waves at extratropical latitudes are difficult to identify by climatological analysis of the temperature and horizontal wind fields from radiosonde profiles using traditional analysis methods here we show that by analyzing ascent rate profiles we define a new variable vertical fluctuation energy ve we can identify convection sources in climatological analyses analysis of a 9 year time series 1998 2006 of united states high vertical resolution radiosonde data shows that ve maximizes in summer in midlatitudes within the troposphere 2 8 9 km and peaks at local afternoon early evening during the summer over most of the contiguous united states furthermore the apparent dominant vertical wavelength based on fourier analysis of the low pass filtered ascent rate fluctuations also increase and decrease with ve both on diurnal and seasonal timescales ve in the lower stratosphere however does not show this same relationship to convection but analysis of the vertical wavelength does show some of the features seen in tropospheric ve unlike midlatitude stations ve within both the troposphere and the lower stratosphere over tropical western pacific island stations is highly correlated with convective precipitation and inversely correlated with outgoing longwave radiation we interpret this difference by using the 4 d gravity wave regional or global ray tracer ray tracing model with a source spectrum representative of a convection source

implementation of a gravity wave source spectrum parameterization dependent on the properties of convection in the whole atmosphere community climate model waccm 1 this article describes the implementation of a new source spectrum parameterization for convectively generated gravity waves in the whole atmosphere community climate model version 2 waccm2 this parameterization is active in the tropics and is interactive with the underlying convection the gravity wave spectrum at each model grid point and time step is determined based on the convective heating depth tropospheric wind and the convective heating rate this parameterization introduces more realistic spatial and temporal distributions of gravity wave activity and yields gravity wave characteristics related to the underlying wave source we present the estimated gravity wave momentum flux phase speed spectra at 100 hpa as well as the gravity wave drag in the middle and upper atmosphere using the new source spectrum representation we also show the effects of this more physically based parameterization on the structure of the stratospheric and mesospheric semi annual oscillations of the zonal wind compared to a simulation with uniform gravity wave distribution we conclude that the interaction of the gravity wave parameterization with tropospheric characteristics is a positive step toward a more physical representation of the middle and upper atmosphere

response of baroclinic life cycles to barotropic shear abstract cyclonic barotropic shear of incrementally increasing magnitude is imposed on an idealized midlatitude jet and the life cycles of baroclinically unstable wavenumber 6 perturbations growing on these jets are studied when the barotropic shear parameter passes a critical value the life cycle makes an abrupt transition from anticyclonic to cyclonic behavior the abrupt transition in behavior is most evident in the barotropic decay of eddy kinetic energy in the structure of the eddies as seen in potential vorticity maps and in the nature of the zonal flow accelerations produced during the life cycle it is suggested that the abrupt transition can be interpreted as arising from a positive feedback between eddy propagation and eddy induced zonal flow accelerations dependences on the zonal scale of the eddy are also investigated wavenumber 8 exhibits cyclonic behavior while wavenumber 4 exhibits anticyclonic behavior for all values of cyclonic shear considered this dependence on zonal scale is c

using satellite observations to constrain parameterizations of gravity wave effects for global models abstract small scale gravity waves are common features in atmospheric temperature observations in satellite observations these waves have been traditionally difficult to resolve because the footprint or resolution of the measurements precluded their detection or clear identification recent advances in satellite instrument resolution coupled to innovative analysis techniques have led in the last decade to some new global datasets describing the temperature variance associated with these waves such satellite observations have been considered the best hope for quantifying the global properties of gravity waves needed to constrain parameterizations of their effects for global models although global maps of averaged gravity wave temperature variance have now been published from a variety of different instruments on earth orbiting platforms these maps have not provided the needed constraints the present paper first summarizes what has been learned from traditional temporally and spatially averaged analyse

characteristics of gravity waves with short vertical wavelengths observed with radiosonde and gps occultation during dawex darwin area wave experiment 1 we conducted radiosonde soundings during three intensive observation periods iop of the darwin area wave experiment dawex campaign in october to december 2001 and measured humidity temperature and wind velocity up to 30 35 km every 3 hours for 40 times at three sites in each iop we analyzed height profiles of kinetic ek and potential ep energy per unit mass caused by gravity waves with vertical wavelengths less than 3 km the wave energy was clearly enhanced between 15 20 km and 25 30 km and it was considerably depressed at 20 25 km between the two height regions this feature was seen at all the sites during the three iops different types of wave activity seem to contribute to the enhancement of ek and ep below 20 km and at 25 30 km height distribution of the wave energy at 20 30 km seems to correlate with the structure of the mean zonal wind shear we also analyzed a latitude height section of ep in october to december 2001 using gps occultation data with the champ satellite collected around the dawex sites 80 180 e and 30 n to 30 s compared with the latitude distribution of ep with gps the dawex results were slightly smaller and larger at 20 25 km and 25 30 km respectively the longitude variation of ep is also analyzed from the gps data at 10 15 s which generally agreed well with the dawex results at both 20 25 km and 25 30 km

generation and trapping of gravity waves from convection with comparison to parameterization some parameterizations of gravity wave mean flow forcing in global circulation models gcms add realism by describing wave generation by tropospheric convection because the convection in gcms is itself a parameterized process these convectively generated wave parameterizations necessarily use many simplifying assumptions in this work the authors use a realistic simulation of wave generation by convection described in previous work which was validated by observations from the darwin area wave experiment dawex to test these assumptions and to suggest some possible improvements to the parameterizations in particular the authors find that wave trapping in the troposphere significantly modifies the spectrum of vertically propagating waves entering the stratosphere above convective wave sources and offer a linear method for computing wave transmission and reflection effects on the spectrum suitable for inclusion in the parameterizations the wave fluxes originate from both a time varying heating mechanism and an obstacle effect mechanism acting in the simulation methods for including both mechanisms in the parameterizations are described waves emanating from the obstacle effect remain very sensitive to the depth of penetration of latent heating cells into an overlying shear zone which will continue to make it difficult to accurately parameterize in a gcm where the convective cells are not resolved

vhf profiler observations of winds and waves in the troposphere during the darwin area wave experiment dawex 1 a vhf atmospheric radar wind profiler was used to study tropospheric winds during the darwin area wave experiment dawex the profiler which operated at a frequency of 54 1 mhz was located at pirlangimpi garden point 11 4 s 130 5 e on the tiwi islands observations were made regularly up to heights near 8 km with maximum heights occurring when convective activity was strongest mean winds observed between october and december 2001 are in good agreement with conditions that prevailed across northern australia during this period during the first two intensive observation periods iop during october and november the zonal and meridional wind components were westward and northward respectively with stronger values in november by the time of iop3 in mid december the zonal flow was eastward a pattern that is typical of the australian monsoon fluctuations in the three wind components for periods less than 3 hours are analyzed for iop2 in november when strong convective storms hectors occurred on all afternoons over the tiwi islands the fluctuations which are ascribed to convectively generated gravity waves show a correspondingly strong diurnal cycle with horizontal wind variances peaking between 8 and 12 m2s 2 in the early afternoon in the lower troposphere variances are only 2 m2s 2 in the early morning hours a power spectral analysis shows that oscillations with ground based periods between 8 and 17 min are especially prominent during hector events the profiler observations are compared with a numerical model study of gravity wave generation by convection on 17 november 2001 there is a satisfactory degree of agreement between the behavior of the model and profiler oscillations both as a function of height and time

darwin area wave experiment dawex field campaign to study gravity wave generation and propagation 1 a field campaign studying aspects of the atmospheric dynamics from the ground to the lower thermosphere was conducted in australia during october to december 2001 this darwin area wave experiment dawex involved participation from nine institutions in the usa japan and australia and focused on studying wave perturbations in the middle atmosphere in conjunction with detailed observations of tropospheric moist convection the experiment involved extended deployment of six airglow imagers medium frequency radars and a boundary layer wind profiler tropospheric convection was observed by meteorological radars including the australian bureau of meteorology research centre c band polarized radar this paper provides an overview of the experimental setup and also discusses the basic meteorological fields during the experiment including some examples of convective activity near darwin wind and temperature fields extending up into the lower thermosphere were constructed on the basis of campaign observations and the ukmo global meteorological analysis these fields can be used for the modeling of gravity wave propagation through the middle atmosphere

seafloor spreading anomalies in the magsat field of the north atlantic we show that a simple magnetization model for the crust of the north atlantic basin will explain the major anomalies of the magsat intermediate wavelength magnetic field over the basin our seafloor spreading model incorporates anomalous skewness and enhanced magnetization near the spreading axis and at the cretaceous quiet zones the anomalies over the young oceanic crust indicate a decrease in susceptibility away from the spreading axis the enhanced magnetization over the cretaceous quiet zone could be due to an increased main field intensity or a change in the spreading center processes during the cretaceous though similar observations are not readily apparent for other basins the north atlantic correlation indicates that an investigation of the remanent magnetization of the ocean crust could provide a standard for the evaluation of the intermediate wavelength magnetic field the models presented in this paper demonstrate that a sizable percentage of the intermediate wavelength energy in the magsat field is derived from sources within the earth s crust it also demonstrates that remanent magnetization is an important source of the magsat field over oceanic areas we seek to demonstrate in this paper that although components of the main field are likely to contribute to the intermediate wavelength scalar field we can correlate the anomaly field over the north atlantic to the well studied thermal remanent magnetization of the oceanic crust with a minor component of induced magnetization associated with younger oceanic crust

on the existence of storm tracks abstract given that middle latitude weather systems transport heat in a manner such as to weaken the baroclinicity that is thought to be crucial to their growth it is perhaps surprising that concentrated regions of such eddy activity i e storm tracks are found in the northern hemisphere winter the existence and possible self maintenance of storm tracks is investigated using a linear stationary wave model with storm track region forcings taken from data averaged over a number of winters it is found that the direct thermal effect of the eddies does indeed act against the existence of the storm track their vorticity fluxes lead to some reduction of this effect it is argued that the mean diabatic heating in the storm track region is an indirect eddy effect this heating is found to maintain the mean maximum in baroclinicity in the region further the mean low level flow induced by the eddy effects is such as to enhance the warm western oceanic boundary currents that are crucial to the existence of t

gregarious tropical convection abstract a beat source with a vertical profile like that of observed tropical mesoscale convective systems mcss is shown to cause through inviscid gravity wave dynamics upward displacement at low levels in a mesoscale region surrounding the heating typical values are 10 30 area contraction at the surface everywhere within 270 km of the heating 6 h after it starts as a result conditions near an existing mcs but beyond the area of mcs outflow become more favorable for the development of additional convection this theory predicts that cloud clusters should be gregarious infrared satellite imagery confirms that almost half of the cold cloudiness observed in a month over the oceanic warm pool region was contributed by just 14 objectively defined multiday supercluters

marine magnetic anomalies over the lord howe rise and the tasman sea implications for the magnetization of the lower continental crust abstract marine geophysical and geological data provide evidence for a continental origin of the lord howe rise a submarine plateau east of australia our marine geomagnetic data reveal a long wavelength anomaly associated with the western margin of the rise forward and inverse modeling shows that the anomalies can be best explained by strong magnetization of the lower crust of the lord howe rise a geomagnetic map of the northern tasman sea that includes dampier ridge another elongated piece of presumably continental crust shows positive anomalies over areas of continental origin and negative ones over the deep sea basin this indicates higher values of the product of magnetization times thickness for the continental crust than for the oceanic crust three dimensional model calculations show that the high magnetization in the lower crust of eastern australia is limited to a maximum depth of 15 20 km this is in accordance with geothermal results that indicate the curie isotherm is at a shallow depth there the anomalies of the tasman sea are negative due to the negative part of the edge anomaly on the seaward side of the continental margin it is not necessary to hypothesize strong reversed magnetization in the lower oceanic crust and upper mantle

thermoremanent magnetization of the oceanic lithosphere inferred from a thermal evolution model implications for the source of marine magnetic anomalies abstract thermoremanent magnetization models of the oceanic crust and the uppermost part of the oceanic mantle are derived on the basis of a typical thermal evolution model of the oceanic lithosphere using different magnetic blocking temperature ranges the magnetization of the basaltic layer 2a has a similar pattern but with a lower intensity to that of the standard sea floor spreading magnetic model however the magnetization patterns of the lower crust and upper mantle are skewed so that in a given polarity period the total magnetization of a lithospheric column which is created early in the period is greater than that of a column which is created later in the period magnetic anomalies of the magnetization models are calculated at sea level and at magsat elevation for slow moderate and fast sea floor spreading velocities although the magnetization models are derived based on purely physical reasoning without using any a priori information about the observed marine and magsat magnetic anomalies they do yield magnetic anomalies which are similar to the observed anomalies the models explain several features of the marine and magsat anomalies the anomalous skewness of marine magnetic anomalies is explained in terms of the magnetization of the oceanic lower crust the magnetization of the oceanic upper mantle may account for the anomalous skewness observed over the cretaceous quiet zones in the atlantic ocean the skewness of the magnetic anomalies calculated at sea level decreases with the increase in the sea floor spreading velocity

magnetic structure of the upper kilometer of the marine crust at deep sea drilling project hole 504b eastern pacific ocean deep sea drilling project hole 504b is located in 5 9 m y crust in the eastern pacific ocean about 200 km south of the costa rica rift at 1076 m subbasement it is the deepest penetration of marine crust yet achieved we present here magnetic data from this hole especially from the recently cored leg 83 section which constitutes the lowermost 500 m these data when combined with those of other studies yield not only the deepest but also the most detailed and comprehensive picture of marine magnetic structure at a single site currently available the basement rocks from hole 504b can be divided into three major magnetic units the upper units the top 500 m or so of basement are essentially similar to other shallow marine basement sections consisting of a mixture of various types of extrusive basalt low temperature altered titanomagnetite is the dominant magnetic carrier and the magnetic properties of these rocks are comparable to other shallow marine basalts below the upper units is a 200 m transition zone consisting of a mixture of extrusives and dikes high temperature hydrothermal alteration much of it greenschist facies grade has caused oxidation exsolution of primary titanomagnetite as well as its partial or total replacement by silicates the dominant magnetic carrier is an fe rich titanomagnetite magnetically indistinguishable from pure magnetite the combination of oxidation exsolution and silicate replacement results in a very low natural remanent magnetization nrm the remaining 300 m are in the upper portion of the sheeted dike complex primary titanomagnetite is exsolved but alteration is less intense than in the transition zone nrm values are substantially higher than those of the transition zone due to less silicate replacement of primary titanomagnetite coupled with an uncertain contribution from secondary magnetite which occurs as a silicate alteration product the nrm magnitude is sufficiently high that the sheeted dike complex in this location at least may make a significant if not substantial contribution to the magnetic anomaly overall the magnetic properties of this hole especially the lower 500 m are strongly influenced by postemplacement alteration and may bear little or no resemblance to their values upon initial cooling as hydrothermal temperatures do not appear to have exceeded 400 c the nrm in the lower two sections is apparently a chemical rather than a thermal remanence as such models of marine magnetic structure that combine cooling models of the crust with the assumption that magnetic remanence is purely of thermal origin do not appear sufficient to adequately predict the properties of the deeper crust a better understanding of the nature of postemplacement alteration in the marine crust is required especially its characteristics at high temperatures and its effects on magnetic properties

westward generation of eastward moving tropical convective bands in toga coare mechanisms responsible for westward generation of eastward moving tropical convective bands in the tropical ocean and global atmosphere coupled ocean atmosphere response experiment toga coare are investigated using a two dimensional numerical cloud model sequential generation of new convective bands to the west of an old eastward moving convective band is successfully simulated in an environment of a convectively active day during toga coare characterized by west winds at low levels and strong easterlies aloft it is concluded that the westward generation of new convective bands is explained by a gravity wave mechanism two westward propagating modes excited below the convective cells moving westward relative to the convective bands appear to play an important role a slow propagating mode 15 ms 21 excited by a shallow convective band is ducted in the troposphere under an unstable layer of small richardson number containing its critical level a fast propagating mode 25 m s21 excited by a deep convective band is ducted in the troposphere under the remaining region of the convective cell containing its critical level these two modes propagate horizontally to the west and promote the growth of shallow convection into long lived convective bands a dry model with thermal forcing representing the convective cell showed that preferential excitation of westward propagating waves below the convective cell is due to westward motion and ascension of the convective cell a comparative simulation without the critical level confirms the proposed gravity wave mechanism

impact of a spectral gravity wave parameterization on the stratosphere in the met office unified model abstract the impact of a parameterized spectrum of gravity waves on the simulation of the stratosphere in the met office unified model um is investigated in the extratropical mesosphere the gravity wave forcing acts against the mean zonal wind and it dominates over the resolved wave forcing in the extratropical stratosphere the gravity wave forcing gives a small acceleration in the direction of the mean zonal wind both summer and winter stratospheric jets have improved maximum strength and tilt with height when the parameterized gravity wave forcing is included although the southern winter jet is still more vertically aligned than in observational analyses the timing of the seasonal breakdown of the southern winter vortex is also improved by the addition of gravity wave forcing in the tropics the most obvious impact is that the model reproduces the quasi biennial oscillation qbo with a realistic mean and range of periods it also reproduces most of the observed asymmetries between the easterl

suppression of baroclinic instability in horizontally sheared flows abstract baroclinic instability in the presence of a meridionally sheared barotropic component to the basic flow is studied using a two level model the inclusion of a linear shear results in merldionally confined normal modes with growth rates much reduced compared to the unsheared case similar results hold for more complicated situations such as a baroclinic jet with barotropic shear the results are applied to life cycle calculations in which it is shown that the nonlinear decay phase of a baroclinic disturbance sets up barotropic shears whichtend to suppress further baroclinic developments an example of the southern hemisphere winter mean flow is found to be strongly stabilized by virtue of a strong sheared barotropic part of the zonal flow