observational evidence of slantwise convective adjustment abstract attempts were made during two field experiments to fly instrumented aircraft along absolute momentum m surfaces as a means of accurately determining slantwise convective stability the application of this technique appears to have been quite successful we present the results of four such efforts conducted in the ascent regions of midlatitude cyclones observed during the new england winter storms and genesis of atlantic lows experiments in three of the four cases the atmosphere was almost exactly neutral to slantwise ascent while being quite stable to vertical displacements in the fourth case the atmosphere departed from neutrality but was also substantially drier evidently due to subsidence we find excellent agreement between assessments of stability based on the m surface flights and on cross sections constructed from rawinsonde observations on the basis of these results i hypothesize that slantwise convective neutrality is characteristic of the ascent regions of baroclinic cyclones and

sedimentary signatures and processes during marine bolide impacts a review abstract studies of submarine impact craters resulting from impacts of comets or asteroids demonstrate that the presence of water and the physical properties of target rocks have a major influence on sedimentary processes associated with meteorite impacts this results in difference in sedimentary signature of bolide impacts in marine environments compared to subaerial impact craters in subaerial impacts the targets are commonly hard rocks frequently of igneous and or metamorphic origin whereas in submarine impacts the targets are mostly unconsolidated or poorly lithified sediments or sedimentary rocks with high volumes of pore water such differences result in variability in crater morphology and in sedimentary processes inside and outside the impact area impacts in shallow water marine neritic environments produced craters with low or absent rims and wide and shallow brims as characterize by both the montagnais on the scotian shelf the mjolnir in the barents sea 45 and 40 km in diameter respectively and the chesapeake bay 90 km in diameter lack of elevated rims is thought to be the result of current reworking and resurge of the water back into the excavated cavity as the water in the crater is vaporized during this process resurge gullies can be cut across the rim while mass and debris flows turbidites and other gravity deposits are produced as results of tsunami and crater wall and central high collapse during and after the crater excavation stage such deposits are found both within and outside the crater structure the only difference between gravity deposits triggered by an impact or other rare events such as earthquakes is the admixture of various melt particles and possible enrichments in iridium in the former impacts near the shelf edge may cause partial collapse of the continental margin as shown by the montagnais and chicxulub impacts some of the gravity and debris flows generated by margin collapse may be channelized with final deposits up to several hundred meters thick extending for hundreds of kilometers from the impact site other impact features such as shatter cones tektites spherules and ni spinels shocked quartz isotropication and partial melts are common to both submarine and subaerial impacts theoretical calculations of the destructive forces of mega tsunami waves triggered by meteorite impact in the ocean greatly exceed those based on geologic evidence a dearth of gigantic tsunami evidence for the chicxulub impact outside of the gulf of mexico where from theoretical modelling the maximum near bottom orbital velocity of water flow crossing the deep north atlantic basin should have been 1 m s could be the result of the mitigating effect of the bathymetry of surrounding area causing wave diffraction and interference calculated maximum horizontal orbital velocity near the seafloor at the shelf for the montagnais impact is 22 m s for a 200 m high wave 5 5 m s for a 50 m high wave at 500 km decreasing to 0 5 m s at 1000 km strong enough to scour the deep ocean bottom and produce distinct erosion surfaces and disconformities in marine sedimentary record however lack of cores across impact horizon prevents confirmation of occurrence of such bottom water flows

numerical modeling for target water depth estimation of marine target impact craters 1 marine impacts can develop a crater in the seafloor if the target water depth is shallow in relation to the size of the impactor the geology of this marine target crater is influenced by the layer of water in the upper part of the target the influence increases with increased water depth the target water depth is essential when calculating the magnitude of the impact event as the seafloor crater merely expresses some of the expended energy the target water depth for a marine target crater is often estimated by facies analysis of sediments related to the impact crater unfortunately often with ambiguous results we propose to combine the conventional methods with numerical modeling of the target water depth based on the special target water related features of the crater we used geological data from the lockne crater as constraints in a simulation with the sova hydrocode the simulations were done for three different target water depths 200 m 500 m and 1000 m within the likely depth range for the middle ordovician sea in which the impact occurred in order to obtain the minimum estimated size of the lockne crater by an asteroid impact at 20 km s the water depth must have been slightly less than 1 km and the impactor radius must have been about 400 m it was not possible to generate a crater with the geological features of lockne for target water depths of less than about 500 m however the ratio may be further constrained by factors such as impactor density impact angle and impact velocity

large impact crater in the barents sea seismic profiles in the central barents sea record a conspicuous structure exhibiting gross morphological and structural characteristics closely resembling those of large impact craters this feature named the mjolnir structure is 39 km in diameter and of jurassic early cretaceous age it probably formed by the impact of an asteroid or comet 0 7 2 5 km in diameter into a shallow sea underlain by 5 km of sedimentary section if the impact interpretation is correct mjolnir is one of the largest well preserved craters in the errestrial record

short baroclinic waves of small amplitude in a moist environment baroclinic instability at short wavelengths is investigated using a primitive equation non hydrostatic two dimensional numerical model in which the vertical shear is uniform and in which moisture is present in the lower part of the troposphere the height of the transition between saturated and unsaturated air determines the vertical and horizontal scales of the most unstable modes as the transition region is raised the most unstable wavelength increases until the large scale moist eady wave extending to the tropopause prevails

transitional morphology in west deuteronilus mensae mars implications for modification of the lowland upland boundary abstract west deuteronilus mensae which lies along the lowland upland boundary of mars at 45 latitude 345 longitude exhibits both gradational l a rossbacher 1985 in models in geomorphology woldenberg ed pp 343 372 allen unwin boston and fretted terrain r p sharp 1973 j geophys res 78 4073 4083 boundary types to the west the boundary is gradational and to the east it is defined by fretted terrain the two types of boundaries do not merge with one another within the west deuteronilus mensae region however the gradational boundary materials appear to overlap the fretted terrain by about 500 km contacts between units associated with the gradational boundary and the fretted terrain boundary can both be recognized in the region of overlap the fretted terrain can be identified as much as 300 km north of the gradational boundary in this region lowland units associated with the gradational boundary embay canyons of the fretted terrain in a topographically conformal fashion changes in the fretted terrain across the gradational boundary from uplands to lowlands include a reduction of canyon wall slopes and depths such that the fretted terrain north of the gradational boundary appears mantled but not obscured there are at least two major classes of processes which might explain the lateral overlap 1 erosion of stratified upland terrain and 2 deposition of plains materials onto the sloping upland margin and fretted terrain erosion of stratified upland terrain does not adequately explain the plainward decrease in crater densities across the gradational boundary nor is it consistent with evidence that lowland plains material appears to onlap the sloping upland margin in several places of the possible plains emplacement mechanisms eolian deposition would not produce the sharp apparently topographically conformal gradational unit contacts volcanics plains emplacement would not preserve the complex geometry of the underlying fretted terrain sediment deposition in either a liquid or ice covered sea could produce the draped appearance of the fretted terrain outflow channels along the lowland upland boundary particularly those of the circum chryse and west elysium regions may have flooded the northern lowlands to depths of tens to hundreds of meters the gradational unit contacts may represent the shorelines of such a sea the scale of characteristic morphologies along these contacts may require an unfrozen condition of sufficient duration to allow lacustrine style wave erosion and redistribution of material along the contacts our limited understanding of the martian paleoclimate and h 2 o inventories allows the possibility of clement periods in the past and other geological evidence e g small valley networks and outflow channels strongly suggests an extensive role of liquid water

sedimentological analysis of resurge deposits at the lockne and tvaren craters clues to flow dynamics the lockne and tvren craters formed about 455 million years ago in an epicontinental sea where seawater and mainly limestones covered a crystalline basement the target water depth for tvaren apparent basement crater diameter d 2 km was probably not over 150 m and for lockne d 7 5 km recent best fit numerical simulations suggest the target water depth of 500 700 m lockne has crystalline ejecta that partly cover an outer crater 14 km diameter apparent in the target sediments tvren is eroded with only the crater infill preserved we have line logged cores through the resurge deposits within the craters in order to analyze the resurge flow the focus was clast lithology frequencies and size sorting we divide the resurge into resurge proper with water and debris shooting into the crater and ultimately rising into a central water plume anti resurge with flow outward from the collapsing plume and oscillating resurge not covered by the line logging due to methodological reasons with decreasing flow in diverse directions at lockne the deposit of the resurge proper is coarse and moderately sorted whereas the anti resurge deposit is fining upwards and better sorted the tvren crater has a smoothly fining up section deposited by the resurge proper and may lack anti resurge deposits at lockne the content of crystalline relative to limestone clasts generally decreases upwards which is the opposite of tvren this may be a consequence of factors such as crater size i e complex versus simple and the relative target water depth the mean grain size i e the mean phi value per meter and standard deviation i e size sorting for both craters can be expressed by the equation 0 60 1 25

spatiotemporal development of irreversible mixing in midlatitude baroclinic wave life cycles morphology energetics and nonisentropic mixing activity the significance of nonisentropic irreversible mixing processes is diagnosed for idealized simulations of synoptic scale baroclinic wave life cycles along the subtropical jet stream using a nonhydrostatic anelastic mesoscale model subject to a free slip surface boundary condition a variety of morphological features of mixing is identified such as a mesoscale columnar vortex associated with the onset of frontal fracture episodic overturnings along the surface fronts wrinkling of the tropopause and injection of tropospheric air into the stratosphere the evolution of the degree of nonisentropic irreversible mixing is first analyzed by computing the change of the base component of potential energy that cannot be converted into kinetic energy the structure of the mixing activity is also diagnosed through inspection of spatiotemporal changes of entropy to demonstrate that the surface fronts are by far the most active regions of such activity this activity is found to be primarily longitudinal extending from the surface to the lower stratosphere to form a three dimensional spiral in the synoptic scale cyclone and along the fronts however an exceptional region also exists along the warm front where the structure becomes primarily transverse in the mature phase of frontal development in a model including an explicit representation of small scale turbulence in all simulations the net transfer of mass and heat across the tropopause is from troposphere to stratosphere the maximum transfer occurs when the observed climatological level of stratification contrast is assumed between stratosphere and troposphere the same climatological choice leads to a minimum net irreversible mixing which occurs primarily at earth s surface

wintertime observations of subtropical mode water formation within the gulf stream 1 we study the structure of subtropical mode water stmw within the eastward flowing gulf stream as it forms during strong winter cooling shipboard observations using seasoar and adcp reveal that while active mixing by gravitational instabilities is common large vertical and lateral shears of the gulf stream play a central role in determination of the modes of active mixing evidence is presented that low static stability and large vertical shear can combine to cause slantwise convection symmetric instabilities while the large anticyclonic shears to the south of the gulf stream core can cause low absolute vorticity and precondition the ertel potential vorticity to be small and more susceptible to instabilities the area of active mixing driven by surface forcing in the presences of shear occupies a swath 50 90 km wide immediately south of the gulf stream core at the northern edge of the sargasso sea

a wide cold frontal rainband and its relationship to frontal topography a wide cold frontal rainband associated with a vigorous surface cold front was investigated using conventional meteorological data and the velocity and reflectivity data from high resolution doppler radar it was found that the movement of the rainband was closely related to the velocity of the frontal surface and that the substructural features of the rainband corresponded closely with the topographical features of that surface the evolution of the topography of the frontal surface is described and some processes that might affect frontal topography and therefore the structure and movement of rainbands are examined

marine target craters on mars an assessment study observations of impact craters on earth show that a water column at the target strongly influences lithology and morphology of the resultant crater the degree of influence varies with the target water depth and impactor diameter morphological features detectable in satellite imagery include a concentric shape with an inner crater inset within a shallower outer crater which is cut by gullies excavated by the resurge of water in this study we show that if oceans large seas and lakes existed on mars for periods of time marine target craters must have formed we make an assessment of the minimum and maximum amounts of such craters based on published data on water depths extent and duration of putative oceans within contacts 1 and 2 cratering rate during the different oceanic phases and computer modeling of minimum impactor diameters required to form longlasting craters in the seafloor of the oceans we also discuss the influence of erosion and sedimentation on the preservation and exposure of the craters for an ocean within the smaller contact 2 with a duration of 100 000 yr and the low present crater formation rate only 12 detectable marine target craters would have formed in a maximum estimate with a duration of 0 8 gyr as many as 1400 craters may have formed an ocean within the larger contact 1 meridiani with a duration of 100 000 yr would not have received any seafloor craters despite the higher crater formation rate estimated before 3 5 gyr on the other hand with a maximum duration of 0 8 gyr about 160 seafloor craters may have formed however terrestrial examples show that most marinetarget craters may be covered by thick sediments ground penetrating radar surveys planned for the esa mars express and nasa 2005 missions may reveal buried craters though it is uncertain if the resolution will allow the detection of diagnostic features of marine target craters the implications regarding the discovery of marine target craters on mars is not without significance as such discoveries would help address the ongoing debate of whether large water bodies occupied the northern plains of mars and would help constrain future paleoclimatic reconstructions

when a cosmic impact strikes the sea bed most bolides that collide with the earth hit the sea limited knowledge about marine target impacts hampers predictions about their perilousness this study presents geological features that are particular to craters formed at sea the features are most likely a result of the influence of the target on the cratering process marine target craters form only if the target sea is shallow enough to admit sufficient kinetic energy into the sea bed when the crater diameter is large compared to the water depth the crater resembles its counterparts that are formed on land craters formed in deeper water are concentric and often lack melt sheets and rim walls but have deposits and radial gullies formed by the resurge of the sea impacts on the deep shelf are probably much more energetic than is suggested by the dimensions of the preserved crater

the degradation of impact craters in maja valles and arabia mars photoclinometry is being utilized to quantify the degree of degradation suffered by martian impact craters 1 to 5 km diameter range in the arabia and maja valles regions results indicate that present crater depths can vary substantially from crater depths expected for the original fresh craters at all sizes examined localized regions of high moderate and low degradation of the order of 102 km2 have been delineated using the percentage change in crater depth diameter between current and assumed fresh crater morphology regions of low degradation occur randomly across these regions and probably represent recently produced impact craters which have undergone little degradation since their formation aeolian activity including current mantling deposits appears to dominate the processes degrading craters in the arabia region with fluvial and impact processes playing a secondary role in the maja valles region crater degradation is associated primarily with fluvial outflow channel activity with aeolian and impact processes contributing secondary effects to the destruction of impact craters

a study of candidate marine target impact craters in arabia terra mars previous workers have proposed that a northern ocean existed early during martian geologic history and the shorelines of that ocean would coincide roughly with the crustal dichotomy that divides the smooth northern lowlands with the cratered southern highlands arabia terra is a region on mars that straddles the crustal dichotomy and several proposed shorelines are located in the area shallow marine impact craters on mars likely would exhibit features like those on earth including characteristic morphological features that are distinctly different from that of craters formed on land common attributes of terrestrial marine impact craters include features of wet mass movement such as gravity slumps and debris flows radial gullies leading into the crater depression resurge deposits and blocks of dislocated materials crater rim collapse or breaching of the crater wall a central peak terrace or peak ring terrace and subdued topography an indicator of both age and possible flood inundation immediately following impact in this article these features have been used to evaluate craters on mars as to a possible marine origin this study used a simple quantification system to approximately judge and rank shallow marine impact crater candidates based on features observed in terrestrial analogs based on the quantification system 77 potential shallow marine impact craters were found within an area bounded by 20 n and 40 n as well as 20 w and 20 e nine exemplary candidates were ranked with total scores of 70 or more in a second smaller study area impact craters of approximately similar size and age were evaluated as a comparison and average total scores are 35 indicating that there is some morphological difference between craters inside and outside the proposed shorelines results of this type of study are useful in helping to develop a general means of classification and characterization of potential marine craters

formation of resurge gullies at impacts at sea the lockne crater sweden the lockne crater in sweden is a marine target crater formed in a shelf sea approximately 460 ma ago the crater structure consists of an inner crater surrounded by an outer inclined surface that extends to almost 12 km from the center marine craters differ in several respects from craters formed on land one special feature is the formation of resurge gullies excavated by the erosional force of the resurging sea water after the impact the formation of these gullies strongly depends on the ratio crater rim height to water depth as well as on the size of the impact structure such gullies are known from very few marine target craters at the lockne impact site four gullies are identified each of which cuts radially through the rim of the outer crater the rapid collapse of that part of the crater cavity which formed in the seawater resulted in forceful flooding of the crater the resurging seawater not only contained fallback ejecta on its way towards the cavity on the sea bottom it incorporated fractured lithologies from the sea bottom as well this entrained material disintegrated during transport and constitutes today the dominantly monomict lower part of the resurge sequence the resurge flood was highly turbulent highly erosive and developed to a probable hyperconcentrated flow or a possible water flood the erosion in the gullies proceeded as headward erosion down to the transition zone between the brecciated and the less disintegrated crystalline basement

monsoons itczs and the concept of the global monsoon earth s tropical and subtropical rainbands such as intertropical convergence zones itczs and monsoons are complex systems governed by both large scale constraints on the atmospheric general circulation and regional interactions with continents and orography and coupled to the ocean monsoons have historically been considered as regional large scale sea breeze circulations driven by land sea contrast more recently a perspective has emerged of a global monsoon a global scale solstitial mode that dominates the annual variation of tropical and subtropical precipitation this results from the seasonal variation of the global tropical atmospheric overturning and migration of the associated convergence zone regional subsystems are embedded in this global monsoon localized by surface boundary conditions parallel with this much theoretical progress has been made on the fundamental dynamics of the seasonal hadley cells and convergence zones via the use of hierarchical modeling approaches including aquaplanets here we review the theoretical progress made and explore the extent to which these advances can help synthesize theory with observations to better understand differing characteristics of regional monsoons and their responses to certain forcings after summarizing the dynamical and energetic balances that distinguish an itcz from a monsoon we show that this theoretical framework provides strong support for the migrating convergence zone picture and allows constraints on the circulation to be identified via the momentum and energy budgets limitations of current theories are discussed including the need for a better understanding of the influence of zonal asymmetries and transients on the large scale tropical circulation

explosive volcanism in the tharsis region global evidence in the martian geologic record 1 a global equatorial set of layered deposits on mars has been reexamined with mars global surveyor data the stratigraphy morphology and erosional characteristics of units separated by thousands of kilometers are remarkably similar to each other and consistent with a widespread episodic subaerial source of fine grained material this study is the first to propose a genetic link between many units that have traditionally been considered on a local or regional level including the medusae fossae formation valles marineris interior deposits chaotic terrain terra meridiani and arabia materials mapping relationships suggest that all of these units are post noachian in age and formation may have continued until the recent past thinning of deposits with increasing distance from the tharsis rise suggests that much of these materials may be volcanic ash flows and air fall from explosive eruptions in the tharsis region analogous to those produced by terrestrial plinian eruptions contemporary martian winds would preferentially transport ash from an eruption plume near tharsis east or west along the equator depending on the season erosion has removed much of the original volcanic ash and only isolated layered stacks remain sometimes in topographic depressions that inhibit erosion modeling indicates that widespread dispersion of ash after an eruption is easily attainable in present or past atmospheric conditions consistent with the hypothesis that the far reaching deposits may be composed of ash from past explosive eruptions in the vicinity of tharsis

limits on the extent of the solsticial hadley cell the role of planetary rotation abstractthe role of planetary rotation in limiting the extent of the cross equatorial solsticial hadley cell shc is investigated using idealized simulations with an aquaplanet general circulation

baroclinic eddy equilibration under specified seasonal forcing baroclinic eddy equilibration under a northern hemisphere like seasonal forcing is studied using a modified multilayer quasigeostrophic channel model to investigate the widely used quick baroclinic eddy equilibration assumption and to understand to what extent baroclinic adjustment can be applied to interpret the midlatitude climate under aslowly varyingseasonalforcing the eddyand meanflow seasonalbehavioris characterized by four clearly divided time intervals an eddy inactive time interval in summer a mainly dynamically determined eddy spinup time interval starting in midfall and lasting less than one month and a quasi equilibrium time interval for the zonal mean flow available potential energy from late fall to late spring with a mainly external forcing determined spindown time interval for eddy activity from late winter to late spring the baroclinic adjustment can be clearly observed from late fall to late spring the sensitivity studyof the eddyequilibrationto the time scale of the external forcing indicatesthat the time scale separation between the baroclinic adjustment and the external forcing in midlatitudes is only visible for external forcing cycles one year and longer in spite of the strong seasonality of the eddy activity similar to the observations a robust potential vorticity pv structureisstillobservedthroughalltheseasons however itisfoundthatbarocliniceddyisnottheonly candidatemechanismtomaintaintherobustpvstructure theroleoftheboundarylayerthermalforcingand the moistconvectionin maintaining the lower levelpv structureisdiscussed theadjustment and thevertical variation of the lower level stratification play an important role in all of these mechanisms

role of water in the formation of the late cretaceous wetumpka impact structure inner gulf coastal plain of alabama usa the effect of shallow marine water 30 100 m deep in the late excavation and early modification stages of a marine target crater 5 km in diameter as exemplified by the late cretaceous wetumpka impact structure in alabama usa is manifest in the early collapse of a weak part of the rim excavation flow and connate marine water are interpreted to be factors in this collapse this partial rim collapse catastrophically emplaced an upper structure filling unit of broken and redistributed sedimentary target formations which presently mantles the deeper fallback breccia deposits within the structure furthermore rim collapse flow facilitated the formation of a structurally modified extrastructure terrain which is located outside and adjacent to the collapsed rim segment this extrastructure terrain appears to be the product of extensive slumping of poorly consolidated target sedimentary formations

reexamining the dynamics of short scale diabatic rossby waves and their role in midlatitude moist cyclogenesis abstract as a step toward a more comprehensive study of the physical processes that underlie explosive cyclogenesis a two dimensional semigeostrophic model with a now standard parameterization of latent heat release is used to diagnose the structure energetics and propagation characteristics of short scale diabatic normal modes in a moist baroclinic atmosphere with the eady basic state upon revisiting the inviscid problem it is found that when a thermodynamically consistent vertical profile of latent heat release is used the short wave cutoff vanishes and growth rates become independent of zonal wavelength for zonal wavelengths shorter than approximately 1900 km the destabilized short scale modes identified previously as diabatic rossby waves owe their existence to the continuous generation of potential vorticity by moist processes associated with warm air advection rising motion and latent heat release to determine if these short scale diabatic rossby wave modes continue to grow at small

oceanic impacts a growing field of fundamental geoscience the importance of oceanic impacts of collisional events and resulting energy release are briefly described data collection methods from the eltanin a mesosiderite projectile deep water impact are presented

shock metamorphism on the ocean floor numerical simulations abstract impact craters are unknown in deep ocean basins important reasons for this are that the ocean crust is younger than continents and the impacts are weakened by the water column laboratory experiments can show specific features of water impacts however impact velocities are too small

the role of inertial instability in determining the location and strength of near equatorial convection there are two major organized cloud configurations in the vicinity of the equator where there is a small cross equatorial surface pressure gradient convection is close to the equator and is generally tied to the location of the lowest sea level pressure slp and warmest sea surface temperature sst in agreement with arguments based upon simple thermodynamical considerations however when there is a substantial cross equatorial pressure gradient such as occurs in the monsoon regions organized convection appears off the equator in the summer hemisphere equatorward of the slp minimum and not necessarily collocated with the warmest ssts thus in this instance simple thermodynamical considerations alone cannot explain 2he location of the convection in this situation the zero absolute vorticity contour 0 also lies in the summer hemisphere therefore between the equator and the 0 contour is a region of locally anticyclonic absolute vorticity and an inertially unstable regime it is argued that the convection results from the low level divergence convergence doublet centred about the 0 contour which is the mitigating response to the inertial instability the associated latitude height secondary circulation should provide subsidence suppressed convection over the equator and rising motion enhanced convection to the north of the zero absolute vorticity contour signatures of the inertial instability predicted by theory are found in observations supporting the hypothesis wherever a strong cross equatorial pressure gradient exists the 0 contour bisects a maximum in the divergent wind field divergence is found equatorward of the zero contour and convergence on the poleward side latitude height cross sections show strong local meridional circulations with maximum rising motion on the poleward side of 0 as the regions where the rising motions occur are conditionally unstable there is deep convection and the vertical circulations extend throughout the troposphere it is noted that the intensity of the off equator convection is deeper and probably stronger than convection located at the equator this is probably because the convection associated with the inertial instability is more efficient necessary conditions for the location of near equatorial convection are listed arguments are presented whereby inertial instability is established as the cause rather than an effect of off equatorial convection these include an outline of the sequence of processes leading up to the convection the factors that limit the encroachment of the 0 contour into the summer hemisphere are discussed and an explanation for the existence of the low level westerly monsoon wind maximum is suggested the possible role played by the instability mechanism or the lack of it in coupled model simulations that produce seasonally migrating and or double itczs in the eastern pacific ocean is discussed finally it is proposed that the instability mechanism is important in the initiation of westward moving disturbances found in the eastern pacific and in determining active and break periods in the summer indian monsoon

mesoscale dynamics of cold fronts structures described by dropsoundings in fronts 87 the fronts 87 project was a european experiment to make mesoscale observations to deduce the dynamics of active cold fronts approaching north west europe with a minimum of orographic influences in this paper data from dropsoundings made by the meteorological office s c 130 research aircraft are analysed the aircraft flew a pattern with 4 or 5 runs oriented approximately at right angles to the front but with each run displaced in the along front direction by about 100 km the dropsondes gave soundings to a height of approximately 7 km with a cross frontal spacing of 20 km at best and 60 km on average between 30 and 50 soundings were made in each event over an area of 500 500 km2 these data are unique in their mesoscale resolution over a synoptic scale region this was made possible by the ability to track up to 5 sondes in the air at any time several new aspects of cold front dynamics have been identified from these data the approach here is to treat each run as an instantaneous cross frontal section the discussion is centred on the hypotheses postulated before the experiment these were intended to allow verification or refutation of aspects of models of frontal dynamics such as those embodied in semi geostrophic theory a minimal and purely objective analysis has been performed on the data presented here but the high resolution of the dropsonde observations permits evaluation of differentiated quantities such as potential vorticity pv with some confidence issues raised here include the degree of thermal wind balance the structure of conserved variables such as equivalent potential temperature and absolute momentum the cross frontal circulation and the role of diabatic processes such as the evaporation of snow and the potential vorticity structure on the mesoscale and its interpretation the intention is to present an integrated view of the dynamical structure of fronts in the light of theoretical rather than conceptual or airflow models

formation process of intermediate water in baroclinic current under cooling the formation process of intermediate water in baroclinic current under cooling is investigated using a nonhydrostatic numerical model after baroclinic instability develops into finite amplitude in a short time strong downdrafts with a horizontal scale of 1 km are generated near the density front and subduct surface water to depths 400 m along isopycnals as a result patches of ventilated water of 10 20 km horizontal scale with anticyclonic circulation are formed at intermediate depths combined effects of baroclinic instability and convection are key dynamics for these phenomena convection acts as an initiator for baroclinic instability at the onset and accelerates its subsequent growth by reducing stratification developed baroclinic wave forms an intense density front and downdraft along isopycnals is generated through the frontogenetic process density change due to convection intensifies this frontal downdraft by strengthening the geostrophic forcing tendency to destroy the geostrophic balance and by reducing potential vorticity static stability further symmetric instability induced by the density change due to convection and intensified by the frontogenetic process drives slantwise convection which in turn enhances the frontal downdraft consequent downward velocity becomes 20 times as large as that of the frontal downdraft without convection cooling and twice larger than that of pure convection since the intensified frontal downdraft moves its position with time and induces divergent flow at depths a patch of ventilated water with a horizontal scale much larger than that of the frontal downdraft is formed the role of convection cooling in the formation process of intermediate water in this context is to enhance the frontal downdraft rather than to deepen the mixed layer this scenario is quite different from the one realized when baroclinic instability and convection do not coexist

north polar region craterforms on mars geometric characteristics from the mars orbiter laser altimeter abstract this study focuses on topographic characterization of ice associated north polar region impact craters and several enigmatic polar craterforms using new information from the mars orbiter laser altimeter mola an instrument aboard the mars global surveyor mgs orbiter we find that for ice associated craters the topography reveals several surprising results not previously apparent from orbital images alone first geometric properties for several impact craters associated with ice and frost deposits suggest that cavity infill is as high as 80 of reconstructed preerosion levels second craters associated with ice also demonstrate unique cavity geometries relative to their nonpolar counterparts finally in some cases ice associated impact features are anomalously deep on the basis of depths modeled from the best available scaling laws we suggest that burial of these impact features by either episodic advance of the polar cap margin or by continuous deposition at the highest rates previously estimated for the north polar region of mars has occurred subsequent stripping has exhumed the features leaving behind cavity infill deposits and a few of these display topographic levels above the surrounding preimpact surface in at least one case cavity interior deposits show layering with a typical thickness of 10 m suggesting episodic deposition and ablation of materials as in the polar layered terrain in addition to ice associated craters we investigated the topography of several enigmatic polar craterforms in particular a few craterforms within 150 km of the permanent north polar cap appear to resemble simple effusive lava shield volcanoes found on earth their geometric properties cannot be reconciled with previous suggestions that they were manifestations of martian hydromagmatic processes i e maar volcanism mola s initial measurements of impact craters and other craterforms in the north polar latitudes of mars support the concept of a geologically recent surface with evidence of effusive volcanism and enhanced sedimentation in addition the measured impact crater depths for ice associated craters suggest at least a few cases of enhanced excavation and thus a possibly weaker target relative to the typical northern hemisphere plains

geometric relationships of pristine martian complex impact craters and their implications to mars geologic history 1 based on the depths of pristine complex craters on mars we have estimated the final post modification phase depth dr to diameter d function of martian complex craters we suggest that the dr d function determined here holds globally for complex craters 12 km diameter the global nature of this function may be due to the stability of slopes in the highly fractured rock that makes up the steep interior walls of post modification phase craters the stability of such slopes should be controlled mainly by such factors as the nature of the fractures and not target strength this function is an essential benchmark for studies that utilize crater geometry as their basis for example the function can be used in gradation history of mars to predict the magnitude of degradation for instance an example 20 km diameter middle early noachian age crater that initially formed in the mid latitudes of mars with a depth of 1800 m instead of 1404 m as predicted by the fresh craters function has lost 1450 m of its depth over its history 350 m in the noachian 400 m during an intense episode of degradation at the end of the noachian and 700 m thereafter no evidence of the period of intense degradation is found in the high latitudes although those regions appear to have been affected by a process that reduced crater depth soon after they form

constraints on early events in martian history as derived from the cratering record the shapes and densities of crater size frequency distribution curves are used to constrain two major events early in martian history termination of high obliteration rates and viability of the multiple impact origin of the crustal dichotomy distribution curves of fresh craters superposed on uplands intercrater plains and ridged plains display shapes and densities indicative of formation prior to the end of heavy bombardment this observation correlates with other geologic evidence suggesting a major change in the erosional regime following the last major basin size impact i e argrye in addition the multisloped nature of the curves supports the idea that the downturn in the crater size frequency distribution curves reflects the size frequency distribution of the impactors rather than being the result of erosion the crustal dichotomy formed prior to the heavy bombardment intermediate epoch based on distribution curves of knobby terrain if the dichotomy resulted from a single gigantic impact this observation places constraints on when this event happened an alternate theory for dichotomy formation the multiple impact basin idea is questioned since distribution curves of large basins as well as heavy bombardment era units are not represented by a 3 differential power law function this study finds fewer basins missing on mars compare to the moon and mercury than previously reported the area covered by these missing basins is less than that covered the northern plains

lithostratigraphy and biostratigraphy of upper eocene and lower oligocene strata in southwest alabama and southeast mississippi abstract abstract upper eocene and lower oligocene strata in the eastern gulf coast region are represented by western clastic dominated deposits of mississippi and eastern carbonate dominated deposits of georgia and northern florida these lithofacies intergrade and intertongue across alabama the complexity of the lithofacies changes in the region has caused problems in the recognition of geological ages the stratigraphic position of the eocene oligocene boundary as determined by macrofossils and benthic microfossils is different from that position indicated by planktonic foraminifers the four members of the yazoo clay north twistwood creek clay cocoa sand pachuta marl and shubuta clay members the lateral equivalent of the shubuta clay member crystal river formation and the overlying red bluff clay bumpnose limestone and their associated foraminifers were studied from exposures at st stephens quarry in washington county alabama at little stave creek in clarke county alabama and on the chickasawhay river in wayne county mississippi based on vertical distributions of planktonic foraminifers at these localities the eocene oligocene boundary occurs at the top of the yazoo clay the lowermost two members of the yazoo clay were not placed in any specific foraminiferal biozones at this time due to the paucity of planktonic foraminifers in these beds but these units have been assigned to the globigerinatheka semiinvoluta interval zone by barker in blow 1979 the distributions of planktonic foraminifers in the pachuta marl and shubuta clay members at both localities allow assignment of these units to the globorotalia cerroazulensis s l interval zone while the red bluff clay bumpnose limestone is assigned to the pseudohastigerina micra interval zone

mjolnir structure an impact crater in the barents sea a systematic search for impact indicators was conducted on a core of late jurassic early cretaceous sedimentary strata from the vicinity of the proposed mjolnir impact structure barents sea a 0 8 m thick section of the core was found to contain unequivocal indicators of meteoritic impact shocked quartz grains and a strong enrichment in iridium the ejecta bearing strata were discovered only 30 km north northeast of the structure within a stratigraphic interval corresponding to the seismically defined deformation event at mjolnir further study of this unusually well presented impact crater ejecta layer pair may help constrain poorly understood aspects of large magnitude meteorite impacts into the oceans 14 refs 4 figs

a numerical study of typhoon megi 2010 part i rapid intensification abstracttyphoon megi 15w was the most powerful and longest lived tropical cyclone tc over the western north pacific during 2010 while it shared many common features of tcs that crossed luzon island in the northern philippines megi experienced unique intensity and structural changes which were reproduced reasonably well in a simulation using the advanced research weather research and forecasting model arw wrf with both dynamical initialization and large scale spectral nudging in this paper processes responsible for the rapid intensification ri of the modeled megi before it made landfall over luzon island were analyzed the results show that megi experienced ri over the warm ocean with high ocean heat content and decreasing environmental vertical shear the onset of ri was triggered by convective bursts cbs which penetrate into the upper troposphere leading to the upper tropospheric warming and the formation of the upper level warm core in turn cbs with their roots inside of the eyewall in