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

remote trigger of deep convection by cold outflow over the taiwan strait in the mei yu season a modeling study of the 8 june 2007 case abstractin this study the heavy rainfall event over central taiwan during the mei yu season on 8 june 2007 is investigated with an emphasis on the triggering mechanism for the deep convection that produced the rain observations indicate that there existed two lines of forcing with convection prior to the rain one over the northern taiwan strait along the mei yu front and the other over the southern taiwan strait yet the convection in question developed over the central strait between these two lines in an unstable environment with strong westerly vertical wind shear this motivated the authors to carry out the present study the cloud resolving storm simulation cress of nagoya university was used and the event was reproduced at a horizontal grid size of 2 km including the initiation of new convection over the central strait at the correct location and time the model results suggest a crucial role played by the series of active persistent and propagating storms in the southern strait along the

long lived mesoscale systems in a low convective inhibition environment part i upshear propagation abstractpart ii of this study of long lived convective systems in a tropical environment focuses on forward tilted downshear propagating systems that emerge spontaneously from idealized numerical simulations these systems differ in important ways from the standard mesoscale convective system that is characterized by a rearward tilted circulation with a trailing stratiform region an overturning updraft and a mesoscale downdraft in contrast to this standard mesoscale system the downshear propagating system considered here does not feature a mesoscale downdraft and although there is a cold pool it is of secondary importance to the propagation and maintenance of the system the mesoscale downdraft is replaced by hydraulic jump like ascent beneath an elevated forward tilted overturning updraft with negligible convective available potential energy therefore the mesoscale circulation is sustained almost entirely by the work done by the horizontal pressure gradient and the kinetic energy available from

structure and evolution of the 22 february 1993 toga coare squall line organization mechanisms inferred from numerical simulation abstract mechanisms responsible for meso and convective scale organization within a large tropical squall line that occurred on 22 february 1993 during the tropical ocean global atmosphere coupled ocean atmosphere response experiment are investigated using a three dimensional numerical cloud model the squall line occurred in an environment typical of fast moving tropical squall lines characterized by moderate convective available potential energy and moderate to strong vertical shear beneath a low level jet with weak reverse vertical shear above a well simulated aspect of the observed squall line is the evolution of a portion of its leading convective zone from a quasi linear to a three dimensional bow shaped structure over a 2 h period this transition is accompanied by the development of both a prominent mesoscale vortex along the northern edge of the 40 60 km long bow shaped feature and elongated bands of weaker reflectivity situated rearward and oriented transverse to the leading edge within enha

a numerical study of the stratiform region of a fast moving squall line part i general description and water and heat budgets abstract a two dimensional nonhydrostatic cloud model is applied to the simulation of a tropical squall line that occurred on 23 june during the copt 81 experiment owing to the use of an ice parameterization scheme the simulation reproduces many interesting features of the stratiform part observed with doppler radars in particular this includes the dynamical thermodynamical and microphysical structures of the stratiform part different parts are clearly identified from the simulation and observations a leading convective zone 40 km wide with large precipitation a developed stratiform zone stretching over 150 km with moderate precipitation between these two regions a transition zone 20 km wide giving only light precipitation and a forward anvil near 12 km the mean horizontal circulation is characterized by two mean flows the front to rear flow that represents upward and rearward injection of boundary layer air and the underlying rear to front flow the simulated vertical velocity except in th

wave cloud lines over northwest australia the frequency of occurrence and formation mechanisms of wave cloud lines off the northwest coast of australia are investigated prior to the present study little was known about these wave cloud lines apart from being spectacular atmospheric phenomenon in their own right these wave clouds can have a role in the secondary initiation of convection and can be a hazard to low flying aircraft a climatology of wave clouds produced from visible satellite imagery suggests two main types of cloud lines form over northwest australia the first are bore like waves similar in structure to the morning glory of northeast australia and occur at least 2 to 3 times per month throughout the entire year the second type are convectively generated cloud lines which are more circular in shape appear to originate from convective storms and occur at least 0 5 to 1 5 times per month during the wet season high resolution nested simulations are performed with the met office unified model for case studies of each type of wave the bore like waves occurred in the presence of synoptic scale low level southeasterly flow and a heat low along the northwest coast of australia at night the offshore southeasterlies accelerate into the heat low and collide with the onshore sea breeze the southeasterlies override the sea breeze and the wave cloud lines form at the leading edge of this front the convectively generated waves radiate outwards from the convective storms producing compensating subsidence and adiabatic warming these waves take the form of n 2 mode wave fronts which span the entire depth of the troposphere and are similar in structure to waves produced by deep convection which are described in previous studies

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

impact of soil moisture and convectively generated waves on the initiation of a west african mesoscale convective system a mesoscale convective system mcs case study was observed over northeast mali as part of the african monsoon multidisciplinary analysis amma on 31 july 2006 observations of this case suggest that the soil moisture heterogeneity and atmospheric gravity waves emitted from a parent mcs were important trigger mechanisms for this system this study uses high resolution met office unified model metum simulations to assess the importance of the synoptic circulation land surface and gravity waves in the initiation and development of the mcs during the early afternoon shallow convection developed over a region of dry soil within a synoptic scale convergence zone which was caused by the confluence of the southerly monsoon flow with winds associated with the circulation around the saharan heat low two pronounced waves were emitted from a nearby parent storm and propagated towards the convergence zone when the second wave reached the location of the shallow convection deep convection was immediately initiated further convective cells developed later in the afternoon over dry soil many adjacent to strong soil moisture gradients these aggregated with the main storm which later developed into the case study mcs a comparison of model simulations with without the soil moisture heterogeneity and gravity waves shows that the synoptic scale circulation and convergence zones specified by the atmospheric analysis were the most important factors for the successful simulation of the mcs if the location of the initiation of the system is to be forecast accurately the land surface that is the soil moisture must be represented adequately in order to reproduce the timing of the secondary initiation of convection correctly the model must be able to capture gravity waves that are emitted by existing systems

forced gravity waves and the tropospheric response to convection we present theoretical work directed toward improving our understanding of the mesoscale influence of deep convection on its tropospheric environment through forced gravity waves from the linear hydrostatic non rotating incompressible equations we find a two dimensional analytical solution to prescribed heating in a stratified atmosphere which is upwardly radiating from the troposphere when the domain lid is sufficiently high we interrogate the spatial and temporal sensitivity of both the vertical velocity and potential temperature to different heating functions considering both the near field and remote responses to steady and pulsed heating we find that the mesoscale tropospheric response to convection is significantly dependent on the upward radiation characteristics of the gravity waves which are in turn dependent upon the temporal and spatial structure of the source and the assumed stratification we find a 50 reduction in tropospherically averaged vertical velocity when moving from a trapped i e low lid to upwardly radiating i e high lid solution but even with maximal upward radiation we still observe significant tropospheric vertical velocities in the far field 4 hours after heating ends we quantify the errors associated with coarsening a 10 km wide heating to a 100 km grid in the way a general circulation model gcm would observing a 20 reduction in vertical velocity the implications of these results for the parameterisation of convection in low resolution numerical models are quantified and it is shown that the smoothing of heating over a grid box leads to significant in grid box tendencies due to the erroneous rate of transfer of compensating subsidence to neighbouring regions further we explore a simple time dependent heating parameterisation that minimises error in a parent gcm grid box albeit at the expense of increased error in the neighbourhood

mechanisms for the mesoscale organization of tropical cloud clusters in gate phase iii part i shallow cloud bands abstract mesoscale cloud clusters are a frequently observed feature of the tropical atmosphere and are primarily responsible for the observed large scale vertical mass flux given that the forcing for such convection comes from widely separated horizontal scales boundary layer motions on the scale of 1 km and weak ascent on a scale of over 1000 km the persistent organization of cloud clusters into the scale 10 50 km presents an important problem in this article part i two and three dimensional numerical simulations of convection under a capping inversion at 2 km are presented to demonstrate that the mechanism responsible for mesoscale organization of clouds requires neither deep convection nor large scale forcing the case study used as a basis for these simulations is one of several instances reported by lemone and meitin of mesoscale cloud bands during gate phase iii these observations are remarkable in that the cloud bands are shallow yet possess a rather large horizontal periodicity in the ra

role of gravity waves in triggering deep convection during toga coare the role of gravity waves in the initiation of convection over oceanic regions during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare experiment is investigated first an autocorrelation method is applied to infrared temperature observations of convective events from satellite images it reveals that new deep convective cells often occur a few hours after a previous intense event at a typical distance of a few hundred kilometers such fast moving modes faster than 15 m s 21 are interpreted as the trace of gravity waves excited by previous convection and contributing to trigger further convection second the specific case of 11 12 december 1992 during which an active squall line is generated after the collapse of a previous mesoscale convective system mcs nearby is analyzed with a nonhydrostatic model the triggering of the second mcs is well reproduced explicitly owing to the use of the two way interactive grid nesting the convective source appears to emit pulses of gravity waves on a wide range of small scales on the contrary the troposphere response to the convective source exhibits a spectral simplicity a slowly evolving mode characterized by an ascent in the pbl and a compensating subsidence in the free troposphere favors shallow convection and inhibits deep convection respectively traveling modes propagating away from the convective source are characterized by a fast mode 50 m s21 and a slower mode 25 ms 21 associated with the convective and stratiform development of the source respectively in agreement with previous studies a budget analysis reveals the different factors leading to the deep convection triggering first an active pbl characterized by strong surface fluxes and mean ascent initiates shallow convection lasting about 2 h inhibited above by the subsiding motions maintaining a dry layer second horizontal advection of moist and cooler air at midlevels and detrainment from cumuli contribute to destroy the dry air layer capping the shallow convective layer finally vertical advection induced by the gravity waves passage modulates the vapor and temperature evolution ascending phases favor moistening and cooling whereas subsiding phases stop these effects delaying the deep convection onset the triggering occurs after a strong subsidence when the ascent phases of deep and shallow modes are combined

observation of a quasi 2 day wave during toga coare abstract detailed structure of the quasi 2 day oscillation observed in the active phase of the madden julian oscillations during the intensive observation period of tropical ocean and global atmosphere coupled ocean atmosphere response experiment toga coare iop was described a variety of observational platforms is used including high resolution gms infrared histogram rain rate estimate from toga and mit radar measurements upper air soundings and boundary layer profiler winds from the integrated sounding system and surface data from the imet buoy the quasi 2 day mode had a westward propagation speed of 12 15 day 1 a horizontal wavelength of 25 30 longitude a coupling with the westward propagating n 1 inertio gravity waves was hypothesized from the space time power spectral distribution of the cloud field the wind disturbance structure was consistent with the hypothesis the vertical wave structure had an eastward phase tilt with height below 175 hpa and vice versa above indicating the wav

tomographic imaging of the shallow crustal structure of the east pacific rise at 9 30 n compressional wave travel times from a seismic tomography experiment at 9 30 n on the east pacific rise are analyzed by a new tomographic method to determine the three dimensional seismic velocity structure of the upper 2 5 km of oceanic crust within a 20 18 km2 area centered on the rise axis the data comprise the travel times and associated uncertainties of 1459 compressional waves that have propagated above the axial magma chamber a careful analysis of source and receiver parameters in conjunction with an automated method of picking p wave onsets and assigning uncertainties constrains the prior uncertainty in the data to 5 to 20 ms the new tomographic method employs graph theory to estimate ray paths and travel times through strongly heterogeneous and densely parameterized seismic velocity models the nonlinear inverse method uses a jumping strategy to minimize a functional that includes the penalty function horizontal and vertical smoothing constraints and prior model assumptions all constraints applied to model perturbations are normalized to remove bias we use the tomographic method to reject the null hypothesis that the axial seismic structure is two dimensional three dimensional models reveal a seismic structure that correlates well with cross and along axis variations in seafloor morphology the location of the axial summit caldera and the distribution of seafloor hydrothermal activity the along axis segmentation of the seismic structure above the axial magma chamber is consistent with the hypothesis that mantle derived melt is preferentially injected midway along a locally linear segment of the rise and that the architecture of the crustal section is characterized by an en echelon series of elongate axial volcanoes approximately 10 km in length the seismic data are compatible with a 300 to 500 m thick thermal anomaly above a midcrustal melt lens such an interpretation suggests that hydrothermal fluids may not have penetrated this region in the last 103 years asymmetries in the seismic structure across the rise support the inferences that the thickness of seismic layer 2 and the average midcrustal temperature increase to the west of the rise axis these anomalies may be the result of off axis magmatism alternatively the asymmetric thermal anomaly may be the consequence of differences in the depth extent of hydrothermal cooling

scale selection in locally forced convective fields and the initiation of deep cumulus abstract deep cumulus dynamics has often been treated as an initial value problem where the long time effect of surface energy fluxes are neglected initiation is often assumed to follow from a strong localized deformation of the flow field which is elsewhere quiescent in nature however the atmosphere is rarely found in an undisturbed condition just prior to the inception of deep growth one likely cause of widespread motions is the natural modal response of the environment to surface energy fluxes which results in a field of disturbances evidence is presented in this paper for the possible existence of a class of solutions when deep convection is allowed to evolve in the context of a thermally forced field of shallow convection this class of solutions is neglected when one visualizes the growth of severe local storms in term of buoyant bubbles in an otherwise tranquil atmosphere considering deep cumulus initiation as a field problem severely limits the concept of an isolated cloud individual clou

secondary initiation of multiple bands of cumulonimbus over southern britain ii dynamics of secondary initiation summary the convective storm initiation project csip pilot field programme took place in the summer of 2004 in southern england during this field campaign a case of a cold pool outflow initiating an arc of convection downwind of the generating storm the primary storm was observed three further arcs were initiated further downwind of this first arc these arcs all later gave significant rainfall over south east england results from the modelling studies described in this paper show that gravity waves generated by the primary storm may have been responsible for initiating the further three arcs of convective showers that were observed the modelled primary storm generates waves with a range of vertical wavelengths these are separated by dispersion with the higher order modes with the largest vertical wavelengths travelling fastest approximately 30 m s 1 the fastest two modes suppressed convection and later modes increased boundary layer depth and so initiated convection the multiple peaks from non hydrostatic effects in the third mode may have been responsible for the two main arcs observed and the fourth mode may have been responsible for the third arc the amplitude of the modelled waves at low levels was much larger downwind of the primary storm than upwind and only initiated convection there which agrees with the observations of the arcs forming downwind of the primary storm this asymmetry appears to be caused by the moving wave source and asymmetric wave trapping finally we discuss the implications for numerical weather prediction nwp reducing the resolution of the model and varying the magnitude of the wind speed used shows that a non hydrostatic high resolution 1k m nwp model should be able to capture such wave processes if the generating storm is forecast or analysed correctly

high vp vs ratio saturated cracks or anisotropy effects 1 we measured vp vs ratios of thermally cracked westerly granite thermally cracked carrara marble and 4 porosity fontainebleau sandstone for an effective mean pressure ranging from 2 to 95 mpa samples were fluidsaturated alternatively with argon gas and water 5 mpa constant pore pressure the experimental results show that at ultrasonic frequencies vp vs ratio of water saturated specimen never exceeded 2 15 even at effective mean pressure as low as 2 mpa or for a lithology for which the poisson s ratio of minerals is as high as 0 3 calcite in order to check these results against theoretical models we examine first a randomly oriented cracked medium with dispersion but without anisotropy and second a medium with horizontally aligned cracks with anisotropy but without dispersion the numerical results show that experimental data agree well with the first model at high frequency vp vs ratios range from 1 6 to 1 8 in the dry case and from 1 6 to 2 2 in the saturated case the second model predicts both vp sv and vp sh to vary from 1 2 to 3 5 depending on the raypath angle relative to the crack fabric in addition perpendicular to the crack fabric a high vp vs ratio is predicted in the absence of shear wave splitting from these results we argue the possibility that high vp vs ratio 2 2 as recently imaged by seismic tomography in subduction zones may come from zones presenting important crack anisotropy the cumulative effects of crack anisotropy and high pore fluid pressure are required to get vp vs ratios above 2 2 citation wang x q a schubnel j fortin e c david y gueguen and h k ge 2012 high vp vs ratio saturated cracks or anisotropy effects geophys res lett 39 l11307 doi 10 1029 2012gl051742

the role of environmental shear and thermodynamic conditions in determining the structure and evolution of mesoscale convective systems during toga coare abstract a collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the tropical ocean global atmosphere toga coupled ocean atmosphere response experiment coare the soundings were constructed primarily from aircraft data below 5 6 km and primarily from radiosonde data at higher altitudes the well documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended as noted by earlier investigators nearly all convective bands occurring in environments with appreciable shear below a low level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low level shear at a speed close to the wind maximum when there is appreciable shear at middle levels 800 400 mb convective bands form parallel to the shear with

the role of convectively generated gravity waves on convective initiation a case study abstracta case study of a convection initiation ci event involving a mesoscale gravity wave is presented this severe convection event occurred in east china on 5 june 2009 high frequency automatic weather station aws data visible satellite data and doppler radar data were combined to depict the features of the gravity wave and the development of several convection centers the gravity wave was manifested by a surface pressure dip and surface wind shift propagating westward away from the early convection the pressure dip propagated at a speed of 30 m s 1 which is comparable with that in previous observational studies of convectively generated gravity waves a special focus is on the initiation of a deep convection cell in anhui province which resulted in 25 deaths surface observations showed two precursors before ci including a convergence line and wind shift at the eastern end of the convergence line high resolution numerical simulations with the weather research and forecasting wrf model

vertical atmospheric flow on titan as measured by the hasi instrument on board the huygens probe 1 on january 14 2005 the huygens probe descended on the surface of titan the largest moon of saturn and the only moon in the solar system with a substantial atmosphere after the deployment of the main parachute and the release of the heat shield at an altitude of about 150 km the local pressure and temperature were measured by hasi huygens atmospheric structure instrument all the way down to the surface these measurements have now been used to determine the vertical component of the atmospheric flow along the trajectory of the probe the average flow direction is rising below about 40 km and sinking above that with features that loosely match the strong shears in the horizontal wind profile obtained by radio telemetry the prevalence of sinking motion in the stratosphere is an unexpected result for the given latitude suggesting a layered atmosphere with a large thermally indirect cell wiggles superposed on the main upward flow are evidence of inertial instability characteristic of equatorial regions pointing to a latitudinal shear of zonal wind throughout the lower atmosphere

melt to mush variations in crustal magma properties along the ridge crest at the southern east pacific rise the determination of along axis variations in melt properties within the crustal axial magma chamber beneath fast spreading axes is important for understanding melt delivery from the mantle eruption history along the ridge crest and the process of crustal accretion seismic reflection images1 2 3 4 have shown the molten sill to be continuous along the ridge crest for many tens of kilometres with varying widths 250 4 500 m but variations in its seismic properties and thickness have remained elusive despite several attempts to constrain these properties5 6 7 here we report that the melt sill along the southern east pacific rise which is about 50 m thick undergoes abrupt changes in its internal properties ranging from pure melt to mush the 60 km long ridge crest segment near 14 00 s is characterized by three 2 4 km sections containing pure melt embedded within a magma chamber rich in mush these small pure melt pockets may represent a fresh supply of magma from the mantle capable of erupting and forming the upper crust conversely the 80 90 of the magma chamber which is mushy is unlikely to erupt and may influence the lower crustal accretion

vertical mode and cloud decomposition of large scale convectively coupled gravity waves in a two dimensional cloud resolving model this paper describes an analysis of large scale o 1000 km convectively coupled gravity waves simulated using a two dimensional cloud resolving model the waves develop spontaneously under uniform radiative cooling and approximately zero mean flow conditions with wavenumber 2 of the domain appearing most prominently and right moving components dominating over left moving components for random reasons the analysis discretizes the model output in two ways first a vertical mode transform projects profiles of winds temperature and heating onto the vertical modes of the model s base state atmosphere second a cloud partitioning algorithm sorts sufficiently cloudy grid columns into three categories shallow convective deep convective and stratiform anvil results show that much of the tilted structures of the waves can be captured by just two main vertical spectral bands each consisting of a pair of vertical modes the slow modes have propagation speeds of 16 and 18 m s 1 and roughly a full wavelength vertical structure through the troposphere while the fast modes have speeds of 35 and 45 m s 1 and roughly a half wavelength structure deep convection anomalies in the waves are more or less in phase with the low level cold temperature anomalies of the slow modes and in quadrature with those of the fast modes owing to the characteristic life cycle of deep convective cloud systems shallow convective heating peaks 2 h prior to maximum deep convective heating while stratiform heating peaks 3 h after the onset of deep convection in the waves is preceded by a gradual deepening of shallow convection lasting a period of many hours results of this study are in broad agreement with simple two mode models of unstable large scale wave growth under the name stratiform instability differences here are that 1 the key dynamical modes have speeds in the range 16 18 m s 1 rather than 23 25 m s 1 owing to a shallower depth of imposed radiative cooling and 2 deep convective heating as well as stratiform heating is essential for the generation and maintenance of the slow modes

origin and transport of tropical cirrus clouds observed over paramaribo suriname 5 8 n 55 2 w 1 the intertropical convergence zone itcz passes twice a year over tropical suriname bringing two wet and two dry seasons during a pilot study campaign in suriname cirrus clouds were observed with a mobile aerosol raman lidar marl and with balloon sondes containing a frost point hygrometer called snow white over the period october november 2004 these observations are used to study the origin of cirrus clouds and the dynamical processes that determine their transport using european centre for medium range weather forecasts ecmwf operational analyses the height of cirrus occurrence is in phase with the height of the cold point tropopause with maximum heights occurring during northern hemisphere winter that are about 2 km above the minimum values in summertime the occurrence of cirrus often corresponds with a northerly meridional wind flow in a layer underneath the tropopause also when the itcz lies to the south in the period january may ecmwf analyses point out that inertial instability flow in the form of vertically stacked meridional circulation cells in the upper troposphere ut can explain the transport of these cirrus events also evident is that radiative cooling of a moist layer transported in the ut leads to a thermal wind in the form of an easterly westerly jet associated with the southward northward transport of moist air an interactive play between the inertial instability and thermal wind mechanisms explains many of the observed features of cirrus occurrence over suriname the observed cirrus mostly originates from the itcz or from deep convective centers to the south that form during the early summer monsoon

the lagrangian parcel dynamics of moist symmetric instability abstract a simple model describing the slantwise ascent of a two dimensional horizontal air tube subject to moist symmetric instability is developed under the assumptions that the froude number is small and that mixing is absent it is shown that in general the horizontal velocities attained by the tube are comparable to those of the mean flow and that vertical velocities of up to a few meters per second are possible the tube ascends slantwise in such a way that its buoyancy remains nearly zero unless the environment is my very nearly moist adiabatic in which case ascent at an angle of 45 to the vertical is preferred results of the analysis support the contentions of bennetts and hoskins and emanual that moist symmetric instability is the cause of some mesoscale rainbands in a companion paper it is demonstrated that the stability of the moist baroclinic atmosphere to two dimensional slantwise displacements of arbitrary magnitude can be approximately assessed by reversibly lifting parcel along sur

spatial distribution and temporal evolution of crustal melt distribution beneath the east pacific rise at 9 10 n inferred from 3 d seafloor compliance modeling determining the melt distribution in oceanic crust at mid ocean ridges is critical to understanding how magma is transported and emplaced in the crust seafloor compliance deformation under ocean wave forcing is primarily sensitive to regions of low shear velocity in the crust making it a useful tool to probe melt distribution analysis of compliance data collected at east pacific rise between 9 and 10 n through 3 d numerical modeling reveals strong along axis variations in the lower crustal shear velocities as well as temporal variation of crustal shear velocity near 9 48 n between measurements spanning 8 years compliance measured across the rise axis at 9 48 n and 9 33 n suggest a deep crustal low velocity zone beneath the ridge axis with a low vs vp ratio consistent with melt in low aspect ratio cracks or sills changes in compliance measured at 9 48 n between years 1999 and 2007 suggest that the melt fraction in the axial crust decreased during this interval perhaps following the 2005 2006 seafloor eruption this temporal variability provides direct evidence for short term variations of the magmatic system at a fast spreading ridge

a case study on the far field properties of propagating tropospheric gravity waves abstractmesoscale gravity waves were observed by barometers deployed as part of the usarray transportable array on 29 june 2011 near two mesoscale convective systems in the great plains region of the united states simultaneously airs satellite data indicated stratospheric gravity waves propagating away from the location of active convection peak perturbation pressure values associated with waves propagating outside of regions where there was precipitation reached amplitudes close to 400 pa at the surface here the origins of the waves and their relationship to observed precipitation are investigated with a specialized model study simulations with a 4 km resolution dry numerical model reproduce the propagation characteristics and amplitudes of the observed waves with a high degree of quantitative similarity despite the absence of any boundary layer processes surface topography or moist physics in the model the model is forced with a three dimensional time dependent latent heating cooling field that

horizontal wind disturbances induced by inertial instability in the equatorial middle atmosphere as seen in rocketsonde observations 1 rocketsonde observations at kwajalein 8 7 n 167 7 e were used for investigation of horizontal wind disturbances induced by inertial instability two characteristic cases were found in the 1978 1979 northern winter during which the limb infrared monitor of the stratosphere lims made observations in both the cases the eastward component and the northward component of horizontal winds are negatively correlated in the vertical direction near the stratopause level which is thought to be due to inertial instability the wind disturbances have a wavelength of about 10 km and their maximum amplitudes are estimated as over 10 ms 1 examinations of the lims temperatures support a possibility that the wind disturbances are induced by inertial instability it was revealed that temperature disturbances characteristic of inertial instability called pancake structures appear in the same height and time as the wind disturbances do the relationship between the rocketsonde wind disturbances and the lims pancake structures is 90 out of phase as expected by the theory of inertial instability in addition it was confirmed that the two inertial instability events follow enhancements of the midlatitude planetary wave this is consistent with a mechanism of the inertially unstable disturbances in the equatorial region inferred in some observational and numerical studies analyses of rocketsonde data were extended to the entire observation period at kwajalein which suggests that the occurrence of inertially unstable disturbances is biased toward the winter season however at some other stations in the tropics wind disturbances which may be induced by inertial instability do not appear so frequently as they do at kwajalein this seasonality and regionality of inertially unstable disturbances would be due to the seasonality and regionality of planetary wave breakings at midlatitude

gravity waves compensating subsidence and detrainment around cumulus clouds abstract gravity waves play an important role in the redistribution of heat and moisture in a deep convecting cloud field we explore this role in a two dimensional numerical experiment on a simple moist convecting system consisting of an isolated long lasting nonprecipitating cloud in a calm atmosphere with no surface forcing the cloud develops a horizontally averaged density variation with height which is neutrally buoyant with respect to a moist adiabatic the buoyancy difference between the cloud and the undisturbed sounding produces circulations that can be understood as spreading gravity waves which adjust the environmental buoyancy to be equal to the cloud buoyancy by compensating subsidence unlike the circulations inside clouds this adjustment takes place without turbulent mixing hence the buoyancy adjustment time t1 during which the environment comes into rough buoyant equilibrium with the clouds is much shorter than the mixing time t2 which it takes a tracer initially concentrated at s

equatorial atmospheric waves and their association to convection equatorial wave systems and their relationships with convective activity are analyzed in the western and central pacific regions during the coupled ocean atmosphere response experiment coare intensive observation periods the study uses geostationary meteorological satellite infrared temperature observations and the operational european centre for medium range weather forecasts analyses supplemented with coare observations spectral and complex principal component analysis are applied to the data using the linear theory of equatorially trapped waves as a guideline the existence of three types of waves is detected in the 7 10 day period range n 5 1 rossby waves are found to the east of the date line in a region of weak convective activity over the western equatorial pacific where intense convection occurs the 7 10 day waves do not possess the general characteristics of linear rossby waves but they are strongly linked to the active phases of westerly wind bursts and of convection analysis of the meridional wind reveals intense mixed rossby gravity waves with a mean 5 day period and westward phase and eastward group velocities over the western pacific the convection is found to be strongly correlated with the antisymmetric structure of the divergent field as predicted by the linear theory in the 200 hpa divergence field n 5 1 gravity waves are visible having periods shorter than 2 5 days they rapidly propagate about 25 m s21 both westward and eastward and have strong correlations with convective clusters

a case study of gravity waves convective storms interaction 9 may 1979 abstract an analysis is presented of a series of severe storms which occurred in the north central united states on 9 may 1979 and whose spatial distribution and movement correlate well with observed gravity waves two gravity wave trains of 2 1 3 mb amplitude 2 5 3 3 h period and 240 265 km horizontal wavelength were isolated through power spectra analysis and cross correlation techniques applied to national weather service barograph traces the wave trains propagated in the 200 direction which coincided with the jet axis with a phase velocity of 20 30 m s 1c and within a 300 km wide band the storms were identified on enhanced infrared goes satellite pictures with the help of radar summaries these convective systems initially developed in nebraska and propagated north northeast at 25 m s 1 revealing wave like characteristics with a separation of 300 400 km the convective systems were closely linked to the observed wave trains with cell intensity height and associated rainfall maximized at the wa

structure and evolution of the 22 february 1993 toga coare squall line numerical simulations abstract in this study a numerical cloud model is used to simulate the three dimensional evolution of an oceanic tropical squall line observed during the tropical ocean global atmosphere coupled ocean atmosphere response experiment and investigate the impact of small scale physical processes including surface fluxes and ice microphysics on its structure and evolution the observed squall line was oriented perpendicular to a moderately strong low level jet salient features that are replicated by the model include an upshear tilted leading convective region with multiple updraft maxima during its linear stage and the development of a 30 km scale midlevel vortex and associated transition of the line to a pronounced bow shaped structure in this modeling approach only surface flukes and stresses that differ from those of the undisturbed environment are included this precludes an unrealistically large modification to the idealized quasi steady base state and thus allows us to more easily isolate effects of

diabatic divergence profiles in western pacific mesoscale convective systems abstract heating in the atmosphere can be expressed as diabatic divergence d which is nearly equal to the actual horizontal divergence in tropical convection high quality profile measurements from airborne doppler radar purls in ten mesoscale convective systems mcs observed during toga coare are examined and the mean profile is compared with rawinsonde array measurements young convective features have strong near surface convergence while older cells with better developed downdrafts and stratiform precipitation areas have their peak convergence aloft in the mean then surface flow is only weakly convergent or oven divergent so that the main convergence into mcss is deep and peaked aloft with a sharp melting convergence at 0 c divergence prevails above 10 km altitude but was undersampled by the radar unusual but well sampled observations in the purl dataset include a persistent widespread profile feature in one well sampled mcs a cyclone rainband oscillatory reverberations c

gravity waves in shear and implications for organized convection abstract it is known that gravity waves in the troposphere which are often excited by preexisting convection can favor or suppress the formation of new convection here it is shown that in the presence of wind shear or barotropic wind the gravity waves can create a more favorable environment on one side of preexisting convection than the other side both the nonlinear and linear analytic models developed here show that the greatest difference in favorability between the two sides is created by jet shears and little or no difference in favorability is created by wind profiles with shear at low levels and no shear in the upper troposphere a nonzero barotropic wind or equivalently a propagating heat source is shown to also affect the favorability on each side of the preexisting convection it is shown that these main features are captured by linear theory and advection by the background wind is the main physical mechanism at work these processes should play an important role in the organization of

interannual and longer term variability of upper air circulation in the northeast brazil tropical atlantic sector the ncep ncar 1958 1997 upper air data set along with surface ship observations have been analyzed for the march april extremum of the annual cycle in the tropical atlantic sector and core of the northeast brazil nordeste rainy season 1 in the long term mean the intertropical convergence zone itcz overlies a band of warmest surface waters and is accompanied by ascending motion and upper tropospheric divergent outflow directed mainly into a band of upper tropospheric convergence and subsidence over the tropical north atlantic 2 during years of extreme drought in the nordeste the band of warmest surface waters is displaced northward and concomitant with this is a northward shift of the strongest lower tropospheric convergence ascending motion and upper tropospheric divergent outflow 3 during the low warm as compared to the high cold so phase the bands of warmest surface waters most intense convection and largest lower tropospheric convergence ascending motion and upper tropospheric outflow are all shifted northward similar to the pattern characteristic of the nordeste drought years 4 long term trends are documented by a southward displacement of the bands of most intense convection largest lower tropospheric convergence ascending motion and upper tropospheric divergence and increasing cloudiness over the equatorial atlantic the study thus substantiates the upper air processes instrumental for the interannual and decadal scale variability in the surface climate of the region

inertial instability flow in the troposphere over suriname during the south american monsoon 1 weekly sonde observations in suriname supported by ecmwf analyses and a linear stability analysis are used to analyze the recurrence of inertial instability as the itcz migrates over land during the south american monsoon a layer of cool moist air from the atlantic ocean is then advected southward over suriname in the shape of a cold front displacing the warmer air over the continent the return flow northward by the upper branch of the hadley cell is a region where inertial instability pervades due to cross equatorial advection of anticyclonic vorticity and the proximity of the subtropical jet as it migrates closer to the equator this unstable region evidently leads to the episodical formation of a meridional sub cell below the tropopause where the damping is calculated to be strong enough to stabilize flow at smaller vertical scales and yet weak enough to allow the observed cell recurrence at approximately the inertial frequency of the underlying latitudes this instability should also contribute to the hadley cell formation through northward acceleration in the upper branch the moist saturated conditions in the lower troposphere do allow inertial instability here but the high damping values within the boundary layer suggests that the observed southward acceleration in the lower branch of the hadley cell has a cause other than inertial instability

severe downslope windstorm calculations in two and three spatial dimensions using anelastic interactive grid nesting a possible mechanism for gustiness abstract the clark nonhydrostatic anelastic code is extended to allow for interactive grid nesting in both two and three spatial dimensions tests are presented which investigate the accuracy of three different quadratic interpolation formulae which are used to derive boundary conditions for the fine mesh model application of the conservation condition of kurihara and others is shown to result in significant improvements in the treatment of interactive nesting a significant improvement in the solutions for interactive versus parasitic nesting is also shown in the context of forced gravity wave flow this result for the anelastic system is in agreement with the earlier results of phillips and shukla who considered the hydrostatic shallow water system of equations the interactive nesting model is applied to the simulation of the severe downslope windstorm of 11 january 1972 in boulder using both two and three spatial dimensions the three dimensional simulation results in a gustiness signature in the

interactions between upper and lower tropospheric gravity waves on squall line structure and maintenance abstract using a simplified thermodynamic sounding and variable vertical wind shear we investigate the role of gravity waves on the structure and propagation of a simulated two dimensional squall line based on an observed squall line environment the modeled troposphere has been divided into three distinct thermodynamic layers these consist of an absolutely stable atmospheric boundary layer abl an elevated well mixed layer and an upper tropospheric layer of intermediate stability we find the mixed layer to have a dual role it has a reduced stability and thus provides abundant buoyancy for the convective scale updrafts and it provides an ideal layer to trap meso scale 20 200 km wave energy generated in the stable layers the generated waves thus have a significant and lasting impact on the simulation we also find this thermodynamic structure to be conducive to both strong surface wind perturbations and long lived squall lines experiments that vary the vertical wind shear profile demonstrate

cloud top evolution of tropical oceanic squall lines from radar reflectivity and infrared satellite data abstract precipitation estimation over the tropical oceans is commonly performed using passive infrared ir measurements of cloud top brightness temperature from geostationary satellites to infer the location of deep convection it has been recognized in recent years that the majority of tropical precipitation is produced by mesoscale convective systems mcss however the relationship between the ir cloud top patterns associated with mcss and the underlying precipitation is not well understood the assumption that the coldest cloud tops are associated with deep active convection has been central to the characterization of cloud system motion and organization and to many ir based rainfall retrievals previous studies suggested that this view may be oversimplified when applied to propagating convective systems such as squall lines the goal of this study was to understand the evolution of the cold cloud associated with tropical oceanic squall line mcss and to discuss the implications for the retrieva

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

the influence of cross equatorial pressure gradients on the location of near equatorial convection tomas and webster note that in regions where there is a substantial cross equator surface pressure gradient there is locally anticyclonic absolute vorticity on the low pressure side of the equator and thus the flow in this region meets the parcel criterion for inertial instability they hypothesize that the atmospheric response to this absolute vorticity distribution is a convergence divergence doublet in the boundary layer the convergence centre results in strong convection and thus is very important in determining the strength and location of convection i e the intertropical convergence zones whereas the divergence centre results in suppressed convection in this work a zonally symmetric model of the boundary layer is developed to investigate this hypothesis further an analysis of a linearized version of the model indicates that although the observed flow in these regions meets the parcel criterion for instability it does not satisfy the linear stability criteria because of the stabilizing influence of dissipation and the finite vertical scale much greater shear than that observed is required for linear instability it is noted however that when the parcel criterion is met one of the nonlinear terms which was neglected in the linear analysis may have an important influence on the flow several experiments are performed integrating the full nonlinear model relaxing back to a pressure distribution having a cross equatorial gradient to test whether the observed vorticity wind and convergence distributions can be well simulated it is found that the simulations reach a steady state in which there is a region of locally anticyclonic absolute vorticity on the low pressure side of the equator this absolute vorticity distribution results in an anomalous acceleration of the meridional wind and a convergence divergence doublet similar to that observed by tomas and webster this response is explained as resulting from accelerations that act to bring the zonal flow out of geostrophic balance as it passes through the region of locally anticyclonic absolute vorticity the response of the meridional wind to the strength of the pressure gradient is quasi linear for small and somewhat larger values of forcing but between these values there is a range where the response is highly nonlinear i e large increases in the strength of the meridional wind result from small changes in the pressure gradient it is suggested that this nonlinear response may play a role in the observed sudden onset of the monsoon circulations the nonlinear model is also forced using pressure distributions taken from observations the resulting steady state simulated vorticity wind and convergence distributions closely match the observations for the cases of pressure distributions taken from the east pacific and east atlantic during july when forced using the pressure distribution from the indian ocean region during july the model produces an unrealistically strong meridional wind response some improvement is obtained by including the latitudinally dependent zonal pressure gradient force which is moderately strong in this region finally the model is forced using a pressure distribution taken from observations in the central pacific ocean during july in this case the cross equator pressure gradient is near zero

multiscale variability of deep convection in relation to large scale circulation in toga coare deep convection over the indo pacific oceanic warm pool in the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare occurred in cloud clusters which grouped together in regions favoring their occurrence these large groups of cloud clusters produced large scale regions of satellite observed cold cloud top temperature this paper investigates the manner in which the cloud clusters were organized on time and space scales ranging from the seasonal mean pattern over the whole warm pool region to the scale of individual cloud clusters and their relationship to the large scale circulation and sea surface temperature sst the dominant convective variability was associated with the intraseasonal oscillation iso a large eastward propagating ensemble of cloud clusters marked the iso s progress the meridional structure of the iso was strongly affected by the seasonal cycle with a southward shift from the northern hemisphere in october november to the southern hemisphere in january february the sst had an intraseasonal signal in lagged quadrature with the cold cloudiness and rainfall in coare the sst increased decreased during the convectively suppressed active phases of the iso enhanced low level westerly winds occurred toward the later stages of the enhanced convection periods of the iso though not always centered at the equator the strongest westerlies tended to be located between two synoptic scale cyclonic gyres which were often not symmetric about the equator in the low level wind field this asymmetry in the anomalous equatorial low level westerlies may have different implications for the oceanic response in the coupled atmosphere ocean system than those centered on the equator the cyclonic gyres contained highly concentrated deep convection and in four cases the gyres developed into tropical cyclones within the envelope marking the convectively active phase of the iso cloud clusters were frequently concentrated into westward propagating disturbances with a local periodicity of similar to 2 days these 2 day disturbances have been identified in earlier spectral studies and appear to be related to westward propagating inertio gravity waves in coare they typically contained numerous cloud clusters which underwent a distinct diurnal cycle most of the cloud clusters embedded in the 2 day disturbances moved westward though some were stationary and a few moved eastward a cloud duster tracking program identified groups of clusters lime dusters that exhibited continuity in time and space in the most convectively active period of the iso the tracking program identified almost the entire iso cloud ensemble as a long lasting trackable superconvective system this observation indicates the lack of a distinct scale separation between convection and large scale disturbances during the most intense convective periods in coare

the initiation and horizontal scale selection of convection over gently sloping terrain abstract two and three dimensional numerical simulations were performed to investigate the scale selection and initiation of both moist and dry convection over gentle western and gentle eastern slopes where the latter represents an idealization of the eastern colorado region of the great plains of north america this work extends earlier studies of thermally forced convection by considering a model framework that is large enough to resolve both the convective scale dynamics as well as the larger scale dynamics within which the convection is embedded as a result the scale interaction problem leading to the selection of the dominant deep modes of the troposphere and consequent convection initiation is more realistically treated the main physical mechanisms involved in the initiation of convection in these studies are the usual boundary layer instabilities leading to the development of eddies and or shear aligned rolls the excitation of gravity waves by the boundary layer motions interacting with the fr

two day disturbances in the equatorial western pacific from november 1992 to february 1993 westward propagating disturbances with scales of 1000 1500 km frequented the equatorial western pacific they propagated at about 16 m s 1 and their passage was accompanied by a two day oscillation in high cloudiness their convection was most intense when the lower troposphere was cool and the level of free convection was relatively low the disturbances propagated westward at about 18 m s 1 faster than the lower tropospheric flow this rapid propagation may have been the result of a coupling between the disturbance s convection and the 20 m s 1 vertical mode the signal of this mode was apparent in vertical profiles of temperature and wind and the most intense convection began when the mode dynamically cooled the lower troposphere lowering the level of free convection

the synoptic setting and possible energy sources for mesoscale wave disturbances abstract thirteen case studies of mesoscale wave disturbances characterized by either a singular wave of depression or wave packets with periods of 1 4 h horizontal wavelengths of 50 500 km and surface pressure perturbation amplitudes of 0 2 7 0 mb are reviewed to isolate common synoptic features for these cases and to shed light on possible energy sources for the waves a strong thermal inversion in the lower troposphere north of a frontal boundary and a jet streak propagating toward a ridge axis in the upper troposphere are commonly observed in all the cases in general the area of wave activity is bounded by the jet axis to the west or northwest a surface front to the southeast an inflection axis between the trough and ridge axes to the southwest and the ridge axis to the northeast the conditions specified by lindzen and tung as being necessary to form a wave duct which include the existence of the lower tropospheric inversion seem to be met in many of these cases this suggests that a du

paleointensity applications to timing and extent of eruptive activity 9 10 n east pacific rise placing accurate age constraints on near axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges geomagnetic paleointensity of submarine basaltic glass sbg holds promise for placing quantitative age constraints on near axis flows in one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful sbg paleointensity estimates from 189 near axis 4 km sites at the east pacific rise 9 10 n paleointensities range from 6 to 53 t and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field samples within and adjacent to the axial summit trough ast have values approximately equal to or slightly higher than the present day samples out to 1 3 km from the ast have values higher than the present day and samples farther off axis have values lower than the present day the on axis samples 500 m from the ast provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique results from samples collected near a well documented eruption in 1991 1992 suggest there may be a small negative bias in the paleointensity estimates limiting resolution of the dating technique possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain anomalously high magnetic unblocking temperatures leading to a small cooling rate bias and or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories paleointensity remains useful in approximating age differences in young flows and a clear along axis paleointensity contrast near 9 50 n is suggestive of a 150 200 year age difference paleointensity values of off axis samples are generally consistent with rough age interpretations based on side scan data furthermore spatial patterns in the paleointensity suggest extensive off axis flow emplacement may occur infrequently with recurrence intervals of 10 20 kyr results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows with flow size linked to time between eruptions