radiative dynamical consequences of dry tongues in the tropical troposphere abstract dry layers are frequently observed in atmospheric soundings from the climatologically humid western pacific warm pool region some 2400 soundings from the toga coare field program were objectively examined for humidity drops layers in which relative humidity decreases rapidly with height indicative of the bases of dry layers these occur throughout the lower and middle troposphere with frequency peaks near 550 the 0 c level 800 and 950 mb a composite constructed from these sounding data indicates the following almost all dry layers are too dry and not warm enough to be interpreted as conservative vertical displacements rather they apparently consist of filaments or tongues of low moist static energy air advected into the column often from the subtropics dry tongues are anomalously virtually warm near their bases with a slight cool layer below that is they sit atop sharp stable layers or inversions the authors hypothesize that radiation is responsible for the thermal structure o

a theory for strong long lived squall lines abstract we study herein the mechanics of long lived line oriented precipitating cumulus convection squall lines using two and three dimensional numerical models of moist convection these models used in juxtaposition enable us to address the important theoretical issue of whether a squall line is a system of special long lived cells or whether it is a long lived system of ordinary short lived cells our review of the observational literature indicates that the latter is the most consistent paradigm for the vast majority of cases but on occasion a squall line may be composed of essentially steady supercell thunderstorms the numerical experiments presented herein show that either type of squall line may develop from an initial line like disturbance depending on the magnitude and orientation of the environmental shear with respect to the line with shallow shear oriented perpendicular to the line a long lived line evolves containing individually short lived cells our analysis of this type o

interannual variability of deep convection over the tropical warm pool satellite data show that the total amount of deep convection was nearly constant from year to year over the warm pool region of the tropical indian and pacific oceans but the spatial distribution varied in the 1986 1987 warm el nino southern oscillation event the maximum of deep convective activity was over the high sea surface temperature sst of the west central pacific with a local minimum over the eastern indian ocean in the 1988 1989 cold enso event the maximum convective activity switched to the eastern indian ocean with a minimum over the west central pacific the sst changed very little over the eastern indian ocean from year to year and the centers of convective activity were always collocated with the four month mean low level westerlies in each year the latitude of the cloudy region of the intraseasonal oscillation iso varied interannually the interannual differences in deep convective activity over the warm pool were accounted for almost exclusively by the occurrence or nonoccurrence of extremely large long lasting deep mesoscale cloud systems cloud tops 208 k with horizontal dimensions 300 700 km such cloud systems occurred more frequently than normal over the western pacific during the warm enso event and over the eastern indian ocean during the cold event they occurred more frequently over the tropical eastern indian and western pacific oceans than over the maritime continent thus accounting for the seesaw pattern of the observed cold cloudiness between the two oceanic regions on both interannual and intraseasonal timescales

a numerical study of the genesis of extratropical convective mesovortices part i evolution and dynamics abstract the purpose of this study is to understand the genesis of extratropical convective mesovortices and the large scale environmental features that influence the vortex formation a hypothesis is proposed that mesovortices form in the stratiform rain regions of mesoscale convective systems mcss due to the reduction of static stability that reduces the effective local rossby radius in such regions a conceptual model of the mesoscale convective cyclogenesis is introduced which describes the three stages of the mesovortex formation a modified version of the pennsylvania state university national center for atmospheric research three dimensional hydrostatic mesoscale model is used to simulate mesovortex genesis in analytically generated pre mcs large scale environments the model simultaneously incorporates parameterized convection and a grid resolvable convective scheme containing the effects of hydrostatic water loading condensation evaporation freezing melting and sublimation a control si

numerical simulation of an intense squall line during 10 11 june 1985 pre storm part ii rear inflow surface pressure perturbations and stratiform precipitation abstract an intense rear inflow jet surface pressure perturbations and stratiform precipitation associated with a squall line during 10 11 june 1985 are examined using a three dimensional mesoscale nested grid model it is found that the large scale baroclinity provides favorable and deep rear to front flow within the upper half of the troposphere and the mesoscale response to convective forcing helps enhance the trailing extensive rear inflow however latent cooling and water loading are directly responsible for the generation of the descending portion of the rear inflow the role of the rear inflow is generally to produce convergence ahead and divergence behind the system and thus assist the rapid acceleration of the leading convection when the prestorm environment is convectively favorable and the rapid dissipation of the convection when encountering unfavorable conditions in this case study the rear inflow jet appears to have caused the splitting of the surface wake low as well as the organized

the evolution of the 10 11 june 1985 pre storm squall line initiation development of rear inflow and dissipation abstract mesoscale analysis of surface observations and mesoscale modeling results show that the 10 11 june squall line contrary to prior studies did not form entirely ahead of a cold front the primary environmental features leading to the initiation and organization of the squall line were a low level trough in the lee of the rocky mountains and a midlevel short wave trough three additional mechanisms were active a southeastward moving cold front formed the northern part of the line convection along the edge of cold air from prior convection over oklahoma and kansas formed the central part of the line and convection forced by convective outflow near the lee trough axis formed the southern portion of the line mesoscale model results show that the large scale environment significantly influenced the mesoscale circulations associated with the squall line the qualitative distribution of along line velocities within the squall line is attributed to the larger scale circulations associated with the le

dynamical ocean forcing of the madden julian oscillation at lead times of up to five months abstractthe authors show that a simple three dimensional ocean model linearized about a resting basic state can accurately simulate the dynamical ocean response to wind forcing by the madden julian oscillation mjo this includes the propagation of equatorial waves in the indian ocean from the generation of oceanic equatorial kelvin waves to the arrival of downwelling oceanic equatorial rossby waves in the western indian ocean where they have been shown to trigger mjo convective activity simulations with idealized wind forcing suggest that the latitudinal width of this forcing plays a crucial role in determining the potential for such feedbacks forcing the model with composite mjo winds accurately captures the global ocean response demonstrating that the observed ocean dynamical response to the mjo can be interpreted as a linear response to surface wind forcing the model is then applied to study primary madden julian events which are not immediately preceded by any mjo activity or by any apparen

structure and evolution of numerically simulated squall lines abstract using a three dimensional numerical cloud model we investigate the effects of vertical wind shear on squall line structure and evolution over a wide range of shear magnitudes depths and orientations relative to the line we find that the simulated squall lines are most sensitive to the magnitude of the component of shear perpendicular to the line and that we may reproduce much of the range of observed structures by varying this single parameter for weak shear a line of initially upright to downshear tilted short lived cells quickly tilts upshear producing a wide band of weaker cells extending behind the surface outflow boundary for moderate to strong shear the circulation remains upright to downshear tilted for longer periods of time with vigorous short lived cells confined to a relatively narrow band along the system s leading edge at later times however these systems may also weaken as the circulation tilts upshear for strong deep shears oriented obliquely to the line the squal

an integrated view of the 1987 australian monsoon and its mesoscale convective systems i horizontal structure this observational study is organized around three issues the forcing of the monsoon flow by embedded deep convection the thermodynamic conditions that support the convection and the structures of the mesoscale convective systems mcss that constitute much of the deep convection the monsoon convective cloudiness occurred predominantly offshore over the earth s warmest waters week long periods of widespread deep convection caused cycles of monsoon spin up culminating in tropical cyclone formation momentum transports by the convection also created smaller scale vortex pairs in the upper troposphere sounding data suggest that the convection was modulated by low level processes not by large scale deep forced ascent triggering by mesoscale boundary layer cold pool boundaries and coastlines determined specifically where convection occurred while several positive feedbacks acted to keep regional thermodynamic conditions favourable hence monsoon convection once initiated persisted as self exciting superclusters composed at any instant of many distinct mcss the observed mcss all contained areas of deep convection and stratiform precipitation areas stratiform precipitation usually evolved in place from convective cells although precipitation also occasionally fell from overhanging anvils created by upper level shear the mcss within similar synoptic wind environments tended to have similar mesoscale structures this fact reflects the effects illustrated herein of environmental winds upon the relative motions of cold pools which trigger new convective cells and stratiform precipitation areas which evolve from old convective cells

organization of convection within the madden julian oscillation convection associated with the madden julian oscillation mjo is organized over a broad range of time and space scales within a large scale envelope of activity the large scale envelope of convection is characterized as an eastward propagating disturbance predominantly of zonal wavenumber 2 which is confined over the warmest waters of the indian and western pacific oceans large scale convection is shifted toward the disturbance s temperature anomaly which appears to result from frictional effects in the boundary layer synoptic and mesoscale convective activity is enhanced within the wet phase of the large scale envelope the increase at synoptic scales occurs uniformly over all scales but little difference in the character of the variability is observed between dry and wet phases the enhanced mesoscale activity within the wet phases exhibits a marked tendency to be organized into westward propagating clusters with periods near 2 days and half wavelengths of about 1000 km the role of these modes for the evolution of the mjo is discussed

geochronological constraints on tourmaline formation in the western fold belt of the mount isa inlier australia evidence for large scale metamorphism at 1 57 ga boron metasomatism during two deformational events resulted in tourmaline formation in parts of late palaeoproterozoic eastern creek volcanics of the western fold belt of the mount isa inlier despite low u pb ratios 1 as measured by laser ablation icp ms when the step leaching process was applied to the tourmaline it process extracted pb fractions with 206 pb 204 pb up to 76 illustrating the method s ability to efficiently separate radiogenic and common pb that are hosted in different crystal lattice sites tourmaline from two syn d2 metamorphic hydrothermal veins at anderson s lode and eldorado yield similar pb step leaching ages of 1573 12 and 1577 48 ma respectively tourmaline from a post d2 but pre d3a vein at king was dated at 1528 51 ma although there is evidence to suggest some contribution from u rich inclusions i e zircon and monazite to some leach steps the isochrons are thought to provide robust ages for hydrothermal tourmaline formation during two discrete thermal events this is supported by estimates of pb closure temperatures in tourmaline derived from ionic porosity models the 1575 ma ages for d2 also reflect the main metamorphic event in the western fold belt but are 40 myr older than accepted age estimates for this episode the revised timing for peak metamorphism tectonism in the western fold belt corresponds with the age of metamorphism in the eastern fold belt suggesting broadly coeval tectonic histories across the entire inlier from at least 1600 ma

a study of the response of deep tropical clouds to mesoscale processes three dimensional numerical experiments abstract a three dimensional numerical cloud ensemble model has been developed to investigate the collective feedback effects of cloud systems on the large scale environment an observed large scale lifting is imposed continuously in the model small amplitude random perturbations in the form of temperature fluctuations are also continuously fed into the model at low levels to simulate random thermals over the tropical ocean the model allows several clouds of various sizes to develop simultaneously inside the domain an integration of the model is made for six hours of simulated time in order to allow large numbers of convective clouds to develop following each simulation the collective feedbacks of cloud systems on the large scale temperature moisture and horizontal momentum fields are computed horizontal and time averages of various relevant variables are also computed to elucidate the statistical properties of the clouds the model was applied to a case of a well defined itcz rainband over the eas

a mesovortex within a near equatorial mesoscale convective system during toga coare abstract airborne doppler radar observations are used to investigate the internal structure of a midlevel mesovortex that developed within the rear part of the stratiform precipitation region of a mature to decaying mesoscale convective system this system composed of several convective elements occurred on 13 december 1992 on the eastern side of the intensive flux array of the tropical ocean global atmosphere coupled ocean atmosphere response experiment near the equator 2 s the mesovortex contained several features that were previously observed in other tropical and midlatitude systems radar observations show that the closed wind circulation at midlevels had a horizontal dimension of 150 km and was coincident with a marked rear inflow at the central rear of the stratiform region where a notch pattern clearly identified the rear dry air intrusion it was associated with positive vertical vorticity anticyclonic for the southern hemisphere and could be classified as a cooling induced vortex maxi

convection over the pacific warm pool in relation to the atmospheric kelvin rossby wave abstract deep convection over the western tropical pacific warm pool is analyzed in terms of its relation to the atmospheric kelvin rossby wave which dominates the large scale flow during the austral summer the study uses doppler radar data collected by aircraft and ship radars during different time periods in the tropical ocean global atmosphere coupled ocean atmosphere response experiment to characterize the mesoscale circulations of organized convective cloud systems occurring throughout the season the study focuses on convection in two contrasting phases of the wave the westerly onset region just west of the point within the wave where low level easterlies change to westerlies and the strong westerly region or westerly wind burst lying between the large scale counterrotating gyres of the kelvin rossby wave in the westerly onset region the zonal wind component had midlevel easterlies overlying low level westerlies in the strong westerly region a deep layer of westerlies extended from the

organized convective systems in the tropical western pacific as a process in general circulation models a toga coare case study we examine the large scale effects of organized convective systems in the tropical western pacific observed during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare in a case study approach we examine realizations of a supercluster associated with the onset of the december 1992 westerly wind burst in the t213 operational medium range weather forecasting model of the european centre for medium range weather forecasts ecmwf we idealize a supercluster as a hierarchy of three interacting scales namely organized cumulonimbus c 1 mesoscale convective systems c 2 and the supercluster component c 3 it is shown that the ecmwf model represents this hierarchy as a c 3 like surrogate whose influence dominates the effect of parametrized convection this causes over prediction of the model tendencies which in the case of zonal momentum is explained in elementary terms the structure of the resolved scale momentum flux is explained by moncrieff s 1992 archetypal theory of organized convection which has been verified against observations and cloud resolving model data sets the parametrization of subgrid scale convective momentum flux in the ecmwf model based on a momentum mixing concept produces subgrid scale tendencies that are physically different from transports associated with cumulonimbus convection in a shear flow we outline a strategy for parametrizing the momentum flux by the c 1 component based on the archetypal model the c 2 component which is part resolved and part parametrized is at odds with the assumptions of scale separation underpinning parametrization it is argued that this component should be represented as part of the prognostic treatment of convectively generated cirrus finally we suggest cloud resolving modelling studies to further quantify the structure and large scale impact of superclusters in a westerly wind burst environment ranging from idealized models to models having data assimilation capability

three dimensional kinematic and microphysical evolution of florida cumulonimbus part iii vertical mass transport mass divergence and synthesis abstract a statistical technique is employed to examine the evolving properties of the ensemble small scale variability of high resolution radar data collected in a multicellular florida thunderstorm this paper examines vertical mass transport and mass divergence and synthesizes these observations with results from the first two parts of the study into a self consistent conceptual model that describes the convective to stratiform transition of the storm vertical mass transport distributions indicate that the more numerous weak and moderate strength upward and downward velocities not the few strongest accomplished most of the vertical mass transport in the storm hence most of the mass of precipitation is condensed outside the areas of intense upward motion these data thus suggest a change in the way we think about convection although the few regions of strongest vertical motion play a part in the overall storm evolution by dispersing particles throughout the depth of the storm it is the more preva

structure and evolution of the 22 february 1993 toga coare squall line aircraft observations of precipitation circulation and surface energy fluxes abstract this study documents the precipitation and kinematic structure of a mature eastward propagating oceanic squall line system observed by instrumented aircraft during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare doppler radar and low level in situ observations are used to show the evolution of the convection from an initially linear nnw sse oriented convective line to a highly bow shaped structure with an embedded low to midlevel counterclockwise rotating vortex on its northern flank in addition to previously documented features of squall lines such as highly upshear tilted convection on its leading edge a channel of strong front to rear flow that ascended with height over a rear inflow that descended toward the convective line and a pronounced low level cold pool apparently fed from convective and mesoscale downdrafts from the convective line rearward the observations of this system showed distinct multiple maxima in updraft strength with

rear inflow in squall lines with trailing stratiform precipitation abstract the relative airflow and accompanying precipitation structure of squall lines trailed by mesoscale regions of stratiform rain are examined with emphasis on the occurrence of rear inflow i e the intrusion of environmental air into these storms across the trailing precipitation boundary three cases from kansas and oklahoma provide examples of strong rear inflow which crosses the back edge of the stratiform precipitation area at relative speeds exceeding 10 m s 1 the vertical profiles of relative flow at the trailing precipitation boundary of these three systems were remarkably similar with the rear inflow confined to a jet like layer centered at about 550 mb doppler radar data for two of these cases showed that the rear inflow jet occupied a continuous channel extending from middle levels at the back edge of the stratiform region to lower levels of the leading convective region where it merged with outflow from convective downdrafts to bolster the leading gust front strong front to rea

kinematic characteristics of air flowing into and out of precipitating convection over the west pacific warm pool an airborne doppler radar survey air flowing into and out of 25 cloud systems over the west pacific warm pool was sampled by doppler radars on board two noaa wp 3d aircraft and the ncar electra aircraft during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare in convective cells updraught inflows and downdraught outflows tended to be positioned adjacent to each other at sharp updraught downdraught interfaces sloping at various angles and directions the updraught inflows were sometimes shallow and sometimes deep often extending well above the boundary layer downdraught outflow magnitudes were usually stronger than updraught inflows and the horizontal directions of the inflow and outflow were frequently offset from each other at angles of 90 in stratiform precipitation regions the dominant airflow features were descending mid level inflows located at the bases of anvils and often extending into the interior of the precipitation system these stratiform inflows originated at levels between 5 and 10 km and descended to about 3 km a few extended down to the surface stratiform inflows were typically strong and sandwiched between weaker stratiform outflows at upper and lower altitudes the stratiform region inflow and outflow directions were frequently offset by 90 in the horizontal the mid level inflow currents entered the stratiform regions horizontally from a direction similar to that of the large scale ambient wind the mid level stratiform inflow tended to occur in the same vertical plane as the convective downdraught outflow although the mid level stratiform inflow appeared to be slowed by horizontal pressure gradients before connecting with the convective downdraught outflow in other respects the mesoscale systems differed from a simplified two dimensional model of airflow the systems moved primarily by discrete propagation and updraughts and downdraughts tended to have orthogonal horizontal relative airflow

kinematic and thermodynamic characteristics of the flow over the western pacific warm pool during toga coare abstract rawinsonde and satellite infrared radiation ir data from the tropical ocean global atmosphere toga coupled ocean atmosphere response experiment coare are used to investigate mean and transient behavior and horizontal variability of the atmosphere over the western pacific warm pool infrared data for the 4 mo intensive observing period iop and vertical motion fields indicate that the intensity of convection height of maximum upward motion and sst all increased from west 140 e to east across the coare domain iop mean ir data show a double itcz intertropical convergence zone structure north and south of the intensive flux array ifa centered at 2 s 156 e although marked variability in the patterns occurred on a month to month basis three prominent westerly wind bursts occurred over the ifa during the 4 mo iop in association with the intraseasonal oscillations isos strong upward motion usually occurred 1 3 weeks prior to the peak low level westerlies subsidence dominated when

three dimensional evolution of simulated long lived squall lines abstract simulations of squall lines using nonhydrostatic convection resolving models have been limited to two dimensions or three dimensions with the assumption of along line periodicity the authors present 3d nonhydrostatic convection resolving simulations produced using an adaptive grid model where the lines are finite in length and the restriction to along line periodicity is removed the base state for the simulations is characterized by weak shallow shear and high convective available potential energy cape an environment in which longlived midlatitude mesoscale convective systems mcss are observed the simulated systems bear strong resemblance to many observed systems suggesting that large scale forcing absent in the horizontally homogeneous environment is not needed to produce many of the distinguishing features of midlatitude mcss in simulations without coriolis forcing the presence of line ends leads to mature symmetric systems characterized by a central region of strong convectio

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

momentum transport by a line of cumulonimbus abstract the vertical transport of horizontal momentum normal to a line of cumulonimbus observed during gate on 14 september 1974 is against the vertical momentum gradient contrary to the predictions of mixing length theory data from repeated aircraft passes normal to the line s axis at heights from 0 15 to 5 5 km are used to document the flux and determine its source the flux is concentrated in roughly a 25 km wide active zone just behind the leading edge of the line in kilometer scale convective updrafts accelerated upward by buoyancy and toward the rear of the line by mesoscale pressure forces the fall in mesoscale pressure from the leading edge to the rear of the active zone is mainly hydrostatic resulting from relatively high virtual temperatures and the 60 degree tilt of the leading edge from the vertical with the clouds at the surface well ahead of those aloft evaluation of the terms in the momentum flux generation equation confirms that the above process reflected by the velocity buoyan