multiple variscan nappes in limousin western massif central france geophysical constraints to the geological model and geodynamic implications abstract the upper crustal structure of limousin western massif central france results from tectonic superposition of major variscan high grade metamorphic crystalline nappes one of them containing well preserved relicts of an early paleozoic oceanic lithosphere three of these nappes outcrop in the studied area as evidenced by surface geological observations their overall present synclinorium structure is confirmed by the gravimetric data results from three short refraction seismic profiles recorded in this area are totally compatible either with the local tectonic model or with the global one recently proposed for limousin although it would be an over interpretation to derive a geological model from the only seismic profiles presented this study shows that short seismic profiles are critical for 3 d extrapolation of the geological surface data in old orogenic regions as ophiolites are located within a peculiar unit representing an early major thrust contact the origin of the various nappes above and below may be defined in terms of plate tectonics

potential vorticity diagnosis of a tropopause polar cyclone long lived coherent vortices located near the tropopause are often found over polar regions although these vortices are a commonly observed feature of the arctic and can have lifetimes longer than one month little is known about the mechanisms that control their evolution this paper examines mechanisms of intensity change for a cyclonic tropopause polar vortex tpv using an ertel potential vorticity epv diagnostic framework results from a climatology of intensifying cyclonic tpvs suggest that the essential dynamics are local to the vortex rather than a consequence of larger scale processes this fact motivates a case study using a numerical model to investigate the role of diabatic mechanisms in the growth and decay of a particular cyclonic vortex a component wise breakdown of epv reveals that cloud top radiational cooling is the primary diabatic mechanism that intensifies the tpv during the growth phase increasing amounts of moisture become entrained into the vortex core at later times near hudson bay allowing the destruction of potential vorticity near the tropopause due to latent heat release to become comparable to the radiational tendency to create potential vorticity

the impact of extratropical transition on the downstream flow an idealized modelling study with a straight jet the interaction of a tropical cyclone undergoing extratropical transition et with the midlatitude synoptic scale flow is investigated using full physics numerical experiments with idealized initial conditions the emphasis is on the impact on the midlatitude flow downstream of the et event the midlatitude flow is represented by a balanced straight jet stream as the tropical cyclone approaches the jet a ridge trough couplet and a distinct jet streak form in the upper level flow a midlatitude cyclone develops rapidly downstream of the et system and the further evolution is characterized by downstream baroclinic development based on hovmoller diagrams the upper level development is interpreted as the excitation and subsequent dispersion of a rossby wave train on the potential vorticity gradient associated with the jet the characteristics of this wave train are sensitive to the structure of the jet and to moist processes in the midlatitudes the tropical cyclone undergoing et acts as a sustained forcing for the wave train and the structure of the et system impacts the development most significantly one to two wavelengths downstream of et piecewise inversion of potential vorticity complemented by the partitioning of the flow into its rotational and divergent parts is applied to assess the impact of the et system quantitatively both the cyclonic circulation and the outflow of the tropical cyclone are important contributors to the formation and amplification of the ridge trough couplet the outflow anomaly reduces the eastward motion of the ridge trough couplet significantly and thus promotes phase locking between the tropical cyclone and the upper level pattern copyright 2008 royal meteorological society

mechanisms controlling the downstream poleward deflection of midlatitude storm tracks abstractthe atlantic and pacific storm tracks in the northern hemisphere are characterized by a downstream poleward deflection which has important consequences for the distribution of heat wind and precipitation in the midlatitudes in this study the spatial structure of the storm tracks is examined by tracking transient cyclones in an idealized gcm with a localized ocean heat flux the localized atmospheric response is decomposed in terms of a time and zonal mean background flow a stationary wave and a transient eddy field the lagrangian tracks are used to construct cyclone composites and perform a spatially varying pv budget three distinct mechanisms that contribute to the poleward tilt emerge transient nonlinear advection latent heat release and stationary advection the downstream evolution of the pv composites shows the different role played by the stationary wave in each region in the region where the tilt is maximized all three mechanisms contribute to the poleward propagation of the

monsoon extratropical circulation interactions in himalayan extreme rainfall extreme precipitation and flood episodes in the himalayas are oftentimes traced to synoptic situations involving connections between equatorward advancing upper level extratropical circulations and moisture laden tropical monsoon circulation while previous studies have documented precipitation characteristics in the himalayan region during severe storm cases a comprehensive understanding of circulation dynamics of extreme precipitation mechanisms is still warranted in this study a detailed analysis is performed using rainfall observations and reanalysis circulation products to understand the evolution of monsoon extratropical circulation features and their interactions based on 34 extreme precipitation events which occurred in the western himalayas weh during the period 1979 2013 our results provide evidence for a common large scale circulation pattern connecting the extratropics and the south asian monsoon region which is favorable for extreme precipitation occurrences in the weh region this background upper level large scale circulation pattern consists of a deep southward penetrating midlatitude westerly trough a blocking high over western eurasia and an intensifying tibetan anticyclone it is further seen from our analysis that the key elements of monsoon midlatitude interactions responsible for extreme precipitation events over the weh region are 1 midlatitude rossby wave breaking 2 west northwest propagation of monsoon low pressure system from the bay of bengal across the indian subcontinent 3 eddy shedding of the tibetan anticyclone 4 ageostrophic motions and transverse circulation across the himalayas and 5 strong moist convection over the himalayan foothills furthermore high resolution numerical simulations indicate that diabatic heating and mesoscale ageostrophic effects can additionally amplify the convective motions and precipitation in the weh region

boundary layer friction in midlatitude cyclones results from an idealized three dimensional baroclinic life cycle model are interpreted in a potential vorticity pv framework to identify the physical mechanisms by which frictional processes acting in the atmospheric boundary layer modify and reduce the baroclinic development of a midlatitude storm considering a life cycle where the only non conservative process acting is boundary layer friction the rate of change of depth averaged pv within the boundary layer is governed by frictional generation of pv and the flux of pv into the free troposphere frictional generation of pv has two contributions ekman generation which is directly analogous to the well known ekman pumping mechanism for barotropic vortices and baroclinic generation which depends on the turning of the wind in the boundary layer and low level horizontal temperature gradients it is usually assumed at least implicitly that an ekman process of negative pv generation is the mechanism whereby friction reduces the strength and growth rates of baroclinic systems although there is evidence for this mechanism it is shown that baroclinic generation of pv dominates producing positive pv anomalies downstream of the low centre close to developing warm and cold fronts these pv anomalies are advected by the large scale warm conveyor belt flow upwards and polewards fluxed into the troposphere near the warm front and then advected westwards relative to the system the result is a thin band of positive pv in the lower troposphere above the surface low centre this pv is shown to be associated with a positive static stability anomaly which rossby edge wave theory suggests reduces the strength of the coupling between the upper and lower level pv anomalies thereby reducing the rate of baroclinic development this mechanism which is a result of the baroclinic dynamics in the frontal regions is in marked contrast with simple barotropic spin down ideas finally we note the implications of these frictionally generated pv anomalies for cyclone forecasting

the role of moist processes in the formation and evolution of mesoscale snowbands within the comma head of northeast u s cyclones abstract the role of moist processes in regulating mesoscale snowband life cycle within the comma head portion of three northeast u s cyclones is investigated using piecewise potential vorticity pv inversion modeling experiments and potential temperature tendency budgets snowband formation in each case occurred along a mesoscale trough that extended poleward of a 700 hpa low this 700 hpa trough was associated with intense frontogenetical forcing for ascent a variety of pv evolutions among the cases contributed to midlevel trough formation and associated frontogenesis however in each case the induced flow from diabatic pv anomalies accounted for a majority of the midlevel frontogenesis during the band s life cycle highlighting the important role that latent heat release plays in band evolution simulations with varying degrees of latent heating show that diabatic processes associated with the band itself were critical to the development and maintenance of the band however changes in the meso

a case study of the cutoff process and latent heating effect in an upper level cold core low during the mei yu season in east asia abstractusing observational data and model sensitivity test the present study diagnosed the evolution of an upper level trough over 2 7 may 1998 during the mei yu season in east asia with a focus on the role of latent heating downstream from the trough on the cutoff process and the formation of the cold core low ccl compared with conceptual models based on dry dynamics the formation process of this cutoff low col was inconsistent with the model of migratory jet streak trough interaction but the evolution of the upper level trough including its narrowing and rotation of axis and the scale of the resultant col generally agreed with the theory of interaction between trough and mean westerly flow with anticyclonic background shear east southeast of the trough active and persistent convection appeared and induced 1 divergent ageostrophic flow near the tropopause 2 an increase in mid to upper level thickness due to effective heating and 3 amplification of an upper level ridge downstream thro

dynamical aspects of the life cycle of the winter storm lothar 24 26 december 1999 dynamical aspects of the life cycle of the winter storm lothar 24 26 december 1999 are investigated with the aid of the european centre for medium range weather forecasts analysis data and mesoscale model simulations neither of these datasets capture the full amplitude of the observed extreme pressure fall and surface wind speeds but they do help identify a range of key dynamical and physical features that characterize the development of this unusual event the analysis and interpretation is primarily based upon the evolution of the lower and upper level potential vorticity pv field complemented by three dimensional trajectory calculations lothar originated in the western atlantic and travelled as a shallow low level cyclone of moderate intensity towards europe this translation took place below and slightly to the south of a very intense upper level jet and was accompanied by continuous and intense condensational heating that sustained a pronounced positive low level pv anomaly not unlike the concept of a diabatic rossby wave no significant pv anomalies were evident at the tropopause level during this early phase of the life cycle the surface cyclone intensified rapidly when the shallow cyclone approached the jet stream axis the circulation induced by the diabatically produced low tropospheric pv anomaly on steeply sloping isentropic surfaces that transect the intense upper level jet contributed significantly to the rapid formation of a narrow and deep tropopause fold this stratospheric pv anomaly virtually merged with the diabatically produced ephemeral pv feature to form a vertically aligned tower of positive pv at the time of maximum storm intensity a sensitivity study with a dry adiabatic hindcast simulation shows no pv tower configuration and only a very weak surface development and confirms the primary importance of the cloud diabatic heating for the tropopause fold formation and the rapid bottom up intensification of lothar a comparison of the anomalously high atlantic sea surface temperatures in december 1999 with the water vapour source regions for the latent heat release that accompanied the rapid intensification phase of lothar shows a close relationship this is of importance when discussing the possible implications of climate variability and change on the development of north atlantic winter storms copyright 2002 royal meteorological society

a potential vorticity based study of the role of diabatic heating and friction in a numerically simulated baroclinic cyclone abstract a particularly intense case of western atlantic baroclinic cyclogenesis was investigated in this study specifically the roles of latent heat of condensation and surface friction were examined from the potential vorticity or pv thinking perspective the methodology used for this study involves three key components 1 a full physics mesoscale model which provides a continuous and dynamically consistent dataset and provides full user control over physical processes 2 a partitioned pv integration which temporally integrates the accumulation of pv due to various physical processes in the model s eulerian framework and 3 the piecewise inversion method of davis and emanuel which calculates the balanced wind and mass field associated with particular pv anomalies potential vorticity features obtained through the partitioned integration technique were inverted to yield their direct contributions to the total circulation in addition sensitivity studies were carried out to determine the overal

frontal wave cyclogenesis in the north atlantic a climatological characterisation the ambient conditions during cyclogenesis on fronts in the north atlantic na are characterised climatologically in the era interim dataset these secondary cyclones which grow from frontal waves are typically small in scale shallow and deepen rapidly and pose a challenge to forecasters and numerical weather prediction models frontal wave development has been studied intensively in the past based on observations e g fastex real case studies or analytical models in this study frontal wave cyclogenesis is studied from a climatological perspective and is detected by a combination of two automated identification methods for the period between 1979 and 2014 a climatology of frontal wave cyclogenesis is presented horizontal composites provide an estimate of the climatological mean of conditions during frontal wave cyclogenesis which may prove useful in the assessment of the realism of proposed idealized models of frontogenesis the findings are also compared with assumptions made in earlier idealized studies of frontal waves the composites reveal coherent structures of diabatically generated low level pv in the lower troposphere neutral moist static stability at cyclogenesis and an ambient lower troposphere close to saturation the vertical pv structure in the east na is more vertically aligned than in the west na and pv anomalies from the upper and lower levels reach deeper into the mid troposphere which indicates an enhanced phase lock between the two anomalies previous studies suggest the existence of an upper limit of along frontal stretching above which frontal wave development is strongly suppressed the presented multi decadal analysis confirms its existence in a range between 0 6 and 0 8 10 5 s 1 finally a climatology of along frontal stretching for the na is presented and compared to the fraction of cyclogenesis forming on frontal waves

tropical plumes and extreme precipitation in subtropical and tropical west africa three cases of extreme precipitation in october january and march april over subtropical north western and tropical west africa have been selected in order to study the moisture transports precipitation generation mechanisms and large scale dynamics involved all cases show strong tropical extratropical interactions and elongated cloud bands extending from the tropics into the subtropics usually referred to as tropical plumes tps investigations are based on observational data and high resolution output from simulations using the university of wisconsin nonhydrostatic modeling system trajectory analyses show that moisture is transported from the tropical easterly mid level flow the monsoonal southerlies and the north easterly trade winds to provide precipitable water and potential instability at mid tropospheric levels the instability is released through ascent related both to weak quasi geostrophic forcing to the east of an upper level trough and to the strong inertially unstable and highly divergent upper level subtropical jet stj accompanying the tp in the january and march cases the passage of a precursor upper level trough over the same location precedes the development and initiates the moisture transport from the deep tropics the associated north westerly cold advection supports the generation of precipitation by triggering convection over the tropical atlantic ocean whose outflow feeds into the stj over africa and by enhancing the vertical mass transport within the intertropical discontinuity over and near west africa in october when the still active african monsoon facilitates the extraction of tropical moisture a wave in the tropical easterlies and strong trade winds from the southern hemisphere are dominant factors low level cyclogenesis and frontogenesis only play roles in the late stages of the developments the elongated mostly positively tilted potential vorticity pv streamers associated with the observed troughs form as a result of an equatorward transport of high pv air downstream of a large ridge over the north atlantic the rapid amplification of the ridge is achieved through a combination of negative horizontal pv advection and diabatic reduction of upper level pv through latent heating within a cloud band that forms in connection with an explosive baroclinic development near the east coast of north america copyright 2005 royal meteorological society

on a threefold classification of extratropical cyclogenesis a recently proposed extension to the twofold extratropical cyclogenesis classification scheme of petterssen and smebye is discussed a third class of extratropical cyclone type c is described in which initial development is controlled by a pre existing upper level potential vorticity pv anomaly in its early stages such a system is indistinguishable from the classical type b cyclone of the petterssen and smebye scheme however subsequent development cannot be understood in terms of a co operative interaction of the upper level feature with a low level baroclinic zone rather the cyclogenetic dynamics of type c systems are dominated by the action of strong midlevel latent heating such heating can generate important anomalies of pv that act to suppress the formation of a low level thermal anomaly and that interact destructively with the pre existing upper level feature candidate type c events are identified using recently developed height attributable quasi geostrophic vertical motion diagnostics the application of one such diagnostic to a climatology of instantaneous cyclonic features suggests that type c events may occur with reasonable frequency the generic behaviour of system types in the proposed threefold classification scheme is compared with the actual dynamics of some cyclones from the fronts and atlantic storm track experiment fastex the analysis is based on piecewise pv inversions and numerical simulations this approach is able to provide a good description of a case of standard type b development consistent with the qualitative description of petterssen and smebye and with the quasi geostrophic diagnostics within the same framework two systems are discussed whose behaviours do not fit the simple a b classification but are consistent with the proposed type c mechanism copyright 2003 royal meteorological society

the linkage between the warm and the cold conveyor belts in an idealized extratropical cyclone abstractthis study continues the investigation of airstreams in idealized moist baroclinic waves and addresses the formation of the cold conveyor belt ccb its linkage to the warm conveyor belt wcb and their impact on the development of a midlatitude cyclone the ccb is identified as a coherent bundle of trajectories characterized by weak ascent and a strong increase of potential vorticity pv along the flow in contrast to the wcb defined as the trajectories with maximum ascent the authors illuminate the role of the two conveyor belts in the formation of two strong pv anomalies that form in the upper wcb negative pv anomaly and lower troposphere ccb positive pv anomaly respectively and thereby establish a link between these airstreams and relevant aspects of the dynamics of extratropical cyclones the ccb moves close to the surface along the colder side of the bent back front and experiences a pv increase as it passes below a region of maximum latent heat release at midtropospheric level

geologic geochemical and geophysical consequences of plume involvement in the emeishan flood basalt province prevolcanic kilometer scale lithospheric doming in the emeishan large igneous province southwest china allows us to evaluate the spatial and temporal consequences of uplift on the paleogeography geology geochemistry and geophysics of the region systematic spatial variations are observed across the domal structure in the distribution and thickness of clastic and carbonate sediments the extent of erosion thickness and chemistry of volcanic rocks and the crust mantle structure these features which are best explained by a mantle plume may be used to track older plume sites in the geologic record

potential vorticity diagnostics of a mei yu front case abstract the present study selects the mei yu frontal case of 12 13 june 1990 over southeastern china and performs potential vorticity pv diagnostic analysis to discuss the mechanism responsible for the intensification and maintenance of frontal vorticity the mei yu front had typical characteristics at the western section and was shallow with weak baroclinity but strong horizontal shear vorticity for this particular case results of piecewise pv inversion indicate that latent heat release associated with both deep convections and stratiform clouds was responsible for the frontal vorticity and the apparent frontogenesis near 0000 utc 13 june was driven almost entirely by an outbreak of deep convections along the front through the conditional instability of the second kind cisk mechanism proposed by cho and chen the positive feedback between the mei yu frontogenesis and cumulus latent heating in which the front provided low level convergence and helped organize the convection while latent heatin

reexamining the cold conveyor belt abstract despite the popularity of the conveyor belt model for portraying the airflow through midlatitude cyclones questions arise as to the path of the cold conveyor belt the lower tropospheric airflow poleward of and underneath the warm front some studies beginning with carlson s analysis of the eastern u s cyclone of 5 december 1977 depict the cold conveyor belt moving westward reaching the northwest quadrant of the storm turning abruptly anticyclonically rising to jet level and departing the cyclone downstream hereafter the anticyclonic path other studies depict the cold conveyor belt reaching the northwest quadrant turning cyclonically around the low center and remaining in the lower troposphere the cyclonic path to clarify the path of the cold conveyor belt the present study reexamines carlson s analysis of the cold conveyor belt using an observational and mesoscale numerical modeling study of the 5 december 1977 cyclone this reexamination raises several previously unappreciated

layer wise attribution of vertical motion and the influence of potential vorticity anomalies on synoptic development a methodology is presented for attributing vertical motion to potential vorticity pv in a explicit manner using the quasi geostrophic omega equation the methodology is then used to determine the contribution individual pv anomalies make to the synoptic development it is shown how the total vertical motion may be decomposed into self generating and interaction terms the former correspond to vertical motion that arises from one pv region alone whilst each of the interaction terms represent vertical motion due to the interaction of two different regions of pv the method is applied to a number of cyclones studied in the fronts and atlantic storm tracks experiment in order to determine the dynamical role of individual pv regions in cyclogenesis for purposes of comparison cyclones were chosen to be representative of the types a b and c of an extended petterssen smebye scheme devised by deveson browning and hewson in terms of our new pv method the type a and b cyclones exhibit only subtle differences with the upper level layer of pv playing a larger role in the case of the type b cyclone a much larger difference is apparent between the type a and b cyclones on the one hand and the type c cyclone on the other in the type c cyclone the upper level pv self generating term plays an important role whereas in the type a and b cases the influence of the upper level pv is largely restricted to the interaction terms in all of the cases the vertical motion attributable to the interaction between the low level pv and boundary thermal gradient was found to be large hence from the viewpoint of instantaneous attribution we conclude that the low level pv is always important copyright 2003 royal meteorological society

a diagnostic study of the flateyri avalanche cyclone 24 26 october 1995 using potential vorticity inversion abstract the evolution of a deep north atlantic cyclone which caused devastating avalanches in northwest nw iceland in october 1995 was investigated as the main tool for this investigation potential vorticity analysis was used this allows the quantification and comparison of the roles of different processes that contribute to the cyclone deepening at different stages interpretation of potential vorticity inversions and isentropic air trajectories yields the following picture of the cyclone development the thermal field over the north atlantic had acquired strong west east gradients due to a combination of advection of cold air southeastward from a cold cyclonic gyre south of iceland and advection of warm air northward on the westward flank of a warm anticyclonic ridge over central europe a low level baroclinic wave forming just south of ireland was rapidly reinforced due to interaction with a descending high value upper level potential vorticity anomaly and was isentropically advected from the