climatology and dynamics of the link between dry intrusions and cold fronts during winter part i global climatology cold fronts are a primary feature of the day to day variability of weather in the midlatitudes and feature in conceptual extratropical cyclone models alongside the dry intrusion airstream here the climatological frequency and spatial distribution of the co occurrence of these two features are quantified and the differences in cold front characteristics intensity size and precipitation when a dry intrusion is present or not are calculated fronts are objectively identified in the ecmwf era interim dataset for the winter seasons in each hemisphere and split into three sub types central fronts within a cyclone area trailing fronts outwith the cyclone area but connected to a central front and isolated fronts not connected to a cyclone these are then associated with dry intrusions identified using lagrangian trajectory analysis trailing fronts are most likely to be associated with a di in both hemispheres and this occurs more frequently in the western parts of the major storm track regions isolated fronts are linked to dis more frequently on the eastern ends of the storm tracks and in the subtropics all front types when co occurring with a di are stronger in terms of their temperature gradient are much larger in area and typically have higher average precipitation therefore climatologically the link with dis increases the impact of cold fronts there are some differences in the statistics of the precipitation for trailing and isolated fronts that are further investigated in part ii of this study from the front centred perspective

on the movement and low level structure of cold fronts abstract this paper presents a review of theoretical and observational studies relating to the low level structure of cold fronts and explores the factors that are pertinent to frontal motion observational studies have shown that in some cases surface cold fronts move at speeds faster than the normal component of the wind at all levels in the lower troposphere and therefore propagate other case studies have shown that the low level flow immediately behind the front and normal to it is faster than the front and that the front has the local structure of a gravity current its speed of movement being well determined by the gravity current speed equation them different types of behavior are related to results of recent theoretical studies and the mechanism by which fronts can propagate is elucidated it is shown that a necessary requirement for propagation is the existence of an alongfront temperature gradient we question the relevance of the gravity current speed equation in general despite its appar

on the role of inertial instability in stratosphere troposphere exchange near midlatitude cyclones abstractin simulations of midlatitude cyclones with the university of wisconsin nonhydrostatic modeling system uwnms mesoscale regions with large negative absolute vorticity commonly occur in the upper troposphere and lower stratosphere utls overlying thin layers of air with stratospheric values of ozone and potential vorticity pv these locally enhanced stratosphere troposphere exchange ste events are related to upstream convection by tracing negative equivalent potential vorticity epv anomalies along back trajectories detailed agreement between the patterns of negative absolute vorticity pv and epv each indicators of inertial instability in the utls is shown to occur in association with enhanced ste signatures results are presented for two midlatitude cyclones in the upper midwest where convection develops between the subpolar and subtropical jets mesoscale regions of negative epv air originate upstream in the boundary layer as they are transported through convection epv becomes incre

convective mixing in a tropopause fold we present a case study of the passage of a tropopause fold over the uk behind a cold front with emphasis on the mixing caused by convection extending into the fold the event took place on 15 16 january 1999 and was the subject of intensive observations using the met office c130 aircraft and the mesosphere stratosphere troposphere mst radar at aberystwyth here we concentrate on radar and satellite observations during the afternoon of 16 january when the surface cold front had passed over the uk a tongue of moist air moved north eastwards over wales at 700 hpa at this time which because of the very dry air in the fold above resulted in potential instability the resulting convection was clearly observed in noaa satellite images the mst radar depicted the passage of the cold front and tropopause fold as a layer of high echo power and vertical wind shear ascending with time spectral widths showed the fold to be free of turbulence until 1200 utc on 16 january when convection was observed reaching into the frontal zone and generating turbulence eddy dissipation and diffusivity rates of 8 6 mw kg 1 and 8 5 m2s 1 respectively were derived for this event to place these figures in context they are compared with corresponding rates derived for sixteen other passages of tropopause folds over the radar each resulting from shear rather than convective instability the convective event is found to be comparable to the strongest shear events and to correspond to moderate turbulence as experienced by an aircraft this process is of potential importance for atmospheric chemistry because it mixes boundary layer air directly with stratospheric air over a timescale of 1 2 hours copyright 2004 royal meteorological society

climatology and dynamics of the link between dry intrusions and cold fronts during winter part ii front centred perspective the conceptual picture of an extratropical cyclone typically includes a cold front and a dry intrusion di behind it by objectively identifying fronts and dis in ecmwf era interim data for 1979 2014 part i quantified the climatological relationship between cold fronts and dis driven by the finding that front intensity and frontal precipitation are enhanced in the presence of dis here we employ a front centred perspective to focus on the dynamical and thermodynamical environment of cold fronts with and without dis in the northern hemisphere winter distinguishing between trailing fronts that connect to a parent cyclone and isolated fronts examples of dis behind each type illustrate the baroclinic environment of the trailing front and the lack of strong temperature gradients across the isolated front composite analyses of north atlantic and north pacific fronts outline the major differences in the presence of dis compared to similar fronts but without dis in their vicinity the magnitude and spatial structure of the modification by dis depends on the front intensity yet generally with dis trailing fronts occur with stronger slp dipole deeper upper tropospheric trough stronger 10 m wind gusts enhanced ocean sensible and latent heat fluxes in the cyclone cold sector and heavier precipitation isolated weak fronts exhibit similar behaviour with different spatial structure this study highlights the central role of dis for shaping the variability of fronts and their associated environment and impact

break up of a stratospheric streamer observed by mst radar the break up of a stratospheric streamer over the british isles is described using satellite mesosphere stratosphere troposphere mst radar and in situ observations together with european centre for medium range weather forecasts analyses the streamer started as a thin ribbon of high potential vorticity to the west of the british isles as this was strained by the background flow it broke up into a series of mesoscale vortices mst radar and ozonesonde observations delineated a tropopause fold to the west of the initial streamer taking the form of a thin layer of stratospheric air embedded in a deep layer of descending upper tropospheric air the fold appears to have been mixed out by small scale turbulence as the streamer broke up this break up also generated strong inertia gravity waves which broke just above the tropopause introducing extensive mixing to the lowermost stratosphere one of the mesoscale vortices returned over the mst radar showing a very indistinct tropopause typical of a synoptic scale cut off low this vortex was also responsible for the development of a rain band on its eastern flank which brought significant rainfall to the british isles at a time when the synoptic flow pattern was strongly anticyclonic

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

tropopause folds in era interim global climatology and relation to extreme weather events tropopause folds are intimately linked to upper level frontogenesis and jet stream dynamics they play an important role for stratosphere troposphere exchange the dynamical coupling of upper and lower tropospheric levels and for generating severe weather events this study presents a global climatology of tropopause folds using era interim reanalysis data from 1979 to 2012 and a refined version of a previously developed 3 d labeling and fold identification algorithm this algorithm objectively separates stratospheric and tropospheric air in complex situations e g in regions with strong low level inversions and in extratropical cyclones where diabatically generated potential vorticity anomalies typically occur three classes of tropopause folds are defined shallow medium and deep and their geographical distribution vertical extent and seasonal cycle are investigated most shallow folds occur along the subtropical jet stream in agreement with previous studies hot spots of medium and deep tropopause folds are found west of australia and along the coast of antarctica in the southern hemisphere and around the east coast of north america in the northern hemisphere seasonal cycles show maxima in winter for all fold classes medium and deep folds are frequently associated with surface wind gust and precipitation extremes as quantified for folds over the southern indian ocean wind gust extremes occur mainly in an elongated band upstream and equatorward of folds whereas precipitation extremes occur mainly east and poleward of folds overall in the considered region about 20 of medium folds and 33 of deep folds are associated with surface wind or precipitation extremes in the vicinity of the fold

diagnostic study of a narrow cold frontal rainband and severe winds associated with a stratospheric intrusion a diagnostic study of a severe wind event associated with a cold front is presented based on output from an operational numerical weather prediction model together with radar and satellite imagery and detailed surface observations an intrusion of stratospheric air initially with a potential vorticity of 2 pv units is shown as part of a backward tilted filament to have plunged down to the lower troposphere where it was associated with a narrow cold frontal rainband at its leading edge and produced severe wind gusts up to 70 knots a close correspondence in time and space is revealed between these surface events and the appearance in the model of a pocket of diluted stratospheric air with pv 1 at the 2 km level the stratospheric intrusion produced two distinguishable wind surges one behind the narrow cold frontal rainband and another mainly dry about 200 km farther behind the strength of the winds was attributable to the downward transport of momentum as part of the large scale dynamics local moist processes played only a minor role at coarse resolution the radar echo from the narrow cold frontal rainband resembled line convection but it is shown that there were significant differences between this event and the archetypal form of line convection often seen in the british isles

fragmentation of stratospheric intrusions evidence is presented pointing to the existence of rich and coherent subsynoptic and mesoscale flow features at tropopause levels these features are related to and evolve from the classical v shaped intrusions of stratospheric air down to tropospheric elevations it is shown that intrusions can develop into elongated 2000 3000 km and slender 200 km streamers and that thereafter such a streamer can roll up to form a train of stalactite shaped vortex subentities with an accompanying substantial thinning of the intervening filament in addition there are indications that the vortices themselves can develop a spirallike interior structure of interleaved stratospheric and tropospheric air these inferences are based upon two independent but complementary sources analysis of the potential vorticity distribution on tropopause transcending isentropic surfaces derived from the analysis fields of the european centre for medium range weather forecasts either directly or indirectly using a contour advection technique and imagery from the water vapor channel of the european space agency meteosat 4 satellite streamers were observed to occur with a frequency of approximately one per week over central and southern europe during the winter of 1991 1992 the fragmentation is linked to the instability or self development of a filament of enhanced potential vorticity and it can modify or instigate surface weather systems moreover by inducing a substantial and rapid enlargement of the intrusion s surface area it greatly enhances the potential for local irreversible mixing of stratospheric and tropospheric air

variability of precipitation along cold fronts in idealized baroclinic waves abstractprecipitation patterns along cold fronts can exhibit a variety of morphologies including narrow cold frontal rainbands and core and gap structures a three dimensional primitive equation model is used to investigate alongfront variability of precipitation in an idealized baroclinic wave along the poleward part of the cold front a narrow line of precipitation develops along the equatorward part of the cold front precipitation cores and gaps form the difference between the two evolutions is due to differences in the orientation of vertical shear near the front in the lower troposphere at the poleward end the along frontal shear is dominant and the front is in near thermal wind balance while at the equatorward end the cross frontal shear is almost as large at the poleward end the thermal structure remains erect with the front well defined up to the midtroposphere hence updrafts remain erect and precipitation falls in a continuous line along the front at the equatorward end the cores form

a lagrangian based analysis of extratropical cyclones ii a detailed case study a detailed case study analysis is undertaken of the occurrence of significant coherent ensembles of trajectories cets that accompany atlantic cyclogenesis the approach is based upon the lagrangian based analysis scheme introduced in part i the analysis reveals the existence of several distinct moist ascending and dry descending cets two types of moist ascending cets associated with strong condensational processes ascend from the boundary layer of the warm sector either in the close vicinity of the cyclone centre or in the trailing cold frontal region the latter is the main contributor to the precipitation along the cold front during the early phase of the development another type of moist cets distinguished by maximum water vapour flux travels at low levels through the warm sector and ascends to mid tropospheric levels in the warm frontal region where it produces strong precipitation around the time of maximum storm intensity cets with an upper level origin are linked to the classical dry intrusion different cets correspond to strong descent of upper level tropospheric air that penetrated to the lower troposphere the movement of stratospheric air toward the centre of the maturing cyclone and an event of stratosphere to troposphere exchange upstream of the upper level trough two different visualizations of time sequences of the moist ascending cets indicate that they represent well defined flexible tubes or airstreams that exist over a time period comparable with the duration of the main cyclone intensification they provide a translating flexible but coherent river bed for the quasi continuous ascent of moist boundary layer air to the upper troposphere these tubes are akin to the concept of warm conveyor belts and the present analysis method objectively diagnoses their geometrical and physical characteristics and potential vorticity pattern some consideration is also given to the relationship of cets and tropospheric rivers and to possible shortcomings of the approach

secondary cyclogenesis along an occluded front leading to damaging wind gusts windstorm kyrill january 2007 abstractwindstorm kyrill affected large parts of europe in january 2007 and caused widespread havoc and loss of life in this study the formation of a secondary cyclone kyrill ii along the occluded front of the mature cyclone kyrill and the occurrence of severe wind gusts as kyrill ii passed over germany are investigated with the help of high resolution regional climate model simulations kyrill underwent an explosive cyclogenesis south of greenland as the storm crossed poleward of an intense upper level jet stream later in its life cycle secondary cyclogenesis occurred just west of the british isles the formation of kyrill ii along the occluded front was associated i with frontolytic strain and ii with strong diabatic heating in combination with a developing upper level shortwave trough sensitivity studies with reduced latent heat release feature a similar development but a weaker secondary cyclone revealing the importance of diabatic processes during the formation of kyrill ii kyrill ii moved

a five year radar based climatology of tropopause folds and deep convection over wales united kingdom abstracta five year 2006 10 radar based climatology of tropopause folds and convective storms was constructed for wales united kingdom to determine how deep moist convection is modulated by tropopause folds based on the continuous high resolution data from a very high frequency vhf wind profiling radar located at capel dewi wales 183 tropopause folds were identified tropopause folds were most frequent in january with a secondary maximum in july based on data from the u k weather radar network a climatology of 685 convective storms was developed the occurrence of convective storms was relatively high year round except for an abrupt minimum in february april multicellular lines 43 5 were the most common morphology with a maximum in october followed by isolated cells 33 1 with a maximum in may september and nonlinear clusters 23 4 with a maximum in november january convective storms were associated with 104 56 8 of the tropopause folds identified in this study with the associ

microphysical properties of cold frontal rainbands observations have been obtained within an intense precipitation rates 50 mm h 1 narrow cold frontal rainband ncfr embedded within a broader region of stratiform precipitation in situ data were obtained from an aircraft which flew near a steerable dual polarisation doppler radar the observations were obtained to characterise the microphysical properties of cold frontal clouds with an emphasis on ice and precipitation formation and development primary ice nucleation near cloud top 55 degrees c appeared to be enhanced by convective features however ice multiplication led to the largest ice particle number concentrations being observed at relativelyhigh temperatures 10 degrees c the multiplication process most likely rime splintering occurs when stratiform precipitation interacts with supercooled water generated in the ncfr graupel was notably absent in the data obtained ice multiplication processes are known to have a strong impact in glaciating isolated convective clouds but have rarely been studied within larger organised convective systems such as ncfrs secondary ice particles will impact on precipitation formation and cloud dynamics due to their relatively small size and high number density further modelling studies are required to quantify the effects of rime splintering on precipitation and dynamics in frontal rainbands available parametrizations used to diagnose the particle size distributions do not account for the influence of ice multiplication this deficiency in parametrizations is likely to be important in some cases for modelling the evolution of cloud systems and the precipitation formation ice multiplication has significant impact on artefact removal from in situ particle imaging probes

hydraulic conductivity in upland blanket peat measurement and variability a key parameter used in wetland hydrological and landform development models is hydraulic conductivity head recovery tests are often used to measure hydraulic conductivity but the calculation techniques are usually confined to rigid soil theory this is despite reports demonstrating the misapplication of rigid soil theory to non rigid soils such as peats while values of hydraulic conductivity calculated using compressible techniques have been presented for fenland peats these data have never to the authors knowledge been compared to such calculations in other peat types head recovery tests slug withdrawal were performed on piezometers at depths ranging from 10 cm to 80 cm from the surface on north pennine blanket peats results were obtained using both rigid and compressible soil theories allowing comparison of the two techniques compressible soil theory gives values for hydraulic conductivity that are typically a factor of five times less than rigid soil calculations hydraulic conductivity is often assumed to decrease with depth in upland peats but at the study site in the northern pennines it was not found to vary significantly with depth within the range of peat depths sampled the variance within depth categories was not significantly different to the variance between depth categories showing that individual peat layers did not have characteristic hydraulic conductivity values thus large lateral and vertical differences in hydraulic conductivity over short distances creates problems for modelling but may help account for the high frequency of preferential flow pathways within what is otherwise a low matrix hydraulic conductivity peat hydraulic conductivity was found to vary significantly between sampling sites demonstrating that hillslope or catchment scale variability may be more important than plot scale variability values for compressibility of the peats are also reported these generally decline with depth and also vary significantly between sampling sites there are implications for the way in which measurements of hydraulic conductivity and other properties of blanket peat are interpreted as the effects of environmental change in one part of a peat catchment may be very different to those in another

origin of string patterns in boreal peatlands the detailed stratigraphical analyses supported by field observations on mires in labrador and northern minnesota reported here provide the basis for an explanation of the development of the surface pattern of alternating peat ridges strings and linear depressions or pools flarks in boreal fens the developmental sequence involves drainage impediment and the concomitant initiation of peat accumulation development of linear patterns as a result of changing hydrological conditions and differential rates of peat accumulation of hummock and hollow plant communities and subsequent enlargement and coalescence of pools through the degradation of the underlying peat and the intervening ridges we show here that many of the processes may be active in the development of the pool patterns on raised bogs as well as on fens

some outstanding issues in the geology of lebanon and their importance in the tectonic evolution of the levantine region abstract the concept of pervasive and disruptive late neogene quaternary strike slip tectonics in lebanon has made establishing a satisfactory pre neogene palaeogeographic and palaeotectonic history for the area difficult research has allowed clarification of three fundamental issues that permit the setting of lebanon against its adjacent regions to be more firmly assigned the first issue is the identification of the significance of the 100 km long nne ssw trending monocline of the western lebanon flexure this feature is interpreted as a modified and rejuvenated early mesozoic hinge line that is presumed to be the northern continuation of the carmel sinai hinge line to the south the second issue is the correlation of the mount lebanon bekaa and anti lebanon complex with the adjacent intracontinental palmyride fold belt of syria through recognition in each of common divisions in structural style between a higher northern zone dominated by broadly spaced box folds and a lower southern zone with more numerous closely spaced and more internally complex folds the third issue involves the identification of major two stage late cretaceous to palaeogene syrian arc tectonism within lebanon resolution of these issues suggests a tectonic model for the region whereby the levantine region formed in two main phases the first of these was the multiphase late palaeozoic early mesozoic rifting of the area which imposed a fundamental extensional fabric over much of the region the second tectonic phase was the subsequent inversion of these extensional structures in the two stage syrian arc deformation of senonian and late palaeogene age which gave rise to the main lebanon structures it is this syrian arc tectonism rather than the late cenozoic strike slip that was probably the main structuring agent which affected the lebanon area the transpression from the braided dead sea fault motion although locally complex is an essentially neotectonic overprint on an already existing fabric offset of syrian arc structures in lebanon suggests that the total sinistral motion of the yammouneh fault segment of the dead sea fault is around 47 km

the frontal hydraulic head a micro α scale 1 km triggering mechanism for mesoconvective weather systems abstract measurements from the noaa boulder atmospheric observatory bao 300 m tower the national center for atmospheric research ncar sabreliner aircraft and the noaa goes 5 satellite give evidence for the cross front scale collapse of nonprecipitating surface cold frontal zones to horizontal distances of 1 km or less the leading edges of these frosts possess the characteristic structure of density current flows an elevated hydraulic head followed by a turbulent wake vertical motions at the frontal heads exceed 5 m s 1 at 300 m agl the ascent at the frontal head may act as a 1 km scale triggering mechanism for the release of potential instability and the formation of intense squall line mesoconvection

tropopause folds and cross tropopause exchange a global investigation based upon ecmwf analyses for the time period march 2000 to february 2001 1 a new methodology is proposed to identify folds of the dynamical tropopause taken as the 2 potential vorticity pv units pvu isosurface from global analysis data sets from the european centre for medium range weather forecasts ecmwf it consists of a three dimensional subdivision of the atmosphere into stratospheric and tropospheric parts and a subsequent examination of multiple tropopause crossings in vertical profiles of the analyses the method is applied to a 1 year period starting in march 2000 seasonal mean fold frequency distributions show that folds occur preferentially in the subtropics with maximum frequencies of about 30 pronounced maxima are located in the subtropical bands that extend from 20 e to 120 e on both hemispheres during summer generally subtropical folds are comparatively shallow deep folds occur most frequently in midlatitudes during winter over eastern north america and the western north atlantic with maximum frequencies of about 1 detailed investigation of the relationship between individual folds and cross tropopause exchange events reveals that on the average individual folds in the extratropics are associated with larger exchange mass fluxes compared to the subtropics however due to the much higher frequency of subtropical folds the percentage of exchange events that are associated with tropopause folds is larger near 20 40 latitude 50 70 than further poleward 20 30 during winter and 10 20 during summer this indicates that tropopause folds are the key feature for cross tropopause exchange in the subtropics whereas in the extratropics other tropopause structures are at least of equal importance

on the mesoscale structure of surface wind and pressure fields near tornadic and nontornadic cold fronts abstractobservations from a mesoscale network of automatic weather stations are analyzed for 15 u k cold fronts exhibiting narrow cold frontal rainbands ncfrs seven of the ncfrs produced tornadoes a time compositing approach is applied to the minute resolution data using the radar observed motion vectors of ncfr precipitation segments interpolated onto a 5 km grid the analyses resolve much of the small mesoscale structure in surface wind temperature and pressure fields postfrontal winds varied substantially between cases tornadic ncfrs exhibited a near 90 wind veer and little or no reduction in wind speed on ncfr passage these attributes were generally associated with large vertical vorticity horizontal convergence and vorticity stretching at the ncfr nontornadic ncfrs exhibited smaller wind veers and or marked decreases in wind speed across the ncfr and weaker vorticity convergence and vorticity stretching in at least four tornadic ncfrs increases in vorticity stretching preceded tor

observations of the mesoscale sub structure in the cold air of a developing frontal cyclone observations from 58 dropwindsondes released in a mesoscale array during the fronts 92 experiment are interpreted in the context of satellite imagery to derive the mesoscale structure and evolution of parts of a frontal cyclone developing over the eastern north atlantic a conceptual model involving the intertwining of dry intrusion and cloud head flows is corroborated and is used to provide the framework for interpreting the detailed mesoscale behaviour in the cold air two distinct dry intrusions were responsible for two cold fronts trailing south westwards from the tip of the cloud head both were surface features at the beginning of our study but the leading one evolved into an upper front with mid level convection as the dry intrusion responsible for it overran the warm sector areas of both high and low potential vorticity were indicated within the dry intrusions upon encountering a critical level of zero system relative velocity at the top of the moist boundary layer the dry intrusions arrival was associated with the development of multiple dry and moist laminae near the top of the boundary layer the vertical wavelength of the laminae was about 1 km and they extended over 200 km in the front normal direction with a slope of typically 1 in 60 although most parts of the laminae were subsaturated their circulations combined with the double structure of the dry intrusions to produce multiple shallow cloud lines within the boundary layer these formed as an extension of the south western tip of the cloud head associated with the developing cyclone possible mechanisms for generating the observed structures are discussed

structure of an internal bore and dissipating gravity current as revealed by raman lidar abstract detailed moisture observations from a ground based raman lidar and special radiosonde data of two disturbances associated with a dissipating gust front are presented a synthesis of the lidar data with conventional meteorological data in conjunction with theoretical calculations and comparison to laboratory studies leads to the conclusion that the disturbances seen in both the lidar and accompanying barograph data represent a weak gravity current and an associated undular bore the disturbances display excellent coherence over hundreds of kilometers upstream of the lidar site bore formation occurs at the leading edge of the gust front coincidentally with the rapid weakening of the gravity current analysis suggests that the bore was generated by the collapse of the gravity current into a stable nocturnal inversion layer and subsequently propagated along this wave guide at nearly twice the speed of the gravity current the raman lidar provided detailed measurements of the vertical structure o

a general circulation model study of a tropopause folding event at middle latitudes the uk universities global atmospheric modelling programme general circulation model ugcm has been run at high horizontal resolution to investigate a tropopause folding event at middle latitudes geopotential height potential vorticity and specific humidity distributions were diagnosed from the model and showed a realistic representation of the synoptic development of the cut off low associated with the tropopause fold the model also successfully portrayed the frontogenesis associated with tropopause folding and represented both the upper level frontal region and the corresponding jet streaks as they progressed through the region of the developing trough a series of vertical cross sections at the ends of the jet streaks demonstrated that a fold could be captured by the ugcm using potential vorticity and specific humidity although more small scale features were evident in the specific humidity contours than in the corresponding potential vorticity fields the corresponding baroclinic zone was also resolved although both the slope and thickness were rather larger than sometimes observed both the spatial and temporal extent of the fold compared well with observations in addition the model simulated the ageostrophic winds in the entrance exit and middle streak regions a direct circulation was obtained at the entrance and an indirect circulation at the exit region of the jet streak in good agreement with theory and with observational data a combination of confluence cold advection and curvature was responsible for the forcing of the ageostrophic motion at the jet entrance whereas the forcing at the jet exit was attributed mainly to diffluence and curvature q vectors were employed to investigate the vertical motion in the vicinity of the fold and showed the precise positions of descent corresponding to the fold along the entire length of the jet streak finally a tropopause fold around a jet that was not subject to the development of strong baroclinic instabilities was reproduced by the model it was smaller in size than the fold of the cyclonic trough a q vector analysis confirmed the position of the small fold along the jet and revealed large scale ascending motion that may have been responsible for the small size of fold the possibility of using the model to yield a realistic estimate of the transfer of air from the stratosphere to the troposphere in a tropopause folding event at mid latitudes is discussed

dry intrusions lagrangian climatology and dynamical impact on the planetary boundary layer abstractdry air intrusions dis are dry deeply descending airstreams from the upper troposphere toward the planetary boundary layer pbl the significance of dis spans a variety of aspects including the interaction with convection extratropical cyclones and fronts the pbl and extreme surface weather here a lagrangian definition for di trajectories is used and applied to ecmwf interim reanalysis era interim data based on the criterion of a minimum descent of 400 hpa during 48 h a first global lagrangian climatology of di trajectories is compiled for the years 1979 2014 allowing quantitative understanding of the occurrence and variability of dis as well as the dynamical and thermodynamical interactions that determine their impact dis occur mainly in winter while traveling equatorward from 40 50 latitude dis typically reach the lower troposphere with maximum frequencies of 10 in winter in the storm track regions as well as over the mediterranean sea arabian sea and eastern north pa

revised stratigraphy of the upper cretaceous and cenozoic series of lebanon based on nannofossils abstract the aim of these investigations was to revise the geological map of lebanon calcareous nannofossils were studied from a predominantly marly senonian maastrichtian and cenozoic series about 900 spot samples were collected from exposed strata this lithologically homogenous stratigraphic interval is poorly subdivided on the existing geological maps the present study allows the precise determination of hiatuses and tectonic events palaeocene upper eocene upper oligocene and lower miocene units were identified for the first time in lebanon

hydrogen and oxygen isotopic records in monthly scales variations of hydrological characteristics in the different landscape zones of alpine cold regions monthly scale variations in stable isotopic composition in precipitation and various water bodies were analyzed in this study using data collected from a field station experimental observation was performed to analyze processes in the glacier snow zone alpine cold desert zone marsh meadow zone alpine shrub zone and mountain grassland zone based on the isotopic and hydrochemical analyses of glacier snow frozen soil groundwater and other water sources in the headwater catchment of the alpine cold region delta o 18 and delta d values of precipitation and various water bodies in mafengou river basin exhibited strong seasonal variations precipitation and various water bodies were rich in delta o 18 and delta d during the wet season and relatively negative in delta o 18 and delta d during the dry season the isotopic content of various water bodies in mafengou river basin was dependent on temperature elevation of the recharge area and amount of precipitation lower temperatures were typically associated with lower delta o 18 and delta d values and are believed to be mainly responsible for the seasonal patterns observed in the isotopic values of various water bodies in mafengou river basin in the alpine cold region seasonal variations were larger in streams where recent precipitation is the main source of the flow and smaller in streams where groundwater is the dominant source of water c 2013 published by elsevier b v

palaeoenvironmental evolution of the late miocene palaeolake at zahle bekaa valley lebanon abstract neogene non marine sedimentary rocks of the bekaa valley lebanon are microfossil rich and show minimal lateral facies changes and post depositional diagenetic alteration across continuous exposures in this paper we analyze the sedimentary facies and microfossils of three closely spaced stratigraphic sections near the town of zahle the sedimentary sequence is divided from base to top into three intervals 1 a massive conglomerate unit interpreted as alluvial fan deposits 2 fossiliferous yellowish marls and limestone interpreted as perennial lacustrine deposits and 3 an organic rich marlstone intercalated with volcanic ash horizons interpreted as a palustrine interval representing an overall shallowing sequence intervals 2 and 3 contain a diverse microfossil assemblage comprising charophytes terrestrial molluscs a freshwater bivalve ostracods as well as fish and mammal remains the palaeoenvironmental characteristics and evolution of these lacustrine deposits are inferred through facies analysis and through comparison of microfossil tolerances with the ecological requirements of their nearest living relatives results suggest that during the miocene the bekaa valley was occupied by a relatively shallow stable oligotrophic freshwater or slightly oligohaline lake that evolved to a very shallow eutrophic lake with a dense palustrine vegetation belt the palaeolake ultimately regressed to the south due to climatic changes and tectonic stresses leaving the lacustrine deposits exposed in the area of zahle as an erosional surface

geochemistry and hydrology of a calcareous fen within the savage fen wetlands complex minnesota usa abstract savage fen is a wetlands complex at the base of north facing bluffs in the minnesota river valley the complex includes 27 8 hectares of calcareous fen that host rare calciphile plants whose populations are declining in minnesota water and sediment compositions in the calcareous fen were studied to gain a better understanding of the hydrologie system that sustains the rare vegetation groundwater in the fen is a calcium magnesium bicarbonate type with circumneutral ph values the groundwater composition is the resuit of interactions among water dissolved and gaseous carbon species carbonates and ion exchangers shallow groundwater is distinguished from deep groundwater by smaller concentrations of chloride sulfate magnesium and sodium and larger concentrations of calcium bicarbonate hydrogen sulfide and ammonium magnesian calcite is the prevalent carbonate in unconsolidated sedimentary fill beneath the fen and is an important source and sink for dissolved calcium magnesium and inorganic carbon calcite concentrations just below the water table are small because aerobic and anaerobic oxidation of organic matter increase the partial pressure of carbon dioxide pco2 decrease ph and cause calcite to dissolve thick calcite accumulations just above the water table in the root zone of calciphile plants resuit from water table fluctuations and attendant changes in pco2 groundwater beneath savage fen recharges in lakes and ponds south of the fen and upwells to the surface within the fen water at the water table is a mixture of upwelling groundwater and water near the surface that flows downslope from higher elevations in the fen changes in oxygen and hydrogen isotope compositions of shallow groundwater indicate that the proportion of upwelling groundwater in shallow groundwater decreases downgradient in the calcareous fen encroachment of reed grasses into the calcareous fen may reflect human caused disturbances in the recharge area

87sr 86sr as a tracer of groundwater discharge and precipitation recharge in the glacial lake agassiz peatlands northern minnesota a contrast in the 87sr 86sr ratio of atmospherically derived sr 0 710 and sr from regional bedrock 0 730 allows quantification of the proportions of precipitation recharge and groundwater discharge into peat columns of bogs and fens in the glacial lake agassiz peatlands two contrasting bog fen pairs were studied the lost river peatland located over a regional groundwater discharge zone and the red lake peatland situated atop a hydrologic divide where recharge is expected to occur bog pore waters consist of 90 100 precipitation in the upper 2 m below which they mix with groundwater adjacent fens consist mostly of groundwater ranging from 30 near the surface to 100 with depth at the red lake peatland mixing relations suggest an additional sr source that may be peat mineralization distinct 87sr 86sr and ca sr ratios for groundwater discharging into bog and fen sites are indicative of distinct groundwater sources and are likely indicative of flow paths at two different length scales

elemental distribution and pyrite occurrence in a freshwater peatland alberta a continental minerotrophic peatland in alberta was studied for its botanical petrographical mineralogical and geochemical characteristics changes in peat stratigraphy and mineralogy are related and development toward oligotrophic conditions is recorded elemental concentration and distribution vary in the peat profile reflecting the influence of natural and anthropogenic factors on peat geochemistry many elements al as ba co cr fe mn th v accumulate in the zone of water table fluctuation as a result of changes in redox chemistry the relative elemental concentration at the surface and near surface e g as cr sb th ti v may also be attributed to atmospheric deposition related to anthropogenic activity chemical analysis al fe mg na si ti also reveals the presence of a non visible volcanic ash rich layer near the base of the peat profile possibly mazama tephra this study provides evidence of pyrite formation in a freshwater peat forming environment associated with high sul

attribution and its application to mesoscale structure associated with tropopause folds it is important in meteorology to be able to establish a link between various features this link in its strongest form would be referred to as cause and effect but this is extremely difficult to establish for atmospheric flows a weaker but nonetheless powerful link is attribution which attempts to provide a way to associate various fundamental features such as potential vorticity anomalies to weather events involving regions of ascent attribution is not new and it is implicit in the way that atmospheric evolution is usually described in this paper an attempt is made to define more precisely what are the steps and assumptions being made in such a method these ideas become more concrete by applying them to a simple model of a two dimensional front at which there is a tropopause fold the aim is to find the vertical motion attributable to the descended tropopause fold in an extra tropical cyclone this is the first step in a longer programme of research designed to provide a test of the hypothesis that such a descended tropopause can trigger mesoscale phenomena such as convection and cyclones

hydrological processes and chemical characteristics of low alpine patterned wetlands south central new zealand summary patterned wetlands in new zealand are not only rare but they are distinguished from their northern hemisphere counterparts by their vegetation and the supply of rainfall which is relatively free of pollution hydrological processes in two low alpine patterned wetlands 5 and 220 ha on block faulted uplands in south central south island new zealand were investigated using hydraulic head gradients and chemical and isotope data there was no significant spatial variability in chemical parameters within either wetland although the larger wetland is characterised by a lower trophic status than the smaller one ca2 concentrations and ph of surface water of both wetlands are within the range of that of northern hemisphere ombrotrophic mesotrophic bogs and poor fens pool water chemistry only partially overlaps with the lower end of the ca2 distribution for the northern hemisphere wetlands likely to be caused by the low level of ca2 in aeolian dust the ph in the pools of the small wetland was generally higher probably as a result of differing organic acid production by the sphagnum and sedge species both of which were common and dominant or sub dominant in the wetlands some temporal variability was observed possibly due to seasonal variations in plant uptake and or rainfall a steady increase in na concentrations in the pools over 4 years was tentatively attributed to an excess of supply from rainfall and or groundwater cation and stable isotope data indicate strong evaporation in pools and surface water despite the high ambient relative humidity and mixing within the top of the wetland sequence stratigraphy indicates that peat formation has been interrupted by flooding or runoff bringing mineral matter into the wetlands particularly in the earlier development stages of the larger wetland the peat sequence is thicker in the smaller wetland reaching up to 2 8 m with limited mineral input during wetland development the relatively low ph and ionic concentrations in the contemporary water and peat sequence of the larger wetland indicate that it has become isolated from runoff and mineral input

a severe frontal rainband part iii derived thermodynamic structure abstract pressure buoyancy and virtual potential temperature perturbations are calculated from wind fields derived from doppler radar data taken in a surface cold front the dynamics of the front are similar to a density current this hypothesis is also suggested by accompanying numerical simulations of cold air outflows the updraft at the leading edge of the cold air mass is maintained in conjunction with an upward directed pressure force the average maximum updraft is in excess of 7 m s 1 without any appreciable potential instability present in the undisturbed warm sector sounding the buoyancy and virtual potential temperature data reveal a front with a substantial fraction of the cooling taking place within the first 2 km of a frontal zone thus the aspect ratio width depth of the front even after the filtering associated with the interpolation and retrieval process is slightly less than one the frontogenesis for the shear in the along front wind and the thermal gradient are discussed the g

observation of a cold front with rope cloud abstract observations are presented of a cold front which passed over the florida peninsula and produced a rope cloud visible on satellite imagery the low level structure of the leading edge of the front is revealed with data from the 150 m tower at cape canaveral air force station it is shown that the leading edge of the front is very similar to thunderstorm gust fronts and the speed is predicted well by the gust front speed equation this indicates that the rope cloud is directly analogous to the arc cloud sometimes observed along thunderstorm gust fronts further the rope cloud appears to be a less intense manifestation of the forced convection that produces the narrow cold frontal rainbands observed in some other cold front studies

sting jets in extratropical cyclones a review this paper reviews the current state of knowledge of sting jets in extratropical cyclones sting jets were formally identified in 2004 by the pioneering work of keith browning reviewing this and subsequent studies we define the sting jet as a coherent air flow that descends from mid levels inside the cloud head into the frontal fracture region of a shapiro keyser cyclone over a period of a few hours leading to a distinct region of near surface stronger winds it therefore lies above the cold conveyor belt during some stage its life but at least in some cases descends to reach the top of boundary layer ahead of the cold conveyor belt it is not attributed to a specific mechanism in this definition we conclude that it is likely that a continuum of sting jet descent and speed up mechanisms exists at one extreme is balanced descent partly associated with frontolysis in the frontal fracture region more horizontally small scale and stronger frontolytic descent may occur associated with weak stability to slantwise convective downdraughts instability to slantwise convective downdraughts may occur in some systems leading to multiple slantwise convective downdraughts associated with the release of conditional symmetric instability and even possibly symmetric instability the global climatology of sting jets and the interaction between the atmospheric boundary layer and sting jets are revealed as specific areas where more research is needed finally we describe eight myths and misunderstandings that exist in the current literature with the aim of guiding future research into the sting jet phenomenon