structure of a frontal cyclone a diagnostic study of a mid latitude cyclone has been carried out using routinely available numerical weather prediction model products and imagery the cyclone was intense but not exceptionally so and the results are believed to have some generality the detailed description of the cyclone structure just before it began to deepen rapidly integrates concepts from a number of researchers into a common framework a dominant feature of the cyclone close to its centre was the cloud head a region of cloud with a sharp convex outer edge which formed poleward of the main polar front cloud band the cloud head was caused by two flows that entered it from the east ascending and fanning out within it one flow the cold conveyor belt brought low wet bulb potential temperature w air back into the cloud head from low levels ahead of the warm front the other flow was due to high w air that peeled off from the base of the main warm sector airflow i e part of the warm conveyor belt and travelled in the boundary layer back towards the cyclone centre first undercutting dry air that had earlier descended from the upper troposphere called a dry intrusion and then ascending at the upper boundary of the cloud head above the cold conveyor belt the transverse circulation that gave the ascent within the cloud head also led to the cold front fracturing along its length into two separate sharp surface cold fronts with a more diffuse frontal region in between frontal fracture the two sharp surface cold fronts were associated with narrow cold frontal rainbands line convection one of the line convection segments forming the southern edge of the cloud head the overrunning of the dry intrusion in the region of the frontal fracture led to a structure known as a split front in which an upper level humidity front began to run ahead of the position of the surface cold front a feature of the cyclone at this intermediate stage in its evolution was that a large proportion of the precipitation was being generated by ascent of relatively cold air the cold conveyor belt in the cloud head as part of the thermally indirect circulation at the left exit of a developing secondary upper level jet

the central anomaly magnetic high implications for ocean crust construction and evolution the central anomaly magnetic high camh is a long lineated short wavelength 10 km wide at the sea surface magnetic anomaly overlying many of the world s midocean spreading centers this anomaly is particularly well developed over the northern juan de fuca ridge including the northern symmetrical and endeavour segments the ubiquitous presence of this zone of high magnetic contrast directly in the zone of active oceanic plate formation is a key to our understanding of near axis crustal plate evolution recently obtained deep tow magnetometer data from segments of the juan de fuca spreading center have caused us to revise the present models of the source of the camh and to review the implications the new model would have on our understanding of the evolution of the magnetic structure of oceanic plates in this model we attribute the near axis magnetization contrast to tectonically controlled low temperature alteration of the upper vertical section of extrusive basement extensive and systematic fissuring of the upper extrusive rocks allows penetration of sea water and rapid alteration of the magnetic minerals in fissured areas approximately 3 km off axis the correlation between fissured crustal blocks and near bottom magnetic lows and unfissured crustal blocks with magnetic highs which is observed at the axis is also seen in a single off axis tow over an outer flanking ridge

propagating rifts and continental breakup a simple quantitative model is proposed whereby continental breakup starts with a phase of continental rifting and continues with the propagation of rifts toward locked zones these zones are deformed in the rifting process while oceanic crust is generated in the wake of the rift tip the deformed zones will cause overlaps when a continental reassembly is attempted the model accounts for the shape and distribution of magnetic quiet zones at ocean continent boundaries and for the detailed geometry of some v shaped zones of oceanic magnetic anomalies the former provide an estimate of the scale at which the hypothesis of rigid plates fails and the latter are used to measure the velocity of rift propagation illustrations are provided for present rifting in the red sea and gulf of aden and for early rifting of the south atlantic and breakup of pangea

thermal structure and airflow in a model simulation of an occluded marine cyclone abstract a very fine mesh model simulation of the ocean ranger storm of february 1982 is used to study the thermal structure and airflow in an intense marine cyclone in particular the study investigates the structures of the occluded front and the secluded pool of warm air in the simulated cyclone and examines the formation of these structures with the help of a large number of air trajectories it was found that in the model simulation the largest thermal gradient occurred along the occluded front not along the warm or cold fronts and that the intense gradient was a product of strong warm and occludofrontogenesis in the inflowing air the occluded front was embedded within air that was earlier located within the baroclinic zone ahead of the low center not air that was in proximity to the historical warm and cold fronts the warm air in the seclusion likewise originated in the baroclinic zone ahead of the low the seclusion at low levels resulted from a tongue of slower moving relatively warm air be

3 d inversion of deep tow magnetic data on the atlantis ii deep red sea hydrothermal and geodynamic interpretation abstract to study the magnetic signature of hydrothermal oceanic crust areas data were collected using a magnetometer towed a few hundred meters above the sea floor within an acoustic transponder net the experiment was performed in the red sea over the atlantic ii deep near 21 20 n 38 05 e this deep created during the brunnes epoch is well known for its intense hydrothermal activity and the presence of hot brines overlying metalliferous sediments the magnetic field was inverted to obtain the distribution pattern of crustal magnetization including the effects of topography and irregular depth of the magnetic sensor the procedure however does not produce any unique solution it is approximated using probabilistic linear methods tarantola and valette 1982 which require the introduction of constraints in the form of gaussian probability density functions the resultant magnetization pattern then follows a gaussian law the mean and standard deviation are then computed the results show contrary to what has been suggested formerly rona 1978a b that the strongest magnetization as well as the most frequent hydrothermal and magmatic occurrences are found in the southwest basin the high magnetic intensities are thus associated with a recent magmatism which initiates the hydrothermal circulation two hypotheses explain this 1 1 the freshness of the basalts and the low temperature alteration are the principal causes of magnetization contrasts and represent respectively the discharge and recharge zones of the hydrothermal plumbing 2 2 in the general framework of the red sea opening by progressive tearing the formation of the atlantis ii was extremely fast according to the elastic rebound theory reid 1911 this hypothesis is applicable to some other deeps

seismic gravity and magnetic surveys in the central part of the red sea their interpretation and implications for the structure and evolution of the red sea abstract while there is unanimous agreement that the axial part of the red sea has witnessed seafloor spreading since the early pliocene opinions diverge considerably with regard to the situation in the main trough and shelves new high precision data comprising palaeomagnetic seismic gravity magnetic and bathymetric measurements from the central and northern parts of the red sea have been examined to discriminate between the earlier proposed hypotheses and to formulate an evolutionary model for the red sea the results show that oceanic crust extends for about 80 km at 19 n on each side of the spreading axis symmetrical seafloor spreading magnetic anomalies are recognized out to anomaly 3a 6 m y b p between 18 30 and 19 45 n anomaly 2a between 19 45 and 20 n and near 21 n anomaly 2 near 22 n and only anomaly 1 near 23 30 n although anomaly 3a is the oldest recognizable magnetic anomaly the extent and configuration of the oceanic basement as determined from seismic measurements indicate that spreading has started and continued without a hiatus since 10 12 m y b p prior to the initiation of oceanic accretion an extension of 40 55 km i e 50 70 has taken place in the proterozoic continental lithosphere whereas a separation of 70 160 km has occurred during the seafloor spreading phase several lines of evidence indicate that the shelves and the marginal parts of the main trough are of continental nature

jet streaks in the gulf stream mesoscale alongstream speed changes of the gulf stream are diagnosed from an array of current meters at depths 400 700 and 1000 m near 688w during the development of steep ratio of amplitude to wavelength o 1 meanders speed maxima jet streaks are generally found between trough and crest axes in steep meanders with local speed minima near the trough and crest axes speed changes along streamlines can be quite dramatic speed changes along the jet axis between jet streaks and local minima in excess of 0 60 0 40 and 0 35 m s 21 are observed at depth 400 700 and 1000 m respectively this is in comparison with peak speeds in a frontal coordinates system mean of 1 22 0 67 and 0 28 m s21 at depth 400 700 and 1000 m respectively from a previous study the presence of the jet streaks can be explained kinematically as a superposition of the gulf stream and barotropic vortices the development of these jet streaks in relation to the developing steep meanders differs from the canonical picture of jet streak baroclinic wave development in the atmospheric jet stream in that the jet streaks in the gulf stream are predominantly fixed in place with respect to meanders as they steepen

mineralogy and geochemistry of epigenetic features in metalliferous sediment atlantis ii deep red sea finely laminated and undisturbed except in the area of the southwest basin in this part bedding resedimentation and brecciation are common in addition some sediment cores contain vein minerals precipitated in open fissures which crosscut unlithified metalliferous sediments these veins are the conduits by which new hot brine vents into the deep vein mineralogy is dominated by anhydrite talc smectite pyrite sphalerite and chalcopyrite vertical zonation due to cooling and reaction of the incoming brine as it rises through the metalliferous sediment near the southwest basin west basin transition metalliferous sediment has been locally recrystallized to a hematite magnetite pyroxene assemblage probably due to the intrusion of basalt into the metalliferous sediments modified journal abstract

on the dependence of boundary layer ventilation on frontal type 1 case study simulations with idealized tracers have been used to determine the relationship between the dynamics and conceptual representations of different midlatitude frontal systems and the amount distribution and time scale of boundary layer ventilation by these systems the key features of ventilation by a kata and ana cold frontal system are found to be quantitatively and also often qualitatively similar to the main ventilation pathways which are the conveyor belts cloud head and other convection the conveyor belts and cloud head occur within cloud implying that they can be identified using satellite imagery differences in the transport by the two systems can be related to their conceptual representations and include a sensitive dependence on the diurnal cycle for the kata but not the ana cold frontal case

three dimensional inversion of marine magnetic anomalies on the equatorial atlantic ridge st paul fracture zone delayed magnetization in a magmatically starved spreading center the st paul fracture zone fz in the equatorial atlantic is interrupted by three intratransform ridge itr spreading centers a detailed magnetic survey corrected for the diurnal variations using a moored magnetic station six submersible dives and three bottom towed video camera tracks provide data on the most eastern itr 0degrees37 n 25degrees27 w visual observations and submersible sampling displayed a high ultramafic volcanic ratio supporting the assumption that the itr is in a magmatically starved state volcanics were mainly found on the rift valley floor from 4700 to 4000 m and as a thin cap 160 m on the top of the eastern rift crest 2700 m most of the rift walls consist essentially of serpentinized peridotites and gabbros the magnetic data show a well defined ridge centered anomaly a generalized inversion method was applied to the field data to calculate the crustal equivalent magnetization assuming that the seafloor is broken down into elementary cells of 1x1x0 5 km 3 which fit the topography the average of absolute value of equivalent magnetization is 2 7 a m 1 the width of the central normal polarity brunhes epoch is wider at least 34 km than that indicated by the nuvel 1 kinematic model 24 5 km this 40 excess is believed to be significant and is thought to be the result of prolonged chemical remanent magnetization acquired during the serpentinization of peridotites in a magmatically starved accretion segment we suggest that peridotites could continue to acquire magnetization as long as tectonic activities facilitate the circulation of seawater in the upper mantle

magnetic properties and opaque mineralogy of drilled submarine intrusive rocks summary measurements of natural remanent magnetization nrm curie temperature ferrimagnetic and paramagnetic susceptibility saturation induced and remanent magnetizations coercive forces alternating field properties and viscous magnetization are reported for 50 submarine intrusive rocks drilled during legs 30 37 and 45 of dsdp the collection includes doleritic sill rocks fresh and serpentinized cumulate gabbros and serpentinized cumulate peridotites and serpentinized lherzolites believed to have originated in layers 2b 3b and 4 respectively magnetite with a curie temperature between 520 and 580 c is the principal magnetic mineral in all samples there is no indication of maghemitization or of metamorphism to greenschist facies or above the magnetite in the doleritic and cumulate gabbros is a product of deuteric alteration of titanomagnetite and pyroxene the stable nrm is a primary trm the magnetite in the serpentinized rocks is a secondary product of serpentinization the stable nrm is a crm in most rock types the magnetite is of single domain or pseudo single domain size and soft components of nrm are small the magnetite grain size in some of the doleritic gabbros is much coarser these rocks acquire large viscous magnetizations which however are readily removed by alternating field cleaning the cleaned nrms of legs 30 and 45 rocks have approximately dipole inclinations but the nrms of most of the mutually intruded leg 37 units have been dispersed by tectonic rotations the doleritic gabbros and the serpentinized peridotites have stable directionally coherent nrms 10 3emu cm 3 in intensity their counterparts in layers 2b and 3b 4 are likely contributors to oceanic magnetic anomalies layer 3b cumulate gabbros and tectonic serpentinites in the middle and upper crust are less likely anomaly sources

dynamical aspects of the october storm 1987 a study of a successful fine mesh simulation a diagnostic study of a successful numerical simulation of the october storm 1987 is described this good non operational forecast was obtained using the meteorological office s fine mesh model run from a revised analysis at 00 gmt on 15 october 1987 the meteorological office s mesoscale model was also run using initial conditions at 12 gmt 15 october obtained by interpolating the above fine mesh forecast fields to examine the benefits of very high horizontal resolution 15km grid length diagnostic maps and cross sections derived are used to address two scientific issues i the role of weak moist slantwise stability or alternatively positive slantwise convective potential energy ii the dynamical significance of the striking cloud structure the cloud head seen in the satellite imagery early in the storm s life it appears that the baroclinic zone in which the storm develops is highly receptive to forcing by an upper trough and the warm boundary layer air just ahead of the main surface front is characterized by substantial slantwise convective available energy the cloud head is identified with a region of intense slantwise ascent in the warm air just above the main frontal zone its great meridional extent is associated with the upper tropospheric outflow and horizontal spreading of air which has recently undergone rapid slantwise ascent the low level temperature field exhibits throughout the development period a remarkable shape preserving thermal ridge of considerable intensity the sensitivity of the forecast to the removal of surface energy fluxes and latent heat of condensation are investigated separately it is found that on the timescale of the simulation surface energy fluxes have negligible impact on the explosive development on the other hand the contribution of latent heat release to the development appears to dominate the dry baroclinic instability process

a characteristic life cycle of upper tropospheric cyclogenetic precursors during the experiment on rapidly intensifying cyclones over the atlantic erica abstract this paper documents a characteristic life cycle of upper tropospheric precursors to surface cyclogenesis observed during the field phase of the experiment on rapidly intensifying cyclones over the atlantic erica december 1988 february 1989 this life cycle begins with the development of an elongated region of lower dynamic tropopause that forms in association with an intensifying midtropospheric jet front over central north america the elongated disturbance subsequently compacts into a more circular configuration prior to crossing the east coast of north america and frequently is associated with rapid surface cyclogenesis offshore a representative example of the life cycle outlined above is documented through a detailed case study of the upper precursor associated with the second erica intensive observation period iop 2 cyclone emphasis is placed upon i description of the tropopause structure and evolution during the upper precursor life cycle ii diagnosis of mechanisms leading to t

multi scale description of a sahelian synoptic weather system representative of the west african monsoon a reference case of a sahelian weather system observed during the hydrological atmospheric pilot experiment hapex sahel in august 1992 is described from a seasonal viewpoint as well as from synoptic and convective system viewpoints it is shown that the case study is representative of the climatology at all these scales and presents many interacting scales and physical processes at intraseasonal scale the monsoon onset is characterized by an abrupt shift of precipitation together with a latitudinal migration of the african easterly jet aej and convection at the month and day scales the convective activity occurs in an apparent zonal break of the tropical easterly jet the month of august 1992 exhibits intense synoptic activity the vorticity field is characterized by northerly dry and southerly wet components located at 850 hpa on each side of the aej their intraseasonal modulation on a period of 20 to 40 days leads to active and break phases of the synoptic activity around 21 august the 700 hpa vorticity field features the propagation of a typical easterly wave with a westward propagation of a cyclonic circulation followed by an anticyclonic circulation convective activity occurs mainly ahead of the 700 hpa vorticity maximum with the formation of a squall line on air mountains propagating south westward at 15 m s 1 the convective system propagates about twice as fast as the vortex core in contrast with the convection in the european centre for medium range weather forecasts re analysis which stays in phase with the vorticity the squall line corresponds to the largest contributor to the systems passing in august 1992 over the hapex sahel region its environmental conditions and its effects on the atmosphere including the surface parameters are presented copyright 2002 royal meteorological society

characterization of excess pore pressures at the toe of the nankai accretionary complex ocean drilling program sites 1173 1174 and 808 results of one dimensional modeling 1 elevated fluid pore pressures play a critical role in the development of accretionary complexes including the development of the decollement zone in this study we used measured permeabilities of core samples from ocean drilling program odp leg 190 to develop a permeability porosity relationship for hemipelagic sediments at the toe of the nankai accretionary complex this permeability porosity relationship was used in a one dimensional loading and fluid flow model to simulate excess pore pressures and porosities simulated excess pore pressure ratios as a fraction of lithostatic pressure hydrostatic pressure using the best fit permeability porosity relationship were lower than predicted from previous studies we then tested sensitivity of excess pore pressure ratios in the underthrust sediments to bulk permeability lateral stress in the prism and a hypothetical low permeability barrier at the decollement our results demonstrated significant increase in pore pressures below the decollement with lower bulk permeability such as obtained by using the lower boundary of permeability porosity data or when a low permeability barrier is added at the decollement in contrast pore pressures in the underthrust sediments demonstrated less sensitivity to added lateral stresses in the prism although the profile of the excess pore pressure ratio is affected both simulations with lateral stress and a low permeability barrier at the decollement resulted in sharp increases in porosity at the decollement similar to that observed in measured porosities furthermore in both scenarios maximum excess pore pressure ratios were found at the decollement suggesting that either of these factors would contribute to stable sliding along the decollement

metabasalts axial hot springs and the structure of hydrothermal systems at mid ocean ridges knowledge of the chemical transfer and mineralogical transformations that occur when sea water reacts with basalt at elevated temperatures and pressures can be used along with geological and geophysical data to deduce the typical structure and evolutionary sequence for hydrothermal systems within the oceanic crust along the axis of a mid ocean ridge studies of metabasalt and metadiabase dredged from fault scarps along the axial valley of the mid atlantic ridge reveal consistent relationships among the bulk chemistry of the altered rocks and their secondary mineralogy mineral abundances and mineral compositions especially for chlorite these relationships can be interpreted in terms of the distribution of alteration with respect to time temperature and water rock ratio in young crust assemblages of chl ab ep act chl ab ep act qtz chl ab qtz and chl qtz are produced at successively higher effective sea water to rock ratios within the temperature range of the greenschist facies 250 to 450 c toward higher ratios chlorites tend to become more mg rich pillow basalts of layer 2 are commonly altered under these conditions by sea water on the descending rather than the ascending limb of a convection system this type of alteration which reflects moderately high water rock ratios may characteristically occur above a still partially molten magma chamber as the magma chamber solidifies and is then penetrated more pervasive alteration of the deeper part of layer 2 and of layer 3 occurs at lower water rock ratios producing solutions such as those recovered from springs on the galapagos rift and the east pacific rise at 21 n localized upwelling limbs of the convection system can produce veins filled with quartz sulfides and fe rich chlorite

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

flow rates in the axial hot springs of the east pacific rise 21 n implications for the heat budget and the formation of massive sulfide deposits abstract the first in situ measurements of flow rates in submarine hot springs temperatures between 275 and 350 c were made in the hot springs of the east pacific rise at 21 n during november 1981 the flow rates ranged between 0 7 and 2 4 m s we estimate that the total rates at which heat and mass are transported from the three hot spring areas obs national geographic and southwest vents are ca 2 2 108 w and 150 kg of fluid s respectively the very high rate of heat loss precludes the existence of hot springs as steady state features of the east pacific rise at 21 n we estimate that hot springs are active for a maximum of ca 40 000 years chemical and or mechanical clogging of the vents can drastically reduce the lifetime of the hot springs using a simple model of plume development 1 we calculated that less than 3 of the sulfide particles that are entrained in the hydrothermal plumes at 21 n settle from the plumes before dispersal by a lateral submarine current at a height of ca 250 m above the seafloor 2 the time that is required to form the sulfide mounds that surround the hot springs by the accumulation of sulfide particles that settle from the plumes is ca 70 85 years the current rate at which sulfide particles settle from the hydrothermal plumes and accumulate near the hot springs appears to be too small to lead to the formation of large massive sulfide deposits such as the mavrouni deposit on cyprus 3 within the time limits that are suggested by our thermal energy balance calculations

the effect of large scale flow on low level frontal structure and evolution in midlatitude cyclones abstract observational and modeling studies documented in the literature indicate that the large scale flow has an important effect on the structure and evolution of low level fronts in midlatitude cyclones the purpose of this paper is to address the role of the large scale flow on low level cyclone frontal structure and evolution through a combined observational and idealized modeling approach analyses of two observed cyclone cases embedded in large scale diffluence and confluence respectively are presented to illustrate two possible cyclone frontal structures and evolutions specifically the cyclone moving into a diffluent high amplitude ridge becomes meridionally elongated and possesses a strong meridionally oriented cold front and a weak warm front the cold front rotates into the warm front forming an occluded front in the manner of the norwegian cyclone model as indicated by the narrowing of the thermal ridge connecting the warm sector to the cyclone center in contrast the cyclone moving i

mesoscale gravity waves and their environment in the central united states during storm fest abstract in an effort to better understand mesoscale gravity waves in winter storms in the central united states their frequency of occurrence wave characteristics the general conditions under which they occur and their effects upon the weather mesoscale surface and rawinsonde data as well as radar and satellite imagery collected during the storm scale operational and research meteorology fronts and experimental system test are analyzed in addition factors affecting the ability of objective surface map analysis to properly represent the waves are investigated thirteen coherent pressure pulse events with amplitudes of 0 2 4 0 mb and periods of 1 6 h were identified in the surface pressure data during the 6 weeks of the project involving 34 of the total hours investigated a variety of wave types occurred including wavelets wave trains and singular waves the three largest amplitude events were analyzed in detail using autospectral analysis and a barnes time to space conversion objective analysis

a diagnostic study of jet streaks kinematic signatures and relationship to coherent tropopause disturbances abstract a diagnostic study is conducted of the kinematics and evolution of upper level jet streaks representative of three of the four phases in the shapiro conceptual model of a jet streak progressing through a synoptic scale baroclinic wave over north america the three phases selected for consideration apply to those segments of the wave pattern where jet streaks are relatively straight the 1200 utc 2 december 1991 case trough over ridge considers a strong jet streak located over eastern north america and constitutes the bulk of the study the other two cases which also concern jet streaks over north america are from 0000 utc 11 november 1995 northwesterly flow and 0000 utc 28 october 1995 southwesterly flow kinematic signatures consistent with the classic four quadrant conceptual model for a straight jet streak are evident in all three cases although flow curvature and thermal advection lead to significant departures from this conceptual model the position of the jet streak within the s

intraseasonal variability of wintertime frontal activity and its relationship with precipitation anomalies in the vicinity of south america the intraseasonal variability of the frontal activity and its connection with the variability of the atmospheric circulation and precipitation in the southern hemisphere is studied the frontal activity is defined as the relative vorticity times the local temperature gradient a band pass filter was applied to retain the intraseasonal timescales an empirical orthogonal function analysis was applied to the filtered frontal activity anomalies the two main modes show positive and negative centers located mainly over the southern pacific ocean and south american sector and are in quadrature with each other a similar pattern was found when the main modes of intraseasonal variability of the 500 hpa geopotential height were projected on the frontal activity suggesting that the variability of fronts are influenced by the variability of the large scale atmospheric circulation moreover the precipitation anomalies projected on the main modes of both frontal activity and 500 hpa geopotential height show similar structures especially over the southern pacific ocean and south america which may indicate that the variability of fronts controls the variability of precipitation the lagged regression of the time series of the frontal activity areally averaged over one of the centers of action against the frontal activity anomaly field shows at lags 8 and 8 a similar pattern suggesting a period of around 17 days for each mode moreover lagged regression between times series of frontal activity and precipitation anomalies reveals an opposite pattern between southeastern south america and southern chile being precipitation anomalies over these two regions anti correlated due to the frontal activity

large scale antecedent conditions associated with the 12 14 march 1993 cyclone superstorm 93 over eastern north america abstract the results of a multiscale analysis of the 12 14 march 1993 superstonn ss93 over eastern north america are presented a time sequence of overlapping 10 day time mean 5oo hpa geopotential height and anomaly composites shows that the northern hemisphere nh flow pattern from 18 february to 15 march 1993 is characterized by 1 three persistent troughs situated over eastern asia and the northwestern pacific over eastern north america and over northwestern africa and southwestern europe eastward to central russia and 2 a massive blocking anticyclone located over the central and eastern atlantic beginning 8 9 march 1993 the planetary scale flow amplifies substantially the explosive ss93 cyclogenesis and the transport of cold air to very low latitudes occurs a few days later as the nh available potential energy content after peaking on 9 march 1993 decreases by about 6 7 a dynamical tropopause analysis is used to track coherent transient potential vorticity pv anomalies and show their q

chemical and isotopic 87sr 86sr δ18o δd constraints to the formation processes of red sea brines abstract about twenty deeps filled with hot brines and or metalliferous sediments are located along the red sea axis these brines present a well suited framework to study the hydrothermal activity in such a young ocean the present study outlines the results of a geochemical approach combining major trace element and isotopic oxygen hydrogen strontium analyses of brines in six of the deeps to evaluate different processes of brine formation and to compare the evolution of each deep important heterogeneities in temperature salinity hydrographic structure and chemistry are recorded each brine having its own characteristics the intensity of hydrothermal circulation varies among the deeps and ranges from being strong atlantis ii and nereus to weak port soudan and even to negligible valdivia and suakin and it varies along the entire red sea axis these observations do not favour a unique formational model for all of the brines for example the brines of the suakin deep appear to have been formed by an old sea water which dissolved evaporite beds without significant fluid circulation and hydrothermal input while others such as atlantis ii or nereus deeps appear to be dominated by hydrothermal influences a striking feature is the absence of a relationship between the position of the deeps along the axis and their evolutionary maturity

interpretation of satellite imagery of a rapidly deepening cyclone model output and observational data are used to interpret in detail satellite images of a distinctive cloud system several hundred kilometres long which was observed before and during rapid cyclonic development that occurred over england conceptual models are forwarded which account for the structure of the major cloud areas and upper level moisture fields as observed by water vapour imagery immediately prior to and following the onset of vigorous cyclogenesis the cloud system corresponded to what r b weldon refers to as a baroclinic leaf cloud and was characterized by ana cold frontal ascent the baroclinic leaf lay within an area of enhanced ascent ahead of a major upper level trough a jet streak which originated upstream propagated rapidly around the base of the upper trough a tongue of dry air centred on the left flank of this jet streak lay immediately upstream of the leaf cloud and had a major influence on its subsequent development this intrusion of dry air was well represented by the meteorological office fine mesh model which showed it as a pronounced tongue of recently descended air with high potential vorticity which originated in the upper troposphere and lower stratosphere rapid cyclogenesis occurred as the dry intrusion overran low level air of high w within a warm conveyor belt at the southern tip of the leaf cloud interaction of the dry intrusion and baroclinic leaf cloud is shown to be particularly important in accounting for the detailed distribution of weather it also suggests a way of pinpointing the location and timing of vigorous cyclogenesis

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

dynamical characterization of the fastex cyclogenesis cases summary a series of diagnostics based on the automatic tracking of cyclones is applied to the 25 life cycles captured during the fastex field experiment the tracking enables a setting of the various cases into a common frame of reference that moves with the system time filtering is used to further separate the events into a basic state and perturbations information is obtained on environmental properties such as strain or baroclinicity combining these diagnostics it is shown that instability mechanisms based on the existence of potential vorticity strips or the control of these instabilities by deformation may be involved in the initial genesis of a number of cases but do not seem to characterize specific types nor to determine a systematic behaviour on the other hand it is shown that rapid deepening phases leading eastern atlantic systems to their maximum amplitude happens suddenly and at the same time as crossing the jet stream even in the absence of a clear additional upper level feature for cold air systems development means meeting a diffluent zone this is unlike the american east coast systems that are associated with an exit entrance complex another remarkable feature is the generation of a frontal waveas a re sult of the parent vorticity maximum being apparently stretched and split this has occurred in 25 of cases a number of other properties of the fastex cases are shown such as several occurrences of large

a composite study of the interactions between tropical cyclones and upper tropospheric troughs abstract the objective of this study is to understand how interactions with upper tropospheric troughs affect the intensity of tropical cyclones the study includes all named atlantic tropical cyclones between 1985 and 1996 to minimize other factors affecting intensity change times when storms are over subcritical sea surface temperatures 26 c or near landfall are removed from the sample a trough interaction is defined to occur when the eddy momentum flux convergence calculated over a 300 600 km radial range is greater than 10 m s 1 day 1 the trough interaction cases are separated into four composites i favorable superposition tropical cyclone intensifies with an upper tropospheric potential vorticity pv maximum within 400 km of the tropical cyclone center ii unfavorable superposition iii favorable distant interaction upper pv maximum between 400 and 1000 km from the tropical cyclone center and iv unfavorable distant interaction for comparison two additional composites are cre

a simple model for the synoptic analysis of cold fronts a split cold front model with an over running upper front or humidity front ahead of a surface front or humidity front is shown to provide a useful representation of the principal characteristics of many katafronts the use of such a model leads to more consistent analyses especially of cloud and rain than are achieved using unmodified classical concepts

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

the structure and evolution of a simulated midlatitude cyclone over land abstract using output from a mesoscale model simulation this paper describes the evolution of the three dimensional temperature and humidity structures of an intense cyclone that developed over the eastern half of the united states during 14 16 december 1987 some specific findings include the following the occlusion process at the surface appeared similar to the classical paradigm with the cold front catching up with the warm front aloft a warm type occlusion structure formed as an upper baroclinic zone merged with the surface based occluded front the frontogenesis associated with the cold and warm fronts was initially continuous but as the system evolved a break in the frontogenesis along the northern section of the cold front developed a single frontogenesis feature appeared to support both the warm and occluded fronts a quasi linear region of convective activity was observed in advance of the surface cold front this convective line was associated with an upper level humidity front wh

fastex iop 18 a very deep tropopause fold i synoptic description and modelling the life cycle of a very deep tropopause fold 820 hpa is documented with aircraft and ship observations during the intensive observing period 18 of the fronts and atlantic storm track experiment fastex the initial setting involves a coherent tropopause disturbance and an associated arctic tropopause fold the confluence episode that results from the phasing up of the tropopause disturbance and a southern ridge ends in the formation of an intense jet streak the dynamics of which are associated with the development of a polar tropopause fold a diagnostic analysis suggests that the final dramatic stratospheric intrusion is the consequence of the vertical superposition of the arctic and polar tropopause folds the mesoscale non hydrostatic meso nh model is used to discuss this hypothesis mixing of the passive stratospheric tracer within the marine boundary layer is investigated with sensitivity tests which unplug in turn the model physical parametrizations finally upper level forcings associated with the development of the tropopause fold are investigated in detail in a companion paper

sensitivity of cyclogenesis to sea surface temperature in the northwestern atlantic during the intensive observation period 15 13 15 february 1997 of the fastex experiment a major cyclone crossed the atlantic ocean from the newfoundland basin to southern iceland its surface low center deepened by 17 hpa in 7 h when the perturbation crossed the north atlantic current nac from cold 38c to warm water 158c to elucidate the role of sea surface temperature sst and air sea fluxes in the dynamics of oceanic cyclones three nonhydrostatic mesoscale simulations were performed the first one is a control experiment with a realistic sst field describing in detail the oceanic front associated with the nac system the two following simulations are sensitivity experiments where the sst front is removed the first one uses a uniformly cold sst equal to 38c and the second one uses a uniformly warm sst equal to 158c the frontogenetic function and the vertical velocity sources in the lower atmospheric layers of the three simulations were diagnosed in the control simulation the surface heat fluxes were found to be negative in the perturbation warm sector and positive in the region behind the cold front as reported by numerous authors this pattern of surface heating and cooling did not intensify the cyclone except in the occlusion when the phasing with the sst front occurs this configuration enhances the horizontal gradient of surface buoyancy flux across the occlusion which increases the buoyancy flux source of vertical velocity w when the sst front is removed the surface heat fluxes are strongly affected in magnitude and in spatial variability the marine atmospheric boundary layer mabl stability the convective activity the warm advection in the core of the wave and the heating depth are strongly affected by the different surface flux fields there are several consequences i the uniform ssts tend to decrease the cold front intensity of the wave ii a weaker buoyancy flux source of vertical velocity is found above a uniform cold sst across the occlusion in comparison with the control case and iii surprisingly a weaker w buoyancy flux source is also found above a uniform warm sst because of a higher heating depth vertical velocity depends not only on the buoyancy flux forcing but also on the thermal wind the turbulent momentum and the thermal wind imbalance forcings the thermal wind forcing and the thermal wind imbalance forcing were the most sensitive to the sst compared to the turbulent momentum forcing this result means that i the feed back of the ageostrophic circulation induced by the surface is greater on the kinematic forcings than on the turbulent forcings and ii the turbulent momentum forcing does not play a crucial role in cyclogenesis above a uniform warm sst the strongest intensity of the occlusion is due to the strongest w thermal wind forcing and w thermal wind imbalance forcing in the mabl in spite of a weaker w buoyancy flux forcing than in the control case this result is explained by the convective activity that increases the low level convergence and vorticity spinup this point means that latent heat release and baroclinicity are in tight interaction in the first 12 h and at the scale of the simulation domain the three cyclones evolve similarly but at a small scale their internal structures diverge strongly and rapidly the scale at which the surface turbulent fluxes act on the dynamics of marine cyclones is therefore important finally the cyclone simulated in the warm sst case developed more rapidly than those simulated in the real and the cold sst cases this behavior is attributed to the strong positive surface heat fluxes because they preconditioned the mabl by moistening and heating the low levels during the incipient stage of the cyclone development

effects of cloud diabatic heating on the early development of the fastex iop17 cyclone a set of numerical simulations is used to study the first development phase of the fastex cyclone a particularly well sampled frontal cyclone system that developed on the trailing front of a primary large scale baroclinic wave during intensive observation period 17 iop17 of fastex the fronts and atlantic storm track experiment the role of diabatically induced vertical circulations is investigated with a combination of diagnostic tools including a generalized omega equation and keyser s psi vector technique applied to the model outputs the full physics simulation shows a good agreement with observed data particularly the splitting of the upper level flow into a pair of jet streaks analysis of the contributions to the vertical ageostrophic transverse circulations near the jet streak pair structure shows that a positive feedback exists between the storm s dynamical configuration and the diabatic processes in the second part of the paper this interpretation is confirmed by a sensitivity analysis excluding cloud processes results suggest that after initial baroclinic growth the storm evolves towards a purely diabatic deepening regime leading to a diabatic rossby wave mechanism as proposed by recent wave cisk conditional instability of the second kind theoretical studies

origin of strong winds in an explosive mediterranean extratropical cyclone abstractduring 2 3 december 2012 the black sea and east coast of romania were affected by a rapidly deepening mediterranean cyclone the cyclone developed a bent back front along which short lived

four dimensional variational analyses of fastex situations using special observations this paper focuses on intensive observation period 17 of the fronts and atlantic storm track experiment fastex the fastex special soundings obtained during that period are added to the conventional dataset and assimilated with a 4 dimensional variational 4d var scheme under development at meteo france results show a consistent use of the high density fastex ship soundings by the 4d var formulation and an improvement brought by this new scheme over 3d var when using only the vertical profiles synchronous with synoptic time the application of the 4d var analysis to dropsondes launched by a system relative flight designed by the joint centre for mesoscale meteorology also produces a fine scale description of sub structures of the mature system

structure of a frontal system over the northeast pacific ocean abstract a frontal system passed over the storm transfer and response experiment project area on 17 november 1980 as revealed by ship born rawinsonde surface and radar information this northeast pacific storm exhibited three components it linked surface and 500 mb troughs with marked surface windshift and associated confluence and horizontal wind shear but exhibiting no surface temperature change a midtropospheric cold katafront and an upper moisture front that was moving ahead more rapidly than the other components the upper moisture front was marked by a moisture gradient across the trailing edge of a large cloud region that moved with the winds at 500 mb or higher due to the horizontal thermal homogeneity of the marine boundary layer the confluence associated with the surface trough did not cause low level cold frontogenesis high w values of the upper portion of the frontal system indicate that air was advected from much farther south important characteristics of frontal system evolution

frontal wave development by interaction between a front and a cyclone application to fastex iop 17 the way new cyclones appear along a baroclinic zone is central to the problem of the prediction of mid latitude cyclones a new barotropic mechanism for frontal wave development is proposed in the present paper the ingredients are a frontal environment characterized by horizontal wind shear and a finite amplitude initial wind perturbation orthogonal to the main deformation axis associated with the front it is found that the initial wind perturbation can be provided by a low level vortex maximum located north westward with respect to the position of the new cyclone interactions between these synoptic features are documented using local energetics the theory is only valid at the very early stage of development after a few hours the main growing phase which is mostly baroclinic will take place a wide set of experiments is performed in order to find the preferred location of the vortex at the initial time in the frame of the barotropic vorticity equation the result is found to be consistent with the fronts and atlantic storm track experiment case studied in this paper frontogenetic forcing provided by a deformation field does not inhibit the perturbation amplification it is also found that the impact of the vortex is significantly larger than potential instabilities developing along the front where the unstable feature is defined as the most unstable singular vector copyright 2004 royal meteorological society

role of the low frequency deformation field on the explosive growth of extratropical cyclones at the jet exit part i barotropic critical region abstract by using new theoretical results on perturbation growth in spatially and temporally complex quasigeostrophic flows this paper investigates the role of the large scale deformation field on extratropical cyclones and especially on their explosive growth in the jet exit region theoretical ideas are tested by decomposing the atmospheric flow into a high and a low frequency part and by analyzing four dimensional variational data assimilation 4dvar reanalysis data of the fronts and atlantic storm track experiment fastex during february 1997 as well as reanalysis data for the end of december 1999 regions where the low frequency deformation magnitude is greater than the absolute value of the low frequency vorticity are shown to correspond to regions where synoptic disturbances at the same level tend to be located these regions in the upper troposphere are intrinsically related to the horizontal inhomogeneities of the low frequency large scale upper tropospheric jet but cannot be detected by look

fastex iop 18 a very deep tropopause fold ii quasi geostrophic omega diagnoses a set of two companion papers is dedicated to the documentation of the life cycle of a very deep tropopause fold 820 hpa during the intensive observing period 18 of the fronts and atlantic storm track experiment fastex in this second part diagnoses of vertical motion using a q vector partitioning in the natural coordinate system that follows the geostrophic wind are analysed the partitioning allows the evaluation of vertical motions associated with forcing mechanisms such as confluence and diffluence thermal advection by the horizontal geostrophic shear shear advection and curvature of the flow the synoptic situation involves the formation of an intense upper level jet streak when an arctic trough and a southern ridge move into phase with each other results assessed in the course of the tropopause fold life cycle show that subsiding vertical motions associated with each of the forcing mechanisms confluence shear advection and curvature overlap on the cyclonic shear side of the entrance region of the jet streak it is shown that an additional effect of shear advection over the confluence a necessary ingredient for the development of deep tropopause folds in two dimensional contexts takes place in the present case study however the forcing mechanisms that contribute mostly to subsiding vertical motions over the warm side of the upper level frontal zone i e with a maximum frontogenetic effect are in order of importance the shear advection and the curvature

modeling consolidation and dewatering near the toe of the northern barbados accretionary complex at the toe of the northern barbados accretionary complex temperature and pore water chemistry data indicate that fluid flow is channeled along the decollement and other shallow thrust faults we examine mechanisms that may prevent consolidation and maintain high permeability over large sections of the decollement high resolution bulk density data from five boreholes show that the decollement is well consolidated at some sites while other sites remain underconsolidated underconsolidated decollement behavior is associated with kilometer scale negative polarity seismic reflections from the decollement plane that have been interpreted to be fluid conduits we use a coupled fluid flow consolidation model to simulate the loading response of a 10 km long by 680 m thick slice of sediment as it enters the accretionary complex the simulations capture 185 ka 5 km of subduction with a load function representing the estimated effective stress of the overriding accretionary prism 3 8 taper angle simulation results of bulk density in the decollement 3 2 km arcward of the deformation front are compared with observations the results show that persistent high pore pressures at the arcward edge of the simulation domain can explain underconsolidated behavior the scenario is consistent with previous modeling results showing that high pore pressures can propagate intermittently along the decollement from deeper in the complex simulated seaward fluxes in the decollement 1 14 cm yr 1 lie between previous estimates from modeling studies of steady state 1 m yr 1 flow maximum simulated instantaneous fluid sources 2 5 10 13 s 1 are comparable to previous estimates the simulations show minor swelling of incoming sediments fluid sources 3 10 15 s 1 up to 3 km before subduction that may help to explain small scale shearing and normal faulting proximal to the protodecollement

a 4d var re analysis of fastex a complete re analysis of the fronts and atlantic storm track experiment 5 january 27 february 1997 is performed it provides a reference description of the life cycle of an ensemble of cyclogeneses over the north atlantic many aspects make this re analysis noteworthy in particular the fastex period is characterized by an exceptional increase in the number of dropsoundings or radiosoundings concentrated on the storm track another striking point of the re analysis is the use of a four dimensional variational 4d var assimilation scheme on such a long period associated with the french arpege action des recherche petite echelle et grand echelle global stretched model that allows a locally high resolution in the area where dense networks of dropsondes were deployed a twin re analysis with the same characteristics but without any fastex data is also performed it shows that the impact of the special fastex data is quite noticeable in the resulting analyses and is lower but still perceptible in the associated short range forecasts 6 h 36 h the impact of fastex observations on a particular fastex case is also shown copyright 2003 royal meteorological society

hydrogeological modeling of porous flow in the oregon accretionary prism venting of pore fluid was recently reported near the front of the oregon accretionary prism and was directly measured at one of the venting sites during alvin dives in this study the authors model the flow of pore fluid in the oregon accretionaly prism they use seismic reflection sections to constrain subsurface structures drill hole results from deep sea drilling project leg 18 to constrain sediment properties and regional tectonic information to constrain the timing and the rate of plate convergence and sediment imbrication and thickening the results of two dimensional time element finite element modeling show that high pore pressures are generated beneath the decollement due to the load of imbricated and thickened sediments diffusive flow at a rate of several millimetres per year is predicted within the accretionary prism if fault zone permeability is discounted the presence of a decollement or permeable fault penetrating the accretionary prism as imaged on the seismic reflection section can drastically alter the patterns of fluid flow and produce focused venting of fluid as observed on the other hand fluid venting along faults may not significantly alter the high pore pressures below the decollement

conceptual models of upper level frontogenesis in south westerly and north westerly flow the conceptual model proposed by shapiro as it is applied to the evolution of an upper level frontal zone within a baroclinic wave is reviewed and its limitations are investigated through previous literature and two casestudies presented in this paper the early stages in the evolutions of these two cases are used to examine specific limitations of this conceptual model 1 upper level frontogenesis in south westerly flow that evolves from a state of equivalent barotropy to a state of cold advection along the front and 2 upper level frontogenesis in northwesterly flow with along front variation in the sign of the thermal advection such that warm advection occurs upstream of cold advection in the thermal trough vector frontogenesis diagnostics for the lagrangian rate of change of the magnitude and direction of the horizontal potential temperature gradient including tilting due to vertical motion are derived these diagnostics are applied to the two cases to examine the maintenance of the potential temperature gradient and the development of cold advection along each upper level front the upper level front in south westerly north westerly flow was maintained primarily by deformation tilting frontogenesis in agreement with previous research the increasing cold advection along the upper level front in both cases was related to an upstream vorticity maximum for the case in south westerly flow the pre existing vorticity maximum approached a downstream equivalent barotropic upper level front in a manner similar to an instant occlusion resulting in cold advection along the length of the upper level front for the case in north westerly flow an intensifying vorticity maximum concentrated the cold advection in the base of the thermal trough as warm advection developed upstream these two cases are compared to upper level fronts in previous literature and a climatology of upper level fronts associated with landfalling cyclones over the eastern north pacific ocean the results indicate that these two cases are typical of early evolutions of upper level fronts that can occur in south westerly and north westerly flow therefore a revised version of the shapiro conceptual model is presented that represents more accurately the early evolutions exhibited in the present and previous studies

punctiform initiation of seafloor spreading in the red sea during transition from a continental to an oceanic rift transition from a continental to an oceanic rift occurs in the red sea by initial emplacement of oceanic crust in regularly spaced hot points which serve as nuclei for axial propagation into segments of oceanic crust accretion and for initiation of seafloor spreading hot points develop above upwelling mantle diapirs caused by density viscosity inversion beneath continental and oceanic rifts

the three dimensional secondary circulation in developing gulf stream meanders from june 1988 until august 1990 an array of 12 inverted echo sounders ies 12 additional ies with pressure gauges to measure the abyssal pressure signal and 12 tall current meter moorings instrumented at depths 400 m 700 m 1000 m and 3500 m was deployed in the gulf stream near 688w the current meter array spanned an area of approximately 260 km by 200 km with nominal horizontal instrument spacing of 56 km in the cross stream direction and 65 km in the downstream direction six events were observed during the two year deployment during which a steep gulf stream meander trough and an abyssal cyclone developed concurrently two of the meander troughs eventually pinched off to form cold core rings as the abyssal flows spun up they became nearly barotropic with strong o 30 cm s21 swirl speeds that advected the main thermocline and helped steepen the meanders although the geostrophic barotropic flow played a large role in the development of the system ageostrophic flow was also required in this study the three dimensional mesoscale ageostrophic flow within the main thermocline is diagnosed during two steep meander trough events and one steep meander crest event it is shown that in addition to the alongstream horizontal ageostrophic flow associated with the curvature of the main jet cyclostrophic speeds greater than 25 cm s21 are not unusual on trough axes there is significant horizontal cross stream ageostrophic flow cross stream ageostrophic flow as high as 18 cm s 21 is diagnosed vertical momentum advection is also shown to be an important contributor to cross stream ageostrophic flow with values as high as 7 cm s 21 diagnosed the patterns of the ageostrophic flow are shown to be tied to the phasing between the upper level meander and the abyssal vortex both rotational and divergent patterns in the ageostrophic flow are evident for parcels on main thermocline isopycnals the ageostrophic flow provides an asymmetric cross frontal flow relative to the symmetric cross frontal flow associated with the barotropic vortices

crustal underplating and evolution in the red sea rift uplifted gabbro gneiss crustal complexes on zabargad and brothers islands the nature of the crust of the red sea outside the oceanic axial trough segments can be inferred from uplifted crustal sections exposed on the islands of zabargad and the brothers in the central and northern red sea zabargad exposes in addition to mantle derived peridotites a crustal section consisting of metagabbros and metapyroxenites associated with silicic gneisses and cut by doleritic dikes seismic velocities of samples from this section are compatible with crustal velocities obtained by seismic refraction experiments in the central and northern red sea the zabargad peridotitesilicic gneiss gabbroic association provides a sample of upper mantle lower crust in a continent before the inception of a major rifting event the petrology and mineral chemistry of the zabargad metagabbros suggest that they were originally part of a basic layered complex which crystallized at relatively high pressure 900 1000 mpa or 9 10 kbar i e over 30 km deep the gneisses have tonalitic trondhjemitic affinity and were originally equilibrated in the garnet granulite facies implying also high pressure and derivation from the lower continental crust this suggests that after the subcrustal continental lithosphere along the proto red sea rift had been consumed possibly by convective thinning but before any significant thinning of the crust basaltic melts intruded the base of the continental crust underplating it with gabbroic complexes thus crustal rifting was probably preceded by a thermal anomaly in the mantle which supports an active rifting hypothesis the gabbro pyroxenite complex and the gneisses underwent subsequently retrometamorphic events under decreasing pt conditions from granulite to amphibolite to greenschist facies implying decompression and uplift probably related to the gradual thinning of the continental crust during rifting during this stage additional gabbroic intrusions and basaltic dikes exemplified by the gabbros and dolerites from the brothers islands were emplaced at decreasing depths beneath the thinned continental crust the northern red sea crust is presently in this stage it is neither oceanic nor continental it lacks a continuous thick basaltic upper crustal layer and is made prevalently of gabbroic and doleritic intrusives with relicts of sialic continental crust subsequently basaltic injections became progressively focused in a narrow axial zone and seafloor spreading initiated at discrete equidistant hot points as observed presently in the central red sea which by axial propagation evolved into nearly continuous zones of oceanic crust accretion as observed presently in the southern red sea processes similar to those proposed here for the evolution of the red sea rift can be documented in passive margins of the atlantic and in some alpine ophiolite complexes

secondary cyclogenesis the initiation phase of a frontal wave observed over the eastern atlantic the initiation phase of a secondary frontal wave in the wake of a primary cyclogenesis event over the atlantic is documented north of the trailing surface cold front and south west of the primary cyclone an intense upper level jet generates a direct and transverse ageostrophic circulation in its entrance region evidence is shown for two conflicting roles of this circulation first stabilization of the front through frontogenetic cross frontal low level convergence and second an increase of the potential for instability of the front through diabatic e g latent heat release potential vorticity pv redistribution in the upward branch of the circulation following saturation of the primary cyclogenesis and the associated along front north eastwards shift of the upper level jet the low level branch of the ageostrophic circulation behaves like a zipper closing on the front while in the upward branch an anomalous strip of pv builds along the cold front isentropic pv maps show that parcles of stratospheric origin embedded in the downward branch of the ageostrophic circulation a tropopause fold may have ended in the vicinity of the strip of anomalous low level pv perhaps contributing to an increase in the potential for instability of the surface front using the domain independent attribution method of bishop it is shown that the frontal wave forms on a part of the cold front where first the along front stretching rate due to the environmental flow decays below the critical threshold value 0 6 10 5 s 1 prescribed by theoretical studies and second the across front frontogenetic component due to the ageostrophic circulation decays this is consistent with semi geostrophic theory where the cross frontal convergence and the stretching deformation are linked through the sawyer eliassen equation results add to other recent observational findings that the potential for instability the evolving environmental flow and the induced ageostrophic convergence each have a crucial role in differentiating between frontal waves that will grow and those that will be suppressed

nonlinear variations of the physical properties along the southern ecuador subduction channel results from depth migrated seismic data abstract we use two high quality pre stack depth migrated multichannel seismic profiles acquired to quantify physical properties variations of underthrust sediments along the first 30 km of subduction off the erosional southern ecuadorian margin seismic data show three zones along the subduction channel referred to as zones i ii and iii characterized by distinct velocity and velocity derived physical properties which are in agreement with values estimated from experimental results of deformation in granular media these three zones result from transformational changes of underthrust sediments governed by fundamentally different physical processes that control their mechanical behavior at increasing confining pressures based on our observations and its comparison with experimental results we argue that the transformations undergone by underthrust sediments as they dip into the subduction zone are the following within zone i progressively increasing velocity and decreasing velocity derived porosity indicates continuous sediment compaction which must be accompanied by effective fluid drainage along the decollement and or across the accretionary wedge the underthrust material is here unconsolidated from a mechanical point of view laboratory experiments indicate that the dominant processes at this range of pressures are grain rolling particle rotation and frictional slip at grain contacts within zone ii velocity and porosity remains constant for 16 km sis 72 and 12 km sis 18 this suggests undrained conditions resulting in growing fluid overpressure at the subduction channel grain deformation is similar to zone i within zone iii velocity increases and porosity falls rapidly indicating sediment compaction and subsequent release of over pressured fluids where grain deformation is likely to be elastic this might be the dominant process until the grains attain their crushing strength resulting in granular cataclasis and eventually in the collapse of the system we suggest that over pressured fluid release may induce hydrofracturation and it is likely to increase inter plate coupling down from zone iii

secondary frontal waves in the north atlantic region a dynamical perspective of current ideas the current understanding of the dynamics of secondary cyclogenesis on frontal systems in the north atlantic region is reviewed these secondary cyclones are often poorly forecast and in extreme cases can have damaging consequences but because of their small scales and potential for rapid growth detailed observations and analyses are limited in this paper recent work is described and compared it is argued that on the synoptic scale the development of primary baroclinic waves is well understood but on the subscales of these systems where the secondaries develop there is a wider variety of mechanisms which may cause or modify cyclone growth this variety may be reflected in a wider range of resulting systems various dynamical models have been postulated secondary frontal waves are seen as the instability of a low level potential vorticity strip or warm band at a front with possible finite amplitude triggering by upper level features latent heating is thought to provide a suitable unstable potential vorticity strip but the degree to which the latent heating contributes to the instability itself is a matter of dissent secondary systems are often shallow so boundary layer processes are important but the influence of these is not well understood laboratory experiments have suggested that the boundary layer will act to suppress instability in general frontogenetic strain flow while acting to intensify the basic state front suppresses wave development in contrast recent work suggests that a frontolytic strain flow may be important in enhancing the development of one class of waves the fastex experiment an international field study of north atlantic secondary cyclogensis has recently attempted to observe these systems in detail it is hoped that the data which have been obtained will help to resolve some of the outstanding issues in this field

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

local partitioning of the overturning circulation in the tropics and the connection to the hadley and walker circulations conceptually it is useful to partition the three dimensional tropical circulation into meridional and zonal components namely the hadley and walker circulations the averaging involved in their definitions can introduce ambiguities these problems can be circumvented by first partitioning the total vertical mass flux into components associated with overturning in the meridional and zonal directions respectively called here the local hadley and local walker circulations defining the local hadley and local walker circulations this way ensures the pair of two dimensional overturning circulations can be added to give the original three dimensional circulation even when the averages are taken over limited domains the method is applied to the vertical motion from the era interim reanalysis for the period 1979 to 2009 one important result is that the local hadley circulation responds much more strongly to enso than the local walker circulation even though the local walker circulation in the central pacific weakens during el nino years and strengthens and widens during la nina years

a technique for diagnosing three dimensional ageostrophic circulations in baroclinic disturbances on limited area domains abstract the methodology developed by keyser et al for representing and diagnosing three dimensional vertical circulations in baroclinic disturbances using a two dimensional vector streamfunction referred to as the psi vector is restricted to f plane channel model geometry the vertical circulation described by the psi vector consists of the irrotational or divergent part of the ageostrophic wind and the vertical velocity a key property of the psi vector is that its projections onto arbitrarily oriented orthogonal vertical planes yield independent vertical circulations allowing separation of a three dimensional vertical circulation into two two dimensional components and thus objective assessment of the extent to which a three dimensional vertical circulation is oriented in a preferred direction here the methodology for determining the psi vector is modified to be suitable for real data applications the modifications consists of reformulating the diagnostic equations to apply to conformal map pr

excess pore pressures within subducting sediments does the proportion of accreted versus subducted sediments matter 1 although previous numerical modeling studies have examined pore pressures in both accretionary and non accretionary subduction zones they have not addressed the question of whether the proportion of accreted versus subducted sediments significantly affects pore pressures this modeling study used consistent parameters to compare excess pore pressures directly below the decollement for varying proportions of accreted sediment within the initial 5 to 12 kilometers arcward of the deformation front subduction zones with lesser accretion thickness show lower simulated excess pore pressures than those with greater accretion thickness due to the ease of fluid escape when overlying wedge sediments are thin however at greater distances arcward of the deformation front the magnitude of excess pore pressures significantly increases and the excess pore pressures peak further arcward as the proportion of underthrust sediments increases this difference suggests that the proportion of accreted versus subducted sediment could potentially affect seismogenesis

oxygen isotope geothermometry and origin of smectites in the atlantis ii deep red sea abstract the sediments underlying the hot brine pool of the atlantis ii deep a localised area of geothermal activity in the red sea comprise a diversity of facies characterised by combinations of one or more of five species assemblages sulphide sulphate silicate oxide and carbonate each including several mineral phases the silicate mineral assemblage is dominated by geothermal authigenic smectites previous studies of these smectites have reported iron rich varieties only nontronite in particular and only one environment of formation in three cores from the southwest basin of the atlantis ii deep of the present study three smectites comprising two species have been distinguished 10 21 evidently from three different environments of formation two of these smectites are nontronites one from sulphide silicate amorphous facies the other from silicate carbonate oxide facies the third is a montmorillonite beidellite from sulphate sulphide silicate oxide facies the oxygen isotopic compositions of samples of the three smectites have been determined from which formation temperatures have been calculated six samples of the anoxic nontronite have formation temperatures in the range 90 140 c a single sample of the oxic nontronite has a formation temperature of about 80 c four samples of the montmorillonite beidellite have formation temperatures in the range 160 200 c the formation temperature range of the two nontronites is intermediate between the temperature of the brine at or prior to discharge up to 250 c 12 and the temperature of the brine pool in the deep about 50 60 c 13 14 the nontronite formation temperature range reflects genesis by combination of isotopically light silica supplied by the incoming brine and isotopically heavier iron oxyhydroxide settling from the upper layers of the brine pool evidently the anoxic nontronite forms at greater depth hotter in the brine pool than the oxic nontronite resulting in a relatively greater contribution from silica but diminished contribution from iron oxyhydroxide in the former compared to the latter the wide range of the formation temperatures for the anoxic nontronite is related to the different actual locations of the samples in the sulphide silicate amorphous facies the formation temperature range of the montmorillonite beidellite is approaching the estimated temperature of the brine at or prior to discharge the montmorillonite beidellite formation temperature range reflects genesis by combination of isotopically light silica and aluminium both supplied by the incoming brine at the site of an active discharge vent the wide range of the formation temperatures for the montmorillonite beidellite may in part reflect a possible thermal event at the brine source likely to have occurred during deposition of the sulphate sulphide silicate oxide facies and which it appears has contributed to the formation of this facies 10 20

flow in the lee of idealized mountains and greenland abstract a series of idealized simulations of flow impinging on large mountains is conducted to investigate the impact of the mountain on the flow far downstream and to shed some light on the effects that greenland may have on airflow over the north atlantic the upstream profiles of wind and stability are kept constant there is no surface friction the rossby number is 0 4 and the nondimensional mountain height nh u is varied from 1 to 6 the maximum sea level pressure deficit the maximum geopotential height deficit and the orographically generated potential vorticity all increase with increased showing no signs of abrupt change as the flow enters the regime of upstream blocking the potential vorticity produced at the mountain is accumulated in vortices that are advected downstream the vortices are associated with a larger pressure gradient to the south of the wake giving rise to stronger westerlies at the surface as well as at upper levels this process can explain how greenland may affe

multiscale structure and evolution of an oklahoma winter precipitation event abstract a significant winter precipitation event occurred on 8 9 march 1994 in oklahoma snow accumulations greater than 30 cm 12 in were measured within a narrow corridor in northern oklahoma on the synoptic scale and mesoscale a correspondence between large snow accumulations and 600 hpa frontogenesis was revealed the precipitation was formed above the cold frontal surface owing to midtropospheric ascent associated with the cross frontal circulation in a region of elevated conditional instability the location of such a narrow corridor of large accumulations was not however disclosed by any patterns in the radar reflectivity data indeed during this event an elongated maximum of snow accumulation was not associated with a persistent band of enhanced reflectivity and vice versa dual polarization and dual doppler radar data allowed for a novel analysis of winter precipitation processes and structures within the context of the larger scale diagnosis it was possible to identify in order of

strain accumulation across the asal ghoubbet rift djibouti east africa the asal ghoubbet rift in the republic of djibouti is a subaerial continuation of the gulf of aden ridge system many geological and tectonic arguments indicate that it represents one of the most typical spreading segments of the world rift system geodetic measurements regularly conducted from 1979 to 1983 across this area show an accumulation of extension in the inner floor of the rift valley with a rate of about 60 mm yr perpendicular to the normal faults that bound the rift periodic leveling surveys across major faults in the rift show vertical creep movements of about 5 to 10 mm yr although the observed deformations could be contamined by postseismic relaxation after a rifting episode in november 1978 the regularity and the magnitude of these movements during the last two years argue for a steady state mechanism of deformation of the rift valley the deformation is uniformly distributed in time and in space inside the inner floor of the rift the necking of the lithosphere in the central part of the rift and the high temperature gradient imply a ductile rheology in the inner floor which is stretched in a steady state process

the life cycle of an extratropical marine cyclone part ii mesoscale structure and diagnostics abstract this is the second of two articles describing the evolving structure and selected physical processes within an intense extratropical marine cyclone observed during the experiment on rapidly intensifying cyclones over the atlantic erica field program part i describes the 24 h frontal cyclone evolution through 6 h horizontal analyses of observations taken by specially deployed observing systems from air land and sea part ii presents frontal scale and precipitation structures and physical processes from analyses based primarily on research aircraft observations taken during three phases of the cyclone s life cycle horizontal analyses at 350 m above ground level describe the cyclone s mesoscale frontal baroclinic structure and associated flow patterns the vertical structure and evolution of the cyclone s cold front warm front and bent back front are illustrated in cross sectional analyses of potential temperature wind velocity potential vorticity pv front relative transverse flow vecto

multiple potential vorticity inversions in two fastex cyclones this paper investigates pre existing synoptic scale disturbances responsible for cyclogenesis using the manipulation of initial conditions through quasi geostrophic potential vorticity qgpv and its inversion the use of a qgpv inversion together with a t63 model allows us to study the sensitivity of some fastex events to several qgpv patterns present within the analysis it has been found that ertel potential vorticity modifications of the analysis are preserved by the dynamical initialization process the qgpv inversion method is applied to cyclones occurring in intensive observation periods iops 11 and 12 of fastex fronts and atlantic storm track experiment to demonstrate which potential vorticity anomalies present within the atmosphere prior to the development are involved in the cyclogenesis the choice of the boundary condition when an upper level precursor is removed from the initial conditions has little impact on the cyclone development the linear interactions advection deformation of the anomaly by its environment and downstream development between these upper level precursors and the surrounding flow dominate the dynamics of iop1 1 cyclone development whereas nonlinear processes are particularly strong in the iop12 case the cyclone development is only weakly sensitive to low level initial structures unless they are shaped like adjoint model sensitivities collocation with another shape has little influence on the development

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

a simplified analysis of parameters controlling dewatering in accretionary prisms abstract one of the many dynamic geologic processes taking place at subduction zones is the compaction driven dewatering of seafloor sediments as they are accreted to the overriding plate the rate of dewatering is equal to the divergence of the sediment matrix velocity field this rate can be estimated analytically if simplifying assumptions are made about the geometry of the prism and motion of the sediments the analytical expression depends only on the sediment accretion velocity thickness of the accreted section prism taper angle and sediment porosity distribution a sensitivity analysis of the solution shows that the fluid production distribution is relatively insensitive to the sediment porosity distribution however the solution is very sensitive to the taper angle of the wedge high angle wedges expel almost all of the incoming water within 20 km while low angle wedges may retain a significant fraction of the incoming water for 50 or more kilometers thickness of the incoming section is also important dewatering of thin accreted sections is more concentrated near the toe of the wedge analyses of transects through the northern barbados makran vancouver and nankai accretionary prisms illustrate a range of dewatering rates and spatial distributions of dewatering

frontogenesis in the presence of small stability to slantwise convection abstract it is often observed that despite the existence of near neutrality to slantwise convection rainbands and snowbands can persist for long periods with narrow intense updrafts producing large quantities of precipitation in many cases this is probably due to the presence of active frontogenesis which as shown here maintains such structures in order to investigate this process we use two dimensional semigeostrophic theory to solve the deformation forced frontogenesis problem for a circumstance in which the stability to moist slantwise convection is small but positive in this case numerical simulation is necessary to determine the evolution of the front several previous studies have described the role of diabatic forcing in modifying the cross frontal circulation by use of the sawyer eliassen equation in particular analytic solutions of that equation for the case of small moist symmetric stability show that a narrow updraft should occur ahead of the maximum geostrophic frontogenesis numeri

gulf of aden axial magnetic anomaly and the curie temperature isotherm the main features of magnetic anomalies over ocean ridges have been explained1 as a corollary of seafloor spreading and geomagnetic reversals oceanic crust is formed in a narrow region becoming magnetized in the direction of the earth s magnetic field as its temperature falls through the curie point of the magnetic minerals present the gulf of aden was one of the first places where reversely magnetized sea floor was recognized2 the seafloor spreading direction and latitude are such that the anomaly due to normal magnetization is negative and slightly skewed positive anomalies were also observed suggesting the presence of reverse magnetization a short wavelength magnetic anomaly which frequently occurs superimposed on the axial magnetic anomaly in the gulf of aden is now described various interpretations are considered the preferred involves a dramatic shallowing of the curie temperature isotherm close to the seafloor spreading centre the interpretation has implications for models of the generation of oceanic lithosphere and for locating possible geothermal areas in rifted regions

the structure and evolution of lower stratospheric frontal zones part 1 examples in northwesterly and southwesterly flow the structure evolution and dynamics of two lower stratospheric frontal zones are examined from a basic state variables perspective the case studies highlight the asynchronous evolution of the lower stratospheric and upper tropospheric frontal portions of upper level jet front uljf systems as well as some substantial differences in lower stratospheric frontal development that occur in southwesterly and northwesterly flow the evolution of the uljf in northwesterly flow was characterized by an initially intense but weakening lower stratospheric front along with an initially weak but intensifying upper tropospheric front throughout the evolution geostrophic cold air advection in cyclonic shear characterized a substantial portion of the lower stratospheric front this circumstance supported subsidence through the local jet core within the cold upper troposphere weakening the lower stratospheric front via tilting this subsidence extended downward below the jet core where it is suggested to have played a role in the early stages of upper tropospheric frontogenesis in the southwesterly flow case the evolution of the uljf was characterized by a strengthening lower stratospheric front and a weakening upper tropospheric front a deep column of upward vertical motion resulted from the superposition of lower tropospheric ascent associated with convection along a surface cold front and upper tropospheric lower stratospheric utls ascent through the jet core coincident with geostrophic warm air advection in cyclonic shear along large sections of the lower stratospheric front the utls ascent located on the cold edge of the lower stratospheric baroclinicity served to intensify the lower stratospheric frontal zone via tilting the implications of these lower stratospheric frontal processes on the topography of the tropopause and downstream sensible weather are discussed copyright 2012 royal meteorological society

fastex iop17 cyclone introductory synoptic study with field data this paper introduces the study of the early development phase of the iop17 frontal cyclone of the fastex field experiment fronts and atlantic storm track experiment with the support of observations from fastex observing platforms a synoptic description highlights the development of mesoscale features of interest in the operational meteo france arpege action de recherche petite echelle grand echelle analysis cycle i e cloud head dry wedge and upper level jet outflow a discussion attempts to isolate two significant differences of the present cyclone life cycle compared with previous well known conceptual models of maritime cyclone life cycles i the indirect role of an upper level precursor in triggering the surface low ii the existence of a diabatic regime taking over from the baroclinic development to sustain the deepening rate of the cyclone these two mechanisms are the subject of two companion papers

interactions between deformation and fluids in the frontal thrust region of the nantroseize transect offshore the kii peninsula japan results from iodp expedition 316 sites c0006 and c0007 integrated ocean drilling program iodp expedition 316 sites c0006 and c0007 examined the deformation front of the nankai accretionary prism offshore the kii peninsula japan in the drilling area the frontal thrust shows unusual behavior as compared to other regions of the nankai trough drilling results integrated with observations from seismic reflection profiles suggest that the frontal thrust has been active since 0 78 0 436 ma and accommodated 13 to 34 of the estimated plate convergence during that time the remainder has likely been distributed among out of sequence thrusts further landward and or accommodated through diffuse shortening unlike results of previous drilling on the nankai margin porosity data provide no indication of undercompaction beneath thrust faults furthermore pore water geochemistry data lack clear indicators of fluid flow from depth these differences may be related to coarser material with higher permeability or more complex patterns of faulting that could potentially provide more avenues for fluid escape in turn fluid pressures may affect deformation well drained sand rich material under the frontal thrust could have increased fault strength and helped to maintain a large taper angle near the toe recent resumption of normal frontal imbrication is inferred from seismic reflection data associated decollement propagation into weaker sediments at depth may help explain evidence for recent slope failures within the frontal thrust region this evidence consists of seafloor bathymetry normal faults documented in cores and low porosities in near surface sediments that suggest removal of overlying material overall results provide insight into the complex interactions between incoming materials deformation and fluids in the frontal thrust region

the occlusion process in a midlatitude cyclone over land abstract a historical review highlights the lack of consensus regarding the structure and evolution of occluded fronts and cyclones in order to better understand the occlusion process this paper and its companion examine a model simulation of the 14 16 december 1987 cyclone over the eastern united states the structure of an occluded front was observed at low levels as the cold front caught up with the warm front while aloft an upperlevel frontal zone appeared to play a role in the occlusion process although the thermal structure of this storm suggested that a cold type occlusion should have formed the model simulation produced a warm type occlusion structure an extensive review of the literature on occluded fronts suggested that cold type occlusions are rare if they exist at al1 finally the occluded front simulated in this case is compared critically to the norwegian t bone and other conceptual models of cyclone evolution

a potential vorticity diagnostic approach to upper level frontogenesis within a developing baroclinic wave the process of tropopause folding is studied in the context of the life cycle of baroclinic waves previous studies of upper level frontogenesis have emphasized the role of the vertical circulation in driving stratospheric air down into the midtroposphere here a potential vorticity based approach is adopted that focuses on the generation of a folded tropopause to facilitate comparison of the two approaches the diagnosis is applied to the upper level front previously simulated and studied by rotunno et al the potential vorticity approach clarifies the primary role played by the horizontal nondivergent wind in producing a fold and explains why folding should be a common aspect of baroclinic development between the trough and upstream ridge prolonged subsidence within a region of weak system relative flow generates a tropopause depression oriented at an angle to the large scale flow the large scale vertical shear then locally increases the slope of the tropopause eventually leading to a tropopause fold in contrast tropopause folding in the base of the trough is caused by the nondivergent cyclonic circulation associated with the surface thermal wave the winds associated with the thermal wave amplify the potential vorticity wave aloft and these winds which decrease with height rapidly generate a tropopause fold within the trough

the 1993 superstorm cold surge frontal structure gap flow and tropical impact in the wake of the eastern united states cyclone of 12 14 march 1993 a cold surge originating over alaska and western canada brought northerlies exceeding 20 m s21 and temperature decreases up to 158c over 24 h into mexico and central america this paper addresses the multiscale aspects of the surge from the planetary scale to the mesoscale focusing on 1 the structure and evolution of the leading edge of the cold surge 2 the reasons for its extraordinary intensity and equatorward extent and 3 the impact of the surge on the tropics specifically on the strength of the trade winds and on the sea surface temperature in the eastern pacific the cold surge was initiated as a developing cyclone over the gulf of mexico and an upstream anticyclone east of the rockies caused an along barrier pressure gradient to form forcing topographically channeled northerlies along the rocky and sierra madre mountains to transport cold air equatorward on the mesoscale the leading edge of the cold surge possessed nonclassical frontal structure for example as the cold surge entered mexico the coldest air and the strongest wind arrived at about 900 hpa before affecting the surface suggestive of a tipped forward leading edge to the surge also satellite imagery and surface observations indicate that the leading edge appeared to be successively regenerated in the warm presurge air the cold surge had characteristics reminiscent of a kelvin wave a tipped forward cold front a pressure jump line a bore and a gravity current but none of these conceptual dynamical models was fully applicable associated with the cold surge gap winds up to 25 m s21 were observed in the gulfs of tehuantepec a tehuantepecer fonseca papagayo and panama owing to the strong cross mountain pressure gradient in the case of the tehuantepecer a rope cloud emanated from the isthmus of tehuantepec and turned anticyclonically consistent with an inertial oscillation on the synoptic and planetary scales the extraordinary equatorward extent of the cold surge was aided by topographic channeling similar to cold air damming by a low latitude upper tropospheric trough and by the lower branch of the secondary circulation associated with a confluent jet entrance region aloft the cold surge also impacted the tropical atmosphere and ocean by contributing to the strengthening of the northeast trade winds over the eastern pacific ocean and by inducing local cooling of the sea surface temperature in the gulfs of tehuantepec and papagayo by about 48 88c

effects of a low level precursor and frontal stability on cyclogenesis during fastex iop17 the fronts and atlantic storm track experiment fastex has provided comprehensive data to document the life cycle of secondary frontal cyclones over the north atlantic and to improve the understanding of cyclogenesis mechanisms this study analyses the processes leading to the triggering of a particularly well sampled frontal cyclone that developed on the trailing cold front of a mature primary cyclone during fastex intensive observation period 17 iop17 between 16 and 20 february 1997 the case features both a classical low level vorticity strip along the primary front where the frontal cyclone develops and more unexpectedly a pre existing continental surface low further west the importance of which is revealed in the pilot study of arbogast and joly in order to study possible cyclogenetic contributions along the primary front the domain independent vorticity divergence attribution technique of bishop is used to partition the flow into contributions from the continental low and from the large scale environment results indicate that the frontal cyclone is triggered where and when along front stretching decreases below the theoretical critical threshold of bishop and thorpe it is shown that the cyclone is developing under normalized strain less than 1 in agreement with other case studies finally the role of the continental low is investigated using a series of numerical forecasts made from different initial conditions manipulated with a potential vorticity inversion tool the wind attributable to the continental low accounts for roughly 30 of the along front stretching decrease the continental low also has a frontolytic effect on the western part of the primary front where its thermal advection pattern favours cyclogenesis in general terms the frontal cyclone triggering seems to support the bishop and thorpe theoretical framework of frontal instability in low strain areas however a more complex picture arises when considering the role of the continental surface low which also acts as a low level precursor

a review of the structure and dynamics of upper level frontal zones abstract this article presents a review of upper level fronts with the intent of synthesizing observational and modeling studies into a conceptual and dynamical description of these fronts and their evolution relative to the life cycle of midlatitude baroclinic waves the discussion begins by tracing present day concepts concerning the structure of upper level frontal systems which are based on composite analyses of radiosonde and aircraft data from their origins in the pioneering analyses of upper air data in the 1930s perspectives from scales both smaller and larger than upper level frontal systems are provided respectively by considering the effects of turbulent processes on frontal structure and dynamics and by relating variations in frontal structure to the evolution of the baroclinic waves that provide the dynamical environment for upper level frontogenesis the dynamics of upper level fronts are shown to comprise the interactions between the primary geostrophic and secondary ageostrophic cir