origins of the reversed flow over the windward alpine foothills during map iop3 and iop8 the synoptic scale situations during the two intensive observing periods iops of the mesoscale alpine programme map characterized by persisting precipitation and down valley flow as observed by the doppler on the wheel radar dow over the two alpine valleys toce and ticino were investigated the first period iop3 24 27 september 1999 was characterized by a south westerly flow and convective precipitation whereas the second period iop8 20 21 october 1999 by a down valley flow and stratiform precipitation during iop3 the down valley northerly flow over the north western part of the po valley was confined within a thin layer of 200 300 m depth during iop8 the northerly flow extended from the alps foothills to the ligurian sea in a deep layer of 1 2 km height the meso nh numerical model with a 2 5 km horizontal mesh grid reproduced the characteristics of both cases from the valley scale to the meso scale sensitivity experiments were furthermore performed in order to highlight the relationships between wet or dry drainage flow channelled by the alpine orography mesoscale upstream flow blocked by the alps and a regional cold pool at the foothills inside the po valley during iop8 the diabatic processes were dominant at local scales in confirmation of steiner results based on the dow observations and they were significant at regional scales upstream of the alps by maintaining a cold pool over the po valley the iop3 low level flow reversal occurred as the flow began to cross over the alps instead of turning around a downslope flow is associated with the blocked regime period upstream of the alps copyright 2006 royal meteorological society

autumnal precipitation distribution on the southern flank of the alps a numerical model study of the mechanisms during the autumn there are frequent events of heavy precipitation on the southern slopes of the meso scale alpine chain moreover the accompanying climatology exhibits a distinctive spatial distribution with some meso scale regions of enhanced precipitation such as the wet spot in the vicinity of lago maggiore possible mechanisms to account for the observed spatial distribution are established using a heuristic model based approach it is shown using a mesoscale model that the response to a steady horizontally uniform and moisture laden flow impinging upon a comparatively realistic representation of the alpine chain can reproduce both the overall meso scale precipitation distribution and its meso scale sub structure the nature of the simulated flow regime over around or intermediate is sensitive to the strength of the ambient geostrophic flow and its modification in the planetary boundary layer contrariwise the simulated precipitation distribution in the lago maggiore wet spot is comparatively insensitive to the changes in direction of the incident geostrophic flow from south to south west and in strength in the range 10 30 m s 1 although there is a marked change in the structure of the near alpine flow this systematic precipitation enhancement appears to be related to the relative strength and compensating effects of an easterly barrier jet tracking along the zonally elongated part of the alps and an air stream channelled between the maritime alps and the apennines further evidence to the above interpretation of the meso scale precipitation distribution is given with simulations conducted using a smoothed and schematic representation of the alpine chain likewise the sensitivity to the presence of a lago maggiore like indentation of the topography is tested and evidence is provided of a further indentation induced precipitation enhancement at the local scale copyright 2004 royal meteorological society

south american antarctic plate motion over the past 50 myr and the evolution of the south american antarctic ridge summary magnetic and bathymetric data from the south american antarctic plate boundary east of the south sandwich trench have been interpreted to produce ocean floor ages spreading rates and directions sam ant motion over the past 50 myr has been slow 10 15 mm yr 1 half rates about 20 ma the spreading direction changed from 120 300 to the present e w these results have been combined with similar data from the south atlantic and southwest indian oceans to calculate six poles and rates of sam ant motion covering the past 50 myr the change at 20 ma appears to have originated the long offset bullard and south sandwich fracture zones the change may have been facilitated or even triggered by ridge crest trench collision along the south scotia ridge east of the south orkney islands this event however could not alone have caused a stable change it is concluded that the global balance of forces may for long periods be imperfectly reflected in plate motions because of the constraining effects of long fracture zone offsets on the directions of plate motion and that the causes of abrupt changes in direction may precede them by several million years

subduction influence on magma supply at the east scotia ridge despite a spreading rate of 65 70 km ma 1 the east scotia ridge has along most of its length a form typically associated with slower rates of sea floor spreading this may be a consequence of cooler than normal mantle upwelling which could be a feature of back arc spreading at the northern end of the ridge recently acquired sonar data show a complex rapidly evolving pattern of extension within 100 km of the south sandwich trench new ridge segments appear to be nucleating at or near the boundary between the south american and scotia sea plates and propagating southwards supplanting older segments the most prominent of these north of 56 30 s has been propagating at a rate of approximately 60 km ma 1 for at least 1 ma and displays a morphology unique on this plate boundary a 40 km long axial high exists at the centre of this segment forming one of the shallowest sections of the east scotia ridge beneath it seismic reflection profiles reveal an axial magma chamber or amc reflector similar to those observed beneath the east pacific rise and valu fa ridge simple calculations indicate the existence here of a narrow 1 km wide body of melt at a depth of approximately 3 km beneath the sea floor from the topographic and seismic data we deduce that a localised mantle melting anomaly lies beneath this segment rates of spreading in the east scotia sea show little variation along axis hence the changes in melt supply are related to the unique tectonic setting in which the south american plate is tearing to the east perhaps allowing mantle flow around the end of the subducting slab volatiles released from the torn plate edge and entrained in the flow are a potential cause of the anomalous melting observed a southward mantle flow may have existed beneath the axis of the east scotia ridge throughout its history

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

a multiwinter analysis of channeled flow through a prominent gap along the northern california coast during caljet and pacjet abstract experimental observations from coastal and island wind profilers aircraft and other sensors deployed during the california land falling jets experiment of 1997 98 and the pacific land falling jets experiment of 2000 01 2003 04 were combined with observations from operational networks to document the regular occurrence and characteristic structure of shallow 400 500 m deep cold airstreams flowing westward through california s petaluma gap from the central valley to the coast during the winter months the petaluma gap which is the only major air shed outlet from the central valley is 35 50 km wide and has walls extending at most a modest 600 900 m above the valley floor based on this geometry together with winter meteorological conditions typical of the region e g cold air pooled in the central valley and approaching extratropical cyclones this gap is predisposed to generating westward directed ageostrophic flows driven by along gap pressure differences two case studies and a fiv

a numerical study on the effects of taiwan topography on a convective line during the mei yu season abstract during the morning hours on 23 may 2002 a convective line associated with a mei yu front brought heavy rainfall along the coast of central taiwan under favorable synoptic conditions of warm air advection and large convective available potential energy cape of over 3000 m2 s 2 doppler radar observations indicated that deep convection was organized into a linear shape with a northeast southwest orientation along the front about 70 km offshore from taiwan over the northern taiwan strait the system then moved toward taiwan at a slow speed of about 4 m s 1 in the present study the effects of taiwan topography on this convective line and subsequent rainfall distribution were investigated through numerical modeling using the nagoya university cloud resolving storm simulator cress at a 2 km horizontal grid size experiments with different terrain heights of taiwan including full terrain ftrn half terrain htrn and no terrain ntrn were performed the control run using full terrain and co

upstream effects of mesoscale mountains abstract the alpine experiment alpex has revealed that low level air is typically diverted around the alps without reaching the mountaintop in pursuit of an understanding of the physical basis of this phenomenon and of its generality we have explored the characteristics of orographic blocking of a rotating continuously stratified quid as revealed in a simple model problem retaining full nonlinear and transient effects hydrostatic dynamics is assumed and the obstacle is taken to be an infinitely long ridge with height h x the key questions treated are the strength of the upstream deceleration of cross mountain flow and the length scale over which the decelerated region extends by means of scale analysis the controlling parameters are found to be the rossby number ro u fl and the froude number fr nhm u where u is the speed of the oncoming flow f is the coriolis parameter l the mountain half width n the brunt vaisala frequency and hm is the maximum mountain height the scale analysis also

double bright band observations with high resolution vertically pointing radar lidar and profilers on 11 may 2010 an elevated temperature inversion associated with an approaching warm front produced two melting layers simultaneously which resulted in two distinct bright bands as viewed from the er 2 doppler radar system a vertically pointing coherent x band radar located in greenbelt md due to the high temporal resolution of this radar system an increase in altitude of the melting layer of approximately 1 2 km in the time span of 4 min was captured the double bright band feature remained evident for approximately 17 min until the lower atmosphere warmed enough to dissipate the lower melting layer this case shows the relatively rapid evolution of freezing levels in response to an advancing warm front over a 2 h time period and the descent of an elevated warm air mass with time although observations of double bright bands are somewhat rare the ability to identify this phenomenon is important for rainfall estimation from spaceborne sensors because algorithms employing the restriction of a radar bright band to a constant height especially when sampling across frontal systems will limit the ability to accurately estimate rainfall

mesoscale organization and structure of orographic precipitation producing flash floods in the lago maggiore region this article investigates the mesoscale precipitation mechanisms affecting the severest flood events observed from 2005 to 2012 in the maggia river located in the lago maggiore region on the southern side of the european alps high resolution volumetric radar data are used to describe the horizontal and vertical structure of precipitation while sounding data depict the airflow conditions the events causing the nine highest peak discharge rates in the river in the last eight years are characterized by the presence of convection which is generally absent in the storms that produced lower peak flow rates during major floods convective cells repeatedly propagate over the watershed over time at large temporal scales precipitation patterns assume the form of an elongated band of precipitation the main reason for the formation of the band is the different direction of the low level and upper level flow in relation to the orientation of the alpine orography when pre frontal moist and conditionally unstable air over the po valley is advected towards the alpine chain by a southsoutheasterly low level jet convection develops over the slopes west of lago maggiore where the low level flow is perpendicular to the terrain orientation convective cells are then transported towards the northeast by the upper level southwesterly steering flow reaching maturity over the maggia watershed this mesoscale process can trigger and maintain moist convection for a long time over the same region statistical analyses show that the strength and stability of the upstream flow at low levels 1 5 km determine to what extent precipitation will be convective a comparison with the mesoscale alpine program intensive observing period map iop precipitation cases shows that map iop 2b and 3 are representative of the type of event that produces the biggest floods in the lago maggiore area

a geological map of the scotia sea area constrained by bathymetry geological data geophysical data and seismic tomography models from the deep mantle abstract the scotia sea is one of the tectonically most complex and least understood back arc basins on earth which partly results from its remote location making the acquisition of data challenging here we provide a review of current publicly available geophysical and geological data in the scotia sea realm including magnetic bouguer gravity anomaly high resolution bathymetric heat flow and reflection seismic data and rock type data from cored and dredged samples with this inter disciplinary data set we performed an offshore geological mapping exercise that allowed us to identify lithologies in the predominantly submerged scotia sea domain cross sections combining crustal structure and mantle tomography enabled us to address some of the still persisting geological challenges in this tectonically complex area the data review revealed that basalt is the dominant lithology in the scotia sea area occupying most of the west and east scotia sea wss and ess andesitic and more felsic lithologies are identified in the central scotia sea css and the northern east scotia sea ess mesozoic palaeozoic metamorphic sedimentary lithologies are reported from the highs along the north and south scotia ridges nsr and ssr these highs originate from a land bridge that until the late mesozoic connected the south american and antarctic continents scarcely available and contradicting data prevent the age determination of several structural units surrounding and located on the scotia plate but our mapping exercise allowed us to confirm the presence of the early oligocene late miocene ancestral south sandwich arc assa in the east of the css setting the minimum age of the older segment of the css crust to eocene earliest oligocene three cross sections cross cutting the scotia sea reveal two high velocity zones indicating cold mantle material one is situated below the structural highs along the ssr which we interpreted as remnant slab material of the assa and another below south georgia and the css which we interpreted as the front of the slab that currently subducts at the south sandwich trench ssat the neutral velocity discontinuity below the wss implies mantle conditions of a recently extinct spreading centre the upper mantle low velocity anomaly below the css is interpreted as warmer toroidal flows around the slab edges of the subducting plate at the ssat we have demonstrated that the geological and geophysical data publicly available today allows us to create offshore geological maps in remote inaccessible offshore domains this is a less time consuming economically advantageous exercise which re uses existing geological and geophysical data for a new purpose it is the most data inclusive study there is today of the scotia sea region and serves as a guideline for future expeditions targeting the css and the structural features along the nsr to identify their age and origin a georeferenced version of the map is provided in the supplementary material

meiofauna towards the south sandwich trench 750 6300m focus on nematodes abstract meiobenthos excluding foraminifers from the south sandwich trench was studied in sediment samples collected by multiple core muc or multiple grab mg sampling stations were chosen along a depth transect that covered the continental slope and abyssal plain towards the trench water depths from 747 to 6319 m total abundance and biomass of meiobenthos ranged between 354 and 1675 ind 10 cm 2 and between 16 3 and 80 8 g c 10 cm 2 respectively standing stock decreased though not linearly with increasing water depth biomasses and densities were situated above the world ocean s regression line of meiobenthic stocks against water depth and higher on the trench floor 354 930 ind 10 cm 2 18 7 g c 10 cm 2 at 6300 m compared to most other oceans trench regions this observation confirms earlier results from the antarctic continental margins that antarctic meiofauna is characterised by dense populations in which nematodes predominate over the other taxonomic groups 87 98 of total abundance taxonomic investigations at genus level revealed that chromadoridae monhysteridae desmodoridae desmoscolecidae xyalidae and cyatholaimidae were dominant most of the nematode genera in total 94 occurred in all depths with only a slight distinction between the shallow daptonema dichromadora and molgolaimus preferentially at 750 2300 m and deep with tricoma preferentially from 3000 m on the shallow communities were comparable to those from the weddell sea continental margin the sediments at 2300 6300 m harboured in big lines similar compositions as abyssal and trench communities worldwide two main morph clusters were distinguished among the south sandwich trench nematodes plump nematodes with average l w 9 belonging to the taxonomic similar selective deposit feeding genera of the desmoscolecida and comparable slender nematodes l w 26 biomass size class 4 to 2 such as monhystera acantholaimus daptonema dichromadora molgolaimus microlaimus and some species from paracanthonchus mixed among mainly selective and epistratum feeding modes they formed the bulk of the trench community the results did not fit well with the general decrease in standing stock with increasing water depth and distance from the continental shelf similarly nematode size length from 124 to 2991 m biomass from 0 023 to 0 042 g dwt did not show clear bathymetric trends the lack of the usual deep sea trends of structural characteristics of meiofauna in general density total biomass taxon composition and functional attributes of nematodes morphometrics biomass trophic guild structure and maturity index is explained by the particularly complexity of the sedimentary environment of the south sandwich trench which was the result of physical processes turbidites bioturbation activity by invertebrate taxa and food supply along the depth transect

effect of stable layer formation over the po valley on the development of convection during map iop 8 during intensive observation period iop 8 of the mesoscale alpine program there was a stable layer of air in the lowest levels of the po valley and just upstream of the apennines in this study the effects of the stable layer on the formation of convection in the southern alpine region were investigated through a series of two dimensional idealized experiments the goals of this study were twofold 1 to determine if stable layer strength affected the placement of convection and 2 to test the notion that the stable layer during iop 8 behaved as an effective mountain to accomplish the first objective three simulations were compared in which the strength of the inversion and low level cooling in the po valley and upstream of the apennines was varied the results of these simulations show that the stronger the inversion and low level cooling the farther south the convection was positioned additionally it was found that convection developed as a result of the formation of a broad region of moist instability over the stable layer cellular convection developed in this region of moist instability the second objective was tested through a simulation where the cold pool was replaced by terrain mmtn as in the reference or stb10 simulation the upslope of the terrain in the mmtn simulation was characterized by a wide zone of moist instability however wave structures to the lee of the apennines were markedly different in the stb10 and mmtn simulations this led to different convective and precipitation patterns with the mmtn simulation exhibiting heavier convection over the po valley while the heaviest convection in stb10 was upstream of the apennines these results suggest that at best the stable layer in the stb10 simulation can only be roughly approximated by terrain

the role of nearshore slope on cross shore surface transport during a coastal upwelling event in gulf of finland baltic sea abstract the wind induced coastal upwelling process often contains a mid phase that involves coherent long living cross shore surface jets of cooler water these jets extend to 40 45 km from the coast and tend to start from particular coastal locations we develop a simple method for evaluating the depth in the water column from where the upwelled water originates we also test the hypothesis that the development of cross shore surface jets may be triggered by some permanent characteristics such as the bathymetry or the slope of the nearshore seabed the analysis is performed for a strong and well documented upwelling in the gulf of finland the baltic sea using high resolution bathymetry data satellite derived sea surface temperature surface currents measured by in situ drifters and properties of water masses in two sampling locations the results indicate that the cross shore jets originate exclusively from the shore sections with much steeper slopes 0 0075 than in the rest of the study area the cooler water most likely origins from intermediate water masses at depths between 15 and 30 m the resulting identification of the source depth of the upwelled water and its spatial location assists in understanding the flux of nutrients during upwelling events and their link to the onset of cyanobacteria blooms

holocene depositional history of the fluvia muga prodelta northwestern mediterranean sea abstract seismic analysis of the fluvia muga prodelta located on the gulf of rosas continental shelf northwestern mediterranean sea is a prograding oblique sigmoidal depositional wedge the internal seismic pattern of this wedge records its sedimentary history developed during the holocene sea level highstand three major types of seismic facies have been identified within the prodeltaic wedge 1 continuous stratified facies that characterizes the proximal prodelta environment 2 discontinuous stratified facies that describes the middle prodelta environment and 3 a transparent facies representing the distal prodelta environment the growth pattern of the prodelta is chiefly controlled by the seasonal character and low water discharge of the muga and fluvia rivers the multiple changes in their mouth locations the local shelf current regime developed in the gulf of rosas embayment and the predominance of gravity induced sedimentary processes

multiscale mountain waves influencing a major orographic precipitation event abstract this study combines high resolution mesoscale model simulations and comprehensive airborne doppler radar observations to identify kinematic structures influencing the production and mesoscale distribution of precipitation and microphysical processes during a period of heavy prefrontal orographic rainfall over the cascade mountains of oregon on 13 14 december 2001 during the second phase of the improvement of microphysical parameterization through observational verification experiment improve 2 field program airborne based radar detection of precipitation from well upstream of the cascades to the lee allows a depiction of terrain induced wave motions in unprecedented detail two distinct scales of mesoscale wave like air motions are identified 1 a vertically propagating mountain wave anchored to the cascade crest associated with strong midlevel zonal i e cross barrier flow and 2 smaller scale 20 km horizontal wavelength undulations over the windward foothills triggered by interaction

growth and mass wasting of volcanic centers in the northern south sandwich arc south atlantic revealed by new multibeam mapping new multibeam swath bathymetric sonar data acquired using an em120 system on the rrs james clark ross supplemented by sub bottom profiling reveals the underwater morphology of a not vert similar 12 000 km2 area in the northern part of the mainly submarine south sandwich volcanic arc the new data extend between 55 45 s and 57 20 s and include protector shoal and the areas around zavodovski visokoi and the candlemas islands groups each of these areas is a discrete volcanic center the entirely submarine protector shoal area close to the northern limit of the arc forms a 55 km long east west trending seamount chain that is at least partly of silicic composition the seamounts are comparable to small subaerial stratovolcanoes in size with volumes up to 83 km3 indicating that they are the product of multiple eruptions over extended periods zavodovski visokoi and the candlemas island group are the summits of three 3 3 5 km high volcanic edifices the bathymetric data show evidence for relationships between constructional volcanic features including migrating volcanic centers structurally controlled constructional ridges satellite lava flows and domes and mass wasting of the edifices mass wasting takes place mainly by strong erosion at sea level and dispersal of this material along chutes probably as turbidity currents and other mass flows that deposit in extensive sediment wave fields large scale mass wasting structures include movement of unconsolidated debris in slides slumps and debris avalanches volcanism is migrating westward relative to the underlying plate and major volcanoes are asymmetrical being steep with abundant recent volcanism on their western flanks and gently sloping with extinct eroded volcanic sequences to their east this is consistent with the calculated rate of subduction erosion of the fore arc

turbulence as a mechanism for orographic precipitation enhancement this study examines the dynamical and microphysical mechanisms that enhance precipitation during the passage of winter midlatitude systems over mountain ranges the study uses data obtained over the oregon cascade mountains during the improvement of microphysical parameterization through observational verification experiment 2 improve 2 november december 2001 and over the alps in the mesoscale alpine program map september november 1999 polarimetric scanning and vertically pointing s band doppler radar data suggest that turbulence contributed to the orographic enhancement of the precipitation associated with fronts passing over the mountain barriers cells of strong upward air motion 2ms 1 occurred in a layer just above the melting layer while the frontal precipitation systems passed over the mountain ranges upstream flow appeared to be generally stable except for some weak conditional instability at low levels in the two improve 2 cases the cells occurred in a layer of strong shear at the top of a low level layer of apparently retarded or blocked flow shown by doppler radial velocity data the shear apparently provided a favorable environment for the turbulent cells to develop the updraft cells appeared at the times and locations where the shear was strongest 10 m s 1 km 1 the richardson number was slightly less than 0 25 at the level where the cells were observed suggesting shear generated turbulence could have been the origin of the updraft cells another possibility is that the rough mountainous lower boundary could have triggered buoyancy oscillations within the stable sheared flow the existence of turbulent cells made possible a precipitation growth mechanism that would not have been present in a laminar upslope flow the turbulent cells appeared to facilitate the rapid growth and fallout of condensate generated over the lower windward slopes of the mountains in a laminar flow over terrain upward motions would be unlikely to produce liquid water contents adequate to increase the density and hence the fall speed of precipitating ice particles by riming the turbulent updraft cells apparently create pockets of higher values of liquid water content embedded in the widespread frontal cloud system and snow particles falling from the parent cloud systems can then rapidly rime within the cells and fall out observations by polarimetric radar and direct aircraft sampling indicate the occurrence of rimed aggregate snowflakes and or graupel in the turbulent layer inasmuch as the shear layer is the consequence of retardation or blocking of the low level crossbarrier flow and the turbulence is a response to the shear the shear induced cellularity is an indirect response of the flow to the topography the turbulence embodied in this orographically induced cellularity allows a quick response of the precipitation fallout to the orography since aggregation and riming of ice particles in the turbulent layer produce heavier more rapidly falling precipitation particles without the turbulent cells condensate would more likely be advected farther up and perhaps even over the mountain range small scale cellularity has traditionally been associated with the release of buoyant instability by the upslope flow our results suggest that cellularity may be achieved even if buoyant instability is weak or nonexistent so that even a stable flow has the capacity to form cells that will enhance the precipitation fallout over the windward slopes

kelvin helmholtz waves in extratropical cyclones passing over mountain ranges kelvin helmholtz billows with horizontal scales of 3 4 km have been observed in midlatitude cyclones moving over the italian alps and the oregon cascades when the atmosphere was mostly statically stable with high amounts of shear and ri 0 25 in one case data from a mobile radar located within a windward facing valley documented a layer in which the shear between down valley flow below 1 2 km and strong upslope cross barrier flow above was large several episodes of kelvin helmholtz waves were observed within the shear layer the occurrence of the waves appears to be related to the strength of the shear when the shear attained large values an episode of billows occurred followed by a sharp decrease in the shear the occurrence of large values of shear and kelvin helmholtz billows over two different mountain ranges suggests that they may be important features occurring when extratropical cyclones with statically stable flow pass over mountain ranges

cross barrier flow during orographic precipitation events results from map and improve abstract ground based and airborne doppler radar data collected during passage of frontal rainstorms over the european alps and cascade mountains of oregon are found to exhibit characteristic cross barrier flow and precipitation patterns a stably stratified layer of blocked or partially blocked low momentum flow below mountain crest level is separated from strong cross barrier flow aloft by a concentrated layer of vertical wind shear this shear layer slopes upward toward the mountain crest and persists for several hours as the storm passes over the mountain range this pattern is remarkably similar from case to case and from one mountain range to the other the orographically enhanced precipitation in these cases exhibits stratiform structure marked by a radar bright band which often drops to a lower height indicating locally cooler conditions immediately adjacent to the windward mountain slopes a terrain induced gravity wave produces strong downslope flow and spillover of precipitation onto the

observed mass transports accompanying upstream orographic blocking during map iop8 a mass budget within a domain of 200 km 300 km encompassing the western end of the po river valley in northern italy is performed to examine the relative contributions of the multiple sources of moist airflow in the formation of precipitation for a case of strong upstream blocking observed during intensive observing period eight iop8 of the mesoscale alpine programme map results of this mass budget made possible by horizontally extensive doppler radar observations covering this upstream region reveal that the bulk of the blocked easterly flow undergoes widespread i e mesoscale slow ascent over a region of relatively flat terrain and is ultimately lifted to a level at which it can apparently be entrained into the overriding unblocked southerly component flow as a result the occurrence of profound orographic blocking did not preclude a portion of this low level air from ultimately surmounting the high alpine crest the contribution to this broad mesoscale circulation of smaller scale downslope flows observed within multiple deep alpine river drainages is also investigated by computing observed mass transports emerging from the mouths of several valleys surrounding the lago maggiore region overall the down valley flow is shown to be a relatively minor contributor to the low level mesoscale transports within the po basin but nonetheless one which is important to obtain mass balance in these budget computations copyright 2006 royal meteorological society

deep orographic storms over the sierra nevada part i thermodynamic and kinematic structure abstract two deep orographic storms were documented over the sierra nevada with an instrumented aircraft and with a single doppler radar in both storms the geographic winds were normal to the barrier with speeds of 15 to 30 m s 1 because of blocking the measured wind component normal to the barrier was substantially less than the geostrophic wind component especially below barrier crest height in the very stable case a barrier jet was present below 1 km agl and had peak winds of 28 m s 1 in the slightly stable case a barrier jet was present at 1 km agl and had peak winds of 21 m s 1 other than the speed and altitude of the barrier jet the other measured characteristics of the airflow and thermal fields were rather similar between these two storms the forced ascent of stable air plus the diabatic process of melting caused the isotherms to descend near the barrier the barrier jet can be expressed by the thermal wind relation and is therefore a quasi geostrophic phenomenon the derived vertical

formation mechanisms for convection over the ligurian sea during map iop 8 abstract the dynamical impacts of an unusually strong stable layer that developed over the po valley and northern ligurian sea during mesoscale alpine program map intensive observation period 8 iop 8 on the formation of convection over the ligurian sea are explored based on numerically simulated equivalent potential temperature wind vectors and by a trajectory analysis of parcels both beneath and above the stable layer it is shown that the stable layer behaved as a material surface or effective mountain to the airstreams impinging on it from the south additional analyses show that the leading edge of the stable layer was collocated with maxima in upward motion and a strong positive moisture flux hence it was further argued and demonstrated through inspection of soundings upstream of the cold dome and trajectory analyses that lifting by the stable layer enhanced convective activities over the ligurian sea finally processes contributing to the maintenance of the stable layer during iop 8 were

seismotectonics and relative plate motions in the scotia sea region source parameter inversion of p and sh waveforms and amplitudes for 20 shallow earthquakes using a singular value decomposition technique provides important constraints on the tectonics and relative plate motions of the scotia sea region the derived focal mechanisms show both thrust and strike slip faulting along the north scotia ridge and normal and strike slip faulting along the south scotia ridge two thrust faulting mechanisms south of tierra del fuego indicate diffuse convergence of the antarctic plate along the western margin of the scotia plate earthquakes are consistent with active subduction and back arc spreading along the south shetland trench and bransfield strait the depths of two earthquakes 35 km and 55 km and one shallow thrust faulting event suggest continued subduction along the south shetland trench but the underthrusting is largely aseismic due to the young plate age and slow subduction rate the magnitude and depth of normal faulting earthquakes along bransfield strait and the south scotia ridge are suggestive of diffuse extension rather than typical organized mid ocean spreading which is generally associated with smaller and shallower earthquakes eight strike slip and thrust faulting events provide well constrained slip vectors showing scotia plate motion relative to the surrounding larger plates using a four plate model scotia sc south sandwich ss antarctica an and south america sa and a fixed sa an euler vector we inverted these data together with published centroid moment tensor slip vectors along the south sandwich trench and published marine magnetic data from the ss back arc spreading center to derive the first quantitative plate motion model for the scotia region the sc sa euler pole is located in the south atlantic and shows large latitudinal uncertainties the sc an pole is better constrained and located in the weddell sea these results predict left lateral strike slip motion with a component of compression along the north scotia ridge left lateral strike slip with a component of extension along the south scotia ridge and east west compression in drake passage although no rate data have been published connecting the scotia plate with the global plate system euler vector magnitudes are obtained through closure the results though poorly constrained suggest relative motion rates of 0 5 cm yr along the north scotia ridge and 1 0 cm yr along the south scotia ridge and in drake passage

orographic signature on multiscale statistics of extreme rainfall a storm scale study 1 rainfall intensity and spatiotemporal patterns often show a strong dependence on the underlying terrain the main objective of this work is to study the statistical signature imprinted by orography on the spatial structure of rainfall and its temporal evolution at multiple scales with the aim of developing a consistent theoretical basis for conditional downscaling of precipitation given the topographic information of the underlying terrain the results of an extensive analysis of the high resolution stage ii doppler radar data of the rapidan storm june 1995 over the appalachian mountains is reported in this study the orographic signature on the elementary statistical structure of the precipitation fields is studied via a variable intensity thresholding scheme this signature is further explored at multiple scales via analysis of the dependence of precipitation fields on the underlying terrain both in fourier and wavelet domains the generalized normal distribution is found to be a suitable probability model to explain the variability of the rainfall wavelet coefficients and its dependence on the underlying elevations these results provide a new perspective for more accurate statistical downscaling of orographic precipitation over complex terrain with emphasis on preservation of extremes

tectonic erosion at the front of the japan trench convergent margin abstract the imaging of a multichannel seismic record was improved by reprocessing using pre stack techniques the reprocessed record shows structures that indicate tectonic erosion and gravity collapse at the front of the japan trench margin much of the lower slope appears to be underlain by a detached coherent block of continental crust the lower slope has failed by mass wasting and the resulting apron of slump debris at the base of the slope has become involved in thrust faulting at the front of the subduction zone slumping continues as long as debris is removed from the front of the margin by subduction and the apron cannot build up sufficiently to stabilize the failing lower slope truncated beds at the base of the upper plate indicate subcrustal erosion as well this probably being the main cause of massive subsidence of the margin subsidence was the cause of oversteepening destabilization and subsequent gravity collapse of the leading edge of the upper plate

geochronology and geochemistry of the northern scotia sea a revised interpretation of the north and west scotia ridge junction abstract understanding the tectonic evolution of the scotia sea is critical to interpreting how ocean gateways developed during the cenozoic and their influence on ocean circulation patterns and water exchange between the atlantic and southern oceans we examine the geochronology and detrital age history of lithologies from the prominent submerged barker plateau of the north scotia ridge metasedimentary rocks of the north scotia ridge share a strong geological affinity with the fuegian andes and south georgia indicating a common geological history and no direct affinity to the antarctic peninsula the detrital zircon geochronology indicates that deposition was likely to have taken place during the mid late cretaceous a tonalite intrusion from the barker plateau has been dated at 49 6 0 3 ma and indicates that magmatism of the patagonian fuegian batholith continued into the eocene this was coincident with the very early stages of drake passage opening the expansion of the proto scotia sea and reorganization of the fuegian andes the west scotia ridge is an extinct spreading center that shaped the scotia sea and consists of seven spreading segments separated by prominent transform faults spreading was active from 30 6 ma and ceased with activity on the w7 segment at the junction with the north scotia ridge reinterpretation of the gravity and magnetic anomalies indicate that the architecture of the w7 spreading segment is distinct to the other segments of the west scotia ridge basaltic lava samples from the eastern flank of the w7 segment have been dated as early mid cretaceous in age 137 93 ma and have a prominent arc geochemical signature indicating that seafloor spreading did not occur on the w7 segment instead the w7 segment is likely to represent a downfaulted block of the north scotia ridge of the fuegian andes continental margin arc or is potentially related to the putative cretaceous central scotia sea

airborne doppler observations of a cold front in the vicinity of vancouver island abstract this study uses airborne doppler radar and flight level measurements from the coastal observations and simulations with topography experiment to examine the detailed mesoscale structure of an oceanic cold front upstream of vancouver island on 13 december 1993 these aircraft observations show that there were dramatic differences in frontal structure and movement between nearshore and offshore regions presumably due to the effects of blocking by the terrain the aircraft observations are considered in two parts since the behavior of the front evolved over the flight period during the early and middle portion of the flight the low level flow east of the front was out of the south southeast in the nearshore region rather than southerly as found farther offshore the nearshore segment of the front was oriented south north and appeared to be stationary the zone of orographic influence was found to extend to a region 20 km offshore and confined primarily to the lowest 1 5 km msl in distinct c

airflow within major alpine river valleys under heavy rainfall summary this study documents the aire ow within major alpine river valleys in the presence of precipitation the analysis seeks a conceptual understanding of the effect of rain on the valley e ow ground based and airborne doppler radar and surface data from the mesoscale alpine programme show that during persistent rainy periods the subsidence caused by melting and evaporation of precipitation particles contributes to the formation of a down valley e ow this down valley eow opposes moist southerly e ow aloft which is lifted over the topographic barrier and in which precipitation particles are formed

orographic modification of convection and flow kinematics by the oregon coast range and cascades during improve 2 abstract this paper describes the kinematic and precipitation evolution accompanying the passage of a cold baroclinic trough over the central oregon coast range and cascades during 4 5 december 2001 of the second improvement of microphysical parameterization through observational verification experiment improve 2 field project in contrast to previously documented improve 2 cases the 4 5 december event featured weaker cross barrier winds 15 20 m s 1 weaker moist static stability nm 0 006 s 1 and convective cells that preferentially intensified over oregon s modest coastal mountain range these cells propagated eastward and became embedded within the larger orographic precipitation shield over the windward slopes of the cascades the weather research and forecasting model version 2 2 at 1 33 km grid spacing was able to accurately replicate the observed evolution of the precipitation across western oregon as a result of the convective cell development the precipitation enhancement over the co

formation and maintenance mechanisms of the stable layer over the po valley during map iop 8 abstract during intensive observation period 8 iop 8 of the mesoscale alpine program a strong stable layer formed over italy s po valley and the northern ligurian sea this stable layer has been shown in previous research to be important for the formation of convection over the ligurian sea and the lack thereof over the po valley and southern slopes of the alps the purpose of this study is to investigate the mechanisms that acted to form and maintain the stable layer during iop 8 this aim is accomplished through inspection of observed data as well as numerical simulations and sensitivity experiments observations and reanalysis data show that starting on 17 october 1999 a relatively cool stable air mass was advected around the eastern side of the alps into the lower atmosphere of the po valley both observational data and model output show this air mass as being blocked as it encountered the western alps thus resulting in an accumulation of cool stable air at low levels in the po valley during th

observations and impacts of upstream blocking during a widespread orographic precipitation event a unique combination of airborne and ground based doppler radar observations stretching continuously from the alps south to the mediterranean sea is assembled to document profoundly blocked airflow and associated modification of precipitation processes upstream of the alps during intensive observing period 8 of the mesoscale alpine programme this diversion of incident southerly low level winds in advance of a deep baroclinic trough served to reduce markedly the transport of warm moist mediterranean air toward the steep alpine slopes heavy rains forecast in this flood prone region thus failed to materialize with lighter though protracted stratiform precipitation extending over the comparatively flat reaches of the adjacent po valley release of convective instability did occur but was displaced well south of the alps over the coastal apennine mountain chain and adjacent waters of the mediterranean sea influences on a variety of scales that evidently led to development of this blocked flow are examined and multiple expressions of upstream blocking are identified these range from pervasive down valley flows observed within multiple deep alpine river drainages to a larger scale cool easterly low level flow channelled alongside the curving south face of the alps which was ultimately extruded southward over the mediterranean observed thermodynamic profiles in these contrasting regimes are presented and related to the observed precipitation structure copyright 2003 royal meteorological society

effects of coastal orography on landfalling cold fronts part ii effects of surface friction abstract the role of surface friction in modifying cold fronts as they make landfall in regions of steep coastal orography is examined by means of idealized simulations both the effects of the surface roughness change at the coast in the absence of orography and the effects of orography are considered flow over a large and abrupt change in surface roughness generates an inertia gravity wave above the boundary layer with characteristics similar to that associated with flow over a plateau deceleration of the cross coast flow occurs over land as well as for a short distance upstream and causes retardation of frontal motion a prescribed northerly postfrontal jet weakens rapidly after landfall maximum vertical motions are several centimeters per second however only small rainfall enhancement is expected since the updraft is very narrow and produces only small vertical displacements friction modifies the flow over the orography by increasing the upstream flow deceleration and reducing the magnitude of

a numerical study of three catastrophic precipitating events over southern france ii mesoscale triggering and stationarity factors in the western mediterranean basin large amounts of precipitation can accumulate in less than a day when a mesoscale convective system mcs stays over the same area for several hours heavy precipitating events hpes in this region especially southern france are not only characterized by significant precipitation rates typically more than 200 mm in less than 24 or 48 hours but also by quasi stationary behaviour the aim of this present study is to use realistic simulations of past events to analyze and better understand the physical mechanisms which lead to the stationarity of hpes over southern france using a high resolution 2 5 km non hydrostatic mesoscale atmospheric model we focused on three hpes 13 14 october 1995 the cevennes case 12 13 november 1999 the aude case and 8 9 september 2002 the gard case the experimental design for simulating realistic structures and evolutions of the hpes with the meso nh model has been presented in a companion paper in that study the evolution of the synoptic patterns in which the hpes occurred were analysed in this paper the importance of mesoscale ingredients such as the moist and conditionally unstable low level jet llj is examined in terms of their ability to focus the deep convection over the same area for several hours the cevennes case is typical of a so called cevenole event in france for which orographic forcing is the primary mechanism which acts to continuously generate convective cells upwind of the massif central the unusual location of the gard event is explained by a cold pool induced by the evaporation of precipitation which acts as a relief like feature to force the conditionally unstable and moist llj to rise this constitutes a quasi stationary forcing as the cold pool is blocked in the rhone valley for the third case diabatic cooling and the massif central act to enhance the precipitation but have almost no impact on the location of the anchoring point of the precipitating system

tectonic activity evolution of the scotia antarctic plate boundary from mass transport deposit analysis the spatial distribution and temporal occurrence of mass transport deposits mtds in the sedimentary infill of basins and submerged banks near the scotia antarctic plate boundary allowed us to decode the evolution of the tectonic activity of the relevant structures in the region from the oligocene to present day the 1020 mtds identified in the available data set of multichannel seismic reflection profiles in the region are subdivided according to the geographic and chronological distributions of these features their spatial distribution reveals a preferential location along the eastern margins of the eastern basins this reflects local deformation due to the evolution of the scotia antarctic transcurrent plate boundary and the impact of oceanic spreading along the east scotia ridge esr the vertical distribution of the mtds in the sedimentary record evidences intensified regional tectonic deformation from the middle miocene to quaternary intensified deformation started at about 15 ma when the esr progressively replaces the west scotia ridge wsr as the main oceanic spreading center in the scotia sea coevally with the wsr demise at about 6 5 ma increased spreading rates of the esr and numerous mtds were formed the high frequency of mtds during the pliocene mainly along the western basins is also related to greater tectonic activity due to uplift of the shackleton fracture zone by tectonic inversion and extinction of the antarctic phoenix ridge and involved changes at late pliocene the presence of mtds in the southern scotia sea basins is a relevant indicator of the interplay between sedimentary instability and regional tectonics

a numerical study of moist stratified flow regimes over isolated topography numerical simulations of moist flows over simply shaped three dimensional mountains were undertaken using a mesoscale meteorological model with the purpose of examining the existence of different flow regimes near obstacles and the transitions among them moving the system in parameter space resulted in different solutions for a particular set of parameters this method was adopted to explore numerically multiple regimes stable for at least meteorologically interesting time periods the evolution of the flow was studied in two different experimental set ups in the first case the height of the obstacle was progressively changed over time in the second case the effect of an advective change of the humidity content inside the channel was analysed in both experiments a dependence on the history of the flow became apparent a comparison among cases of obstacles with the same aspect ratio and non dimensional height but with different horizontal cross sections highlighted the importance of the obstacle geometry in perturbing the upstream flow favouring the persistence of upstream blocked regions in the case of an arc shaped mountain the sensitivity of the reversal flow volume to several control parameters was analysed with additional experiments in particular the importance of the cooling associated with the evaporation of precipitation was emphasized copyright 2004 royal meteorological society

kinematic and thermodynamic structures of sierra barrier jets and overrunning atmospheric rivers during a landfalling winter storm in northern california abstractthis study characterizes kinematic and thermodynamic structures of sierra barrier jets sbjs atmospheric rivers ars and their interaction over the period 14 16 february 2011 when a winter storm made landfall in northern california a suite of scanning and profiling doppler radars rawinsondes and gps receivers is used to document these structures across the central valley and up the western sierra slope to the crest along an 200 km segment of the sierra the winter storm is grouped into two episodes each having an ar that made landfall low level winds in the sbj observed during episode 1 were southeasterly and embedded in a stably stratified air mass along barrier wind speeds u340 reached maximum values of 25 30 m s 1 as low as 0 2 km msl over the central valley and as high as 1 5 km msl over the western sierra slope southwesterly winds associated with the ar overlaid the sbj along an interface that sloped upward from southwest to northeast with a southwestern extent at the western

on the effects of coastline perturbations on coastal currents and fronts abstract surface stress driven coastal currents and upwelling were produced around the outer edge of a cylindrical tank having a conical bottom and a protuberance representing a cape a large standing wave formed at a distance dsw 2rc 0 4 s rc 0 2 downstream of the cape where dsw is the distance between the wave trough and tip of the cape rc is the radius of the cape and g h0 u f s where g g 0 is the reduced gravity h0 is the initial depth of the top layer u is the friction velocity applied to the top surface of the fluid f is the coriolis parameter and s is the distance of the stationary position of the upwelling front at the surface from the wall of the tank the extreme offshore extent of this stable wave trough as seen from particle streaks was asw 3 4rc however at low values of after many rotation periods the standing wave often grew in amplitude producing a cyclone which detached from the wave trough unstable baroclinic waves formed at the upwelled front upstream o

mantle flow in the south sandwich subduction environment from source side shear wave splitting 1 the occurrence of seismic anisotropy in the earth s upper mantle is a global phenomenon related to subcrustal deformation and flow processes the shear wave splitting analysis method has led to a global set of anisotropy maps mainly derived from receiver side analysis remote places with few seismometers deployed remain unexamined source side splitting analysis allows mapping of mantle fabrics in these regions here we investigate seismic anisotropy in the south sandwich islands subduction environment core reflected scs waves recorded at the neumayer seismograph network are corrected for well constrained receiver anisotropy and then analysed for source anisotropy sub slab mantle minerals are aligned horizontally almost parallel to the trench indicating a westward flow around the subducting slab this is consistent with a model of horizontal mantle flow due to slab rollback that was previously inferred from marine and geochemical studies in the back arc region

the structure of cretaceous oceanic crust of the nw pacific constraints on processes at fast spreading centers a 725 km long transect along a flow line in the nw pacific provides new images of the internal structure of cretaceous oceanic crust formed at fast spreading rates reflections from the moho transition zone document changes in crustal travel time along the profile variations with a wavelength of 100 150 km may representing fluctuations in the magma supply with a 2 3 m y periodicity to the south the crust may be several kilometers thicker than normal in places possibly resulting from increased magma supply due to the distal effects of the shatsky hotspot at about the depth expected for the boundary between seismic layers 2 and 3 sub horizontal reflections are imaged over much of the profile the clearest are positive polarity and of an amplitude consistent with a reflection coefficient of 0 1 implying a sharp velocity and density discontinuity northward ridgeward dipping reflections in the lower crust are imaged over much of the profile particularly toward the north away from the region possibly affected by the hotspot these reflections stop at the moho reflection but die out perhaps 1 s beneath top basement they probably represent lithological layering as predicted by models of ductile flow accompanying passive upwelling during plate separation although they could represent secondary shears generated by basal drag during active mantle upwelling beneath the spreading center the discrete character and in places regular spacing every 8 km of these features hint at a cyclicity in the layering and thus at some episodicity in magmatic processes over a timescale of 0 1 0 2 m y

plate motions slab dynamics and back arc deformation back arc extension or compression is often related to a particular behavior of the slab dip change and or forward backward migration with respect to the upper plate slabs act either passively when anchored in the deep mantle whereas back arc deformation accommodates the backward or forward motion of the upper plate with respect to the trench or actively when slab pull increasing with slab age forces the trench to rollback in addition to these two mechanisms i e slab anchor and slab rollback numerous observations support the existence of dynamic mantle flow that can exert an overpressure on one side of the slab causing its forward or rearward migration with respect to the arc based on a compilation of upper plate absolute motion trench absolute motion back arc deformation rate upper plate strain regime and slab age for all oceanic subduction zones excluding any kind of collision with continents arcs or plateaus we have examined how the combined effects of these parameters can account for the observed back arc deformations our main results are 1 a global correlation exists between upper plate absolute motion and back arc deformation i e back arc extension when upper plate retreats and vice versa 2 there are as many advancing trenches as retreating ones with trench motion globally limited to 50 mm y 1 furthermore there is no positive correlation between trench retreat and slab age and 3 upper plate absolute motion often fails to explain the back arc deformation rates and the trench motions observed at several subduction zones as tonga new hebrides sandwich or ryukyu we propose that the trench migration of these subduction zones are on the influence of mantle flows we conclude that spontaneous trench rollback related to slab pull is negligible with respect to upper plate motion back arc deformation regime is mostly controlled by the upper plate absolute motion relative to a partly anchored slab trench motion is limited to 50 mm y 1 even if locally mantle flows force slab to move

deep water sedimentary environments of the northwestern weddell sea and south sandwich islands antarctica abstract two transects have been sampled using short cores multi and box seabed photography video sequences and sediment profile images across the northwestern weddell sea and south sandwich forearc antarctica a total of 12 core stations were examined for sediment structure texture and composition to determine their depositional history four of the core stations from the weddell continental slope abyssal plain and south sandwich forearc and trench were further examined for trace metal analysis ba u mn and pb for sediment source and palaeoproductivity and 210pb for age profiles and depth of mixing by bioturbation the 3055 m station at the base of the weddell continental slope recovered laminated muddy silty contourites with a surface layer of hemipelagites with abundant ice rafted debris possibly originating from the 1995 larsen a and 2002 larsen b ice shelf disintegration this is interpreted as a moderate energy depositional environment influenced by weddell sea bottom water along slope current reworking sediment sediment sources to this station include the direct input of clastic material from ice with variation in sediment sources from in situ current winnowing and along slope transport reflected in the mn li and ba li ratios the 4760 m station on the weddell abyssal plain recovered hemipelagites interpreted as a low energy depositional environment with sediment supplied both from direct ice rafting and pelagic settling during productive seasonally ice free periods the 210pb profiles indicate a sedimentation rate of 0 024 cm year 1 with the 206 207pb isotope ratios suggesting a principle sediment source from ice rafting the 2292 and 6326 m stations from the slope and floor of the south sandwich forearc recovered pelagic and turbiditic sediments this is interpreted as a high energy down slope depositional environment with high surface biogenic productivity and sediment supply leading to localised down slope mass movements on the slope and floor high levels of mn li and elevated u li ratios demonstrate a strong concentration of mn in organic poor sediment the mn redoxcline is present at 10 cm depth the high ba li 500 ppm in the forearc floor core reflects some limited enhanced productivity source sediment for the forearc may in part be derived from the northwest weddell abyssal plain based on the 206 207pb isotope ratios reflecting transport by weddell sea bottom water

dynamics and microphysics of orographic precipitation during map iop3 a dynamical and microphysical four dimensional study of an intense orographic precipitating system is carried out in the frame of map iop3 25 26 september 1999 high precipitation opportunely occurred in the range of the swiss operational doppler radar at monte lema switzerland the us spol polarimetric doppler radar and the french ronsard doppler radar both located near lago maggiore italy radar data have been combined to deduce four dimensional precipitation and wind fields during the most intense precipitation period 1600 2000 utc on 25 september the organization and evolution of the microphysical field have been obtained through an analysis of spol polarimetric data nine hydrometeor classes light moderate and heavy rain hail rain hail and graupel hail mixtures dry and wet snow and ice crystals are inferred by means of a fuzzy logic method initially developed by vivekanandan et al the temporal mean study of reflectivity fields reveals that the precipitation presents a convective pattern and is primarily located on the foothills and over the first mountain mean microphysical fields reveal rain below the 0 c level wet snow ice crystals and dry snow above with an embedded 2 km deep layer of graupel hail mixture this suggests convective microphysical processes temporal series of precipitation and wind fields are then analysed in order to characterize in detail the organization and evolution of the system three stages were identified first an elongated structure growing and intensifying over the lake while moving towards the alps then a spreading of the system over the mountains and finally a weakening over lago maggiore and the mountains it has to be noted that the lake and the first mountainous peaks were important factors in the generation and the intensification of convective cells the lake acts as a secondary moisture source which can favour local convection and mountainous slopes favour updraughts which permit a downwind extension of the system by ejection of precipitating particles according to the fountain particles concept finally through a temporal and spatial microphysical study coalescence below the 0 c level riming and freezing above are concluded to be the major processes in the formation of intense precipitation the essential role of ice phase in the formation and enhancement of precipitation has been highlighted copyright 2005 royal meteorological society

idealized numerical simulations of quasi stationary convective systems over the northwestern mediterranean complex terrain northwestern mediterranean coastal regions are frequently affected by torrential rainfall associated with quasi stationary mesoscale convective systems mcss the present work examines how the characteristics of a conditionally unstable flow impinging on the coastal complex terrain of the northwestern mediterranean can affect the location and intensity of quasi stationary mcss the study is based on idealized simulations but including the major ingredients of northwestern mediterranean heavy precipitation events a moist conditionally unstable marine flow of about 100 km width facing the true terrain composed of the massif central surrounded by the alps and the pyrenees we find that mcss are located upstream of the mountain range with a slow flow whereas with a rapid flow the heaviest precipitation is over the massif central slopes in a similar way when the lateral environment is drier the heaviest precipitation is located upstream whereas a humid environment favours precipitation over the slopes the dominant lifting mechanism is strongly related to the location of the system 1 direct orographic triggering for systems over mountain slopes and 2 cold pool triggering for upstream systems in addition the neighbouring mountains interplay through deflection of the flow and induced low level convergence favoured by slow or dry lateral environments and through cold pool blocking within valleys copyright 2012 royal meteorological society

the role of wind stress curl in jet separation at a cape abstract a high resolution numerical model is used to study the importance of spatial variability in the wind forcing to the separation of a coastal upwelling jet at a cape an idealized topography and wind field based on observations from the cape blanco oregon region are used several simulations are investigated with both the intensity and the spatial structure of the wind forcing varied to isolate the importance of the observed intensification in the wind stress and wind stress curl magnitudes to the separation process a simulation using a straight coast confirms that the presence of the cape is crucial for separation wind stress intensification by itself with zero curl does not aid separation the wind stress curl intensification south of the cape on the other hand is important for controlling details of the process because the positive wind stress curl drives upwelling isotherms in the offshore region tilt upward creating a pressure gradient that sustains an intensification of the southw

mechanisms of intense alpine rainfall abstract numerical studies by the authors and others of the 1994 piedmont flood show that the orographically modified flow was a critical element for the production of extraordinary rainfall to uncover the precise mechanism of orographic rainfall occurring in full physics mm5 simulations of the 1994 piedmont flood the authors carried out simulations with the same real data initial and boundary conditions but with the real orography replaced by an idealized one with excellent agreement between real and idealized orography on the rainfall rate versus time in the piedmont area analysis of the idealized orography simulation provides a clear picture of the model s mechanism of orographically induced rainfall as noted in previous studies of the 1994 piedmont case a moist saturated airflow has a reduced effective static stability and tends to flow over the mountains while an unsaturated airstream is stable and tries to flow around toward the left in the northern hemisphere in the 1994 piedmont case t

effects of the alps and apennines on forecasts for po valley convection in two hymex cases two intensive observation periods iops of the hydrological cycle in the mediterranean experiment hymex are examined in this study iop6 and iop13 were characterized by troughs with associated cold fronts entering the western mediterranean and in both cases organized frontal convection in the po valley was observed these similarities notwithstanding predictability of the po valley convection was limited for iop6 for most of the forecast models available during hymex but not for iop13 using the weather research and forecast wrf model in research mode the present study confirms the relatively good forecast for frontal convection in iop13 is not very sensitive to modeling assumptions in contrast it is found that only a two way nested simulation initialized close to the event was able to produce a realistic representation of the squall line in the po valley for iop6 a comparison between a successful and an unsuccessful simulation of the po valley convection in iop6 suggests the sensitivity lies with the orographic flow modification which was at the threshold between flow over and flow around responses to the maritime alps in particular in the flow over regime downslope winds from the maritime alps and apennines suppress the convergence uplift where the front encounters the barrier wind in the po valley while in the flow around regime it is unimpeded a delicate balance between the competing mechanisms of orographically induced subsidence on the lee side of the apennines and frontal uplift in the po valley is found to be crucial for squall line survival

the kinematic structure of a wasatch mountain winter storm during ipex iop3 the influence of orographic circulations on the precipitation structure of a wasatch mountain winter storm is examined using observations collected during the third intensive observing period iop3 of the intermountain precipitation experiment ipex the event featured the passage of a midlevel 700 550 hpa trough followed 3 h later by a surface trough prior to and during the midlevel trough passage large scale southwesterly flow impinged on the wasatch mountains low level confluence was observed between this southwesterly flow and along barrier southerly flow within 20 40 km of the wasatch mountains this confluence zone which moved toward the wasatch mountains during and following the passage of the midlevel trough was accompanied by low level convergence and precipitation enhancement over the upstream lowlands dual doppler analysis revealed the presence of a shallow along barrier jet near the base of the wasatch mountains that was surmounted by southwesterly cross barrier flow at mid and uppermountain levels this cross barrier flow produced strong 1 2 m s 1 ascent as it interacted with the steep windward slopes of the wasatch mountains where precipitation was roughly double that observed in the lowlands upstream flow deflection and splitting were also observed near the highest terrain features a narrow region of strong subsidence which at times exceede d2ms 1 was found to the lee of the wasatch and based on radar imagery appeared to modulate hydrometeor spillover aloft processes contributing to the evolution of the near barrier flow field including topographic blocking diabatic effects and surface friction contrasts are discussed

notes and correspondence polarimetric radar observation of multiple freezing levels abstract an unusual multiple freezing level event observed with polarimetric radar during the second phase of the improvement of microphysical parameterization through observational verification experiments improve 2 field program is described the event occurred on 28 november 2001 when a warm front moved over the oregon cascade mountains as the front approached an elevated melting layer formed above a preexisting melting layer near ground continued warming of the lower atmosphere eventually dissipated the lower melting layer the polarimetric measurements are used to estimate the height of the freezing levels document their evolution and deduce hydrometeor habits the measurements indicate that when the two freezing levels were first observed melting was incomplete in the upper melting layer and characteristics of particles that passed through the two melting layers were similar as warming progressed the character of particles entering the lower melting layer changed possibly becoming ice pell

effects of coastal orography on landfalling cold fronts part i dry inviscid dynamics abstract in this study the interaction of cold fronts with idealized coastal terrain typical of the western united states and canada is considered two issues are examined first what are the factors that determine the strength of the coastal winds and second what are the orographic effects on the frontal evolution to address these issues the authors utilize a two dimensional boussinesq terrain following coordinate numerical model in which a uniform prescribed flow is forced to move over a plateau the resultant across mountain velocities are characterized by a zone of strongly decelerated flow upstream of the windward slope and a train of inertia gravity waves downstream a barrier jet oriented parallel to the mountain is produced by the coriolis force the variations of the magnitude of the upstream deceleration and the barrier jet over a wide range of froude numbers and rossby numbers are described steady linear theory applied to flow over a plateau shows that the upstream deceleration is dete

mechanisms producing different precipitation patterns over north eastern italy insights from hymex sop1 and previous events during the first hymex special observation period sop1 field campaign the target site of north eastern italy nei experienced a large amount of precipitation locally exceeding the climatological values and distributed among several heavy rainfall episodes in particular two events that occurred during the last period of the campaign drew our attention these events had common large scale patterns and a similar mesoscale setting characterised by southerly low level flow interacting with the alpine orography but the precipitation distribution was very different during intensive observing period iop18 31 october 1 november 2012 convective systems were responsible for intense rainfall mainly located over a flat area of the eastern po valley well upstream of the orography conversely during iop19 4 5 november 2012 heavy precipitation affected only the alpine area in addition to iop18 and iop19 the present study analyses other heavy precipitation episodes that display similar characteristics and which occurred over nei during the autumn of recent years a high resolution 2 km grid spacing non hydrostatic nwp model and available observations are used for this purpose the two different observed precipitation patterns are explained in terms of interaction between the impinging flow and the alps depending on the thermodynamic profile convection can be triggered when the impinging flow is forced to rise over a pre existing cold air layer at the base of the orography in this situation a persistent blocked flow condition and upstream convergence are responsible for heavy rain localized over the plain conversely if convection does not develop flow over conditions are established and heavy rain affects the alps numerical parameters proposed in the literature are used to support the analysis finally the role of evaporative cooling beneath the convective systems is evaluated it turns out that the stationarity of the systems upstream of the alps is mainly attributable to persistent blocked flow conditions while convective outflow slightly modifies the location of precipitation

orographic effects on precipitating clouds 1 precipitation over and near mountains is not caused by topography but rather occurs when storms of a type that can occur anywhere deep convection fronts tropical cyclones form near or move over complex terrain deep convective systems occurring near mountains are affected by channeling of airflow near mountains capping of moist boundary layers by flow subsiding from higher terrain and triggering to break the cap when low level flow encounters hills near the bases of major mountain ranges mesoscale convective systems are triggered by nocturnal downslope flows and by diurnally triggered disturbances propagating away from mountain ranges the stratiform regions of mesoscale convective systems are enhanced by upslope flow when they move over mountains in frontal cloud systems the poleward flow of warm sector air ahead of the system may rise easily over terrain and a maximum of precipitating cloud occurs over the first rise of terrain and rainfall is maximum on ridges and minimum in valleys if the low level air ahead of the system is stable blocking or damming occurs shear between a blocked layer and unblocked moist air above favors turbulent overturning which can accelerate precipitation fallout in tropical cyclones the tangential winds encountering a mountain range produce a gravity wave response and greatly enhanced upslope flow depending on the height of the mountain the maximum rain may occur on either the windward or leeward side when the capped boundary layer of the eye of a tropical cyclone passes over a mountain the cap may be broken with intense convection resulting

the structure and evolution of gap outflow over the gulf of tehuantepec mexico abstract mesoscale model simulations are used to examine the structure and dynamics of a gap outflow event over the gulf of tehuantepec mexico that was associated with a surge of cold air along the eastern slopes of the sierra madre the simulated gap outflow winds emerged from chivela pass reached a maximum speed of 25 m s 1 and turned anticyclonically as they fanned out over the gulf northerly winds were also able to ascend the mountains east and to a lesser extent west of chivela pass indicating that the movement of cold air across the sierra madre was not confined to the pass a mesoscale pressure ridge was aligned along the axis of the gap outflow jet which was flanked to the west by an anticyclonic eddy and to the east by a weaker cyclonic eddy a model derived trajectory along the axis of the outflow jet traced an inertial path with anticyclonic curvature produced primarily by the coriolis acceleration the cross flow pressure gradient acceleration along this trajectory was negligible be

effect of the andes cordillera on precipitation from a midlatitude cold front the effects of the andes cordillera the major mountain range in south america on precipitation patterns of baroclinic systems approaching from the southeast pacific remain largely unstudied this study focuses on a case in late may 2008 when an upper level trough and surface cold front produced widespread precipitation in central chile the primary goal was to analyze the physical mechanisms responsible for the structure and evolution of the precipitation weather research and forecasting wrf model simulations indicate that as an upper level trough approached central chile midtropospheric flow below 700 hpa was blocked by the high topography and deflected poleward in the form of a barrier jet this northerly jet had wind maxima in excess of 15 m s 21 was centered around 925 hpa and extended westward 200 km from the mountains it intersected the cold front which approached from the south near the coast thereby increasing convergence along the frontal surface slowing its equatorward progress and enhancing rainfall over central chile another separate region of heavy precipitation formed over the upwind slopes of the cordillera a trajectory analysis confirmed that the barrier jet moved low level parcels from their origin in the moist southeast pacific boundary layer to the coast when model topography was reduced to twenty percent of its original height the cold front advanced more rapidly to the northeast generated less precipitation in central chile between 338 and 368s and produced minimal orographic precipitation on the upwind andean slopes based on these findings the high topography appears responsible for not only orographic precipitation but also for substantially increasing precipitation totals over the central coast and valley

reflective oceanic crust formed at a fast spreading center in the pacific we present new seismic images of cretaceous crust formed at a fast spreading center in the pacific the high crustal reflectivity observed in these data contradicts the conventional wisdom that accretionary structures formed at fast spreading centers are not seismically detectable subhorizontal reflections can be traced at 600 800 ms two way time below the top of basement for tens of kilometres suggesting the presence of a widespread seismic boundary possibly a structural discontinuity related to the maximum depth of hydrothermal circulation at the spreading center or the base of the sheeted dikes lower crustal reflections dipping dominantly toward the paleo spreading center may represent mafic ultramafic banding similar to that observed in the lower crust of reconstructed ophiolite sections

enhanced magma supply at the southern east scotia ridge evidence for mantle flow around the subducting slab abstract bathymetric and seismic data show that the southernmost segment of the east scotia ridge segment e9 is anomalous in its curved plan form and in the presence of a large axial volcanic ridge avr spreading commenced only within the past 1 million years on this segment which appears to have propagated both northward and southward the presence of a caldera at the summit of the avr indicates that a shallow magma chamber of limited extent existed recently beneath the ridge eruption from this magma chamber probably within the past 0 1 million years may have led to the formation of the avr itself careful examination of seismic reflection profiles suggests that small pockets of magma may still exist beneath the avr magmatism on segment e9 may be enhanced as a result of the flow of shallow mantle around the southern end of the subducting slab beneath the south sandwich arc in a manner similar to that proposed for the northern end this mantle probably carries a slightly enriched hotspot signature and is affected by volatiles released from the edge of the slab both tending to increase the supply of magma to the back arc spreading centre

triggering and evolution of a deep convective system in the mediterranean sea modelling and observations at a very fine scale gaining a deeper physical understanding of the high impact weather events which repeatedly affected the western mediterranean basin in recent years on the coastal areas of eastern spain southern france and northern italy is strongly motivated by the social request to reduce the casualties and the economical impacts due to these highly localized and hardly predictable phenomena in october 2014 an extreme event hit genoa city centre less than 3 years after a very similar event which occurred in november 2011 taking advantage of the availability of both observational data and modelling results at the micro alpha meteorological scale this article provides insights about the triggering mechanism and the subsequent spatio temporal evolution of the genoa 2014 back building mesoscale convective system the major finding is the effect of a virtual mountain created over the ligurian sea by the convergence of a cold and dry jet outflowing from the po valley and a warm and moist low level southeasterly jet within the planetary boundary layer

a review of tertiary climate changes in southern south america and the antarctic peninsula part 2 continental conditions abstract climate changes in southern south america and the antarctic peninsula during the tertiary show a strong correlation with ocean warming and cooling events which are in turn related to tectonic processes during periods of accelerated sea floor spreading and mid ocean ridge activity sea levels rose so that parts of the continents were flooded and forests were destroyed however this was balanced by the large scale release of co2 during volcanic outgassing and carbonate precipitation on the continental shelves which caused rising air temperatures and the poleward expansion of sub tropical and temperate forests cooling episodes generally caused an increase in the north south thermal gradient because of an equatorward shift in climate belts so that the westerly winds intensified and brought higher rainfall to the lower latitudes an increase in wind blown dust caused temperatures to drop further by reflecting sunlight back into space the rising andes range had a marked influence on climate patterns up to the middle miocene it was still low enough to allow summer rainfall to reach central and north central chile but after about 14 ma it rose rapidly and effectively blocked the spill over of moisture from the atlantic ocean and amazon basin at this time the cold humboldt current was also established which together with the andes helped to create the arid diagonal of southern south america stretching from the atacama desert to the dry steppes of patagonia this caused the withdrawal of subtropical forests to south central chile and the expansion of sclerophytic vegetation to central chile however at the same time it intercepted more rain from the northeast causing the effect of the south american monsoon to intensify in northwestern argentina and southern bolivia where forest communities presently occur in patagonia glaciation started as early as 10 5 ma but by 7 ma had become a prominent feature of the landscape and continued apparently uninterruptedly into the pleistocene the antarctic peninsula saw its first mountain glaciation between 45 and 41 ma with major ice sheet expansion commencing at about 34 ma isolated stands of nothofagus forests were still present in low lying areas suggesting that the glaciers were initially wet based but dry based glaciers were established at around 8 ma although temperatures rose briefly during the messinian pliocene transition causing sub antarctic flora to retreat to higher elevations of the transantarctic mountains the present cold polar conditions were finally established by about 3 ma

orographic effects on rainfall in map cases iop 2b and iop 8 although the large scale flow was similar important differences in mesoscale atmospheric structure made the difference between moderately intense rain in intensive observing period iop 2b and relatively light rain in iop 8 in the lago maggiore target area lmta this conclusion is supported here through analysis of data from and numerical simulations of these two cases analysis of the large scale data shows that there was in both cases a moist tongue of southerly flow moving from the west to the east of the alpine south side mesoscale data analysis and numerical simulations suggest that the most important difference between the two cases was the presence of a cold stable air mass in the po valley in iop 8 which persisted through the period in which the large scale moist tongue was progressing eastward and prevented the most humid air from reaching the lmta another difference contributing to the greater rainfall in the lmta in iop 2b was the development of conditional instability with associated convective rain due to the effect of the alps on the eastward passage of the moist tongue in iop 8 the atmosphere remained locally stable throughout the period copyright 2003 royal meteorological society

climatology of barrier jets along the alaskan coast part ii large scale and sounding composites abstract this paper investigates the large scale flow and thermodynamic structures associated with barrier jets along the alaskan coast using the national centers for environmental prediction ncep reanalysis as well as the average wind moisture and thermodynamic soundings at yakutat alaska yak and whitehorse yukon territory canada yxy large scale and sounding composites are constructed for all barrier jets objectively identified around yak using synthetic aperture radar sar imagery during the cool and warm seasons of 1998 2003 during the cool season the jet events are separated into those with sharp upstream wind gradients shock jets highly variable gustlike surface winds variable jets and the other jet events other jets those cool season barrier jets without shock or variable characteristics are associated with an anomalously deep upper level trough approaching the gulf of alaska and an anomalous ridge over western canada and interior alaska the associated surface cyclone a

orographic precipitation forcing along the coast of northern california during a landfalling winter storm abstractthis study documents orographic precipitation forcing along the coastal mountains of northern california during the landfall of a significant winter storm over the period 16 18 february 2004 the primary observing asset is a scanning x band doppler radar deployed on the coast at fort ross california which provides low level e g below 1 km msl horizontal and vertical scans of radial velocity and reflectivity to characterize airflow and precipitation structures further context is provided by a wind profiling radar a radio acoustic sounding system rass balloon soundings buoys a gps receiver and surface meteorological sensors the winter storm is divided into two episodes each having pre cold frontal low level jet llj structures and atmospheric river characteristics episode 1 has a corridor of terrain trapped airflow tta that forms an interface with the llj the interface extends 25 km offshore in a 0 5 km vertical layer and the western edge of this interface near the ocean surf

small scale precipitation elements in midlatitude cyclones crossing the california sierra nevada abstractradar data in some frontal systems passing over the sierra nevada of california show large variance on scales of 10 km the most prominent features are a few kilometers in scale and are similar to small scale precipitation cells embedded in fronts seen over other mountain ranges other frontal systems crossing the sierras are characterized by more uniform air motions updrafts in large variance storms have characteristics of shear induced turbulence although buoyant instability may also contribute large variance storms occur under stronger upstream winds and vertically integrated cross and along barrier moisture fluxes rain gauges indicate that large variance storms have precipitation greater than smaller variance storms stronger horizontal moisture fluxes may provide greater mean upslope condensation rates however it is hypothesized that accelerated microphysical processes are needed to most efficiently convert the condensate into precipitation that falls out on the lower slopes before be

slice of intraoceanic arc insights from the first multichannel seismic reflection profile across the south sandwich island arc we present the first multichannel seismic mcs reflection line that crosses the south sandwich trench south sandwich island arc and east scotia sea backarc basin the line is used in combination with earthquake catalogue data to interpret the strain distribution across the sandwich plate and the relationship of forearc structures to processes operating at the trench the mcs data reveal a 1 2 km high fault scarp associated with a 20 km wide arcward tilted block in the mid forearc these features indicate large scale gravitational collapse and earthquake data are consistent with trench normal extension at shallow depth in this area there is however little evidence of distributed extension within the interior of the sandwich plate the mcs data show a small frontal wedge that achieves its maximum thickness only 18 km from the trench backarc magnetic data mid forearc extension and the small size of the frontal wedge are all consistent with long term and ongoing subduction erosion earthquake data suggest that this erosion is taking place in an environment of low interplate stress

evidence from earthquakes for bookshelf faulting at large non transform ridge offsets migrating non transform offsets which occur along mid ocean ridges when a propagating segment gradually elongates and takes over spreading from a neighbouring doomed rift segment1 3 represent a significant modification of the plate tectonic paradigm4 the migrating offset zone between the two spreading segments is clearly a locus of significant deformation but the mechanism of this deformation has been controversial here we use the source parameters and locations of earthquakes at such offsets to discriminate between previously proposed models2 4 7 earthquakes at large non transform offsets at medium and fast spreading rates show strike slip mechanisms with nodal planes rotated relative to the expected transform fault orientation one set of nodal planes presumably corresponding to the fault planes is parallel to curved sea floor lineaments which show increased rotation as a function of position in the deforming zone a bookshelf faulting model in which initially ridge parallel faults are rotated by simple shear is consistent with the observed lineament orientations focal mechanisms and earthquake distributions

blocking and deflection of airflow by the alps abstract a method of computing low level trajectories from observed sea level pressure is shown to be capable of distinguishing cases of blocked and non blocked flow around the alps a flow splitting parameter s derived from the trajectories is found to be a reasonable criterion for distinguishing blocked from nonblocked cases with a value of s 1 5 serving as a useful threshold for classification in considering eight cases of postfrontal cold flow against the alps only a few can be clearly identified as blocking or noblocking and even these identifications may not pertain to all levels and all segments of the alpine chain the trajectory based classification scheme compares well with sounding and flight level data from the noaa p 3 research aircraft the degree of blocking is shown to be related to the upstream froude number and the strength of the upstream pressure nose a remarkable result is that the observed surface wind patterns appear blocked in all cases considered here we refer to this

damaging southerly winds caused by barrier jets in the cook strait and wellington region of new zealand abstractstrong southerly winds regularly occur in the cook strait region of new zealand occasionally these winds are strong enough to cause severe damage to property and threaten human life one example of a storm containing such winds is the wellington storm which occurred on 20 june 2013 for this case wind speeds in cook strait were stronger than those observed or forecast elsewhere in the storm even though wind speeds of this intensity are rare storms affecting new zealand with central pressures equal to the wellington storm 976 hpa are not uncommon numerical experiments have been carried out to investigate the possible reasons for the exceptional damaging southerly winds dsws occurring in this storm analyses of the simulations showed that dsws in cook strait for this event were actually barrier jets not gap winds as they appeared the strength of barrier jets in cook strait is sensitive to the precise location of the storm center this explains the uncommon occurrence of dsws in cook

precipitation over concave terrain abstract many topographic barriers are comprised of a series of concave or convex ridges that modulate the intensity and distribution of precipitation over mountainous areas in this model based idealized study stratiform precipitation associated with stratified moist airflow past idealized concave ridges is investigated with a focus on windward blocking flow confluence and the associated precipitation enhancement it is found that flow confluence and precipitation enhancement by a concave ridge are controlled by the nondimensional ridge height m m nmhm u where nm is the moist buoyancy frequency hm is the maximum ridge height and u is the wind speed based on which three dynamical regimes can be defined in the linear regime m 0 4 a flow confluence zone is present over the upwind slope of the ridge vertex where precipitation is significantly enhanced the precipitation enhancement is due to the additional updraft driven by the horizontal flow convergence with a considerable contribution fr

a separating coastal upwelling jet at cape blanco oregon and its connection to the california current system the coastal upwelling region near cape blanco oregon 433n o the west coast of the united states was studied using a towed ctd on seasoar a shipboard acoustic doppler current pro ler adcp satellite sea surface temperature maps and surface drifters during august 1995 the equatorward upwelling jet was inshore of the shelfbreak north of cape blanco meandered gently o shore around the cape and then veered sharply o shore just to the south of the cape analysis of vertical sections of density velocity and spicinessa con rmed the separation of an upwelling jet and front the jet was followed from over the shelfbreak north of the cape to 100 km o shore downstream of the cape the separating jet originates from the coastal upwelling jet leaving the shelf but is augmented by an o shore branching of the poleward undercurrent by combining information from adcp streamfunction maps with analysis of hydrographic data including distributions of the tracer like quantity spicinessa a conceptual model of the three dimensional circulation near the cape emerges a mid shelf upwelling jet encounters a coastal promontory then turns o shore stretching and deepening as it crosses the continental margin the jet then turns back shoreward cyclonically where the deepened equatorward ow encounters the top of the poleward undercurrent owing along the continental slope this causes a portion of the undercurrent to turn o shore to join and strengthen the equatorward transport in the separated jet throughout the separation process the coastal upwelling front and jet are continuous robust features separation of a coastal upwelling jet is an important mechanism for cross shelf transport from the coast to the deep ocean cape blanco appears to be the northernmost point where an equatorward jet regularly separates from the coast to become an oceanic jet and thus serves as the northern boundary of the region where an intense meandering current characteristic of the california current system