acceleration of near surface strong winds in a dry idealised extratropical cyclone to understand near surface strong winds in extratropical cyclones a simulation of a dry idealised baroclinic wave is presented the forces that accelerate the winds are analysed using the terms in the horizontal momentum equation two regions of strong near surface winds developed within the simulation one to the east of the low centre and the other to the southwest the flow to the east resembling the cold conveyor belt accelerated as it passed through the strong pressure gradient associated with the warm frontal zone this acceleration was reduced by friction near the surface the winds to the southwest were characterised by three airstreams one airstream consisted of air parcels that started north of the warm front near the surface accelerated north of the warm front encircled the low and continued to accelerate to the southwest of the low the increases in wind speed in this airstream also part of the cold conveyor belt resulted from the along flow pressure gradient force being greater than friction the second airstream consisted of air parcels that descended west of the cold front and maintained their speed during descent until they were slowed either by friction in the boundary layer or as they moved southward against the local pressure gradient between these two airstreams a third airstream was characterised by air parcels that accelerated during descent from the mid to lower troposphere whilst moving around the bent back front this airstream bears some resemblance to the sting jet and was accelerated by the pressure gradient force on descent until it encountered increased friction or moved against the local pressure gradient

an extreme cold surge over the greek peninsula summary this paper is devoted to the detailed description of an extreme cold surge that occurred 3 13 march 1987 over greece this event has been rated as the worst snowfall over the last 100 years and duc to its severily and persistence it paralysed the cconornic and communal life of greece for several days emphasis is given to the initiation phase of this event the structural evolution of the cold surge is analysed using both observations and model results model simulations pcrformed with the colorado state university regional atmospheric modelling system provided the necessary data to diagnose the mesoscale structure of the cold outbreak different mechanisms involved in the structural evolution ofthe cold surge are investigated the gustiness of the observed winds and their significant departure from geostrophy are related to the role of an important isallobaric wind the progression of this surge presented characteristics of a density current while near the eastern slopes of the mountain barriers of continental greece cold air damming occurred leading to an accelerated flow parallcl to the mountains heavy snowfall events over greece are often associated with cold air surges prop agating from northern and central europe through the balkans metaxas 1978 livada tselepidaki 1979 prezerakos and angouridakis 1984 these cold surges are accompanied with the passage of a fast moving cold front from the north towards the south very strong winds especially in the gaps between mountains in northern greece and over the aegean sea a sharp temperature decrease over the whole country and a sharp pressure rise the lifetime of these surges is normally from 1 to 3 days extending up to 10 days in exceptional cases their persistence can cause important problems in economic life since the result ing snowfall can prohibit road transportation on the main traffic routes greek forecasters call these cold fronts balkan fronts due to their origin in the frame of this study this terminology will be retained although the classical term cold surge will also be used interchangeably the forecast of the evolution of these fronts as well as the persistence of cold surges over greece is of great importance not only for the impact of snowfall on economic life but also for the safety of navigation in the aegean sea in gale force winds cold surges accompanied by severe weather phenomena have also been reported in the himalayan region nakamura and murakami 1983 over the usa e g bell and bosart 1988 colle and mass 1995 as well as over central america schultz et al 1997 all the aforementioned studies underlined the fact that cold surges are accompanied by strong lower tropospheric northerly winds sharp decreases in temperature and sharp pressure rises in these studies some interesting features accompanying cold surges were also ad dressed mainly cold air damming near mountain barriers as a result of the interaction of the synoptic scale flow with topography bell and bosart 1988 concluded that the component of the pressure gradient force parallel to the mountain was the primary cause for the appearance of an accelerated flow nearly parallel to the mountains colle and mass 1995 demonstrated that pressure ridging east of the mountain barrier caused ageostrophic northerly winds while cold avection of upslope flow led to the damming of cold air schultz et al 1997 showed that the extraordinary equatorial extent of a cold surge was aided by topographic channelling similar to cold air damming

the significance of linear trends and clusters of fault related mesothermal lode gold mineralization gold deposits from the laverton region western australia form both linear trends and cluster like distributions in map view associated with regional scale shear zone systems clusters of deposits are common and they form 5 5 km domains spaced every 15 to 20 km in contrast there is also a linear trend of mineralization that can be traced continuously for 20 km along a shear zone in detail mineralization is found in a range of different rock types and is hosted in fault shear zone networks that are smaller than the adjacent regionalscale shear zones by several orders of magnitude a mixture of different fault types comprise these mineralized fault shear networks often within the same gold field although thrust and reverse faults are a common component of the richer deposits e g sunrise dam wallaby granny smith the seismogenic behavior of fault systems during mineralization can explain these observations illustrated by simple stress transfer models that show the location of aftershock distributions around different fault types strike slip ruptures on regional scale structures can repeatedly trigger restricted clusters of aftershocks these aftershock clusters describe vertical pipe like domains in three dimensions in contrast regional scale thrust or reverse ruptures trigger continuous more diffuse domains of aftershocks along the full strike of the rupture thus ruptures along regional strike slip structures are able to trigger ruptures on preexisting thrust faultsshears at the deposit scale when they do so the thrust aftershocks generate a vertical domain of elevated permeability over the depth of the crust where the individual flow paths are likely to be highly tortuous increasing the potential for fluid rock interaction or fluid mixing the existence of linear trends and cluster like distributions of deposits at laverton may be consistent with two different ages of mineralization in this gold camp the results also show that distributions of deposits provide an indication of the kinematics of the regional fault systems that they relate to and that stress transfer modeling is a simple non computationally demanding technique that is easily adapted to exploration for fault related mineral deposits

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

prefrontal wind shift lines in the plains of the united states abstract in the central united states cold fronts are sometimes preceded by a surface wind shift by several hours these wind shifts are examined to determine their origins and characteristics a climatology and a case study of an autumn prefrontal wind shift are presented data collected by the recently implemented oklahoma mesonet and a network of wind profilers permit finescale analyses of the autumn prefrontal wind shift it is found that prefrontal wind shifts are associated with both lee troughs and drylines the climatology reveals that in the central united states fronts are most frequently up to 60 of them preceded by wind shifts just east of the rockies where lee troughs and drylines originate this paper focuses on a case study of a prefrontal wind shift associated with a lee trough that forms when a front approaches the lee trough the prefrontal wind shift associated with the lee trough forms in the lee of the mountains but propagates away from the mountains when downslope flow decreas

evolution of mesoscale precipitation band environments within the comma head of northeast u s cyclones abstract this paper explores the mesoscale forcing and stability evolution of intense precipitation bands in the comma head sector of extratropical cyclones using the 32 km north american regional reanalysis hourly 20 km rapid update cycle analyses and 2 km composite radar reflectivity data a statistical and composite analysis of 36 banded events occurring during the 2002 08 cool seasons reveals a common cyclone evolution and associated band life cycle a majority 61 of banded events develop along the northern portion of a hook shaped upper level potential vorticity pv anomaly during the 6 h leading up to band formation lower tropospheric frontogenesis nearly doubles and the conditional stability above the frontal zone is reduced the frontogenesis increase is primarily due to changes in the kinematic flow associated with the development of a mesoscale geopotential height trough this trough extends poleward of the 700 hpa low and is the vertical extension of the surface warm front and surface

on air motion trajectories in cold fronts abstract we examine air parcel trajectories in the two dimensional model for a cold front by reeder and smith these are found to be in close agreement with trajectories deduced from analyses of summertime cool changes in southeastern australia adding further support to the applicability of the numerical model to this kind of cold front the favorable comparison points also to the dynamical consistency of the conceptual model for the cool change which has evolved from the analysis of data from observational experiments

migration of the 3 d deep seismic reflection survey at the ktb location oberpfalz germany abstract a near vertical deep 3 d seismic survey covering an area of approximately 17 85 19 1 km has been carried out across the german continental deep borehole ktb site this survey is the first of this kind to investigate complicated crystalline crust two major problems were encountered during the post stack 3 d migration of these data first the survey region includes areas with strong lateral velocity gradients which are difficult to treat with many migration algorithms velocities in the sedimentary rocks are 3000 m s and in the crystalline rocks 5500 m s secondly the data are sparsely sampled 50 50 m binning but include steeply dipping events with true dips up to 60 to determine the optimum 3 d migration approach three different algorithms have been tested summation modified kirchhoff finite difference and phase shift results of the various tests suggest that one pass 3 d migration algorithms produce noticeably superior results to the much faster two pass methods the combination of steep dips and sparsely sampled data was a problem for the finite difference migration whereas shallow dipping features were imaged well by this technique although the adopted phase shift migration algorithm could not handle lateral velocity gradients it produced excellent results for both shallow and steeply dipping structures both of these migration algorithms are relatively fast by comparison the summation migration algorithm produced overall good results for regions containing steep dips and strong lateral velocity variations but it requires a lot of computing time

seismic imaging of reflectors in the sg4 borehole middle urals russia the upper crust in the middle urals is known to be highly reflective however it is not clear whether the observed reflections originate from lithological boundaries or from fault zones the sg4 borehole located in the tagil synform of the middle urals drilled to 5354 m as of august 1995 presents one opportunity to study the source of this reflectivity the hole has penetrated several different lithologies as well as numerous tectonic or fracture zones the upper 5070 m consist of island arc rocks which have presumably been thrust on top of basalts to allow correlations between available surface seismic studies and the borehole observations vertical and offset seismic profile data were acquired in the borehole over the interval 520 m to 3940 m two shot points were used a near offset one at 135 m from the wellhead and a far offset one at 1845 m from the wellhead after processing the borehole seismic data show numerous reflecting horizons many reflections correlate with major fracture zones while others have no clear correlation with fracture zones nor lithological contrasts the interval between approximately 4450 m and 5100 m which is imaged below the bottom of the survey shows a layered reflective pattern that correlates with the lower part of a flysch unit which ends at 5070 m the basalts below appear to be relatively transparent a prominent east dipping reflector on a w e cdp line about 700 m north of the wellhead is not clearly imaged on the borehole seismic however in a pilot hole which deviates a few hundred metres in the northerly direction from the main hole a marked low velocity zone is observed at the corresponding depth for this reflector indicating its source to be from a fault zone that the fault zone is not clearly observed on the borehole seismic data is attributed to 3d effects

a study of frontal dynamics with application to the australian summertime cool change abstract the dynamics of frontal evolution is investigated in the context of the australian summertime cool change using a two dimensional numerical model the model is essentially the same as that used by reeder and smith but with different initial conditions and with an open rather than periodic flow domain the initial conditions are an idealization of cross sections through a typical preexisting finite amplitude disturbance prior to its amplification to an intense front trough system basically they consist of a warm prefrontal northerly airstream embedded in a zonal shear flow in thermal wind balance the model develops a quasi steady surface cold front during the 24 hours real time of integration and this front is shown to have many features in common with a kinematic model of the australian summertime cool change the latter model was synthesized from observational data on surface cold fronts obtained during the phase i and ii field experiments of the australian cold fronts research program

q along the fennolora profile summary the fennoscandian long range project fennolora recorded refraction data on a profile from northern norway through sweden to the baltic sea the data have been used to interpret complex crust and upper mantle structure to 450 km depth and the data are unique in scandinavia for their extension and quality in this study crustal q has been determined for p waves along the profile using the spectral ratio method q was found to be frequency dependent of the form q q0fm where q0 and m are constants and f is the frequency in order to simplify the presentation and interpretation of the results and to allow comparison to other studies q was also determined assuming independence of frequency q varied by up to a factor of 3 along the profile with generally increasing values going from north to south this might correlate with more complex structure at the northern segment than at the southern segment average crustal q for the whole profile using all data was q 29f0 96 and the corresponding constant q was 460 since most of the energy in the spectra was from 3 to 30 hz it seems that the frequency independent q is comparable to q f at the average frequency at 16 hz q f 415 the sensitivity of q with depth was investigated using pairs of stations with average distance to shot points of 50 150 and 250 km the average frequency independent q values thus obtained were 430 470 and 480 respectively with a somewhat larger variation for the frequency dependent q values however using the present method and data set there is no indication of significant variation of crustal q with depth this is in contrast to many coda q studies which give increased q with depth

impact of subgrid scale orography on equatorial angular momentum budget and the cold surges in a general circulation model abstractthe dynamical relations between equatorial atmospheric angular momentum eaam equatorial mountain torques and cold surges are analyzed in a general circulation model gcm first the authors show that the global eaam budget is well closed in the gcm much better than in the ncep ncar reanalysis they then confirm that the equatorial torques due to the tibetan plateau the rockies and the andes are well related to the cold surges developing over east asia north america and south america respectively for all these mountains a peak in the equatorial mountain torque components precedes by few days the development of a cold surge confirming that the cold surge s preconditioning is dynamically driven by large scale mountains the authors also analyze the contribution of the subgrid scale orography sso parameterizations and find that they contribute substantially to the torques in experiments where these parameterizations are almost entirely reduced over a given massif the authors find th

regional sources of mountain torque variability and high frequency fluctuations in atmospheric angular momentum abstract the sources of high frequency 14 day fluctuations in global atmospheric angular momentum aam are investigated using several years of surface torque and aam data the midlatitude mountain torque associated with the rockies himalayas and andes is found to be responsible for much of the high frequency fluctuations in aam whereas the mountain torque in the tropics and polar regions as well as the friction torque play a much lesser role on these timescales a maximum in the high frequency mountain torque variance occurs during the cool season of each hemisphere though the northern hemisphere maximum substantially exceeds that of the southern this relationship indicates the seasonal role played by each hemisphere in the high frequency fluctuations of global aam a case study reveals that surface pressure changes near the rockies and himalayas produced by mobile synoptic scale systems as they traversed these mountains contributed to a large fluctuation in mountain torque and a notable high fre

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

the structure and evolution of cold surges east of the rocky mountains abstract northerly surges of cold air often move southward along the eastern side of the rockies from southern canada into mexico the strongest surges which generally develop in midwinter are associated with temperature decreases and pressure rises of 20 30 c and 15 30 mb respectively within 24 h surges are usually accompanied by a meridionally elongated pressure ridge and strong low level ageostrophic winds that parallel the terrain the width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters mexico this paper provides a detailed description of a northerly surge to the east of the rocky mountains that occurred on 12 14 november 1986 using both observational and model data the structural evolution of the surge is analyzed in addition the dynamics of the event is explored by diagnosing the momentum thermodynamic energy and vorticity equations to determine the typical synoptic scale evolution of these

climatology of strong intermountain cold fronts abstract motivated by the intensity and severity of winds and temperature falls that frequently accompany rapidly developing cold fronts in northern utah this paper presents a 25 yr climatology of strong cold frontal passages over the intermountain west and adjoining western united states using conventional surface observations and the north american regional reanalysis strong cold frontal passages are identified based on a temperature fall of 7 c or greater in a 2 3 h period a concurrent pressure rise of 3 hpa or greater and the presence of a large scale 700 hpa temperature gradient of at least 6 c 500 km 1 the number of strong cold frontal passages exhibits a strong continental signature with very few events 10 along the pacific coast and more than 200 events east of the continental divide the number of events increases dramatically from the cascade mountains and sierra nevada to northern utah indicating that the intermountain west is a frequent cold front breeding ground a composite of th

cold surges in tropical and extratropical south america the strong event in june 1994 the authors investigate one case of surges of polar air that occasionally propagate into southeastern brazil during wintertime and are harmful to coffee production because of the freezing conditions associated with them the cooling is also observed in southern and with less intensity in western amazonia the event of 26 june 1994 is studied the frost event caused a sharp drop in coffee production and similarly dramatic increases in coffee prices the event was characterized by the presence of an anticyclonic perturbation off the coast of chile that enters south america and crosses the andes mountains south of 458s hours later the anticyclone center tracks equatorward until it reaches 208 308s and then shifts eastward toward the atlantic based on a diagnostic analysis using the national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis a two stage process can be distinguished in the starting period midtropospheric troughing is established east of the andes over central argentina due to stretching and or vorticity advection this troughing is responsible for cold advection along the eastern flank of the andes which produces substantial temperature drops and deepens the upper trough locally the increase in the magnitude of the zonal temperature gradients east of the cold advection zone at levels near 700 hpa is another result of the cooling process this feature would ensure that the cold advection be extended eastward producing an upper level trough local deepening there this intensification would produce an increase in the cyclonic vorticity advection which would tend to produce sea level pressure drops underneath this near surface low pressure area would eventually contribute to the southeastern brazil sb cooling due to the associated southerly winds another feature is the near surface local anticyclonic generation due to midtropospheric cold advection associated with descending motions low level divergence and anticyclonic growth eventually this high pressure would also contribute to produce southwesterlies to transport cold air to sb the paper is divided in two sections the first part is a study of synoptic and climatic aspects of the cold surge episodes by using daily surface climatic observations the purpose of the second part is to analyze the dynamic aspects of this cold episode and to study the cooling mechanisms by using the four times daily surface and upper air ncep ncar reanalysis as well as to look for possible predictors

a review of cold fronts with prefrontal troughs and wind shifts the conceptual model of a classical surface based cold front consists of a sharp temperature decrease coincident with a pressure trough and a distinct wind shift at the surface many cold fronts however do not conform to this model time series at a single surface station may possess a pressure trough and wind shift in the warm air preceding the cold front hereafter called a prefrontal trough and prefrontal wind shift respectively although many authors have recognized these prefrontal features previously a review of the responsible mechanisms has not been performed to date this paper presents such a review ten disparate mechanisms with different frontal structures have been identified from the previous literature these mechanisms include those external to the front i e those not directly associated with the cold front itself synoptic scale forcing interaction with lee troughs drylines interaction with fronts in the mid and upper troposphere and frontogenesis associated with inhomogeneities in the prefrontal air mechanisms internal to the front i e those directly associated with the structure and dynamics of the front include the following surface friction frontogenesis acting on alongfront temperature gradients moist processes descent of air ascent of air at the front and generation of prefrontal bores gravity waves given the gaps in our knowledge of the structure evolution and dynamics of surface cold fronts this paper closes with an admonition for improving the links between theory observations and modeling to advance understanding and develop better conceptual models of cold fronts with the goal of improving both scientific understanding and operational forecasting

trace element and sulfur isotopic evidence for redox changes during formation of the wallaby gold deposit western australia abstract wallaby is a major gold deposit of the st ives gold field of western australia with an estimated resource of 8 million ounces of gold it has a well established paragenesis across five vein sets that displays macroscopic evidence of changing redox through time from hematite to magnetite micro analysis of pyrite from each vein generation shows a progressive and gradual change in redox conditions the sulfur isotope composition has a 34 s range of 7 7 to 9 8 using 3 m spots on the sensitive high resolution iron micro probe stable isotope shrimp si negative values indicative of an oxidized sulfur signature are found in the earliest generation of pyrite which coexists with hematite that also contains high concentrations of as ni zn ag sb cu and pb conversely positive values representative of reduced sulfur are found in later generations of pyrite with lower concentrations of as ni sb cu zn and pb these later pyrite crystals display higher as ni as sb and as bi and lower cu te these geochemical trends are the result of redox controlled transport and partitioning into pyrite of minor and trace elements now within the pyrite structure previous studies suggested a single orogenic event formed the wallaby deposit this is not supported by the present study trace element ratios such as as ni clearly delineate the high au generations and could be used as an exploration tool it is suggested that pyrite from the wallaby gold deposit formed via desulfidation and a gradual change in redox conditions within an evolving fluid and did not result from mixing of two separate fluids as previously advocated utilizing pyrite to link the entire fluid history of wallaby demonstrates the general use of pyrite as a valuable mineral tracer in gold bearing fluid systems

satellite observations of the wind jets off the pacific coast of central america part i case studies and statistical characteristics measurements of near surface winds by the nasa scatterometer nscat from october 1996 through june 1997 are analyzed to investigate the three major wind jets along the pacific coast of central america that blow over the gulfs of tehuantepec papagayo and panama each jet is easily identifiable as locally intense offshore winds in the lee of low elevation gaps through the sierra madre mountain range the jets have relatively narrow cross stream width but often extend several hundred kilometers or more into the pacific the tehuantepec and papagayo jets sometimes merge with the northeast trade winds of the pacific the tehuantepec jet was highly energetic with characteristic timescales of about 2 days events were triggered by high pressures associated with cold surges into the gulf of mexico that originated over the great plains of north america the papagayo and panama jets were much more persistent than the tehuantepec jets the winds at both of these lower latitude locations exhibited a strong seasonal variation with almost exclusively offshore flow from late november 1996 through late may 1997 and periods of onshore flow in october and november during the late stages of the 1996 central american monsoon season superimposed on this low frequency seasonal variation were events with characteristic timescales of a few days based on nscat data the spatial and temporal evolution of major wind events is described in detail for three representative case studies in december 1996 the jets developed sequentially from north to south consistent with the notion that wind events in the two lower latitude jets are associated with cold air outbreaks that trigger the tehuantepec jet a day or so earlier in november 1996 and march 1997 the papagayo and panama jets were strongly influenced by tropical phenomena that had little apparent association with the tehuantepec jet these latter two case studies together with the distinction between the statistical characteristics of the three jets suggest that the papagayo and panama jets are predominantly controlled by a mechanism that is very different from the across gap pressure gradients associated with high pressure systems of midlatitude origin that control the tehuantepec jet

doppler radar observations of non occluding cyclic vortex genesis within a long lived tornadic storm over southern england on 3 november 2009 a long lived tornadic storm developed close to the apex of a small frontal wave moving eastwards over southern england ten tornadoes and several instances of non tornadic wind damage occurred along a narrow path extending for over 120 km the storm passed within 5 km of two of the radars in the met office s network providing an opportunity for detailed observation initial development was in the form of an intense segment of line convection at the northern flank of the frontal wave and associated deepening surface mesolow diameter 60 70 km the line evolved rapidly the southern portion bowed out to form a transient s shaped echo before dissipating the northern section persisted leaving a hook shaped storm which became quasi steady in overall echo shape radars revealed the cyclic development of five discrete low level mesovortices diameter 1 4 km between 1130 and 1330 utc along a zone of intense cyclonic vertical vorticity at the southeastern edge of the storm these vortices intensified rapidly typically from initial detection to maximum observed differential velocity in 10 min some attaining low level differential velocities exceeding 40 m s 1 however significant differential velocities 20 m s 1 were not observed above 2 5 km above ground level and no persistent mid level mesocyclone was present the cyclic behaviour was unusual in that vortices were non occluding moving southeastwards relative to the storm to the right of the storm motion vector away from the core this is in contrast to the more commonly observed evolution of vortex occlusion by storm downdraughts and subsequent leftward and rearward storm relative movement during decay copyright 2011 british crown copyright the met office published by johnwiley sons ltd

assessing the applicability of the tropical convective stratiform paradigm in the extratropics using radar divergence profiles abstractlong term radar observations from a subtropical location in southeastern texas are used to examine the impact of storm systems with tropical or extratropical characteristics on the large scale circulation climatological vertical profiles of the horizontal wind divergence are analyzed for four distinct storm classifications cold frontal cf warm frontal wf deep convective upper level disturbance dc uld and nondeep convective upper level disturbances nc uld dc uld systems are characterized by weakly baroclinic or equivalent barotropic environments that are more tropical in nature while the remaining classifications are representative of common midlatitude systems with varying degrees of baroclinicity dc uld systems are shown to have the highest levels of nondivergence lnd and implied diabatic heating maxima near 6 km whereas the remaining baroclinic storm classifications have lnd altitudes that are about 0 5 1 km lower analyses of climatological mean divergence profiles are also s

pressure dependent seismic reflection amplitude changes in crystalline crust lessons learned at the continental deep drilling site ktb summary we conducted an active seismic experiment aimed at measuring changes in seismic reflection amplitudes as a consequence of fresh water injection and corresponding pressure changes at the german continental deep drilling site ktb the injection took place at the bottom of the 4 km deep pilot borehole in the se2 fault zone in crystalline rock units between the springs of 2004 and 2005 prior to the experiment theoretical calculations indicated a possible increase in the compressional wave reflection coefficient as a result of an injection induced reduction of the seismic velocities within the fault zone despite good repeatability of the emitted source signals the experiment suffered from missing the clear reflection signals expected from the fault zone with regard to seismic data from past experiments applying various data processing steps did not enhance the signals enough to obtain clear reflections or even pressure dependent reflection amplitude changes the signal to noise ratio remains smaller than the effects under observation provided that reflections are present in the data the error bar of the recorded signals is of the order of 100 per cent therefore we conclude that the experiment was not successful in seismically measuring pressure variations however important lessons for land seismic time lapse measurements in crystalline environments have been learned i the source should be capable of emitting frequencies below 30 hz ii the detector array setup proved to be partly questionable because in a scattering environment like the crystalline rocks at the ktb site the incidence of a plane wave precondition might be violated for high frequency signals iii near surface variations of elastic properties likely influence seismic monitoring iv using a step function that is a first order pressure discontinuity to model the subsurface pressure build up is very likely too simple an approach

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

the australian summertime cool change part ii mesoscale aspects abstract observations of four cold frontal systems traversing the coastal region of southeast australia in late spring and early summer are described in terms of process occurring on the mesoscale a conceptual model is presented which summarizes the main results of the data analysis features found in common with other studies of cold fronts include i the multiple line nature of the frontal transition zone ftz ii concentration of cyclonic relative vorticity at a height z 1 to 1 5 km in the rear of the ftz and iii the existence of a prefrontal jet at z 1 5 km northerly in our case southerly in the northern hemisphere the change lines within the ftz and at the leading edge if there is no sea breeze are most probably convective instability lines whose alignment and movement depend on the large scale cloud layer winds the lines are evident as mesoscale cloud bands from satellite imagery and as rainbands from radar at least one of these develops into a vigorous squall line whose cold outflo

an observational and numerical study of a cold front interacting with the olympic mountains during coast iop5 abstract this paper documents the three dimensional flow and precipitation structures associated with a weak cold front interacting with the olympic mountains and the subsequent development of a puget sound convergence zone this study utilizes data collected during coast iop5 the fifth intensive observing period of the coastal observation and simulation with topography field experiment that took place on 11 12 december 1993 one of the most important data sources was a noaa p 3 aircraft which provided flight level data radar reflectivity and doppler winds as it circumnavigated the olympics initially frontal passage along the western foothills of the olympics was accompanied by a 2 3 c temperature drop a rapid wind shift to northwesterlies and an intense line of precipitation 35 45 dbz however the wind shift and associated precipitation structures attenuated when the front began to ascend the windward slopes of the olympics surface and p 3 observations document the deformation of the front

a technique for representing three dimensional vertical circulations in baroclinic disturbances abstract the problem of representing vertical circulations in frontal zones is reexamined with the objective of devising a methodology sufficiently general to apply in situations where these circulations are no longer confined to the cross front transverse vertical plane and therefore must be viewed as fully dimensional the proposed methodology which builds upon the earlier work of hoskins and draghici and of eliassen consists of adopting a vector streamfunction that describes the vertical velocity and the horizontal irrotational flow this generalized streamfunction referred to as the psi vector may be determined uniquely from the vertical velocity field over a limited region provided that suitable lateral boundary conditions on the velocity potential for the irrotational part of the horizontal velocity can be specified a key property of the psi vector is that its projections onto arbitrary orthogonal vertical planes yield two independent vertical circulations providing an objective means for se

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

a comparison of the finescale structures of a prefrontal wind shift line and a strong cold front in the southern plains of the united states abstractthe objectives of this study are to determine the finescale characteristics of the wind and temperature fields associated with a prefrontal wind shift line and to contrast them with those associated with a strong cold front data from a mobile polarimetric x band doppler radar and from a surveillance s band radar temperature profiles retrieved from a thermodynamic sounder and surface observations from the oklahoma mesonet are used to analyze a prefrontal wind shift line in oklahoma on 11 november 2013 data from the same mobile radar and the oklahoma mesonet are used to identify the finescale characteristics of the wind field associated with a strong surface cold front in oklahoma on 9 april 2013 it is shown that the prefrontal wind shift line has a kinematic and thermodynamic structure similar to that of an intrusion elevated density current while the cold front has a kinematic structure similar to that of a classic density current other characteristics of the prefrontal wind shift line

three dimensional baroclinic instability and summertime frontogenesis in the australian region the structure of a mature three dimensional baroclinic wave and its attendant cold front is examined using an idealized numerical model with a simple parametrization of dry buoyant convection resulting from diurnal heating relative flow isentropic analyses of the numerical solution and of an observed australian summertime frontal system are compared it is shown that the model well represents the typical synoptic environment accompanying frontogenesis in the australian region a diagnosis of the three dimensional vertical circulation is obtained using a technique involving a vector streamfunction the circulation in a cross sectional plane parallel with the low level temperature gradient is found to be highly two dimensional with geostrophic confluence constituting the principal frontogenetical forcing at low levels while at upper levels the frontogentical forcing is dominated by geostrophic horizontal shear in a second cross section taken zonally the frontogenetic forcing is found to consist almost solely of geostrophic horizontal shear throughout the entire troposphere however in this case the circulation in the plane of cross section is found to be only qualitatively two dimensional as a significant proportion of the low level postfrontal subsidence is associated with circulation normal to the cross section it is shown that the frontal structure analysed is sensitive to the location of the chosen cross sectional plane and that this in turn provides an explanation for the observed relative shallowness of australian summertime fronts the effects of diurnal heating are important but confined to the surface based mixed layer with the addition of diurnal heating the prefrontal updraught amplifies almost threefold and the frontal temperature contrast over the heated region approximately doubles the diabatic forcing term in the well known sawyer eliassen equation is shown to be negligible this suggests that the increased low level convergence is a reflection of the model s enhanced response to frontogenetic forcing in the presence of low static stability in the surface based well mixed layer finally the numerical model solution is compared with a simpler two dimensional model specifically developed to study australian summertime fronts

topographic effects on a wintertime cold front in taiwan abstract this paper describes an observational and numerical study of an intense wintertime cold front that occurred in taiwan on 8 january 1996 the front was associated with rope clouds at the leading edge and a broad area of stratiform clouds behind the front was blocked by the central mountain range of taiwan and divided into two sections on each side of the mountain range as the cold air moved southward along the east coast the increasing westward coriolis force induced a landward acceleration after the cold air piled up against the mountains a coastal pressure ridge developed the cold air damming yielded a geostrophic balance between the westward coriolis force and the eastward component of the pressure gradient force in the x direction and a southward acceleration in the y direction mainly caused by the southward pressure gradient force component over the taiwan strait southward pressure gradient forces increased when the low level stable cold air was confined over the taiwan strait lead

a mechanism for african monsoon breaks mediterranean cold air surges 1 surges of cold air from the mediterranean into northern africa during the boreal summer are documented and their influence on monsoon breaks is analyzed using tropical rainfall measuring mission rainfall estimates and reanalysis products between 1998 and 2006 6 10 cold air surges occurred each summer with low level temperature anomalies ranging from less than 1 k to over 6 k composite analysis indicates that cold air surges over northern africa persist for 2 10 days and travel equatorward at approximately 5 5 m s 1 which is 0 5 1 5 m s 1 faster than the observed climatological low level meridional flow northern african cold surges have characteristics similar to surges observed elsewhere in the world including a hydrostatically induced ridge of surface pressure and an amplified upper tropospheric ridge trough pattern the african cold surge is preceded by the passage of a shortwave trough and an intensification of the upper tropospheric subtropical westerly jet streak over the mediterranean sea these events are associated with increased confluence in the jet entrance region over the central mediterranean an enhanced direct secondary circulation subsidence and low level ageostrophic northerly flow over northeastern africa composite analysis shows that the passage of a cold surge is associated with an enhancement in convective activity over southern algeria western niger northern mali and mauritania 2 to 5 days before the surge reaches the eastern sahel 17 5 n when northeasterly flow channeled between the atlas and ahaggar mountains strengthens and transports relatively moist air from the western mediterranean and eastern north atlantic over the region and increases moisture convergence over western africa north of 20 n over the eastern sahel of sudan and eastern chad the composite results reveal a break in convective activity and decrease in low level convergence when the surge arrives that persists for about 6 days these results offer great promise for improving the short range prediction of rainfall over northern africa

seismic wave attenuation in metamorphic rocks from vsp data recorded in germany s continental super deep borehole vertical seismic profiling vsp data recorded in the german continental deep borehole ktb between 3 and 6 km have been used to study the attenuation of p waves a vibrator source was used to generate the data in this depth range the lithology is dominated by a metabasite unit amphibolite and metagabbro with various degrees of alteration intercalated with paragneisses the method used to determine attenuation is based on the fit of a straight line to the logarithm of amplitude ratios for variable depth and fixed frequency inspection of the corresponding attenuation curves showed sharp discontinuities which correlate with changes in the seismic source mostly related to changes in location as recorded by a nearby monitor geophone to avoid the uncertainties introduced by these changes the analysis was restricted to a depth interval 3576 4538 m for which the source was not moved even in this case however the monitor traces showed significant variations in their frequency content although not much variation in their peak time domain amplitudes to account for these variations the attenuation coefficients obtained for the actual data were corrected for source variations by subtraction of the attenuation coefficients determined from the analysis of synthetic vsp data computed using source wavelets extracted from the monitor traces after these corrections the values of q for frequencies between 7 8 and 46 9 hz are between 14 and 32 these values are quite low and may be related to the presence of fluids known to exist in the depth range under consideration although scattering processes may also contribute to the high attenuation the variation of q with frequency may be the result of processing artifacts

summertime cold fronts in southeast australia behavior and low level structure of main frontal types abstract the cold fronts research programme cfrp concentrated upon cold frontal systems synoptic scale fronts occurring in southeast australia during late spring and early summer november and december many of these tend to the complex with change lines associated with sea breezes prefrontal squall lines and related cold outflows garratt et al the present paper extends the earlier work to include additional observations of frontal events from field phase 3 of the cfrp held in late 1984 in addition we distinguish between this frontal system type 1 and a frontal system type 2 occurring throughout summer but predominantly late in the season which is often dry with a single major change line identified as a mesoscale coastal front 0bservations from the cfrp and other sources are described covering a wide range of frontal intensities with frontal speeds varying between 5 and 25 m s 1 the nature of the low level flow behind and in the vicinity of the active surface cold front scf is

a short lived cold front in the southwestern united states a cold front developed in arizona in a region of initially small temperature gradient developing to great intensity accompanied by damaging winds over new mexico and then losing strength over the high plains of oklahoma and texas the entire development of frontogenesis and frontolysis occurred in no more than 24 h the initial growth of temperature contrast was attributable mainly to horizontal variation of surface heat flux during the morning with little heating in a region of dense cloud cover and scattered showers in the west and with intense heating in a region of only thin high clouds to the east the accompanying ageostrophic circulation then resulted in a collapse toward discontinuity the frontal zone maintained an approximately steady state for a few hours in early afternoon at this time the westerly component of surface wind just ahead of the zone was not as strong as the eastward motion of the zone the passage of the zone was accompanied by a veering and strengthening of the surface wind so that westerly components were briefly larger than the frontal motion the tendency of the convergent wind field to produce a frontogenesis was evidently balanced by small scale mixing subsequently the pressure trough and surface wind shift propagated eastward more rapidly than the frontal temperature contrast the contrast quickly weakened as the mixing then was unopposed severe convection developed during the evening as the convergent surface wind shift came into contact with humid unstably stratified air

a comparison between frontogenesis in the two dimensional eady model of baroclinic instability and summertime cold fronts in the australian region a two dimensional anelastic numerical model with a simple turbulence parametrization is integrated with eady s linear normal mode solution for an unstable baroclinic wave as initial condition the structure of the surface front which develops after five days of model integration is compared with data on summertime cold fronts or cool changes in south eastern australia obtained during recent field experiments of the australian cold fronts research programme it is shown that the ridge trough structure of an amplifying baroclinic wave and its attendant surface front captures many of the important features of cool changes together with the broad scale flow in which they develop indeed it is argued that the model provides a useful theoretical framework in which the dynamics of the australian summertime cool change may be understood

the structure and evolution of a simulated rocky mountain lee trough abstract this paper describes the life cycle of a lee trough associated with the passage of a baroclinic wave over the rocky mountains based on two overlapping simulations by the pennsylvania state university national center for atmospheric research mesoscale model the evolution of the lee trough may be summarized as follows a surface cyclone and lee trough developed and intensified as an upper level short wave trough approached and moved over the rockies near the low center the lee trough was found to be frontogenetical with the frontogenesis being forced by 1 confluent deformation associated with the wind fields of the developing cyclone and lee trough and 2 differential vertical motion along the eastern slopes of the rocky mountains and the adjacent sloping terrain of the high plains in turn the frontogenesis drove an ageostrophic circulation that sharpened the lee trough the lee trough represented a break in trajectory origin with trajectories ending ahead of the lee trough originating over

planetary and synoptic scale signatures associated with central american cold surges abstract motivated by outstanding issues from a previous case study of a midlatitude cold surge that affected mexico and central america the climatology of central american cold surges is examined in this paper an independently derived listing of 177 cold surge events is employed for which the following properties are tabulated onset date duration time between cold surge events latitude of maximum equatorward penetration min and 48 h maximum surface temperature change at merida mexico t these data show that 75 of the cold surges have durations of 2 6 days the same timescale as mobile disturbances in the westerlies also there does not appear to be any relationship between t and the duration of the event although cold surges that penetrate to low latitudes min 7 10 n have a weak tendency to persist longer than those that do not penetrate to low latitudes min 15 20 n in addition the reding data indicate that the cold surges tend to reach their most equatorward extent where

an observational and numerical study of an intense landfalling cold front along the northwest coast of the united states during coast iop 2 abstract this paper presents an observational and numerical study of an intense wintertime cold front that made landfall along the northwest coast of the united states during iop 2 3 4 dec 1993 of the coastal observation and simulation with topography experiment observations suggest that the offshore frontal zone was associated with two transitions the first characterized by a substantial temperature gradient a nearly vertically oriented wind shift from southwesterly ahead to westerly behind and a convective rainband while the second transition had a slantwise wind shift zone from westerly ahead to northwesterly behind the frontal zone was quite narrow 5 km wide and nearly vertical below 850 mb and its width increased by 1 2 orders of magnitude above 850 mb well before the frontal landfall low level flow to the west of the olympics was associated with geostrophic balance in the cross shore direction and downgradient acceleration in the alongshore direction which contributed to the formation

mesoscale dynamics of the near dryline environment analysis of data from cops 91 abstract a technique is introduced using surface data from triangular networks of adjacent mesonet stations to estimate the terms in the horizontal equation of motion at anemometer level 10 m on either side of the dryline in the southern plains of the united states data from an instrumented surface mesonetwork in the texas panhandle and western and central oklahoma during the cooperative oklahoma profiler studies cops 91 field program were employed for this purpose east of the dryline and surface pressure trough the vertical mixing term had a component normal and to the right of the surface wind in accord with ekman theory west of the dryline and surface pressure trough the vertical mixing term had a component normal and to the left of the surface wind in disagreement with ekman theory it is suggested that disagreements with ekman theory may be due to baroclinic effects in the boundary layer it is also shown that during the day both the westward component of the pressure gradient force and the ea

cold fronts with and without prefrontal wind shifts in the central united states time series of cold fronts from stations in the central united states possess incredible variety for example time series of some cold fronts exhibit a sharp temperature decrease coincident with a pressure trough and a distinct wind shift other time series exhibit a prefrontal trough and wind shift that precedes the temperature decrease associated with the front by several hours in early march 2003 two cold fronts passed through oklahoma city oklahoma okc representing each of the above scenarios the cold front on 4 march was characterized by a coincident sharp wind shift pressure trough and a strong temperature decrease of 108c in 2 min on the other hand the cold frontal passage on 8 march was characterized by a prefrontal wind shift occurring over a 7 h period before the temperature decrease of 108c in 2 h twelve hours before frontal passage at okc both fronts had the same magnitude of the horizontal potential temperature gradient and petterssen frontogenesis by the time of frontal passage at okc the magnitude of the horizontal potential temperature gradient for the 4 march front was double that of the 8 march front and the frontogenesis was nearly four times as great the simultaneity of the surface horizontal potential temperature gradient and deformation and convergence maxima coincident with the wind shift was primarily responsible for the greater strength of the cold front in okc on 4 march compared to that on 8 march whether a prefrontal wind shift occurred was determined by the timing and location of cyclogenesis in the central united states on 4 march a cyclone was adjacent to the slope of the rocky mountains and developed on the cold front as it moved through oklahoma permitting greater frontogenesis and resulting in a cold frontal passage at okc with a simultaneous temperature decrease and wind shift on 8 march the cyclone moved eastward through oklahoma before the arrival of the cold front resulting in a prefrontal wind shift associated with the northerlies behind the cyclone followed by the frontal passage a 2 yr climatology of cold frontal passages at okc supports the results from the two cases above indicating that the timing and location of cyclogenesis was responsible for these two different cold frontal structures these results imply that for situations resembling those of this study the prefrontal trough is not directly associated with the cold front but is caused by external processes related to the lee troughing

seismic reflections from the crystalline crust below the continental deep drilling site ktb modeling and inference on reflector properties 1 the three dimensional 3 d reflection seismic data set iso 89 3d was recorded near the deep borehole ktb in southeastern germany reflections from the se1 reflector and from the top of the erbendorf body eb in the upper crystalline crust can be identified in 5 10 of the single shot sections the reflectors have been first identified in previous studies of stacked and migrated seismic data in this paper the velocity and density variations of these two structures are estimated in a new way using true amplitude single shot vibroseis data the method uses the direct wave pg as a reference phase and models the amplitude ratios of the se1 and eb reflections to pg modeling in this paper uses a combination of ray theory and the reflectivity method and the se1 and the top of the eb are assumed to be obliquely oriented 1 d structures pg modeling shows that a depth dependent velocity function within the uppermost crystalline basement explains the amplitudes and travel times of this phase with sufficient accuracy the largest observed amplitude ratios se1 pg and eb pg are explained by laminated models with strong velocity contrasts and with reflection coefficients of magnitude 0 1 0 2 se1 and 0 05 0 15 eb the total thickness of the reflecting zones is less than 300 m pg amplitude modeling requires low qp factors 100 to a depth of 1 km whereas at larger depths values of several hundred are necessary to keep the se1 and eb velocity contrasts in realistic ranges both reflectors can be interpreted as cataclastic zones for the se1 this interpretation agrees with the view that it is a steeply dipping thrust fault which continues the tectonic franconian lineament into the upper crust we assume that the eb is the fractured top of a high velocity zone at depths below 10 km known from earlier wide angle measurements both reflectors have large weakly reflecting or nonreflecting parts the se1 is nonreflecting at the intersection with the ktb borehole

vertical seismic profile results from the kola superdeep borehole russia multi offset vertical seismic profiles vsps from the kola superdeep borehole sg 3 as part of a larger seismic study of the kola region conducted during the spring of 1992 sample the dipping pechenga complex from 2175 m to 6000 m and contribute to the understanding of reflectivity in crystalline and precambrian environments from the surface to 6000 m the sg 3 borehole penetrates interlayered proterozoic metavolcanic and metasedimentary units and a mylonitic shear zone ranging from greenschist to amphibolite metamorphic grade respectively the kola vsps display a 6 velocity decrease which coincides to a mylonitic shear zone located between 4500 m and 5100 m within the sg 3 borehole seismic interfaces are identified by mode converted energy ps and sp transmissions and reflections in addition to primary seismic phases the vsp shear wave energy is generated at or near the source by vertical vibrators p wave and s wave reflections are generally detected from the same reflecting horizons but increases in relative s wave and sp reflection amplitudes originate at 1900 m 3800 m 4500 m and 5100 m depths these depths coincide with zones of elevated vo vs and may support the presence of free pore fluid which is reported from initial drilling for the proterozoic lithologies sampled by the vsp reflection events result from five mylonitic shear zones and three lithologic contrasts

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 tropospheric gravity waves created by leeside cold fronts abstract the water vapor channel of the geostationary operational environmental satellite goes 8 reveals narrow 30 km wide elongated 500 1000 km bands that propagate southward on the eastern side of the rocky mountains from colorado to texas two events in which surface and wind profiler observations show that these bands are associated with leeside cold fronts are documented in detail and several other cases are summarized the wind profilers observe vertical motions exceeding 1 m s 1 in narrow plumes at the leading edge of the fronts in broader zones in the upper troposphere and in the lower stratosphere these cause vertical displacements of up to 1 km and are responsible for the signature in water vapor images the bands occur when the rocky mountains block either arctic leeside cold fronts coming from the north or northeast or pacific cold fronts coming from the northwest the blocking changes the frontal orientation and disrupts geostrophic thermal wind balance near the terrain modified fron

seismic attenuation above 10 hz in southern california from coda waves recorded in the cajon pass borehole we estimate the attenuation around cajon pass southern california for frequencies above 10 hz and we find total q to exhibit only weak frequency dependence ranging from 800 at 10 hz to 1500 at 100 hz the intrinsic attenuation is approximately twice the level of the scattering attenuation measurements are made using earthquake seismograms recorded at 0 3 km depth using the multiple lapse time window method the results are not dependent on receiver depth and are consistent with previous estimates of q made from direct waves recorded at 2 5 km depth our q values are therefore thought to be representative of the seismogenic crust and the technique used is uncontaminated by the highly attenuating near surface at least above 10 hz we also calculate q from surface data between 1 and 10 hz and find a clear change in the frequency dependence of both intrinsic and scattering attenuation at 10 hz q exhibits strong frequency dependence below 10 hz 1 8 consistent with previous studies in active tectonic regions and only weak frequency dependence at higher frequencies 0 34 this change in behavior renders it unwise to extrapolate q measurements outside the frequency range from which they were derived for example in earthquake source studies possible factors responsible for the apparent change in frequency dependence of q are considered further work is required however to resolve the causes of this change

an overview of barrier winds off southeastern greenland during the greenland flow distortion experiment during the greenland flow distortion experiment barrier flow was observed by an instrumented aircraft on 1 2 5 and 6 march 2007 off southeastern greenland during this time period the barrier flow increased from a narrow jet 15 m s 1 to a jet filling almost the whole of the denmark strait with maximum wind speed exceeding 40 m s 1 dropsonde observations show that the barrier flow was capped by a sharp temperature inversion below mountain height below the inversion was a cold and dry jet with a larger northerly wind component than that of the flow above which was also warmer and more moist thus the observations indicate two air masses below mountain height a cold and dry barrier jet of northern origin and above this a warmer and moister air mass that was of cyclonic origin numerical simulations emphasize the non stationarity of the greenland barrier flow and its dependence on the synoptic situation in the greenland iceland region they show that the barrier jet originated north of the denmark strait and was drawn southward by a synoptic scale cyclone with the strength and location of the maximum winds highly dependent on the location of the cyclone relative to the orography of greenland experiments without greenland s orography suggest a 20 m s 1 enhancement of the low level peak wind speeds due to the presence of the barrier thus the greenland barrier flows are not classic geostrophically balanced barrier flows but have a significant ageostrophic component and are precisely controlled by synoptic scale systems copyright c 2009 royal meteorological society

frontal analysis in the light of abrupt temperature changes in a shallow valley abstract the authors examine selected cases of abrupt temperature changes in a shallow valley in rural oklahoma and examine their wider associations all changes in the valley whether rise or fall are accompanied by a northerly wind shift at or shortly before the time of the abrupt change cases of cooling show a drop of as much as 20 f 11 c in less than 1 h the pattern of surface potential temperature over the central united states indicates that these cases represent ideal cold front passages although little or no weather accompanies them cases of abrupt warming which are entirely nocturnal on the other hand are associated with weak regional temperature gradients and with strong nocturnal cooling prior to the event under nearly clear skies and with light winds the imputed surface inversion does not occur at a nearby urban location and the breakdown is an important factor in the temperature rise at the rural site the important distinction between these cases and the cold front passages that

extratropical tropical interaction during onset of the australian monsoon reanalysis diagnostics and idealized dry simulations abstract the onset of the australian monsoon is examined using i reanalysis data for seasons when enhanced observational networks were available and ii a 15 yr onset composite similar to previous findings onset is characterized by a sudden strengthening and deepening in tropical westerly winds which are overlain with upper tropospheric easterlies all onsets are preceded by up to a 7 day preconditioning period of enhanced vertical motion and moistening during the transition season the 6 weeks prior to onset a number of moist westerly events occur generally they are only sustained for short periods and overlain by upper level westerly winds suggesting an association with midlatitude troughs which protrude into the deep tropics for individual years and for a 15 yr composite monsoon onset is associated with major cyclogenesis events over the southwest indian ocean in the presence of a subtropical jet over the eastern indian ocean the proposed mechanism for extratropical tropical interaction i

characterization of seismic reflections from faults in a crystalline environment schneeberg germany we present an approach for analysing seismic reflections from faults in a crystalline hard rock environment we analysed 3d seismic reflection data for geothermal reservoir characterization acquired in the erzgebirge region germany the seismic image derived from this data set revealed two main features a less pronounced reflector corresponding to a steeply dipping major fault zone roter kamm and a group of pronounced reflectors attributed to the existence of conjugate mineralized faults we analysed these reflections in the pre stack data to characterize the nature and origin of reflectivity this was done by extracting the corresponding waveforms from the raw data and carefully pre processing them including amplitude correction for geometrical spreading and signal to noise enhancement reflection coefficients were derived from the pre processed shot gathers by comparing the amplitudes of the reflected and direct waves synthetic waveform modelling using the reflectivity method has been performed for several model families consisting of one dimensional velocity depth functions with varying velocities densities and thicknesses of the layers a comparison of the modelled and observed waveforms revealed that a reflection coefficient of 0 18 for the conjugate mineralized faults can be explained by single layers with high impedance contrast and a thickness between 30 m and 40 m whereas the reflection from the roter kamm fault zone with a reflection coefficient of 0 23 requires a model consisting of several low velocity layers with a total thickness of up to 100m embedded in a high velocity background model these results are in accordance with the geological interpretation of these reflectors however the characteristics of these reflections vary significantly within the investigation area both in terms of the reflection coefficient and the waveform which is also in agreement with the general lateral variation of fault zone characteristics known from tectonic investigations such as geological mapping of outcrops and fabric analysis

mesoscale modeling of a cold front and its interaction with a diurnally heated land mass abstract a mesoscale numerical model incorporating a detailed planetary boundary layer scheme including momentum heat and moisture exchange with the lower boundary is used to study the change in structure of a dry summertime front as it moves towards a coastline two dimensional experiments show that the diurnal pressure fall over land causes a trough to form in the coastal region ahead of the approaching front frontogenesis at this mesoscale trough leads to the formation of a new front near the coast giving the appearance of strong acceleration of the original frontal system in the offshore region this process can occur over a wide range of times from early afternoon to late evening but no such acceleration occurs for fronts crossing the coast in the early morning hours these results indicate a tendency for more fronts to cross the coastline in the afternoon early evening period than at other times a statistic which is observed in southeast australia and oregon usa they also imply fronts sho

the life cycle of an undular bore and its interaction with a shallow intense cold front abstract the evolution of an undular bore and its associated wind shift spawned by the passage of a shallow surface cold front over the southern great plains of the united states is examined using surface and remote sensing observations along with output from a high resolution numerical model simulation observations show that a separation between the wind shift and thermodynamic properties of the front was induced by the formation of a bore over south central kansas around 0200 utc 29 november 2006 by the time the front bore complex passed through lamont oklahoma approximately 4 h later the bore had reached its maximum intensity and its associated wind shift preceded the trailing baroclinic zone by 20 min within several hours the bore decayed and a cold frontal passage characterized by a wind shift coincident with thermodynamic properties was observed at okmulgee oklahoma thus a substantial transformation in both the structural and dynamical characteristics of the bore as well as its relations

coincident normal incidence and wide angle reflections from the moho evidence for crustal seismic anisotropy abstract we explore the consequences of interpreting wide angle crustal seismic data assuming isotropic models when the real crust may be anisotropic we have used a simple anisotropic model to generate synthetic travel times and have inverted these assuming an isotropic layered model for the crust we show that neglecting the effects of anisotropy can produce estimates of crustal thickness and crustal velocity structure that are significantly in error for realistic levels of anisotropy 10 the error in crustal thickness can be several kilometres and the error in average velocity can be 0 5 km s we show that isotropic inversion of anisotropic data can lead to an apparent mismatch between the position of the moho inferred from normal incidence and wide angle data as an example of such a mismatch we show data acquired over the continental crust offshore the north of scotland the dataset we have modelled includes a wide angle expanding spread profile a conventional ocean bottom seismometer profile a high resolution wide angle onshore offshore experiment a synthetic aperture 16 km offset cdp profile and a conventional deep reflection profile these data show unusually sharp bright and continuous reflections from the moho at all offsets the normal incidence reflection moho and the moho modelled from the wide angle data both show the same lateral structure however they are offset one from the other in normal incidence two way travel time this mismatch is considerably larger than the maximum error expected in the wide angle model and is much greater than the accuracy with which the moho can be picked on the reflection data if we assume that this mismatch is caused by crustal scale seismic anisotropy then the synthetic results indicate that anisotropy in the crust north of scotland is about 7

a proposed method of surface map analysis present surface frontal analyses suffer from the defect that frontal positions are typically not collocated with zones of intense temperature contrast further individuals typically do not agree as to the existence type and location of fronts the author argues that the lack of a surface temperature analysis is mainly responsible for these flaws and it is proposed that such analysis preferably of potential temperature in regions of variable terrain elevation become part of routine procedure such an analysis will reveal nonfrontal baroclinic zones of considerable intensity most cold fronts except the strongest ones are denoted as baroclinic troughs propagating eastward in the prevailing westerly flow it is argued that when a meridional cold front exists in the presence of even a small meridional temperature gradient the wind shift should propagate away from the intense surface temperature gradient which then weakens an explanation is provided based on quasigeostrophic theory it follows that fronts are short lived phenomena

modeled and observed variations in storm divergence and stratiform rain production in southeastern texas abstractstorm divergence profiles observed by an s band doppler radar are compared to ensemble simulations of 10 disparate precipitating systems occurring in warm season weakly baroclinic and strongly baroclinic environments in southeastern texas eight triply nested mesoscale model simulations are conducted for each case using single and double moment microphysics with four convective treatments i e two convective parameterizations and explicit versus parameterized convection at 9 km observed and simulated radar reflectivities are objectively separated into convective stratiform and nonprecipitating anvil columns and comparisons are made between ensemble mean echo coverages and levels of nondivergence lnds in both the model and observations storms occurring in less baroclinic environments have more convective rain area less stratiform rain area and more elevated divergence profiles the model ensemble and observations agree best for well organized leading line trailing stratiform systems

seismic attenuation in the continental crust sw of england summary significant variability in crustal attenuation exists off england s sw coast as shown by birps deep seismic reflection lines the presence of the thick cornubian batholith with no sedimentary cover acting as a low attenuating region compared to normal upper crust with about 300 m of sedimentary cover can account for the observed differences in the strength of signal returning from the lower crust application of the spectral amplitude ratio method to determine the effective quality factor q of the lower crust beneath the granite yields a value of 400 200 a modification of the spectral amplitude ratio method to measure relative q s at different locations indicates that the average q of the upper and middle crust containing the granite is twice that elsewhere although the hypothesis that the lower crustal composition differs significantly beneath the granite cannot be ruled out geological and seismic observations suggest that differential attenuation is the main factor causing variation in the observed reflectivity thus thick granites act as low attenuating windows to the lower crust

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt

high frequency borehole seismograms recorded in the san jcinto fault zone southern california part 2 attenuation and site effects we examine surface and downhole p and s wave spectra from local earthquakes recorded at two borehole seismometer arrays knw bh and pfo bh installed in the southern california batholith region of the san jacinto fault zone by the u s geological survey to assess the influence of the weathered layer on the spectral content of high frequency 2 to 200 hz seismic signals earthquake signals recorded downhole at both sites show significantly improved seismic bandwidth due to both a reduction in ambient noise levels and especially to dramatically increased levels of high frequency signal significant seismic signal is observed up to approximately 190 hz for p waves at knw bh stacked spectral ratios from these signals indicate that the highly weathered near surface between 0 and 150 m at knw bh and pfo bh exerts a much larger influence on seismic signals than deeper between 150 and 300 m material modeling of uphole downhole spectral ratio data suggests q 6 5 and q 9 between 0 and 150 m increasing to q 27 and q 26 between 150 and 300 m an outcrop mounted anza network station deployed approximately 0 4 km from knw bh displays roughly similar high frequency content to the knw bh downhole sensors but it exhibits spectra that are significantly colored by directional resonances low q and low velocity near surface material forms a lossy boundary layer at these borehole sites that is advantageous to the high frequency downhole environment not only are noise levels reduced but reflections from the surface and near surface are greatly attenuated as a result high frequency recordings from below the weathered zone more nearly resemble those recorded in a whole space than would otherwise be expected

avo analysis by offset limited prestack migrations of crustal seismic data abstract the physical causes of reflections from the crystalline crust are still a matter of debate the polarity of the wavelet and the amplitude versus offset avo dependence of seismic reflection data can help to reveal the nature of a reflector in contrast to seismic data from sedimentary environments however it is very difficult to get polarity and avo information from crystalline basement data reflections in prestack data are typically weak incoherent and often obscured by numerous diffractions there is however a better chance of obtaining reliable avo information from prestack migrated data after true amplitude processing as the migration process unravels the jumble of reflections and diffractions and improves the signal noise ratio important steps in this processing sequence are careful editing followed by surface consistent amplitude corrections for source and receiver coupling to the ground migration velocity models can be obtained from refraction seismics and or from migration velocity analyses the migration itself is of kirchhoff type and applied to cof gathers cof common offset to get one migrated section for each cof these cof migrated images can then be used to get avo functions for specific reflections examples from the dekorp near vertical reflection line ktb8502 show some strong reflections from beneath a granite body at about 9 to 11 km depth both the positive and negative reflection coefficients of the reflectors are consistent with an interval velocity model obtained from migration velocity analyses avo functions from these reflections include offsets from zero to 12 km and reflection angles from zero to about 30 absolute reflection coefficients are surprisingly small with maximum values of 0 015 two layer modelling has been done to match the avo dependence of these reflections

travelling waves and bores in the lower atmosphere the morning glory and related phenomena abstract observations of solitary wave and travelling bore like disturbances in the lower atmosphere are reviewed together with relevant theoretical laboratory and numerical studies concerning the structure propagation and genesis of such disturbances primary emphasis is given to a discussion of the morning glory phenomenon in northern australia the example par excellence of these types of disturbance and the best documented observationally to date observations of related phenomena elsewhere are reviewed briefly while such occurrences appear to be less frequent in any single location than in the southern gulf of carpentaria region of northern australia reported occurrences are widespread and there is mounting evidence that such disturbances are more common than is generally supposed the evidence suggests that whenever a suitable wave guide exists for the propagation of disturbances the latter can be generated as the natural response of quite general forcing mechanisms examples being the collision of two sea breezes from opposite directions on a peninsula the interaction between a thunderstorm outflow and a sea breeze front or simply the transformation of a thunderstorm or squall line outflow as it moves into a low level stable layer because of the large vertical displacements they produce the larger amplitude disturbances typically with surface pressure jumps of 1 mb or more must be considered as potential triggers for deep convection if the atmosphere is sufficiently conditionally unstable and the large low altitude wind shears associated with them present a serious hazard to low flying aircraft

gap flow in an alpine valley during a shallow south fohn event observations numerical simulations and hydraulic analogue this paper examines the three dimensional structure and dynamics of southerly hybrid gap mountain flow through the wipp valley wipptal austria observed on 30 october 1999 using high resolution observations and model simulations the observations were obtained during a shallow south fohn event documented in the framework of the mesoscale alpine programme map three important data sources were used the airborne differential absorption lidar leandre 2 the ground based doppler lidar teaco2 and in situ measurements from the national oceanic and atmospheric administration p 3 aircraft this event was simulated down to 2 km horizontal resolution using the non hydrostatic mesoscale model meso nh the structure and dynamics of the flow were realistically simulated the combination of high resolution observations and numerical simulations provided a comprehensive three dimensional picture of the flow through the wipptal in the gap entrance region brenner pass austria the low level jet was not solely due to the channelling of the southerly synoptic flow through the elevated gap part of the wipptal flow originated as a mountain wave at the valley head wall of the brenner pass downstream of the pass the shallow fohn flow had the characteristics of a downslope windstorm as it rushed down towards the inn valley inntal and the city of innsbruck austria downhill of the brenner pass the strongest flow was observed over a small obstacle along the western side wall the nosslachjoch rather than channelled in the deeper part of the valley just to the east further north the low level jet was observed in the centre of the valley approximately halfway between brenner pass and innsbruck where the along axis direction of the valley changes from north to north north west the low level jet was observed to be deflected to the eastern side wall of the wipptal interaction between the stubaier alpen the largest and highest topographic feature to the west of the wipptal and the south westerly synoptic flow was found to be the primary mechanism responsible for the deflection the along and cross valley structure and dynamics of the flow were observed to be highly variable due to the influence of surrounding mountains localized steep slopes within the valley and outflows from tributaries the gschnitztal and the stubaital to the west of the wipptal for that shallow fohn case observations and simulations provided a large body of evidence that downslope flow created thinning thickening fluid and accelerations decelerations reminiscent of mountain wave hydraulic theory along the wipptal two hydraulic jump like transitions were observed and simulated i on the lee slope of the nosslachjoch and ii in the gschnitztal exit region a hydraulic solution of the flow was calculated in the framework of reduced gravity shallow water theory the down valley evolution of the froude number computed using leandre 2 p 3 flight level and teaco2 measurements confirmed that these transitions were associated with super to subcritical transitions copyright 2002 royal meteorological society

interpretation of the reflections in the siljan ring area based on results from the gravberg 1 borehole abstract during 1985 about 80 km of surface multichannel seismic reflection data were collected across the meteorite impact in the siljan ring area in central sweden the area consists mainly of granitic and gneissic rock ranging in age from 1400 to 2000 million years with remnants of palaeozoic sedimentary rocks preserved by downfaulting after a meteorite impact at 360 ma thereby forming a circular ring about 40 km in diameter dolerite intrusions ranging in age from 850 to 1700 million years are also present the seismic data revealed several high amplitude laterally continuous sub horizontal reflections in the northern part of the structure the high amplitude reflections and a possible intermediate low velocity zone were contributing factors in choosing the site for the gravberg 1 deep earth gas test well drilling and vertical seismic profiling vsp found that the reflectors were associated with dolerite sills which had intruded into the granite and which range in thickness from a few metres up to 60 m and with a pre impact area extent of at least 800 km 2 studies of amplitude and frequency versus offset afvo show the observations are compatible with a model of simple granite dolerite granite layering

formation of a coastal barrier jet in the gulf of mexico due to the interaction of cold fronts with the sierra madre oriental mountain range the passage of tree cold fronts cfs over the gulf of mexico were simulated with the weather research and forecasting model to study the mechanisms associated with the formation of a jet parallel to the sierra madre oriental smor mountain range near the port of veracruz dew point temperature td 20 c and available convective potential energy best simulated the horizontal propagation of the intense cfs over the gulf and equivalent potential temperature helped identify the vertical position of the fronts additional simulations with reduced topography showed that mountain heights exceeding 2 5 km where the smor is closest to the coast were crucial for the formation of a coastal barrier jet the froude number fr 18 ms 1 and occurred near the surface 1 km amsl consistent with observations and similar to other barrier jets in the world for the first time the jet near the port of veracruz is documented as a coastal barrier jet the cfs also produced strong tehuano gap winds

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

kinematic and moisture characteristics of a nonprecipitating cold front observed during ihop part i across front structures abstract a wide array of ground based and airborne instrumentation is used to examine the kinematic and moisture characteristics of a nonprecipitating cold front observed in west central kansas on 10 june 2002 during the international h2o project ihop this study the first of two parts is focused on describing structures in the across front dimension coarsely resolved observations from the operational network and dropsondes deployed over a 200 km distance centered on the front are combined with higher resolution observations from in situ sensors doppler radars a microwave radiometer and a differential absorption lidar that were collected across a 40 km swath that straddled a 100 km segment of the front the northeast southwest oriented cold front moved toward the southeast at 8 10 m s 1 during the morning hours but its motion slowed to less than 1 m s 1 in the afternoon in the early afternoon the cold front separated cool air with a northerly component flow of 2 4 m s 1 from a 10 km wide ban

a comparison of two coastal barrier jet events along the southeast alaskan coast during the sarjet field experiment abstract the southeastern alaskan regional jets experiment investigated the structures and physical processes of barrier jets along the coastal fairweather mountains near juneau alaska from 24 september to 21 october 2004 this paper compares in situ aircraft data and high resolution simulations from the first intensive observation period iop1 which featured a nearly terrain parallel barrier jet classical jet with another coastal jet event iop7 that was influenced by offshore directed gap flows at the coast hybrid jet iop1 featured southerly onshore flow preceding a landfalling trough which was blocked by the coastal terrain and accelerated down the pressure gradient to produce a 5 10 m s 1 wind enhancement in the alongshore direction in the lowest 1 km msl in contrast iop7 featured higher surface pressure and colder low level temperatures to the east inland of the study area than did iop1 which resulted in offshore directed coastal gap flow exiting cross sound below 500 m that turned a

equatorial mountain torques and cold surge preconditioning the evolution of the two components of the equatorial mountain torque emt applied by mountains on the atmosphere is analyzed in the ncep reanalysis a strong lagged relationship between the emt component along the greenwich axis tm1 and the emt component along the 90 e axis tm2 is found with a pronounced signal on tm1 followed by a signal of opposite sign on tm2 it is shown that this result holds for themajor massifs antarctica the tibetan plateau the rockies and the andes if a suitable axis systemis used for each of them for the midlatitude mountains this relationship is in part associated with the development of cold surges following these results two hypotheses are made i the mountain forcing on the atmosphere is well measured by the regional emts and ii this forcing partly drives the cold surges to support these a purely dynamical linear model is proposed it is written on the sphere uses an f plane quasigeostrophic approximation and includes the mountain forcings in this model a positive negative peak in tm1 produced by a mountain massif in the northern southern hemisphere is due to a large scale high surface pressure anomaly poleward of the massif at a later stage high pressure and low temperature anomalies develop in the lower troposphere east of the mountain explaining the signal on tm2 and providing the favorable conditions for the cold surge development it is concluded that the emt is a good measure of the dynamical forcing of the atmospheric flow by the mountains and that the poleward forces exerted by mountains on the atmosphere are substantial drivers of the cold surges at least in their early stage therefore the emt time series can be an important diagnostic to assess the representation of mountains in general circulation models 2010 american meteorological society

supracrustal intraplate thickening of variscan basement due to alpine foreland compression results from the superdeep well ktb bohemian massif germany until its final depth of 9101 m the superdeep well ktb kontinentales tiefbohrprogramm der bundesrepublik deutschland drilled through variscan basement rocks which suffered strong brittle ductile and brittle deformation during late and post variscan times investigations of faults and mineralized tension gashes revealed the following deformation stages 1 upper carboniferous subvertical tension gashes 2 upper carboniferous reverse faults 3 cretaceous subhorizontal tension gashes and reverse faults 4 neogene normal faults clear depth dependent variations of metamorphic conditions of the post variscan deformation structures are obvious whereas the upper carboniferous structures show almost no variations with depth this refers to the metamorphic index minerals as well as to the deformation fabrics in quartz we explain this considerable lack of gradients as being the result of reactivative reverse faulting that led to considerable vertical thickening of the upper crust the fault geometry indicates an antiformal stack the frontal ramp of which was drilled by ktb at 7000 m depth there is strong evidence that this frontal ramp referred to as the franconian lineament at the surface rises from a subhorizontal detachment at about 10 km depth which corresponds to the brittle ductile boundary layer of quartz bearing rocks the considerable amount of supracrustal vertical thickening above the franconian lineament results from repeated movements along this detachment since upper carboniferous times it seems reasonable to suppose that in intraplate tectonic settings the brittle ductile boundary layer is prone to form a long lived decoupling horizon along which the upper brittle crust is detached from the viscous middle and lower crust

near surface velocities and attenuation at two boreholes near anza california from logging data to investigate near surface site effects in granite rock we drilled 300 m deep boreholes at two sites which are collocated with stations from the digital array at anza california the first borehole was sited at station knw keenwild fire station which is located along a ridge line about 8 7 km east of the san jacinto fault zone station pfo pinon flat observatory chosen for the second site is another 6 km further to the east of station knw and is located on a gently sloping hillside we logged each borehole for p and s wave velocities as well as for crack density and orientation p waves were generated by striking a plate with a hammer at the surface a tool consisting of weighted anvils driven by compressed air against end plates along a 3 5 m beam was used to generate shear waves signals were recorded downhole with a three component sensor package at 2 5 m intervals from the surface to 50 m depth and at 5 m intervals from 50 m depth to the bottom of the hole velocities were determined by differencing the measured arrival times of first arrivals or peaks over each interval in depth travel times were computed for the first breaks at shallow depths however below about 100 m depth times were computed for the first peaks rather than for first breaks since the first arrival was no longer clearly distinguishable the knw site yielded a shear velocity of 1 9 km sec by only 30 m in depth and reached close to 2 6 km sec at the bottom of the hole p wave velocities at knw were also high at 5 4 km sec starting at 120 m depth the pfo site had similar but slightly higher shear wave velocities the bottom hole shear wave velocity reached 3 0 km sec and the p wave velocity was 5 4 km sec shear wave attenuation was computed using both the pulse rise time and spectral ratio methods at station knw attenuation was significant only in an interval between 17 5 and approximately 40 m in depth over the top 50 m attenuation corresponding to a q of about 8 was obtained a total t of 0 004 sec was measured for this interval pulse rise times also increased rapidly in this zone the spectral ratio data for station pfo yields two peaks in attenuation above 50 m similar to the attenuation found for station knw the peak in attenuation corresponds to a q of about 11 averaged over the top 50 m spectra of the seismic pulses produced by the hammer give good signal between 20 to 80 hz significant motion perpendicular to the polarizations of the first shear wave arrival was recorded within a few meters of the surface apparently the rock structure is sufficiently complicated that body waves are being converted sh to sv at oblique incidence very close to the surface the presence of these elliptical particle motions within a mere few m of the pure shear wave source suggests that the detection of polarizations perpendicular to the main shear arrival at a single location at the surface is not by itself a good method for detecting shearwave splitting within the upper few tens of kilometers of the earth s crust crack densities and orientations were determined from televiewer records these records showed cracks with a preferred direction at station knw and of a greater density than at station pfo at station pfo crack densities were smaller and more diffuse in orientation

east pacific ocean eddies and their relationship to subseasonal variability in central american wind jets subseasonal variability in sea surface height ssh over the east pacific warm pool off central america is investigated using satellite observations and an eddy resolving ocean general circulation model ssh variability is organized into two southwest tilted bands on the northwest flank of the tehuantepec and papagayo wind jets and collocated with the thermocline troughs eddy like features of wavelength similar to 600 km propagate southwestward along the high variance bands at a speed of 9 13 cm s wind fluctuations are important for eddy formation in the gulf of tehuantepec with a recurring interval of 40 90 days when forced by satellite wind observations the model reproduces the two high variance bands and the phase propagation of the tehuantepec eddies our observational analysis and model simulation suggest the following evolution of the tehuantepec eddies on the subseasonal timescale in response to the gap wind intensification a coastal anticyclonic eddy forms on the northwest flank of the wind jet and strengthens as it propagates offshore in the following two to three weeks an energetics analysis based on the model simulation indicates that besides wind work barotropic and baroclinic instabilities of the mean flow are important for the eddy growth both observational and model results suggest a re intensification of the anticyclonic eddy in response to the subsequent wind jet event off papagayo ocean eddy formation is not well correlated with local wind jet variability in both the gulfs of tehuantepec and papagayo subseasonal ssh variability is preferentially excited on the northwest flank of the wind jet factors for this asymmetry about the wind jet axis as well as the origins of wind jet variability are discussed

the zonal men westerlies over the southern hemisphere abstract a brief review and evaluation of various analyses of the southern hemisphere westerlies is given along with the presentation of some relent results several features characterize the westerlies of the southern hemisphere as quite different from those in the northern hemisphere and in the past thew have typically been difficult to reproduce well in general circulation modes they are the double jet structure in winter the stronger midlatitude tropospheric winds in summer than in winter and the ensuing much smaller amplitude of the annual cycle which is associated with a maximum of global atmospheric angular momentum in january new values for the hemispheric angular momentum integrals are than previously reported two estimates of the distribution and strength of the southern westerlies that have been widely used are considered to be seriously biased factors contributing to discrepancies among different results am large natural variability missing data and biases in observing systems and me

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

seismic velocities and q factors in the uppermost crust beneath the sveka profile in finland the sveka deep seismic sounding profile in central finland was shot in 1981 to study the crustal structure of the fennoscandian shield in the contact zone between the archaean and svecokarelian provinces apart from p and s waves well developed rayleigh surface waves r waves with periods of 0 5 2 0 s were recorded from all the shot points in the frequency band 0 5 2 5 hz the r waves were clearly observed on the entire 325 km long profile amplitudes of p s and r waves corrected for charge size were used to study the attenuation of waves and q factors from the records of body and r waves the distributions of seismic velocities and quality factors were determined using the dynamic ray tracing method 2 d modelling and the reflectivity method 1 d modelling in the 2 d model of the uppermost crust the p wave velocities vary from 5 8 5 9 km s at the top to 6 2 6 3 km s at a depth of 6 km the velocity ratio solvpvs varies with depth and distance from 1 67 to 1 74 reaching its maximum value of 1 8 2 0 in the uppermost 200 500 m and values of 1 76 1 80 in the uppermost 1 km the quality factors qp and qs vary in the uppermost 1 km from 20 to 120 reaching a higher value of 140 in the marginal zone of the archaean basement in the depth range of 1 2 km q values of 80 400 were found in the depth range 2 6 km q values of 300 800 were found for the svecokarelian province and 200 400 for the archaean the low values of vs and q in the uppermost 1 km are thought to be the result of rock fracturing the crack density was estimated for the uppermost crust along the profile for the sveka profile we found values of 0 35 0 54 in the uppermost 200 m 0 10 0 32 at a depth of 0 2 1 0 km and 0 1 for depths of 1 4 km the comparison of our seismic data with results of theoretical and laboratory investigations implies that the uppermost crustal rocks are saturated the zones of strong attenuation of r waves coincide with faults and schist zones which are well known from surface geology

comments on unusually long duration multiple doppler radar observations of a front in a convective boundary layer stonitsch and markowski perform multiple doppler radar analyses of a cold front over oklahoma and kansas despite their interesting results their explanations include a number of misconceptions about cold fronts these misconceptions include the proper interpretation of the frontogenesis function the role of entrainment versus differential surface sensible heat flux toward weakening the virtual potential temperature gradient across a cold front a separation of the wind shift from the virtual potential temperature gradient and the factors that affect the motion of the cold front

the san gabriel mountains bright reflective zone possible evidence of young mid crustal thrust faulting in southern california abstract during the los angeles region seismic experiment larse a reflection refraction survey was conducted along a line extending northeastward from seal beach california to the mojave desert crossing the los angeles basin and san gabriel mountains shots and receivers were spaced most densely through the san gabriel mountains for the purpose of obtaining a combined reflection and refraction image of the crust in that area a stack of common midpoint cmp data reveals a bright reflective zone 1 s thick that dominates the stack and extends throughout most of the mid crust of the san gabriel mountains the top of this zone ranges in depth from 6 s 18 km depth in the southern san gabriel mountains to 7 5 s 23 km depth in the northern san gabriel mountains the zone bends downward beneath the surface traces of the san gabriel and san andreas faults it is brightest between these two faults where it is given the name san gabriel mountains bright spot sgmbs and becomes more poorly defined south of the san gabriel fault and north of the san andreas fault the polarity of the seismic signal at the top of this zone is clearly negative and our analysis suggests it represents a negative velocity step the magnitude of the velocity step is approximately 1 7 km s in at least one location an event with positive polarity can be observed 0 2 s beneath the top of this zone indicating a thickness of the order of 500 m for the low velocity zone at this location several factors combine to make the preferred interpretation of this bright reflective zone a young fault zone possibly a master decollement 1 it represents a significant velocity reduction if the rocks in this zone contain fluids such a reduction could be caused by a differential change in fluid pressure between the caprock and the rocks in the sgmbs near lithostatic fluid pressure is required in the sgmbs such differential changes are believed to occur in the neighborhood of active fault zones where fault value action has been postulated less likely alternative explanations for this velocity reduction include the presence of magma and a change in composition to serpentinite or metagraywacke 2 it occurs at or near the brittle ductile transition at least in the southern san gabriel mountains a possible zone of concentrated shear 3 a thin reflection rising from its top in the southern san gabriel mountains projects to the hypocenter of the 1987 m 5 9 whittier narrows earthquake a blind thrust fault earthquake with one focal plane subparallel to the reflection alternatively one could argue that the bends or disruptions in the reflective zone seen at the san gabriel and san andreas faults are actually offsets and that the reflective zone is therefore an older feature possibly an older fault zone

observations of nonclassical frontal propagation and frontally forced gravity waves adjacent to steep topography through the integrated analysis of remote sensing and in situ data taken along the front range of colorado this study describes the interactions that occurred between a leeside arctic front and topographically modulated flows these interactions resulted in nonclassical frontal behavior and structure across northeastern colorado the shallow arctic front initially advanced southwestward toward the front range foothills before retreating eastward then a secondary surge of arctic air migrated westward into the foothills during its initial southwestward advance the front exhibited obstacle like density current characteristics its initial advance was interrupted by strong downslope northwesterly flow associated with a high amplitude mountain wave downstream of the continental divide and by a temporal decrease in the density contrast across the front due to diurnal heating in the cold air and weak cold advection in the warm air the direction and depth of flow within the arctic air also influenced the frontal propagation the shallow obstacle like front actively generated both vertically propagating and vertically trapped gravity waves as it advanced into the downslope northwesterly flow resulting in midtropospheric lenticular wave clouds aloft that tracked with the front because the front entered a region where strong downslope winds and mountain waves extended downstream over the high plains the wave field in northeastern colorado included both frontally forced and true mountain forced gravity waves a sequence of scorer parameter profiles calculated from hourly observations reveals a sharp contrast between the prefrontal and postfrontal wave environments consequently analytic resonant wave mode calculations based on the scorer parameter profiles reveal that the waves supported in the postfrontal regime differed markedly from those supported in the prefrontal environment this result is consistent with wind profiler observations that showed the amplitude of vertical motions decreasing substantially through 16 km above mean sea level msl after the shallow frontal passage

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

the effect of along front temperature variation in a two dimensional primitive equation model of surface frontogenesis abstract the development of frontal zones is examined in a two dimensional primitive equation model of frontogenesis formulated for dry nearly adiabatic and inviscid conditions the model results are interpreted in the context of the general problem of determining the dynamical properties of cold and warm fronts the central hypothesis attributable to eliassen is that cold and warm fronts may be distinguished by the orientation of the cross front thermal wind component and the sense of the associated along front temperature variation three simulations comprising confluent forcing geostrophic contraction in the cross front direction and differing initial specifications of the along front potential temperature gradient are examined in detail in the first simulation referred to as the pure confluence case the along front potential temperature gradient is set to zero establishing a control for specifying along front potential temperature variations respectively characteristic of cold and warm fronts

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