backazimuthal variations of splitting parameters of teleseismic sks phases observed at the broadband stations in germany sks phases observed at broadband stations in germany show significant shear wave splitting we have analyzed sks and skks phases for shear wave splitting from 13 stations of the german regional seismic network grsn from 3 three component stations of the grafenberg array grf and from one austrian station sqta the data reveal strong differences in the splitting parameters fast direction and delay time t from a single event at various stations as well as variations at the individual stations for events with different backazimuths the backazimuthal variations of the splitting parameters at some stations can be explained by two layer anisotropy models with horizontal symmetry axes the best resolved two layer model is the gra1 model upper layer 40 t 1 15s lower layer 115 t 1 95s the upper layer can be attributed to the lithosphere because of the magnitude of the delay time of the upper layer the lower layer must lie within the asthenosphere at other stations splitting parameters are consistent with an anisotropic one layer model for the upper mantle stations near the bohemian massif show fast directions near ew throughout ne germany the directions are oriented nw se the reason for this direction is probably the nearby tornquist teisseyre line the observed fast axes are subparallel to this prominent transeuropean suture zone at stations in southern germany near the alps we observed ene wsw directions below some stations we also found indications of inclined anisotropic layers

on the generation of african squall lines abstract squall lines sls form an important component of the meteorology of northern africa and in particular contribute substantially to rainfall totals their generation requires the existence of a potentially unstable low level supply of moisture overlain by dry desert air and vertical wind shear beneath the midlevel african easterly jet the instability may be released and an sl initiated by factors such as surface heating topography african waves or surface evaporation the relative importance of each of these factors and the means by which they impact on sl generation is reviewed this is followed by a detailed analysis of one month of satellite imagery and surface data for august 1985 over a portion of central northern africa the novelty of our study lies in the temporal resolution of the satellite imagery which with 21 images per day allows the identification of a large number of short lived sls 4 h duration or less on the southern fringes of the sahara these are likely to contribute

relationship between cyclone tracks anticyclone tracks and baroclinic waveguides abstract this study presents observational results concerning the structure and evolution of high frequency fluctuations with periods shorter than 6 days in the geopotential height field the results are based on a statistical analysis of 1000 500 and 250 mb height fields derived from twice daily nmc final analyses over the northern hemisphere for the 20 winters 1964 65 through 1983 84 the disturbances assume the form of waves elongated in the meridional direction with a mean wavelength of 4000 km a westward tilt with height and a mean eastward phase propagation of 12 1 5 m s 1 these results support an interpretation in terms of finite amplitude baroclinic waves whose structure and evolution varies with geographical location over the continents and especially along the eastern slopes of the rockies and the tibetan plateau the waves show marked departures in structure and evolution from those over the oceans in particular we find evidence of a systematic influence of the terrain in steering the

combining satellite and seismic images to analyse the shallow structure of the dead sea transform near the desert transect the left lateral dead sea transform dst in the middle east is one of the largest continental strike slip faults of the world the southern segment of the dst in the arava araba valley between the dead sea and the red sea called arava araba fault af has been studied in detail in the multidisciplinary desert dead sea rift transect project based on these results here the interpretations of multi spectral aster satellite images and seismic reflection studies have been combined to analyse geologic structures whereas satellite images reveal neotectonic activity in shallow young sediments reflection seismic image deep faults that are possibly inactive at present the combination of the two methods allows putting some age constraint on the activity of individual fault strands although the af is clearly the main active fault segment of the southern dst we propose that it has accommodated only a limited up to 60 km part of the overall 105 km of sinistral plate motion since miocene times there is evidence for sinistral displacement along other faults based on geological studies including satellite image interpretation furthermore a subsurface fault is revealed 4 km west of the af on two e w running seismic reflection profiles whereas these seismic data show a flower structure typical for strike slip faults on the satellite image this fault is not expressed in the post miocene sediments implying that it has been inactive for the last few million years about 1 km to the east of the af another now buried fault was detected in seismic magnetotelluric and gravity studies of desert taking together various evidences we suggest that at the beginning of transform motion deformation occurred in a rather wide belt possibly with the reactivation of older n s striking structures later deformation became concentrated in the region of today s arava valley till 5 ma ago there might have been other now inactive fault traces in the vicinity of the present day af that took up lateral motion together with a rearrangement of plates 5 ma ago the main fault trace shifted then to the position of today s af

a tropical squall line observed during the copt 81 experiment in west africa part 1 kinematic structure inferred from dual doppler radar data abstract this paper deals with the analysis of a tropical squall line observed on 22 june 1981 during the copt 81 convection profonde tropicale experiment the present part i is restricted to the study of the kinematic structure of the system which moved in a moderately unstable atmosphere faster than the environmental air at all levels at the observation time the squall line was in a mature stage the explicit description of the airflow within the system was inferred from dual and single doppler radar data the general characteristics of the squall line are found to be very similar to those of tropical squall lines observed during previous experiments such as vhimex or gate a large cloud system composed of an organized convective line ahead of an extensive trailing anvil cloud stratiform rain fast motion and long lasting structure and a well marked gust front signature at ground level the overall airflow presents a three dimensional structure at the leading edge convective scale updrafts a

radar characteristics of continental coastal and maritime convection observed during amma namma ground based radar observations at three distinct geographical locations in west africa along a common latitudinal band niamey niger continental kawsara senegal coastal and praia republic of cape verde maritime are analyzed to determine convective system characteristics in each domain during a 29 day period in 2006 ancillary datasets provided by the african monsoon multidisciplinary analyses amma and nasa amma namma field campaigns are also used to place the radar observations in context results show that the total precipitation is dominated by propagating mesoscale convective systems convective characteristics vary according to environmental properties such as vertical shear cape and the degree of synoptic forcing data are bifurcated based on the presence or absence of african easterly waves in general african easterly waves appear to enhance mesoscale convective system strength characteristics e g total precipitation and vertical reflectivity profiles at the inland and maritime sites the wave regime also resulted in an increased population of the largest observed mesoscale convective systems observed near the coast which led to an increase in stratiform precipitation despite this increase differentiation of convective strength characteristics was less obvious between wave and no wave regimes at the coast owing to the propagating nature of these advecting mesoscale convective systems interaction with the regional thermodynamic and dynamic environment appears to result in more variability than enhancements due to the wave regime independent of location copyright c 2011

modeling of seismic guided waves at the dead sea transform 1 on several recordings of linear seismometer arrays crossing the arava fault af in the middle east we see prominent wave trains emerging from in fault explosions which we interpret as waves being guided by a fault zone related low velocity layer the af is located in the arava valley and is considered the principal active fault of the mainly n s striking dead sea transform system in this section observations of these wave trains are confined to certain segments of the receiver lines and occur only for particular shot locations they exhibit large amplitudes and are almost monochromatic we model them by a two dimensional 2 d analytical solution for the scalar wave field in models with a vertical waveguide embedded in two quarter spaces a hybrid search scheme combining genetic algorithm and a local random search is employed to explore the multimodal parameter space resolution is investigated by synthetic tests the observations are adequately fit by models with a narrow only 3 12 m wide waveguide with s wave velocity reduced by 10 60 of the surrounding rock we relate this vertical low velocity layer with the damage zone of the af since the location of receivers observing and of shots generating the guided waves respectively match with the surface trace of the fault the thickness of the damage zone of the af at least at shallow depths seems to be much smaller than in other major fault zones this could be due to less total slip on this fault

subduction induced strain in the upper mantle east of the mendocino triple junction california we observe splitting of teleseismic shear waves at five stations of the berkeley digital seismic network located east of the mendocino triple junction in northeastern california that is dependent on the arrival direction of the seismic phases the observed variations with back azimuth cannot be explained with laterally varying anisotropy with a horizontal symmetry axis and are attributed to the presence of fabric with an inclined symmetry axis we assume that the anisotropy is caused by the preferred alignment of olivine crystals a grid search over possible orientations of the olivine a axes reveals that south of the mendocino triple junction they dip to the east whereas north of the triple junction they dip to the northeast on the basis of a comparison of the ray paths of our data to the spatial distribution of fast and slow p wave velocity anomalies in the upper mantle we conclude that the anisotropy is located within seismically slow regions and that the directions are controlled by the geometry of a steeply dipping fast p wave velocity anomaly assuming that the fast p velocity anomaly represents subducted slab material we conclude that the fabric beneath the stations north of the triple junction is most likely caused by the differential motion between this rigid strong down going plate and the surrounding mantle south of the triple junction the fabric may have developed while subduction of the farallon plate was still ongoing in this region prior to 6 ma however we prefer to attribute the observations to more recent asthenospheric flow associated with the opening of a slabless window beneath the north american lithosphere the flow is modulated by the presence of rigid lithosphere to the north and east

a synoptic scale wave of 6 9 day period in the atlantic tropical troposphere during summer 1981 abstract using the ecmwf analyses this study documents a tropospheric synoptic scale wave in the tropical atlantic during summer 1981 it has a 6 9 day period and a westward velocity of about 8 5 longitude per day the wave structure has features similar to a mixed rossby gravity wave it is suggested that it may be a normal mode of the earth atmosphere

convection over tropical africa and the atlantic ocean during northern summer part ii modulation by easterly waves abstract using meteosat data and european centre for medium range weather forecast ecmwf analyses we examine easterly waves and their relation with the cloudiness over west africa and the tropical atlantic ocean for three summers june july august and september 1983 85 spectral analysis of the low level meridional wind in the 2 8 5 1 day band reveals maximum wave amplitude near the west african coast at 20 n during august and september the wave amplitude is larger than during june and july and a secondary maximum appears around 7 5 n composites of the mean structure of the wave and the associated cloud modulation reveal consistent relationship between observed cloudiness and ecmwf analyses the phase of the wave modulation of the cloudiness is strongly dependent on the geographical location in response to changes in the mean climatological conditions this phase varies however grossly in four large land and ocean regions centered at 7 5 and 17 5 n respectively for which we derive vertical cro

african easterly wave variability and its relationship to atlantic tropical cyclone activity abstract automatic tracking of vorticity centers in european centre for medium range weather forecasts analyses has been used to develop a 20 yr climatology of african easterly wave activity the tracking statistics at 600 and 850 mb confirm the complicated easterly wave structures present over the african continent the rainy zone equatorward of 15 n is dominated by 600 mb activity and the much drier saharan region poleward of 15 n is more dominated by 850 mb activity over the atlantic ocean there is just one storm track with the 600 and 850 mb wave activity collocated based on growth decay and genesis statistics it appears that the 850 mb waves poleward of 15 n over land generally do not get involved with the equatorward storm track over the ocean instead there appears to be significant development of 850 mb activity at the west african coast in the rainy zone around 10 n 10 w which it is proposed is associated with latent heat release based on the tracking statistics it has been shown th

depth dependent mantle anisotropy below the san andreas fault system apparent splitting parameters and waveforms we measure apparent teleseismic shear wave splitting parameters at several stations of the berkeley digital seismic network bdsn and three temporary broadband stations of the university of california at santa cruz located near the san andreas fault system in northern and central california previously proposed anisotropic models for the region include a two layered structure and a structure with a constant degree of anisotropy and a horizontal fast axis that gradually rotates as a function of depth we significantly increase the number of observations and confirm the existence of depth dependent anisotropy beneath the san andreas fault system we also investigate the extent to which the enhanced data set can constrain the details of the depth dependence using theoretical expressions we determine a suite of two layer models that fit our observations and result in practically indistinguishable apparent splitting parameters the wide range of acceptable models indicates that apparent splitting parameters alone cannot constrain all four parameters that specify the two anisotropic layers upper and lower fast polarization directions and delay times synthetic seismograms for three very different two layer models chosen from our suite of acceptable models and a model with a gradually rotating fast axis show subtle waveform variations and apparent splitting parameters measured from these records do not overlap hence we conclude that waveforms indeed contain additional information about the anisotropic structure we incorporate waveform data by searching for the four splitting parameters of two layer models that minimize the splitting of several records at each station simultaneously the resulting models largely overlap with the models obtained using the theoretical expressions which indicates that the addition of waveform data is not sufficient to uniquely determine all four parameters of a two layer model constraining the upper fast polarization direction to be parallel to the relative motion vector of the pacific and north american plates n35 w allows the values of the other three model parameters to be uniquely determined the lower layer fast polarization direction beneath all stations is approximately e w and the delay times vary from 0 85 to 1 70 s the delay times in the upper layer are smaller and they vary between 0 50 and 1 25 s shear within the uppermost mantle due to the relative plate motion is a satisfactory explanation for the shallow anisotropic signature the sublithospheric e w fast directions may be caused by small scale convection within a slabless window or by motions of the lithosphere relative to the deeper mantle

the shallow velocity structure across the dead sea transform fault arava valley from seismic data 1 tomographic inversion techniques were applied to first arrival traveltimes of refracted p waves to study the shallowest part of the crust in the vicinity of the arava fault af the dead sea transform dst segment between the dead sea and the red sea tomographic inversion techniques were applied to first arrival traveltimes of refracted p waves a 100 km long seismic line was centered on and oriented approximately perpendicular to the af a large number of p wave traveltimes from vibroseis and explosive shots 280 000 were picked manually and used to invert for shallow p wave velocity structure the regularized inversion approach zelt and barton 1998 was used for the tomographic inversion of the traveltimes extensive testing of model and inversion parameters was carried out to derive a reliable p wave velocity model complementary checker board tests indicate that depending on the size of velocity homogeneities the velocity structure is well resolved down to a depth of several kilometers this model represents the first shallow p wave velocity across the whole width of the dst system showing features that correlate well with surface geology and also some buried structures the model further suggests that the af extends vertically downward to at least 3 km the observed variation in upper crustal velocity implies the existence of a simple deformation compatible with a large lateral fault offset from this model a structural and dynamic interpretation of the dst system is then presented the depth extension and geometry of several additional major faults and dst associated shallow sedimentary basins were successfully imaged through the integration of the well known surface geology and nearby boreholes this work again confirms the dst as a typical transform fault system with a dominant strike slip motion confined to a narrow zone

seismic polarization anisotropy beneath the central tibetan plateau sks and skks shear waves recorded on the indepth iii seismic array deployed in central tibet during 1998 1999 have been analyzed for the direction and extent of seismic polarization anisotropy the 400 km long nnw trending array extended south to north from the central lhasa terrane across the karakoram jiali fault system and banggong nujiang suture to the central qiangtang terrane substantial splitting with delay times from 1 to 2 s and fast directions varying from e w to ne sw was observed for stations in the qiangtang terrane and northernmost lhasa terrane no detectable splitting was observed for stations located farther south in the central lhasa terrane the change in shear wave splitting characteristics occurs at 32 n approximately coincident with the transcurrent karakoram jiali fault system but 40 km south of the surface trace of the banggong nujiang suture this location is also near the southernmost edge of a region of high sn attenuation and low upper mantle velocities found in previous studies the transition between no measured splitting and strong anisotropy 2 2 s delay time is exceptionally sharp 15 km suggesting a large crustal contribution to the measured splitting the e w to ne sw fast directions are broadly similar to the fast directions observed farther east along the yadong golmud highway suggesting that no large scale change in anisotropic properties occurs in the east west direction however in detail fast directions and delay times vary over lateral distances of 100 km in both the n s and e w direction by as much as 40 and 0 5 1 s respectively the onset of measurable splitting at 32 n most likely marks the northern limit of the underthrusting indian lithosphere which is characterized by negligible polarization anisotropy taken in conjunction with decades of geophysical and geological observations in tibet the new anisotropy measurements are consistent with a model where hot and weak upper mantle beneath northern tibet is being squeezed and sheared between the advancing indian lithosphere to the south and the tsaidam and tarim lithospheres to the north and west resulting in eastward flow and possibly thickening and subsequent detachment due to gravitational instability in northern tibet crustal deformation clearly follows this large scale deformation pattern

a new groundwater radiocarbon correction approach accounting for palaeoclimate conditions during recharge and hydrochemical evolution the ledo paniselian aquifer belgium abstract the particular objective of the present work is the development of a new radiocarbon correction approach accounting for palaeoclimate conditions at recharge and hydrochemical evolution relevant climate conditions at recharge are atmospheric pco 2 and infiltration temperatures influencing c isotope concentrations in recharge waters the new method is applied to the ledo paniselian aquifer in belgium this is a typical freshening aquifer where recharge takes place through the semi confining cover of the bartonian clay besides cation exchange which is the major influencing process for the evolution of groundwater chemistry particularly in the bartonian clay also mixing with the original porewater solution fossil seawater occurs in the aquifer recharge temperatures were based on noble gas measurements potential infiltration water compositions for a range of possible pco 2 temperature and calcite dissolution system conditions were calculated by means of phreeqc then the sampled groundwaters were modelled starting from these infiltration waters using the computer code netpath and considering a wide range of geochemical processes fitting models were selected on the basis of correspondence of calculated 13 c with measured 13 c the 14 c modelling resulted in residence times ranging from holocene to pleistocene few hundred years to over 40 ka and yielded consistent results within the uncertainty estimation comparison was made with the 13 c and fontes and garnier correction models that do not take climate conditions at recharge into account to date these are considered as the most representative process oriented existing models yet differences in calculated residence times of mostly several thousands of years up to 19 ka are revealed with the newly calculated ages being mostly though not always younger not accounting for climate conditions at recharge pco 2 and temperature is thus producing substantial error on deduced residence times the derived 14 c model ages are correlated with he concentrations measured in the groundwater of the aquifer the obtained residence times show a gap between about 14 and 21 ka indicating possible permafrost conditions which inhibited any groundwater recharge

low level african easterly wave activity and its relation to atlantic tropical cyclogenesis in 2001 abstract this paper provides new information on the low level 850 hpa structure and behavior of african easterly waves aews and relates this information to previous studies individual aews that occurred during june september of 2001 are studied by a synoptic approach that employs hovmoller diagrams wave track maps and case studies the focus is on two aew regimes in the lower troposphere over north africa a dry regime to the north of the african easterly jet aej coincident with the surface position of the monsoon trough near 20 n and a wet regime to the south of the jet coincident with the near equatorial rainbelt near 10 n the following issues are addressed the origin of the waves seen in the two wave regimes relation of the wave activity to the mean positions of the surface monsoon trough and the 600 700 hpa aej collocation of the tracks of the two wave regimes off the african coast and diversity in low level wave behavior that includes merging splitting and dissipation of the cyclonic

trench parallel flow beneath the nazca plate from seismic anisotropy shear wave splitting of s and sks phases reveals the anisotropy and strain field of the mantle beneath the subducting nazca plate cocos plate and the caribbean region these observations can be used to test models of mantle flow two dimensional entrained mantle flow beneath the subducting nazca slab is not consistent with the data rather there is evidence for horizontal trench parallel flow in the mantle beneath the nazca plate along much of the andean subduction zone trench parallel flow is attributale utable to retrograde motion of the slab the decoupling of the slab and underlying mantle and a partial barrier to flow at depth resulting in lateral mantle flow beneath the slab such flow facilitates the transfer of material from the shrinking mantle reservoir beneath the pacific basin to the growing mantle reservoir beneath the atlantic basin trenchparallel flow may explain the eastward motions of the caribbean and scotia sea plates the anomalously shallow bathymetry of the eastern nazca plate the long wavelength geoid high over western south america and it may contribute to the high elevation and intense deformation of the central andes

shear wave anisotropy in the upper mantle beneath the nazca plate in northern chile data from the projecto de investigacion sismologica de la cordillera occidental pisco seismic network and from six broadband seismographs that were operating in northern chile were used to investigate the mantle in the convergent boundary zone between nazca plate and the south american continent for the presence of anisotropy broadband data as well as long period filtered data of teleseismic sks and pks phases were analyzed for the presence of shear wave splitting as a possible indicator for seismic anisotropy in the mantle beneath the pisco network measurable shear wave splitting was observed with maximum delay times between the slow and fast split wave of the order of 1 s splitting of s waves from intermediate depth events located directly beneath the pisco network in the descending nazca plate is generally associated with small delay times of the order of 0 1 s a value typical for the continental crust near vertical scs reflections from two deep earthquakes in argentina and one nearby intermediate depth earthquake have similar splitting parameters as the sks phases this means that the anisotropic zone causing the splitting of the core phases can be constrained to the pacific mantle underlying the subducting nazca plate it probably does not extend deeper than about 260 km the majority of the anisotropy directions inferred from the core phases are parallel to the absolute plate motion apm direction of the nazca plate which is about n80 e at some stations however the fast polarization direction is pointing n160 e nearly parallel to the strike of the trench and the andes which would be compatible with the trench parallel flow model for south america proposed by russo and silver 1994 this direction is observed over an approximately 100 km wide band to the west of the active volcanic zone it may represent either a second anisotropy regime in the mantle a small scale diversion of slab entrained mantle flow or a relatively small area where slab entrainment of mantle flow is reduced or ceases to exist the large number of observed apm parallel fast directions suggests however that the mantle beneath the descending nazca plate in northern chile deforms mainly as the result of slab entrained mantle flow the large variations of anisotropy directions in the andean subduction zone indicate that asthenospheric flow in the pacific mantle has a complex pattern which may vary over scale lengths of a few hundred kilometers and which may be governed by slab morphology

easterly wave regimes and associated convection over west africa and tropical atlantic results from the ncep ncar and ecmwf reanalyses ncep ncar and ecmwf daily reanalyses are used to investigate the synoptic variability of easterly waves over west africa and tropical atlantic at 700 hpa in northern summer between 1979 1995 1979 1993 for ecmwf spectral analysis of the meridional wind component at 700 hpa highlighted two main periodicity bands between 3 and 5 days and 6 and 9 days the 3 5 day easterly wave regime has already been widely investigated but only on shorter datasets these waves grow both north and south of the african easterly jet aej the two main tracks noted over west africa at 5 n and 15 n converge over the atlantic on latitude 17 5 n these waves are more active in august september than in june july their average wavelength phase speed varies from about 3000 km 8 m s 1 north of the jet to 5000 km 12 m s 1 south of the jet rainfall convection and monsoon flux are significantly modulated by these waves convection in the inter tropical convergence zone itcz being enhanced in the trough and ahead of it with a wide meridional extension compared to the 3 5 day waves the 6 9 day regime is intermittent and the corresponding wind field pattern has both similar and contrasting characteristics the only main track is located north of the aej along 17 5 n both over west africa and the atlantic the mean wavelength is higher about 5000 km long and the average phase speed is about 7 m s 1 then the wind field perturbation is mostly evident at the aej latitude and north of it the perturbation structure is similar to that of 3 5 days in the north except that the more developed circulation centers moving more to the north lead to a large modulation of the jet zonal wind component south of the aej the wind field perturbation is weaker and quite different the zonal wind core of the jet appears to be an almost symmetric axis in the 6 9 day wind field pattern a clockwise circulation north of the aej being associated with a counter clockwise circulation south of the jet and vice versa these 6 9 day easterly waves also affect significantly rainfall convection and monsoon flux but in a different way inducing large zonal convective bands in the itcz mostly in the trough and behind it as opposed to the 3 5 day wave regime these rainfall anomalies are associated with anomalies of opposite sign over the guinea coast and the sahelian regions over the continent these waves are more active in june july and in august september over the ocean gate phase i gave an example of such an active 6 9 day wave pattern considered as a sequence of weak easterly wave activity this phase was also a sequence of high 6 9 day easterly wave activity we suggest that the 6 9 day regime results from an interaction between the 3 5 day easterly wave regime maintained by the barotropic baroclinic instability of the aej and the development of strong anticyclonic circulations north of the jet over west africa and both north and south of the jet over the atlantic significantly affecting the jet zonal wind component the permanent subtropical anticyclones azores libya st helena could help initiation and maintenance of such regime over west africa and tropical atlantic based on an a priori period band criterion our synoptic classification has enabled us to point out two statistical and meteorological easterly wave regimes over west africa and tropical atlantic ncep ncar and ecmwf reanalyses are in good agreement the main difference being a more developed easterly wave activity in the ncep ncar reanalyses especially for the 3 5 day regime over the atlantic

case study of an intense african easterly wave abstract the life cycle of an intense african easterly wave aew over the african continent is examined using european centre for medium range weather forecasts ecmwf operational analyses meteosat satellite images and synoptic observations this system the strongest aew of 2000 can be tracked from central north africa into the eastern atlantic ocean where it is associated with the genesis of hurricane alberto synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity pv allowing exploration at the role of multiple scales in the evolution of this aew the authors analysis promotes the division of the aew life cycle into three distinctive phases i initiation the aew development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in sudan this convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical north africa ii baroclini

a preliminary morphology of precipitation systems in tropical northern australia a preliminary morphology of convective systems observed in the vicinity of darwin australia is presented several types of tropical convection during monsoonal and break season flow are identified using specific examples and compared with a range of systems observed worldwide in the general context of the bulk richardson number a significant spectrum of convective activity ranging from low shear and low convective available potential energy cape to high shear and moderate to high cape typical of mid latitude severe storms is identified significant differences between monsoonal flow and break season systems are apparent the formation of convective lines perpendicular to the low level shear and the apparent balance between advancing shallow cold pools and the shear are ubiquitous features convective scale downdraughts and mesoscale descent appear to be responsible for redistribution of 700 mb momentum to the planetary boundary layer pel the motion of the break season squall lines appears to result from an equilibrium between the cold pool and the undisturbed environmental pel flow deviations from this balance are hypothesized to result in the observed propagating and non propagating or slow moving modes examples of these modes are given showing the classical continuous development of new cells in a balanced state along the leading edge of long lived squalls and in the propagating case the discontinuous development of new cells on the cold pool ahead of the squall

characteristics of an african easterly wave observed during namma abstract the evolution of an african easterly wave is described using ground based radar and ancillary datasets from three locations in west africa niamey niger continental dakar senegal coastal and praia republic of cape verde oceanic the data were collected during the combined african monsoon multidisciplinary analyses amma and nasa amma namma campaigns in august september 2006 two precipitation events originated within the wave circulation and propagated with the wave across west africa mesoscale convective systems mcss associated with these events were identified at all three sites ahead of within and behind the 700 mb wave trough an additional propagating event was indentified that originated east of the wave and moved through the wave circulation the mcs activity associated with this event did not show any appreciable change resulting from its interaction with the wave the mcs characteristics at each site were different likely due to a combination of life cycle effects and

geochemical processes and solute transport at the seawater freshwater interface of a sandy aquifer geochemical processes occurring at a seawater freshwater interface were studied in a shallow coastal siliclastic aquifer containing minor amounts of calcite data were collected from 106 piezometers in a 120 m transect from the coastline and landward in the first 40 m from the coastline a wedge of saltwater is intruding below the freshwater aquifer the aquifer is strongly reduced with mineralization of organic matter by methanogenesis in the freshwater aquifer and sulfate reduction dominating in the most seaward part of the saline aquifer the spatial separation of cations in the aquifer indicated a slow freshening process where ca2 from freshwater displaced the marine cations na and mg2 from the exchanger complex the resulting loss of ca2 from solution decreases the saturation state for calcite and possibly causes calcite dissolution a storm flooding event was recorded where pulses of dense seawater sank through the fresh aquifer as a result the terminal electron accepting process switched from methanogenesis to sulfate reduction the pulses of sinking seawater also triggered cation exchange reactions where ca2 was expelled from the exchanger by seawater na and mg2 the released ca2 is being flushed from the aquifer by groundwater flow and this export of ca2 will in the long term cause decalcification of the sediment the water composition in the aquifer is in a transient state as the result of various processes that operate on different timescales oxidation of organic matter occurs continuously but at a rate decreasing on a geological time scale the freshening of the aquifer operates on the timescale of a few years the episodic flooding and sinking of seawater through the aquifer proceeds in the course of days to weeks but occurs irregularly with years in between

small scale sublithospheric continental mantle deformation constraints from sks splitting observations summary it is generally believed that azimuthal seismic anisotropy is an important property of the subcontinental mantle and that this is a relatively small scale phenomenon compared with anisotropy of the oceanic mantle during recent years shear wave splitting has come to be viewed as the most direct diagnostic of azimuthal anisotropy the magnitude of the splitting delay times can often rule out a crustal source of anisotropy however it is not yet clear whether the source of anisotropy lies within the lithosphere or the sublithosphere it is essential to know the scale of lateral variations and the distribution of anisotropy with depth in order to understand the origin of this phenomenon here we describe a way to constrain the depth of the anisotropy by studying the lateral variation of the splitting parameters at neighbouring seismic stations we use sks and skks phases recorded at the nars nl array a relatively dense network of broad band stations separated by about 50 km fresnel zones at different depths are calculated for these phases the depth of the anisotropy is constrained by the criterion that fresnel zones corresponding to different splitting observations should not overlap variability in the splitting measurements for events with different directions of approach recorded at one station and for single events recorded at various stations provides evidence for a non homogeneously distributed source of anisotropy located below the array at different depths we show that this variability in the measurements cannot be due to effects of crustal scattering our results indicate that the anisotropy is partly located in the sublithospheric mantle at about 400 km depth this suggests that the deformation in this region is subject to significant lateral variations

the role of environmental shear and thermodynamic conditions in determining the structure and evolution of mesoscale convective systems during toga coare abstract a collection of case studies is used to elucidate the influence of environmental soundings on the structure and evolution of the convection in the mesoscale convective systems sampled by the turboprop aircraft in the tropical ocean global atmosphere toga coupled ocean atmosphere response experiment coare the soundings were constructed primarily from aircraft data below 5 6 km and primarily from radiosonde data at higher altitudes the well documented role of the vertical shear of the horizontal wind in determining the mesoscale structure of tropical convection is confirmed and extended as noted by earlier investigators nearly all convective bands occurring in environments with appreciable shear below a low level wind maximum are oriented nearly normal to the shear beneath the wind maximum and propagate in the direction of the low level shear at a speed close to the wind maximum when there is appreciable shear at middle levels 800 400 mb convective bands form parallel to the shear with

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

boundary layer mantle flow under the dead sea transform fault inferred from seismic anisotropy lithospheric scale transform faults play an important role in the dynamics of global plate motion near surface deformation fields for such faults are relatively well documented by satellite geodesy strain measurements and earthquake source studies and deeper crustal structure has been imaged by seismic profiling relatively little is known however about deformation taking place in the subcrustal lithosphere that is the width and depth of the region associated with the deformation the transition between deformed and undeformed lithosphere and the interaction between lithospheric and asthenospheric mantle flow at the plate boundary here we present evidence for a narrow approximately 20 km wide subcrustal anisotropic zone of fault parallel mineral alignment beneath the dead sea transform obtained from an inversion of shear wave splitting observations along a dense receiver profile the geometry of this zone and the contrast between distinct anisotropic domains suggest subhorizontal mantle flow within a vertical boundary layer that extends through the entire lithosphere and accommodates the transform motion between the african and arabian plates within this relatively narrow zone

evidence of two regimes of easterly waves over west africa and the tropical atlantic synoptic scale easterly waves at 700 hpa have been studied over west africa and the tropical atlantic for the summers of 1979 1995 spectral analyses fast fourier and wavelet transforms of 700 hpa meridional wind component along 17 5 n enable to point out two band periods between 3 and 5 days and between 6 and 9 days an example of each wave is shown composite analysis confirms the evidence of two such regimes of easterly waves during summer over west africa and the tropical atlantic the 6 9 day wave regime differ from the 3 5 day wave regime by larger anticyclonic cells originated from the libyan and the azores areas and by more northern tracks

three dimensional kinematic and microphysical evolution of florida cumulonimbus part ii frequency distributions of vertical velocity reflectivity and differential reflectivity abstract high resolution radar data collected in florida during the convection and precipitation electrification experiment are used to elucidate the microphysical and kinematic processes occurring during the transition of a multicellular storm from convective to stratiform stages a statistical technique is employed to examine the evolving properties of the ensemble small scale variability of radar reflectivity vertical velocity and differential reflectivity over the entire storm differential radar reflectivity data indicate that the precipitation at upper levels was nearly glaciated early in the storm s lifetime dual doppler radar data show that throughout the storm s lifetime both updrafts and down drafts were present at all altitudes and that most of the volume of the radar echo contained vertical velocities incapable of supporting precipitation size particles thus the ensemble microphysical properties of the storm were increasingly dominated by particles falling in an environment of weak vertic

frequency dependent anisotropy in wellington new zealand shear wave splitting measurements are made using teleseismic s scs and sks waveforms recorded at the gdsn broadband station snzo situated in south karori new zealand the average s and sks delay times are around 2 to 3 s among the highest in the world the average scs delay time is significantly smaller around 1 s this discrepancy appears to be due to differences in dominant frequency the most likely cause of frequency dependent anisotropy is oriented heterogeneities with a scale length much smaller than the wavelength the fast polarizations range between 21 and 79 with an average ne sw direction which is sub parallel to the trend of the local geologic structure and the strike of the hikurangi subduction zone azimuthal variations in delay time which cannot be explained by differences in period may be due to a dipping axis of symmetry or laterally varying anisotropy or a more complicated symmetry system

deep convective system evolution over africa and the tropical atlantic in the tropical african and neighboring atlantic region there is a strong contrast in the properties of deep convection between land and ocean here satellite radar observations are used to produce a composite picture of the life cycle of convection in these two regions estimates of the broadband thermal flux from the geostationary meteosat 8 satellite are used to identify and track organized convective systems over their life cycle the evolution of the system size and vertical extent are used to define five life cycle stages warm and cold developing mature cold and warm dissipating providing the basis for the composite analysis of the system evolution the tracked systems are matched to overpasses of the tropical rainfall measuring mission satellite and a composite picture of the evolution of various radar and lightning characteristics is built up the results suggest a fundamental difference in the convective life cycle between land and ocean african storms evolve from convectively active systems with frequent lightning in their developing stages to more stratiform conditions as they dissipate over the atlantic the convective fraction remains essentially constant into the dissipating stages and lightning occurrence peaks late in the life cycle this behavior is consistent with differences in convective sustainability in land and ocean regions as proposed in previous studies the area expansion rate during the developing stages of convection is used to provide an estimate of the intensity of convection reasonable correlations are found between this index and the convective system lifetime size and depth

three dimensional structure and dynamics of african easterly waves part i observations the mean structure of african easterly waves aews over west africa and the adjacent atlantic is isolated by projecting dynamical fields from reanalysis and radiosonde data onto space time filtered satellite derived outgoing longwave radiation these results are compared with previous studies and an idealized modeling study in a companion paper which provides evidence that the waves bear a close structural resemblance to the fastest growing linear normal mode of the summertime basic state flow over africa there is a significant evolution in the three dimensional structure of aews as they propagate along 10 n across west africa at this latitude convection occurs in northerly flow to the east of the greenwich meridian then shifts into the wave trough and finally into southerly flow as the waves propagate offshore into the atlantic itcz in contrast to the north of the african easterly jet along 15 n convection remains in southerly flow throughout the waves trajectory along 10 n over west africa the location of convection is consistent with the adiabatic dynamical forcing implied by the advection of perturbation vorticity by the mean thermal wind in the zonal direction as in the companion paper offshore and along 15 n the relationship between the convection and dynamics is more complex and not as easily explained in terms of dynamical forcing alone

anisotropy beneath california shear wave splitting measurements using a dense broadband array we have determined the shear wave splitting parameters for a dense network of broad band stations in the western united states using high quality sks and skks waveforms with particularly high spatial resolution in the southern california region covered by the trinet seismic network the alignment of most fast polarization directions can be explained by plate tectonic extensional and compressional events we find that the overall pattern of fast directions agrees well with the pn anisotropy model by hearn that images the uppermost mantle furthermore the measured fast directions are generally orthogonal to the maximum horizontal compressive stress directions as determined from shallow crustal stress indicators world stress map this suggests that the pattern of anisotropy in the western us is predominantly uniform throughout the crust and upper mantle and that a 100 150 km thick layer as estimated from the sks delay time assuming 4 per cent anisotropy of anisotropic material has experienced coherent strain conditions and has undergone a similar deformation history a more detailed investigation reveals small scale lateral variations in anisotropy that are manifested by minor differences in splitting parameters between closely located stations as well as between sks and skks for the same station event pairs we also identify a contrast in splitting parameters between central the greater bay area and southern california in central california our measurements show evidence for variation of splitting parameters with backazimuth while in southern california the pattern of measurements can be fit adequately with a single layer anisotropy model this contrast dominates any consistent effect of the san andreas fault saf we can fit the variation of splitting parameters as a function of polarization azimuth for some stations in the vicinity of the saf better with a two layer anisotropy model than a single layer model with one thin layer having a fast direction parallel to the saf however many alternative models which could incorporate dipping axes of anisotropy lateral variation of anisotropy or a more continuous variation of fast direction with depth would be able to produce a similar fit

shear wave splitting from local events beneath the ryukyu arc trench parallel anisotropy in the mantle wedge we present shear wave splitting measurements from local slab earthquakes at eight seismic stations of the japanese f net array located in the ryukyu arc we obtained high quality splitting measurements for 70 event station pairs and found that the majority of the measured fast directions were parallel to the strike of the trench and perpendicular to the convergence direction splitting times for individual measurements ranged from 0 25 to 2 s most values were between 0 75 and 1 25 s both the fast directions and the split times were similar to results for teleseismic s k ks and s wave splitting at the same stations which suggests that the anisotropy is located in the mantle wedge above the slab we considered several mantle deformation scenarios that would result in predominantly trench parallel fast directions and concluded that for the ryukyu subduction system the most likely explanation for the observations is corner flow in the mantle wedge combined with b type olivine fabric in this model the flow direction in the wedge is perpendicular to the trench but the fast axes of olivine crystals tend to align perpendicular to the flow direction resulting in trench parallel shear wave splitting

simultaneous inversion of shear wave splitting observations from seismic arrays 1 seismic waveforms recorded at high density receiver arrays facilitate the application of new inversion techniques which take advantage of the coherent nature of the observations we use measurements of shear wave splitting parameters from observed sks waveforms along a dense receiver profile and compare them with splitting parameters obtained from numerical waveform modeling through anisotropic earth models we use two different iterative approaches for the inversion of the observed splitting parameters 1 a local optimization technique the downhill simplex method and 2 a global genetic algorithm search in our forward modeling we calculate sks waveforms by a finite difference fd method solving the anisotropic wave equation instead of deriving individual anisotropic models for each station and combining them into one model by the comparison of fd modeling and observations we avoid a direct interpretation of the splitting parameters in terms of medium properties we apply these techniques to the analysis of sks phases recorded along a 100 km profile located at the dead sea transform fault the measured splitting parameters show gradual short scale variations along the profile which depend on frequency lateral and vertical variations of anisotropy are accounted for by two dimensional block structures for the simplex method the complexity of the models the number of anisotropic blocks is increased gradually whereas the number of anisotropic blocks is kept fixed for the genetic algorithm search the anisotropic structures of the best fitting models derived from the two inversion approaches agree well the results support earlier interpretations of the observations in terms of a narrow approximately 20 km wide vertical decoupling zone in the mantle that accommodates the transform motion between the african and arabian plates

precipitation from african easterly waves in a coupled model of the tropical atlantic abstract a regional coupled climate model is configured for the tropical atlantic to explore the role of synoptic scale african easterly waves aews on the simulation of mean precipitation in the marine intertropical convergence zone itcz sensitivity tests with varying atmospheric resolution in the coupled model show that these easterly waves are well represented with comparable amplitudes on both fine and coarse grids of the atmospheric model significant differences in the model simulations are found in the precipitation fields however where heavy rainfall events occur in the region of strong cyclonic shear of the easterly waves only on the higher resolution grid this is because the low level convergence due to the waves is much larger and more realistic in the fine resolution simulation which enables heavier precipitation events that skew the rainfall distributions toward longer tails the variability in rainfall on these time scales accounts for more than 60 70 of the total variability as a

three dimensional structure and dynamics of african easterly waves part iii genesis this paper promotes the view that african easterly waves aews are triggered by localized forcing most likely associated with latent heating upstream of the region of observed aew growth a primitive equation model is used to show that aews can be triggered by finite amplitude transient and localized latent heating on a zonally varying basic state that is linearly stable heating close to the entrance region of the african easterly jet aej is shown to initiate aews downstream the heating leads to an initial trough that reaches the west african coast about 5 7 days later depending on the nature of the heating profile after this a structure that projects strongly onto the leading linear normal mode of the basic state becomes established characterized by a number of westward propagating disturbances that strongly resemble aews the sensitivity of the forced aews to the nature and location of the heating profile is examined aews are most efficiently triggered by heating profiles that establish lower tropospheric circulations close to the entrance region of the aej in the present study this was best achieved by lower tropospheric heating from shallow convection or upper level heating and lower level cooling from a stratiform precipitation profile both profiles have significant heating gradients in the vertical in the mid to lower troposphere this triggering paradigm for the genesis of aews has consequences for the variability and predictability of aews at weather and climate time scales in addition to the nature of the aej often emphasized it is crucial to consider the nature and variability of upstream heating triggers

the propagation and diurnal cycles of deep convection in northern tropical africa the propagation and diurnal cycle of organized convection in northern tropical africa are examined using five years 1999 2003 of digital infrared imagery for may august reduced dimension techniques are used to document the properties of cold clouds proxies for deep convection and precipitation large scale environments are diagnosed from global analyses organized convection in africa consists of coherent sequences or episodes which span an average distance of about 1000 km and last about 25 h a substantial fraction of events exhibits systematic propagation at regional to continental scales while undergoing decay and regeneration episodes with 36 h duration and 1472 km span recur at a one per day interval most episodes have phase speed of 10 20 m s 1 which is faster than most african easterly waves convective episodes tend to initiate in the lee of high terrain consistent with thermal forcing from elevated heat sources average diurnal frequency maxima result from the superposition of local diurnal maximum with the delayed phase arrival of systems propagating from the east propagation occurs with moderate low to mid tropospheric shear which varies with the african easterly jet migration and west african monsoon phases frequent deep convection occurs with local shear maxima near high terrain for the peak monsoon period and for 10 w 10 e where easterly waves and convective systems are frequent 35 of cold cloud episodes occur east of the wave trough compared with about 24 to the west based on the coherent behaviour of organized propagating convection inferences may be made regarding the prediction of precipitation beyond one or two days copyright 2008 royal meteorological society

a diagnostic modelling study of the trailing stratiform region of a midlatitude squall line abstract a kinematic model with continuity equations for sensible heat water vapor cloud water cloud ice rain snow and graupel is used to determine the steady state thermodynamic and microphysical processes associated with the observed mesoscale air motion fields in the trailing stratiform region of the 22 may 1976 oklahoma squall line two versions of the observed air motions are used a low resolution but areally extensive flow pattern derived from the composite sounding data of ogura and liou and a high resolution but less areally extensive flow field obtained from the dual doppler radar analysis of smull and houze model calculations based on the high resolution doppler derived mesoscale air motions show that the location and horizontal scale of the region of most intense stratiform precipitation are determined by the pattern of horizontal transport and fallout of snow and low density graupel particles advected into the stratiform region from the leading line of convective cells the stratiform

investigation of seismic anisotropy beneath the dead sea fault using dense networks of broadband stations 1 shear waveforms from core refracted sks phases recorded at 105 portable stations belonging to the desert and desire campaigns and nine permanent broadband stations of the israel seismological network are analyzed to study polarization seismic anisotropy beneath the region of the dead sea transform fault in the middle east shear wave splitting parameters exhibit variations with back azimuth initial polarization of the incoming sks waves the pattern of this variation is nearly constant along the strike of the fault suggesting a laterally uniform anisotropic structure beneath the dead sea region the modeling of the azimuthal variations of the shear wave splitting parameters and split waveforms yields two layered anisotropic models consisting of an upper layer with nearly n s symmetry axis and a deeper layer with around n25 e symmetry axis the split time is almost equally partitioned between the upper and lower layers allotting a value of 0 6 0 8 s to each layer 2 d finite difference modeling across the southern segment of the dead sea transform fault demonstrates that anisotropic structure in the strike normal direction is relatively uniform the dead sea transform fault appears not to have a significant role in the development of the regional anisotropic fabric the upper anisotropic layer is possibly related to a fossil fabric in the lithosphere inherited from the precambrian pan african orogeny the lower layer may be related to the mantle deformation due to the relative motion between the lithospheric plates and the asthenosphere and possibly affected by the local flow field due to mantle plumes as inferred by other studies

short scale variations of shear wave splitting across the dead sea basin evidence for the effects of sedimentary fill 1 we examine shear wave splitting of sks waveforms collected by a temporary array of 68 stations in the region of the dead sea basin the observed splitting parameters exhibit systematic variations along a dense ew trending 60 km profile across the basin the delay times vary significantly between 1 0 and 2 8 seconds with smaller values in the very center of the profile the fast polarizations are oriented more or less parallel to the strike of the dead sea transform fault and vary between 10 and 20 degrees with respect to north finite frequency waveform modeling reveals that the source region of the small scale lateral variations is likely located within the crust the modeling further shows that purely isotropic velocity variations affect shear wave splitting to a large degree the observed variations of splitting parameters can be explained by the sedimentary fill of the basin and its low isotropic seismic velocities whereas the mantle is uniformly anisotropic our study indicates that precaution must be taken when interpreting short scale lateral variations of shear wave splitting in terms of anisotropic structures in the crust or upper mantle

teleseismic imaging of subaxial flow at mid ocean ridges traveltime effects of anisotropic mineral texture in the mantle summary deformation of peridotite caused by mantle flow beneath an oceanic spreading centre can result in the development of seismic anisotropy traveltime anomalies and shearwave splitting will develop as seismic energy propagates through such an anisotropic region thus providing a signature of the deformation field at depth in this study we investigate the nature of deformation associated with mantle upwelling for two models of flow in the upper 100 km of the mantle the finite strain fields of the passive upwelling model versus the buoyancy enhanced upwelling model are quite different this suggests that mineral aggregates deform differently in the two models thus developing seismic signatures that are distinguishable numerical estimates of the corresponding mineral textures are made using polycrystal theory for olivine with four operative slip systems the activation of a slip system is determined for each grain on the basis of the local critical resolved shear stress the computed grain deformation reflects a balance between stress equilibrium for the aggregate as a whole and strain continuity between neighbouring grains within the aggregate this approach enables a direct link to be made between the model flow fields and the resulting texture development given these mineral orientation distributions elastic parameters are calculated and wavefronts are propagated through the anisotropic structure traveltimes for teleseismic body waves are computed using ray theory and amplitudes are estimated for an across axis profile extending 100 km from the ridge axis relative p wave residuals of up to 1 s are predicted for the buoyant model with on axis arrivals being earliest since near vertical velocities are fastest beneath the axis on axis p wave arrivals for the passive model are half a second earlier than arrivals 60 km off axis and relative delays continue to increase slowly as distance from the ridge increases s wave splitting of almost a second is predicted for the buoyant model whereas less than a half second of splitting is determined for the passive model

modeling anisotropy and plate driven flow in the tonga subduction zone back arc the goal of this study is to determine whether shear wave splitting observed in subduction zone back arc regions the tonga subduction zone in particular can be quantitatively modeled with flow in the back arc mantle driven by the motions of the subducting slab and the upper back arc plate we calculated two dimensional mantle flow models using known tonga plate motions as boundary conditions and assuming a range of uniform and variable viscosity structures shear wave splitting was predicted for the anisotropy due to lattice preferred orientation lpo of olivine and orthopyroxene in the flow model finite strain fields the predicted shear wave splitting provides a good match to the fast directions parallel to the azimuth of subducting plate motion and splitting times 0 5 1 5 s observed in tonga both for models where lpo anisotropy develops everywhere above 410 km and for models where lpo anisotropy is confined to regions of relatively high stress if lpo anisotropy does develop over the entire upper 410 km of the mantle the strength of anistropy induced by a given amount of shear strain must be relatively weak 4 for shear strains of 1 5 with a maximum value of 6 for very large strains the splitting observations are comparably fit by a wide range of different viscosity models anisotropy due to melt filled cracks aligned by stresses in the back arc flow models predicts fast directions roughly normal to observed values and thus cannot alone explain the observed splitting

north atlantic hurricanes contributed by african easterly waves north and south of the african easterly jet abstract a majority of tropical cyclones in the north atlantic develop from african easterly waves aews which originate along both the southern and northern flanks of the midtropospheric african easterly jet aews and aewn respectively the purpose of this note is to identify the contribution of aewss and aewns to north atlantic tropical cyclones that develop from aews applying a manual backtracking approach to identify the genesis locations of aews it was found that the population ratio of tropical cyclones formed from aewss to those formed from aewns is 1 1 2 because the population ratio of aewss to aewns is 1 2 5 the conversion rate of the former aews to tropical cyclones is twice as effective as the latter waves in addition it was found that aewns travel farther and take longer to transform into tropical cyclones than aewss which is likely because the aewns are drier and shallower than aewss an analysis of various terms in the moisture and vorticity budgets reveals that the monsoon trough

the low level structure of african easterly waves in 1995 abstract the existence of african easterly waves aews north of the african easterly jet aej core with maximum amplitude at low levels has been confirmed and clarified using radiosonde data and the u k meteorological office global model analysis from the hurricane season of 1995 at bamako 12 5 n 8 0 w the aews were characterized mainly by maximum amplitudes at the level of the aej around 700 mb whereas at dakar 14 7 n 17 5 w the waves were characterized by maxima between 850 and 950 mb the low level waves to the north of the aej arise in association with baroclinic interactions between the negative meridional potential vorticity pv gradients in the jet core and the positive low level gradient of potential temperature enhanced by the presence of low static stability air north of the aej these waves follow the positive meridional gradients over northern africa in contrast to the jet level aews that follow the meridional pv gradients at the level of the aej cross correlation analysis

radar characteristics of precipitation features in the epic and tepps regions of the east pacific abstract ship based radar data are used to compare the structure of precipitation features in two regions of the east pacific where recent field campaigns were conducted the east pacific investigation of climate processes in the coupled ocean atmosphere system epic 2001 10 n 95 w in september 2001 and the tropical eastern pacific process study tepps 8 n 125 w in august 1997 corresponding july september 1998 2004 tropical rainfall measuring mission trmm precipitation radar pr data are also used to provide context for the field campaign data an objective technique is developed to identify precipitation features in the ship and trmm pr data and to develop statistics on horizontal and vertical structure and precipitation characteristics precipitation features were segregated into mesoscale convective system mcs and sub mcs categories based on a contiguous area threshold of 1000 km2 these features were required to have at least one convective pixel as well as an other nc category com

the non commutivity of shear wave splitting operators at low frequencies and implications for anisotropy tomography summary measurements of the splitting or birefringence of seismic shear waves constitute a powerful and popular technique for characterizing azimuthal anisotropy in the upper mantle the increasing availability of data sets from dense broad band seismic arrays has driven interest in the development of techniques for the tomographic inversion of shear wave splitting data and in comparing splitting measurements with anisotropic upper mantle models obtained from other constraints such as surface wave analysis two different theoretical approaches have been developed for predicting apparent shear wave splitting parameters fast direction and delay time for models that include multiple layers of anisotropy at depth which is useful for comparing azimuthally anisotropic surface wave models with shear wave splitting measurements these approaches differ in one key aspect which is whether or not the shear wave splitting operator can be treated as commutative in this paper we investigate the theoretical source of this discrepancy and show that at frequencies relevant to most studies of upper mantle anisotropy the term that results in the non commutivity of the shear wave splitting operator in the expressions for multiple layer splitting must be retained in contrast the quantity known as the splitting intensity which is closely related to the apparent fast direction and delay time does commute at these frequencies we illustrate these inferences with forward modelling examples and discuss their implications for the tomographic inversion of shear wave splitting measurements the comparison of surface wave models with shear wave splitting observations and the joint inversion of surface wave and shear wave splitting observations for upper mantle anisotropic models

analysis of convection and its association with african easterly waves abstract the association between convection and african easterly wave aew activity over tropical africa and the tropical atlantic during the boreal summer is examined using satellite brightness temperature tb and ecmwf reanalysis datasets spectral analysis using 18 yr of tb data shows significant variance in the 2 6 day range across most of the region within the regions of deep convection this time scale accounts for about 25 35 of the total variance the 2 6 day convective variance has similar amplitudes over western and eastern africa while dynamic measures of aew activity show stronger amplitudes in the west this study suggests that weak aew activity in the east is consistent with initial wave development there and indicates that convection triggered on the western side of the mountains over central and eastern africa near darfur western sudan and ethiopia has a role in initiating aews westward the subsequent development and growth of aews in west africa is associated with stronger cohe

air motions and precipitation structure of an early summer squall line over the eastern tropical atlantic abstract the 28 june 1974 squall line over the ship army of the global atmospheric research programme s atlantic tropical experiment gate has been examined aircraft radar satellite sounding and surface data have been employed in the analysis this squall line is the third gate oceanic squall line to have been subjected to intensive case study analysis the vertical air motions and the relative amounts of convective and stratiform rain in this case were similar to those of the other two lines however this squall line moved more slowly without discrete jumps and the horizontal airflow relative to the system was quite different from that associated with the other two lines except at the surface the strong front to rear relative flow normal to the line observed at most levels in the other cases was absent with the relative flow tending instead to be parallel to the line in the low to middle troposphere relative flow parallel to the line from the north fed a post squall mesoscale downdraft in t

sks splitting measurements beneath northern apennines region a case of oblique trench retreat abstract we present here the new observations of seismic anisotropy obtained from sks birefringence analysis we studied 27 teleseismic earthquakes recorded by the temporary seismic network of retreat project in the northern apennines region for each station event couple we calculate the anisotropic parameters delay time and fast polarization direction by minimizing the energy in the transverse component our measurements confirm the existence of two domains the tuscany domain on the south west with respect to the apennines shows mostly nw se fast axes directions with a rotation toward e w direction moving toward the tyrrhenian sea the adria domain north east of the apennines orogen shows more scattered measurements with prevailing n s to nne ssw directions also with back azimuthal dependence the transition between the two domains is abrupt in the northern part of the study region but more gradual in the southern part measured delay times 1 8 s on average suggest that the detected anisotropy is located principally in the asthenosphere beneath the adria domain where the presence of a double layer structure seems consistent a lithospheric contribution is plausible an interpretation in terms of ongoing mantle deformation suggests a differential evolution of the trench retreat process along the northern apennines orogen the orogen parallel anisotropy in the study region is beneath the inner part of the belt instead of beneath its crest and no orogen normal measurements are found in the tuscany side compared to the anisotropy pattern of the typical slab retreat seen in southern part of the northern apennines in the northernmost one the anisotropy suggests that an oblique trench retreat has occurred possibly linked to northern apennines retreat since 5 ma

crustal structure of the southern dead sea basin derived from project desire wide angle seismic data summary as part of the dead sea integrated research project desire a 235 km long seismic wideangle reflection refraction wrr profile was completed in spring 2006 across the dead sea transform dst in the region of the southern dead sea basin dsb the dst with a total of about 107 km multi stage left lateral shear since about 18 ma ago accommodates the movement between the arabian and african plates it connects the spreading centre in the red sea with the taurus collision zone in turkey over a length of about 1 100 km with a sedimentary infill of about 10 km in places the southern dsb is the largest pull apart basin along the dst and one of the largest pull apart basins on earth the wrr measurements comprised 11 shots recorded by 200 three component and 400 one component instruments spaced 300 m to 1 2 km apart along the whole length of the e w trending profile models of the p wave velocity structure derived from the wrr data show that the sedimentary infill associated with the formation of the southern dsb is about 8 5 km thick beneath the profile with around an additional 2 km of older sediments the depth to the seismic basement beneath the southern dsb is about 11 km below sea level beneath the profile seismic refraction data from an earlier experiment suggest that the seismic basement continues to deepen to a maximum depth of about 14 km about 10 km south of the desire profile in contrast the interfaces below about 20 km depth including the top of the lower crust and the moho probably show less than 3 km variation in depth beneath the profile as it crosses the southern dsb thus the dead sea pull apart basin may be essentially an upper crustal feature with upper crustal extension associated with the left lateral motion along the dst the boundary between the upper and lower crust at about 20 km depth might act as a decoupling zone below this boundary the two plates move past each other in what is essentially a shearing motion thermo mechanical modelling of the dsb supports such a scenario as the desire seismic profile crosses the dst about 100 km north of where the desert seismic profile crosses the dst it has been possible to construct a crustal cross section of the region before the 107 km left lateral shear on the dst occurred

geochemical calculations and observations on salt water intrusions i a combined geochemical minxing cell model abstract a combined geochemical mixing cell flow model has been developed the geochemical model is based on the computer program eq 3 6 wolery 1982 has been modified to include cation exchange reactions and is incorporated in a one dimensional mixing cell model for stationary one dimensional transport dispersion or diffusion is calculated by mixing the cell contents with neighbouring cells relationships between mixing factors and dispersivity and diffusion coefficients are derived the model is used to simulate concentration changes during salt water intrusions fundamental differences are shown to exist between diffusion of salt water from a stagnant water body and dispersive intrusions during marine transgressions or flooding events with dispersive flow characteristic water compositions develop in space and time as a result of cation exchange with diffusion these steep concentration gradients are smoothed out and mixed compositions occur the concept that characteristic concentrations occur during dispersive flow is used to obtain paleohydrological information in a dutch polder the depth of the fresh salt water interface before polder development is deduced from a simulation of observed exchange reactions in the polder groundwater

the influence of plate motions on three dimensional back arc mantle flow and shear wave splitting both the polarization direction of the fast shear waves and the types of deformation within overriding plates vary between the back arc basins of western pacific subduction zones the goal of this study is to test the possibility that motions of the overriding plates may control the patterns of seismic anisotropy and therefore the observed shear wave splitting we calculated three dimensional models of viscous asthenospheric flow driven by the motions of the subducting slab and overriding plates shear wave splitting was calculated for polymineralic anisotropy within the back arc mantle wedge assuming that the anisotropy was created by flow induced strain predicted splitting may differ substantially depending on whether anisotropy is computed directly using polycrystalline plasticity models or is based on the orientation of finite strain a parameter study shows that both finite strain and textural anisotropy developed within three dimensional plate coupled asthenospheric flow models are very heterogeneous when back arc shearing occurs within the overriding plate therefore predicted shear wave splitting varies strongly as a function of plate motion boundary conditions and with ray path through the back arc asthenosphere flow models incorporating plate motion boundary conditions for the tonga southern kuril and eastern aleutian subduction zones produce splitting parameters consistent with observations from each region trench parallel flow generated by small variations in the dip of the subducting plate may be important in explaining observed fast directions of anisotropy sampled within the innermost corner of the mantle wedge

lithosphere structure across the dead sea transform as constrained by rayleigh waves observed during the desert experiment summary the interdisciplinary dead sea rift transect desert project that was conducted in israel the palestine territories and jordan has provided a rich palette of data sets to examine the crust and uppermost mantle beneath one of earth s most prominent fault systems the dead sea transform dst as part of the passive seismic component thirty broad band sensors were deployed in 2000 across the dst for roughly one year during this deployment we recorded 115 teleseismic earthquakes that are suitable for a fundamental mode rayleigh wave analysis at intermediate periods 35 150 s our initial analysis reveals overall shear velocities that are reduced by up to 4 per cent with respect to reference earth model prem to the west of the dst we find a seismically relatively fast but thin lid that is about 80 km thick towards the east shallow seismic velocities are low while a deeper low velocity zone is not detected this contradicts the currently favoured thermomechanical model for the dst that predicts lithospheric thinning through mechanical erosion by an intruding plume from the red sea on the other hand our current results are somewhat inconclusive regarding asthenosphere velocities east of the dst due to the band limitation of the recording equipment in jordan

shear wave anisotropy beneath the andes from the banjo seda and pisco experiments we present the results of a detailed shear wave splitting analysis of data collected by three temporary broadband deployments located in central western south america the broadband andean joint experiment banjo a 1000 km long east west line at 20 s and the projecto de investigacion sismologica de la cordillera occidental pisco and seismic exploration of the deep altiplano seda deployed several hunderd kilometers north and south of this line we determined the splitting parameters fast polarization direction and t splitting delay time for waves that sample the above and below slab regions teleseismic ks and s scs waves from local deep focus events as well as s waves from intermediate focus events that sample only the above slab region all but one of the ks stacks for the banjo stations show e w fast directions with t varying between 0 4 and 1 5 s however for ks recorded at most of the seda and pisco stations and for local deep focus s events north and south of banjo there is a rotation of to a more nearly trench parallel direction the splitting parameters for above slab paths determined from events around 200 km deep to western stations yield small delay times 0 3 s and n s fast polarization directions assuming the anisotropy is limited to the top 400 km of the mantle olivine stability field these data suggest the following spatial distribution of anisotropy for the above slab component as one goes from east where ks reflects the above slab component to west changes from e w to n s and delay times are substantially reduced this change may mark the transition from the brazilian craton to actively deforming e w shortening andean mantle we see no evidence for the strain field expected for either corner flow or shear in the mantle wedge associated with relative plate motion the small delay times for above slab paths in the west require the existence of significant spatially varying below slab anisotropy to explain the ks results the implied anisotropic pattern below the slab is not easily explained by a simple model of slab entrained shear flow beneath the plate instead flow induced by the retrograde motion of the slab in combination with local structural variations may provide a better explanation

observations of seven african easterly waves in the east atlantic during 2006 abstract the african monsoon multidisciplinary analyses amma experiment and its downstream nasa extension namma provide an unprecedented detailed look at the vertical structure of consecutive african easterly waves during august and september 2006 seven easterly waves passed through the namma domain two waves developed into tropical cyclones debby and helene two waves did not develop and three waves were questionable in their role in the development of ernesto florence and gordon ncep global data assimilation system gdas analyses are used to describe the track of both the vorticity maxima and midlevel wave trough associated with each of the seven easterly waves dropsonde data from namma research flights are used to describe the observed wind structure and as a tool to evaluate the accuracy of the gdas to resolve the structure of the wave finally satellite data are used to identify the relationship between convection and the organization of the wind structure results support a necessary d

texture of mantle lithosphere along the dead sea rift recently imposed or inherited seismic anisotropy a property linked to the texture of the mantle rock should be distributed with depth along the trace of the dead sea rift dsr owing to a combination of present day and ancient tectonics using data from four permanent and one temporary seismic observatories we evaluate birefringence splitting of 91 teleseismic core refracted shear waves primarily sks phases we find significant levels of birefringence in the bulk of observed phases we also find that birefringence parameters fast directions and delays vary as a function of source receiver geometry notably the pattern of this directional variation in birefringence is quite similar at all sites we have examined we interpret observed birefringence in sks phases in terms of one and two layer models single layer models for all stations exhibit a fast polarization oriented 12 19 east of north with anisotropy sufficient to generate 1 3 s time delay we find strong evidence for at least two distinct anisotropic layers for the two layer models the upper layers resemble the single layer models showing near north fast polarizations and delays on the order of 1 s three out of four sites show fast polarizations in the lower layer that strike 50 80 cw from north with time delays 0 3 0 6 s one site at the northern end of the dsr displays a higher degree of anisotropy in the lower layer and a more northerly fast polarization overall the lower layers at all sites appear to be consistent with the deformation caused by plate motion relative to the asthenosphere the fabric in the upper layer is sub parallel to the present day transcurrent motion on the dsr but also matches the typical orientation of lithospheric seismic anisotropy in the arabian shield our overall conclusion is that the impact of the dsr on the rock fabric of the mantle lithosphere is probably quite weak