magnetic parameter variations in the chaona loess paleosol sequences in the central chinese loess plateau and their significance for the middle pleistocene climate transition abstract a high resolution rock magnetic investigation was performed on the chaona quaternary loess paleosol sequences in the central chinese loess plateau based on a newly developed independent unturned time scale and magnetic records we reconstructed the history of the east asia monsoons during the last 3 ma and explored the middle pleistocene climate transition mpt rock magnetic results show that the loess layers are characterized by relatively high coercivity and remanent coercivity lower magnetic susceptibility ms and that the paleosol layers are characterized by relatively high ms saturation magnetization and remanent saturation magnetization spectrum analyses indicate that there are various periods in addition to orbital periodicities according to the onset and stable appearance of 100 kyr period we consider that the mpt recorded in this section began at 1 26 ma and was completed by 0 53 ma which differs from previous investigations based on orbitally tuned time scales the forcing mechanism for the mpt was more complicated than just the orbital forcing we conclude that the rapid uplift of the tibetan plateau may have played an important role in the shift of periodicities during the middle pleistocene

frontogenesis and the development of secondary wave cyclones in fastex two major mechanisms of frontogenesis deformation and shear are important in frontal wave cyclone development horizontal deformation can suppress the nonlinear wave development using an analytic model bishop and thorpe have shown that large strain rates inhibit any wave slope amplification for real cases this ambient strain can be measured using the vorticity divergence attribution method developed by bishop this technique permits us to confirm the crucial role of such strain on the evolution of cases of wave development during the fronts and atlantic storm track experiment fastex horizontal shear in the presence of an along front thermal gradient is also an important mechanism of frontogenesis using an eady model joly and thorpe have shown that in cases of large along front thermal gradient frontal waves have growth rates smaller than the front itself and thus would not develop the domain independent attribution method developed by bishop is here extended to a geopotential field partition this leads via a nonlinear balance condition to the estimation of the ambient along front potential temperature gradient the role of such an along front potential temperature gradient is discussed as well as the relative contributions of the two frontogenesis mechanisms for the fastex cases

geomorphic evidence for northeastward expansion of the eastern qilian shan northeastern tibetan plateau abstract the eastern qilian shan is a tectonically active region consisting of a series of faults with bounded intermountain basins and is located within the transition zone between the mainland tibetan plateau and the alax block active deformation affecting the topography in this region can be quantified based on geomorphic indices therefore we applied the geomorphic indices hypsometry and stream power incision model to evaluate the tectonic deformation pattern of the eastern qilian shan hypsometric curves and s a plots of the selected seven rivers were determined hi values of every single sub basins were also calculated the results show that the local topographic evolution of the eastern qilian shan is in pre steady state and will soon reach its equilibrium state according to the analysis of regional climate condition e g rainfall and glaciers distribution and fault characteristics it is suggested that abundant glacial snowmelt water sustains fluvial incision and tectonic deformation from sustained northeastward expansion of the tibetan plateau is the main reason for the landscape growth combining the research of the deep electric structure beneath the gulang boundary zone we claimed that the eastern qilian shan even the northeastern tibet has been growing and expanding northeastward since the middle miocene and its leading edge now is the hexibao sidaoshan fault on the premise of synthetically considering the western and middle parts of the qilian shan it is suggested that the hexibao sidaoshan fault area will rise into a mountain in 1 2 ma

thermoluminescence dating of loess and palaeosols from the lantian section shaanxi province china abstract ten samples of loess and palaeosol collected from the upper part of lantian section shaanxi have been dated using the fine grain 4 11 m tl technique palaeodose values were determined by the regeneration method readhead 1982 the tl ages obtained ranged from 2 4 104 years for the upper malan loess l1 to 1 5 105 years for the upper part of loess l2 of the lishi loess fine grain tl dating does not seem to be suitable for the samples older than 150 ka the uncertainty of the tl age estimate especially for the palaeosol sample may be primarily an effect of disequilibrium of the 238u decay chain and of water content variations since deposition of sediments

river terrace systems in north west europe an archive of environmental change uplift and early human occupation abstract staircases of large scale aggradational river terraces are a notable feature of many valleys in the temperate lattitudes particularly in areas beyond the reach of the erosive activities of pleistocene ice sheets it is now recognized that the cyclic fluctuations of climate during the quaternary have driven the generation of terraces through the direct and indirect influence of both temperature and precipitation on fluviatile activity where fossiliferous deposits are preserved within terrace sequences it is often possible to date these and to correlate them with the oceanic record thus providing an important framework for the evidence of environmental change on land middle and late pleistocene terraces in different areas can commonly be seen to have formed in synchrony with glacial interglacial cycles or with longer periodicity megacycles climatic forcing alone is insufficient to cause terraces to form however uplift is also necessary so that terrace sequences can provide a useful record of crustal movement in northwest europe where some of the best known studies of river terrace sequences have been carried out the fluviatile deposits are also an important repository for palaeolithic artefacts from which a record of early human occupation can be reconstructed

stability and meromixis in a water filled mine pit a field experiment examined stability and stratification in a disused mine fit filled with 1 75 x lo7 m3 of water vertical profiles of conductivity and temperature indicated that due to substantial chemical stratification spring and autumn overturn did not penetrate to the base of the pit although parameterization indicated that double diffusion should be expected we found only circumstantial evidence of associated step structure the available data suggest that groundwater inflow created a warm salty pool of water at the base of the water column giving the appearance of a meromictic structure with a monimolimnion however this pool was noi a consistent feature suggesting both a variable inflow and significant diffusion rates from the temperature data estimates indicated that hypolimnetic vertical eddy diffusivity varied between 1 x 10e7 and 5 x lo m s l the observations identify short term relatively energetic internal wave events that may have a significant impact on this value the present study is useful in two ways first it shows how pit lakes form a natural laboratory fcr a range of processes and second it illustrates how these processes relate to diffusion paranieterization it is clear that reliable parameterization is vital for long term modeling required for prediction of water quality over decadal timescales with the closure of an open cut mining operation come the dual problems of rehabilitation of the mine pit and containment of deleterious material generated by the mine during its life in many cases the pit fills with water either naturally or through mine activities thus an artificial lake of moderate volume is created davis and ashenberg 1989 characterization of the transport processes within the water column is important for two reasons first the pit is likely to contain hazardous levels of dissolved metals and second if vertical transport rates are low then the pit may prove suitable as a storage location for hazardous material water filled mine pit lakes pit lakes have received consideration in the literature but with a biological or geochemical emphasis e g klapper and schultz 1995 crater lakes are the natural system most similar to these artificial water bodies martin 1985 macintyre and melack 1985 pit lakes are unique for several reasons they are often deep relative to natural lakes of similar surface area they have high surrounding walls that often generate a microclimate different to that outside the pit they typically have very thin sediment layers they may undergo substantial and rapid changes in water level and they are likely to contain significant concentrations of dissolved material the final point is important if the dissolved material is environmentally harmful or if it leads to meromixis with these departures from what might be associated with typical natural lakes

fluvial responses to late quaternary climate change in the shiyang river drainage system western china abstract as a drainage system located in arid western china the shiyang river combined with considerable fluvial strata and landform information provides an environmental context within which to investigate fluvial responses to late quaternary climate change sedimentological analysis and optically stimulated luminescence osl dating enabled us to reconstruct the processes and fluvial styles of three sedimentary sequences of the shagou and hongshui rivers in the shiyang drainage system our results present a variety of river behaviors during the late quaternary in these areas in the upstream shiyang river zhangjiadazhuang zjdz profile of the shagou was dominated by aggradation and a meandering channel pattern at 10 6 4 2 ka while a noticeable channel incision occurred at 4 2 ka followed by lateral channel migration in the downstream shiyang river datugou dtg profile of the hongshui was an aggrading meandering river from 39 7 to 7 2 ka while channel incision occurred at 7 2 ka another downstream profile wudunwan wdw of the hongshui was also characterized by aggradation from 22 4 to 4 8 ka however its channel pattern shifted from braided to meandering at 13 ka a discernable downcutting event occurred at 4 8 ka followed by three channel aggradation and incision episodes prior to 1 8 ka the last 1 8 ka has been characterized by modern channel and floodplain development the fluvial processes and styles investigated have a close correlation with late quaternary climate change in the shiyang river drainage during cold phases the wdw reach was dominated by aggradation with a braided channel pattern during warm phases the rivers that we investigated were also characterized by aggradation but with meandering channel patterns channel incision events and changes of fluvial style occurred mainly during climate transitions

stacked 2 6 ma grain size record from the chinese loess based on five sections and correlation with the deep sea δ18o record 1 this study aims to establish a stacked climate record of the quaternary period from the chinese loess sequence and to address the forcing mechanisms for the regional climate history of the loess plateau by correlating the stacked record with a composite 18o record in deep sea sediments a total of 18 352 samples were obtained from five loess sections located at baoji lingtai jingchuan puxian and pingliang in the southern and middle loess plateau these yielded high resolution grain size records between section correlation of these shows that although small depositional hiatuses are present in places within a single section most parts of the sections display near continuous dust deposition throughout the quaternary the grain size records were tuned simultaneously to the theoretical variations in obliquity and precession of the earth s orbit the grain size records plotted on their orbital timescales were then averaged to form a stacked loess grain size time series termed the chiloparts record this resolves most of the orbital timescale paleoclimate events buried in the loess soil sequences of the southern and middle loess plateau and can be used as a regional archive of the pleistocene climate history in the loess plateau comparison of the chiloparts record with a composite marine 18o record shows that for the past 1 8 ma the loess paleosol record can be correlated almost cycle by cycle with the marine record several discrepancies in the climatic events between the two records have also been identified implying that regional forcing mechanisms may have played a part in the climatic evolution of the chinese loess plateau

on the source of midlatitude low frequency variability part i a statistical approach to persistence abstract the forcing of low frequency variability by synoptic transient traveling perturbations is investigated within a quasi geostrophic channel forced by a localized baroclinic jet spontaneously generated baroclinic perturbations grow and decay along a storm track at the end of the track a maximum of low frequency variability is obtained in agreement with atmospheric observations the structure of low frequency variability is studied with a combination of statistical methods using a multivariate red noise model as a random reference we show that the anomalies are preferentially linked with local stationary structures or long wave vacillations according to their location and their sign a systematic study of persistence is conducted with a criterion based on rms of the streamfunction variations the interesting quantity is the probability of persistence which shows a very inhomogeneous distribution in phase space and several separated maxima the composites based on these maxima exhibit the charact

miocene cooling in the northern qilian shan northeastern margin of the tibetan plateau revealed by apatite fission track and vitrinite reflectance analysis apatite fission track and vitrinite reflectance data from phanerozoic rocks of the northern qilian shan and jiuxi basin at the northeastern margin of the tibetan plateau show a complex thermal history with multiple paleothermal events peak paleotemperatures reached in the middle cretaceous suggest that hydrocarbon generation occurred in the basin at that time paleozoic mesozoic samples show evidence of a middle tertiary cooling episode between 20 and 10 ma this thermal fingerprint is significant because stratigraphic evidence for miocene exhumation is overwhelmed by spectacular pliocene pleistocene deformation and erosion we interpret the miocene cooling as recording initial exhumation of the northern qilian shan earlier than other models have proposed i e after 6 5 ma miocene cooling is broadly coincident with cenozoic uplift and exhumation recorded elsewhere in the tibetan tarim region

life cycle simulations of shallow frontal waves and the impact of deformation strain the life cycle of shallow frontal waves and the impact of deformation strain on their development is investigated using the idealized version of the met office non hydrostatic unified model which includes the same physics and dynamics as the operational forecast model frontal wave development occurs in two stages first a deformation strain is applied to a front and a positive potential vorticity pv strip forms generated by latent heat release in the frontal updraught second as the deformation strain is reduced the pv strip breaks up into individual anomalies the circulations associated with the pv anomalies cause shallow frontal waves to form the structure of the simulated frontal waves is consistent with the conceptual model of a frontal cyclone deeper frontal waves are simulated if the stability of the atmosphere is reduced deformation strain rates of different strengths are applied to the pv strip to determine whether a deformation strain threshold exists above which frontal wave development is suppressed an objective method of frontal wave activity is defined and frontal wave development was found to be suppressed by deformation strain rates 0 4 10 5 s 1 this value compares well with observed deformation strain rate thresholds and the analytical solution for the minimum deformation strain rate needed to suppress barotropic frontal wave development the deformation strain rate threshold is dependent on the strength of the pv strip with strong pv strips able to overcome stronger deformation strain rates leading to frontal wave development than weaker pv strips copyright 2006 royal meteorological society

esr dating of quartz extracted from quaternary and neogene sediments method potential and actual limits abstract esr dating of fluvial fluvio marine and beach sediment has been tested using the al centre in quartz grains from a basin and fossil beaches located around the manche channel dating was performed in conjunction with sedimentological stratigraphical and neotectonical studies the technical basis of the method is presented in the light of the study made on recent and fossil sediment in order to establish a procedure for the determination of the palaeodose the application on a neogene basin samples allows to push back the dating esr limits results show that esr dating of sediment can provide chronozons which are necessary to the uderstanding of the history of a basin where classical chronological markers do not exist

cyclic quaternary alluviation and terracing in a nonglaciated drainage basin on the north flank of the qinling shan central china abstract pleistocene alluvial terraces of the nonglaciated ba river drainage basin on the north flank of the qinling shan are capped by a succession of loess units and paleosols that correlate with the standard marine isotope chronology and are used to date the subjacent alluvial gravels alluvial fills were deposited during isotope stages 2 6 8 12 and 16 whereas terracing occurred during interglacial stages 1 5 7 11 and 15 the apparent absence of terraces dating to stage 14 and stage 4 may be due to the lesser intensity of these glaciations compared to that of stage 2 although disruption of the alluvial regime by local tectonism is a likely alternative for the lack of a stage 4 terrace a stage 10 terrace was not positively identified from available exposures its possible absence could be related to post stage 12 uplift aggradational episodes correlate with glaciations and loess deposition whereas degradational episodes correlate with interglaciations or interstades and soil formation implying that climate is the primary control on quaternary paleohydrology this in turn points to variations in the earth s orbital geometry as the major factor that modulates both climate and ultimately the fluvial system in the qinling shan in this region glaciations were dominated by a cold dry winter monsoon climate whereas during interstades and interglaciations a warmer and wetter climate prevailed implying strengthening of the summer monsoon both the loess paleosol and the alluvial records are consistent with climate model simulations spanning the last 18 000 yr that show a change from cold dry conditions during the last glacial maximum to a climate warmer and wetter than present during the first half of the holocene

esr dating studies of palaeo debris flow deposits in dongchuan yunnan province china abstract the ages of palaeo debris flow deposits in dongchuan yunnan province were determined by esr dating they are consistent with a 14 c age estimate the results suggest that the debris flow deposits were formed during humid periods of the late pleistocene simulation experiments and sem analysis show that the resetting of ge centres in quartz is most likely due to collisions of the quartz grains during the process of rapid deposition

simulating internal and external controls on fluvial terrace stratigraphy a qualitative comparison with the maas record palaeohydrological studies indicate that the use of sedimentary palaeoclimatic indicators in fluvial sediments still requires rigorous calibration of the climatic controls on the formation of those indicators an explorative attempt is made using a simulation experiment with the fluver2 model the simulation illustrates that climate control on the erosion sedimentation behaviour of a large fluvial system differs along the fluvial system while in the uplifting hinterland only glacial extremes are recorded as sediments with a relatively limited preservation chance as terraces due to the uplift the more downstream reach towards the hinge zone displays a sensitive fluvial record registering even stadial interstadial dynamics well preserved in an extensive glacial interglacial terrace staircase downstream in a subsiding basin the most complete sedimentary record is preserved which still contains temporal gaps of approximately 20 related to the spatial delay effects of the external control of climate and related base level change a systematic comparison of simulated climate changes and resulting fluvial dynamics illustrates that due to the internal control of fluvial systems a one to one correlation between recorded fluvial dynamics and climate change becomes less the further one moves downstream along the longitudinal profile a qualitative comparison of the simulation results with the maas record reveals an interesting match suggesting that the simulated system dynamics may well apply the maas system

river terrace formation modelling and 3 d graphical simulation a model on river terrace formation is presented written in pascal and run on a vax 8600 the model calculates the influence of a fluvial system on the relief of an area with macroscopical dimensions 10kmx20kmx0 5km over a period of 2 5 million years model input relies on uplift and alternations in discharge and sediment load as a function of climatic changes the output of the model are 3 dimensional grid drawings which visualize the impact of uplift discharge and sediment load on a landscape model formulation is based on empirical information on fluvial systems which was incorporated in the model by means of a slightly adapted way of finite state modelling in which decisions act as thresholds the model is organized using two entities river and landscape with attributes that have values within a specific realistic domain the model produces plausible x y z and x y t plots in the light of existing geomorphological theories the described modelling procedure shows that it is possible to simulate river terrace formation threedimensionally with the use of empirical information

uplift driven valley incision and river terrace formation in southern england outside the limits of middle pleistocene glaciation the river basins of southern england contain long cenozoic terrestrial sedimentary records of greatest importance are the river terrace sequences which contain biostratigraphical and sedimentary evidence that testifies to the high amplitude climatic changes of the quaternary however trends in valley development cannot easily be accommodated within the framework of quaternary climatic change it is argued in this paper that episodic incision and terrace development result from uplift a consequence of the interplay between continuing alpine orogenic movements and erosion driven isostasy using a simple linear model height age an uplift rate of ca 7 cm ka 1 is estimated for the terrace sequence in the upper thames valley this preliminary model is evaluated using the records of adjacent basins 1997 john wiley sons ltd

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

objective classification of extratropical cyclogenesis extratropical cyclones experience vastly different genesis conditions at the first point of their tracks a novel method is introduced to characterize this variability and classify genesis events by computing 30 diagnostic variables that describe the synoptic scale environment of 16029 genesis events in the northern hemisphere extratropics using era interim reanalyses from 2000 2011 these variables are referred to as precursors and include parameters characterizing upper level forcing low level baroclinicity thermodynamic stability surface fluxes and moist processes the genesis events spread over a large portion of the 30 dimensional precursor phase space and no obvious clusters occur which highlights the high variability of cyclogenesis processes and indicates that they form in a continuum rather than a few distinct categories a projection of the genesis events to the first two principle components pc of the precursor phase space allows reducing the dimensionality and introducing a meaningful segmentation of the genesis events in five classes the first two pcs are characterized by upper level forcing e g the amplitude of the upper level potential vorticity pv anomaly and low tropospheric diabatic processes e g precipitation and diabatically produced low level pv respectively the first of the five identified classes constitutes the center of the pc1 pc2 phase space and represents average conditions composites reveal that the four classes of events characterized by large positive or negative scores of pc1 and pc2 occur in distinct and strongly differing flow regimes characterized by the strength of the upper level forcing the structure of the upper level jet and the amplitude of low level moist processes and baroclinicity the four classes also have clearly differing geographical distributions many well known cyclogenesis events fall within classes characterized by strong low level moist processes with or without strong upper level forcing also discussed are the robustness of the method and the linkage to classical concepts of cyclone classifications

rheological differences between archaean and younger crust can determine rates of quaternary vertical motions revealed by fluvial geomorphology surface uplift is widely observed in subaerial regions of continental crust that are distant from plate boundaries often revealed by river terrace staircases with altitude ranges of several hundred metres which typically span the quaternary this characteristic geomorphology has been explained by forcing of flow in the weak lower crustal layer by surface processes non steady state erosion and cyclical loading by ice sheets and sea level fluctuations this study tests this interpretation by showing that analogous river terrace staircases are absent in archaean crust which lacks the weak lower crustal layer found elsewhere

the spatial distribution and evolution characteristics of north atlantic cyclones a climatology of extratropical cyclones is produced using an objective method of identifying cyclones based on gradients of 1 km height wet bulb potential temperature cyclone track and genesis density statistics are analyzed and this method is found to compare well with other cyclone identification methods the north atlantic storm track is reproduced along with the major regions of genesis cyclones are grouped according to their genesis location and the corresponding lysis regions are identified most of the cyclones that cross western europe originate in the east atlantic where the baroclinicity and the sea surface temperature gradients are weak compared to the west atlantic east atlantic cyclones also have higher 1 km height relative vorticity and lower mean sea level pressure at their genesis point than west atlantic cyclones this is consistent with the hypothesis that they are secondary cyclones developing on the trailing fronts of preexisting parent cyclones the evolution characteristics of composite west and east atlantic cyclones have been compared the ratio of their upper to lower level forcing indicates that type b cyclones are predominant in both the west and east atlantic with strong upper and lower level features among the remaining cyclones there is a higher proportion of type c cyclones in the east atlantic whereas types a and c are equally frequent in the west atlantic

landscape evolution of the dabanshan planation surface implications for the uplift of the eastern tip of the qilian mountains since the late miocene abstract a series of erosional surfaces including a planation surface ps and several fluvial terraces are well preserved in the dabanshan mountains northeastern ne tibetan plateau the dabanshan planation surface has been divided into several platform patches by river incision including the dongping ps and the halagu ps and is overlain by late cenozoic sediments furthermore a series of fluvial terraces are inset into the dabanshan ps in order to reconstruct the formation and evolution of the dabanshan ps we analysed two drill cores of late cenozoic sediments from the uplifted dongping ps and halagu ps respectively first we analysed the sedimentary facies to investigate the sedimentary environment during deposition using various integrated methods including the sediment properties grain size and provenance to constrain the termination time of dabanshan ps development we used micromammalian fossil assemblages and magnetostratigraphy based on the overlying strata and the morphometric features we propose that this low relief upland landscape along the eastern tip of the qilian mountains represents preserved remnants of elevated planation surface our sedimentological and geochronological results show that the stable weathering conditions of the dongping ps and the halagu ps were ended at 8 1 ma and 7 5 ma respectively when they changed to a fluviolacustrine sedimentary environment this result reveals that the development of the dongping ps and the halagu ps were terminated at 8 1 ma and 7 5 ma respectively the uplift of the dabanshan ps at 8 6 ma suggests that the initial uplift of the eastern tip of the qilian mountains occurred later than in the western and central segments indicating eastward growth of the qilian mountains over time in addition the accelerated downcutting recorded by the planation surface and its inset river terraces indicates that the eastern tip of the qilian mountains may have experienced accelerated uplift since the late miocene

diagnosis of frontal instabilities over the southern ocean the development of three fronts over the southern ocean is described using seawinds on quikscat scatterometer surface winds and an attribution technique to partition the wind field in three components nondivergent and irrotational components at the scale of the front and the remaining harmonic component or environmental flow induced by the synoptic scale flow the front and the environment in which the front is embedded are analyzed separately a frontal wave is shown to develop out of the first front when the large scale alongfront stretching decreases the environmental flow becomes frontolytic and a connection with the upper levels is established in the second case the stretching remains relatively strong and no frontal wave develops the third front exhibits a developing wave but is not in a favorable configuration with the upper levels the frontal wave does not deepen significantly

rapid late pleistocene incision of atlantic passive margin river gorges the direct and secondary effects of rapidly changing climate caused large rivers draining the atlantic passive margin to incise quickly into bedrock beginning about 35 000 years ago measured in samples from bedrock fluvial terraces 10 beryllium shows that both the susquehanna and potomac rivers incised 10 to 20 meter deep gorges along steep convex lower reaches during the last glacial cycle this short lived pulse of unusually rapid down cutting ended by 13 000 to 14 000 years ago the timing and rate of downcutting are similar on the glaciated susquehanna and unglaciated potomac rivers indicating that regional changes not simply glacial meltwater initiated incision

late miocene pliocene geomorphological evolution of the xiaoshuizi peneplain in the maxian mountains and its tectonic significance for the northeastern tibetan plateau as a remnant elevated peneplain the tangxian planation surface is widely distributed in north china and the ordos plateau further into the tibetan plateau tp the main planation surface can also be traced across this the highest plateau on the earth s surface what the relation is between these two planation surfaces and whether they represent the remnants of a unique planation surface remains open to question as the intermediate region between the low altitude north china and the high altitude tp the maxian mountains and such bilateral basins located in the northeastern tp margin retain a series of well preserved geomorphic features such as planation surfaces pediment surface and the fluvial terraces of the yellow river when and how the xiaoshuizi peneplain was formed are crucial questions to elucidate the geomorphic evolution of the northeastern tp and infer the tectonic history of the tp here we present two parallel red clay loess drill cores from the uplifted xiaoshuizi bedrock peneplain in the maxian mountains which document both the upper formation age of the planation surface and the evolution of asian aridification based on the integration of high resolution magnetostratigraphy analysis and pliocene mammal fossils we demonstrate that the red clay loess succession deposited on the xiaoshuizi peneplain probably began to accumulate at similar to 6 9 ma implying that the xiaoshuizi peneplain had begun to form before this these results support the hypothesis that the xiaoshuizi planation surface is equivalent to tangxian planation surface of north china and main planation surface of the tp furthermore the onset of red clay sequence may indicate that the xiaoshuizi peneplain was ultimately destroyed by the intensive tectonic uplift of the northeastern tp before 6 9 ma taking into account the synchronous late miocene red clay deposition between the xiaoshuizi peneplain and central chinese loess plateau clp we propose that the onset of interior asian aridification occurred in the late miocene and was principally triggered by the rapid uplift of tp

dating of erosion surface and terraces in the eastern qilian shan northwest china the actively deformed foreland of eastern qilian shan mountains contains well preserved geomorphic features such as erosion surfaces river terraces and tectonically uplifted alluvial fans providing suitable archives for research on regional tectonic activities and palaeoclimatic changes these geomorphic surfaces are well dated by using a combination of magnetostratigraphy electron spin resonance thermoluminescence infra red stimulated luminescence radiocarbon dating and correlation with the well established loess palaeosol sequences of china our results show that the erosion surface formed about 1 4 ma ago and the age of river terraces is 1 24 ma 820 860 ka 780 ka 420 440 ka 230 250 ka 140 ka 60 ka and 10 ka respectively valley incision rates of e 0 09 0 25 m ka 1 have been identified the repetitive stratigraphic and geomorphic pattern of these terraces indicates the fluvial sedimentation incision cycles are tightly associated with the 100 ka glacial interglacial climatic cycles copyright c 2006 john wiley sons ltd

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

oblique stepwise rise and growth of the tibet plateau two end member models of how the high elevations in tibet formed are i continuous thickening and widespread viscous flow of the crust and mantle of the entire plateau and ii time dependent localized shear between coherent lithospheric blocks recent studies of cenozoic deformation magmatism and seismic structure lend support to the latter since india collided with asia 55 million years ago the rise of the high tibetan plateau likely occurred in three main steps by successive growth and uplift of 300 to 500 kilometer wide crustal thrust wedges the crust thickened while the mantle decoupled beneath gently dipping shear zones did not sediment infilling bathtub like of dammed intermontane basins formed flat high plains at each step the existence of magmatic belts younging northward implies that slabs of asian mantle subducted one after another under ranges north of the himalayas subduction was oblique and accompanied by extrusion along the left lateral strike slip faults that slice tibet s east side these mechanisms akin to plate tectonics hidden by thickening crust with slip partitioning account for the dominant growth of the tibet plateau toward the east and northeast

the topographic controls on the decadal scale erosion rates in qilian shan mountains n w china abstract the relationships between climate topography and erosion are significant in understanding landscape evolution in order to study this relationship in a tectonically active landscape the details of 11 drainage basins were collected from qilian shan mountains decadal scale erosion rates including the mechanical load and solute load contributions are estimated in natural conditions the calculated erosion rates show that the average erosion rate of qilian shan mountains is about 0 08 mm yr while the variation of annual erosion rates within each basin is significant the changes of topography and climate which potentially control erosion rates are also employed in this paper for correlating analyses correlation analyses indicate that erosion rates are more closely correlated with topographic variables such as mean local relief and mean slope than all of the climatic variables and mean local relief and decadal scale erosion rates show a linear relationship in these tectonically active mountains however some topographic variables like basin area and elongation ratio exert limited influence on erosion rates while others such as basin elevation basin relief and basin roughness show poor correlation the results indicate that topographic control like aspects of the local terrain steepness plays the most important role in spatial distribution of decadal scale erosion rates throughout qilian shan mountains under topographic control some climatic variables like discharge and runoff however could account for the significant variation of annual erosion rates in individual basin when comparing erosion rates on different timescales we found that the decadal scale erosion rates are lower than the long term river incision rates as well as the exhumation rates during early and middle miocene the change to more arid climatic condition since the middle miocene combining with tectonic uplift should attribute to the inconsistent erosion rates over different timescales in qilian shan mountains

frontal wave stability during moist deformation frontogenesis part i linear wave dynamics abstract it has been shown that lower tropospheric potential vorticity zones formed during moist deformation frontogenesis will support growing waves if at some time the frontogenesis ceases in this paper the ways in which these waves are affected by the frontogenetic process are identified observations show that fronts in the eastern atlantic commonly feature saturated ascent regions characterized by zero moist potential vorticity furthermore in many cases the horizontal temperature gradient in the lowest one to two kilometers of the atmosphere is rather weak these features are incorporated in an analytical archetype the dynamical implications of saturated ascent in conditions of zero moist potential vorticity are represented in the model by assuming that adiabatic temperature changes are precisely balanced by diabatic tendencies the observed small temperature gradient at low levels is represented in the model by taking it to be zero in the lowest two kilometers consequently the forcing of the

the role of secondary cyclones and cyclone families for the north atlantic storm track and clustering over western europe secondary cyclones are those that form in association with a pre existing primary cyclone typically along a trailing cold front in previously studied cases they have been shown to cause extreme damage across europe particularly when multiple cyclones track over the same location in rapid succession known as cyclone clustering to determine the dynamical relationship between primary and secondary cyclones over the north atlantic a frontal identification algorithm is partnered with a cyclone identification method to objectively identify secondary cyclones in 35 extended winter periods using reanalysis data cyclones are grouped into cyclone families consisting of a single primary cyclone and one or more secondary cyclones this paper aims to quantify the differences between secondary and primary cyclones over the north atlantic and how cyclone families contribute to episodes of cyclone clustering across western europe secondary cyclones are shown to occur most frequently in the central and eastern north atlantic whereas primary cyclones are commonly found over the western north atlantic cyclone families have their strongest presence over the north atlantic ocean and contribute more than 50 of cyclones over the main north atlantic storm track a final category solo cyclones which are not associated with cyclogenesis on any connected fronts are most commonly identified over continental regions as well as the mediterranean sea primary cyclones are associated with the development of an environment that is favourable for secondary cyclone growth enhanced rossby wave breaking following primary cyclone development leads to an increase in the upper level jet speed and a decrease in low level stability secondary cyclogenesis commonly occurs in this region of anomalously low stability close to the european continent during periods of cyclone clustering secondary cyclones are responsible for approximately 50 of the total number of cyclones the increase in jet speed and decrease in static stability initiated by the primary cyclones acts to concentrate the genesis region of secondary cyclones and direct the cyclones that form along a similar track while there is an increase in the secondary cyclogenesis rate near western europe during periods of european clustering the basin wide secondary cyclogenesis rate decreases during these periods thus the large scale environment redistributes secondary cyclones during periods of clustering rather than increasing the total number of secondary cyclones

chinese loess and the paleomonsoon abstract this paper presents a general review of the recent research advances of the east asian paleomonsoon based mainly on studies of the loess soil sequences in the chinese loess plateau in the last 2 6 million years the paleomonsoonal history may be divided into about 166 events on a time scale of the earth s orbital variations during the last glacial period millenial scale oscillations of the monsoon system were prominent which can be fairly well correlated with the dansgaard oeschger cycles recognized in the greenland ice cores the monsoonal rainfall belt has experienced a wide repeated advance retreat change during the glacial interglacial cycles of the pleistocene both temporal and spatial changes of the monsoon system in the quaternary could have been linked closely to global ice volume variations

yellow river terrace sequences of the gonghe guide section in the northeastern qinghai tibet implications for plateau uplift abstract the uplift of the tibetan plateau was a significant event in terms of global landforms and climate but the exact nature of the uplift process remains contested among geologists in the uplifted inland area of the plateau river terrace formation has been controlled mainly by tectonic uplift which means that analyses of river terraces are an excellent means of investigating the uplift process in this paper we establish the terrace sequence of the yellow river in the gonghe guide section of the northeastern tibetan plateau making use of field investigations gravel analysis and electron spin resonance esr dating terraces t3 t9 of the numbered sequence formed at 0 13 0 18 0 23 0 41 0 85 0 93 and 1 32 ma moving backwards in time and upwards from the valley floor whereas terraces t11 t20 formed at 1 71 1 75 1 88 1 94 2 01 2 12 2 23 2 31 2 36 and 2 47 ma this suggests that the yellow river existed in gonghe and guide basins for at least the last 2 47 ma the incision rates of the yellow river in the gonghe guide section indicate that there were three distinct phases of uplift of the northeastern tibetan plateau occurring at different rates with an average uplift rate of 0 26 mm yr during the quaternary these results support the multi stage uplift model which states that the tibetan plateau has experienced continuous uplift since 8 ma but contradicts both the early uplift theory which holds that the uplift of the tibetan plateau occurred mainly before the pliocene and the notion of a late and rapid uplift of the plateau that began at 3 6 ma

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

evidence for a new geomagnetic reversal from lava flows in idaho discussion of short polarity reversals in the brunhes and late matuyama polarity chrons k ar ages and paleomagnetic data for basalt samples from a new core hole site e at the idaho national engineering laboratory inel indicate that the age of the reversed polarity event recorded in snake river plain lavas is older than 465 50 ka 1000 years before present reported previously by champion et al 1981 nine basalt flows eight with normal polarity and one with reversed polarity were recognized in the site e core hole the flows above and below the reversed flow have ages of 491 80 ka and 580 93 ka respectively the inclination of the paleomagnetic field direction of the reversed flow at site e agrees with the inclination of reversed flows elsewhere at inel which have an age of 565 14 ka these reversed flows were previously thought to be correlative with the emperor event we suggest that this polarity event is an older event which we name the big lost reversed polarity subchronozone and subchron a review of data documenting short reversal records from volcanic and sedimentary rocks shows that there is evidence for eight polarity subchrons in the brunhes and two besides the jaramillo in the late matuyama these 10 short subchrons begin to indicate the many short events that cox 1968 hypothesized must exist if polarity interval lengths have a poisson distribution these events are true subchrons not excursions and may or may not have low associated paleointensities although low field strengths might explain why the reversal process aborts the mean sustained polarity interval length since late matuyama chron time is 90 000 years the similarity of this number with the 105 year period of the earth s orbital eccentricity suggests anew that linkage between geomagnetic paleoclimatic and possible underlying earth orbital parameters should be evaluated

stream terrace genesis implications for soil development genesis of three distinct types of stream terraces can be understood through application of the concepts of tectonically induced downcutting base level of erosion complex response threshold of critical power diachronous and synchronous response times and static and dynamic equilibrium climatic and tectonic stream terraces are major terraces below which flights of minor complex response degradation terraces can form these three types of terraces can be summarized by describing a downcutting aggradation renewed downcutting sequence for streams with gravell bedload by tectonically induced downcutting streams degrade to achieve and maintain a dynamic equilibrium longitudinal profile at the base level of erosion lateral erosion bevels bedrock beneath active channels to create major straths that are the fundamental tectonic stream terrace landform aggradation events record brief reversals of long term tectonically induced downcutting because they raise active channels they may be considered as major the result of climatic perturbations or minor the result of complex response model types of perturbations climatically controlled aggradation followed by degradation leaves an aggradation surface this type of fill terrace tread is the fundamental climatic stream terrace landform aggradation surfaces may be buried by subsequent episodes of deposition unless intervening tectonically induced downcutting is sufficient for younger aggradation surfaces to form below older surfaces raising of the active channel by either tectonic uplift or by climatically induced aggradation provides the vertical space for degradation terraces to form first in alluvial fill and then in underlying bedrock along tectonically active streams these are complex response terraces because they result from interactions of dependent variables within a given fluvial system pauses in degradation to a new base level of erosion and or minor episodes of backfilling lead to formation of complex response fill cut and strath or of fill terraces fill cut terraces are formed in alluvium they are complex response terraces because they are higher than the base level of erosion good exposures and dating are needed to distinguish static equilibrium complex response minor strath terraces from dynamic equilibrium tectonic major straths strath terraces may be regarded as complex response terraces where degradation rates between times terrace tread formation exceed the long term uplift rate for the reach based on ages and positions of tectonic terraces late quaternary global climatic changes control aggradation events and even the times of cutting of major tectonic straths because the base level of erosion can not be attained during times of climatically driven aggradation degradation events most terrace soils form on treads of climatic and complex response terraces aggradation surfaces may provide an ideal flight of terraces on which to study a soils chronosequence each aggradation event is recorded by a single relict soil where tectonically induced downcutting is sufficient to provide clear altitudinal separation of the terrace treads multiple paleosols are typical of tectonically stable regions where younger aggradation events spread alluvium over treads of older climatic terraces pedons on a climatic terrace in a small fluvial system commonly are roughly synchronous variations of soil properties that can be attributed to temporal differences will be minor compared to altitudinally controlled climatic factors climatic terraces of adjacent watersheds also should be roughly synchronous correlatable variations of soil properties that can be attributed to temporal differences will be minor compared to lithologic and climatic factors between different watersheds such generalizations may not apply to basins with sufficient relief that geomorphic responses to climatic changes occur at different and overlapping times and to large rivers whose widely separated reaches are characterized by different response times to climatic perturbations soils on climatic terraces of distant watershedswill not be synchronous if their respective aggradation events occur during full glacial times and interglacial times soils on some complex response terraces may be diachronous within a given fluvial system and typically are diachronous between watersheds

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

quaternary glaciation of the bailang river valley qilian shan abstract the qilian shan on the northeast margin of the qinghai tibetan plateau is weakly influenced by the asian monsoon until recently the quaternary glacial geology of this region has been poorly understood this paper describes a sequence of quaternary glacial deposits in the upper reaches of the bailang river using electron spin resonance esr thermoluminescence tl and radiocarbon dating tills loess buried soils and landforms were dated the oldest till was dated 463 ka bp by esr a younger till and its outwash terrace were dated to 135 and 130 ka bp the loess on this outwash terrace was dated to 141 7 11 4 ka bp at its base and 43 7 3 5 ka bp in its central part by tl dating a buried soil on a younger till was dated at 6920 78 ka bp using 14c the glacial landforms and these dating results show that glacial advances occurred during the little ice age the neoglacial mis 2 4 mis 6 and mis 12 glaciation during mis 12 implies that the qilian mountains were rising coevally with the qinghai tibetan plateau and were probably at a sufficient elevation for glaciation since at least 463 ka bp

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

mesoscale dynamics of a deepening secondary cyclone in fastex iop16 three dimensional structure retrieved from dropsonde data the mesoscale dynamics of a secondary cyclone sampled during the field phase of the fronts and atlantic storm track experiment fastex intensive observation period iop 16 are documented using airborne dropsonde data the method proposed to recover the mesoscale analytical three dimensional 3 d dynamic fields from the dropsonde measurements is evaluated using simulated fields airborne in situ measurements and airborne doppler radar measurements these simulations and comparisons indicate that the method is able to resolve the mesoscale structures within the studied secondary cyclone both in the regions of clear air and precipitation it appears however that the most accurate description of the 3 d dynamic fields in both these regions would be obtained by combining doppler and dropsonde measurements the studied secondary cyclone is a fast moving frontal wave that developed on the trailing cold front of a parent low situated over greenland using the dropsonde derived 3 d fields the mesoscale flows involved in the organization of the secondary cyclone are described the potential vorticity and equivalent potential vorticity fields deduced from the basic analytical fields indicate that the atmosphere is roughly neutral to moist slantwise instability this configuration is known to significantly promote frontogenesis the vertical configuration of the jets and circulations within this secondary cyclone are compared with the coupling uncoupling scheme of shapiro a slight coupling is found between the thermally indirect circulation and the ageostrophic circulation the obtained coupling is however not as developed as in the ideal case described by shapiro which may explain the small upward velocities in our case airborne doppler radar and dropsonde measurements are combined so as to infer the multi scale processes involved in the mesoscale and convective scale organization of the atlantic secondary cyclones and to evaluate the new emerging theoretical interpretations of secondary cyclogenesis

late pleistocene slip rate on a blind thrust in the western qilian shan nw china abstract in the western front of the qilian shan ne tibetan plateau a series of 5 fluvial t1 t5 terraces developed along the baiyang river are deformed across the shiyougou anticline which is related to the yumen beidahe yb blind thrust in this study we measure the deformed terraces across the anticline using differential gps which provides data for us to reconstruct the thrust fault geometry the ages of terrace abandonment are constrained by 10be exposure dating for t2 t4 and t5 to be 8 6 1 7 ka 21 6 1 4 ka and 48 4 2 1 ka respectively a four segment fault model is inferred with changing dips at depths of 0 3 0 1 km 2 2 0 1 km and 5 9 0 9 km combining terrace ages with the fault geometry model we derive a vertical slip rate of 0 9 0 1 mm yr and a shortening rate of 1 9 0 5 mm yr for the yb blind thrust this result suggests that the northern frontal thrust system accommodates only 30 of the total shortening rates across the western qilian shan the river incision rate has increased from 1 5 mm yr to 5 2 mm yr from the late pleistocene to the holocene the mismatch between river incision and rock uplift is likely related to climate change since the last glacial maximum

magnetostratigraphic dating of river terraces rapid and intermittent incision by the yellow river of the northeastern margin of the tibetan plateau during the quaternary up to 23 paleosols have been identified in loess sequences that overlie fluviatile sediments on seven terraces of huang he the yellow river and daxia he in the linxia basin at the northeastern margin of the tibetan plateau the magnetostratigraphic record in the oldest sequence appears to be fairly complete and spans the entire quaternary it includes evidence for the jaramillo and olduvai normal polarity subchrons within the matuyama chron as well as the cobb mountain and stage 54 formerly gilsa events age constraints within the brunhes are provided by 14c and thermoluminescence optical stimulated luminescence dating the magnetostratigraphic chronology of the quaternary sediments indicates that terraces were formed at about 1 66 1 4 1 1 0 6 0 12 0 055 and 0 01 ma the terraces may represent climatically induced pauses in long term tectonically induced incision by the rivers on the basis of terrace heights and ages incision rates were highest from 1 66 to 1 4 ma 1 m kyr decreased from 1 4 to 0 12 ma 0 1 m kyr and increased from 0 12 ma to present 0 75 m kyr if river incision is attributed solely to uplift of the tibetan plateau then this uplift appears to have been rapid as well as intermittent following the first evidence of the existence of huang he in this area at about 1 75 ma however the effects of uplift cannot be fully separated from incision caused by changes in climate copyright 1997 by the american geophysical union

stream response to quaternary tectonic and climatic change evidence from the upper weihe river central china abstract seven terraces of upper weihe river in the longxi basin on the north flank of the qinling mountains are capped with a succession of loess units and paleosols that correlate with the standard marine isotope chronology based on fieldwork and terrace ages which were determined using loess paleosol stratigraphy paleomagnetic optical stimulated luminescence osl and radiocarbon dating the results show that the fluvial fills were deposited during marine oxygen isotope stages miss 22 20 18 16 12 6 4 and 2 respectively whereas terracing occurred during glacial interglacial transitions this in turn points to eccentricity related climatic changes that may have driven recurrent incision and aggradation processes and significantly influenced the formation of staircase terraces of weihe river through the direct and indirect influence of both temperature and precipitation on fluviatile activity on the basis of terrace ages and its heights incision rates were enhanced from 865 to 412 ka 0 27 m ka decreased from 412 to 128 ka 0 09 m ka and increased from 128 ka to present 0 32 m ka

quaternary climate change and the formation of river terraces across growing anticlines on the north flank of the tien shan china nested stream terraces warped upward over actively growing anticlines along the north flank of the tien shan in western china appear to record alternating phases of valley widening and incision differences of relative heights between remnants of four separate strath terraces along one river and between two such terraces along another reach 100 to 120 m over the crests of the anticlines we infer that this spacing is due to alternating stages of valley widening and rapid incision associated with climate changes with a periodicity of 100 kyr the crests of the anticlines appear to emerge from the aggrading flanks of the anticlines at an average rate of about 1 mm a the maximum heights of 25 and 35 10 m for the lowest terraces above their projected initial profiles imply ages of roughly 25 kyr and 35 kyr 10 kyr hence they suggest that flood plains which were abandoned to form the terraces developed adjacent to active stream beds during the last glacial period when climates were relatively

frontal wave cyclogenesis in the north atlantic a climatological characterisation the ambient conditions during cyclogenesis on fronts in the north atlantic na are characterised climatologically in the era interim dataset these secondary cyclones which grow from frontal waves are typically small in scale shallow and deepen rapidly and pose a challenge to forecasters and numerical weather prediction models frontal wave development has been studied intensively in the past based on observations e g fastex real case studies or analytical models in this study frontal wave cyclogenesis is studied from a climatological perspective and is detected by a combination of two automated identification methods for the period between 1979 and 2014 a climatology of frontal wave cyclogenesis is presented horizontal composites provide an estimate of the climatological mean of conditions during frontal wave cyclogenesis which may prove useful in the assessment of the realism of proposed idealized models of frontogenesis the findings are also compared with assumptions made in earlier idealized studies of frontal waves the composites reveal coherent structures of diabatically generated low level pv in the lower troposphere neutral moist static stability at cyclogenesis and an ambient lower troposphere close to saturation the vertical pv structure in the east na is more vertically aligned than in the west na and pv anomalies from the upper and lower levels reach deeper into the mid troposphere which indicates an enhanced phase lock between the two anomalies previous studies suggest the existence of an upper limit of along frontal stretching above which frontal wave development is strongly suppressed the presented multi decadal analysis confirms its existence in a range between 0 6 and 0 8 10 5 s 1 finally a climatology of along frontal stretching for the na is presented and compared to the fraction of cyclogenesis forming on frontal waves

a 900 k y record of strath terrace formation during glacial interglacial transitions in northwest china the timing of the development of strath terraces with respect to climatic variability remains equivocal previous studies attribute strath terrace formation to glacial or interglacial climates or to variations in sediment and water fluxes that cause lateral erosion followed by vertical incision a chronology of strath terrace formation spanning similar to900 k y has been generated on the basis of loess paleosol couplets and paleomagnetic thermoluminescence and radiocarbon dating of strath terraces in the qilian shan of northeastern tibet repetitive stratigraphic and geomorphic patterns on each terrace indicate that they formed during glacial interglacial transitions long term bedrock incision rates and inferred rock uplift rates appear steady and unrelated to strath formation over the past 900 k y

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

late quaternary fluvial terraces near the daocheng ice cap eastern tibetan plateau the timing of terrace formation relative to the glacial interglacial cycle and what factors control that timing such as changes in climate and or uplift are controversial here we present a study of the terraces along the yazheku river using electron spin resonance esr dating and analysis of the sedimentary characteristics in order to establish the timing of terrace formation and to assess the forcing mechanisms that have been proposed the yazheku river flows in glacial trough leading from the haizi shan on the eastern tibetan plateau the range was uplifted during the quaternary and repeatedly glaciated by ice caps the four highest major terraces t5 t4 t3 and t2 are the result of both climatic and tectonic influences strath terraces t5 t2 were created during haizi shan glacial expansions during mis 16 12 6 and 3 4 respectively the major aggradation phases of the four terraces occurred during the deglaciations at the ends of mis 16 12 6 and 2 down cutting which led to the generation of the four terraces immediately followed the deposition of the t5 t2 gravel units these incisions occurred during the transitions between mis 16 15 mis 12 11 mis 6 5 and mis 2 1

the relative efficiency and influence of glacial and fluvial erosion on tibetan plateau landscapes abstract driven by climate change periods of dominating glacial and fluvial erosion have both sculpted mountain ranges during the quaternary to understand the evolution of mountain landforms there is considerable interest in knowing which erosion agent leads to more incision how glacial thermal regime impacts erosion and landscape development and whether glacial erosion can result in increased relief to investigate the relative efficiency of glacial and fluvial erosion and the influence of glacial thermal regime on topography we compare two different mountain ranges the qionglai shan monsoonal temperate glaciation and lenglongling subcontinental glaciation in western china we use a 1 d model to construct presumptive fluvial longitudinal profiles if glaciation had not occurred and compare these with the actual glacial profiles to obtain relative incision rates of fluvial and glacial erosion results demonstrate that glacial incision is smaller than fluvial incision in lenglongling and larger than fluvial incision in the qionglai shan which may be influenced by different tectonics and glacier types topographic analysis of glacial and fluvial valleys with similar drainage areas show that glacial erosion results in valley widening local valley relief increase induced by isostatic uplift and slope gradient increase in lenglongling however there is no local valley relief and slope increase observed in the qionglai shan which may reflect slope process responses to threshold hillslope limiting the height above valley bottoms

landscape disequilibrium on 1000 10 000 year scales marsyandi river nepal central himalaya in an actively deforming orogen maintenance of a topographic steady state requires that hillslope erosion river incision and rock uplift rates are balanced over timescales of 10 5 10 7 years over shorter times 10 5 years hillslope erosion and bedrock river incision rates fluctuate with changes in climate on 10 4 year timescales the marsyandi river in the central nepal himalaya has oscillated between bedrock incision and valley alluviation in response to changes in monsoon intensity and sediment flux stratigraphy and 14 c ages of fill terrace deposits reveal a major alluviation coincident with a monsoonal maximum ca 50 35 ky bp cosmogenic 10 be and 26 al exposure ages define an alluviation and reincision event ca 9 6 ky bp also at a time of strong south asian monsoons the terrace deposits that line the lesser himalayan channel are largely composed of debris flows which originate in the greater himalayan rocks up to 40 km away the terrace sequences contain many cubic kilometers of sediment but probably represent only 2 8 of the sediments which flushed through the marsyandi during the accumulation period at f10 4 year timescales maximum bedrock incision rates are f7 mm year in the greater himalaya and f1 5 mm year in the lesser himalayan mahabarat range we propose a model in which river channel erosion is temporally out of phase with hillslope erosion increased monsoonal precipitation causes an increase in hillslope derived sediment that overwhelms the transport capacity of the river the resulting aggradation protects the bedrock channel from erosion allowing the river gradient to steepen as rock uplift continues when the alluvium is later removed and the bedrock channel re exposed bedrock incision rates probably accelerate beyond the long term mean as the river gradient adjusts downward toward a more equilibrium profile efforts to document dynamic equilibrium in active orogens require quantification of rates over time intervals significantly exceeding the scale of these millennial fluctuations in rate d 2003 elsevier b v all rights reserved

low slip rates and long term preservation of geomorphic features in central asia in order to understand the dynamics of the india asia collision zone it is important to know the strain distribution in central asia whose determination relies on the slip rates for active faults1 2 3 4 5 many previous slip rate estimates of faults in central asia were based on the assumption that offset landforms are younger than the last glacial maximum 20 kyr ago 6 7 8 9 10 11 in contrast here we present surface exposure ages of 40 to 170 kyr obtained using cosmogenic nuclide dating for a series of terraces near a thrust at the northern margin of the tibetan plateau combined with the tectonic offset the ages imply a long term slip rate of only about 0 35 mm yr 1 for the active thrust an order of magnitude lower than rates obtained from the assumption that the terraces formed after the last glacial maximum our data demonstrate that the preservation potential of geomorphic features in central asia is higher than commonly assumed

terminations and their correlation with solar insolation in the northern hemisphere a record from a loess section in northwest china a high resolution loess section in northwest china is analyzed for the major climate terminations and their correlation with solar insolation over the past 0 8 ma based on age controls and a grain size age model proposed by porter and an porter s c an z s 1995 correlation between climate events in the north atlantic and china during the last glaciation nature 375 305 308 a timescale for the shagou loess section is constructed which is independent of orbital tuned results the timescale is similar to other loess sections and the mid point ages of the boundary between a paleosol and its underlying loess layer are very close to that of terminations recorded in the marine record shagou section records indicate that grain size changed sharply during the termination process terminations occurred at the time of the maximum or increase of july radiation in the northern hemisphere at high latitudes suggesting that insolation might be a factor affecting terminations in the chinese loess this result supports the milankovitch hypothesis however variations in the amplitude of the insolation and grain size records are not proportionally matched so that the mechanism remains unsolved

the impact of deformation strain on the formation of banded clouds in idealized modelling experiments experiments are performed using an idealized version of an operational forecast model to determine the impact on banded frontal clouds of the strength of deformational forcing low level baroclinicity and model representation of convection line convection is initiated along the front and slantwise bands extend from the top of the line convection elements into the cold air this banding is attributed primarily to m adjustment the cross frontal spreading of the cold pool generated by the line convection leads to further triggering of upright convection in the cold air that feeds into these slantwise bands secondary low level bands form later in the simulations these are attributed to the release of conditional symmetric instability enhanced deformation strain leads to earlier onset of convection and more coherent line convection a stronger cold pool is generated but its speed is reduced relative to that seen in experiments with weaker deformational strain because of inhibition by the strain field enhanced low level baroclinicity leads to the generation of more inertial instability by line convection for a given capping height of convection and consequently greater strength of the slantwise circulations formed by m adjustment these conclusions are based on experiments without a convective parametrization scheme experiments using the standard or a modified scheme for this model demonstrate known problems with the use of this scheme at the awkward 4 km grid length used in these simulations copyright 2008 royal meteorological society

crustal uplift in southern england evidence from the river terrace records abstract much of the past work on the quaternary rivers of northwest europe has been concerned with river terraces which characterise almost every valley while these terraces are undoubtedly striking features of the landscape the incision achieved by quaternary rivers is equally significant in terms of river behaviour and for an understanding of the factors affecting landform development during the quaternary this paper examines the incision achieved during the quaternary by the thames in both its upper and lower catchments and by the hampshire avon in southern england valley incision rates of ca 0 07 0 10 m ka 1 have been identified although in the lower catchment of the thames these have been enhanced by additional incision in response to glacio isostasy and valley shortening a model is proposed in which regional uplift is recognized as the primary cause of incision by these quaternary rivers possible mechanisms for regional uplift are considered

relative and absolute dating of land surfaces abstract the dating of land surfaces has long posed problems for geomorphologists relative methods stratigraphic geomorphic topographic are sound and convincing exhumed forms may complicate identification and relationships for both epigene and etch forms have been buried and exhumed but in tectonically undisturbed areas the higher surfaces are older than those preserved at lower levels also surfaces have an age range the relationship of surfaces with volcanic deposits old shorelines and genetically related sedimentary sequences provides sound ages and correlation with dated duricrusts and faults is also useful there are no temporal limits to relative dating for the methods are equally applicable to the dating say of proterozoic surfaces as of those of pleistocene age the disadvantage of such methods is that the necessary evidence is frequently either not preserved or not exposed the so called absolute physical numerical methods and especially those based on exposure age dating with in situ cosmogenic radionuclides are appealing because they produce direct numerical ages and appear to be widely applicable but there are severe temporal limitations and sampling problems complicate and may invalidate interpretation absolute age determinations must be consistent with the stratigraphic and geomorphologic settings the best results are obtained when physicists and earth scientists pool their knowledge and experience a background in local and regional geology is especially important

the matuyama brunhes and upper jaramillo transitions recorded in a loess section at weinan north central china abstract the paleomagnetic records of the matuyama brunhes mb and upper jaramillo uj polarity reversals have been obtained from a loess section at weinan shaanxi province in china an investigation of the rock magnetic properties using stepwise irm acquisition hysteresis loops thermomagnetic analysis and x ray diffraction on magnetic extracts identifies pseudo single domain magnetite as the carrier of the main fraction of the remanence associated with maghemite and hematite the total duration of the directional change based upon independent loess accumulation rates is about 5000 yr for the mb reversal and about 3200 yr for the uj reversal these values are consistent with previous estimates obtained from marine or lacustrine sediments apart from a few intermediate points observed on north america mb and in the pacific uj the transitional virtual geomagnetic pole vgp paths are situated in two longitudinal bands over eastern asia and australia mb and the americas uj the bulk of the uj record and the part of the mb record over the americas are not consistent with the hypothesis of a record arising from smoothing of pre and post transitional directions 1 a reappraisal of the statistical approach of mcfadden et al 2 shows that the longitudinal distribution of the records of reversals is consistent with the hypothesis of genuine geographically preferred sectors for the paths independent of site longitude with a probability higher than 0 45 when the two weinan records are considered thus these results obtained from wind blown loess sustain the hypothesis of two preferential longitudinal bands for reversing transitional vgps suggested from the study of marine and lacustrine sediments

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

rates and kinematics of active shortening along the eastern qilian shan china inferred from deformed fluvial terraces in the eastern qilian shan a flight of fluvial terraces developed along the jinta river valley are deformed across the nanying anticline four individual fluvial terraces are preserved at different elevations above the river and higher terrace treads are draped by systematically thicker aeolian loess optically stimulated luminescence dating of deposits at the base of the loess provides constraints on the timing of surface abandonment terraces were abandoned at 69 4 ka b p t4 57 4 ka b p t3 and between 34 3 ka b p t2 respectively differential gps measurement of the terrace profile across the anticline allows reconstruction of subsurface fault geometry we model terrace deformation above a listric thrust fault with a tip line at 2 2 0 1 km depth and whose dip shallows systematically to 23 3 degrees at depth of 5 8 1 1 km combining terrace ages with this model of fault geometry we estimate a shortening rate of 0 8 0 2 mm a across the nanying fold and a shortening rate of similar to 0 1 mm a across the mountain front fault since similar to 70 ka b p this rate suggests that the frontal fault system along the eastern qilian shan accomplishes crustal shortening at rates of approximately 0 9 0 3 mm a during late pleistocene time

frontal instability generated by tropospheric potential vorticity anomalies the development of secondary disturbances on a two dimensional front is described using a linear stability analysis a two stage process is envisaged in which first strong frontogenesis produces condensation at the front this leads to the generation of a lower tropospheric zone of high potential vorticity pv lying along the front as frontogenesis weakens the second stage is entered in which an essentially steady front is susceptible to the growth of linear normal modes along the front owing to the existence of the pv anomaly or equivalently the frontal rainband thus for a typical band of width 160 km a frontal wave of some 800 km is produced with a growth rate of about 1 day 1 the dynamics of these modes is examined by considering both the realistic frontal flow as well as idealized strips of pv anomalies the idealized case is potentially applicable to a variety of meteorological situations such as upper jet streaks useful insight is gained from an analysis of the energetics of the modes this shows that for narrow frontal pv anomalies the waves main energy source is the basic kinetic energy and further they have a negative vertical heat flux recent observations suggest that frontal waves are indeed characterized by such a heat flux as most frontal zones involve significant condensation and therefore are likely to exhibit a lower tropospheric pv maximum it is stressed that studies of frontal waves need to include this aspect of the frontal structure

diminutive frontal waves a link between fronts and cyclones abstract a number of recent publications have dealt with cyclone identification and tracking following on this paper extends the typical cyclone life cycle back in time to embrace a new feature called a diminutive frontal wave one aim is to improve predictability by extending tracks this is particularly important for small cyclonic windstorms which can often be missed in postprocessed output from operational ensemble and climate runs the recognition of diminutive waves requires a new front relative low level vorticity partition the parts are labeled frontal vorticity and disturbance vorticity and are computed respectively from front parallel and cross front low level wind components a diminutive frontal wave then lies wherever there is a local along front maximum in the disturbance vorticity computations require local coordinates these are conveniently provided at all grid points by objective front diagnostics analysis of cyclone type transitions over the north atlantic in opera

increasing nd isotopic ratio of asian dust indicates progressive uplift of the north tibetan plateau since the middle miocene the time and rate of tibetan uplift is critical in understanding both the geodynamics of plateau growth and the influence of mountain building on climate change models predict that the uplift of the north tibetan plateau ntp occurred later than uplift of the south and central tibetan plateau uplift of the ntp is believed to have initiated the aridity in the interior of asia and to have critically affected the evolution of asian monsoon and ultimately global climate the history of ntp uplift has remained uncertain however scenarios range from early miocene to the past million years this work provides new evidence by relating the nd isotopic evolution of asian dust to ntp uplift the sources of the asian dust are located in the arid lands between the ntp and the central asia orogen mass balance calculations indicate that the decreasing e nd values from 8 2 to 10 4 for pacific dust since 15 ma reflects increasing relative contribution of ntp materials 40 to 70 to the dust source as paleoelevation in the central asia orogen has largely remained constant the increasing relative ntp detritus contribution is interpreted to reflect the progressive surface uplift of the ntp at a rate of at least 0 12 mm yr 1 since 15 ma

differential rock uplift along the northeastern margin of the tibetan plateau inferred from bedrock channel longitudinal profiles abstract bedrock channel profile analysis is a robust reconnaissance tool for examining the rates and patterns of deformation in active orogens here steepness and concavity indices were extracted from bedrock channels draining the northern qilian mountains based on the universal shear stress incision model the profile analysis reveals a systematic spatial distribution of steepness and concavity indices the average steepness index for channels draining the western part is approximately 1 6 times higher than that for the eastern part through empirical calibration of erosion coefficient k the influence of channel width rock strength discharge and sediment flux are identified and an approximately 1 1 1 2 times difference in k values is found between the west and east under the hypothesis that lithologic resistance and sediment flux have no contribution correlating profile analyses with parameter calibration an approximate 2 3 times increase in the rock uplift rate is found from the eastern part to the western part of the orogen the integration of this study with previous work in the eastern part of the mountains indicates that cross strike 2 4 times and along strike 2 3 times differential rock uplift both exist in the northeastern margin of the tibetan plateau in addition the abnormal values of concavity are apparently associated with the channels transecting large thrust fault zones thus the shortening deformation may play a dominant role in controlling the differential rock uplift through folding and thrusting of the northeast directed thrust faults across the northeastern margin of the tibetan plateau

quaternary drainage evolution of the datong river qilian mountains northeastern tibetan plateau china abstract drainage basins are useful recorders of tectonics and climate the datong river which drains the qilian mountains china is one of the most elongated river basins of the world how this unusual basin configuration was established remains unknown despite this area being ideal for assessing the controlling effects of tectonics and climate on the drainage evolution in this study we reconstruct the history of the drainage evolution of the datong river using integrated provenance methods u pb dating heavy mineral and gravel compositions and river terrace dating the drainage system of the modern datong river began to form at 1100 ka and the drainage basin was limited to the area of the lower modern datong river since then it has gradually extended through headward erosion the river channelized into the menyuan basin between 620 ka and 420 ka and it captured the upper datong river between 130 ka and 60 ka we suggest that the rapid uplift of the qilian mountains disrupted and longitudinally deflected the transverse rivers over the past 1100 ka additionally intensified precipitation and glacial meltwater enhanced the river incision and captured the middle and upper datong river during 620 420 ka and 130 60 ka respectively this model shows that the direction of drainage pattern evolution is controlled by tectonics and that the timing of drainage reorganization is controlled by climate change in the active fold thrust belt

late quaternary river incision rates and rock uplift pattern of the eastern qilian shan mountain china abstract the uplift rate and pattern of the qilian shan mountain which lies in the northeast margin of the tibetan plateau are key to understanding the dynamics of crustal deformation in the northeastern tibetan plateau in this work late quaternary terraces in seven river valley sections in the eastern qilian shan mountain are studied to determine terrace abandonment ages by osl optical simulating luminescence and terrace heights by gps global positioning system surveys river incision rates since 76 ka bp are calculated ranging from 0 37 mm a to 1 49 mm a distribution of river incision rates shows that higher river incision rates 0 85 1 49 mm a are located at the rivers upstream reaches where mean mountain elevation is 4000 m and relatively lower incision rates 0 32 0 62 mm a are located at the mountain front where the mean mountain elevation is

loess stratigraphy in central china abstract loess sequences in central china provide a detailed record of climate changes during the last 2 5 million years at least 44 major shifts from glacial to interglacial conditions occurred during this time in china prolonged loess deposition lasting more than 40 millennia occurred eleven times with only four of these intervals in brunhes chron the climate oscillations took place with increased frequency between 2 4 and 0 5 ma compared to the last half million years major shifts toward cooler and drier climates occurred at about 2 4 ma 1 2 ma and 0 5 ma while less pronounced shifts are dated to 1 65 ma 0 8 ma and 0 2 ma three of the last four reversals of the earth s magnetic field whose lithostratigraphic position has been detail are recorded in a glacial deposit the low field magnetic susceptibility of loess and soils which reflects the concentration of ultrafine magnetite is related to the degree of pedogenesis and serves as a proxy measure of climate the accumulation rate of the magnetite was fairly constant throughout the last 1 2 million years and can serve as a measure of time susceptibility records from localities near xifeng and luochuan located 160 km apart are compared and used to define informal lithostratigraphic units in the loess plateau of china

resolving time space histories of late cenozoic bedrock incision along the upper colorado river usa abstract the upper colorado river basin drains the western slope of the rocky mountains province of north america and hosts a rich record of fluvial terraces and volcanic deposits that provide an archive of bedrock river incision since the late miocene here we present new geochronology from fluvial deposits and associated volcanic units using multiple methods luminescence cosmogenic burial 40ar 39ar basalt and detrital sanidine and u pb detrital zircon to reconstruct a detailed record of bedrock incision in the upper colorado river over timescales ranging from miocene to late pleistocene late miocene ca 11 8 ma basalt flows perched 800 1700 m above the colorado river and the oldest known ancestral colorado river deposits preserved beneath basalt flows on grand mesa provide the basis for average long term 107 yr incision rates rates range from 110 160 m ma between grand junction downstream and glenwood canyon upstream with rates decreasing upvalley intermediate scale 105 106 yr incision rates are 200 320 m m y over the past 1 3 ma and these rates are faster than the average over the past 8 11 ma short term

slip rate at the north eastern front of the qilian shan china terra nova 22 180 187 2010 abstract we constrain the slip rate for the frontal thrust of the qilian shan north eastern tibet by combining structural investigations satellite imagery topographic profiling and 10be exposure dating we surveyed two terrace levels and from each we took 6 7 samples in profiles dug to depths of 2 m these constrain inheritance and determine the precise ages of abandonment of the terraces 29 9 7 8 ka for the upper terrace and 16 3 4 4 ka for the lower one topographic profiles with a determination of burial of the footwall by sediment yield offsets of the surfaces of 96 4 4 4 and 40 1 2 8 m the average vertical rate is 2 8 1 3 mm a 1 with a horizontal slip rate of 2 5 mm a 1 this rate is higher than those determined farther north on similar structures but is consistent with the gps velocity field and accounts for roughly half of the geodetic shortening across the qilian shan

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

a classification of fastex cyclones using a height attributable quasi geostrophic vertical motion diagnostic a systematic and objective procedure is developed for applying the simple cyclone classification scheme of petterssen and smebye this method uses a height attributable solution of the quasi geostrophic equation to identify and quantify the relative importance of upper and lower tropospheric forcing and also the time trend in the horizontal spacing of the forcing at these two levels by applying this classification method to a sample of cyclogenesis events during their maximum intensification stage from the fronts and atlantic storm track experiment field experiment the type a and type b scheme of petterssen and smebye is reproduced and extended to include a type c type c consists of upper level dominated cyclones that form at high latitudes and in their initial stages resemble comma cloud type polar lows detailed examples of each of the three types are presented some cyclones can undergo more than one period of development and it is found that each development period can be classified as a different cyclogenesis category a or b these cyclones are classified as hybrid type a b there is some evidence that cyclone forecast accuracy depends on cyclone type thereby suggesting the potential for this method to be used to assign confidence levels for forecasts of cyclogenesis produced by numerical weather prediction models type b cyclones appear more difficult to predict because their development depends initially on the interaction between significant features in the upper and lower troposphere copyright 2002 royal meteorological society t d hewson s contribution is crown copyright

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

bedrock incision rock uplift and threshold hillslopes in the northwestern himalayas the topography of tectonically active mountain ranges reflects a poorly understood competition between bedrock uplift and erosion dating of abandoned river cut surfaces in the northwestern himalayas reveals that the indus river incises through the bedrock at extremely high rates 2 12 mm yr 1 in the surrounding mountains the average angles of hillslopes are steep and essentially independent of erosion rate suggesting control by a common threshold process in this rapidly deforming region an equilibrium is maintained between bedrock uplift and river incision with landsliding allowing hillslopes to adjust efficiently to rapid river down cutting

the role of the environmental flow in the development of secondary frontal cyclones the impact of the environmental flow on the development of secondary frontal cyclones is investigated several case studies are examined as examples of secondary frontal cyclone events observed in the north atlantic western europe sector a simple measure of growth is defined to chart their development the vorticity attribution technique of bishop is utilized to calculate the action of the large scale environmental flow on the fronts in particular the environmental along front stretching shown to be important in theoretical models of frontal instabilities is calculated the role of the environmental deformation appears to be crucial as part of a baroclinic life cycle stretching deformation acts to build up a front but suppress along front waves if the stretching rate diminishes barotropic instabilities may then break out diagnostics are examined to try to ascertain the growth mechanisms at work in each frontal cyclone case a range of values for the commonly prescribed deformation frontogenesis and shearing frontogenesis parameters are calculated

asymmetrical river valleys and their tectonic significance in the maxianshan area ne tibetan plateau abstract the geometry of river networks is particularly sensitive to active tectonic deformation morphotectonic analyses conducted using geomorphic indices and numerical models are useful methods with which to identify such deformation the northeastern ne margin of the tibetan plateau is characterized by active tectonics easily erodible bedrock and well developed valleys and is thus an ideal region for studying the effects of tectonics on the landscape evolution here we firstly identify the systemic asymmetrical valleys in the maxianshan area ne tibetan plateau to quantitatively analyze the asymmetrical characteristics of these valleys identified in the southern maxianshan 33 parallel streams were investigated by using two drainage basin analytical techniques namely the transverse topographic symmetry factor t and asymmetry factor af our results show that the mean t value is 0 14 magnitude and 319 08 direction and the majority of the af results are

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

paleomagnetic chronology and paleoenvironmental records from drill cores from the hetao basin and their implications for the formation of the hobq desert and the yellow river abstract reconstructing the cenozoic environmental history of hetao basin in the northern part of the ordos plateau in north china is important not only for revealing the evolution of the yellow river but also for understanding the formation of the hobq desert here we present the stratigraphic framework of drill core dr01 with length of 2503 18 m and the results of magnetostratigraphic and esr dating and multi proxy analyses of drill core wedp05 with length of 274 60 m from the hetao basin the magnetostratigraphic and esr results indicate that core wedp05 spans the last 1 68 ma stratigraphic sequence of core dr01 indicates that the hetao area was uplifted and eroded during the early cenozoic before subsiding to form a sedimentary basin subsequently the basin was a fluvio lacustrine environment during the pliocene and then experienced alternating desert and fluvio lacustrine conditions during the quaternary sedimentary facies and multi environmental proxy analyses of core wedp05 indicate that the basin was occupied by a fluvio lacustrine system during the following intervals 1 47 1 30 ma 1 17 1 07 ma 0 68 0 60 ma and from 0 47 ma to the last interglacial and that a desert environment developed during the lake regression phases of 1 30 1 17 ma 1 07 0 68 ma and 0 60 0 47 ma the presence of aeolian sand at the base of core wedp05 suggests that the origin of the hobq desert can be traced back to the early pleistocene and resulted from the erosion and transportation of exposed fluvio lacustrine sediments by near surface winds associated with the asian winter monsoon a large river channel in the hetao basin may have existed as early as the pliocene which was occupied by the yellow river when its upper reaches formed by at least 1 6 ma subsequently at least since 1 2 ma the yellow river formed its drainage system around the hetao basin and controlled the paleoenvironment evolution of the basin

differential impact of small scaled tectonic movements on fluvial morphology and sedimentology the huang shui catchment ne tibet plateau abstract the aim of this paper is to investigate the morphological implications of the interplay between tectonic movements at different rates timescales and spatial extent in a catchment of intermediate size the huang shui river a main tributary of the yellow river in the ne tibet plateau river incision started from a peneplain like surface that developed in post miocene pliocene times at the end of the tertiary a general tectonic uplift of the tibetan plateau initiated river incision this general uplift also caused local fragmentation of the huang shui catchment into blocks kilometers or maximally a few tens of kilometers of local extent that have subsided and or uplifted relative to one another fluvial deposition of 30 m in the subsiding blocks contrasts with erosion and formation of gorges in the uplifted blocks incision into the former peneplain was not continuous but a staircase of terraces developed under climatic influences terrace deposits are sometimes capped by interglacial soils or soil derived material apparently terrace incision occurred at the transition to the next cold period the different rates of uplift and subsidence of the individual blocks resulted in the simultaneous development of erosion and accumulation terraces of different sizes within the same catchment even within the same tectonic block this makes it impossible to connect the terraces of the different blocks except for the three youngest terraces representing the last 300 000 years thus illustrating the uniform tectonic history of the catchment since that time

an obliquity controlled early pleistocene river terrace record from western turkey investigation of the pleistocene sequence of the gediz river western turkey has revealed a record of early pleistocene river terraces eleven terraces spanning the interval from 1 67 to 1 245 million years ago mis 59 37 are preserved beneath basaltic lava flows the high number of terraces over this short time period reflects high frequency sedimentation incision cycles preserved due to the fortuitous combination of relatively high uplift rates 0 16 mm yr 1 and progressive southwards valley migration comparison of this record with odp967 from the eastern mediterranean basin suggests a link between the production of terraces and obliquity driven 41 000 year climate cycles in the early pleistocene