holocene aeolian activity in the southwestern kalahari desert southern africa significance and relationships to late pleistocene dune building events little is known about the timing of dune building episodes in the southwestern kalahari desert previously arid phases when dune activity occurred have been inferred from gaps in chronologies derived from information that attests to former humid periods to redress this optically stimulated luminescence osl dating was applied to sediment samples taken from a range of aeolian landforms in the sw kalahari four periods of dune building are identified in the late pleistocene and holocene holocene aeolian activity at 6 ka and 1 2 ka led to the construction of localised dune forms in contrast to late pleistocene times when the extensive field of linear dunes that dominates the region was emplaced late holocene aridity was however sufficient to allow rapid emplacement of the lunette dune at witpan the record of aeolian activity is evaluated in the context of humid chronologies and an ocean atmosphere model of holocene climate change with good agreement obtained it is concluded that the application of

cross equatorial influence of winter hemisphere subtropical cold surges abstract two case studies are presented of winter hemisphere cold surges affecting the summer hemisphere tropics one case has been chosen from each of the northern and southern hemispheres in both instances the subtropical rise in pressure is tracked equatorward this leads to a pressure rise at the equator and the establishment of a west east pressure gradient at low latitudes in the opposite hemisphere these effects lead in turn to an enhanced cross equatorial component of flow and enhanced monsoon westerly flow in the summer hemisphere both enhanced wind flow effects are observed through a deep layer from the surface up to at least 500 mb in both cases the sequence of events also includes the development of a tropical cyclone in the summer hemisphere monsoon trough the generality of the above sequence of events is investigated with time series data for several seasons it is shown that day to day changes in equatorial pressure are significantly correlated to pressure changes in the winter hemis

a first look at the structure of the wave pouch during the 2009 predict grip dry runs over the atlantic abstractin support of the national science foundation pre depression investigation of cloud systems in the tropics nsf predict and national aeronautics and space administration genesis and rapid intensification processes nasa grip dry run exercises and national oceanic and atmospheric administration hurricane intensity forecast experiment noaa ifex during the 2009 hurricane season a real time wave tracking algorithm and corresponding diagnostic analyses based on a recently proposed tropical cyclogenesis model were applied to tropical easterly waves over the atlantic the model emphasizes the importance of a lagrangian recirculation region within a tropical wave critical layer the so called pouch where persistent deep convection and vorticity aggregation as well as column moistening are favored for tropical cyclogenesis distinct scenarios of hybrid wave vortex evolution are highlighted it was found that easterly waves without a pouch or with a shallow pouch did not develop although not all wav

observations and predictability of a nondeveloping tropical disturbance over the eastern atlantic abstracta strong african easterly wave aew left the west african coast in early september 2014 and operational global numerical forecasts suggested a potential for rapid tropical cyclogenesis of this disturbance in the eastern atlantic despite the presence of a large region of dry air northwest of the disturbance analysis and in situ observations show that after leaving the coast the closed circulation associated with the aew trough was not well aligned vertically and therefore low level or midlevel dry air was advected below or above respectively areas of closed circulation gps dropwindsonde observations highlight the dry air undercutting the midlevel recirculation region in the southwestern quadrant this advection of dry air constrains the spatial extent of deep convection within the aew trough leading to the vortex decaying as the column continues to be displaced horizontally losing vertical alignment this enables increased horizontal advection of dry air into the system further limiting

westward propagating predecessors of monsoon depressions abstract by examining maps of 24 h change of sea level pressure during july and august over the 10 year period 1969 78 we find that 45 87 of the 52 lows and depressions that formed in the bay of bengal were associated with predecessor disturbances coming from the cast in 12 23 instances the predecessor was associated with a typhoon or named tropical storm in the south china sea while the remaining 33 64 were weaker systems originating over a broad region of land and sea from examination of time sections over the same period from eastern thailand to the burmese coast we identified 50 westward moving disturbances with consider able vertical extent of which 64 developed into lows or depressions on reaching the bay in 60 of the 50 instances the disturbance could be traced to the south china sea 32 being typhoons and named tropical storms and 28 weaker circulations the remaining 40 of the 50 disturbances appeared to originate over land for the three stations along the section line cr

northern hemisphere 500 hpa trough merger and fracture a climatology and case study abstract the results of an objective climatology of 500hpa trough merger defined as the amalgamation of two initially separate vorticity maxima and trough fracture defined as the splitting of a single vorticity center into two separate vorticity centers are presented for the northern hemisphere the data source is the gridded national meteorological center nmc now the national centers for environmental prediction analyses available on cd rom on the nmc operational octagonal grid 381 km resolution at 60 n for the period september 1957 to may 1989 a total of 2152 3053 merger fracture events are identified merger and fracture events tend to occur preferentially in the band of middle latitude westerlies merger events exhibit a tendency to avoid major mountainous regions and show a somewhat higher frequency of occurrence downstream of these regions fracture events cluster somewhat over eastern ocean basins the southwestern united states and northwestern africa while also avoiding major mount

downstream fining in contrasting reaches of the sand bedded yellow river compared to downstream fining of a gravel bedded river little field evidence exists to support the process of downstream fining in large fine sand bedded rivers in fact the typically unimodal bed sediments of these rivers are thought to produce equal mobility of coarse and fine grains that may discourage downstream fining to investigate this topic we drilled 200 sediment cores in the channel beds of two fine grained sand bedded reaches of the yellow river a desert reach and a lower reach and identified a fine surface layer fsl developed over a coarse subsurface layer csl in the 3 m thick bed deposits in both reaches downstream the thickness of the fsl increased while that of the csl decreased comparison of the depth averaged median grain sizes of the csl and the fsl separately in both reaches shows a distinct downstream fining dependence to the median grain size which indicates that at a large scale of 600 800 km the csl shows a significant downstream fining but the fsl shows no significant trends in downstream variations in grain size this result shows that fine sediment supply 0 08 mm median grain size from wind borne sediments and cross desert tributaries can interrupt the fsl and coarsen the channel bed surface locally copyright 2011 john wiley sons ltd

the johnstown flood of july 1977 a long lived convective system abstract the heavy rains responsible for the disastrous flash floods new johnstown pennsylvania on the night of 19 20 july 1977 are shown to be part of a large quasi circular mesoscale convective complex this complex can be traced back to an origin in south dakota nearly 96 h earlier the complex tends to become more compact and intense during the night with a radius of 150 km and to expand in response to new and peripheral convection during the day on the small synoptic scale the system is characterized by a pool of cool air at the surface and throughout the lower troposphere just north of the centroid of the rainstorm by a cyclonic circulation of substantial intensity in the lower and middle troposphere and by anticyclonic circulation in the upper troposphere airflow relative to the moving storm is southeasterly at the surface veering rapidly to southwesterly in the boundary layer and then to westerly in the upper troposphere substantial convective instability is present throughout the env

long term morphodynamic changes of a desert reach of the yellow river following upstream large reservoirs operation abstract china s yellow river has experienced its dramatically decreasing trend for the flow discharge since the construction and operation of large reservoirs located upstream this low flow regulation has triggered a severe aggradation of the ulan buh desert channel of the yellow river because the declining flow exhibits no capability to scour and carry away large amount input of desert sands from the ulan buh desert twenty monitoring cross sections documented the ulan buh desert channel has experienced its increasing aggradational trend in conjunction with its lateral migration decreasing trend from 1966 to 2005 which is opposite to the normal pattern of aggradation with deepening or symmetrical infilling for a channel located downstream of a reservoir the channel aggradation can also be identified two stages slow aggradation and rapid aggradation slow aggradation is characterized by the channel bed elevation rising 9 5 cm on average between 1968 and 1985 which responded to the operation of the liujiaxia reservoir during this period the flow discharge was similar to pre dam flow conditions but the sediment transport reduced to half of its pre dam value because of about 0 24 108 t of desert sands entering the channel from the ulan buh desert annually this dilute flow indicated not to scour the channel as expected but contrarily to cause the channel aggraded rapid aggradation followed completion of the longyangxia reservoir with the channel bed elevation rising by 73 cm on average between 1986 and 2005 in this period the combined regulation of liujiaxia and longyangxia reservoirs has caused the flow discharge decreasing dramatically which is more beneficial for accumulation of the desert sands 0 19 108 t yr 1 on average in the desert channel and led to the channel aggradation rate accelerated rapidly

external influences on hurricane intensity part iii potential vorticity structure abstract the interaction of hurricane elena 1985 with a baroclinic wave was reexamined using both potential vorticity pv and a formulation for eliassen palm fluxes in cylindrical coordinates the hurricane began to deepen rapidly as a narrow upper level positive pv anomaly became nearly superposed over the low level center the intensification appeared to represent an evaporation wind feedback activated by constructive interference of the pv anomalies enhanced convection associated with this process eroded the upper pv anomaly and prevented it from crossing the hurricane and reversing the intensification the eliassen palm flux divergence showed that maximum eddy activity remained in the upper troposphere prior to the reintensification this activity was produced in part because the action of the outflow anticyclone of the hurricane contributed to synoptic scale wave breaking the upper level pv anomaly that approached the center was much narrower in extent than the original synoptic scale trough th

reevaluation of the aeolian sand flux from the ulan buh desert into the upper yellow river based on in situ monitoring abstract aeolian sand transport into rivers is a crucial factor influencing channel sediment dynamics and fluvial processes in rivers that flow through desert areas few studies have examined the dynamic processes controlling factors and the amount of aeolian sand transported into river channels here we report a case study of aeolian sand transport into in the upper reaches of the yellow river as it flows along the eastern margin of the ulan buh desert which is a major desert in northern china the present study is based on systematic monitoring of wind flow characteristics aeolian sediment dynamics saltation creep and suspension and the migration of typical sand dunes mobile semi anchored and anchored from 2013 to 2016 the relationship between wind speed and wind blown sand transport in the study area was determined the relationship between the rate of aeolian sand transport saltation and creep and the wind speed 2 m above the ground surface can be depicted with a polynomial relation when the wind speed is 5 7 m s and with a power function when the wind speed is

the three dimensional structure of synoptic scale disturbances over the tropical atlantic abstract twice daily analyses of low level and 200 mb winds over the tropical atlantic region archived by the national hurricane center are used to diagnose the structure of synoptic scale disturbances in the 3 5 day period band the large scale disturbances extracted by a complex empirical orthogonal function technique are found to have a preferred shift at 200 mb relative to the low level troughs of somewhat less than one quarter cycle the presentation concentrates on july 1975 during which a repeated series of strong disturbances propagated through the region the relationship between these disturbances and systems in the eastern pacific is discussed an analysis of the vorticity propagation characteristics for the disturbances during the month indicates a very different balance from level to level at the lower level advection by the mean wind plays a major rate at 200 mb the meridional advection of mean vorticity is more inportant rawinsonde data from several island stations are used to res

fluvial aeolian interactions part i modern systems two modern fluvial aeolian depositional systems great sand dunes national monument colorado and the mojave river wash california are remarkably similar in spite of different climates sizes fluvial sediment textures and relative directions of aeolian and fluvial transport dune growth and migration and deflation of blowouts create 8 10 m of local relief in unflooded aeolian landscapes there are six prominent fluvial aeolian interactions 1 fluvial flow extends into the aeolian system until it is dammed by aeolian landforms 2 interdune areas overbank interdunes upstream of aeolian dams and alongside channels are flooded 3 water erodes dunes alongside channels and interdunes 4 flood waters deposit sediment in interdune areas 5 fluvially derived groundwater floods interdunes interdune playas 6 wind erodes fluvial sediment and redeposits it in the aeolian system unique and characteristic sediments are deposited in overbank interdunes and in interdune playas reflecting alternate fluvial and aeolian processes and rapidly changing flow and salinity conditions these fluvial aeolian interdune deposits are characterized by irregular concave up bases and flat upper surfaces containing mudcracks or evaporite cement characteristic low relief surfaces form in aeolian systems as an effect of flooding fluvial deposits are resistant to aeolian deflation aeolian sand is preserved when flood sediments are deposited around the bases of the dunes thus repetitive fluvial and aeolian aggradation tends to be stepwise as interdune floors are suddenly raised during floods the effects of flooding should be easy to recognize in ancient aeolianites even beyond the area covered with overbank muds

the effect of vertical shear on tropical cyclone intensity change abstract the effect of vertical shear on tropical cyclone intensity change is usually explained in terms of ventilation where heat and moisture at upper levels are advected away from the low level circulation which inhibits development a simple two layer diagnostic balance model is used to provide an alternate explanation of the effect of shear when the upper layer wind in the vortex environment differs from that in the lower layer the potential vorticity pv pattern associated with the vortex circulation becomes tilted in the vertical the balanced mass field associated with the tilted pv pattern requires an increased midlevel temperature perturbation near the vortex center it is hypothesized that this midlevel warming reduces the convective activity and inhibits the storm development previous studies have shown that diabatic heating near the storm center acts to reduce the vertical tilt of the vortex circulation these studies have also shown that there is an adiabatic process that acts to redu

tropical storm formation in a baroclinic environment abstract an analysis is presented of the large scale conditions associated with the initial development of tropical storm diana september 1984 in a baroclinic environment ordinary extratropical wave cyclogenesis began along an old frontal boundary east of florida after 0000 utc 7 september and culminated in tropical cyclogenesis 48 h later water vapor satellite imagery showed that the initial cyclogenesis and incipient tropical storm formation was nearly indistinguishable from a classical midlatitude development cyclogenesis occurred in three stages a large scale cold trough and associated frontal system crossed the atlantic coast while a small potential vorticity maximum aloft fractured off the main trough and stalled over central florida in the first stage as the main trough sheared off eastward cyclogenesis began along the southwestern end of the stalled frontal zone east of florida anticyclogenesis to the north in the wake of the shearing trough allowed a surge of cooler and drier air to flo

suspension of large concentrations of sands existing methods based on the diffusion theory have been extended to determine the sediment concentration distribution for large concentrations of sand the modifications for volumetric concentrations ranging from 4 to 20 include decreased fall velocity and increased viscosity and specific weight of the suspension the sediment concentration profiles are sensitive to the velocity profiles and the fall velocity of sediment particles in turbulent flows either a power law or a two layered logarithmic law can be used to describe the velocity profiles and the sediment difiusivity the proposed method accurately depicts the deviation from the rouse equation at large concentrations of sands and the agreement with sediment concentration profiles measured by einstein and chien is excellent the proposed numerical solution gradually reduces to the relationships of karim and kennedy and rouse as the volumetric concentration decreases below four percent

a late pleistocene linear dune dam record of aeolian fluvial dynamics at the fringes of the northwestern negev dunefield abstract a late pleistocene aeolian fluvial record within a rare vegetated linear dune like structure at the fringe of the northwestern negev dunefield israel provides direct evidence of dune damming dynamics within the structure and its environs study methods included high resolution morphology and stratigraphy micromorphology and sedimentological analyses chronology was based on eight archaeological sites from the structure and the inqua dune atlas chronologic database low energy fine grained fluvial deposits underlying the structure and extending from its flanks indicate deposition by low energy hyper concentrated flows in a floodplain environment and later in water bodies that formed by dune damming of a mid sized drainage basin interbedded sand with fine grained deposits within the linear structure indicates interchanging dominances between aeolian sand incursion and seasonal floods sand deposition during dune elongation led to structure growth and dune damming of its drainage system that in turn formed water bodies and upstream fine grained deposition following seasonal floods calculations of current sediment yields indicate that fine grained deposits accretion up to the structure s brim could possibly have rapidly occurred over a total time span of decades however artifacts dating to the geometric kebaran 17 5 12 9 cal kyr bp and harifian 12 9 11 2 cal kyr bp archaeological periods on the structure s surface indicates intermittent repetitive and short term camping utilizing adjacent water bodies over a time period of 4000 5000 years fluctuating high winds and precipitation during a time window of increased fluvial availability of fine grained sediment from the hinterland generated ample fine grained deposition after 11 cal kyr bp the abundance and recurrence of dammed water bodies decreased when reduced wind power constrained dune dam maintenance after sediment accommodation space dissipated fluvial flow of the drainage basin led to dune dam destruction and partial fluvial erosion of the fine grained deposits that continues today

large scale influences on tropical cyclogenesis in the western north pacific abstract objectively analyzed data from the european centre for medium range weather forecasts are used to examine the large scale aspects of the formation of tropical cyclones it is hypothesized that tropical cyclogenesis occurs when external atmospheric forcing on the synoptic or larger scale provides uplift through a deep layer enhancing convection in a region with environmental conditions favorable for genesis emphasis is placed on the roles of upper level troughs low level wind surges preexisting tropical cyclones and propagating wave disturbances in triggering tropical cyclogenesis composites of the 200 hpa and 850 hpa flows reveal the presence of both upper level troughs and low level wind surges respectively prior to genesis in the composites the wind surges also appear to be related to the presence of a prior circulation located approximately 2000 km to the west of the genesis location an examination of the individual cases demonstrates that approximately 85 of all storms had either

the role of synoptic scale flow during tropical cyclogenesis over the north atlantic ocean abstract the synoptic scale flow during tropical cyclogenesis and cyclolysis over the north atlantic ocean is investigated using compositing methods genesis and lysis are defined using the national hurricane center nhc now known as the tropical prediction center best track data genesis lysis occurs when nhc first last identifies and tracks a tropical depression in the final best track dataset storm centered composites are created with the analysis of the tropical oceanic lower level atoll 900 hpa and 200 hpa winds for june november produced by nhc for the years 1975 93 results show that significant regional differences exist in 200 hpa flow during genesis across the atlantic basin composites of genesis in the western part of the basin show a 200 hpa trough ridge located to the west east of the atoll disturbance in the eastern half of the basin composites of genesis show a sprawling 200 hpa ridge centered northeast of the atoll disturbance the major axis of this elliptically shaped 20

the contribution of aeolian processes to fluvial sediment yield from a desert watershed in the ordos plateau china in arid zones many active aeolian dunes terminate at ephemeral and perennial desert rivers the desert rivers show very high rates of sediment transport that cause deleterious downstream effects on the river system and ecology high sediment loading has been attributed to severe water erosion of sparsely covered watersheds during infrequent but heavy rainfall although aeolian erosion is known to lead to high rates of wind blown sand transport direct confirmation of whether the aeolian processes accelerate or inhibit fluvial sediment loss is lacking here we show that an aeolian fluvial cycling process is responsible for the high rate of suspended sediment transport in a sudalaer ephemeral desert channel in the ordos plateau of china frequent aeolian processes but low frequency once every 3 years on average flooding occur in this region wind blown saltating grains appeared to be unable to cross the desert channel because of interruption of channel induced recirculating air flow and therefore tended to settle in the channel during the windy seasons leading to channel narrowing during flooding this narrowed channel was found to yield a threefold increase in suspended sediment loading and a 3 4 fold increase in the weight percentage of the 0 08 0 2 mm sediment fraction on 18 july 2012 loss of stored aeolian sand due to channel erosion accounted for about half of the total sediment yield in this watershed these findings show that aeolian processes play an essential role in accelerating the sediment yield from a watershed characterized by aeolian fluvial interplay and also suggest that the drier the region and the greater the aeolian process the more the aeolian process contributes to fluvial sediment yield copyright 2013 john wiley sons ltd

on the formation of amex tropical cyclones irma and jason abstract the amex observational dataset with its high temporal and spatial resolution has been used to study the formation and structure of tropical cyclones irma and jason these systems developed and evolved entirely within the experiment s special observing network the study is mostly based upon six hourly numerical analyses of the mass and wind fields on 11 vertical levels over a 1 25 lat long grid the systems are traced from prior to the formation of a resolvable closed surface circulation to when they were operationally classified as tropical cyclones the discussion focuses on the synoptic to cyclone scale changes during formation time sections of various kinematic variables together with an index of deep convection obtained from digital satellite cloud imagery are used to trace the development both systems developed during active phases of the monsoon and initially were of maximum intensity in the middle troposphere low level spinup occurred in three stages the first stage was associate

atlantic subtropical storms part i diagnostic criteria and composite analysis abstract subtropical cyclones st have only recently gained attention as damaging weather systems a set of criteria for identifying and classifying these systems is introduced here and employed to identify 18 st cases forming in the 1999 2004 hurricane seasons to be classified as an st these systems must have near surface gale force winds and show hybrid structure for more than one diurnal cycle the 18 st cases are partitioned into four classes based upon their genesis environments genesis over waters with sst in excess of 25 c is observed in almost 80 of warm season cases in contrast with only 55 in an st climatology presented in a companion study the low shear magnitude constraint recognized for tropical cyclogenesis is less apparent for st formation with over 50 forming in the two partitions characterized by shear in excess of 10 m s 1 this relatively high shear environment corresponds to equatorward intrusion of upper troughs over the relatively warm sst present in the mid late hurricane s

a study of the structural transformation of the african easterly waves abstract it is hypothesized that few african waves become tropical storms because they typically have an unfavorable thermal structure that cannot be sufficiently transformed during its propagation across the atlantic ocean by the condensational heating under normal moisture conditions this hypothesis is tested in the context of a moist multilevel quasi geostrophic model the longitudinal variation of the background zonal flow over the atlantic ocean is incorporated in the model as a progressively changing zonal flow that an easterly wave would encounter as it propagates across the oceanic region the dependence of the evolution of such a wave field for 10 days upon the various model parameters is evaluated by means of numerical integration a detailed comparison of the structural and energetic properties of the wave field in nondeveloping and developing cases supports the hypothesis specifically the initial model easterly wave field has a cold core at the trough region centered at the lower tropospher

bed sediment particle size characteristics and its sources implication in the desert reach of the yellow river a 1000 km desert reach of the yellow river flows through an extensive aeolian desert region in recent years the river bed of this reach has shown an aggradational trend leading to substantial social economic and other environmental problems the desert reach of the yellow river provides an opportunity for us to study the sediment particle size characteristics and its sources transport and sedimentation processes in a large desert river to investigate this topic 111 shallow sediment core samples from the desert reach and some typical surface sediment samples were also systematically collected from the main reach and potential source areas in the upper yellow river basin for particle size analysis the analysis of sediment cores in the desert reach indicated that a fine sediment layer had developed over a coarse sediment layer in the 1 to 4 m thick bed deposits aeolian sand the ten tributaries and coarse fraction from the riverbank provide material for the development of the coarse sediment layer in the sediment cores while the relatively fine sediment 0 05 mm coming from the loess plateau our results provide a meaningful reference for the management of desert rivers around the world

synoptic scale influence of the saharan air layer on tropical cyclogenesis over the eastern atlantic abstract the formations of hurricane andrew tropical storm ernesto and hurricane luis which occurred respectively during the 1992 1994 and 1995 hurricane seasons over the eastern atlantic have been investigated by utilizing the european centre for medium range weather forecasts ecmwf gridded data analyses these cases were selected to illustrate the contrasting influences of the saharan air layer sal on tropical cyclogenesis analyses results show that tropical storm ernesto 1994 and hurricane luis 1995 formed from the merger of the low level 925 hpa and midlevel 700 hpa vortices over the eastern atlantic within the monsoon trough enhanced by surges in the trades midlevel vortices associated with each case appear to evolve from african wave troughs enhanced by cyclonic shear vorticity of the midtropospheric jet which existed to the south of an sal anticyclonic eddy as an elongated wind maximum vorticity budget calculations suggest that vortex stretching dominated the enhancement of

downstream fining through selective particle sorting in an equilibrium drainage network the phenomenon of downstream fining has been attributed to both particle abrasion and selective particle sorting the latter is generally considered to play the dominant role within resistant lithologies it has been recognized that tributaries can disrupt fining patterns however few downstream fining studies have considered the entire fluvial network structure here we combine a theory for selective transport with a model of river basin evolution in order to simulate the dynamics of selective sorting throughout a drainage network previous numerical modeling studies of single thread or braided channels have treated downstream fining as a phenomenon driven by differential deposition rates we show however that in an eroding drainage network downstream fining emerges as a natural dynamic adjustment to variable water sediment and energy inputs even under conditions of uniform size distribution in sediment flux thus although selective deposition and abrasion clearly can and do play a role in some fluvial systems neither is necessary to produce downstream fining within a drainage network

clouds in tropical cyclones abstract clouds within the inner regions of tropical cyclones are unlike those anywhere else in the atmosphere convective clouds contributing to cyclogenesis have rotational and deep intense updrafts but tend to have relatively weak downdrafts within the eyes of mature tropical cyclones stratus clouds top a boundary layer capped by subsidence an outward sloping eyewall cloud is controlled by adjustment of the vortex toward gradient wind balance which is maintained by a slantwise current transporting boundary layer air upward in a nearly conditionally symmetric neutral state this balance is intermittently upset by buoyancy arising from high moist static energy air entering the base of the eyewall because of the radial influx of low level air from the far environment supergradient wind in the eyewall zone and or small scale intense subvortices the latter contain strong erect updrafts graupel particles and large raindrops produced in the eyewall fall out relatively quickly while ice splinters left

anatomy of an observed african easterly wave in july 2006 the detailed structure of an african easterly wave aew observed during the amma field campaign is analysed the wave was present from 25 to 29 july 2006 a complex circulation pattern was observed the overall structure of convection and the positive vorticity of the trough region had an elongated inverted v appearance wrapped around an area of low winds and clear skies satellite imagery showed that the aew was a significant influence on the modulation of convection on the large scale the wave was identified initially through its strong signature on soil moisture and convection the aew structure observed was not anticipated and has not been discussed in previous literature in addition wave tracking using a hovmoller diagram of meridional winds did not detect the wave and a hovmoller of vorticity showed the wave moved at a slower speed than other aews in july new schematics explaining the structure are presented describing the case as observed by satellites and analysed by a limited area version of the met office unified model it is proposed that the positive vorticity branches of the inverted v can be regarded as analogous to atmospheric fronts with characteristic gradients in winds and thermodynamic properties acting as locations for enhanced convection the implications of the new case are discussed in relation to previous theory and it is suggested that the accepted model of an idealised aew is incomplete and should be extended to include more complex structures copyright 2011 royal meteorological society and british crown copyright the met office

tropical storm development from african easterly waves in the eastern atlantic a comparison of two successive waves using a regional model as part of nasa amma 2006 abstract two successive african easterly waves aews from august 2006 are analyzed utilizing observational data the european centre for medium range weather forecasts reanalysis and output from the national center for atmospheric research national oceanic and atmospheric administration weather research and forecasting model wrf to understand why the first wave does not develop over the eastern atlantic while the second wave does the first aew eventually forms hurricane ernesto over the caribbean sea but genesis does not occur over the eastern atlantic the second wave although weaker than the first over land leaves the west african coast and quickly intensifies into tropical storm debby west of the cape verde islands this study shows that the environmental conditions associated with the first aew s passage inhibited development these conditions include strong low and midtropospheric vertical wind shear owing to a stronger than normal african easterly jet lower than normal relative humidity a

responses of the braided channel to reduced discharge and lateral inputs of aeolian sand in the ulan buh desert reach of the upper yellow river the ulanbuh desert reach belongs to the upper reaches of the yellow river located in inner mongolia autonomous region china which has developed a sandy bed this reach that flows across aeolian dunes field of the ulanbuh desert has caused large lateral infusion of aeolian sands into riverbed an abrupt addition of the bedload and a sharp channel widening forming a typical braided channel in addition to traditional driving factors such as discharge suspended sediments and slope here we also focus on the lateral infusion of aeolian sands into the yellow river which may have a significant impact on channel form to further recognize the desert braided channel change here based on a long time series of field dataset from 1966 to 2013 results show that lateral infusion of aeolian sand is the same as discharge as a significant factor to channel form which is mainly controlled by aeolian activity and discharge conditions before the early 1990s higher discharge and greater intensity of aeolian activity caused a frequent lateral movement and large lateral inputs of aeolian sands into river this reach showed an unsteady and wider braided channel with a higher braiding intensity from the early 1990s to 2013 a reduced discharge that caused a decreasing lateral movement coupled with the decreasing aeolian activity have resulted in the main flow of the yellow river far away from sand dunes which led to a reduction in lateral inputs of aeolian sands thus the braided channel of the ulanbuh desert reach presented a shrinkage state gradually changing from a wider and unsteady braided channel to a narrower and relative steady channel form

is the tropical atmosphere conditionally unstable abstract we examine the mean and standard deviation of the bouyancy of lifted boundary layer parcels from a large sample of soundings from the western equatorial pacific the inclusion of condensate loading in the definition of buoyancy is emphasized as is the precise originating level of parcels in the subcloud layer we confirm the observation of betts that those parts of the tropical atmosphere experiencing deep convection are nearly neutral to adiabatic parcel ascent from the subcloud layer when adiabatic condensate loading is included in the definition of buoyancy parcels lifted from the top of the subcloud layer are more nearly neutral than those originating near the surface and atmospheres with higher levels of free convection lfcs are closer to neutral than those with low lfcs we explore possible reasons for these observations and discuss their implications for tropical meteorology

anti clockwise hysteresis in suspended sediment concentration during individual storms holbeck catchment yorkshire england abstract analysis of the flow sediment hysteresis shape may assist in determining the sediment source area in a small basin a clockwise hysteresis occurs when sediment is derived from the bed and banks of the channel or areas adjacent to the channel whereas an anti clockwise hysteresis occurs when the upper part of the slopes is the sediment source area

eolian fluvial interaction in the page sandstone middle jurassic in south central utah usa a case study of erg margin processes abstract channel and lens shaped deposits of non eolian red sandstone are enclosed within the eolian leche e member of the page sandstone in south central utah detailed analysis of lithology and geometry and regional correlation of the red sandstone deposits suggests that the channel shaped scours and in filling deposits were formed by ephemeral stream processes in contrast to earlier interpretations that suggested a marine estuarine origin we hypothesize that ephemeral streams transporting volcanic debris flowed toward the north and northeast along the western edge of the page erg local avulsion possibly caused by eolian damming of an adjacent drainage led to stream flow into low areas of the page erg entrainment of loose eolian sand combined with fluid loss via infiltration led to a rapid increase in sediment fluid ratio during flow events initially resulting in sandy debris flow deposition subsequent floods entrained less material from the underlying more cohesive debris flow deposits had a lower sediment water ratio and hence left deposits containing some evidence of flow turbulence eventually stream drainage avulsed again probably back to a more northerly course into the carmel sea fluvial deposition within the erg ceased and eolian deposits covered the now intercalated fluvial complex

aeolian and fluvial evidence of changing climate and wind patterns during the past 100 ka in the western simpson desert australia abstract sediments near finke in central australia provide evidence of late quaternary evolution and the interaction of aeolian and fluvial systems in response to changing climate in the western simpson desert thermoluminescence tl dating is used to develop a chronology of aeolian and alluvial activity and to identify differences in sand provenance quaternary alluviation in the finke valley at this location occurred at or prior to c 90 ka and with no surviving evidence of subsequent activity until the holocene in contrast to rivers in the eastern part of the lake eyre basin substantial alluviation took place here in the early to mid holocene probably due to reactivation of the northern monsoon and its partial penetration into central australia the regional dunefield near finke consists of linear dunes largely reworked and aligned during the last glacial 30 12 ka as part of the great anticlockwise whorl of dunes in central australia the oldest dated source bordering dunes from the finke river are bright red in colour were deposited at c 100 ka and are now buried beneath paler source bordering dunes the latter consist of two exposed units a lower one of unknown orientation that dates at 17 9 ka and an upper unit aligned to the northwest and parallel to the prevailing winds that dates at 5 0 ka the tl signature and sediment texture of the oldest source bordering dunes show them to have probably been derived from weathered alluvial red beds of similar or greater age nearby and to have contributed abundant sand to the regional dunefield immediately north of the river in contrast sand from the younger paler source bordering dunes appears to be from a different source and to have only recently extended into the regional field the finke region would seem to be in a pivotal position for the study of palaeowind patterns that have created and modified australia s whorl of continental dunes the 30 18 ka regional linear dunes near finke are oriented almost due north their cross sectional asymmetry steeper eastern slopes suggests a response to southwesterly or westerly sand transporting winds between 18 and 10 ka these winds appear to have shifted to their present southeast orientation during the past 5 ka from dune ages alignments and asymmetry it is proposed here that there was a northward shift in the wind pattern at finke by about 100 150 km 1 1 5 of latitude during the last glacial but that through this period the regional dune pattern has remained remarkably stable

aeolian process induced hyper concentrated flow in a desert watershed ephemeral desert channels are characterized by very high rates of sediment transport during infrequent flood events here we show that aeolian process induced hyper concentrated ahc flows occur in the sudalaer desert watershed in the ordos plateau of china which primarily transport 0 08 0 25 mm non cohesive aeolian sand and have a peak suspended sediment concentration of 1 1 1 4 x 10 6 mg l 3 aeolian sand supply and storage in the channel play a crucial role in causing hyper concentrated flow our results indicate that non cohesive aeolian sand can be entrained from the bed and suspended in the turbulent flow when the channel bed slope exceeds a critical threshold 0 0003 we also show that if the frequency ratio of wind blown sandstorms to rainstorms t w t p exceeds beta gamma gamma 0 alpha p v 3 a l where alpha is the wind blown sand transport coefficient beta is the runoff coefficient gamma gamma 0 is the increase in suspension concentration caused by addition of aeolian sands p is the density of rainstorms v is the wind speed of sandstorms a is the runoff generating area l is the aeolian sand filled channel length an ahc flow occurs during the passage of a flood in a desert channel since high frequency aeolian processes provide an adequate quantity of transportable sediment and promote ahc flow most of the infrequent rainfall induced floods occurring in arid zones can develop as ahc flows c 2014 elsevier b v all rights reserved

the global population of mesoscale convective complexes a global set of 714 mesoscale convective complexes is compiled and some of the common properties of the convective systems are identified and examined from a global perspective the data set includes date of occurrence time of first storms initiation maximum extent termination duration cold cloud shield areas and tracks from initiation to termination it is found that the typical convective complex is nocturnal generates a cold cloud shield area of approximately 350 000 km2 and persists for about 10 h the largest systems and most persistent systems tend to occur near the summer solstices for the globe about 400 systems occur each year primarily over land areas most systems develop in favoured zones although some activity occurs over every continent except antarctica and all major oceans the concentration of activity into favoured zones indicates that there must be special dynamic and or thermodynamic conditions necessary for convection to organize into convective complexes activity is strongly tied to the solar day and shifts from 35 s in early january to about 50 n during the boreal summer and back to 35 s by december within the northern hemisphere there is a pronounced poleward migration as the jet stream shifts northward relatively little migration occurs in the ocean dominated southern hemisphere where the subtropical jet remains quasi stationary over the convective complex regions the nocturnal life cycles copious rainfall large cloud shields and great frequency of mesoscale convective complexes suggest that they may be significant contributors to the global hydrologic cycle and earth system energy budget

synoptic and frontal scale influences on tropical transition events in the atlantic basin part i a six case survey abstract the process by which a baroclinic vertically sheared extratropical cyclone is transformed into a warm core vertically stacked tropical cyclone is known as tropical transition six recent tropical transitions of strong extratropical precursors in the subtropical north atlantic are compared to better understand the manner by which some of the canonical structures and dynamical processes of extratropical cyclones serve to precondition the cyclone for transition all six transitions resulted from the interaction between a surface baroclinic zone and an upper level trough during the extratropical cyclogenesis of each storm a period of intense near surface frontogenesis along a bent back warm front occurred to the northwest of each sea level pressure minimum within the resultant circulation diabatic redistribution of potential vorticity pv promoted the growth of a low level pv maximum near the western end of the warm front concurrently the upper level pv anomaly associated with each trough w

mesoscale observations of the genesis of hurricane dolly 1996 abstract recent numerical studies of tropical cyclone genesis suggest a new paradigm for how the surface vortex is established based on a highly nonaxisymmetric mechanism involving the interaction of low level cyclonic circulations generated by deep cumulonimbus convection a reexamination of mesoscale observations during the genesis of hurricane guillermo 1991 confirms the presence of multiple cyclonic circulations more recently airborne doppler radar wind observations during the genesis of atlantic hurricane dolly 1996 also reveal multiple lower to middle tropospheric mesoscale cyclonic circulations during sequential 15 20 min compositing periods a particularly well organized but initially weak mean tangential wind of 7 m s 1 low level cyclonic vortex embedded within the pre dolly tropical disturbance is observed coincident with deep vertically penetrating cumulonimbus convection the earliest observations of this vortex show the peak circulation near 2 km height with a mean diameter of 30 4

lacustrine sediments at narabeb in the central namib desert namibia abstract deep within the hyper arid namib desert at narabeb which is today isolated by 100 m high linear dunes is a 36 m section of lacustrine mudstones and interbedded sands the slightly calcareous and laminated sands of this sequence are composed mainly of rounded to subrounded lightly stained quartz that is similar to the sands of the underlying tertiary tsondab sandstone formation and the surrounding dunes of the namib sand sea the six calcareous mudstone units in this sequence are variably laminated and sandy contain halite filled fractures and have been radiocarbon dated at 20 000 26 000 yr b p the interrelationship of sediments at this site with middle to early stone age implements suggests an age of more than 20 000 years whereas 234u 230th dating of the basal mudstone gave an age of 210 000 260 000 yr b p the calcareous mudstones were deposited in a lake at the former end point of the tsondab river which was prevented from reaching the atlantic ocean by n s trending linear dunes of the namib sand sea the interbedded sands were deposited during low water or dry stages of the lake sometime before 14 000 yr b p the terminus of the tsondab river shifted eastward from narabeb in response to the northward encroachment of linear dunes and the present end point was eventually established at tsondab vlei 38 km to the east lacustrine deposition at narabeb just as at tsondab vlei today probably occurred mainly in response to precipitation in the headwaters of the tsondab river although no major climatic change is necessary to explain the sequence at narabeb the clustering of radiocarbon dates on carbonate from the namib desert in the periods 20 000 35 000 yr b p and 10 000 14 000 yr b p suggests an increase in precipitation during those times

erosion and sediment yields as influenced by coupled eolian and fluvial processes the yellow river china based on data from the middle yellow river a model of erosion and sediment yield is proposed to describe coupled eolian and fluvial processes in a transitional zone from and to sub humid climates and to explain rapid erosion and a high sediment yield in the zone in the study area wind action predominates from march to june which erodes weathered bedrock and transports eolian sand to gullies river channels and floodplains in the following summer especially from july to september rainstorm runoff in gullies and river channels transports large quantities of fine loessic material in the form of hyperconcentrated flow as a result most of the previously stored eolian sand and material supplied by mass wasting of loess can be transported to the major tributaries and the main stream of the yellow river resulting in the high specific sediment yield there exists all optimal grain size composition which maximizes suspended sediment concentration in the study area resulted from the combined wind water processes c 2005 elsevier b v all rights reserved

hyper concentrated flow response to aeolian and fluvial interactions from a desert watershed upstream of the yellow river abstract aeolian and fluvial interactions afis are critical earth surface processes in arid zones especially in desert watersheds the response of hyper concentrated flows to seasonal alternate afis shows very high rates of sediment transport and has important environmental and ecological consequences from local to global scales here we present the aeolian processes induced hyper concentrated ahc flows that occurred in the sudalaer desert watershed upstream of the yellow river which primarily transport non cohesive coarse aeolian sand 0 08 mm and show a peak suspended sediment concentration ssc of 1 1 to 1 4 106 mg l 3 our field study and theoretical analysis indicate that non cohesive coarse aeolian sand downstream in the channel can be entrained from the bed and can be suspended in the turbulent flow by the significant runoff generated upstream with a ssc 0 value of 0 5 106 mg l 3 severe aeolian processes can provide an abundant coarse sediment supply in the channel which once entrained can also trigger and promote the development of ahc flows we define for the first time the ratio of the weight percentage of coarse sediment to fine sediment c 0 08 mm c

cluster analysis of downscaled and explicitly simulated north atlantic tropical cyclone tracks abstracta realistic representation of the north atlantic tropical cyclone tracks is crucial as it allows for example explaining potential changes in u s landfalling systems here the authors present a tentative study that examines the ability of recent climate models to represent north atlantic tropical cyclone tracks tracks from two types of climate models are evaluated explicit tracks are obtained from tropical cyclones simulated in regional or global climate models with moderate to high horizontal resolution 1 0 25 and downscaled tracks are obtained using a downscaling technique with large scale environmental fields from a subset of these models for both configurations tracks are objectively separated into four groups using a cluster technique leading to a zonal and a meridional separation of the tracks the meridional separation largely captures the separation between deep tropical and subtropical hybrid or baroclinic cyclones while the zonal separation segregates gulf of mexico and ca

tropical cyclone formation in a sheared environment a case study abstract the development of hurricane danny 1997 from depression to hurricane was examined using cloud to ground lightning data reconnaissance aircraft data and satellite imagery vertical wind shear between 850 and 200 hpa of 5 11 m s 1 produced persistent downshear convective outbreaks that became progressively more intense and closer to the center during the development early in the period the storm intensified steadily in the presence of this downshear convection during the last and most intense outbreak a second vortex appeared to develop within the convection evidence is presented that the new downshear vortex became the dominant vortex and absorbed the original based on these events it is hypothesized that the presence of moderate vertical wind shear accelerated the early development process equivalent potential temperature fields within 500 m of the surface were examined only well after the period of vortex interaction did the characteristic mature tropical cyclone radial profile of eq

a model to study the grain size components of the sediment deposited in aeolian fluvial interplay erosion watershed abstract aeolian fluvial interplay erosion areas with complex dynamics and physical sources are the main suppliers of coarse sediment in the yellow river understanding the composition distribution and sources of deposited sediments in such areas is of great importance for the control of sediment transport in rivers in this paper a typical aeolian fluvial interplay erosion watershed the dongliu gully was studied and the frequency distribution curves of sediments deposited in the stream channel were fitted using the weibull function sources of deposited sediment in the stream channel were analyzed based on the law of the conservation of matter results showed that the hilly zone accounted for 78 of deposited sediments which were dominated by material with a median grain size d 50 of 0 093 mm and the desert zone accounted for 22 of deposited sediments which were dominated by material with a d 50 of 0 01 mm wind erosion dynamics accounted for 72 of deposited sediments while water erosion dynamics accounted for only 28 this research provides a theoretical basis for the control and management of rivers with high sediment content

age stratigraphy and deposition of near k t siliciclastic deposits in mexico relation to bolide impact examination of 10 k t boundary sections in northeastern and east central mexico and new data presented from 7 sections permit the following conclusions 1 the globally recognized k t boundary and mass extinction in planktic foraminifera is stratigraphically above and separated by a thin marl layer of maastrichtian age from the siliciclastic deposit that is commonly interpreted as a short term hours to days k t impact generated tsunami deposit a similar relationship between the k t boundary and siliciclastic or breccia deposits is observed at brazos river in texas beloc in haiti and poty quarry in brazil 2 stratigraphic control indicates that deposition of the siliciclastic member occurred sometime during the last 150 k y of the maastrichtian and ended at least several thousand years prior to the k t boundary 3 at least four discrete horizons of bioturbation have been observed within the siliciclastic deposit that indicate episodic colonization by invertebrates over an extended time period 4 the glass and spherule rich unit which has been linked to the haiti spherule layer and the chicxulub structure is at the base of the siliciclastic deposit and thus significantly predates the k t boundary event the stratigraphic separation of the k t boundary and siliciclastic deposits and the evidence of long term deposition between them suggests the presence of two events 1 a globally recognized k t boundary impact event marked by ir anomaly and the mass extinction and 2 a caribbean event impact or volcanic and probably linked to the chicxulub structure that predates the k t boundary and is marked by glass and siliciclastic or breccia deposits

william river an outstanding example of channel widening and braiding caused by bed load addition the lower william river in northwestern saskatchewan canada presents an excellent and unambiguous example of rapid channel adjustment to abrupt additions of sandy bed load a relatively narrow and deep single channel stream as it flows northward to lake athabasca the river picks up a 40 fold increase of bed load over a 27 km reach as it encounters a large dune field just south of the lake as a result of the large infusion of eolian sand the channel develops a thoroughly braided pattern while undergoing a 5 fold increase in width and a 10 fold increase in width depth ratio

electrically active hot towers in african easterly waves prior to tropical cyclogenesis abstract it has been hypothesized that intense convective scale hot towers play a role in tropical cyclogenesis via dynamic and thermodynamic feedbacks on the larger scale circulation in this study the authors investigate the role that widespread and or intense lightning producing convection i e electrically hot towers present in african easterly waves aews may play in tropical cyclogenesis over the east atlantic ocean the 700 hpa meridional wind from the ncep ncar reanalysis dataset was analyzed to divide the waves into northerly southerly trough and ridge phases the aews were subsequently divided into waves that developed into tropical storms i e developing and those that did not develop into tropical storms i e nondeveloping finally composites were created using various ncep variables lightning data gathered with the zeus network and worldwide lightning location network wwlln and brightness temperature data extracted from the nasa global merged infrared brightness temperatur

debris flow benches dune contact deposits record paleo sand dune positions in north panamint valley inyo county california debris flows debouching onto the alluvial fan at the north end of panamint valley california have been episodically impounded behind sand dunes resulting in boulder strewn nearly flat topped deposits in irregular basins upslope of the dunes whose upper surface is higher than the adjacent fan surface upslope migration of the dune field over and beyond these deposits eventually leaves them as debris flow benches rising above the general fan surface these features are therefore dune contact forms analogous to ice contact forms such as kame terraces in that both involve deposition against ephemeral barriers benches punctuate the alluvial fan surface for 5 km downfan from the modern dune field clast seismic velocities of boulders on these benches indicate that bench ages increase monotonically with distance from the present dunes implying that the dune field has migrated up the fan because the oldest bench is below the altitude of the highest pluvial lake shoreline in panamint valley gale stage ca 50 ka and slightly above the latest lakeshore i stage ca 14 ka it seems likely that the dunes originated near the shore of the latest lake and have moved upfan at an average rate of 0 8 m yr

an air sea interaction theory for tropical cyclones part ii evolutionary study using a nonhydrostatic axisymmetric numerical model abstract in part i of this study an analytical model for a steady state tropical cyclone is constructed on the assumption that boundary layer air parcels are conditionally neutral to displacements along the angular momentum surfaces of the hurricane vortex the reversible thermodynamics implied by this assumption allows the mature storm to be thought of as a simple carnot engine acquiring heat at the high temperature ocean surface and losing heat near the low temperature tropopause although the oceanic heat source is universally recognized as the sine qua non for the mature hurricane there is also wide acceptance of conditional instability of the second kind cisk which makes no specific reference to surface heat fluxes as the formative mechanism this ambivalence is seen in that all numerical simulation studies find it essential to have transfer from the ocean surface yet all start from a conditionally unstable atmosphere the hypothesis put forward in part i based on the steady state theory is t