observations of the nocturnal boundary layer associated with the west african monsoon a set of night time tethered balloon and kite measurements from the central sahel 15 2n 1 3w in august 2005 were acquired and analyzed a composite of all nights data was produced using boundary layer height to normalize measured altitudes the observations showed some typical characteristics of nocturnal boundary layer development notably a strong inversion after sunset and the formation of a low level nocturnal jet later in the night on most nights the sampled jet did not change direction signifiantly during the night the boundary layer thermodynamic structure displayed some variations from one night to the next this was investigated using two contrasting case studies from the period in one of these case studies 18 august 2005 the low level wind direction changed signficantly during the night this change was captured well by two large scale models suggesting that the large scale dynamics had a significant impact on boundary layer winds on this night for both case studies the models tended to underestimate near surface wind speeds during the night a feature which may lead to an underestimation of moisture flux northwards by models

the effects of large organic debris on sediment processes and stream morphology in vermont abstract research conducted on a small third order stream in the green mountains of vermont suggests that large organic debris lod has an important influence on areal sorting and storage of sediment spacing of pool riffle sequences and channel geometry large organic debris forms local base levels that trap over 26 cubic meters of sediment and cause over 14 percent of the vertical drop along a 412 meter study reach on tiger brook vermont the local base levels created by large organic debris accumulations are the most important control on sediment storage between large bedrock knickpoints along the study reach focusing fine grained sediment deposition in areas stable at low and moderate discharges the micro steps created by these lod local base levels dominate gradient and sediment storage at short time scales between the slowly evolving large bedrock macrosteps the relation that exists among the volumes of standing timber adjacent to the channel lod and sediment stored behind lod seems to indicate that large organic debris causes a negative feedback type mechanism where channel degradation leads to increased standing timber recruitment and large organic debris sediment storage sites

african easterly waves in 30 day high resolution global simulations a case study during the 2006 namma period 1 in this study extended range 30 day high resolution simulations with the nasa global mesoscale model are conducted to simulate the initiation and propagation of six consecutive african easterly waves aews from late august to september 2006 and their association with hurricane formation it is shown that the statistical characteristics of individual aews are realistically simulated with larger errors in the 5th and 6th aews remarkable simulations of a mean african easterly jet aej are also obtained nine additional 30 day experiments suggest that although land surface processes might contribute to the predictability of the aej and aews the initiation and detailed evolution of aews still depend on the accurate representation of dynamic and land surface initial conditions and their time varying nonlinear interactions of interest is the potential to extend the lead time for predicting hurricane formation e g a lead time of up to 22 days as the 4th aew is realistically simulated citation shen b w w k tao and m l c wu 2010 african easterly waves in 30 day high resolution global simulations a case study during the 2006 namma period geophys res lett 37 l18803 doi 10 1029 2010gl044355

cycles and propagation of deep convection over equatorial africa abstractlong term statistics of organized convection are vital to improved understanding of the hydrologic cycle at various scales satellite observations are used to understand the timing duration and frequency of deep convection in equatorial africa a region with some of the most intense thunderstorms yet little has been published about the propagation characteristics of mesoscale convection in that region diurnal subseasonal and seasonal cycles of cold cloud proxy for convective precipitation are examined on a continental scale organized deep convection consists of coherent structures that are characteristic of systems propagating under a broad range of atmospheric conditions convection is triggered by heating of elevated terrain sea land breezes and lake breezes coherent episodes of convection result from regeneration of convection through multiple diurnal cycles while propagating westward they have an average 17 6 h duration and 673 km span most have zonal phase speeds of 8 16 m s 1 p

3d crosshole ert for aquifer characterization and monitoring of infiltrating river water the hydrogeological properties and responses of a productive aquifer in northeastern switzerland are investigated for this purpose 3d crosshole electrical resistivity tomography ert is used to define the main lithological structures within the aquifer through static inversion and to monitor the water infiltration from an adjacent river during precipitation events and subsequent river flooding the river water resistivity increases as a consequence the electrical characteristics of the infiltrating water can be used as a natural tracer to delineate preferential flow paths and flow velocities the focus is primarily on the experiment installation data collection strategy and the structural characterization of the site and a brief overview of the ert monitoring results the monitoring system comprises 18 boreholes each equipped with 10 electrodes straddling the entire thickness of the gravel aquifer a multichannel resistivity system programmed to cycle through various four point electrode configurations of the 180 electrodes in a rolling sequence allows for the measurement of approximately 15 500 apparent resistivity values every 7 h on a continuous basis the 3d static ert inversion of data acquired under stable hydrological conditions provides a base model for future time lapse inversion studies and the means to investigate the resolving capability of our acquisition scheme in particular it enables definition of the main lithological structures within the aquifer the final ert static model delineates a relatively high resistivity low porosity intermediate depth layer throughout the investigated aquifer volume that is consistent with results from well logging and seismic and radar tomography models the next step will be to define and implement an appropriate time lapse ert inversion scheme using the river water as a natural tracer the main challenge will be to separate the superposed time varying effects of water table height temperature and salinity variations associated with the infiltrating water

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

using hydrogeophysical methods to assess the feasibility of lake bank filtration abstract lake tisso is the fourth largest lake in denmark with sufficient continuous surface water input making lake bank filtration an optimal surface water extraction method provided that there are suitable shallow aquifers with a good hydraulic connection to the lake the shallow subsurface under the lake was mapped by five waterborne electrical resistivity tomography ert and eight ground penetrating radar gpr surveys to locate potentially suitable aquifers distributed temperature sensing dts was used for the general characterisation of the hydraulic connection between the lake and aquifer the waterborne ert surveys showed several zones of coarser material along the shore extending up to five meters depth below the lake surface while on land ert surveys confirmed that these zones extend to the lakeshore with similar thickness both waterborne ert and gpr profiles agreed that at a selected field site the highest possibility of coarse sediments is 60 140 m from the lakeshore lakebed temperatures measured in december 2016 may and june 2017 all indicated potential groundwater discharge to the lake at approximately 135 m from the shoreline supporting the results of geophysical surveys showing coarser sediments in that area a 2d numerical flow model of the area with a geological setup based on geophysical information and slugtest data also showed upward groundwater discharge peaks at 60 and 140 m from the shoreline confirming the findings of the field surveys thus this study shows that the combination of waterborne ert and gpr surveys with dts measurements is a fast and efficient way to assess the feasibility of lake bank filtration in these settings this approach of combined hydrogeophysical methods is robust and has a potential to site lake bank filtration areas not only next to large rivers where coarse sediments are found in abundance but even in different sedimentary environments where suitable layers for lake bank filtration are more difficult to identify

mesoscale interactions in tropical cyclone genesis with the multitude of cloud clusters over tropical oceans it has been perplexing that so few develop into tropical cyclones the authors postulate that a major obstacle has been the complexity of scale interactions particularly those on the mesoscale which have only recently been observable while there are well known climatological requirements these are by no means sufficient a major reason for this rarity is the essentially stochastic nature of the mesoscale interactions that precede and contribute to cyclone development observations exist for only a few forming cases in these the moist convection in the preformation environment is organized into mesoscale convective systems each of which have associated mesoscale potential vortices in the midlevels interactions between these systems may lead to merger growth to the surface and development of both the nascent eye and inner rainbands of a tropical cyclone the process is essentially stochastic but the degree of stochasticity can be reduced by the continued interaction of the mesoscale systems or by environmental influences for example a monsoon trough provides a region of reduced deformation radius which substantially improves the efficiency of mesoscale vortex interactions and the amplitude of the merged vortices further a strong monsoon trough provides a vertical wind shear that enables long lived midlevel mesoscale vortices that are able to maintain or even redevelop the associated convective system the authors develop this hypothesis by use of a detailed case study of the formation of tropical cyclone oliver observed during toga coare 1993 in this case two dominant mesoscale vortices interacted with a monsoon trough to separately produce a nascent eye and a major rainband the eye developed on the edge of the major convective system and the associated atmospheric warming was provided almost entirely by moist processes in the upper atmosphere and by a combination of latent heating and adiabatic subsidence in the lower and middle atmosphere the importance of mesoscale interactions is illustrated further by brief reference to the development of two typhoons in the western north pacific

the vertical structure of mesoscale convective vortices abstract simulations of two cases of developing mesoscale convective vortices mcvs are examined to determine the dynamics governing the origin and vertical structure of these features although one case evolves in strong vertical wind shear and the other evolves in modest shear the evolutions are remarkably similar in addition to the well known mesoscale convergence that spins up vorticity in the midtroposphere the generation of vorticity in the lower troposphere occurs along the convergent outflow boundary of the parent mesoscale convective system mcs lateral transport of this vorticity from the convective region back beneath the midtropospheric vorticity center is important for obtaining a deep column of cyclonic vorticity however this behavior would be only transient without a secondary phase of vorticity growth in the lower troposphere that results from a radical change in the divergence profile favoring lower tropospheric convergence following the decay of the nocturnal mcs subsequent con

case study of a developing african easterly wave during namma an energetic point of view abstract the west african perturbation that subsequently evolved into hurricane helene 2006 during nasa s african monsoon multidisciplinary analysis namma 15 august 14 september 2006 and amma s third special observing period sop 3 15 29 september 2006 has been simulated with the nonhydrostatic meso nh model using parameterized convection the simulated disturbance evolved over west africa and the adjacent eastern tropical atlantic through interactions between different processes at the convective scale mesoscale and synoptic scale the aim of this paper is to quantify the energetics of the simulated disturbance a set of energy equations is first developed in the hydrostatic case to solve the limitations of lorenz s analysis when applied to a finite domain it is shown that this approach is also valid in the compressible and in the anelastic case in order to apply it to the meso nh results application to the simulated pre helene disturbance allows one to determine the most important terms in

geoelectrical evidence of bicontinuum transport in groundwater bicontinuum models and rate limited mass transfer rlmt explain complex transport behavior e g long tailing and rebound in heterogeneous geologic media but experimental verification is problematic because geochemical samples represent the mobile component of the pore space here we present geophysical evidence of rlmt at the field scale during an aquifer storage and recovery experiment in a fractured limestone aquifer in charleston south carolina we observe a hysteretic relation between measurements of pore fluid conductivity and bulk electrical conductivity this hysteresis contradicts advective dispersive transport and the standard petrophysical model relating porefluid and bulk conductivity but can be explained by considering bicontinuum transport models that include first order rlmt using a simple numerical model we demonstrate that geoelectrical measurements are sensitive to bicontinuum transport and rlmt parameters which are otherwise difficult to infer from direct hydrologic measurements

the mean dynamical and thermodynamic fields for july 1989 over tropical north africa and their relationship to convective storm activity abstract dynamic and thermodynamic fields from ecmwf analyses have been presented for the tropical north african region for july 1989 except for the region equatorward of 10 n it has been shown that the pattern of mean low level e has a striking correspondence with the propagating storm statistics obtained from meteosat in that region with mean high e corresponding to more propagating storm activity and vice versa considerable correspondence is also found with the cold cloud duration pattern and observed rainfall the low level e pattern poleward of 13 n has been shown to be strongly influenced by the low level meridional wind v the v 0 contour at low levels for example has been shown to have a marked northwest to southeast tilt between about the zero meridian and 25 e superimposed on this is a marked sinusoidal variation in the latitudinal position of the contour associated with the orography in this region the low level e pattern is consistent with these variations with higher values of

the influence of african easterly waves on convection over tropical africa and the east atlantic abstractusing data from the tropical rainfall measuring mission trmm the modulation of convection by african easterly waves aews is investigated over regions of the east atlantic and tropical africa to explain the modulation of convection the large scale environment lift moisture conditional instability and shear is also examined as a function of aew phase in each region over semiarid portions of tropical africa unconditional rain rates are greatest in the northerly phase of aews due to the strong adiabatic forcing for ascent along the guinea coast the western coast of africa and over the east atlantic where forcing for ascent is weaker rainfall is shifted toward the trough where the air is moist significant contrasts in the characteristics of convection as a function of aew phase comparable in magnitude to regional contrasts are also observed in all regions large and high echo top convective systems are more sensitive to aew phase than small and low echo top systems in semiarid region

structures and hydrologic function of soil landscapes with kettle holes using an integrated hydropedological approach the hummocky post glacial soil landscapes with kettle holes as internal drainage systems are characterized by ponds that trap lateral fluxes in topographic depressions a quantitative description is mostly limited by the unknown complexity of hydraulically relevant soil and sediment structures this paper is focussing on a structure based approach to identify relevant field scale flow and transport processes illustrative examples demonstrate extreme variations in water table fluctuation for adjoining kettle holes explanations require a pedohydrologic concept of the arable soil landscape identification of structures is based on geophysical methods and soil hydraulic measurements electrical resistivity imaging yields 0 5 m scale spatial structures that correspond with soil texture distributions electromagnetic induction provides larger scale field maps that reflect major soil and sediment features results of both methods correspond within the limits of the different spatial resolutions with geophysical exploration methods colluvial areas with textural differences between upper and deeper soil layers coarse textured sediment lenses and stony colluvial regions around kettle holes are identified as potentially relevant flow structures the colluvial fringe around the pond seems to be a sensitive area with important lateral exchange fluxes tensiometer measurements perpendicular to this boundary indicate hydraulic gradients directed from the pond towards the partially saturated soil the localized infiltration of trapped water in kettle holes can control large fractions of ground water recharge and may have implications for the fate of agricultural chemicals in post glacial landscapes while surface and subsurface hydraulic structures may be inferred using minimal invasive techniques better understanding of processes and properties governing lateral exchange fluxes between pond and surrounding soil are required

hydrodynamics of floodplain wetlands in a chalk catchment the river lambourn uk variations in floodplain channel water levels and valley floor groundwater levels measured in piezometers and boreholes are examined at selected points along the course of the river lambourn a chalk river in southern england a local alluvial gravel aquifer in the valley bottom is associated with numerous small wetlands that extend over much of the river s perennial profile variations in hydraulic gradient between local borehole levels and or floodplain channel water levels are described for three sites in the seasonal section of the channel at bockhampton east garston and west shefford the results indicate that observed groundwater levels are closely associated with flows from discrete springs at the margins of the channel and floodplain however as the floodplain widens and the alluvial gravel aquifer increases in size the gravel aquifer accounts for a substantial down valley component of groundwater flow with a diffuse vertical water flux in the lower catchment the exchange of flows between the gravel aquifer and the river enables some attenuation of floodplain water table variability providing a stable hydrological regime for valley bottom wetlands catchment controls upon the local valley bottom wetland regime are demonstrated with the application of a simple groundwater model developed using modflow the model is used to simulate groundwater discharge to the river in the upper and lower catchment in addition to the water level regime at selected points in the valley bottom in the lower catchment the results demonstrate the importance of taking catchment scale water flow into account when managing isolated wetlands in a permeable catchment

discharge of nitrate containing groundwater into a coastal marine environment the discharge of nitrate containing fresh groundwater from a sandy coastal aquifer and into the adjacent shallow marine environment was investigated near esbjerg at the northern end of the wadden sea in denmark the geological structure of the coastal site was determined from drilling data and geoelectrical methods such as multi electrode profiling mep the distribution of shoreface sediment containing freshwater was mapped using mep underwater multi electrode profiling umep and the electrical conductivity measured on pore waters freshwater discharge fluxes were measured using seepage meters and estimated from observed head gradients and measured hydraulic conductivities in the aquifer sediments the nitrate distribution in the coastal sediments was obtained from water samples a groundwater flow model was established to quantify the groundwater flow and travel times and the distribution of freshwater discharge along the coastline results show that on the watershed scale the fresh groundwater discharge is controlled by sand filled buried channels that connect the aquifer with the shoreface sediments the freshwater discharge along the coastline is at this scale probably best estimated by the groundwater flow model however at a more detailed scale the distribution of freshwater discharge in the intertidal zone is controlled by small scale geological heterogeneity and models are unable to predict what happens at a small scale on the beach and offshore for that purpose umep seepage meters pore water sampling and local hydraulic gradients are more useful these measurements indicate that the freshwater discharge occurs in distinct zones and that the highest discharge is near the high tide line decreasing rapidly in a seaward direction nitrate is abundant in the shallow groundwater of the coastal aquifer and is also present in the discharging fresh groundwater at certain patches along the coast however on average very little nitrate is observed in the freshwater discharging at the coast the maximum travel time of groundwater through the aquifer until it discharges at the coast is 100 years and since nitrate leaching from soils has only taken place during the last 40 50 years part of the fresh groundwater discharging at the coast must be free of nitrate

atlantic tropical cyclogenesis a three way interaction between an african easterly wave diurnally varying convection and a convectively coupled atmospheric kelvin wave abstractthis paper explores a three way interaction between an african easterly wave aew the diurnal cycle of convection over the guinea highlands ghs and a convectively coupled atmospheric equatorial kelvin wave cckw these interactions resulted in the genesis of tropical storm debby over the eastern tropical atlantic during late august 2006 the diurnal cycle of convection downstream of the ghs during the month of august is explored convection associated with the coherent diurnal cycle is observed off the coast of west africa during the morning later convection initiates over and downstream of the ghs during the afternoon these convective features were pronounced during the passage of the pre debby aew the superposition between the convectively active phase of a strong cckw and the pre debby aew occurred shortly after merging with the diurnally varying convection downstream of the ghs the cckw aew interaction preceded tropical cyclogenesis by 18 h the cckw provided a favorable environment

seasonal variation of moisture content in unsaturated sandstone inferred from borehole radar and resistivity profiles understanding the processes controlling recharge to aquifers is critical if accurate predictions are to be made on the fate of contaminants in the subsurface environment in order to understand fully the hydrochemical mechanisms in the vadose zone it is essential that the dynamics of the hydrology can be suitably characterised the correlation between moisture content and both bulk dielectric and resistivity properties of porous media is well established using suitably placed sensors in boreholes detailed depth profiles of dielectric and resistivity behaviour have been monitored over a period of two years at a triassic sherwood sandstone aquifer field site at hatfield england the borehole borehole transmission radar and borehole resistivity profiles show a significant correlation through appropriate petrophysical relationships derived from core samples seasonal dynamics of the vadose zone are seen to illustrate the migration of wetting and drying fronts over the monitoring period at a second field site in eggborough located 17 km from hatfield similar temporal changes in moisture content in the sandstone were observed using borehole radar profiles travel times of seasonal wetting fronts through the sandstone at both sites appear to be approximately 2 m per month the retardation of this front propagation in the top 3 m is also common to both sites suggesting that pollutant transport may be principally controlled by near surface sediments the results have important consequences to existing groundwater modelling programmes that are being utilised to predict transfer of agricultural chemicals through the vadose zone

macropores as preferential flow paths in meander bends macropores are subsurface connected void spaces caused by processes such as fracture of soils micro erosion and fauna burrows they are common near streams e g hyporheic and riparian zones and may act as preferential flow paths between surface and groundwaters affecting hydrologic and biogeochemical processes we tested the hydrologic function of macropores by constructing an artificial macropore within the saturated zone of a meander bend open macropore om and later filling its upstream end partially filled macropore pfm for each treatment we injected saline tracer at an upgradient monitoring well within the meander and monitored downgradient hydraulics and tracer transport pressure transducers in monitoring wells indicated hydraulic gradients within the meander were 32 higher perpendicular to and 6 higher parallel to the macropore for the om than for the pfm additionally hydraulic conductivities measured via falling head tests were 29 to 550 times higher along the macropore than in nearby sediment we used electrical conductivity probes in wells and electrical resistivity imaging to track solute transport transport velocities through the meander were on average 9 and 21 higher per temporal moment analysis and observed tracer peak respectively for the om than for the pfm furthermore temporal moments of tracer breakthrough analysis indicated downgradient longitudinal dispersion and breakthrough tracer curve tailing were on average 234 and 182 higher for the om respectively this suggests the om enabled solute transport at overall shorter timescales than the matrix but also increased tailing our results demonstrate the importance of macropores to meander bend hydrology and solute transport copyright 2012 john wiley sons ltd

internal processes within the african easterly wave system the internal processes within an african easterly wave aew system involving mass dynamic and water vapour fields are investigated using era i reanalysis in order to highlight the interactions between convection and aews the budgets of heat moisture and momentum are analysed during the different phases of aews detected using synoptic scale precipitable water anomalies as proposed by poan et al 2013 the strong climatological meridional gradient of moisture present in the sahel impacts the shape of the apparent heat source and humidity sink aew events over the sahel are associated with a meridional shift of the intertropical convergence zone itcz large exchanges of momentum by small scale convective transport are also highlighted between the low and mid levels contributing to the reinforcement of the aew circulation at 600 hpa and the damping of the monsoon flow this also appears as a possible mechanism for the vertical tilt of the meridional wind associated with aews heat budget computation in the southern flank of the west african heat low hl region where such aews occur reveals that the heating anomalies are mainly driven by the horizontal advections the vertical circulation acts as a precursor which initiates the heat transport in the lower troposphere however weaker turbulent mixing also participates in the development of these anomalies especially in the boundary layer these budgets ultimately allow the distinct contributions of diabatic and adiabatic processes to be determined

crosshole ip imaging for engineering and environmental applications induced polarization ip imaging is a promising tool in engineering and environmental studies application of this technique for near surface investigations has previously been limited by incomplete understanding of the physicochemical controls on the ip response together with a lack of appropriate methods for data inversion as laboratory studies have shown description of ip in terms of complex electrical conductivity enables access to various structural characteristics pertinent to practical issues such as subsurface lithology definition hydraulic permeability estimation or hydrocarbon contaminant mapping in particular analysis in terms of real and imaginary conductivity components offers improved lithological characterization since surface polarization effects are separated from electrolytic and surface conduction effects an occam type ip inversion algorithm based on complex algebra is described which accounts for these advances in ip interpretation by directly solving for complex conductivity results from crosshole applications at two case study sites demonstrate the suitability of the ip imaging approach for subsurface characterization in the first case study the imaging results correlate with the observed complex sequence of quaternary sediments at a waste disposal site characterization of the polarizability of these sediments offers significant value in lithological differentiation in the second case study the results of ip imaging at a hydrocarbon contaminated site illustrate the potential of the method in environmental studies the hydrocarbon location is clearly evident from the ip image and a markedly different response is observed at an uncontaminated region of the site by adopting empirical structural electrical relationships images of textural and hydraulic properties are estimated as a step toward improved quantitative characterization the success of the method for these contrasting applications supports further investigation into understanding the physical and chemical processes that control observed ip

origin and propagation of a disturbance associated with an african easterly wave as a precursor of hurricane alberto 2000 abstract in this study it is proposed that mesoscale convective complexes mccs and a mesovortex mv were embedded within a wavelike disturbance over north africa that led to the genesis of hurricane alberto 2000 the wavelike disturbance observed may be classified as an african easterly wave aew based on the cloud top area and brightness values observed from infrared satellite data four genesis and three lysis stages are identified within a cycle of moist convection associated with the pre alberto disturbance the availability of water vapor is the most essential factor controlling the convective cycle of the pre alberto disturbance over the african continent the presence of significant topography also contributes to the generation or decay of the associated mccs through regulation of the water vapor supply further analysis of meteosat satellite imagery reveals that the incipient disturbances for 23 of 34 eastern atlantic tropical cyclones originated from the ethiopian highlands eh region du

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

resistivity and percolation study of preferential flow in vadose zone at bokhorst germany traditional nondestructive resistivity techniques have been applied in combination with tracer displacement and conventional soil moisture recording methods i e buried tensiometer and time domain reflectometry tdr for studying flow processes at an arable site in bokhorst germany three water infiltration experiments were carried out using tap water spiked with a nonreactive tracer at different concentrations the study aimed at exploring the capabilities of these combined techniques for tracing the preferential movement of water in the uppermost 1 5 m of the highly heterogeneous vadose zone the results illustrate that the applied tensiometer and tdr techniques can detect the relatively fast flow process at their position points and thus only tentatively trace the preferential flow the additional application of the resistivity method can trace the preferential flow paths continuously along the plane of measurements the results of the individual applied methods complement and confirm each other

genesis of hurricane julia 2010 within an african easterly wave low level vortices and upper level warming abstractwhile a robust theoretical framework for tropical cyclogenesis tcg within african easterly waves aews has recently been developed little work explores the development of low level meso scale vortices llvs and a meso scale surface low in relation to deep convection and upper tropospheric warming in this study the development of an llv into hurricane julia 2010 is shown through a high resolution model simulation with the finest grid size of 1 km the results presented expand upon the connections between llvs and the aew presented in previous studies while demonstrating the importance of upper tropospheric warming for tcg it is found that the significant intensification phase of hurricane julia is triggered by the pronounced upper tropospheric warming associated with organized deep convection the warming is able to intensify and expand during tcg owing to formation of a storm scale outflow beyond the rossby radius of deformation results confirm previous ideas by demonstrating that th

potential vorticity generation by west african squall lines abstractthe west african summer monsoon features multiple complex interactions between african easterly waves aews moist convection variable land surface properties dust aerosols and the diu

electrical resistivity imaging of hydrologic flow through surface coal mine valley fills with comparison to other landforms surface coal mining has altered land cover near surface geologic structure and hydrologic processes of large areas in central appalachia usa these alterations are associated with changes in water quality such as elevated total dissolved solids which is usually measured via its surrogate specific conductance sc the sc of valley fill effluent streams is a function of fill construction methods materials and age yet hydrologic studies that relate these variables to water quality are sparse due to the difficulty of conducting traditional hydrologic studies in mined landscapes we used electrical resistivity imaging eri to visualize the subsurface geologic structure and hydrologic flow paths within a valley fill eri is a noninvasive geophysical technique that maps spatiotemporal changes in resistivity of the subsurface we paired eri with artificial rainfall experiments to track infiltrated water as it moved through the valley fill results indicate that eri can be used to identify subsurface geologic structure and track advancing wetting fronts or preferential flow paths our results suggest that the upper portion of the fill contains significant fines whereas the deeper profile is primarily large rocks and void spaces water tended to pond on the surface of compacted areas until it reached preferential flow paths where it appeared to infiltrate quickly down to 15 m depth in 75 min eri applications can improve understanding of how fill construction techniques influence subsurface water movement and in turn may aid in the development of valley fill construction methods to reduce water quality effects

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

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

prediction formula for the spectral wave period t m 1 0 on mildly sloping shallow foreshores during the last decades the spectral wave period tm 1 0 has become accepted as a characteristic wave period when describing the hydraulic attack on coastal structures especially over shallow foreshores in this study we derive an empirical prediction formula for tm 1 0 on shallow to extremely shallow foreshores with a mild slope the formula was determined based on flume tests and numerical calculations mainly for straight linear foreshore slopes it is shown that the wave period increases drastically when the water depth decreases up to eight times the offshore value the bed slope angle influences the wave period slightly for short crested wave fields the strong increase of tm 1 0 starts closer to shore at smaller water depths than for long crested wave fields

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

energy dispersion in african easterly waves the existence of an upstream eastward group velocity for african easterly waves aews is shown based on single point lag regressions using gridded reanalysis data from 1990 to 2010 the eastward energy dispersion is consistent with the direction of ageostrophic geopotential flux vectors a local eddy kinetic energy eke budget reveals that early in the life cycle of aews growth rate due to geopotential flux convergence exceeds baroclinic and barotropic growth rates later in the life cycle eke decay due to geopotential flux divergence cancels or exceeds baroclinic and barotropic growth a potential vorticity pv budget is used to diagnose tendencies related to group propagation although both upstream and downstream group speeds are possible because of the reversal in the mean meridional pv gradient upstream propagation associated with the positive poleward pv gradient dominates wave packet evolution analogous to the concept of downstream development of midlatitude baroclinic waves new aews develop preferentially upstream of the older ones thus providing a mechanism for seeding new waves it is suggested that these results are also relevant to aew intermittency and storm track structure

analysis of african easterly wave structures and their role in influencing tropical cyclogenesis abstract composite structures of african easterly waves aews that develop into named tropical cyclones in the atlantic are compared and contrasted with nondeveloping aews using the 40 yr ecmwf re analysis era 40 data and satellite brightness temperature between 1979 and 2001 developing aews are characterized by a more distinctive cold core structure two days before reaching the west african coast as they move westward the convective activity increases further in the vicinity of the guinea highlands region at the same time the aew trough increases its vorticity at low levels consistent with a transformation toward a more warm core structure before it reaches the ocean as the aew moves over the ocean convection is maintained in the trough consistent with the observed tropical cyclogenesis the nondeveloping aew has a similar evolution before reaching the coast except that the amplitudes are weaker and there is less convective activity in the guinea highlands region the nondeveloping aew composite

absolute and convective instability of the african easterly jet abstractthe stability of the african easterly jet aej is examined using idealized numerical simulations it is found that a zonally homogeneous representation of the aej can support absolute instability in the form of african easterly waves aews this finding is verified through a local energy budget which demonstrates the presence of both upstream and downstream energy fluxes these energy fluxes allow unstable wave packets to spread upstream and downstream relative to their initial point of excitation this finding is further verified by showing that the ground relative group velocity of these wave packets has both eastward and westward components in contrast with normal mode instability theory which emphasizes wave growth through energy extraction from the basic state the life cycle of the simulated aews is strongly governed by energy fluxes convergent fluxes at the beginning of the aew storm track generate new aews whereas divergent fluxes at the end of the storm track lead to their decay i

monitoring groundwater surface water interaction using time series and time frequency analysis of transient three dimensional electrical resistivity changes 1 time lapse resistivity imaging is increasingly used to monitor hydrologic processes compared to conventional hydrologic measurements surface time lapse resistivity provides superior spatial coverage in two or three dimensions potentially high resolution information in time and information in the absence of wells however interpretation of time lapse electrical tomograms is complicated by the ever increasing size and complexity of long term three dimensional 3 d time series conductivity data sets here we use 3 d surface time lapse electrical imaging to monitor subsurface electrical conductivity variations associated with stage driven groundwater surface water interactions along a stretch of the columbia river adjacent to the hanford 300 near richland washington usa we reduce the resulting 3 d conductivity time series using both time series and time frequency analyses to isolate a paleochannel causing enhanced groundwater surface water interactions correlation analysis on the time lapse imaging results concisely represents enhanced groundwater surface water interactions within the paleochannel and provides information concerning groundwater flow velocities time frequency analysis using the stockwell s transform provides additional information by identifying the stage periodicities driving groundwater surface water interactions due to upstream dam operations and identifying segments in time frequency space when these interactions are most active these results provide new insight into the distribution and timing of river water intrusion into the hanford 300 area which has a governing influence on the behavior of a uranium plume left over from historical nuclear fuel processing operations

the interaction between moist diabatic processes and the atmospheric circulation in african easterly wave propagation an objective tracking algorithm is used to characterise the three dimensional structure of african easterly waves aews in era interim reanalysis and a met office unified model um simulation a special focus is dedicated to the coupling of dynamical aspects of the wave and moist convection the relation between baroclinic features of the wave and latent heating is explored latent heating at and slightly ahead of the wave trough is found to reinforce and sustain the anomalous wave circulation through potential vorticity pv generation and vortex stretching the coupling of moist processes and the circulation takes place mainly through moisture convergence at lower mid tropospheric levels between 850 hpa and 500 hpa these findings are confirmed and examined in more detail in a case study of a strong aew based on high resolution um simulations pv tracers are used to investigate how different moist diabatic processes invigorate the wave again moisture anomalies are found to be the main contributors to generating small scale convergence centres and updrafts ahead of the trough at mid tropospheric levels although buoyancy effects are ultimately responsible for the convective uplift the results suggest that mesoscale circulations associated with the aew dynamics are crucial in creating the small scale moist static instabilities and vortices which are essential for the aew maintenance boundary layer mixing and advection from the northern sahel may create pockets of high pv air around the trough in some instances but this mechanism of wave sustainment needs further investigation

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

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

two contrasting african easterly wave behaviors abstractthe dominant structural variability of african easterly waves aews is explored using an empirical orthogonal function eof approach the structure of aews is obtained by projecting the w

sinkhole risk assessment by ert the case study of sirino lake basilicata italy abstract the presence of natural or artificial lakes and reservoirs that can drain because of natural phenomena can generate catastrophic events affecting urban and agricultural areas next to the source area therefore geophysical prospecting techniques have been applied in the study of sirino lake which during the last century was affected by the sudden opening of small sinkholes resulting in the almost total draining of the lake and in the sudden increase of water flow rates of distal springs two electrical resistivity tomographies erts were carried out across the lake using electrode arrays located on land and across the water body self potential sp data were acquired around the lake shore and the surrounding area the geophysical prospecting contributed significant data toward explaining the unique hydrogeological characteristics of the lake integration of geophysical geological hydrogeological and geomorphological data allowed us to estimate the thickness of the lacustrine deposits beneath the lake to describe the main patterns of the subsurface fluid flows in the area and to identify possible water escape routes causing the piping phenomena

mesoscale convective systems and nocturnal rainfall over the west african sahel role of the inter tropical front a convection permitting regional model simulation for august 2006 and observations are evaluated to better understand the diurnal cycle of precipitation over the sahel in particular reasons for a nocturnal rainfall maximum over parts of the sahel during the height of the west african monsoon are investigated a relationship between mesoscale convective system mcs activity and inter tropical front itf dryline dynamics is revealed over 90 of the sahel nocturnal rainfall derives from propagating mcss that have been associated with topography in earlier studies in contrast in this case study 70 90 of the nocturnal rainfall over the southern sahel 11 n 14 n west of 15 e is associated with mcss that originate less than 1000 km upstream to the north and east in the afternoon in a region largely devoid of significant orography this mcs development occurs in association with the sahel itf combined with atmospheric pre conditioning daytime surface heating generates turbulent mixing that promotes planetary boundary layer pbl growth accompanied by a low level reversal in the meridional flow this enhances wind convergence in the low level moist layer within 2 3 of latitude of the equatorward side of the itf mcss tend to form when this vertical mixing extends to the level of free convection and is accompanied by a mid tropospheric african easterly wave disturbance to the east this synoptic disturbance enhances the vertical wind shear and atmospheric instability over the genesis location these results are found to be robust across the region

electrical resistivity imaging of the architecture of substream sediments 1 the modeling of fluvial systems is constrained by a lack of spatial information about the continuity and structure of streambed sediments there are few methods for noninvasive characterization of streambeds invasive methods using wells and cores fail to provide detailed spatial information on the prevailing architecture and its continuity geophysical techniques play a pivotal role in providing spatial information on subsurface properties and processes across many other environments and we have applied the use of one of those techniques to streambeds we demonstrate through two examples how electrical resistivity imaging can be utilized for characterization of subchannel architecture in the first example electrodes installed in riparian boreholes and on the streambed are used for imaging under the river bed the thickness and continuity of a highly permeable alluvial gravel layer overlying chalk in the second example electrical resistivity images determined from data collected using electrodes installed on the river bed provide a constrained estimate of the sediment volume behind a log jam vital to modeling biogeochemical exchange which had eluded measurement using conventional drilling methods owing to the boulder content of the stream the two examples show that noninvasive electrical resistivity imaging is possible in complex stream environments and provides valuable information about the subsurface architecture beneath the stream channels

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

coherent synoptic disturbances in the australian monsoon abstractcoherent synoptic scale weather systems within the australian monsoon are identified and tracked in the isentropic potential vorticity pv field from the ecmwf interim reanalysis era interim dataset during the southern hemisphere summer the resulting dataset is then used to compile statistics and synoptic composites of australian monsoon disturbances on average a synoptic system is found in the region every 2 5 days however the time interval between consecutive events is highly variable meaning that the synoptic activity in the australian monsoon is not well represented by commonly employed spectral techniques the analysis reveals that most synoptic systems originate within the australian monsoon but at the 315 k level approximately 700 hpa a significant proportion of systems are first detected near the east coast of the continent where extratropical rossby waves are observed to frequently break the average australian monsoon weather system propagates from east to west at approximatel

intermittent african easterly wave activity in a dry atmospheric model influence of the extratropics abstracta dynamical model is constructed of the northern summertime global circulation maintained by empirically derived forcing based on the same dynamical code that has recently been used to study african easterly waves aews as convectively triggered perturbations thorncroft et al leroux and hall in the configuration used here the model faithfully simulates the observed mean distributions of jets and transient disturbances and explicitly represents the interactions between them this simple gcm is used to investigate the origin and intraseasonal intermittency of aews in an artificially dry no convection context a long integration of the model produces a summertime climatology that includes a realistic african easterly jet and westward propagating 3 5 day disturbances over west africa these simulated waves display intraseasonal intermittency as the observed aews also do further experiments designed to discern the source of this intermittency in the model show that the simulated waves are

observing lingering hyporheic storage using electrical resistivity variations around stream restoration structures crabby creek pa time lapse geophysical surveys can map lingering hyporheic storage by detecting changes in response to saline tracer tracer tests were conducted in crabby creek an urban stream outside philadelphia to examine the influence of stream restoration structures and variable sediment thickness we compared electrical resistivity surveys with extensive well sampling 57 wells in two 13 5 m long reaches each with a step drop created by a j hook the two step drops varied in tracer behaviour based on both the well data and the geophysical data the well data showed more variation in arrival time where the streambed sediment was thick and was more uniform where sediment was thin the resistivity in the reach with thin sediment showed lingering tracer in the hyporheic zone both upstream and downstream from the j hook in the second reach where the sediment was thicker the lingering tracer in the hyporheic zone was more extensive downstream from the j hook the contrasting results between the two reaches from both methods suggested that sediments influenced hyporheic exchange more than the step at this location resistivity inversion differed from well data in both reaches in that it showed evidence for tracer after well samples had returned to background mapping lingering tracer either upstream or downstream of a step we conclude that resistivity surveys may become an important tool for hyporheic zone characterization because they provide information on the extent of slow moving fluids in the hyporheic zone which have the potential to enhance chemical reactions copyright 2012 john wiley sons ltd

the influence of the mjo on upstream precursors to african easterly waves abstractthe madden julian oscillation mjo produces alternating periods of increased and reduced precipitation and african easterly wave aew activity in west africa this study documents the influence of the mjo on the west african monsoon system during boreal summer using reanalysis and brightness temperature fields mjo related west african convective anomalies are likely induced by equatorial kelvin and rossby waves generated in the indian ocean and west pacific by the mjo which is consistent with previous studies the initial modulation of tropical african convection occurs upstream of west africa near the entrance of the african easterly jet aej previous studies have hypothesized that an area to the east of lake chad is an initiation region for aews called the trigger region in this study this area exhibits significant intraseasonal convection and wave activity anomalies prior to the wet and dry mjo phases in the west african monsoon region in the trigger region cold tropospheric tempera

dynamics of river aquifer interactions along a chalk stream the river lambourn uk this paper describes the spatial and temporal pattern of groundwater flow accretion to the river lambourn a 234 km2 chalk catchment of the west berkshire downs uk which has been largely unaffected by groundwater abstraction variations in the discharge measured at four fixed gauges in the catchment coupled with information on the length of flowing channel over the period 1983 2001 are used to describe regional patterns in flow accretion mean catchment accretion generally exceeds 0 15 m3 s 1 km 1 but there are significant differences between perennial reaches indicating how the combination of local structural controls and seasonal changes in the drainage net affect flow accretion data from current meter surveys were used to determine the spatial variability in flow accretion 505 paired observations along 12 reaches between 1 and 2 95 km in length indicated a consistent spatial trend in accretion accretion was high in upstream and downstream channel reaches and in middle reaches where dry valleys intersected the main valley a flow accretion index was developed to describe the relationship of flow accretion in each of the 12 study reaches to catchment discharge the relationship varied from a strong positive correlation with catchment discharge two reaches a weak positive correlation three reaches a strong negative correlation two reaches to no relationship to catchment discharge four reaches the results highlight the need to reconsider the usual assumption of uniform or uniformly increasing flow accretion in chalk catchments moreover they emphasize the importance of catchment topography and illustrate how flow accretion in individual reaches may vary between high and low groundwater levels

two dimensional geomorphological characterization of a filled abandoned meander using geophysical methods and soil sampling abstract using geophysical methods for the geomorphological characterization of subsurface features has numerous advantages over traditional exploration methods because of their noninvasive and rapid nature in this study we compared the results of four geophysical methods with each other we also discuss their possibilities and limitations in a geomorphological investigation electrical resistivity tomography ert refraction seismic rs ground penetrating radar gpr and multichannel analysis of surface waves masw methods were applied at an abandoned meander in northern saxony to map a predefined structure by combining these methods we were able to characterize and delineate subsurface features of the abandoned meander including a point bar a channel and a cutbank core samples obtained from sonic drilling were used to validate the findings of both seismic methods however we found that electrical resistivity tomography and ground penetrating radar lacked penetration depth and could only be used to resolve shallower subsurface layers the ert gpr rs and masw can be used to gather images of subsurface structures the masw in particular provides supplementary information about the channel s internal structure with respect to lateral and vertical resolution besides fluvial morphological features we also detected inverse velocity structures within the channel this allowed us to characterize the abandoned meander using information about its layer distribution and material composition however we were only able to characterize and delineate the subsurface features of the abandoned meander by combining all of the aforementioned methods

a climatological study of transient mean flow interactions over west africa a statistical study is made of the relationship between intraseasonal variations in mid tropospheric flow over west africa and transient eddy activity during the june september monsoon season in order to investigate the interaction between the african easterly jet aej african easterly waves aews and convection ncep2 reanalyses are used together with outgoing long wave radiation olr data from june september 1979 2007 intraseasonal variability in the 600 hpa zonal wind is isolated using a 10 120 day filter the leading empirical orthogonal function eof describes north south displacements of the jet axis episodes of aew activity over the continent are evaluated in terms of the perturbation kinetic energy pke of the less than six day high pass filtered wind at 700 and 850 hpa pke and olr are also filtered for the intraseasonal signal lag covariance analyses reveal a two way interaction between the aej and synoptic scale transients prior to episodes of enhanced transient activity the aej is strengthened in the jet entrance region during and following these episodes the aej is strengthened on the northern flank of the jet exit region e vector analysis shows that this displacement of the aej is consistent with forcing by transients that have an aew like structure analysis of olr shows that periods of enhanced transient activity over west africa are associated with enhanced convection over the region at intraseasonal time scales and are correlated with prior episodes of enhanced convection to the east in the convective trigger region identified by thorncroft et al copyright 2009 royal meteorological society

derivation of lowland riparian wetland deposit architecture using geophysical image analysis and interface detection for groundwater surface water interactions to be understood in complex wetland settings the architecture of the underlying deposits requires investigation at a spatial resolution sufficient to characterize significant hydraulic pathways discrete intrusive sampling using conventional approaches provides insufficient sample density and can be difficult to deploy on soft ground here a noninvasive geophysical imaging approach combining three dimensional electrical resistivity tomography ert and the novel application of gradient and isosurface based edge detectors is considered as a means of illuminating wetland deposit architecture the performance of three edge detectors were compared and evaluated against ground truth data using a lowland riparian wetland demonstration site isosurface based methods correlated well with intrusive data and were useful for defining the geometries of key geological interfaces i e peat gravels and gravels chalk the use of gradient detectors approach was unsuccessful indicating that the assumption that the steepest resistivity gradient coincides with the associated geological interface can be incorrect these findings are relevant to the application of this approach in settings with a broadly layered geology with strata of contrasting resistivities in addition ert revealed substantial structures in the gravels related to the depositional environment i e braided fluvial system and a complex distribution of low permeability putty chalk at the bedrock surface with implications for preferential flow and variable exchange between river and groundwater systems these results demonstrate that a combined approach using ert and edge detectors can provide valuable information to support targeted monitoring and inform hydrological modeling of wetlands

revealing the temporal dynamics of subsurface temperature in a wetland using time lapse geophysics summary there is growing recognition of the need to understand the subsurface hydrological mechanisms within wetlands given their potentially significant role in the hydrological and biogeochemical function of catchments as well as their effects on the wetland habitat itself conventional subsurface hydrological sampling and monitoring techniques are often limited in such environments because of the invasive nature of such approaches and the sensitivity of the environment in this study we use ground penetrating radar gpr and electrical resistivity tomography ert to characterise the stratigraphy at a small riparian wetland site then through time lapse ert measurements over a 12 month period we demonstrate how changes in resistivity may provide additional value about localised recharge we compare direct measurements of water table depth and pore fluid electrical conductivity and temperature with subsurface resistivity derived from the ert measurements and reveal that temporal fluctuations in temperature of the subsurface dominates the change in resistivity over the monitored period from this we use the ert images of resistivity to estimate spatio temporal changes in subsurface temperature and thus infer localised zones of groundwater recharge due to suppressed seasonal variation in temperature although we focus here on one 2 d vertical profile through the wetland the results highlight the potential value of non invasive time lapse geoelectrical surveys for mapping 3 d thermal patterns within a wetland environment

radar observations of convective system variability in relationship to african easterly waves during the 2006 amma special observing period abstract a radar based analysis of the structure motion and rainfall variability of westward propagating squall line mesoscale convective systems slmcss in niamey niger during the african monsoon multidisciplinary activities amma 2006 special observing period is combined with an analysis of 700 mb hpa winds and relative vorticity to study the relationship between slmcss and african easterly waves aews radar results show that slmcss were the most important rainmakers in niamey and accounted for about 90 of the rainfall despite being present less than 17 of the time analysis of the 700 mb synoptic scale flow revealed that during the 2006 west african monsoon season the african easterly jet vacillated between about 10 and 15 n on time scales of 1 2 weeks aews followed the jet as it vacillated north and south thereby producing two preferred paths for aews propagating past niamey s longitude a northern track along 8 16 n and a southern track along 2 6 n it was found that niamey slmcss oc

numerical simulation of episodes of organized convection in tropical northern africa abstracta large domain convection permitting numerical model is used to simulate episodes of deep convection which are generated during the day over the ethiopian highlands and then propagate westward over the eastern and central sahel region 5 20 n of northern africa the simulation comprises 12 5 days within the african monsoon multidisciplinary analysis amma field campaign in 2006 during this period long lived precipitation episodes that survived beyond a single diurnal cycle occurred in the lee of the ethiopian highlands only every 2 3 days in both the simulation and observations this contrasts with some other latitudinal corridors in the lee of major topography such as the central united states where long lived heavy precipitation episodes frequently occur on successive nights the intermittency of long lived events for the current case occurs despite regular daily triggering of convection along the upstream orography and is linked to strong lower tropospheric stabilization and reduction o

observed structure of mesoscale convective systems and implications for large scale heating mesoscale convective cloud systems that produce large amounts of rain in mid latitudes and most of the rain in the tropics consist of a combination of convective and stratiform cloud the convective regions contain numerous deep cells that are often but not always arranged in lines the stratiform region is an outgrowth of the convective ensemble it lies adjacent to the convective region and is seeded by ice particles detrained from convective towers sometimes it lies to the rear of a propagating convective line while on other occasions it surrounds the convection the heating of the large scale environment by a mesoscale convective system is affected by both the convective and stratiform regions although processes such as melting and radiation are important the net heating by a system is dominated by condensation and evaporation associated with vertical air motions this paper reviews recent observational evidence regarding the mean vertical motion profiles in the convective and stratiform regions of mesoscale convective systems and the implications of these mean motions for the vertical distribution of heating of the large scale environment in both the convective and the stratiform regions vertical motions have been determined by various techniques including composites of rawinsonde and aircraft wind data single and dual doppler precipitation radar analyses and wind profiler observations in stratiform regions these data consistently show mean vertical velocity that is upward in the upper troposphere and downward in the lower troposphere the level separating upward from downward motion is located from 0 to 2 km above the 0 c level depending on location within the stratiform region diagnostic calculations show that these vertical motion profiles imply heating of the upper troposphere and cooling of lower levels by stratiform region processes data on vertical motions in the convective regions are less consistent from case to case these data sometimes indicate that the mean vertical velocity in convective regions is maximum in the lower troposphere in other cases the data indicate a maximum in the high troposphere diagnostic calculations show that heating profiles diagnosed from these two types of profile are quite different the first having a maximum of heating in the lower troposphere while the second has a maximum in the middle troposphere although it is difficult to determine whether or not the differences in estimates arise from different types of observations analysis methods or sampling strategies it seems likely that they stem from differences in the large scale environment of the different mesoscale systems the ubiquitous occurrence of stratiform regions in mesoscale convective systems and hurricanes together with their consistent heating profiles which systematically concentrate heating in upper levels while cooling lower levels are a major consideration in evaluating the interaction of mesoscale systems with the large scale environment however the consistency of the stratiform profiles from case to case indicates that the variability of net convective plus stratiform heating profiles from case to case lies primarily in the variation of the convective region profiles from one case to another it is suggested that future work should focus on the convective region vertical profiles of vertical motion and heating and on the large scale environmental factors that may control the variation of these profiles from case to case

bedrock detection using 2d electrical resistivity imaging along the peikang river central taiwan article i nfo we use the two dimensional 2d electrical resistivity imaging eri method to detect the contact between sediment and bedrock in a modern fluvial system we performed eight 2d electrical resistivity surveys along the peikang river in kuohsing area central taiwan with a pole pole array of 1 m electrode spacing to produce a spatial resolution of about 1 m the resistivity data were inverted into subsurface electrical structures using the least squares inversion techniques then the contact between the bedrock and sediment was found by using an image processing technique called laplacian edge detection led which represents an objective approach in interpreting various geophysical images numerical modeling demonstrates that isoresistivity lines are not in good agreement with the hypothetical interfaces between the bedrock and sediment but the zero lines of the laplacian operation are strikingly accurate our results indicate a well defined boundary in the resistivity structure that can be used to estimate the quantity of sediments covering bedrock thus highlighting the utility of this technique in studies of landscape evolution sediment transport and sediment budgets

the hydraulic conductivity structure of gravel dominated vadose zones within alluvial floodplains the floodplains of many gravel bed streams have a general stratigraphy that consists of a layer of topsoil covering gravel dominated subsoil previous research has demonstrated that this stratigraphy can facilitate preferential groundwater flow through focused linear features such as paleochannels or gravelly regions within the vadose zone these areas within the floodplain vadose zone may provide a route for interactions between the floodplain surface and alluvial groundwater effectively extending the hyporheic zone across the floodplain during high stream stage the objective of this research was to assess the structure and scale of texture heterogeneity within the vadose zone within the gravel subsoils of alluvial floodplains using resistivity data combined with hydraulic testing and sediment sampling of the vadose zone point scale and broad scale methodologies in combination can help us understand spatial heterogeneity in hydraulic conductivity without the need for a large number of invasive hydraulic tests the evaluated sites in the ozark region of the united states were selected due to previous investigations indicating that significant high conductivity flow zones existed in a matrix which include almost no clay content data indicated that resistivity corresponded with the fine content in the vadose zone and subsequently corresponds to the saturated hydraulic conductivity statistical analysis of resistivity data and supported by data from the soil sampling and permeameter hydraulic testing identified isolated high flow regions and zones that can be characterized as broad scale high hydraulic conductivity features with potentially significant consequences for the migration of water and solutes and therefore are of biogeochemical and ecological significance

groundwater flow and mixing in a wetland stream system field study and numerical modeling summary we combined electrical resistivity tomography ert on land and in a stream with zone based hydraulic conductivities from multi level slug testing to investigate the local geological heterogeneity of the deposits in a wetland stream system the detailed geology was incorporated into a numerical steady state groundwater model that was calibrated against average head observations the model results were tested against groundwater fluxes determined from streambed temperature measurements discharge varied up to one order of magnitude across the stream and the model was successful in capturing this variability water quality analyses from multi level sampling underneath the streambed and in the wetland showed a stratification in groundwater composition with an aerobic shallow zone with oxygen and nitrate top 3 m overlying a reduced anoxic zone while no 3 concentrations up to 58 mg l 1 were found in the top of the aquifer and immediately underneath the streambed no no 3 was detected deeper within the aquifer an inverse relationship between no 3 and so 4 2 suggests that pyrite oxidation takes place in the deeper parts of the aquifer simulated flow path lines showed very different origins for deeper groundwater samples no nitrate reduction is believed to occur in the shallow zone where oxygen is present and the residence time is on the order of 1 year nitrate reduction can however occur in the deeper parts which are oxygen free and where the residence time is on the order of 7 years a simulation with a homogeneous model did not match the observations nearly as well as a heterogeneous model based on ert and a spatially distributed hydraulic conductivity furthermore the origin of the sampled groundwater could not have been predicted from groundwater hydraulic head and the groundwater chemistry alone the presented approach of integrating such methods in groundwater surface water exchange studies proved efficient to obtain information of the controlling factors

geophysical conceptualization of a fractured sedimentary bedrock riverbed using ground penetrating radar and induced electrical conductivity summary bedrock rivers exhibit very different hydraulic and ecological regimes compared to alluvial rivers groundwater surface water interaction in rivers that flow directly on sedimentary bedrock surfaces with exposed fracture and conduit networks is generally based on alluvial river conceptual models however the dual and triple porosity systems of sedimentary rock e g matrix fractures and vugs leads to highly variable hydraulic pathways and groundwater velocities that in turn result in processes that are very different than those derived from alluvial concepts relative to alluvial rivers there has been very little direct examination of dynamic interactions such as fluxes groundwater velocities and mechanisms affecting water quality across the channel interface this is largely because bedrock rivers are difficult to instrument and monitor given their heterogeneous and anisotropic flow systems in this study we evaluate the capacity of non invasive low impact ground penetrating radar and frequency domain electromagnetic induction methods capable of detecting discrete fracture and conduit features mapping complex distributions of lithostratigraphic interfaces and providing information about formation properties along a partially exposed section of the eramosa river located in ontario canada geophysical data are supported by direct hydrogeologic and geologic information obtained from a nearby borehole piezometer cluster and a continuous 15 m rock core valuable insights were gained into the lateral extent and geometry of structurally controlled whydrostratigraphic units e g dissolution enhanced fractures epikarst and karst features which are expected to strongly influence groundwater and surface water interaction along the channel an integrated application of gpr and emi methods was necessary to fully understand the complex geometry of geologic boundaries and karst features within the shallow bedrock aquifer the identification of epikarst below fluvial sediment and its relationship to the formation of dissolution features exposed fracture conduit networks and sinkholes represents the basis for understanding the dynamic hydraulic responses between the surface water and the shallow bedrock aquifer

detecting submarine groundwater discharge with synoptic surveys of sediment resistivity radium and salinity 1 a synoptic geophysical and geochemical survey was used to investigate the occurrence and spatial distribution of submarine discharges of water to upper nueces bay texas the 17 km survey incorporated continuous resistivity profiling measurements of surface water salinity temperature and dissolved oxygen and point measurements of dissolved ra isotopes the survey revealed areas of interleaving vertical fingers of high and low conductivity extending up through 7 m of bay bottom sediments into the surface water located within 100 m of surface salinity and dissolved ra maxima along with peaks in water temperature and lows in dissolved oxygen these results indicate either brackish submarine groundwater discharge or the leakage of oil field brine from submerged petroleum pipelines

saline tracer visualized with three dimensional electrical resistivity tomography field scale spatial moment analysis 1 cross well electrical resistivity tomography ert was used to monitor the migration of a saline tracer in a two well pumping injection experiment conducted at the massachusetts military reservation in cape cod massachusetts after injecting 2200 mg l of sodium chloride for 9 hours ert data sets were collected from four wells every 6 hours for 20 days more than 180 000 resistance measurements were collected during the tracer test each ert data set was inverted to produce a sequence of 3 d snapshot maps that track the plume in addition to the ert experiment a pumping test and an infiltration test were conducted to estimate horizontal and vertical hydraulic conductivity values using modified moment analysis of the electrical conductivity tomograms the mass center of mass and spatial variance of the imaged tracer plume were estimated although the tomograms provide valuable insights into field scale tracer migration behavior and aquifer heterogeneity standard tomographic inversion and application of archie s law to convert electrical conductivities to solute concentration results in underestimation of tracer mass such underestimation is attributed to 1 reduced measurement sensitivity to electrical conductivity values with distance from the electrodes and 2 spatial smoothing regularization from tomographic inversion the center of mass estimated from the ert inversions coincided with that given by migration of the tracer plume using 3 d advective dispersion simulation the 3 d plumes seen using ert exhibit greater apparent dispersion than the simulated plumes and greater temporal spreading than observed in field data of concentration breakthrough at the pumping well

electrical resistivity tomography of vadose water movement cross borehole electrical resistivity tomography ert was used to image the resistivity distribution before and during two infiltration experiments in both cases water was introduced into the vadose zone and the change in resistivity associated with the plume of wetted soil was imaged as a function of time the primary purpose of this work was to study the capabilities and limitations of ert to image underground structure and ground water movement in the vadose zone a secondary goal was to learn specifics of unsaturated flow in a complex geologic setting tomographs of electrical resistivity taken before infiltration image coarser well drained soils sands and gravels as more resistive zones whereas finer grained soils silts and clays which hold more water by capillarity are imaged as more conductive images of changes in resistivity during infiltration show growth of the water infiltration plume with time that is consistent with known geology in the ert images we see the effects of capillary barriers and infer differences between capillary driven flow through fine sediments and gravity driven flow through very permeable sediments images are consistent with numerical flow simulations using hydrological parameter values consistent with soil types inferred from well logs ert can be a useful tool to monitor movement of circuitous moisture fronts in a heterogeneous field setting that would go undetected by borehole measurements

relationship between tropical easterly waves and precipitation during the north american monsoon abstract relationships between tropical easterly waves tews and precipitation over mexico and the united states are examined during the north american monsoon nam the national centers for environmental prediction national center for atmospheric research reanalysis data are used to identify 137 tews that cross mexico north of 20 n after monsoon onset over a 31 yr period from 1975 to 2005 mean precipitation anomalies over two day periods both before and after tew passage are determined using climate prediction center daily precipitation analyses results indicate that positive precipitation anomalies occur along the west coast of mexico and extending into the west central united states in association with tew passage negative precipitation anomalies are found in the south central united states these precipitation anomaly patterns share many similarities to precipitation anomaly patterns previously defined in association with gulf surge events on longer time scales correlations between the total nu

merger of african easterly waves and formation of cape verde storms there are two easterly wave tracks over west africa one to the south and the other to the north of the african easterly jet aej a northern wave sometimes merges with a southern wave near the coast of west africa the dynamic and thermodynamic evolution of the waves during the merger and the impacts of the merger on subsequent storm development are examined in this study three groups of storms are identified non merger developers merger developers and merger non developers relative to non merger developers merger developers have a weaker circulation near the surface prior to the merger but the merger leads to a stronger and deeper wave pouch which is more conducive to tropical cyclogenesis the dryness of the northern disturbances does not seem to hinder the storm development composite analysis shows that merger developers are associated with a warmer and broader heat low over west africa and a more extensive aej which helps to maintain the northern waves over the east atlantic merger developers and non developers resemble each other prior to the merger but dry air intrusion west of the wave trough in the middle and upper troposphere contributes to the non development of some mergers the dry air in non developers is related to a strong and broad upper tropospheric trough and the associated subsidence west of the low level wave pouch the trough advects dry air from the extratropical atlantic in the middle and upper troposphere and also induces a strong westerly vertical shear composites of aerosol optical depth suggest that the saharan air layer has a limited contribution to the dryness in the vicinity of the non developers

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

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

potential vorticity anomalies associated with squall lines abstract this study involves observations and model simulations of potential vorticity anomalies in the wake of midlatitude squall lines using data from the oklahoma kansas pre storm experiment we analyze potential vorticity fields near two squall lines one with and one without a trailing stratiform region from this observational analysis we suggest that squall lines with trailing stratiform regions can leave large positive midtropospheric potential vorticity anomalies in their wake to further interpret these observations we consider a two dimensional version of semigeostrophic theory formulated in isentropic and geostrophic coordinates which results in a simple potential pseudodensity inverse potential vorticity equation using apparent heat source fields that model those computed diagnostically from pre storm data we find that theory does indeed predict large midtropospheric potential vorticity anomalies for model squall lines with a trailing stratiform region but not for model squall lines t

climatology of sign reversals of the meridional potential vorticity gradient over africa and australia abstract a 10 yr climatology 1986 95 was performed using ecmwf gridded analyses on isentropic surfaces to identify regions where the lower tropospheric meridional potential vorticity pv gradient changes sign across africa and australia during their respective summer seasons while an african sign reversal has been documented no similar study has been performed for the australian region which also has desert on the poleward side of open ocean in each hemisphere a northward decrease of pv is sufficient to produce a sign reversal it was found that pv decreases northward in the lower troposphere across northern australia with the maximum reversal on the 315 k surface it had comparable magnitude but smaller zonal extent 3000 km than that on the 320 k surface in africa 5000 km in each region the sign reversal was associated with cyclonic pv anomalies on the equatorward side and anticyclonic anomalies on the poleward side olr was used as a proxy for deep convective heating in order to evaluate

intense convective systems in west africa and their relationship to the african easterly jet for may september 1998 convective systems in west africa were identified from observations by the tropical rainfall measuring mission satellite microwave imager at 85 ghz using re analysis data the 10 day average position of the african easterly jet aej was diagnosed from the 700 hpa zonal winds the distance from each convective system s centroid to the axis of the aej was calculated each convective system s minimum brightness temperatures were ranked so that intense convective systems were defined as those in the 10th percentile or lower the weak 10th percentile and intense convective systems were represented statistically as two separate populations the weak by the skewed gumbel distribution and the intense by the normal distribution from may to august the peak in activity of weak convective systems remained south of 10 n but shifted east of 10 e the peak in activity of the intense convective systems followed the seasonal migration of the aej northwards and became increasingly separate from the peak in activity of weak convective systems the majority of both weak and intense convective systems occurred within 0 50 of high terrain the high convective available potential energy high shear aej environment in the vicinity of high terrain appeared to have the greatest probability of generating intense convective systems in the study area copyright 2006 royal meteorological society

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

maintenance of the african easterly jet the maintenance of the african easterly jet aej has been examined using a zonally symmetric general circulation model with simple parametrizations it is shown that the aej is maintained in association with two diabatically forced meridional circulations one associated with surface fluxes and dry convection in the saharan heat low region and one associated with deep moist convection in the intertropical convergence zone equatorward of this the heat low heating which reaches the height of the aej around 700 mb is particularly important in maintaining the aej and its associated meridional gradients in potential vorticity it is concluded that the mean observed aej results from a combination of the diabatically forced meridional circulations which maintain it and easterly waves which weaken it

imaging and characterisation of subsurface solute transport using electrical resistivity tomography ert and equivalent transport models abstract we assess the usefulness of electrical resistivity tomography ert in imaging and characterising subsurface solute transport in heterogeneous unconfined aquifers a field tracer experiment was conducted at the krauthausen test site germany the spatial and temporal evolution of the injected nabr solute plume was monitored in a 2d ert image plane located downstream of the injection well for 90 days since ert maps changes in bulk electrical conductivity the reconstructed images at selected time intervals are first converted to solute concentration maps by postulating a linear relation the concentration maps are then analysed using an equivalent convection dispersion model cdm which conceptualises the aquifer as a homogeneous medium with a uniform mean flow velocity as demonstrated by associated synthetic model studies ert resolution in terms of recovered equivalent dispersivities is limited due to spatial smoothing inherent to the imaging algorithm since for heterogeneous media local concentrations within the plume deviate from those predicted by the equivalent cdm we also interpret the ert derived pixel breakthrough curves in terms of an equivalent stream tube model stm the stm represents transport in the aquifer by a set of 1d convection dispersion processes allowing the degree of mixing and the heterogeneity of transport within the plume to be quantified we believe that the observed tracer plume is satisfactorily described by the equivalent cdm probably because the tracer plume was small relative to the heterogeneity scale of the aquifer even though application of the stm revealed some deviation from the ideal homogeneous case the equivalent dispersivity in the stm matches the longitudinal dispersivity of the cdm closely consistent with predominantly homogeneous mixing however the stm analysis illustrates how ert results can be used to quantify the variability of parameters relevant to flow and transport in heterogeneous aquifers

comparison between two case studies of developing and nondeveloping african easterly waves during namma and amma sop 3 absolute vertical vorticity budget abstract two west african disturbances observed in august and september 2006 during the national aeronautics and space administration african monsoon multidisciplinary analysis namma and the special observing period 3 amma sop 3 have been simulated using the meso nh numerical model with explicit convection the first disturbance spawned hurricane helene 2006 off the west african coast and the second one referred to as perturbation d though relatively intense failed to develop over the continent each case was associated with a well defined african easterly wave aew trough with embedded growing and decaying convective activity of various size duration and intensity the aim of this work is to investigate the contribution of these convective systems in the generation and maintenance of cyclonic vorticity associated with the aew trough with respect to the synoptic scale processes the absolute vorticity budgets are analyzed during the continental and oceanic transition stages of these aew

initiation of a mesoscale convective complex over the ethiopian highlands preceding the genesis of hurricane alberto 2000 1 a tropical disturbance which would later become hurricane alberto 2000 was traced back in time as a mesoscale convective complex mcc to the ethiopian highlands eh where the mcc first developed meteosat 7 imagery indicates that the mcc develops during the late afternoon and evening of 28 july 2000 and that a mesovortex mv was evident on the morning of 29 july 2000 a preliminary mesoscale model simulation features the development of two areas of maximum relative vorticity in the middle troposphere with the maturation of one of the vorticity maxima into a significant mv the higher level vorticity center of the simulated mv tracks westward and is nearly collocated with the vortex signature in satellite imagery while the lower level center remains near the lee side of the eh the vortex signature in satellite imagery was traceable to the cyclogenesis stage over the eastern atlantic ocean

on the relationship between african easterly waves and the african easterly jet this idealized modeling study investigates how convectively triggered african easterly waves aews are influenced by the intraseasonal variability of the african easterly jet aej a set of 10 day averaged zonally varying basic states is constructed with the ncep 2 reanalysis 1979 2006 a primitive equation model is used to simulate linear aews on each of these basic states using the same idealized convective heating localized over the darfur mountains as an initial trigger it is shown that the transient response depends strongly on the basic state with the same trigger many configurations of the aej fail to produce a wave disturbance while others produce strong easterly wave structures necessary conditions for the development of strong waves can be characterized by a strong jet a strong vertical shear or a strong and extended potential vorticity reversal in strong wave cases the jet is extended to the south and west and the jet core is aligned with the maximum of surface westerlies maximizing the vertical shear the pattern that is optimal for generating easterly waves also closely resembles the dominant mode of variation of the aej revealed by an empirical orthogonal function eof analysis of the set of basic states

trains of african easterly waves and their relationship to tropical cyclone genesis in the eastern atlantic abstractin this study the relationship between trains of african easterly waves aews and downstream tropical cyclogenesis is studied based on 19 summer seasons july september from 1990 to 2008 of era interim reanalysis fields and brightness temperature from the cloud user archive the signature of aew troughs and embedded convection are tracked from the west african coast to the central atlantic the tracked systems are separated into four groups i systems originating from the north zone of the midtropospheric african easterly jet aej ii those coming from the south part of aej iii systems that are associated with a downstream trough located around 2000 km westward termed duo systems and iv those that are not associated with such a close downstream trough termed solo systems by monitoring the embedded 700 hpa filtered relative vorticity and 850 hpa wind convergence anomaly associated with these families along their trajectories it is shown that the duo generally have stronger dynam

the impact of deep convection on the west african summer monsoon climate a regional climate model sensitivity study the role of the representation of deep convection on key elements of the west african summer monsoon climate is addressed using the regional climate model regcm3 two simulations in which a scheme of deep convection is activated and then turned off are performed and intercompared results show that the presence of deep convective heating along the intertropical convergence zone sustains increased lower level baroclinicity favoring intensification of the jet core and leading to a more realistic african easterly jet in addition although the isentropic potential vorticity ipv is lower when the convection scheme is switched off african easterly waves aews are still generated and propagate westwards but they dissipate around the west coast substantial differences between the two simulations occur mainly at the 6 to 9 day time scale over land when much weaker activity is simulated in the absence of convection this indicates that orographic friction and low level large scale moisture convergence generating high values of latent heat and ipv may play the dominant role in the genesis and growth of aews and that deep convection acts to strengthen the overall wave activity and to favor their west coast development copyright 2011 royal meteorological society

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

interactions between convection and a moist vortex associated with an extreme rainfall event over southern west africa abstractan intense mesoscale convective system mcs in the guinea coast region caused one of the highest ever recorded daily rainfall amounts at the nigerian station abakaliki on 12 june 2016 223

the impact of moist processes on the african easterly jet african easterly wave system the african easterly jet easterly wave aej aew system was explored in an idealised model prescribed zonally symmetric surface temperature and moisture profiles determine the aej which becomes established through meridional contrasts in dry and moist convection as in previous studies a realistic aej developed with only dry convection including moist processes increased its development rate but reduced its speed and meridional extent aews grew through barotropic baroclinic conversions negative meridional potential vorticity pv gradients arose in the zonally symmetric state through the intrusion of the low pv saharan boundary layer since moist processes strengthened this significantly through diabatically generated pv in the intertropical convergence zone moist aews were three times stronger larger barotropic conversions and faster aej development increased the moist wave growth rate jet level and northerly low level amplitudes grew but in the moist case the low level amplitudes weakened as the aew interacted with convection consistent with their absence from observations during the peak monsoon striking dependencies between the aej aew and rainfall existed two time scales governed their evolution depending on the transfer coefficients 1 the aej s replenishment rate influenced by heat fluxes and 2 the wave growth rate by damping and the slower jet development rate moist aews were characterized by intermittent growth decay with growth preceded by increased mean rainfall and later weakening aejs these dependencies established an internal 8 10 day variability consistent with intra seasonal observations of 9 day rainy sequences this internal variability offers an alternative explanation to the previously proposed external forcing and a new view of the moist aew life cycle copyright 2009 royal meteorological society