electrical structure in thunderstorm convective regions 1 mesoscale convective systems electric field e soundings through convective regions of mesoscale convective systems mcss are examined in this paper ten e soundings through updrafts in mcs convective regions and five soundings in mcs convective regions outside updrafts are used to show that a typical electrical structure exists in this region these 15 e soundings plus one other previously published sounding which is included in this analysis comprise all known soundings in the convective region of mcss the basic charge structure identified in mcs updrafts consists of four charge regions alternating in polarity with the lowest region positive and the highest region negative the basic charge structure outside updrafts in mcs convective regions has six charge regions alternating in polarity with a positive charge region lowest maximum e magnitudes of both polarities are larger and are located at lower heights in soundings outside updrafts compared to those within updrafts excepting the upper positive charge inferred charge regions are shallower and have larger charge densities outside updrafts the center of the main negative charge is lower and warmer outside updrafts 5 5 km 6 2 c than within updrafts 6 9 km 15 7 c these features inside and outside updrafts are incorporated in a new conceptual model of mcs convective region charge structure also an expanded conceptual model is developed using previously published data from other parts of mcss this more complete conceptual model shows the typical electrical charge airflow and reflectivity features in the stratiform region the transition zone and the convective region of midlatitude mesoscale convective systems

kinematic microphysical and electrical aspects of an asymmetric bow echo mesoscale convective system observed during steps 2000 1 five hours of detailed observations of the asymmetric bow echo mesoscale convective system on 11 12 june 2000 during the steps project are presented data sources included polarimetric and doppler radars a vhf lightning mapper and the national lightning detection network nearly every 99 vhf lightning source occurred within 10 km of the convective line charge identification within the much of the convective line revealed evolution during the first 3 h from a normal polarity dipole upper positive charge near 9 km msl 30 c over mid level negative charge near 7 km 16 c to a normal polarity tripole with the addition of lower positive charge near 4 km 1 c during the final 2 h the charge structure resembled an inverted dipole with negative charge overlaying positive when the bow echo occurred during hour 2 part of the cells associated with severe winds developed an inverted tripolar charge structure and produced predominantly positive cloud to ground lightning stratiform lightning was infrequent normally initiating in the convective line and propagating rearward along two pathways a downward sloping upper pathway near 9 km and a more common constant altitude lower pathway near 6 km 10 c some flashes initiated within the stratiform region during this time but only after stratiform reflectivities increased significantly in the mixed phase region these flashes initiated near the bright band 4 km and tapped negative charge above and positive charge below possible mechanisms to explain the observations are discussed

adaptation of the near surface wind to the development of sand transport a model of wind blown sand transport is described with particular emphasis on the feedback between the grain cloud and the near surface wind the results from this model are used to develop owen s 1964 hypothesis that the grain layer behaves so far as the flow outside it is concerned as increased aerodynamic roughness whose height is proportional to the thickness of the layer the hypothesis is developed to show the influence this dynamic roughness has on the turbulent boundary layer above the saltation layer two processes are identified which influence the path of the system towards equilibrium the first is the feedback between the near surface wind and the grain cloud in which the quantity of sand transported is limited by the carrying capacity of the wind the second is due to the temporal development of an internal boundary layer in response to the additional roughness imposed on the flow above the grain layer by the grain cloud a similarity is noted between the temporal response of a turbulent boundary layer to sand transport and the spatial response of a turbulent boundary layer downstream of a step increase in surface roughness finally it is noted that the work may have important implications for transport rate prediction in unsteady winds

theory and observations of controls on lightning flash size spectra previous analyses of very high frequency vhf lightning mapping array lma observations relative to the location of deep convective updrafts have noted a systematic pattern in flash characteristics in and near strong updrafts flashes tend to be smaller and more frequent while flashes far from strong vertical drafts exhibit the opposite tendency this study quantitatively tests these past anecdotal observations using lma data for two supercell storms that occurred in oklahoma in 2004 the data support a prediction from electrostatics that frequent breakdown and large flash extents are opposed an energetic scaling that combines flash rate and flash area exhibits a 5 3 power law scaling regime on scales of a few kilometers and a maximum inflashenergyatabout10km thespectralshapeis surprisinglyconsistentacrossarangeofmoderatetolarge flash rates the shape of this lightning flash energy spectrum is similar to that expected of turbulent kinetic energy spectra in thunderstorms in line with the hypothesized role of convective motions as the generator of thunderstorm electrical energy the correspondence between kinematic and electrical energy spectra suggests that advection of charge bearing precipitation by the storm s flow including in turbulent eddies couples the electrical and kinematic properties of a thunderstorm

numerical simulation of saltating particles in atmospheric boundary layer over flat bed and sand ripples 1 in this work we numerically simulated the saltating particles in a turbulent boundary layer over flat bed and sand ripples by using natural sand grains in a wind tunnel we obtained the initial conditions for the simulation and also verified the correctness of the numerical model we carefully analyzed the numerically simulated saltating particle movement over the two sand beds and we found the following 1 the aeolian sand transport is a dynamic equilibrium process on both sand beds and it took longer to reach equilibration on the sand ripples than on the flat bed 2 according to the mass flux profile at the trough of the sand ripples there is a maximum mass flux at about 4 cm height in the leeward section the mass flux increases with height below 4 cm and decreases with height above 4 cm 3 the wind profile near the surface is modified by saltating particles on the two different sand beds and the flow field characteristics of the sand ripples are more complex than that of the flat bed

evolution of the total lightning structure in a leading line trailing stratiform mesoscale convective system over houston texas 1 line normal vertical cross sections of houston lightning detection and ranging ldar vhf radiation sources and radar reflectivity provide new insights into the three dimensional total lightning structure and evolution of a leading line trailing stratiform llts mesoscale convective system mcs over houston texas on 31 october 2005 previous research examining only the mature stage of an mcs showed that the overwhelming majority of vhf lightning sources occurred in the convective region with a lightning pathway extending rearward and descending in altitude into the stratiform region this descending pathway was most likely associated with small charged ice particles advected from the convective line unlike previous research the lightning pathway observed during the evolution of the mcs on 31 october 2005 initially extended rearward 40 km at a nearly constant height of 9 10 km in less than an hour the lightning pathway evolved into a sloped pathway similar to that found by previous research with a horizontal extent between 50 to 60 km and downward descent of 4 to 5 km during the lightning pathway evolution radar analysis showed an increase of reflectivity in the midlevels of the stratiform region consistent with increased depositional and aggregational growth above the melting layer broadening increased range of values of radar reflectivity above the melting layer was most likely an indication of a strengthening mesoscale updraft in the stratiform region this strengthening updraft may have contributed to an increase in the growth of the small charge carrying ice crystals giving them a greater fall speed in addition the mesoscale updraft may have promoted an environment conducive to local stratiform region charge generation in the mixed phase region just above the melting layer

laboratory measurement of electrification of wind blown sands and simulation of its effect on sand saltation movement 1 this article presents a measurement of electrification generated by wind blown sands in a field wind tunnel and a numerical methodology to simulate the effect of electrification on the sand saltation movement after the mutual couple interaction between the sand movement and the wind flow is taken into account the measured data of electric charge on the uniform sands in the wind tunnel tests show that the sign of electric charge either negative or positive is mainly dependent on the diameter size of sand particles i e negative charge is gained when the diameter is smaller than 250 m and positive charge is obtained if the diameter is larger than 500 m and that for both uniform and mixed sands the average charge to mass ratio decreases with increasing the wind velocity and increases with height from sand bed meanwhile the measurement of electric field in wind sand cloud related to the electric charge displays that the magnitude of electric field increases generally as the wind velocity and the height increase and the direction of the field is always upwardly vertical to the earth s surface which is opposite to that of the fair weather field in order to exhibit the effect of electrification on sand saltation movement a theoretical model by considering the mutual coupling interaction between wind flow and sand movement is proposed after the electric force exerted on the moving sands is considered through solving the nonlinear coupling dynamic equations by a proposed program the effect of electrification on sand saltation motion e g trajectory is discussed quantitatively after that its effect on wind sand transport flux sand ejecta flux and wind profile is also displayed the results show that the prediction for the bagnold s kink is good agreement with the measurement in literature

electrical structure in thunderstorm convective regions 3 synthesis in this paper results from nearly 50 electric field soundings through convective regions of mesoscale convective systems mcss isolated supercells and isolated new mexican mountain storms are compared and synthesized these three types of thunderstorm convection are found to have a common basic electrical structure within convective updrafts the basic charge structure has four charge regions alternating in polarity and the lowest is positive outside updrafts of convection there are typically at least six charge regions alternating in polarity and the lowest is again positive among the three storm types there are differences in the heights and temperatures at which the basic four charge regions are found in updrafts the height temperature of the center of the main negative charge region averages 6 93 km 16 c in mcs convective region updrafts 9 12 km 22 c in supercell updrafts and 6 05 km 7 c in new mexican mountain storm updrafts in updraft soundings through all three storm types the center height of the main negative charge region increases with increasing average balloon ascent rate and updraft speed at a rate of about 0 3 km per 1 m s 1 with a correlation coefficient of 0 94 a schematic illustrates the basic four and six charge structure for thunderstorm convective regions and it is offered as an improved model for thunderstorm charge structure

steady state saltation in air coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain surface impact process experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced including a diminution of the sub saltation layer mean wind speed as the friction speed increases in this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free stream shear stresses and the grain mass flux increases with friction speed uf less rapidly than uf3

two phase measurements of wind and saltating sand in an atmospheric boundary layer abstract wind blown sand movement is a particle laden two phase flow related to wind erosion in which the velocity distributions of both wind and sand are of particular interest in the present study two types of natural sand one collected from the pohang beach diameter d 200 300 m in south korea and the other from the taklimakan desert d 100 125 m in china were tested in a simulated atmospheric boundary layer a high speed digital camera system was used to capture images of the saltating sand particles at 2000 fps with an exposure time of 1 3000 s instantaneous velocity fields of the saltating sand particles were obtained using a particle tracking velocimetry ptv method from these data the particle resultant velocity volume concentration and streamwise mass flux were estimated as a function of height the results reveal that the resultant particle velocity has an approximate log linear profile with vertical height both the particle concentration and streamwise mass flux decay dramatically in the near surface region z z

wind forces and related saltation transport the effect of several wind characteristics on sand transport was studied in three experiments in north burkina faso west africa the first experiment is used to analyse the relation between wind speed and shear stress fluctuations across height the second experiment is used to study the relation of these wind characteristics with saltation transport for fourteen convective storms registered during the rainy seasons of 2002 and 2003 the effect of sampling time is studied for two of these convective storms the third experiment relates the turbulent structures of four convective storms to saltation transport wind speed measurements were undertaken with two sonic anemometers and sediment transport was measured by two saltiphones the sampling frequency was either 8 or 16 hz the sonic frame of reference was rotated according to a triple rotation horizontal fluctuations showed a fairly good correlation with height because the wind speed at both sensors was affected by the same vortices the correlation coefficients ranged from 0 42 when the distance between the sensors was 1 75 m to 0 92 when the distance was 0 25 m the instantaneous reynolds stress had the weakest correlation correlation coefficient of 0 05 at 1 75 m between the sensors and 0 56 at 0 25 m between the sensors because the momentum at 2 m above the soil surface is transported by different eddies than those close to the ground this also explains the fairly good correlation coefficients between the horizontal components of the wind and saltation compared to the poor correlations between instantaneous reynolds stress and saltation an increase in sampling time did not have much impact on these correlation coefficients up to sampling periods of about 30 s however this sampling interval would be too coarse to describe the vertical wind component adequately the classification of the moments of shear stress into the turbulent structures sweeps ejections inward and outward interactions showed that the mean saltation flux is higher at sweeps and outward interactions than at ejections and inward interactions also saltation occurred more often during sweeps and outward interactions than during ejections and inward interactions

formation of charge structures in a supercell abstract lightning mapping electric field and radar data from the 26 may 2004 supercell in central oklahoma are used to examine the storm s charge structure an initial arc shaped maximum in lightning activity on the right flank of the storm s bounded weak echo region was composed of an elevated normal polarity tripole in the region of precipitation lofted above the storm s weak echo region later in the storm two charge structures were associated with precipitation that reached the ground to the left of the weak echo region six charge regions were inferred with positive charge carried on hail at the bottom of the stack farther forward in the storm s precipitation region four charge regions were inferred with negative charge at the bottom of the stack there were different charge structures in adjacent regions of the storm at the same time and regions of opposite polarity charge were horizontally adjacent at the same altitude flashes occasionally lowered positive charge to ground from the forwa

seasonal and spatial variations of snow chemistry on mount logan yukon canada three ice cores were recovered on or near mount logan yukon canada at 3017 4135 and 5340 ma s l in 2002 prior to ice core drilling we collected snow pit and shallow core samples from mount logan in 2001 to study seasonal and spatial variations of snow chemistry we dug snow pits at six sites between 2420 and 5340 m a s l before the beginning of the melt season with the exception of a pit at 3180 m a s l where the melt season had just started but had affected only the near surface stratigraphy three of the pits were extended deeper with a shallow core the snow pit and core samples were analyzed for ion chemistry and 18 o a series of depth profiles of ions and 18 o shows spatial variations though characteristic peaks can usually be traced across all the profiles concentrations and deposition fluxes of na and cl which are mainly of sea salt origin decrease with altitude on the other hand deposition fluxes of no 3 so 4 2 ca 2 and nh 4 show a weak positive relationship with elevation below the summit plateau stable isotopes 18 o decrease with altitude with a distinctive jump between 3200 and 4500 m a s l as was reported previously stable isotopes 18 o cl ch 3 so 3 msa na and ca 2 show clear seasonal variations which would enable us to date the cores by annual layer counting

the relationship between precipitation and lightning in tropical island convection a c band polarimetric radar study one of the primary scientific objectives of the maritime continent thunderstorm experiment was to study cloud electrification processes in tropical island convection in particular the coupling between ice phase precipitation and lightning production to accomplish this goal a c band polarimetric radar was deployed in the tropics 11 68s 130 88e for the first time accompanied by a suite of lightning measurements using observations of the propagation corrected horizontal reflectivity and differential reflectivity along with specific differential phase rain and ice masses were estimated during the entire life cycle of an electrically active tropical convective complex known locally as hector over the tiwi islands on 28 november 1995 hector s precipitation structure as inferred from these raw and derived radar fields was then compared in time and space to the measured surface electric field cloud to ground cg and total lightning flash rates and ground strike locations during hector s developing stage precipitating convective cells along island sea breezes were dominated by warm rain processes no significant electric fields or lightning were associated with this stage of hector despite substantial rainfall rates aided by gust front forcing a cumulus merger process resulted in larger taller and more intense convective complexes that were dominated by mixed phase precipitation processes during the mature phase of hector lightning and the surface electric field were strongly correlated to the mixed phase ice mass and rainfall merged convective complexes produced 97 of the rainfall and mixed phase ice mass and 100 of the cg lightning as hector dissipated lightning activity rapidly ceased as evidenced from the multiparameter radar observations the multicell nature of hector resulted in the continuous lofting of supercooled drops to temperatures between 2108 and 2208c in discrete updraft cores during both the early and mature phases the freezing of these drops provided instantaneous precipitation sized ice particles that may have subsequently rimed and participated in thunderstorm electrification via the noninductive charging mechanism

field measurement and numerical modelling of aeolian mass flux distributions on a sandy beach the vertical and horizontal distributions of aeolian mass flux were measured at oceano dunes california and these data were used to evaluate a numerical model of saltation grain size analyses showed that the distributions of the modal sediment size class corresponded closely to those of the total sediment population and modelling thus focused on replicating the distributions of the mean grain size although much previous work has assumed that the mean launch speed of saltating particles varies in proportion to shear velocity simulations using a constant mean launch speed were found to yield the closest approximations to the mass flux distributions observed in the field both exponential and gamma distributions of launch velocity produced realistic simulations although the latter approach required the inclusion of an additional reptation component to achieve good results a range of mean launch angles and an equivalent sphere correction were also found to generate comparable results providing the other input parameters could be varied freely all the modelling approaches overestimated the proportion of mass flux occurring at the bottom of the vertical distributions and underestimated the proportion occurring at the upwind end of the horizontal distributions no theoretical shortcoming that would account for these small but systematic discrepancies could be identified and experimental error thus represents a more plausible explanation the conclusion that mean grain launch speeds are essentially constant and independent of shear velocity suggests that the additional kinetic energy extracted by grains under more energetic wind conditions is largely transferred to the bed and that increases in the transport rate are therefore driven primarily by the ejection of additional grains it is suggested that the kinetic energy of rebounding grains is constrained by the ability of the bed to resist deformation equivalent to a plastic limit hence grains of larger mass diameter rebound from the bed at lower speeds and follow shorter lower trajectories as has been widely observed previously

electrical structure in thunderstorm convective regions 2 isolated storms electric field e soundings through the convective regions of two types of isolated thunderstorms are examined analysis of seven soundings through strong updrafts of isolated supercell storms show that the basic e structure there has three e peaks a positive peak near 8 km height a negative peak near 10 5 km and a positive peak near 12 km strong updraft soundings are those with average balloon ascent rates in excess of 10 m s 1 the basic charge structure in strong updrafts of supercells has four charge regions of alternating polarity the lowermost charge is positive between about 4 and 8 km and the uppermost region is negative all the supercell updraft soundings are incomplete due to balloon burst or lightning induced instrument destruction below cloud top six supercell soundings that ascended outside strong updrafts are used to show that the e and charge structures there are more complex than within the strong updrafts analysis of 15 new or previously published soundings through small new mexican mountain thunderstorms indicates that the basic e structure in or near their convective cores consists of three e peaks a lower positive peak at about 5 km height a midlevel negative peak near 6 5 km and an upper positive peak near 9 5 km the basic charge structure near the center of new mexican storm convection has four charge regions alternating in polarity with a positive charge region lowest soundings in new mexican convection that did not ascend near the convective cores show more complex e profiles and charge structures

electrical and polarimetric radar observations of a multicell storm in telex abstract on 28 29 june 2004 a multicellular thunderstorm west of oklahoma city oklahoma was probed as part of the thunderstorm electrification and lightning experiment field program this study makes use of radar observations from the norman oklahoma polarimetric weather surveillance radar 1988 doppler three dimensional lightning mapping data from the oklahoma lightning mapping array lma and balloon borne vector electric field meter efm measurements the storm had a low flash rate 30 flashes in 40 min four charge regions were inferred from a combination of lma and efm data lower positive charge near 4 km and midlevel negative charge from 4 5 to 6 km msl from 0 to 6 5 c were generated in and adjacent to a vigorous updraft pulse further midlevel negative charge from 4 5 to 6 km msl and upper positive charge from 6 to 8 km from 6 5 to 19 c were generated later in quantity sufficient to initiate lightning as the updraft decayed a negative screening layer was present near the storm top

application of thermal anemometry and high frequency measurement of mass flux to aeolian sediment transport research abstract recent research on sediment transport by temporally varying winds has stimulated the need for high frequency measurements of wind velocity turbulence and transport rate conventional technologies yielding only temporally averaged estimates of wind velocity and mass flux are unable to resolve the transitional and self regulating behaviour of the aeolian saltation system in unsteady winds the characteristics calibration and potential of thermal velocity probes suitable for research on aeolian transport are analysed recent developments in particle impact sensing and the use of sand trap force transducer combinations are reviewed and the design of a new optical mass flux sensor is presented an upper frequency limit of about 1 hz is shown to be essential to transport rate relations in unsteady forcing winds or during intermittent transport higher frequencies are required if reynolds stress or potentially influential flow structures and sediment fluid interactions in the saltation cloud are to be resolved armoured hot wire sensors measure u component air velocity to 1 hz in grain clouds and some less robust two dimensional thermal sensors may be used at higher frequencies to determine bed shear from reynolds stress u w during sand transport by wind grain impact devices and automated sand traps provide high frequency measurements of transport rate but they are intrusive and their output signals may be ambiguous this paper reports the design and testing of a new instrument system providing high frequency measurements of aeolian sand transport it comprises a non intrusive optical mass flux sensor synchronized to an array of armoured hot wire anemometers the sensor uses a linear image array to detect at a sampling frequency of 4 40 hz the transit of wind driven particles through a laser sheet transverse to the flow in a wind tunnel the physics of the sensor have been rigorously explored in static grain tests and calibrations to mass flux its principal advantages over existing devices are its non intrusive design and ability to measure near bed mass flux with precision at high frequency example applications demonstrate that this instrumentation is able for the first time to recover high frequency aspects of the temporal behaviour of aeolian saltation previously revealed only by numerical models

simulation of wind blown sand movement and probability density function of liftoff velocities of sand particles 1 accurately describing the probability density function pdf of liftoff or initial velocities of wind blown sand ejecting from a sand bed is fundamental to understanding the mechanisms of wind blown sand movement our objective was to investigate the efficacy of developing the pdf of liftoff velocities based on wind tunnel measurements of sand flux and wind speed profile on the basis of experimental results an optimization algorithm to numerically search for the pdf was developed by multiobjective optimization associated with the saltation model through fitting all sets of the pdf numerical results of optimization with the least squares method an empirical formula of the pdf was obtained to show the efficacy and accuracy of the formula of the pdf obtained using this approach simulations of the sand flux and wind speed profile using selected pdfs of liftoff velocities were performed with the developed function and functions previously proposed by other researchers by comparing the suite of predicted pdfs with the experimental measurements we found that although all the predictions agree reasonably well with the measured data the method proposed herein for obtaining the pdf in this paper results in a more accurate pdf for predicting sand flux and wind speed profile than other tested functions these results validate the approach of obtaining the pdf of liftoff velocities based on wind tunnel measurements

a steady state model of wind blown sand transport a model for steady state eolian saltation is presented in which computation of sand grain trajectories in the wind and of the wind velocity profile modified by saltating grain drag forces is combined with experimental data on grain bed collisions in an iterative simulation scheme which resembles the time development of natural saltation qualitative features of eolian saltation are reproduced in the simulations including a height in the saltating layer at which the wind velocity remains roughly constant with changing free stream friction velocity and an approximate separation of the saltating population into successively hopping grains and reptating or creeping grains the numerical solutions give rise to more general arguments in support of the conclusions that fluid entrainment of grains is unimportant in steady state saltation that the mass flux profile obeys roughly an inverse power law dependence on height and that the vertically integrated mass flux dependence on the free stream friction velocity

on the efficiency of vertical array aeolian field traps the efficiency of three vertical array field traps for measuring the distribution of aeolian mass transport with height was compared with measurements with an isokinetic trap the vertical traps seem to work adequately efficiency 80 at heights greater than 15 mm above the bed closer to the bed however the open array trap catches about 70 the two continuous arrays only about 50 because most of the transport takes place close to the surface the overall efficiency of the field traps ranges from 50 to 70 based on these wind tunnel tests we have developed a low single compartment wedge shaped trap its shape in combination with flow straighteners at the entrance minimizes stagnation effects so that the efficiency is 80 our data indicate that with the wedge trap used in combination with the vertical array aberdeen trap we can sample mass transport in the field during periods with unidirectional winds with an overall efficiency of the order of 80

the role of streamline curvature in sand dune dynamics evidence from field and wind tunnel measurements abstract field measurements on an unvegetated 10 m high barchan dune in oman are compared with measurements over a 1 200 scale fixed model in a wind tunnel both the field and wind tunnel data demonstrate similar patterns of wind and shear velocity over the dune confirming significant flow deceleration upwind of and at the toe of the dune acceleration of flow up the windward slope and deceleration between the crest and brink this pattern including the widely reported upwind reduction in shear velocity reflects observations of previous studies such a reduction in shear velocity upwind of the dune should result in a reduction in sand transport and subsequent sand deposition this is not observed in the field wind tunnel modelling using a near surface pulse wire probe suggests that the field method of shear velocity derivation is inadequate the wind tunnel results exhibit no reduction in shear velocity upwind of or at the toe of the dune evidence provided by reynolds stress profiles and turbulence intensities measured in the wind tunnel suggest that this maintenance of upwind shear stress may be a result of concave unstable streamline curvature these additional surface stresses are not recorded by the techniques used in the field measurements using the occurrence of streamline curvature as a starting point a new 2 d model of dune dynamics is deduced this model relies on the establishment of an equilibrium between windward slope morphology surface stresses induced by streamline curvature and streamwise acceleration adopting the criteria that concave streamline curvature and streamwise acceleration both increase surface shear stress whereas convex streamline curvature and deceleration have the opposite effect the relationships between form and process are investigated in each of three morphologically distinct zones the upwind interdune and concave toe region of the dune the convex portion of the windward slope and the crest brink region the applicability of the model is supported by measurements of the rate of sand transport and the change of the dune surface in the field

vertical profiles of aeolian sand mass flux abstract vertical profiles of the horizontal mass flux of blown sand are investigated experimentally using a passive vertical array in a wind tunnel considering lower sampling efficiency of the sand trap in the near bed region this investigation is complemented by the measurements of the longitudinal profiles of mass flux made using a horizontal sand trap the experiments were conducted with two test sands and five different stream velocities in the upper part of the vertical profile the measured data exhibit an exponential decay distribution with a positive deviation occurring in the near bed region the measured longitudinal profiles are similar to the measured vertical profiles linking both profiles and the modes of sand transport it is possible that saltating sand grains give rise to the well known exponential decay distribution of mass flux and that creeping and reptating grains force a deviation from it a simple equation applicable for both the vertical and the longitudinal sand mass flux variations is introduced and the parameters are estimated from experimental data

lightning location relative to storm structure in a leading line trailing stratiform mesoscale convective system 1 horizontal and line normal vertical cross sections and composite images of dallas fort worth lightning detection and ranging ldar ii vhf radiation sources and radar reflectivity over a 30 min period provide a unique perspective on lightning pathways within a leading line trailing stratiform llts mesoscale convective system mcs on 16 june 2002 the overwhelming majority of vhf lightning sources occurred within the leading convective line in a bimodal pattern in the vertical assuming that vhf source density maxima were most likely associated with positive charge then the ldar ii observations suggest that the gross charge structure of the convective region of the mcs was characterized by a tripole with net positive charge centered at 4 5 km agl 3 c and 9 5 km agl 35 c and net negative charge centered roughly in the relative minimum of the vhf source density maximum at 7 km agl 17 c a persistent lightning pathway and inferred positive charge zone sloped rearward by 40 50 km and downward by 4 5 km from the upper vhf source maximum in the convective line through the transition zone and into the radar bright band of the stratiform region in the stratiform region vhf lightning sources and inferred positive charge were concentrated in three layers centered at 4 5 6 and 9 km agl 2 c 11 c and 31 c respectively consistent with past electric field studies of symmetric llts mcss positive cloud to ground lightning flashes in the stratiform region were initiated in the convective line and followed the slanting pathway from the top of convective cores to the stratiform precipitation where they were horizontally extensive layered and highly branched the sloping lightning pathway was identical to hypothetical trajectories taken by snow particles these observations provide further support for the advection of charge on snow along the sloping pathway and the in situ generation of charge in the horizontal lightning layers as primary contributors to electrification and positive lightning production rearward of the convective line

total lightning signatures of thunderstorm intensity over north texas part i supercells abstract it is shown that total lightning mapping along with radar and national lightning detection network nldn cloud to ground lightning data can be used to diagnose the severity of a thunderstorm analysis of supercells some of which were tornadic on 13 october 2001 over dallas fort worth texas shows that lightning detection and ranging ldar ii lightning source heights quartile median and 95th percentile heights increased as the storms intensified most of the total cloud to ground and intracloud lightning occurred where reflectivity cores extended upward within regions of strong reflectivity gradient rather than in reflectivity cores a total lightning hole was associated with an intense nontornadic supercell on 6 april 2003 none of the supercells on 13 october 2001 exhibited a lightning hole during tornadogenesis the radar and ldar ii data indicated updraft weakening the height of the 30 dbz radar top began to descend approximately 10 min 2 volume scans before tornado touchdow

evolution of lightning activity and storm charge relative to dual doppler analysis of a high precipitation supercell storm abstracta high precipitation tornadic supercell storm was observed on 29 30 may 2004 during the thunderstorm electrification and lightning experiment observational systems included the oklahoma lightning mapping array mobile balloon borne soundings and two mobile c band radars the spatial distribution and evolution of lightning are related to storm kinematics and microphysics specifically through regions of microphysical charging and the location and geometry of those charge regions lightning flashes near the core of this storm were extraordinarily frequent but tended to be of shorter duration and smaller horizontal extent than typical flashes elsewhere this is hypothesized to be due to the charge being in many small pockets with opposite polarities of charge close together in adjoining pockets thus each polarity of lightning leader could propagate only a relatively short distance before reaching regions of unfavorable electric potential in the anvil however lightning extended tens of kilome

electrical and kinematic structure of the stratiform precipitation region trailing an oklahoma squall line abstract an electric field sounding through the transition zone precipitation minimum that trailed an oklahoma squall line on 18 june 1987 provides information about the electrical structure within a midlatitude trailing stratiform cloud a single doppler radar analysis concurrent with the flight depicts a kinematic structure dominated by two mesoscale flow regimes previously identified in squall line systems a strong midlevel front to rear flow coinciding with the stratiform cloud layer and a descending rear inflow that sloped from 6 5 km agl at the stratiform cloud s trailing edge to 1 5 km agl at the convective line electric field magnitudes as high as 113 kv m 1 were observed by the electric field sounding which reveals an electric field structure comparable in magnitude and complexity to structures reported for convective cells of thunderstorms the charge regions inferred with an approximation to gauss law have charge density magnitudes of 0 2 4 1 nc m 3 and vertical thicknesses of 130 1160 m