observed and numerically simulated structure of a mature supercell thunderstorm abstract through the interactive use of doppler radar analyses and a three dimensional numerical storm simulation the detailed structure of a supercell tornadic storm is analyzed this storm named the del city storm occurred in central oklahoma on 20 may 1977 the storm exhibits certain important features which are essential to maintaining its longevity and which promote the storm s transition to its tornadic phase these features are strongly influenced by the rotational character of the storm separates the precipitation from the updraft and which orients the resulting downdrafts to which reinforce low level convergence along the gust front and sustain the storm analyses of air parcel and rain trajectories within the storm provide a detailed visualization of this internal structure these trajectories reveal that air parcels rising through the cyclonically rotating updraft actually turn anticyclonically with height owing to the influence of the storm relative environmental wind field downdraft traj

collective effects of organized convection and their approximation in general circulation models abstract the collective effects of organized convection the environment were estimated using a two dimensional two way nested cloud resolving numerical model with a large outer domain 4500 km as initial conditions the authors used an idealized environment of the onset stage of the december 1992 westerly wind burst that occurred during the tropical oceans global atmosphere coupled ocean atmosphere response experiment two key aspects relating to convective parameterization were examined first the transports sources and sinks of heat moisture and momentum were derived using the model produced dataset in particular the total momentum flux compares well with moncrieff s dynamical theory second the bulk energetics of the cloud system were examined using the model produced dataset the authors found that the shear generation of kinetic energy is comparable to the buoyancy generation and dominates the sum of the buoyancy and water loading generation this means that in addition to the thermodynam

numerical simulation of a midlatitude squall line in two dimensions abstract a two dimensional anelastic cloud model was used in attempts to numerically replicate the observed structure of a midlatitude squall line initial conditions were adapted from observations of the 22 may 1976 oklahoma line model simulations were made with and without considering the ice phase of water these model storms have within the constraints of the model s geometry replicated the basic multicellular character and general line normal airflow typical of these lines in addition the structure and intensity of the subcloud cold air pool and the propagation speeds developed by the storms appear to be reasonable further it was found that the initial conditions chosen resulted in model storms which were not only long lasting but also essentially repetitive indicating that a state of quasi equilibrium had been attained the storms did not decay because the environmental conditions ahead of the storms were favorable and essentially unchanged during the course of the simulations the inclusio

reaction transport modelling of a freshening aquifer tertiary ledo paniselian aquifer flanders belgium abstract the semi confined ledo paniselian eocene aquifer in flanders belgium is recharged in the areas with higher topography where it is covered by the bartonian clay initial conditions in these sediments were marine hydrogeochemical modelling of the groundwater type distribution in the aquifer with a reaction transport model considers recharge of fresh cahco3 water by downward flow through the overlying clay in the aquifer in an upstream direction progressively more freshened water types are found a typical nahco3 type occurs upstream of the brackish fresh water interface the different stages of cation exchange produce a chromatographic sequence of groundwater types which agrees well with observations cation exchange processes occurring mainly in the percolated clay result in a na increase and peaks of k nh 4 and mg2 in the aquifer which are spatially separated as a result of groundwater flow calcite equilibrium gypsum dissolution in the clay and sulphate reduction in the aquifer have also been included in the modelling

structure and evolution of numerically simulated squall lines abstract using a three dimensional numerical cloud model we investigate the effects of vertical wind shear on squall line structure and evolution over a wide range of shear magnitudes depths and orientations relative to the line we find that the simulated squall lines are most sensitive to the magnitude of the component of shear perpendicular to the line and that we may reproduce much of the range of observed structures by varying this single parameter for weak shear a line of initially upright to downshear tilted short lived cells quickly tilts upshear producing a wide band of weaker cells extending behind the surface outflow boundary for moderate to strong shear the circulation remains upright to downshear tilted for longer periods of time with vigorous short lived cells confined to a relatively narrow band along the system s leading edge at later times however these systems may also weaken as the circulation tilts upshear for strong deep shears oriented obliquely to the line the squal

a 40 ka record of temperature and permafrost conditions in northwestern europe from noble gases in the ledo paniselian aquifer belgium the ledo paniselian aquifer in belgium offers unique opportunities to study periglacial groundwater recharge during the last glacial maximum lgm as it was located close to the southern boundary of the ice sheets at that time groundwater residence times determined by 14c and 4he reveal a sequence of holocene and pleistocene groundwaters and a gap between about 14 and 21 ka indicating permafrost conditions which inhibited groundwater recharge in this paper a dataset of noble gas measurements is used to study the climatic evolution of the region the derived recharge temperatures indicate that soil temperatures in the periods just before and after the recharge gap were only slightly above freezing supporting the hypothesis that permafrost caused the recharge gap the inferred glacial cooling of 9 5 c is the largest found so far by the noble gas method yet compared to other palaeoclimate reconstructions for the region recharge temperatures deduced from noble gases for the cold periods tend to be rather high most likely this is due to soil temperatures being several degrees higher than air temperatures during periods with extended snow cover thus the noble gas derived glacial cooling of 9 5 c is only a lower limit of the maximum cooling during the lgm some samples younger than the recharge gap are affected by degassing possibly related to gas production during recharge in part of the recharge area especially during times of melting permafrost the findings of this study such as the occurrence of a recharge gap and degassing related to permafrost and its melting are significant for groundwater dynamics and geochemistry in periglacial areas copyright 2010 john wiley sons ltd

effects of vertical wind shear on the cumulus transport of momentum observations and parameterization abstract dynamical effects of organized cumulus convection on its environment with vertical wind shear are studied analyses of the wind field and momentum budget residual for mesoscale convective systems observed during sesame and pre storm reveal systematic differences in the vertical transport of horizontal momentum between mesoscale convective complex mcc and squall line cases in the mcc a distinct minimum in wind speed appears over the area of intense convection and the momentum budget residual acts to decelerate the environmental wind and to reduce the upper level vertical shear therefore the inferred vertical transport of momentum in the mcc is downgradient in the upper layer on the other hand in the squall line there is no wind speed minimum and the upper level vertical shear of the line normall component of the environmental wind increases as convection develops thus the vertical transport of momentum normal to the squall line is upgradient in the upper layer although the transport of

three dimensional simulations of cumulus congestus clouds on gate day 261 abstract schlesinger s 1978 three dimensional cumulus model is applied to showering congestus clouds on day 261 of gate adjustments are made to the microphysical and turbulent parameterizations the former to be consistent with coalescence growth of warm rain a cylindrical initial perturbation in approximate agreement with gate observations was run with a characteristic thermodynamic sounding and four different wind profiles the four wind profiles were 1 the observed three dimensional flow 2 uniplanar winds 3 unidirectional winds and 4 zero synoptic flow a fifth run was made with the observed three dimensional wind and a slightly moistened destabilized sounding characteristic of a cumulonimbus environment model results are compared with each other and with observations to analyze the effects of varying shear and altered sounding relationships between shear mesovortices and dynamic entrainment are examined as well as the model clouds impact on the environment as a function of shear the

complex effective depth of the ocean bottom the effective depth approximation is extended to allow for energy loss on reflection at an attenuating low shear speed solid ocean bottom the resulting complex effective depth leads to a novel and efficient algorithm for finding the exact complex eigenvalues for the normal modes in isovelocity water over an attenuating fluid or solid bottom while avoiding numerical searches the method gives rapid normal mode solutions for shallow water propagation including the effects of leaky modes

saltwater intrusion and nitrate pollution in the coastal aquifer of dar es salaam tanzania dar es salaam quaternary coastal aquifer is a major source of water supply in dar es salaam city used for domestic agricultural and industrial uses however groundwater overdraft and contamination are the major problems affecting the aquifer system this study aims to define the principal hydrogeochemical processes controlling groundwater quality in the coastal strip of dar es salaam and to investigate whether the threats of seawater intrusion and pollution are influencing groundwater quality major cations and anions analysed in 134 groundwater samples reveal that groundwater is mainly affected by four factors dissolution of calcite and dolomite weathering of silicate minerals seawater intrusion due to aquifer overexploitation and nitrate pollution mainly caused by the use of pit latrines and septic tanks high enrichment of na and cl near the coast gives an indication of seawater intrusion into the aquifer as also supported from the na cl signature on the piper diagram the boreholes close to the coast have much higher na cl molar ratios than the boreholes located further inland the dissolution of calcite and dolomite in recharge areas results in ca hco3 and ca mg hco3 groundwater types further along flow paths ca2 and na ion exchange causes groundwater evolution to na hco3 type from the phreeqc simulation model it appears that groundwater is undersaturated to slightly oversaturated with respect to the calcite and dolomite minerals the results of this study provide important information required for the protection of the aquifer system

a study of the response of deep tropical clouds to mesoscale processes three dimensional numerical experiments abstract a three dimensional numerical cloud ensemble model has been developed to investigate the collective feedback effects of cloud systems on the large scale environment an observed large scale lifting is imposed continuously in the model small amplitude random perturbations in the form of temperature fluctuations are also continuously fed into the model at low levels to simulate random thermals over the tropical ocean the model allows several clouds of various sizes to develop simultaneously inside the domain an integration of the model is made for six hours of simulated time in order to allow large numbers of convective clouds to develop following each simulation the collective feedbacks of cloud systems on the large scale temperature moisture and horizontal momentum fields are computed horizontal and time averages of various relevant variables are also computed to elucidate the statistical properties of the clouds the model was applied to a case of a well defined itcz rainband over the eas

the effective depth approximation for sound propagation in shallow water over a sediment layer and a hard rock basement this paper considers an effective depth approximation for an ocean waveguide with a bottom in which a sediment layer overlies a hard rock basement the primary application of this approximation is to the calculation of three dimensional propagation over sloping bottoms it is shown that at low frequencies the resonances in the sediment layer cause the effective depth to oscillate rapidly at high frequencies when the attenuation in the sediment layer becomes significant the effective depth tends to zero and the acoustic propagation is dependent on the modal attenuation this can be incorporated into the effective depth theory by allowing the effective depth to be a complex number defined in terms of the impedance of the water sediment interface comparison with exact calculations shows that the effective depth approach gives a good approximation for the lower order modes but does not include the effects of the higher order modes which are shown to make a significant contribution to the fine scale stru

parameterization of convective precipitation in mesoscale numerical models a critical review abstract current approaches for incorporating cumulus convection into mesoscale numerical models are divided into three groups the traditional approach utilizes cumulus parameterization at convectively unstable points and explicit nonparameterized condensation at convectively stable points the fully explicit approach uses explicit methods regardless of stability the hybrid approach parameterizes convective scale updrafts and downdrafts but detrains a fraction of parameterized cloud and precipitation particles to the grid scale this allows the path and phase changes of such particles to be explicitly predicted over subsequent time steps the traditional approach provides the only alternative for numerical models with grid spacing too large to resolve mesoscale structure as grid spacing falls below 50 km the traditional approach becomes increasingly likely to violate fundamental scale separation requirements of parameterization particularly if mesoscale organization of convection is parameterize

a numerical study of the vertical transport of momentum in a tropical rainband abstract the vertical transport of horizontal momentum in a convective tropical rainband is studied using a two dimensional cloud ensemble model twelve simulations are made under the same large scale conditions the vertical transports of v momentum parallel to the rainband are essentially the same in all of the simulations even though the structure of the clouds is different in each of the runs the magnitude of the v memomentum transport by clouds is fairly large it takes only half of a day to smooth out the tropical low level easterly jet parallel to the rainband if no other processes am operating the vertical transports of u momentum perpendicular to the rainband are quite different in all of the simulations this difference can be explained by the dissimilarities in the distributions of horizontal momentum associated with various cloud configurations the simulated vertical transports of horizontal momentum are compared with those computed with the schneider and lindzen scheme the results su

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

numerical simulation of tropical cumulus clouds and their interaction with the subcloud layer abstract a two dimensional numerical model suitable for simulating an ensemble of cumulus clouds has been developed it differs from similar models in its greater emphasis on turbulent processes in the boundary layer and in clouds in the model cloud scale dynamics are coupled with a third moment turbulence closure a turbulence scale condensation scheme is used to parameterize the cloud water mixing ratio we studied the response of tropical cumulus clouds to imposed large scale vertical advection by doing two simulations one with upward large scale vertical velocity the disturbed case and one without any the undisturbed case but otherwise identical deep cumulus clouds formed in the disturbed case while only shallow clouds formed in the undisturbed case time averaged and horizontally averaged heat and moisture budgets from the simulations show that the subcloud layer scl in the disturbed case was strongly affected by cumulus circulations and rain evaporation both cumulus updrafts and down

modeling study of a tropical squall type convective line abstract a multidimensional and time dependent cloud scale model is used to investigate the dynamic and micro physical processes associated with convective and stratiform regions within a tropical squall type convective line the evolution of the total convective and stratiform portions of rainfall is also estimated by using model output a three dimensional version of the model covers a horizontal domain about 96 96 km2 frequently the horizontal extent of an observed stratiform region is over a few hundred kilometers therefore a two dimensional version of the model with a 512 km horizontal length is also used to incorporate a complete stratiform region two dimensional model result recapture many interesting features as observed in particular the fractional portion of stratiform rain as well as its fractional area coverage are in good agreement with observations a significant amount of ice particles melted to rain near the freezing level in the trailing part of the modeled squall system during i

a palaeoclimatic record from the ledo paniselian aquifer in belgium indications for groundwater recharge and flow in a periglacial environment abstract understanding the mechanisms of groundwater recharge and flow in periglacial environments remains enigmatic methane releases from melting permafrost in a warming world are a hot topic in contemporary climate research noble gas analyses have been proposed to study methane driven degassing in lakes as well as in permafrost terrains the ledo paniselian aquifer is shown to provide an analogue for such processes from the last period of global warming after the lgm it is postulated that recharge from methane bubbling thermokarst lakes to groundwater took place and can be recognized by a specific signature in the groundwater the ledo paniselian aquifer offers unique opportunities to study periglacial groundwater recharge during the lgm as it was located in the permanent permafrost area to the south of the ice sheet at that time it contains holocene and pleistocene groundwaters which are separated by a recharge gap referring to permafrost during the lgm this is confirmed by noble gas temperature ngt data in the samples immediately upstream of this gap several potential indicators for the specific methanogenic recharge conditions from thawing permafrost are being put forward based on noble gases chemical and isotopic data immediately upstream of the age gap groundwaters show strikingly deviating hco3 so42 13c 18o and 2h compared to the regional trend while their 14c model ages are too young and ngts seem too high these samples are affected by degassing which is not an artefact of sampling and which according to their radiogenic helium concentrations occurred early on during recharge or shortly thereafter the origin of these samples could be explained by the conditions prevailing as the permafrost thawed and groundwater recharge could resume it is inferred that the degassed samples were infiltrated during or shortly after the lgm when recharge from methane bubbling thermokarst lakes to groundwater took place associated to the melting permafrost

cloud resolving modeling of tropical cloud systems during phase iii of gate part i two dimensional experiments abstract a formal framework is established for the way in which cloud resolving numerical models are used to investigate the role of precipitating cloud systems in climate and weather forecasting models emphasis is on models with periodic lateral boundary conditions that eliminate unrealistic numerically generated circulations caused by open boundary conditions in long term simulations defined in this formalism is the concept of large scale forcing and the cloud environment interactions that are consistent with the periodic boundary conditions two dimensional numerical simulations of the evolution of cloud systems during 1 7 september 1974 in phase iii of the global atmospheric research program atlantic tropical experiment gate are conducted based on the above formalism a simple technique is used to force an anelastic cloud resolving model with evolving large scale horizontal wind field and large scale forcing for the temperature and moisture obtained from the gate data the 7 day period selected i

complex reflection phase gradient as an inversion parameter for the prediction of shallow water propagation and the characterization of sea bottoms in this paper a quantity is proposed referred to as the complex reflection phase gradient whose use in a matched field inversion procedure allows for the rapid extraction of first order geo acoustic information about the sea bottom it is based on the observation that at low grazing angles the reflection phase and bottom loss for a wide range of sea bottom types commonly exhibits an approximate linear relationship to the vertical component of the acoustic wave number at the seabed the real part of this quantity specifies the rate at which the reflection phase varies with vertical acoustic wave number while the imaginary part quantifies the rate of change of bottom loss despite being defined with just two real parameters it is shown that it provides an accurate prediction of the sound field for a wide range of bottom types in addition its measurement permits an estimate to be made for the input impedance to the seabed in the zero grazing angle limit and in the case of a homogeneous elastic half space of known density the compressional and shear wave speed the main advantage of the two parameter seabottom representation is that each parameter is readily inverted from comparatively few acoustic pressure measurements the usefulness of the technique is illustrated by the results from computer simulated acoustic pressure measurements made at just eleven sensors in a simple shallow water channel and results from a 10 cm deep laboratory channel at frequencies between 10 khz and 75 khz

a coherent ray model of plane wave reflection from a thin sediment layer plane wave reflection from a thin elastic solid sediment layer over a hard basement is modeled by a simple ray model that includes only two ray families a family of compressional waves that transit the sediment layer any number of times before returning to the water and a similar family that includes doubly converted compressional waves the latter are compressional waves that are converted to shear waves at the basement make one or more round trip transits of the sediment layer and are converted back into compressional waves at the basement the theory is applied to homogenous sediment layers with low shear speeds and it provides valuable insight into the physical processes governing reflection from several seabed environments of interest the coherent sum of all the rays belonging to these families leads to a closed form expression for the overall reflection coefficient that includes several characteristic features also simulated by full wave theoretical numerical models such as bottom loss res

geochemical and palaeohydrological controls on the composition of shallow groundwater in the netherlands with the exception of the south of the country the netherlands has a strong bipartite hydrogeology the holocene part with a coastal dune belt and confining top layer of clay and peat further inland and the pleistocene where thick phreatic aquifers dominate this research aimed to ascertain the geochemical and palaeohydrological controls on the composition of shallow groundwater in 27 regions close to 6000 groundwater analyses were grouped and interpreted in terms of 1 salinity 2 redox status 3 acid base and carbonate status and 4 natural nutrients nh4 and po4 the a priori classification into geographical regions and geological formations revealed many statistically significant differences in medians even for geologically or geographically related data groups the compound specific interpretation indicates that there are geogenically controlled systematic differences in groundwater composition at the regional scale the imprint of the geological sediments on the groundwater composition decreases in the order marine estuarine via limnological fluvial to aeolian the imprints with respect to ph and carbonate status natural nutrients and redox status are not necessarily interrelated the vertical stratification in groundwater composition turns out to be often limited at the regional scale due to mutual occurrence of infiltrating and exfiltrating groundwater in regions and either the presence of a highly reactive holocene confining top layer or temporal changes in contamination in the holocene part the salinity is controlled by the palaeoenvironmental conditions during the holocene and by the recharge origin the average cl concentration decreases from estuarine via lagoonal to the former zuider sea which was a bay the most reduced states and also the highest nutrient concentrations and highest co2 pressure are related to the presence of holocene marine sediments in the confining top layer degradation of marine derived organic matter as a nutrient source is likely more intense in the holocene deposits than that of peat and sedimentary remnants of terrestrial plants a broad range in ph carbonate status and redox status is encountered in the pleistocene part here the palaeohydrological evolution in terms of carbonate leaching together with the geological controls on the calcareous nature of the shallow sedimentary deposits cause regional differences in ph calcite saturation and silicate weathering one region with late pleistocene limnological deposits has deviating groundwater characteristics and appears more similar to the holocene part of the netherlands furthermore reactive fe is not abundant in all pleistocene fluvial sediments nor is it maximally mobilised as not all anoxic groundwater in these sediments is siderite saturated this leads to considerable intra and inter region variability 2013 elsevier ltd

heat moisture and momentum budgets of isolated deep midlatitude and tropical convective clouds as diagnosed from three dimensional model output part ii sensitivity to ice phase and small changes in ambient shear strength and low level moisture supply abstract this project uses a three dimensional anelastic cloud model with a simple ice phase parameterization to evaluate the feedback of isolated deep convective clouds over a horizontal scale comparable to one grid cell in typical mesoscale numerical weather prediction models a more specific focus in this paper is the sensitivity of the feedback to modest changes in the initial vertical wind shear intensity and low level moisture supply as well as to the ice phase two parallel sets of comparative simulations are run for a quasi steady severe oklahoma supercell thunderstorm in strong vertical wind shear versus a weaker less persistent and narrower tropical atlantic cumulonimbus with a slowly decaying and pulsating updraft in much weaker shear the horizontal reynolds averaging approach of anthes is adopted to diagnose the budgets for heat moisture and horizontal momentum several similarities and differences between the midlatitude and tropical control experiments were delineated in part i the ma

the recharge of glacial meltwater and its influence on the geochemical evolution of groundwater in the ordovician cambrian aquifer system northern part of the baltic artesian basin abstract the geochemical evolution of groundwater in the ordovician cambrian aquifer system in the northern part of the baltic artesian basin bab illustrates how continental glaciations have influenced groundwater systems in proglacial areas the aquifer system contains water that has originated from various end members recent meteoric water glacial meltwater and relict na cl brine the saline formation water that occupied the aquifer system prior to the glacial meltwater intrusion has been diluted by meltwaters of advancing retreating ice sheets the diversity in the origin of groundwater in the aquifer system is illustrated by a wide variety in 18 o values that range from 11 to 22 5 these values are mostly depleted with respect to values found in modern precipitation in the area the chemical and isotopic composition of groundwater has been influenced by mixing between waters originating from different end members in addition the freshening of a previously saline water aquifer due to glacial meltwater intrusion has initiated various types of water rock interaction e g ion exchange carbonate mineral dissolution