convectively forced internal gravity waves results from two dimensional numerical experiments two dimensional numerical simulations were performed to investigate the nature of tropospheric internal gravity waves of the type which are observed to occur above active thermal convection over an unstable boundary layer these gravity waves are believed to be excited by a combination of pure thermal forcing and by the boundary layer eddies and cumulus clouds acting as obstacles to the flow in the presence of mean environmental wind shear large amplitude internal gravity waves were obtained in the simulations with amplitudes and horizontal scales similar to the 12 june 1984 aircraft observations over western nebraska this was a day with strong wind shear in the lowest 3 km above the ground and with scattered cumulus clouds topping the boundary layer the simulations show that there is significant difference between the early time solutions as might be predicted by linear theory and late time solutions for the boundary layer eddy structure a layer interaction occurs in which gravity waves of the stable layer are excited by the boundary layer convection there is evidence to suggest that this layer interaction occurs both with and without shear but that it is stronger in the presence of low level shear results indicate that shear or the obstacle effect is a more efficient generator of gravity waves than is the pure thermal forcing the simulations show that the gravity waves initially forced by the boundary layer eddies lead to a feedback mechanism that acts to organize the boundary layer eddies and the cumulus convection the solutions suggest that the character of fair weather convection moist or dry is a non local problem involving at times the full depth of the troposphere the clouds produced in the simulations have very little influence on the wave field or boundary layer eddy structure as they are relatively small cumuli on the other hand the clouds are strongly influenced by the interactions between the wave and eddy fields upshear growth of cumulus clouds similar to that which is frequently observed in nature is reproduced in the simulations the development of feeder feeder is used here in a dynamical sense only clouds on typically the upshear side of the cloud is found to be a result of the interaction between the gravity wave field and the dry and moist convection the relative phase velocity between the gravity waves and the cloud plays a crucial role in determining the character of the cumulus cloud growth in the present simulations these simulations suggest that the dynamics both internal and external to the boundary of a cumulus cloud is a complicated mix between wave dynamics and the usually considered convection dynamics a brief discussion of the implications of the present results to cloud boundary baroclinic instability dynamics is also presented

hurricanes and climate in the caribbean during the past 3700 years bp a multiproxy analysis of lacustrine sediments cored in grand case pond at saint martin north of the lesser antilles archipelago reveals three distinct climatic periods for the last 3700 years from 3700 to 2500 yr cal bp and from 1150 yr cal bp to the present carbonate mud deposition occurred in connection with pond lowstands these periods were also punctuated by severe drought events marked by gypsum laminae and hurricane landfalls leading to marine sand inputs into the pond the intermediate time interval from 2500 to 1150 yr cal bp is typified by black organic mud deposition suggesting that hypoxic to anoxic conditions prevailed at the pond bottom these were probably linked with a perennial pond highstand and reflect more uniform and wetter climatic conditions than today the carbon isotopic composition of the ostracod perissocytheridea bisulcata shows that the lowest 13c values are recorded during the hypoxic periods as a consequence of bacterial recycling of isotopically depleted org

a dual doppler radar study of longitudinal mode snowbands part i a three dimensional kinematic structure of meso γ scale convective cloud systems within a longitudinal mode snowband abstract observations of snowstorms were carried out around ishikari bay hokkaido japan from december 1991 to february 1992 on 15 january 1992 a longitudinal mode snowband was observed by a dual doppler radar system the snowband was composed of meso scale 20 km convective cloud systems the three dimensional kinematic structures and organization processes of the meso scale systems were studied in detail strong band parallel winds appeared in the upper north rear of the meso scale systems since their speed was faster than that of the cellular radar echoes the winds formed rear to front currents increasing in volume and speed the rear to front currents developed into meso scale and penetrated toward the lower front and lower south of the systems consequently the rear to front currents caused strong meso scale convergence and the enhancement of updrafts at their leading edges the transport of the band parallel momentum greatly contributed to the successive development of new convec

a method for estimating momentum fluxes of deep precipitating convection using profiling doppler radar 1 this paper outlines a novel approach for obtaining estimates of momentum fluxes for deep precipitating convection this method relies on data collected by vertically incident profiling doppler radars but may be applied to aircraft or space based radar systems this work represents a significant step forward toward the evaluation of convective momentum transports across regional scales using data from operational satellite systems e g the tropical rainfall measuring mission trmm instrument the new method is demonstrated as convective momentum flux terms and are estimated for deep convection occurring during the third convection and atmospheric moisture experiment camex 3 which was conducted during august and september 1998 these terms contain the most information on the momentum transported in the updrafts and downdrafts of cumulus convection key to the method is that the tilt and orientation of the convective drafts relative to earth s surface are quantified from radar information into perturbation horizontal wind components once an accurate storm motion estimate is provided for perturbation vertical velocities the evaluation of drop size diameters from radar reflectivities and the subsequent evaluation of terminal velocities of hydrometeors and draft motions are necessary information toward applying this technique doppler velocity measurements from the upward looking rd 69 disdromet disdrometer joss waldvogel disdrometer calibrated 915 mhz profiler are used to test the procedures for estimating w the method is tested against numerically simulated convection using the university of wisconsin nonhydrostatic modeling system uw nms mesoscale and cloud resolving model the radar estimated momentum fluxes are found to match in sign and relative magnitude through the depth of the convective clouds studied compared to those obtained from model simulation

mid neoproterozoic 830 750 ma isotope stratigraphy of australia and global correlation abstract carbon and strontium isotope studies of numerous drillcore and outcrop sections within individual australian neoproterozoic basins in a tectonic and sedimentologic framework have allowed the construction of isotopic records for the interval 830 750 ma this combined with the discovery of a restricted stratigraphic range for the acritarch cerebrosphaera buickii in mid neoproterozoic strata of australia makes possible correlation with successions in canada namibia and spitsbergen that part of the mid neoproterozoic succession in spitsbergen which contains c buickii was previously considered younger than the sturtian glacial epoch but this study has indicated that the akademikerbreen group is older than that a correlation scheme is proposed along five tie lines and is based on a correlation between australia and canada at 830 ma and australia spitsbergen and namibia at 760 ma the lowest ever recorded seawater 87 sr 86 sr ratios in the upper shaler supergroup of canada 0 70561 0 70670 compare with ratios in evaporites of the 830 ma bitter springs formation amadeus basin australia 0 70568 0 70720 comparable 13 c carb records support a correlation at about 760 ma a correlation is made between the heaviest mid neoproterozoic 13 c carb values of 7 2 upper burra group adelaide rift complex and 8 1 kanpa formation western officer basin in australia 8 5 in spitsbergen backlundtoppen formation akademikerbreen group and 8 5 in namibia ombombo subgroup congo craton the stratigraphic range of c buickii in australia and spitsbergen supports this correlation a compilation of 13 c carb and 87 sr 86 sr records between about 830 and 750 ma from australia canada namibia and spitsbergen reveals numerous negative excursions which are not associated with preserved glacial deposits it is necessary therefore to consider additional models to the snowball earth to explain the large and rapid carbon isotopic excursions

observations of longitudinal rolls in a near neutral atmosphere abstract aircraft and satellite data an used to study the structure of longitudinal roll vortices in a nearly neutral zi l 1 2 where zi is the inversion height and l is the monin obukhov length boundary layer over the ice covered bering sea during february steam fog formed over cracks and leads in the ice was used as a tracer to delineate the various scales of roll motion seen in satellite images the satellite information combined with aircraft data collected by the noaa p 3 indicated the presence of a hierarchy of roll vortex motions it is suggested that interactions of the various scales of motion resulted in certain scales dominating in one area and other scales dominating in another two kilometer wavelength variations an attributed to the inflection point instability mechanism while 12 15 km variations seen to have been reinforced by the upstream topography on the chukotka peninsula organization of the fog banks on scales of 30 km was also present and may be attributable to resonant subharm

a single doppler radar study of horizontal roll convection in a lake effect snow storm abstract the boundary layer structure and circulation over lake michigan were probed with a doppler radar and an aircraft during a lake effect snow storm on 9 december 1978 evidence is presented that observed wind parallel echo bands and satellite cloud bands resulted from horizontal roll convection in the boundary layer the organization of precipitation within the roll field is discussed and the roll geometry is compared with cases of dry horizontal roll convection reported in the literature

a stable isotope study of microbial dolomite formation in the coorong region south australia abstract the coorong region of south australia is a classic location for the study of modern dolomite formation a number of ephemeral lakes here contain dolomite whilst other lakes in close proximity contain aragonite with minor magnesite our previous field and experimental work has revealed that sulphate reducing bacteria play a key role in overcoming kinetic inhibitors to dolomite formation in this area we here use a combination of oxygen carbon and sulphur isotopes to help to further constrain the processes leading to dolomite precipitation in modern hypersaline environments we report isotopic data from four dolomite and three aragonite containing lakes 34 s data for dissolved lake and porewater sulphates reveal 34 s enrichment of residual sulphate in all seven lakes consistent with bulk closed system microbial fractionation by sulphate reducing bacteria calculations using temperature dependent oxygen isotope fractionation factors between dolomite and the solution from which it precipitated are consistent with a strong microbial component to carbonate precipitation in these lakes a comparison of 18 o data from dolomite and aragonite sediment and from their associated lakewaters indicates seasonal mineral precipitation occurs late in the evaporative cycle of the lakes within a yoghurt like saturated mud horizon coincident with the zone of sulphate reduction surprisingly 13 c data from the carbonates do not show the characteristic wide range of values often associated with organogenic carbonates but instead show that these bacterially mediated precipitates incorporate carbon primarily from the inorganic lakewater reservoir partially diluted by an organic component a coorong type microbial model of dolomite formation may be particularly applicable to for example precambrian sedimentary dolomites where benthic microbial communities which are today restricted to extreme environments dominated the shallow marine ecosystem prior to the emergence of metazoa

thermodynamic variability within the convective boundary layer due to horizontal convective rolls abstract data from the convection and precipitation electrification cape experiment conducted during the summer of 1991 are used to examine and quantify the horizontal variability of temperature and moisture within the convective boundary layer cbl potential temperature variations were only about 0 5 k while variations in water vapor mixing ratio values of 1 5 2 5 g kg 1 were observed throughout the cbl using radar aircraft and sounding data it is shown that horizontal convective rolls are the likely cause of these variabilities the enhanced moisture occurred within the roll updraft regions thus rolls were transporting moist air from the surface upward the observed cloud base heights obtained from cloud photogrammetry were produced from the highest moisture values within the roll updraft regions since the roll ascending branches contained moisture values that were most representative of the observed cloud base heights it is likely that measurements from within the roll updrafts would prov

observations of the sea breeze front during cape part i single doppler satellite and cloud photogrammetry analysis abstract an analysis of two sea breeze events on 6 august an onshore flow event and 12 august an offshore flow event 1991 is presented using single doppler observations satellite images and cloud pictures collected during the convection and precipitation electrification cape experiment documentation of the alongfrontal variability at the leading edge of the sea breeze circulation is presented for the first time the horizontal structure of the front was strongly modulated by the near perpendicular intersections of horizontal convective rolls developing in the ambient air out ahead of the sea breeze on 12 august these intersection points also appeared to be preferred locations for cloud development along the front horizontal convective rolls were also documented on 6 august however their orientation was nearly parallel to the sea breeze front as a result extended sections of these rolls appeared to have merged with the front as it propagated inland rather than having distinct intersection po

a new location of holocene dolomite formation lake hayward western australia holocene dolomite forms in the sediment of lake hayward a small permanent hypersaline lake in the clifton preston lakeland system western australia the geomorphological setting of dolomite formation in lake hayward is similar to the coorong region in south australia unlike in the coorong region dolomite in lake hayward does not form as a direct precipitate from the lake water but is of diagenetic origin this can be deduced from the following features 1 the dolomite occurs only below 60 70 cm from the sediment water interface 2 dolomite occurs as luminescing cement and 3 dolomite has pristine well formed rhomb shaped crystals the source of magnesium for dolomitization is probably from the concentration on inflowing groundwater by evaporation and the selective removal of calcium by chemical and biological aragonite calcite precipitation

stable isotope analyses of no2 no3 and n2o in the hypersaline ponds and soils of the mcmurdo dry valleys antarctica abstract nitrous oxide n 2 o is produced in significant quantities in the soils and lakes of the mcmurdo dry valleys antarctica unraveling the mechanisms of n 2 o production in these soils and ponds is of great interest due to the extreme arid and cold conditions which are hostile to life recent studies have shown production of n 2 o having unique stable isotopic signatures in certain dry valley soils through abiotic reduction of nitrate no 3 and nitrite no 2 on active surfaces of fe ii containing minerals a process known as chemodenitrification in this study 15 n and 18 o of n 2 o no 2 and no 3 as well as the n 2 o site preference sp were measured at three sites to evaluate the role of chemodenitrification in n 2 o production the 15 n and 18 o values in no 3 indicated an atmospheric source while 15 n values in no 2 150 were indicative of abiotic reactions instead of finding unique sp values for n 2 o at dry valley sites sp values mostly fell within the range associated with microbial n 2 o production mechansims the 15 n and 18 o of n 2 o were also within a range of values expected for various biological n 2 o production mechanisms however efforts to detect biological activity in don juan pond djp a hypersaline pond in the wright valley have been largely unsuccessful we consider two possible scenarios for n 2 o production at djp 1 abiotic production in the pond or 2 biological production in nearby freshwater and transport to the pond although little is known about the isotopic systematics of abiotic n 2 o production these results indicate that if the observed n 2 o was produced by an abiotic mechanism its isotopic signature is indistinguishable from that expected from a mixture of several microbial processes and thus the formation pathway cannot be determined from isotopic composition alone

multiproxy paleoaltimetry of the late oligocene pliocene oiyug basin southern tibet the stable isotope compositions of carbonate and organic samples from the oiyug basin in southern tibet allows for model calculations of the oligocene to pliocene paleoelevation of the south central tibetan plateau we measured the oxygen isotope composition of pedogenic and lacustrine calcite dolomite and siderite and the hydrogen isotope composition of n alkanes from plant waxes to reconstruct the 18o and d values of oiyug basin paleometeoric water calculated water isotope values from oiyug basin carbonate and organic samples respectively are in close agreement suggesting the preservation of an unaltered paleometeoric water isotopic signal in these archives late oligocene middle miocene paleoelevation estimates from groundwater pedogenic calcite and lacustrine dolomite indicate basin elevations of 4 1 km 1 2 1 6 km plant wax n alkanes d and lacustrine siderite 18o compositions of middle miocene 15 ma samples indicate paleoelevations of 5 1 km 1 3 1 9 km this estimated elevation is similar to the 5 4 km paleoelevation estimate based on fossil floral physiognomy from the same stratigraphic interval calculated late miocene pliocene paleoelevation estimates derived from the 18o composition of lacustrine marls and carbonate siderite concretions as well as the d from plant wax n alkanes indicate a mean elevation of 5 5 km 1 4 2 0 km at 5 ma although calculated mean paleoelevations for the oiyug basin all fall within the errors associated with the model calculations the close agreement of the different paleoelevation proxies provides an additional degree of confidence in the fidelity of the calculated paleoelevations calculated paleoelevations indicate a possible increase in oiyug basin elevations of 1 4 km between the early and late miocene given the modern oiyug basin elevation of 4 3 km study results allow for a possible 1 km decrease in elevation since the early pliocene these findings in conjunction with other tibetan paleoaltimetry studies are consistent with tectonic models supporting high elevations of the tibetan plateau since the initiation of india asia collision during the eocene and subsequent late cenozoic extensional collapse

hydromagnesite replacement of biomineralized aragonite in a new location of holocene stromatolites lake walyungup western australia holocene stromatolites are described from lake walyungup a coastal hyposaline lake in south western australia at summer low water this groundwater fed depression comprises two permanent shallow water bodies and an ephemeral southern pool set within an areally extensive littoral zone of variably cemented carbonate crust up to 5 m of organic rich carbonate mud has been deposited within each of these basins in less than 7000 years stromatolites rim the water bodies with individual columns up to 2 m tall stromatolite capped tepee structures in subparallel alignment are widespread in the littoral crust suggesting a linkage between stromatolite growth and zones of groundwater discharge lake walyungup stromatolites regardless of external morphology and setting are coarsely laminated and have aragonitic mesoclot microfabrics these microfabrics are similar to those from lithified portions of active thrombolitic microbialites from nearby lake clifton hydromagnesite is a minor to subdominant phase up to 47 wt of the carbonate mineral assemblage in lake walyungup it occurs mainly in the littoral zone as a diagenetic replacement of precursor aragonite particularly within the mesoclot fabric of stromatolites but also in sediments strandline and dune sand crusts derived mainly from erosion of stromatolites in contrast with nonreplaced and impermeable inorganic aragonitic cements stromatolite mesoclots are microper meable micropermeability is inferred to facilitate hydromagnesite diagenesis dolomite is also present in minor amounts as a pore fill in stromatolites and as a subdominant to dominant up to 100 wt phase in thin mudcracked micrite layers within the crust package the layered dolomite may be precipitated directly from the lake water major element abundance of the lake water is na mg2 k ca2 for cations and cl so42 hco3 co32 for anions compared to other nearby coastal lakes lake walyungup has a high ph 9 0 and an extremely high molar mg ca ratio of 90 groundwater in the area has a mg ca ratio generally less than 1 the unusual mg ca ratio in lake walyungup is partially a result of in lake processes with additional minor contribution of mg2 sourced from basal marine sand because no mg rich bedrock source has been found in the region

hydrochemistry and nutrient cycling in yalgorup national park western australia lakes clifton hayward and preston western australia are part of an east west chain of 11 ground water fed lakes that make up the clifton preston lakeland system their hydrochemistries exhibit a large range in salinities 14 207 kg m 3 total dissolved solids tds both seasonally within each lake and among lakes the chemistry of all the lake waters in the system is an nacl so4 brine similar to seawater composition however all three lakes have less so4 br sr mn and probably fe than seawater the concentrations of ca and hco3 vary seasonally the variation of ca and hco3 in all three lakes suggests that calcium carbonate is precipitated during the late summer and early autumn when evaporation is most intense in the upper and lower water layers of lake hayward the concentrations of conservative ions such as na cl and k decrease in winter and increase in summer indicating dilution by winter rain and concentration via evaporation in the summer concentrations of total fe and mn in the lower layer increase in winter suggesting release from the sediments and or decay of the benthic microbial mat on the lake floor tin total inorganic nitrogen po4 p ratios and tn total nitrogen po4 p ratios are high in all lakes suggesting that p is the limiting nutrient in the system observed increases of the macroalgae cladophora in lake clifton suggest that any potential increase in the po4 load to the lake will be taken up by the invigorated growth of these algae and may not be detected by measuring lake water po4 concentrations the increased growth of cladophora is detrimental to the health of the well established microbialite community on the eastern shore of lake clifton

the effects of water temperature stratification and biological activity on the stable isotopic composition and timing of carbonate precipitation in a hypersaline lake the 180 of carbonate minerals in a seasonally stratified hypersaline lake are 2 3 pdb and are inconsistent with calculated 18o values for carbonate in equilibrium with the lake water during the spring and winter when the lake is stratified the calculated 18o of carbonate that would precipitate from the water column is less than that measured in the sediment because of freshwater dilution in the mixolimnion and high temperatures up to 46 c in the monimolimnion thermodynamic calculations indicate that the lake is supersaturated with respect to carbonate phases throughout the year however the 18o data indicate that carbonate is precipitated during the late summer and autumn when the lake is unstratified biologic activity is high and the water temperature is moderate the 13c values of the calcium carbonate minerals precipitated in the lake are heavier than the dissolved inorganic carbon values 13cdic of the lake water and cannot be explained by kinetic fractionation indicating a biologic control on primary carbonate precipitation the 13c of dolomite found in the lake sediments are in equilibrium with porewater 13cdic suggesting a high degree of porewater interaction in dolomitization both the 18o and 13c compositions of the dolomite suggest that it formed in equilibrium with current porewater but not in equilibrium with the coexisting calcium carbonate minerals

temporal variations in lake water temperature paleoenvironmental implications of lake carbonate δ18o and temperature records abstract 18 o and clumped isotope measurements of lacustrine carbonates provide important records of past terrestrial climate and paleoelevation however one of the primary challenges interpreting these data is constraining the relationship between mineral formation temperature and seasonal and mean annual climate we examined surface water temperature records from 88 lakes across the globe to develop transfer functions that relate seasonal water surface temperature to mean annual air temperature these transfer functions provide a means of reconciling proxy measures of water temperature with annual climatic conditions mean annual surface water temperature is related to mean annual air temperature maat 0 0318 t water 2 2 195 t water 12 607 r 2 0 96 independent of lake size with consistently higher mean water temperatures than air n hemisphere spring april june mean lake surface temperatures maat 0 0097 t water 2 1 379 t water 8 227 r 2 0 94 are only slightly warmer than maat for lakes in all climate zones while summer water temperatures jja maat 0 0055 t water 2 1 476 t water 18 915 r 2 0 90 may be 10 20 c warmer than maat for cold climate lakes comparison of carbonate stable isotopic data from sites with different timescales of carbonate formation such as found along latitude or elevation gradients can lead to large over or underestimates of past temperature accurate paleoclimatic interpretation of isotopic proxies or use of temperature proxy constraints i e fossil leaves to estimate lake temperature and calculate water 18 o thus requires appropriate application of water temperature air temperature transfer functions particularly when considering ancient cool high elevation lake systems

oxygen isotopes in meteoric calcite cements as indicators of continental paleoclimate because meteoric water 18 o values decrease with decreasing ambient temperature and increasing latitude 18 o values of meteoric calcite cement should exhibit a similar relation with paleolatitude and be an indicator of continental paleotemperatures to test this we compiled isotopic and paleolatitude data for 20 meteoric cements and nine speleothems ranging in age from devonian to modern and in paleolatitude from 3 5 to 83 mean 18 o values for meteoric cements and speleothems both show the same negative correlation with paleolatitude the 18 o vs latitude trend for these carbonates is almost identical to that predicted for modern inland environments but differs from the trend for coastal environments this suggests that the ground water controlling the ultimate composition of meteoric cement is derived predominantly from inland recharge if it is assumed that the modern meteoric water 18 o vs temperature relation is valid for the past and that insignificant evaporation occurred prior to carbonate precipitation then coastal and inland paleotemperatures can be calculated from the 18 o values of meteoric calcite 18 o mcl and seawater 18 o sw by using the equations t coastal 13 3 32 6 0 231 0 0613 18 o mcl 18 o sw and t inland 17 8 16 2 0 572 0 1233 18 o mcl 18 o sw where t is temperature in c calcite precipitated from coastal meteoric water at temperatures between 0 and 25 c will exhibit a narrow range in 18 o 6 to 4 where 18 o sw 0 the 18 o of calcite precipitated from inland meteoric water will be sensitive to paleotemperature ranging from 14 to 5 where 18 o sw 0 for temperatures of 0 to 25 c

recent human induced trophic change in the large and deep perialpine lake lucerne switzerland compared to historical geochemical variations this study investigates the sedimentological and geochemical changes that occurred during the last 2200 years in the meromictic lake lucerne switzerland one of the largest freshwater lakes of central europe the stable isotope composition 13c and 18o values of bulk carbonates is compared to changes in grain size distribution clay and silt fraction natural trace element input titanium and thorium concentrations and organic material abundance corg nitrogen and phosphorus and composition c n ratios and hydrogen and oxygen indexes a drop in carbonate accumulation and in the 18o values of sediments between ca ad 500 and 700 followed a large and consistent rise in chemical weathering marked by increases in the silicate clay fraction and in crustal element concentrations during the following millennium there was a long term decreasing trend in the lithogenic trace element input and in the phosphorus loading suggesting decreasing terrigeneous input from runoff water the major sedimentological change over the studied period occurred after ca ad 1800 with a significant increase in the erosion driven silt fraction and in the sedimentation rate during the last century human induced increase in nutrient input to the lake highly enhanced the accumulation of organic matter in sediment changes in nutrients and oxygen conditions in the hypolimnion of lake lucerne during the eutrophication period i e the last 40 years highly modified the geochemical fluxes compared to the relatively stable oligotrophic conditions that prevailed during the previous 2000 years before the 19th century climate driven meromixis had a limited impact on the organic matter flux to the sediments but the accumulation of carbonate considerably decreased during periods of lower mechanical erosion rates and high chemical weathering rates

shear instability wave along a snowband instability structure evolution and energetics derived from dual doppler radar data abstract this article presents a detailed analysis of a meso scale 17 km wavelength shear instability wave along a snowband using a series of dual doppler radar data the wave developed along a low level shear line that formed under the strain wind field caused by an adjacent mesoscale vortex the horizontal wind shear across the line was largest at lower levels and the eddy component horizontal winds and the retrieved pressure anomaly showed a bottom intensified structure as well the resultant vertical pressure gradient force was found to be responsible for the enhancement of alternating updrafts and downdrafts that were subsequently related to the formation of the reflectivity core gap structure of the wave eddy kinetic energy eke budgets of the evolving disturbance were investigated using time series of retrieved kinematic and thermodynamic data the wave grew at an approximately constant growth rate for about 40 min from its onset the eke in this quasi linear growth period was primarily gen

the stable isotope composition of dead sea waters abstract dead sea waters are moderately enriched in18o the degree of enrichment constitutes a balance between the dilution by freshwater influx and the isotope fractionation which accompanies evaporative water loss and vapour exchange with the atmospheric moisture modelling of the seasonal cycle and long term trends of 18o in response to the changes in the environmental parameters shows that the major control is exercised by the salinity of the surface waters through its effect on the vapour pressure gradient between the lake s surface and the atmosphere the steady state isotopic composition of the more saline brines tends towards less enriched18o values this fact can explain the relatively high 18o levels encountered in the lisan formation which was deposited from lake lisan the less saline pleistocene precursor of the dead sea

sources and flux of natural gases from mono lake california abstract the ability to identify a formation mechanism for natural gas in a particular environment requires consideration of several geochemical factors when there are multiple sources present four primary sources of methane have been identified in mono lake two of these sources were associated with numerous natural gas seeps which occur at various locations in the lake and extend beyond its present boundary the two other gas sources result from current microbiological processes in the natural gas seeps we observed flow rates as high as 160 moles ch4 day 1 and estimate total lakewide annual seep flux to be 2 1 106 moles ch4 geochemical parameters 13ch4 dch4 ch4 c2h6 c3h8 and 14ch4measurements revealed that most of the seeps originate from a paleo biogenic 13ch4 about 70 natural gas deposit of pleistocene age which underlies the current and former lakebed gas seeps in the vicinity of hot springs had in combination with the biogenic gas a prominent thermogenic gas component resulting from hydrothermal alteration of buried organic matter current microbiological processes responsible for sources of natural gas in the lake included pelagic meth anogenesis and decomposition of terrestrial grasses in the littoral zone methanogenesis in the pelagic sediments resulted in methane saturation 2 3 mm at 50 cm 13ch4 about 85 interstitial sulfate decreased from 133 mm at the surface to 35 mm by 110 cm depth indicating that sulfate reduction and methanogenesis operated concurrently methane diffused out of the sediments resulting in concentrations of about 50 m in the anoxic bottom waters methane oxidation in the oxic anoxic boundry lowered the concentration by 98 but values in surface waters 0 1 1 3 m were supersaturated with respect to the atmosphere the 13ch4 range 21 8 to 71 8 of this unoxidized residual methane was enriched in 13c relative to methane in the bottom water and sediments average outward flux of this methane was 2 77 107 moles yr 1 a fourth but minor source of methane 13ch4 55 2 was associated with the decomposition of terrestrial grasses taking place in the lake s recently expanded littoral zone

geochemical evaluation of fenghuoshan group lacustrine carbonates north central tibet implications for the paleoaltimetry of the eocene tibetan plateau abstract a sedimentologic petrologic and geochemical evaluation of lacustrine carbonates from the eocene oligocene fenghuoshan group of north central tibet provides insight into the paleoenvironmental and paleolimnological setting of the hoh xil basin during the mid tertiary fenghuoshan lacustrine rocks consist primarily of carbonate mudstones and wackestones these carbonates are generally less than 3 m in thickness and are intercalated with siliciclastic lacustrine and fluvial alluvial plain deposits individual limestone beds coarsen and contain increasing amounts of siliciclastic material upward fenghuoshan carbonates also contain abundant ostracode bivalve and charaphyte fossils sedimentologic evidence suggests that carbonate deposition occurred in shallow relatively short lived lacustrine systems that were subsequently filled by alluvial deposits of coeval fluvial systems stable c and o isotopic analysis of fenghuoshan carbonates show 18o values ranging from 11 7 to 10 3 vienna pee de

past changes to isotopic and solute balances in a continental playa clues from stable isotopes of lacustrine carbonates in many salt lakes around the world the relative abundance of preserved authigenic minerals is different from that predicted from solute mass balance calculations conventional mass balance models assume that chloride behaves conservatively over long periods of time and fail to take into account the role of diffusion deflation and fractional crystallisation dissolution of salts an alternative approach is to use oxygen isotopes as these reflect directly water molecule rather than solute concentrations and have the added advantage of providing a palaeohydrological record in lacustrine carbonates we present a steady state stable isotope model in conjunction with stable isotopic measurements of sub surface brines regional groundwaters and carbonate deposits from the lake malata lake greenly playa complex in south australia to estimate the apparent leakage and palaeoleakage from these superficially closed playa lakes the steady state model calculations using the present and compositions of the lake brines and inflowing groundwater suggest that the apparent present day leakage for the complex is 75 to 90 of inflow 35 times that calculated from cl and br under such conditions only low magnesian calcite precipitates and the lake water experiences reduced effects of evaporation gas and vapour exchange and consequently reduced isotopic and chemical enrichment further our model shows that calcite becomes increasingly mg rich until leakage is reduced to 55 to 70 of inflow a condition favourable for dolomitisation and of the lacustrine carbonates show excursions on the order of 5 over the length of a 2 3 m core indicating that the lake complex has varied from being throughflow dominated presence of low mg calcite relatively depleted in and to evaporation dominated high mg calcite dolomite relatively enriched in and throughout late quaternary our estimates of leakage fractions are consistent with the observed mineralogical suite but there remains a discrepancy between apparent closure indicated by the presence of the highly saline brine reservoir 1 leakage and high rates of throughflow inferred from stable isotope data we propose that the brine was formed by winter time re solution of a seasonal halite crust which forms during summer dominated evaporative discharge recirculation of the secondary brine and mixing with regional groundwater may decouple the solute cycle from the water cycle in many playa lakes the end result is a partially mixed brine characterised by nearly conserved solutes but with isotopic signatures indicative of brine rainfall groundwater interactions

the relationship between radar reflectivities and clouds associated with horizontal roll convection on 8 august 1982 abstract horizontal convective rolls hcrs and cloudstreets that occurred in northeastern colorado are studied using doppler radar data cloud photography and vertical soundings examination of the environment in which the hcrs form indicates a boundary layer wind speed that is much lower than those measured in previous studies evidence shows that the hcrs form in a well defined boundary layer yet the cloudstreets that develop seem to exist within a stable layer above a relationship between the radar reflectivity echoes and the cloudstreets is firmly established the echoes are aligned directly under the cloudstreets the echoes at lower levels are shown to be caused by convergence of particulate matter into updraft portions of the hcr circulation yet evidence indicates the cloud level echoes are due to backscatter from refractive index inhomogeneities at the surfaces of rising thermals examination of the echoes show that the strongest and tallest echoes are associated with cloud formation and tha

large eddy simulation of turbulent flow in a marine convective boundary layer with snow abstract a large eddy simulation les model with ice phase included has been used to study the marine convective boundary layer filled with snow extensions to moeng s les model include the diagnosis of cloud ice mixing ratio snow precipitation and the parameterization of detailed microphysical processes model simulations are compared with cold air outbreak field observations over lake michigan as well as with the liquid phase les results for the same atmospheric conditions the buoyancy flux and vertical velocity variance profiles generated by the ice phase les are found to be more consistent with the observations than those generated by the liquid phase les results the incorporation of the ice phase into the les model has also improved the agreement of vertical velocity skewness sw between observations and les model results it has also been found that the presence of precipitation and the associated microphysical processes has a significant effect on the structure of the convective boundary

mesoscale shallow convection in the atmosphere this paper is a review of the observational experimental theoretical and numerical studies of mesoscale shallow convection msc in the atmosphere typically msc is 1 to 2 km deep has a horizontal length scale of a few to a few tens of kilometers and takes distinctive planforms linear and hexagonal the former is called a cloud street roll or band while the latter is called mesoscale cellular convection mcc comprising three dimensional cells msc is characterized by its shape horizontal extent convective depth and aspect ratio the latter is the ratio of the horizontal extent to that in the vertical for cells the horizontal extent is their diameter whereas for rolls it is their spacing rolls usually align along or at angles of up to 10 from the mean horizontal wind of the convective layer with lengths from 20 to 200 km widths from 2 to 10 km and convective depths from 2 to 3 km the typical value of aspect ratio ranges from 2 to 20 rolls may occur over both water surface and land surfaces mesoscale convective cells may be divided into two types open and closed open cell circulation has downward motion and clear sky in the cell center surrounded by cloud associated with upward motion closed cells have the opposite circulation both types of cell have diameters ranging from 10 to 40 km and aspect ratios of 5 to 50 and both occur in a convective layer with a depth of about 1 to 3 km both the magnitude and direction of horizontal wind in the convective layer change little with height msc results from a complex and incompletely understood mix of processes these processes are outlined and their interplay is examined through a review of theoretical and laboratory analyses and numerical modeling of msc