convective momentum transport by rainbands within a madden julian oscillation in a global nonhydrostatic model with explicit deep convective processes part i methodology and general results abstractthe convective momentum transport cmt properties of 13 215 rainbands within a madden julian oscillation mjo event simulated by a global nonhydrostatic model are examined cmt vectors which represent horizontal accelerations to the mean winds due to momentum flux convergences of deviation winds are derived for each rainband the cmt vectors are composited according to their locations relative to the mjo center while a similar number of rainbands are detected in the eastern and western halves of the mjo convective envelope cmt vectors with large zonal components are most plentiful between 0 and 20 to the west of the mjo center the zonal components of the cmt vectors exhibit a coherent directionality and have a well organized three layer structure positive near the surface negative in the low to midtroposphere and positive in the upper troposphere in the low to midtroposphere where the longitudinal difference in the mean zonal wind across the mjo is 10 m s 1 on average the net acceler

identifying sediment sources in a gullied catchment using natural and anthropogenic radioactivity the ratio of the lithogenic nuclides 226ra and 232th indicates that the major source of sediment in transport in a gullied catchment located in the head waters of whiteheads creek new south wales australia is the gully itself this observation is supported by 7be and 137cs concentrations which indicate that the sediment is derived from erosion of either gully floor or gully wall material while subcatchment 7be and 137cs data show sheet rill and gully erosion are occurring in the upstream catchments sediment from these sources is either being trapped before entering the gully or the volume of sediment is small compared to that derived from the gully itself the differences in the 226ra to 232th ratio between the suspended solids collected on the rising stage and samples of sediment deposited during the falling stage at the outlet of the catchment indicate a change in the source of sediment during storm events the results presented here demonstrate the applicability of radionuclide tracers in providing quantitative estimates of the relative contributions of individual catchments to the sediment moving within a drainage network and in determining whether or not the sediments are mainly derived from subsoil or topsoil sources

momentum transport by a line of cumulonimbus abstract the vertical transport of horizontal momentum normal to a line of cumulonimbus observed during gate on 14 september 1974 is against the vertical momentum gradient contrary to the predictions of mixing length theory data from repeated aircraft passes normal to the line s axis at heights from 0 15 to 5 5 km are used to document the flux and determine its source the flux is concentrated in roughly a 25 km wide active zone just behind the leading edge of the line in kilometer scale convective updrafts accelerated upward by buoyancy and toward the rear of the line by mesoscale pressure forces the fall in mesoscale pressure from the leading edge to the rear of the active zone is mainly hydrostatic resulting from relatively high virtual temperatures and the 60 degree tilt of the leading edge from the vertical with the clouds at the surface well ahead of those aloft evaluation of the terms in the momentum flux generation equation confirms that the above process reflected by the velocity buoyan

parameterization of the vertical transport of momentum by cumulus clouds part i theory abstract a scheme is developed to parameterize the vertical transport of momentum by cumulus clouds in which the effect of a cloud induced pressure field is included in addition a new form for momentum exchange between clouds and their environment is used the scheme incorporates a simple cloud model which includes both updraft and downdraft and specifies the cloud dynamic fields to enable us to determine the mean pressure gradient force across the clouds it is shown that in a typical environment with vertical wind shear the pressure gradient force is along the direction of the wind shear the equation governing cloud mean momentum together with the diagnostic equation for cloud induced pressure field is solved by iteration to evaluate quantitatively the horizontal pressure gradient force across the cloud and determine the cloud mean momentum application of the parameterization scheme to gate convective events will be presented in part ii of this paper

a cold path for the gulf stream troposphere connection abstractthe mechanism by which the gulf stream sea surface temperature sst front anchors a band of precipitation on its warm edge is still a matter of debate and little is known about how synoptic activity contributes to the mean state in the present study the influence of the sst front on precipitation is investigated during the course of a single extratropical cyclone using a regional configuration of the met office unified model the comparison of a control run with a simulation in which sst gradients were smoothed brought the following conclusions a band of precipitation is reproduced for a single extratropical cyclone and the response to the sst gradient is dominated by a change of convective precipitation in the cold sector of the storm several climatological features described by previous studies such as surface wind convergence on the warm edge or a meridional circulation cell across the sst front are also reproduced at synoptic time scales in the cold sector based on these results a s

a numerical study of the parametrization of deep tropical convection a two dimensional mesoscale cloud model is used to simulate an ensemble of deep tropical convective clouds using gate data random heating perturbations near the sea surface initiate the development of a population of clouds which grow under the influence of specified large scale radiative and surface forcing budgets of convective heating and moistening are presented and the model found to be a reasonable representation of the atmosphere the model is used to investigate the parametrization of cumulus convection in large scale models equations governing the effect of convection upon the large scale atmosphere are derived in full and compared with approximate forms used by previous workers these forms are the basis of many parametrization schemes simple representations of cumulus heating q1 cumulus moistening q2 and dynamical transports for use in parametrization schemes in large scale atmospheric models are studied the importance of shallow convection in determining the vertical structure of q2 is recognized the dependence of convective rainfall upon atmospheric structure and large scale processes the closure problem is investigated the importance of downdraughts is emphasized and difficulties in the calculation of q2 identified study of parametric forms for cumulus momentum transports indicates that the cumulus friction hypothesis is inadequate it is suggested that present closure schemes are too restrictive

quantifying hyporheic exchange in a tidal river using temperature time series 1 an investigation into groundwater surface water interaction gswi beneath a large tidally influenced river was conducted to determine the effect of tides on the development of a hyporheic zone hz and to quantify mixing of river water and groundwater temperature measurements coupled with independent hydraulic head measurements were used to detect groundwater flow within the riverbed gwsi under tidal forcing produced a 1 m deep hz time averaged riverbed temperature profiles displayed a distinct compressed convex pattern clear evidence of net groundwater discharge however the instantaneous time series data indicate that riverbed temperatures were affected by tidal forcing to a depth of 1 m heat transport modeling revealed that instantaneous velocities within the shallow sediments of the riverbed are rather high creating a zone of vigorous exchange during either a flooding or ebbing tide furthermore the magnitude of the tidal pressure gradient was found to be significantly greater than the pressure gradient expected across 0 8 m high dunes evidence that bed form driven exchange under these conditions and this scale of observation did not contribute to the development of the hz conditions for exchange induced by shear and current bed form are favorable during ebbing tidal conditions only flow paths are therefore limited in depth exchange flow paths in an estuary setting are complex they are limited in duration and space and dominated by tidal pumping

experimental investigation of the thermal time series method for surface water groundwater interactions 1 diel temperature fluctuations have been used to quantify vertical water flow through saturated sediments with 1 d analytical heat models the underlying transport equation relies on assumptions that could be violated using field temperature records to test the capability of this method a hydraulic laboratory experiment was designed a mass of fully water saturated homogeneous sand was exposed to 19 different uniform pressure gradients inducing steady state darcy velocities 0 q 25 m d 1 an areal heating grid generated steady sinusoidal thermal forcing multipoint sediment temperature responses demonstrated increasing spatial variability for increasing velocities this introduced horizontal temperature gradients velocities calculated from heat tracing were compared to velocities independently obtained from solute slugs heat and solute derived velocities agreed for q 3 m d 1 but heat derived velocities were consistently larger at higher velocities temperature amplitude and phase derived velocities revealed significant scatter when compared to solute velocities this scatter reduced when amplitude and phase derived velocities were compared for each sensor pair the variability in heat derived velocities therefore represents a spatially variable flow field in the sand however amplitude and phase derived velocities deviated from a 1 1 relationship at higher velocities this can partly be explained by longitudinal thermal dispersivity and partly by enhanced thermal spreading due to horizontal temperature gradients originating from nonuniform flow this is surprising given the homogeneous sand and transition zone transport conditions pet 0 7 these findings have implications for the quantification of velocities from field records because field conditions are likely more heterogeneous which would exacerbate the effects found in this study

on the causes of mid pliocene warmth and polar amplification the mid pliocene 3 to 3 3 ma ago is a period of sustained global warmth in comparison to the late quaternary 0 to 1 ma ago and has potential to inform predictions of long term future climate change however given that several processes potentially contributed relatively little is understood about the reasons for the observed warmth or the associated polar amplification here using a modelling approach and a novel factorisation method we assess the relative contributions to mid pliocene warmth from elevated co2 lowered orography and vegetation and ice sheet changes the results show that on a global scale the largest contributor to mid pliocene warmth is elevated co2 however in terms of polar amplification changes to ice sheets contribute significantly in the southern hemisphere and orographic changes contribute significantly in the northern hemisphere we also carry out an energy balance analysis which indicates that that on a global scale surface albedo and atmospheric emmissivity changes dominate over cloud changes we investigate the sensitivity of our results to uncertainties in the prescribed co2 and orographic changes to derive uncertainty ranges for the various contributing processes

design and installation of a novel automatic erosion monitoring system process inference in geomorphology is hindered by a lack of information on the true temporal distribution of contemporary erosional and depositional activity to tackle this problem a low cost automatic monitoring system based on the photo electronic erosion pin peep sensor has been developed the peep is essentially an array of photosensitive cells enclosed within a transparent tube and connected by cable to a datalogger when inserted into an eroding landform subsequent retreat of the face exposes more photosensitive cells to light which increases peep voltage output deposition decreases sensor outputs the logged signals thus reveal the magnitude timing and frequency of erosion and deposition events with much greater precision than has hitherto been possible measurement principles electronic and physical design calibration field installation problems and prospects and pilot results from a river bank erosion site are discussed the peep system appears to have great potential for disentangling competing hypotheses in geomorphological process studies and in building and testing erosion and sediment transport models of high temporal resolution

antarctic climate during the middle pliocene model sensitivity to ice sheet variation significant controversy exists surrounding the nature of the antarctic ice sheet during the period of middle pliocene warmth this paper outlines the output from three sensitivity experiments for the middle pliocene using the ukmo gcm and the usgs prism2 data set different east antarctic ice sheet configurations were prescribed in each modelling experiment conforming to a dynamic sea level 35 m higher than at present intermediate pliocene control sea level 25 m higher than at present and stable sea level 12 15 in higher than at present scenario for the nature of the antarctic ice sheet during the middle pliocene all three scenarios would have provided middle pliocene sea levels across the spectrum of estimates provided by known geological data model outputs indicate that surface temperature increases are greatest in the pliocene dynamic experiment climatic conditions for the pliocene control and stable experiments remain harsh over antarctica with surface temperatures rising above freezing mean 4degreesc for a maximum of 2 months per year in contrast mean surface temperatures from the pliocene dynamic experiment rise above freezing mean 10degreesc for 4 months of the year climatic outputs from all of the pliocene experiments were used in a biome model biome 4 for the pliocene control and stable experiments biome 4 predicted the occurrence of exclusively tundra type vegetation on antarctica for the dynamic experiment climatic conditions ameliorated sufficiently to allow biome 4 to predict deciduous taiga montane forests on antarctica thus the results from the dynamic experiment are consistent with reported pliocene nothofagus occurrences on antarctica in the future more sophisticated palaeoclimate modelling studies need to employ atmospheric general circulation models gcms coupled to dynamic ocean and ice sheet models this will allow the gcm to predict rather than prescribe an antarctic ice sheet configuration in equilibrium with reconstructed middle pliocene sea surface temperatures such an approach may also shed light on how the antarctic ice sheet responded to climate variability during the middle pliocene c 2002 elsevier science b v all rights reserved

numerical analysis of river channel processes with bank erosion this paper presents a numerical analysis of river channel processes with bank erosion the model can be used for investigating both bed deformation and bankline shifting in 2d plan form the basic equations are used in a moving boundary fitted coordinate system and a new formulation of nonequilibrium sediment transport is introduced to reproduce the channel processes the model was applied to examine the morphological behavior of experimental channels temporal changes in the plan form in a meandering channel can be classified into two patterns meander developing and meander straightening comparison of the observed and calculated results indicates that the model is applicable to both channel changes under various hydraulic conditions on the basis of the numerical findings the paper clarifies the influence of hydraulic variables on the location of bank erosion and bed scouring the model also was used to investigate the effect of alternate bars on bank erosion and to investigate the development of channel meandering from an initially straight channel

el nino southern oscillation pliocene climate and equifinality it has been suggested that during the pliocene ca 5 1 8ma an el nino state existed as a permanent rather than an intermittent feature that is the tropical pacific ocean was characterized by a much weaker east west gradient than today one line of inquiry used to investigate this idea relates modern el nino teleconnections to pliocene proxy data by comparing regional differences in precipitation and surface temperature with climate patterns associated with present day el nino events assuming that agreement between pliocene data and observations of modern el nino events supports this interpretation here we examine this assumption by comparing outputs from a suite of mid pliocene climate simulations carried out with the uk met office climate model regional patterns of climate change associated with changes in model boundary conditions are compared with observed el nino southern oscillation teleconnection patterns our results indicate that many of the proposed permanent el nino surface temperature and precipitation patterns are observable in mid pliocene climate simulations even when they display variability in tropical pacific sea surface temperatures ssts or when forced with a modern east west sst gradient our experiments highlight the possibility that the same outcome may be achieved through different initial conditions equifinality an important consideration for reconstructed patterns of regional mid pliocene climate

coupled groundwater flow and heat transport simulation for estimating transient aquifer stream exchange at the lowland river spree germany subsurface flow and heat transport near freienbrink ne germany was simulated in order to study groundwater surface water exchange between a floodplains aquifer and a section of the lowland river spree and an adjacent oxbow groundwater exfiltration was the dominant process and only fast surface water level rises resulted in temporary infiltration into the aquifer the main groundwater flow paths are identified based on a 3d groundwater flow model to estimate mass fluxes across the aquifer surface water interfaces a 2d flow and heat transport modelling approach along a transect of 12 piezometers was performed results of steady state and transient water level simulations show an overall high accuracy with a spearman coefficient 0 9996 and root mean square error rmse 0 008 m based on small groundwater flow velocities of about 10 7 to 10 6 ms 1 mean groundwater exfiltration rates of 233 l m 2 d 1 are calculated short periods of surface water infiltration into the aquifer do not exceed 10 days and the infiltration rates are in the same range the heat transport was modelled with slightly less accuracy 0 8359 and rmse 0 34 c in contrast to the predominant groundwater exfiltration surface water temperatures determine the calculated temperatures in the upper aquifer below both surface water bodies down to 10 m during the whole simulation period these findings emphasize prevailing of heat conduction over advection in the upper aquifer zones which seems to be typical for lowland streams with sandy aquifer materials and low hydraulic gradients moreover this study shows the potential of coupled numerical flow and heat transport modelling to understand groundwater surface water exchange processes in detail copyright 2013 john wiley sons ltd

notes and correspondence convective eddy momentum tendencies in long cloud resolving model simulations abstract domain average momentum budgets are examined in several multiday cloud resolving model simulations of deep tropical convection in realistic shears the convective eddy momentum tendency f neglected in many global circulation models looks broadly similar in two and three dimensional simulations it has a large component in quadrature with the mean wind profile tending to cause momentum profile features to descend this component opposes and exceeds in magnitude the corresponding large scale vertical advective tendency which would tend to make features ascend in convecting regions the portion of f in phase with the mean wind is isolated by vertically integrating f u yielding a kinetic energy tendency that is overwhelmingly negative the variation of this energy damping with shear flow kinetic energy and convection intensity measured by rain rate gives a cumulus friction coefficient around 40 to 80 per centimeter of rain in 3d runs large scatter reflects the effects of varying co

the impact of convection on enso from a delayed oscillator to a series of events abstract the ncar community climate system model version 3 ccsm3 exhibits persistent errors in its simulation of the el nino southern oscillation enso mode of coupled variability the amplitude of the oscillation is too strong the dominant 2 yr period too regular and the width of the sea surface temperature response in the pacific too narrow with positive anomalies extending too far into the western pacific two changes in the parameterization of deep convection result in a significant improvement to many aspects of the enso simulation the inclusion of convective momentum transport cmt and a dilution approximation for the calculation of convective available potential energy cape are used in development integrations and a striking improvement in enso characteristics is seen an increase in the periodicity of enso is achieved by a reduction in the strength of the existing short circuited delayed oscillator mode the off equatorial response is weaker and less tropically confined largely as a

water table fluctuations within the floodplain of the river severn england abstract in contrast to extensive research on hydrological processes operating in headwater basins there has been relatively little attention paid to the hydrological processes that occur on the floodplains of lowland rivers this dearth of information is all the more surprising given current interest in the use of floodplains as buffer zones between farmland and the riverine environment in the previous paper water resour res 36 2000 2517 bates et al reported preliminary results from a field site on the floodplain of the river severn in shropshire uk a large lowland river by british standards piezometric data suggested that during out of bank conditions a reverse groundwater ridge develops in the floodplain subsurface and results in strong groundwater flux velocities directed towards the base of hillslopes adjoining the floodplain bates et al showed that the impact of such ridges was to switch off hillslope inputs to the riparian zone they hypothesised that this occurred when surface inundation approached the back of the floodplain in this paper we provide a more detailed analysis of the events considered by bates et al and extend the analysis to the more common in bank flood condition in total five events are considered two out of bank and three in bank these were chosen to represent a wide range of event magnitudes antecedent conditions and local patterns of rainfall and runoff the analysis demonstrates that the reverse groundwater ridging process identified by bates et al also occurs during in bank events hillslope inputs to the floodplain are also switched off in these events if the flood stage is high in smaller floods water continues to move from slope to floodplain although coupling between slope and channel is only re established later in the recession we conclude that contrary to the conclusions of bates et al this process switching is not necessarily dependent on surface inundation approaching the back of the floodplain whilst the paper broadly confirms the operation of the simple reverse groundwater ridging process described by bates et al it shows that antecedent conditions local rainfall and runoff and flood stage all act to complicate this basic pattern lastly we consider the implications for catchment water quality of the newly identified processes

a preliminary morphology of precipitation systems in tropical northern australia a preliminary morphology of convective systems observed in the vicinity of darwin australia is presented several types of tropical convection during monsoonal and break season flow are identified using specific examples and compared with a range of systems observed worldwide in the general context of the bulk richardson number a significant spectrum of convective activity ranging from low shear and low convective available potential energy cape to high shear and moderate to high cape typical of mid latitude severe storms is identified significant differences between monsoonal flow and break season systems are apparent the formation of convective lines perpendicular to the low level shear and the apparent balance between advancing shallow cold pools and the shear are ubiquitous features convective scale downdraughts and mesoscale descent appear to be responsible for redistribution of 700 mb momentum to the planetary boundary layer pel the motion of the break season squall lines appears to result from an equilibrium between the cold pool and the undisturbed environmental pel flow deviations from this balance are hypothesized to result in the observed propagating and non propagating or slow moving modes examples of these modes are given showing the classical continuous development of new cells in a balanced state along the leading edge of long lived squalls and in the propagating case the discontinuous development of new cells on the cold pool ahead of the squall

closure of the panama seaway during the pliocene implications for climate and northern hemisphere glaciation the panama hypothesis states that the gradual closure of the panama seaway between 13 million years ago 13 ma and 2 6 ma led to decreased mixing of atlantic and pacific water masses the formation of north atlantic deep water and strengthening of the atlantic thermohaline circulation increased temperatures and evaporation in the north atlantic increased precipitation in northern hemisphere nh high latitudes culminating in the intensification of northern hemisphere glaciation nhg during the pliocene 3 2 2 7 ma here we test this hypothesis using a fully coupled fully dynamic ocean atmosphere general circulation model gcm with boundary conditions specific to the pliocene and a high resolution dynamic ice sheet model we carry out two gcm simulations with closed and open panama seaways and use the simulated climatologies to force the ice sheet model we find that the models support the panama hypothesis in as much as the closure of the seaway results in a more intense atlantic thermohaline circulation enhanced precipitation over greenland and north america and ultimately larger ice sheets however the volume difference between the ice sheets in the closed and open configurations is small equivalent to about 5 cm of sea level we conclude that although the closure of the panama seaway may have slightly enhanced or advanced the onset of nhg it was not a major forcing mechanism future work must fully couple the ice sheet model and gcm and investigate the role of orbital and co2 effects in controlling nhg

the recent history of erosion and sedimentation on the southern tablelands of southeastern australia sediment flux dominated by channel incision abstract as in other regions colonised from europe within the last few centuries australia s vegetation and soils have been dramatically changed by clearing cropping and grazing in southeastern australia particularly on the southern tablelands the impacts of european settlement are clearly manifested by channel incision by using stratigraphic and documentary evidence in conjunction with aerial photographs sediment budgets for the post settlement period have been constructed to define the fluxes and stores of sediment for each of the major geomorphic components of the 136 km2 catchment of jerrabomberra creek near canberra using these budgets and some plausible assumptions it has been possible to approximate the history of both the sediment delivery ratio and sediment yield for this catchment while the quantities estimated in this analysis are approximations the trends through time are credible sediment yield increased rapidly to a peak after european settlement and has returned to a level between the peak and the pre european value the delivery ratio has followed a similar trend the most general conclusion to emerge is that in this landscape both the total sediment flux and the sediment yield of the catchment have been dominated by channel erosion this result is contrary to the findings in many parts of the world where sheet and rill erosion dominates the fluxes the soil conservation implication of these results is clear to control off site effects of erosion the focus must be on the channels

a methodology for targeting palaeo proxy data acquisition a case study for the terrestrial late miocene when planning the acquisition of new palaeo proxy data for reconstructing past climates many factors influence the decision of where the proxies are to be collected one such influence is the likelihood of recording a significant climate change relative to the modern another consideration which is less often considered is the desire to target regions in which there is significant uncertainty in numerical model predictions of past climate change new proxy data in these regions enables a more rigorous test of model simulations and results in better constraints on the various boundary conditions used to force the models in this paper we present a methodology for targeting new palaeo proxy data based on model simulations we use the terrestrial late miocene 11 6 to 5 3 ma as a case study the late miocene climate provides insights into future climate behavior as it is believed to have been considerably warmer than the modern we carry out a suite of late miocene atmosphere only general circulation model simulations which we initially evaluate relative to an example dataset of terrestrial temperature and precipitation in terms of targeting future data acquisition we locate those regions in the model which exhibit the largest change in temperature and or precipitation relative to the pre industrial we also locate those regions which display the largest variability between the model simulations because new data in these regions are most likely to provide a strict test of the reliability of the model results among other regions the amazon basin the chad basin central canada and northern india are identified as potentially useful areas to collect data

modelling pliocene warmth contribution of atmosphere oceans and cryosphere the relative role of the atmosphere oceans and cryosphere in contributing towards middle pliocene warmth ca 3 ma bp is investigated using the hadcm3 coupled ocean atmosphere general circulation model the model was initialised with boundary conditions from the usgs prism2 data set and a pliocene atmospheric co2 level of 400 ppmv and run for 300 simulated years the simulation resulted in a global surface temperature warming of 3degreesc compared to present day in contrast to earlier modelling experiments for the pliocene surface temperatures warmed in most areas including the tropics 1 5degreesc compared with present day the model predicts a general pattern of ocean warming 1 5degreesc in both hemispheres to a depth of 2000 m below which no significant differences are noted sea ice coverage is massively reduced up to 90 the flow of the gulf stream north atlantic drift is up to 100 mm s 1 greater in the pliocene case analysis of the model predicted meridional streamfunction suggests a global pattern of reduced outflow of antarctic bottom water aabw up to 5 sv a shallower depth for north atlantic deep water formation and weaker thermohaline circulation 3 sv the decrease in aabw occurs mainly in the pacific rather than atlantic ocean model diagnostics for heat transports indicate that neither the oceans nor the atmosphere are transporting significantly more heat in the pliocene scenario rather these results indicate that the major contributing mechanism to global pliocene warmth was the reduced extent of high latitude terrestrial ice sheets 50 reduction on greenland 33 reduction on antarctica and sea ice cover resulting in a strong ice albedo feedback these results highlight the need for further studies designed to improve our knowledge regarding pliocene terrestrial ice configurations

sea surface temperatures of the mid piacenzian warm period a comparison of prism3 and hadcm3 it is essential to document how well the current generation of climate models performs in simulating past climates to have confidence in their ability to project future conditions we present the first global in depth comparison of pliocene sea surface temperature sst estimates from a coupled ocean atmosphere climate model experiment and a sst reconstruction based on proxy data this enables the identification of areas in which both the climate model and the proxy dataset require improvement in general the fit between model produced sst anomalies and those formed from the available data is very good we focus our discussion on three regions where the data model anomaly exceeds 2 c 1 in the high latitude north pacific a systematic model error may result in anomalies that are too cold also the deeper pliocene thermocline may cause disagreement along the california margin either the upwelling in the model is too strong or the modeled thermocline is not deep enough 2 in the north atlantic the model predicts cooling in the center of a data based warming trend that steadily increases with latitude from 1 5 c to 6 c the discrepancy may arise because the modeled north atlantic current is too zonal compared to reality which is reinforced by the lowering of the altitude of the pliocene western cordillera mountains in addition the model s use of modern bathymetry in the higher latitudes may have led the model to underestimate the northward penetration of warmer surface water into the arctic 3 finally though the data and model show good general agreement across most of the southern ocean a few locations show offsets due to the modern land sea mask used in the model additional considerations could account for many of the modest data model anomalies such as differences between calibration climatologies the oversimplification of the seasonal cycle and differences between sst proxies i e seasonality and water depth new sst estimates from data sparse and regionally important areas will greatly enhance our ability to judge model performance

warmer tropics during the mid pliocene evidence from alkenone paleothermometry and a fully coupled ocean atmosphere gcm 1 traditional reconstructions of sea surface temperatures ssts produced by the pliocene research interpretations and synoptic mapping prism group indicate that mid pliocene surface ocean temperatures were unchanged or slightly cooler than modern at the tropics and low latitudes and significantly warmer at higher latitudes particularly in the north atlantic this change in the latitudinal pattern of ssts has been attributed to enhanced meridional ocean heat transport generated by more vigorous surface ocean gyres and or thermohaline circulation thc this study derives new sst estimates using the alkenone paleothermometer in the pacific ocean these estimates combined with published alkenone sst data from two atlantic sites are located in tropical and subtropical regions where the distribution of prism sites is sparse a fully coupled ocean atmosphere general circulation model oagcm running for the mid pliocene was also used to predict sst significant differences are noted between absolute prism ssts interpolated to our core locations and alkenone derived ssts these differences may be related to errors in the prism and alkenone paleothermometry estimates or to regional differences between localities where data was collected however the available alkenone and model based sst estimates are consistent in the sign of temperature change they predict and provide the first indication of warmer ssts during the mid pliocene in the tropics and subtropics this contrasts with prism s estimates of unchanged or slightly cooler ssts for the same geographical regions the pattern of ssts produced by alkenone estimates and modeling is not characteristic of that produced by enhanced meridional ocean heat transport or thc instead the pattern is similar to that which might be expected as a result of higher concentrations of atmospheric co2 which would act to warm the oceans at the tropics and other latitudes furthermore model diagnostics indicate that reduced sea ice and terrestrial ice sheet extents and a strong ice albedo feedback played a major role in forcing mid pliocene warmth further work aimed at validating these conclusions should concentrate on expanding the mid pliocene sst data set particularly at low latitudes as well as undertaking model model comparisons to assess model dependency of the results

river width adjustment i processes and mechanisms in 1993 a task committee tc of the asce was established to study the hydraulics bank mechanics and modeling of width adjustment in alluvial channels the work of the tc in reviewing width adjustment processes and mechanisms is reported in this paper a companion paper presents the findings of the tc with regard to width adjustment modeling this paper first establishes the geomorphic context within which width adjustments occur and it demonstrates that width adjustment may take place over a wide range of scales in time and space in the past engineering analyses of channel width have tended to concentrate on prediction of the equilibrium width for stable channels most commonly the regime extremal hypothesis and rational mechanistic approaches are used and these are reviewed herein more recently attention has switched to channels that are adjusting their morphology either due to natural instability or in response to changes in watershed land use river regulation or channel engineering chara

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

simulation of the madden julian oscillation in a coupled general circulation model part ii the role of the basic state abstract in part i of this study it was shown that air sea coupling had a positive impact on some aspects of the simulation of the madden julian oscillation mjo by a gcm however errors in the basic state climate of that gcm appeared to be preventing the mjo related convection from propagating into the west pacific in this paper the actual impact of these errors will be addressed an integration of a flux adjusted version of the coupled model has been performed which has reduced basic state errors in the west pacific in this version of the coupled gcm the mjo does propagate into the west pacific the simulation of the mjo by a coupled model with the same atmospheric component but a different ocean gcm is also analyzed this coupled gcm has similar systematic errors in low level zonal wind and precipitation to the model studied in part i but with warmer ssts results from this experiment together with the other available evidence suggest that it is the errors in the low level zonal wind component

effect of anisotropy and groundwater system geometry on seepage through lakebeds 2 numerical simulation analysis abstract the interaction of lakes and groundwater is controlled partly by the geologic framework through which the water flows two interrelated geometric factors of the groundwater system that affect flow are overall geometry of the system and anisotropy of the porous media within the system numerical simulation analysis was made for variations in the coefficient of anisotropy for each of several lake and groundwater settings having different geometric configurations these analyses indicate that for a given geometric setting as the anisotropy of geologic materials decreases seepage from a lake decreases and depth of the local groundwater flow system associated with the lake increases transformation of scale of groundwater systems that have anisotropic media to isotropic equivalents results in a change in the overall geometry because of the different slopes of the water table and lakebed resulting from the scale transformations a series of numerical experiments were made for various geometric configurations for a given anisotropy these analyses indicate that as thickness of the groundwater system decreases relative depth of the local flow system increases and seepage from the lake decreases

dynamic streambed fluxes during rainfall runoff events observed 3 d temperature distributions within a streambed were used to analyze the effects on exchange fluxes between groundwater and the stream during rainfall runoff events by combining a dense vertical and lateral monitoring network of streambed temperatures with coupled surface subsurface 3 d flow and heat transport modeling we demonstrate how temperature can be used directly as a calibration target three model setups with different hydraulic conductivity distributions were evaluated in an optimization approach using temperature and hydraulic head data the hydraulic conductivity distributions were based on slug test surveys within the streambed and aquifer a detailed characterization of the hydraulic conductivity of the streambed and aquifer is needed to accurately simulate observed temperatures hence the most sensitive parameters the vertical hydraulic conductivity and the thermal conductivity were calibrated within different conductivity zones of the heterogeneous model simulated exchange fluxes across the streambed showed variations up to a factor of four within just a meter such differences may not have been correctly predicted using 1 d heat transport models due to lateral conduction amongst the different flow paths during the rainfall runoff event fluxes decreased substantially 50 due to a decrease in the hydraulic gradient with increased stream stage although no flow reversals were observed during the studied conditions it is possible that these can occur during larger rainfall runoff events we show that with the current sampling and modeling techniques 3 d temperature data can be used to estimate dynamic exchange in heterogeneous flow fields encountered in the field

estimating the spatial extent of unsaturated zones in heterogeneous river aquifer systems the presence of unsaturated zones at the river aquifer interface has large implications on numerous hydraulic and chemical processes however the hydrological and geological controls that influence the development of unsaturated zones have so far only been analyzed with simplified conceptualizations of flow processes or homogeneous conceptualizations of the hydraulic conductivity in either the aquifer or the riverbed we systematically investigated the influence of heterogeneous structures in both the riverbed and the aquifer on the development of unsaturated zones a stochastic 1 d criterion that takes both riverbed and aquifer heterogeneity into account was developed using a monte carlo sampling technique the approach allows the reliable estimation of the upper bound of the spatial extent of unsaturated areas underneath a riverbed through systematic numerical modelling experiments we furthermore show that horizontal capillary forces can reduce the spatial extent of unsaturated zones under clogged areas this analysis shows how the spatial structure of clogging layers and aquifers influence the propensity for unsaturated zones to develop in riverbeds where clogged areas are made up of many small spatially disconnected patches with a diameter in the order of 1 m unsaturated areas are less likely to develop compared to riverbeds where large clogged areas exist adjacent to unclogged areas a combination of the stochastic 1 d criterion with an analysis of the spatial structure of the clogging layers and the potential for resaturation can help develop an appropriate conceptual model and inform the choice of a suitable numerical simulator for river aquifer systems

a proposed method of bank erosion vulnerability zonation and its application on the river haora tripura india abstract in this paper a new rs gis based simple method has been proposed for estimating bank erosion this method does not need intense field investigation and can provide erosion vulnerability zonation for the entire river the method uses eight parameters i e rainfall erosivity lithological factor bank slope meander index river gradient soil erosivity vegetation cover and anthropogenic impact meteorological data gsi maps srtm dem 30 m horizontal resolution liss iii 23 5 m resolution and google images have been used to determine rain erosivity lithological impact bank slope meander index river gradient vegetation cover and anthropogenic activities soil map of the nbsslp national bureau of soil survey and land use planning india has been used for assessing soil erosivity index by integrating the individual values of those six parameters out of those eight parameters the first two parameters remained constant for the particular study area a bank erosion vulnerability zonation map of the river haora tripura india 23 37 23 53 n and 91 15 91 37 e has been prepared the values have been compared with the existing behi nbs method of 60 spots and also with field data of 30 cross sections covering the 60 spots taken along a 51 km stretch of the river within indian territory and we found that the estimated values are matching with the existing method as well as with field data the whole stretch has been divided into five hazard zones i e very high high moderate low and very low hazard zones and they are cover 5 66 16 81 40 82 29 67 and 9 04 km respectively

a new gravity wave drag scheme incorporating anisotropic orography and low level wave breaking impact upon the climate of the uk meteorological office unified model a new anisotropic gravity wave drag parametrization scheme which represents high drag states modelled on hydraulic jump flow blocking and internal wave reflection theory including trapped lee waves is presented the scheme is shown to represent the breaking of waves over mountains better than previous schemes by comparison with mesoscale simulations of pyrex case studies extended simulations of the meteorological office unified model at climate resolution are presented showing the impact of the new scheme and a combination of the scheme with a new orographic roughness parametrization results show a changed distribution of surface gravity wave surface stress mostly due to mountain anisotropy and a greater tendency for lower troposphere wavc drag inclusion of the orographic roughness parametrization halves the gravity wave stress the combined effect of these schemes for climate integrations being a slight improvement to the tropospheric flow

a permanent el nino like state during the pliocene the pliocene may have been characterized by permanent el nino like conditions initial modeling studies suggest that this may have contributed to pliocene warmth the termination of this state may have influenced northern hemisphere glaciation nhg we use the hadley centre coupled model version 3 to examine the role of the oceans and ocean structure on pliocene warmth a permanent el nino like state is not predicted annual mean sea surface temperatures in the eastern equatorial pacific at ocean drilling program sites 847 and 851 increase by 1 71 c and 1 15 c respectively however el nino southern oscillation events are clearly expressed by the model sensitivity tests indicate that a prescribed permanent el nino like condition increases annual global mean surface temperatures by a maximum of 0 6 c if the pliocene was characterized by such a condition it is questionable that it provided a major contribution to global warmth and therefore unlikely that the termination of this state contributed significantly to the onset of nhg

river patterns and their meaning abstract rivers are largely responsible for shaping the earth s continental landscapes river patterns the spatial arrangements of channels in the landscape are determined by slope and structure at site and sector scale channel morphology varies spatially and in time but river patterns and drainage texture or the frequency of stream lines per unit area together determine the intricacy or otherwise of topography most river patterns evolve through natural selection slope induces the formation of such patterns as parallel radial and distributary while structure produces straight angular trellis and annular arrangements once established patterns tend to persist nevertheless at many sites the usual patterns have been disturbed and patterns that are anomalous in terms of slope and structure have been produced by diversion tectonism volcanism glaciation mass movements and human activities by antecedence superimposition inheritance or underprinting by the persistence of deeply eroding rivers which encounter alien structures and by climatic change river patterns provide clues to underlying structure and to the chronology of events they have also proved significant in the search for minerals

the effect of herbaceous plant communities and soil textures on particle erosion of alluvial streambanks soil texture and plants are understood to be important determinants of erosion however there is a paucity of research on the quantitative effect of various soils plants or the interaction of the two on streambank erosion this study measures the influence of herbaceous plant communities and sandy loam loam and clay loam soils on particle erosion rates it demonstrates that the interaction of plants and soil are important seventy one samples from the banks of 5 streams were tested using a flume multivariate regression analysis was used to analyze a dependent variable of erosion rate with independent variables of root density soil texture and plant communities autocorrelation was treated with the maximum chi square method nebraska sedge carex nebrascensis and baltic rush juncus balticus communities had the lowest erosion rates followed by mixed sedge carex lanuginosa c rostrata and others then mixed grass poa pratensis with deschampsia caespitosa communities silt had a negative effect on erosion for the nebraska sedge baltic rush and mixed grass plant communities but had no effect with the mixed sedge community due to a silt by sedge interaction an increase in percent clay correlated with an increase in erosion as percentages of clay or silt increased root volume density declined root volume density was correlated negatively with erosion clay to silt ratio correlated negatively with erosion thus knowledge of both vegetation and soil texture are necessary to predict or manage strembank erodibility

analysis of cloud resolving model simulations for scale dependence of convective momentum transport abstractwe use 3d cloud resolving model crm simulations of two mesoscale convective systems at midlatitudes and a simple statistical ensemble method to diagnose the scale dependency of convective

mesoscale momentum budget in a midlatitude squall line a numerical case study abstract a meso scale momentum budget and its effect on larger scale mean flow in a midlatitude mesoscale convective system are investigated using a numerical simulation of an intense squall line that occurred during 10 11 june 1985 pre storm it is found that the momentum generation normal to the line associated with the latent heating and cooling contributes most significantly to the momentum budget and determines the meso scale internal structure and evolution of the squall line the momentum generation along the line contributes to the initial development of a mesovortex but has little effect on the final vertical structure of the along line flow both vertical and horizontal momentum advection have significant contributions particularly to the vertical mixing of the along line flow and this component of the horizontal momentum is locally transported down gradient it is also found that for midlatitude convective systems convectively generated downdrafts can play as prominent a role as updrafts i

the effect of horizontal pressure gradients on the momentum transport in tropical convective lines part ii lagrangian calculations abstract the movement of a set of lagrangian parcels in the two dimensional pressure field associated with a tropical convective line is considered the results are compared with observational characteristics of gate quasi two dimensional convective lines it is shown that for the composite slow moving gate convective line it is necessary to account for horizontal pressure gradients in order to obtain realistic momentum flux the role of vertical pressure gradients and initial conditions for an air parcel s movement is studied based on the results some suggestions concerning proper formulation of parameterization of the convective momentum flux are made

parameterization of convective momentum transport in highly baroclinic conditions the organization of convection by vertical shear strongly affects the transport of horizontal momentum yet the concept of organization has received little attention in convective parameterization here the focus is on open cellular convection in cold air outbreaks under strongly baroclinic conditions that occur to the rear of midlatitude cyclones the principles derived herein are envisaged to apply to baroclinic advection at large in similar shear flows open cellular convection forced by surface fluxes of sensible and latent heat and organized by unidirectional shear is simulated using a three dimensional cloud resolving numerical model the effects of the in cloud pressure field organization and shear on momentum transport are quantified for the simulated clouds a simple analytic model of blocked overturning provides dynamical insight into downgradient momentum transport by the simulated three dimensional cumulonimbus ensemble the overturning circulation having vorticity of the same sign as the ambient shear maintains the far field flow while the blocking effect decelerates in cloud momentum the combined effect is to maintain the mean flow somewhat below its undisturbed value the numerical results and analytic predictions are used to evaluate and interpret two parameterization schemes for convective momentum transport used in operational global weather prediction and climate models at the u k meteorological office and the european centre for medium range weather forecasts finally the authors comment on how the downgradient momentum transport by three dimensional cumulus convection contrasts with momentum mixing concepts as well as countergradient momentum transport by two dimensional squall lines

convective momentum transport and perturbation pressure field from a cloud resolving model simulation this study uses a 2d cloud resolving model to investigate the vertical transport of horizontal momentum and to understand the role of a convection generated perturbation pressure field in the momentum transport by convective systems during part of the tropical ocean and global atmosphere coupled ocean atmosphere response experiment toga coare intensive observation period it shows that convective updrafts transport a significant amount of momentum vertically this transport is downgradient in the easterly wind regime but upgradient during a westerly wind burst the differences in convective momentum transport between easterly and westerly wind regimes are examined the perturbation pressure gradient accounts for an important part of the apparent momentum source in general it is opposite in sign to the product of cloud mass flux and the vertical wind shear with smaller magnitude examination of the dynamic forcing to the pressure field demonstrates that the linear forcing representing the interaction between the convective updrafts and the large scale wind shear is the dominant term while the nonlinear forcing is of secondary importance thus parameterization schemes taking into account the linear interaction between the convective updrafts and the large scale wind shear can capture the essential features of the perturbation pressure field the parameterization scheme for momentum transport by zhang and cho is evaluated using the model simulation data the parameterized pressure gradient force using the scheme is in excellent agreement with the simulated one the parameterized apparent momentum source is also in good agreement with the model simulation other parameterization methods for the pressure gradient are also discussed

organized convective systems in the tropical western pacific as a process in general circulation models a toga coare case study we examine the large scale effects of organized convective systems in the tropical western pacific observed during the tropical ocean global atmosphere coupled ocean atmosphere response experiment toga coare in a case study approach we examine realizations of a supercluster associated with the onset of the december 1992 westerly wind burst in the t213 operational medium range weather forecasting model of the european centre for medium range weather forecasts ecmwf we idealize a supercluster as a hierarchy of three interacting scales namely organized cumulonimbus c 1 mesoscale convective systems c 2 and the supercluster component c 3 it is shown that the ecmwf model represents this hierarchy as a c 3 like surrogate whose influence dominates the effect of parametrized convection this causes over prediction of the model tendencies which in the case of zonal momentum is explained in elementary terms the structure of the resolved scale momentum flux is explained by moncrieff s 1992 archetypal theory of organized convection which has been verified against observations and cloud resolving model data sets the parametrization of subgrid scale convective momentum flux in the ecmwf model based on a momentum mixing concept produces subgrid scale tendencies that are physically different from transports associated with cumulonimbus convection in a shear flow we outline a strategy for parametrizing the momentum flux by the c 1 component based on the archetypal model the c 2 component which is part resolved and part parametrized is at odds with the assumptions of scale separation underpinning parametrization it is argued that this component should be represented as part of the prognostic treatment of convectively generated cirrus finally we suggest cloud resolving modelling studies to further quantify the structure and large scale impact of superclusters in a westerly wind burst environment ranging from idealized models to models having data assimilation capability

sawmill brook an example of rapid geomorphic change related to urbanization the watershed of sawmill brook is undergoing urbanization that has increased the frequency of bankfull discharge the increased runoff is causing extensive bank erosion in the main channel that has increased the size of the bed material and the rate of bedload discharge the increased frequency of moderate floods the channel widening and the change in the sediment transport regime is causing a change from a meandering to a braided channel pattern with continued urbanization of the basin the present disequilibrium of the channel will be enhanced resulting in an unstable channel pattern for sawmill brook

the past is a guide to the future comparing middle pliocene vegetation with predicted biome distributions for the twenty first century during the middle pliocene the earth experienced greater global warmth compared with today coupled with higher atmospheric co 2 concentrations to determine the extent to which the middle pliocene can be used as a test bed for future warming we compare data and model based middle pliocene vegetation with simulated global biome distributions for the mid and late twenty first century the best agreement is found when a middle pliocene biome reconstruction is compared with a future scenario using 560 ppmv atmospheric co 2 in accordance with palaeobotanical data all model simulations indicate a generally warmer and wetter climate resulting in a northward shift of the taiga tundra boundary and a spread of tropical savannahs and woodland in africa and australia at the expense of deserts our data model comparison reveals differences in the distribution of polar vegetation which indicate that the high latitudes during the middle pliocene were still warmer than its predicted modern analogue by several degrees however our future scenarios do not consider multipliers associated with long term climate sensitivity changes in global temperature and thus biome distributions at higher atmospheric co 2 levels will not have reached an equilibrium state as is the case for the middle pliocene by the end of this century

seasonal migration of itcz precipitation across the equator why can t gcms simulate it 1 the secondary meridional circulation induced by convective momentum transport cmt within the ascending branch of the hadley circulation is a missing dynamical mechanism that can cause common failure of general circulation models gcms in simulating seasonal migration of the intertropical convergence zone itcz precipitation maximum across the equator this failure is manifested by the model bias that the precipitation peak remains north of the equator during november march the cmt induced secondary circulation characterized by strong downward motion along the equatorial belt and upward motion south and north of the belt tends to modify the meridional distribution of precipitation with the strongest impacts during boreal winter and spring a 20 year gcm simulation with the cmt parameterization successfully reproduces the observed seasonal migration of the itcz precipitation across the equator with the peaks near 8 n during boreal summer and near 8 s during boreal winter

vegetation cover in a warmer world simulated using a dynamic global vegetation model for the mid pliocene in this study we employ the triffid top down representation of interactive flora and foliage including dynamics dynamic global vegetation model dgvm and the hadley centre atmospheric general circulation model version 3 hadam3 gcm to investigate vegetation distributions and climate vegetation feedbacks during the mid pliocene and examine the implications of these results for the origins of hominid bipedalism the triffid model outputs support extant palaeoenvironmental reconstructions for the mid pliocene provided by the prism group pliocene research interpretations and synoptic mapping compared to the pre industrial triffid simulates a significant increase in forest cover during the mid pliocene composed of needle leaf trees in the higher latitudes of the northern hemisphere and broad leaf trees in other regions needle leaf trees extend from the arctic coast into the northern mid latitudes the fractional coverage of bare soil declines in north africa the arabian peninsula australia and southern south america a pattern that is consistent with prism s assertion of less extensive arid deserts a significant increase in the fractional coverage of both broad leaf trees in africa and south america in the mid pliocene scenario is not indicative of a major expansion of tropical rainforests rather it represents an expansion of general woodland type habitats the principal impact of using a dgvm on the gcm predicted climatology for the mid pliocene is to reduce minimum and maximum temperature extremes thus reducing the seasonality of temperature over wide regions the predicted pliocene expansion in broad leaf trees in africa is difficult to reconcile with the savannah hypothesis for the evolution of hominid bipedalism rather the results lend credence to an alternative hypothesis which suggests that bipedalism evolved in wooded to forested ecosystems and was for several million years linked to arborealism

parametrization of momentum transport by convection ii tests in single column and general circulation models diagnostics derived from cloud resolving model simulations in part i of this study relating to the vertical transport of horizontal momentum by convection are used to develop a parametrization of convective momentum transports for deep convection based upon the mass flux convection scheme discussed by gregory and rowntree the importance of cloud pressure gradients to the flow within the cloud is emphasised and a simple method of representing their effect is suggested the scheme is able to reproduce the fluxes derived from the cloud resolving model studies where cloud organization by the flow is unimportant inclusion of the parametrization in a version of the meteorological office unified model demonstrates that convective momentum transports play a large role in the momentum balance of the atmosphere generally simulation of the mean atmospheric circulation by the unified model is improved by the inclusion of such transports

pliocene climate variability northern annular mode in models and tree ring data the northern annular mode nam and its regional expression the north atlantic oscillation nao are the primary interannual oscillatory systems in the northern hemisphere in the modern climate nam has been linked with a number of weather extremes including large terrestrial temperature increases and extreme snowfalls its connection to climate is more controversial although a change in its behaviour between 1970 and 2000 coincided with rapid temperature increases in the northern hemisphere the north atlantic and nordic seas are a key component in pliocene climate showing the largest increases from modern sea surface temperature to understand these changes and the ability of climate models to reproduce them we must consider simulations of the nam here we show that existing mid pliocene simulations exhibit large changes to the mean state and variability in the north atlantic and a significant dampening of the nao through sensitivity experiments this change is primarily attributed to the impact of the lowering of the rocky mountains as the timing of rocky mountains uplift is still disputed simulations of the north atlantic region contain significant uncertainty particularly relating to interannual variability and its climate feedbacks new high temporal resolution climate proxy data is required to test these model reconstructions here we report new annual resolution data and an analysis of climate variability from fossil tree rings these fossils from sites in the canadian arctic provide support for a strong north atlantic oscillation during the pliocene

the march 1997 westerly wind event and the onset of the 1997 98 el nino understanding the role of the atmospheric response in a previous study the effect of the march 1997 westerly wind event wwe on the evolution of the tropical pacific ocean was studied using an ocean general circulation model gcm the response was characterized by i a cooling of the far western pacific 0 88c ii a rapid eastward displacement of the warm pool 2000 km in a month and iii a weak warming of the central eastern pacific along the path of the oceanic kelvin wave excited by the wwe 0 58c in this study the atmospheric response to these aspects of the sea surface temperature sst response are investigated using an atmospheric gcm forced with the sst anomalies from the ocean only experiments the results have demonstrated that the three aspects of the sst anomaly field generated by the wwe

the influence of the shear induced pressure gradient on thunderstorm motion abstract in the present investigation we propose a simple theory to explain how a veering environmental wind shear vector can cause an initially symmetric updraft to grow preferentially to the right of the shear vector and acquire cyclonic rotation the explanation offered is based on linear theory which predicts that interaction of the mean shear with the updraft produces favorable vertical pressure gradients along its right flank to asses the validity of linear theory for large amplitude updrafts the three dimensional shallow anelastic equations are numerically integrated using a simple parameterization for latent heating within a cloud and the linear and nonlinear forcing terms are separately analyzed these results suggest that although the nonlinear effects strongly promote splitting of the updraft the linear forcing remains the dominant factor in preferentially enhancing updraft growth on the right flank we believe this differential forcing is a major contributor to the observed predominance o

magnitude of climate variability during middle pliocene warmth a palaeoclimate modelling study abstract the nature and magnitude of climatic variability during the period of middle pliocene warmth ca 3 29 2 97 ma is poorly understood we present a suite of palaeoclimate modelling experiments incorporating an advanced atmospheric general circulation model gcm coupled to a q flux ocean model for 3 29 3 12 and 2 97 ma bp astronomical solutions for the periods in question were derived from the berger and loutre bl2 astronomical solution boundary conditions excluding sea surface temperatures ssts which were predicted by the slab ocean model were provided from the usgs prism2 2 2 digital data set the model results indicate that little annual variation 0 5 c in ssts relative to a control experiment occurred during the middle pliocene in response to the altered orbital configurations annual surface air temperatures also displayed little variation seasonally surface air temperatures displayed a trend of cooler temperatures during december january and february and warmer temperatures during june july and august this pattern is consistent with altered seasonality resulting from the prescribed orbital configurations precipitation changes follow the seasonal trend observed for surface air temperature compared to present day surface wind strength and wind stress over the north atlantic north pacific and southern ocean remained greater in each of the pliocene experiments this suggests that wind driven gyral circulation may have been consistently greater during the middle pliocene the trend of climatic variability predicted by the gcm for the middle pliocene accords with geological data however it is unclear if the model correctly simulates the magnitude of the variation this uncertainty is derived from a the relative insensitivity of the gcm to perturbation in the imposed boundary conditions b a lack of detailed time series data concerning changes to terrestrial ice cover and greenhouse gas concentrations for the middle pliocene and c difficulties in representing the effects of climatic history in snap shot gcm experiments

constraints on saturn s tropospheric general circulation from cassini iss images abstract an automated cloud tracking algorithm is applied to cassini imaging science subsystem high resolution apoapsis images of saturn from 2005 and 2007 and moderate resolution images from 2011 and 2012 to define the near global distribution of zonal winds and eddy momentum fluxes at the middle troposphere cloud level and in the upper troposphere haze improvements in the tracking algorithm combined with the greater feature contrast in the northern hemisphere during the approach to spring equinox allow for better rejection of erroneous wind vectors a more objective assessment at any latitude of the quality of the mean zonal wind and a population of winds comparable in size to that available for the much higher contrast atmosphere of jupiter zonal winds at cloud level changed little between 2005 and 2007 at all latitudes sampled upper troposphere zonal winds derived from methane band images are 10 m s 1 weaker than cloud level winds in the cores of eastward jets and 5 m s 1 stronger on either side of the jet core i e eastward jets appear to broaden with increasing altitude in westward jet regions winds are approximately the same at both altitudes lateral eddy momentum fluxes are directed into eastward jet cores including the strong equatorial jet and away from westward jet cores and weaken with increasing altitude on the flanks of the eastward jets consistent with the upward broadening of these jets the conversion rate of eddy to mean zonal kinetic energy at the visible cloud level is larger in eastward jet regions 5 2 10 5 m 2 s 3 and smaller in westward jet regions 1 6 10 5 m 2 s 3 than the global mean value 4 1 10 5 m 2 s 3 overall the results are consistent with theories that suggest that the jets and the overturning meridional circulation at cloud level on saturn are maintained at least in part by eddies due to instabilities of the large scale flow near and or below the cloud level

a new parameterization for shallow cumulus convection and its application to marine subtropical cloud topped boundary layers part i description and 1d results abstract a new parameterization of shallow cumulus convection is presented the parameterization consists of a mass flux scheme based on a buoyancy sorting entrainment detrainment plume model the mass flux scheme is coupled to a 1 5 order turbulence closure model with an entrainment closure for convective boundary layers model performance is verified using single column model simulations at relatively high vertical resolution of pure trade cumulus convection and a cumulus to stratocumulus transition mixing rates cloud cover and vertical flux profiles as deduced from previously published large eddy simulation studies are well reproduced by the parameterization the model is used to demonstrate that height variations of lateral mixing rates can be successfully captured by a simple implementation of a buoyancy sorting mechanism in the updraft cloud model a companion paper describes the implementation of the scheme in a mesoscale model

the effect of horizontal resolution on indian monsoon depressions in the met office nwp model monsoon depressions are synoptic scale features that are responsible for a significant fraction of the rain over northern india during the summer monsoon season and as such it is important to quantify their structure and behaviour in numerical weather prediction models it is known that increasing model resolution is strongly correlated with improved forecasts in the short term and global circulation in the longer term as well as better representation of tropical cyclones here we explore the sensitivity of depressions to changes in resolution using the met office unified model seven nwp case studies of depressions from 2013 15 were run at eight resolutions corresponding to equatorial grid spacing of between 16 and 208 km and compared with data for the same events from trmm and era interim reanalysis we found that at the low resolution end of the spectrum increases in resolution led to improvements in the composite structure but with diminishing returns the model also persistently overestimated the depression intensity in particular the wind speed and the warm core aloft with the source appearing to originate in the mid troposphere the sensitivity of the diurnal cycle to resolution was also explored the stratiform component was found to be very well represented by the model whereas the convective component was described quite poorly improvement in most components of structure with increasing model resolution were marginal beyond n320 63 km and n512 39 km for dynamic and thermodynamic fields respectively

a unified convection scheme unicon part i formulation abstractthe author develops a unified convection scheme unicon that parameterizes relative i e with respect to the grid mean vertical flow subgrid vertical transport by nonlocal asymmetric turbulent eddies unicon is a process based model of subgrid convective plumes and mesoscale organized flow without relying on any quasi equilibrium assumptions such as convective available potential energy cape or convective inhibition cin closures in combination with a relative subgrid vertical transport scheme by local symmetric turbulent eddies and a grid scale advection scheme unicon simulates vertical transport of water species and conservative scalars without double counting at any horizontal resolution unicon simulates all dry moist forced free and shallow deep convection within a single framework in a seamless consistent and unified way it diagnoses the vertical profiles of the macrophysics fractional area plume radius and number density as well as the microphysics production and evaporati

high resolution simulation of mean convection and its intraseasonal variability over the tropics in the mri jma 20 km mesh agcm abstract tropical mean convection and its organization on different spatiotemporal scales in a simulation using the meteorological research institute japan meteorological agency mri jma global atmospheric general circulation model agcm at 20 km resolution tl959l60 has been investigated comparison with two lower resolution simulations of 120 km tl159l40 and 180 km tl95l40 shows that convection climatological fields and moist stability over the indian ocean and south pacific convergence zone are better represented in tl959l60 than in lower resolution simulations however the simulated three dimensional structure of the walker and hadley cells and the vertical structure of convective heating do not show marked improvement relative to lower resolution simulations the amplitude and phase speed of convectively coupled equatorial waves show that although the kelvin waves the observed hierarchical structure of cloud clusters associated with the madden julian oscillation mjo convection and th

determining sources and transit times of suspended sediment in the murrumbidgee river new south wales australia using fallout 137cs and 210pb sediment budgets typically require an estimate of the proportional yield from erosion sources to sediments in transport and storage this becomes increasingly difficult as catchments become larger and erosion storage and deposition processes become more complex we demonstrate how fallout radionuclides can be used to estimate the proportional contributions to sediment load from a tripartite classification of erosion sources in a large catchment the mid murrumbidgee 13 500 km2 the three major potential sources of sediment within this catchment are cultivated lands 22 of the surface area uncultivated pastoral lands 78 and the numerous channels and gullies found in this region concentrations of the fallout radionuclides 210pbex and 137cs in representative samples from each of these three sources are significantly different employing these values in a simple mixing model shows that the largest contribution of material is currently derived from subsoil channel gully sources alterations to the suspended sediment 210pbex signature by in situ labeling and decay are also considered applying the model with different concentrations of 210pbex decayed as a function of residence time in channels suggests that the mean residence time of fine grained material within this system is 10 5 years however differences in 137cs concentrations observed between flood and low flow sediments and the presence of the short lived cosmogenic 7be suggest that residence time of some of this fine grained material may be of the order of only weeks to months

a new technique for the automatic monitoring of erosion and deposition rates the rates and processes of erosion and deposition of soils and sediments are subjects of widespread and increasing concern in the earth and environmental sciences process inference from field studies however has been hampered by a lack of information on the precise magnitude frequency and timing of erosional and depositional activity because no automatic monitoring technique has hitherto been available i describe here an automatic photo electronic erosion pin peep system which apparently for the first time allows quasi continuous time series of erosion and deposition data to be collected example results from a river bank site show how the peep system helps to define the true temporal distribution of geomorphological change quantify the erosional impact of individual forcing events and discriminate between competing hypotheses of process control in erosional and depositional contexts the system should thus allow more effective testing of erosion models of high temporal resolution and facilitate a more rigorous linking of catchment sediment output to supply dynamics

triggering of el nino by westerly wind events in a coupled general circulation model two ten members ensemble experiments using a coupled ocean atmosphere general circulation model are performed to study the dynamical response to a strong westerly wind event wwe when the tropical pacific has initial conditions favourable to the development of a warm event in the reference ensemble cref no wind perturbation is introduced whereas a strong westerly wind event anomaly is introduced in boreal winter over the western pacific in the perturbed ensemble cwwe our results demonstrate that an intense wwe is capable of establishing the conditions under which a strong el nino event can occur first it generates a strong downwelling kelvin wave that generates a positive sea surface temperature sst anomaly in the central eastern pacific amplified through a coupled ocean atmosphere interaction this anomaly can be as large as 2 5 c 60 days after the wwe secondly this wwe also initiates an eastward displacement of the warm pool that promotes the occurrence of subsequent wwes in the following months these events reinforce the initial warming through the generation of additional kelvin waves and generate intense surface jets at the eastern edge of the warm pool that act to further shift warm waters eastward the use of a ten members ensemble however reveals substantial differences in the coupled response to a wwe whereas four members of cwwe ensemble develop into intense el nino warming as described above four others display a moderate warming and two remains in neutral conditions this diversity between the members appears to be due to the internal atmospheric variability during and following the inserted wwe in the four moderate warm cases the warm pool is initially shifted eastward following the inserted wwe but the subsequent weak wwe activity when compared to the strong warming cases prevents to further shift the warm pool eastwards the seasonal strengthening of trade winds in june july can therefore act to shift warm waters back into the western pacific reducing the central eastern pacific warming this strong sensitivity of the coupled response to wwes may therefore limit the predictability of el nino events as the high frequency wind variability over the warm pool region remains largely unpredictable even at short time lead

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

evolution of a subgrid deep convection parametrization in a limited area model with increasing resolution the adaptation of a convective parametrization to high resolution has been successfully performed to a certain point by refinements in the representation of the entrainment detrainment and some other features of the original general circulation model oriented scheme but passing to resolutions below 10 km requires dispensing with some fundamental hypotheses starting with quasi equilibrium and the assumption of negligible up and downdraught mesh fractions a prognostic scheme has been used for the draughts vertical velocities and mesh fractions the prognostic closure of the convective scheme has many benefits but additional enhancements are required starting with a coherent treatment of the cloud condensates and a more integrated combination of the different schemes producing condensation cloud and precipitation copyright 2005 royal meteorological society

parametrization of momentum transport by convection i theory and cloud modelling results summary the theory of the mass flux approach to parametrization of convective momentum transport is presented a cloud resolving model is used to simulate momentum transport by ensembles of deep convective clouds in two very different regimes a mid latitude cold air outbreak and tropical convection forced by convergence idealized unidirectional wind profiles are used to simplify interpretation of the results diagnostics relevant to the parametrization problem are presented and it is shown that for both regimes the approximations inherent in the parametrization equations are reasonable the pressure gradients inside clouds play an important role in determining horizontal velocities in the clouds for the cold air outbreak with linear shear the results suggest that these pressure gradients are proportional to the shear and to the up downdraught mass fluxes sensitivity studies suggest that the results are not very sensitive to resolution or parametrization of subgrid scale processes giving some confidence that the results are reasonably accurate for the tropical case with a low level jet the pressure gradients change sign with the wind shear in part i1 of this paper these results will be used to develop and validate a mass flux parametrization of convective momentum transport which is tested in single column and global versions of the meteorological office unified model

the measurement of river bank erosion and lateral channel change a review a detailed review and chronological survey is presented of the various techniques which have been used for the measurement of river bank erosion and channel change the techniques are classified according to the time scales involved long intermediate and short and each is discussed with respect to accuracy and repeatability the methods covered include sedimentological evidence botanical evidence historical sources planimetric resurvey repeated cross profiling erosion pins and terrestrial photogrammetry prospects for future developments are also discussed

downstream fining in large sand bed rivers abstract downstream fining of bed sediments is a well known phenomenon in gravel bed rivers but also occurs in large sand bed rivers the underlying processes however are not necessarily the same due to the difference in sediment mobility grain size distribution width and geographical setting downstream fining studies focusing on large sand bed rivers are scarce nevertheless in this paper the processes that affect downstream fining in gravel bed rivers are reviewed and it is evaluated to what extent they are relevant for large sand bed rivers furthermore several new downstream fining processes are discussed ending in a conceptual model of downstream fining in large sand bed rivers in deriving the conceptual model downstream fining processes are divided into three categories abrasion selective transport and sediment addition extraction abrasion rates in large sand bed rivers are small due to the durable lithologies the small grain size the high degree of rounding the dominance of grinding as abrasion process and the dominant suspended transport mode of the bed sediment only in the upstream part of large sand bed rivers abrasion rates may be somewhat higher selective transport rates in large sand bed rivers everywhere overrun abrasion rates but are small compared to selective transport rates in gravel bed rivers because the unimodal sediments in sand bed rivers exhibit stronger hiding exposure effects during threshold of motion conditions but especially because many sand bed rivers though not all are in state of fully mobilised transport making differences in threshold of motion between coarse and fine grains relatively unimportant selective transport in these rivers is probably mainly the result of the presence of suspended load transport in combination with the effects of dune and perhaps bend sorting which are all processes that act on a relatively large scale within sand bed rivers the degree of selective transport probably decreases downstream due to the change in mixture bimodality and sorting efficiency there are indications that coarse grains may sometimes even be more mobile than fine grains in the downstream part of sand bed rivers note that in contrast to abrasion selective transport only produces a stable downstream fining pattern if the river profile is concave generally a stronger profile concavity leads to a higher aggradation rate and a stronger downstream fining trend sediment addition and extraction processes can obscure the effects of abrasion and selective transport in sand bed rivers overbank sedimentation for instance may significantly decrease the downstream fining rate in large sand bed rivers because especially fine grains are removed from the bed dredging can also strongly affect the downstream fining trend depending on the volume and the grain size of the sediment involved meander migration may cause a nett loss of coarse grains from the channel in aggrading circumstances increasing the downstream fining rate river bifurcations can cause a discontinuity in the downstream fining pattern because bend sorting upstream of the bifurcation causes the river branch that originates in the outer bend of the main channel to be much coarser than the other branch all these addition and extraction processes not only have a direct effect on the grain size composition of the river bed but also indirectly influence downstream fining by their effect on the longitudinal bed profile other sediment addition and extraction processes such as tributary confluences non alluvial sediment sources and dumping are less common in large sand bed rivers but can locally have a great effect a full determination of the relative importance of all downstream fining processes in large sand bed rivers requires extensive field measurements the construction of detailed fractionwise sediment balances and the development of numerical downstream fining models that incorporate all processes described above

coupling of convective momentum transport with convective heating in global climate simulations abstract the effects of convective momentum transport cmt on global climate simulations are examined in this study comparison between two sets of 20 yr 1979 98 integration using the ncar community climate model version 3 ccm3 illustrates that the inclusion of cmt in the convection scheme systematically modifies the climate mean state over the equatorial region the convective momentum tendencies slow down the equatorward flow at higher latitudes near the surface and weaken the equatorial convergence and convection this reduces the convective heating and drying around the equator and produces an improved meridional distribution within the upward branch of the hadley circulation the major heating peak during the boreal winter is moved to south of the equator at about 10 s which is closer to the heat budget residuals of the ecmwf reanalysis data the responses of meridional wind to the reduced heating result in the secondary meridional circulation within the intertropical convergence zone

a comprehensive mass flux scheme for cumulus parameterization in large scale models abstract observational studies indicate that a mass flux approach may provide a realistic framework for cumulus parameterization in large scale models but this approach through the introduction of a spectral cloud ensemble leads normally to rather complex schemes in this paper the question is addressed whether much simpler schemes can already provide realistic values of the thermal forcing by convection under various synoptic conditions this is done through verifying such a scheme first on data from field experiments for periods of tropical penetrative convection gate marshall islands tradewind cumuli atex bomex and extratropical organized convection sesame 79 and then in a nwp model the scheme considers a population of clouds where the cloud ensemble is described by a one dimensional bulk model as earlier applied by yanai et al in a diagnostic study of tropical convection cumulus scale downdrafts are included various types of convection are represented i e penetrative convection in c

in situ quantification of spatial and temporal variability of hyporheic exchange in static and mobile gravel bed rivers seepage meters modified for use in flowing water were used to directly measure rates of exchange between surface and subsurface water in a gravel and cobble bed river in western pennsylvania usa allegheny river qmean 190 m3 s and a sand and gravel bed river in colorado usa south platte river qmean 9 7 m3 s study reaches at the allegheny river were located downstream from a dam the bed was stable with moss algae and river grass present in many locations median seepage was 0 28 m d and seepage was highly variable among measurement locations upward and downward seepage greatly exceeded the median seepage rate ranging from 2 26 upward to 3 76 downward m d at the south platte river site substantial local scale bed topography as well as mobile bedforms resulted in spatial and temporal variability in seepage greatly in exceedence of the median groundwater discharge rate of 0 24 m d both upward and downward seepage were recorded along every transect across the river with rates ranging from 2 37 to 3 40 m d despite a stable bed which commonly facilitates clogging by fine grained or organic sediments seepage rates at the allegheny river were not reduced relative to those at the south platte river seepage rate and direction depended primarily on measurement position relative to local and meso scale bed topography at both rivers hydraulic gradients were small at nearly all seepage measurement locations and commonly were not a good indicator of seepage rate or direction therefore measuring hydraulic gradient and hydraulic conductivity at in stream piezometers may be misleading if used to determine seepage flux across the sediment water interface such a method assumes that flow between the well screen and sediment water interface is vertical which appears to be a poor assumption in coarse grained hyporheic settings copyright 2011 john wiley sons ltd

characterization of momentum transport associated with organized moist convection and gravity waves abstract tropical convection is inherently multiscalar involving complex fields of clouds and various regimes of convective organization ranging from small disorganized cumulus up to large organized convective clusters in addition to being a crucial component of the atmospheric water cycle and the global heat budget tropical convection induces vertical fluxes of horizontal momentum there are two main contributions to the momentum transport the first resides entirely in the troposphere and is due to ascent descent and organized circulations associated with precipitating convective systems the second resides in the troposphere stratosphere and farther aloft and is caused by vertically propagating gravity waves both the convective momentum transport and the gravity wave momentum flux must be parameterized in general circulation models yet in existing parameterizations these two processes are treated independently this paper examines the relationship between the convective momentum transport and

the tropospheric response to tropical and subtropical zonally asymmetric torques analytical and idealized numerical model results abstractthe tropospheric response to prescribed tropical and subtropical zonally asymmetric torques which can be considered as idealizations of vertical momentum transfers by orographic gravity waves or convection is investigated the linear analytical gill model response to westward upper tropospheric torques is compared to the response to a midtropospheric heating which is a familiar point of reference the response to an equatorial torque projects onto a kelvin wave response to the east that is of opposite sign to the response to the east of the heating at upper levels in contrast the torque and heating both produce rossby gyres of the same sign to the west of the forcing and the zonal mean streamfunction responses are identical when the forcings are shifted into the northern hemisphere the streamfunction responses have opposite signs there is upwelling in the southern northern hemisphere in response to the torque heating the nonlinear response to westward torques was explored in idealized