Horizontal Dispersion Research Articles

Characterisation of the muon beams for the Muon Ionisation Cooling Experiment

A novel single-particle technique to measure emittance has been developed and used to characterise seventeen different muon beams for the Muon Ionisation Cooling Experiment (MICE). The muon beams, whose mean momenta vary from 171 to 281 MeV/c, have emittances of approximately 1.5--2.3 \pi mm-rad horizontally and 0.6--1.0 \pi mm-rad vertically, a horizontal dispersion of 90--190 mm and momentum spreads of about 25 MeV/c. There is reasonable agreement between the measured parameters of the beams and the results of simulations. The beams are found to meet the requirements of MICE.

Submesoscale impacts on horizontal and vertical transport in the Gulf of Mexico

[1] Impacts of submesoscale processes on transport are investigated numerically in an energetic mesoscale flow with an ocean model run at two horizontal resolutions, 1 and 5 km. The focus is the northwestern Gulf of Mexico, where the Loop Current eddies are surrounded by smaller vortices. By increasing the horizontal resolution, the number and strength of submesoscale eddies and vorticity filaments within the mixed layer increase dramatically and with them the vertical velocities. Inside the coherent eddies and at their peripheries, increased vertical velocities for increasing resolution are associated to near-inertial motions and they are not limited to the mixed layer, but are found at all depths. Horizontal velocities, on the contrary, are similar. Lagrangian isobaric tracers are deployed close to the surface and at 100 m, and three-dimensional, neutrally buoyant particles are released close to the surface, at the base of the mixed layer and at 100 m. The modeled horizontal dispersion curves for each deployment depth are independent of the kind of particles and of horizontal resolution. Close to the ocean surface, however, convergence zones, generated by submesoscale ageostrophic motions and resolved at 1 km resolution, influence the details of the tracer distributions. Vertical dispersion increases by several folds for increasing resolution at all depths explored, with the largest differences found close to the surface. Therefore, submesoscales processes play a fundamental role in driving vertical transport in eddy-dominated flows, both within and below the mixed layer, for times comparable to the Eulerian time scale.

The reduction of plankton biomass induced by mesoscale stirring: A modeling study in the Benguela upwelling

Recent studies, both based on remote sensed data and coupled models, showed a reduction of biological productivity due to vigorous horizontal stirring in upwelling areas. In order to better understand this phenomenon, we consider a system of oceanic flow from the Benguela area coupled with a simple biogeochemical model of Nutrient-Phyto-Zooplankton (NPZ) type. For the flow three different surface velocity fields are considered: one derived from satellite altimetry data, and the other two from a regional numerical model at two different spatial resolutions. We compute horizontal particle dispersion in terms of Lyapunov exponents, and analyzed their correlations with phytoplankton concentrations. Our modeling approach confirms that in the south Benguela there is a reduction of biological activity when stirring is increased. Two-dimensional offshore advection and latitudinal difference in primary production, also mediated by the flow, seem to be the dominant processes involved. We estimate that mesoscale processes are responsible for 30–50% of the offshore fluxes of biological tracers. In the northern area, other factors not taken into account in our simulation are influencing the ecosystem. We suggest explanations for these results in the context of studies performed in other eastern boundary upwelling areas.

Simulations of Nearshore Particle-Pair Dispersion in Southern California

Abstract Knowledge of horizontal relative dispersion in nearshore oceans is important for many applications including the transport and fate of pollutants and the dynamics of nearshore ecosystems. Two-particle dispersion statistics are calculated from millions of synthetic particle trajectories from high-resolution numerical simulations of the Southern California Bight. The model horizontal resolution of 250 m allows the investigation of the two-particle dispersion, with an initial pair separation of 500 m. The relative dispersion is characterized with respect to the coastal geometry, bathymetry, eddy kinetic energy, and the relative magnitudes of strain and vorticity. Dispersion is dominated by the submesoscale, not by tides. In general, headlands are more energetic and dispersive than bays. Relative diffusivity estimates are smaller and more anisotropic close to shore. Farther from shore, the relative diffusivity increases and becomes less anisotropic, approaching isotropy ~10 km from the coast. The degree of anisotropy of the relative diffusivity is qualitatively consistent with that for eddy kinetic energy. The total relative diffusivity as a function of pair separation distance R is on average proportional to R5/4. Additional Lagrangian experiments at higher horizontal numerical resolution confirmed the robustness of these results. Structures of large vorticity are preferably elongated and aligned with the coastline nearshore, which may limit cross-shelf dispersion. The results provide useful information for the design of subgrid-scale mixing parameterizations as well as quantifying the transport and dispersal of dissolved pollutants and biological propagules.

Trajectory measurements and correlations in the final focus beam line at the KEK Accelerator Test Facility

The Accelerator Test Facility 2 (ATF2) commissioning group aims to demonstrate the feasibility of the beam delivery system of the next linear colliders (ILC and CLIC) as well as to define and to test the tuning methods. As the design vertical beam sizes of the linear colliders are about few nanometers, the stability of the trajectory as well as the control of the aberrations are very critical. ATF2 commissioning started in December 2008, and thanks to submicron resolution beam position monitors (BPMs), it has been possible to measure the beam position fluctuation along the final focus of ATF2 during the 2009 runs. The optics was not the nominal one yet, with a lower focusing to make the tuning easier. In this paper, a method to measure the noise of each BPM every pulse, in a model-independent way, will be presented. A method to reconstruct the trajectory's fluctuations is developed which uses the previously determined BPM resolution. As this reconstruction provides a measurement of the beam energy fluctuations, it was also possible to measure the horizontal and vertical dispersion function at each BPMs parasitically. The spatial and angular dispersions can be fitted from these measurements with uncertainties comparable with usual measurements.

Horizontal advection and dispersion in a stratified shelf sea: The role of inertial oscillations

The role played by inertial motions in horizontal dispersion within the thermocline of a broad, mid-latitude shelf sea is examined through the analysis of a deliberately released dye tracer. Our analysis is of the horizontal and vertical evolution over 40h of a dye tracer injected into the seasonally stratified thermocline of the Celtic Sea on the NW European Shelf. The inferred diapycnal diffusivity was 1.3–1.5×10−5m2s−1, and the radial horizontal diffusivities of the depth integrated dye patch ranged from 1.9 to 4.0m2s−1. The inferred vertical diffusivity is in agreement with microstructure based estimates, and the depth integrated horizontal diffusivity is broadly in agreement with previous dye release derived estimates made over similar scales and time periods. Asymmetry in the horizontal evolution of the dye patch was evident. We argue that mean shear dispersion was responsible for lateral elongation of the dye patch, particularly between hours 23 and 35 after release, during which time horizontal diffusivity along the major axis, Ka, exceeded that along the minor axis, Kb, by more than a factor of 10. We further show that along-patch shear was predominantly a result of differential advection between a deep residual flow to the south-east and an oscillating wind-driven surface Ekman layer. In this region of strong low frequency (inertial) shear a time dependent model of shear dispersion (Young et al., 1982) was able to account for the observed rate of horizontal dispersion calculated on the target isopycnal surface.

Turbulence disturbs vertical refuge use byChaoborus flavicanslarvae and increases their horizontal dispersion

Summary Small-scale turbulence is a key factor in increasing encounter rates but also dispersing prey patches of planktonic organisms and can thus contribute to their distribution. Our hypotheses were that turbulence can (i) disturb the vertical refuge use of fourth instar Chaoborus flavicans larvae and (ii) also disperse them horizontally. Both were tested experimentally, across a gradient of turbulence. The vertical experiments were conducted in 46-L cylinders, providing the larvae a vertical refuge by darkening the bottom layer. The horizontal experiments were conducted in a 200-L aquarium, providing them a horizontal refuge by darkening one end of the aquarium. With both set-ups, two light treatments were tested: uniform darkness as a control and the bottom layer of the cylinder or either end of the aquarium darkened to provide a refuge. In addition to non-turbulent treatment, five different root-mean-square (RMS) velocities were generated, ranging from 0.6 to 10.2 cm s−1 in the vertical and to 10.9 cm s−1 in the horizontal experiment. Increasing turbulence had a significant effect on the distribution of C. flavicans larvae in both vertical and horizontal directions and increased the proportion of larvae in the non-preferred illuminated environment. In the vertical direction, the proportion of larvae in the refuge decreased linearly with increasing turbulence, showing a decline from 84% under calm conditions to 61% at the highest RMS velocity (10.2 cm s−1). The effect of turbulence was more prominent in the horizontal direction; without turbulence, 86% of the larvae occupied the refuge, but the proportion declined to 32% at 5.4 cm s−1 RMS velocity. In experiments with uniform darkness, the distribution of chaoborids was not affected by turbulence either in the vertical or in the horizontal direction. Our results indicate that turbulence has substantial effects on refuge use and distribution of C. flavicans larvae. Increasing turbulence can greatly diminish the ability of prey species to use their vertical refuges. Once away from their vertical refuge, chaoborids are highly vulnerable to horizontal dispersion, thus upsetting their horizontal escape behaviour. Small-scale turbulence is therefore likely to have considerable effects on food-web dynamics in lakes.

RLINE: A line source dispersion model for near-surface releases

This paper describes the formulation and evaluation of RLINE, a Research LINE source model for near-surface releases. The model is designed to simulate mobile source pollutant dispersion to support the assessment of human exposures in near-roadway environments where a significant portion of the population spends time. The model uses an efficient numerical integration scheme to integrate the contributions of point sources used to represent a line-source. Emphasis has been placed on estimates of concentrations very near to the source line. The near-surface dispersion algorithms are based on new formulations of horizontal and vertical dispersion within the atmospheric surface layer, details of which are described in a companion paper (Venkatram et al., 2013). This paper describes the general formulations of the RLINE model, the meteorological inputs for the model, the numerical integration techniques, the handling of receptors close to the line source, and the performance of the model against developmental data bases and near-road concentrations from independent field studies conducted along actual highway segments.

Short-lived radium isotopes on the Scotian Shelf: Unique distribution and tracers of cross-shelf CO2 and nutrient transport

Radium (Ra) isotopes have become an established tool for investigating mixing rates on continental shelves, and more recently have been used to quantify the release of dissolved compounds enriched in pore-waters into the water column. We present results from Ra sampling of the Scotian Shelf region of the Canadian northwestern Atlantic Ocean, which reveal cross-shelf Ra distributions that are unique compared to other coastal regions. We explain the observations of lower 224Ra activities near the coast, relatively high activities at large distances offshore (>100km), and gradients in both offshore and onshore directions by referring to regional geomorphology, as well as shelf bathymetry and circulation patterns. Vertical mixing above shallow offshore banks allows for Ra enrichments in offshore surface waters, while horizontal dispersion of this bottom-generated signal can transport Ra off the shelf break in surface waters, and towards the shore beneath the surface mixed layer. On the Scotian Shelf, estimating horizontal diffusivity (KX) using a 1-D Ra diffusion model is limited by the presence of two distinct Ra sources (land and sediment), by three dimensional shelf topography, as well as by complex hydrodynamic conditions. Here, a numerical 2-D diffusion model reproduces Ra distributions on the shelf using both coastal and benthic sources. Horizontal and vertical mixing coefficients are then calculated by minimizing deviations between model output and observations. Onshore gradients in CO2 and nutrient species combined with model-derived KX values can yield onshore carbon and nutrient fluxes in subsurface waters, which in turn supply the CO2 outgassing from the Scotian Shelf. Our results thus provide constraints for cross-shelf transports of carbon and nutrients on the Scotian Shelf in order to guide mass balance or model based budget approaches in future studies.

Particulate pollution in Ulaanbaatar, Mongolia

The World Health Organization (WHO) listed the air pollution in Ulaanbaatar (Mongolia) among the top 5 cities with the worst air quality in the world. Air quality in the winter season reaches highs of 750 μg/m3 for daily average fine particulates (PM) due to increased coal combustion and lower mixing heights (<200 m), coupled with the city’s geography surrounded by mountains, which further restricts the vertical and horizontal dispersion of the pollutants. The annual average concentrations in 2010–2011 ranged 136 ± 114 μg/m3 (the WHO guideline for fine PM is 10 μg/m3). The single largest source of particulate pollution in Ulaanbaatar is coal and biomass combustion in households and heat-only boilers, followed by power plants. In this paper, we present sector-specific emissions for 2010 accounting for 62,000 tons of fine PM, 55,000 tons of sulfur dioxide, and 89,000 tons of nitrogen oxide emissions. The inventory is spatially disaggregated at 0.01° resolution on a GIS platform for use in a chemical transport model (ATMoS). The modeled concentrations for the urban area ranged 153 ± 70 μg/m3, when overlaid on gridded population, resulted in estimated 1,000–1,500 premature deaths per year due to outdoor air pollution. This study also highlights the linkages between indoor and outdoor air pollution. In these harsh temperate conditions, with 50 % of the emissions originating from Ger households, they are as big a health risk for indoor air quality as they are for outdoor air quality. Any intervention improving combustion efficiency or providing clean fuel for these stoves will have a combined benefit for indoor air quality, outdoor air quality, and climate policy. The analysis shows that aggressive pollution control measures are imperative to protect the population in Ulaanbaatar from excess exposure levels, and implementation of control measures like the introduction of heat efficient stoves, clean coal for heating boilers, and urban transport planning will result in significant health benefits, which surpass any costs of institutional, technical, and economic interventions.

Maintenance of Coastal Surface Blooms by Surface Temperature Stratification and Wind Drift

Algae blooms are an increasingly recurrent phenomenon of potentially socio-economic impact in coastal waters globally and in the coastal upwelling region off northern Baja California, Mexico. In coastal upwelling areas the diurnal wind pattern is directed towards the coast during the day. We regularly found positive Near Surface Temperature Stratification (NSTS), the resulting density stratification is expected to reduce the frictional coupling of the surface layer from deeper waters and allow for its more efficient wind transport. We propose that the net transport of the top layer of approximately 2.7 kilometers per day towards the coast helps maintain surface blooms of slow growing dinoflagellate such as Lingulodinium polyedrum. We measured: near surface stratification with a free-rising CTD profiler, trajectories of drifter buoys with attached thermographs, wind speed and direction, velocity profiles via an Acoustic Doppler Current Profiler, Chlorophyll and cell concentration from water samples and vertical migration using sediment traps. The ADCP and drifter data agree and show noticeable current shear within the first meters of the surface where temperature stratification and high cell densities of L. polyedrum were found during the day. Drifters with 1m depth drogue moved towards the shore, whereas drifters at 3 and 5 m depth showed trajectories parallel or away from shore. A small part of the surface population migrated down to the sea floor during night thus reducing horizontal dispersion. The persistent transport of the surface bloom population towards shore should help maintain the bloom in favorable environmental conditions with high nutrients, but also increasing the potential socioeconomic impact of the blooms. The coast wise transport is not limited to blooms but includes all dissolved and particulate constituents in surface waters.

Upgrade of the 1.2 GeV STB ring for the SR utilization in Tohoku University

An electron accelerator complex, 300 MeV linac and 1.2 GeV booster synchrotron, had been routinely operated as a user facility at Tohoku University until the Great East Japan Earthquake damaged the accelerator seriously. In the last year some budgets were approved to partially reconstruct the accelerator. For the booster ring, some old power supplies of magnets are replaced. Furthermore some quadrupole magnets are also replaced to the combined function magnets of which sextupole component is included. Modifying the ring optics so as to introduce the horizontal dispersion into the position of combined magnet, this replacement will make it possible to correct the chromaticity. There has been no sextupole in the ring, so that the ring current is significantly limited due to head-tail instability. Hence this upgrade will bring the new capability into the ring as a synchrotron light source. Presently it is planned to utilize SR from the 1.3 T bending magnet for educational purpose in the field of SR application such as XAFS. The photon flux of 5.6E12 photons/s/mrad2/0.1%BW will be obtained around the peak energy region of 1 keV for this beam line in the case of 200 mA operation with the emittance of 160 nmrad. In addition to the beam line from bending magnet, a 2 m long straight section is also reserved for an insertion device for future application.

New Climatic Indicators for Improving Urban Sprawl: A Case Study of Tehran City

In the modern world, the fine balance and delicate relationship between human society and the environment in which we exist has been affected by the phenomena of urbanisation and urban development. Today, various environmental factors give rise to horizontal dispersion, spread and growth of cities. One of the most important results of this is climatic change which is directly affected by the urban sprawl of every metropolis. The aim of this study is to identify the relationship between the various horizontally distributed components of Tehran city and changes in essential microclimate clusters, by means of the humidex index. Results showed that, when the humidex was calculated for each of the obtained clusters, it was evident that it had increased with time, in parallel with Shannon’s entropy, as a consequence of the average temperature and relative humidity of each cluster. At the same time, results have shown that both temperature and relative humidity of the study area are related with urban sprawl, urbanisation and development, as defined by Shannon’s entropy and, in consequence, with humidex. In consequence, this new concept must be considered in future research works to predict and control urban sprawl and microclimate conditions in cities.

Solute dispersion in the coastal boundary layer of southern Lake Michigan

We evaluate a three‐dimensional, nested‐grid nearshore model of Lake Michigan for its ability to describe key aspects of hydrodynamics and solute transport using data from a field study conducted in summer 2008. Velocity comparisons with observations from five bottom‐mounted ADCPs at different depths in the coastal boundary layer (CBL) show that the numerical model was able to simulate currents and flow reversals accurately within the inertial boundary layer, however model accuracy reduced close to the shoreline. Power spectra of observed and simulated velocity time series at different locations showed that the hydrodynamic model was able to describe the energy contained in the inertial scales, but over‐predicted turbulent dissipation rates. As a result, model‐predicted values of energy in the smaller, dissipation scales were lower compared to values calculated from observations in the CBL. Differences between energy contained in the observed and simulated velocity spectra increased as the shoreline is approached. Observations showed that vertical variations in the alongshore and cross‐shore velocities were dominated by inertial waves. Inaccuracies in representing energy dissipation rates and processes not explicitly described in the hydrodynamic model (e.g., anisotropy, waves) could potentially contribute to errors in describing transport in the CBL. Measurements from a continuous dye release experiment from a riverine outfall were described using a nearshore model with a mean horizontal dispersion coefficient of 5.6 m2/s. Improved representations of physical processes (such as turbulence, internal waves and wave‐current interactions) can be expected to provide better descriptions of solute transport in the CBL.

Strong particle dispersion by weakly dissipative random internal waves

AbstractSimple stochastic models and direct nonlinear numerical simulations of three-dimensional internal waves are combined in order to understand the strong horizontal particle dispersion at second-order in wave amplitude that arises when small-amplitude internal waves are exposed to weak dissipation. This is contrasted with the well-known results for perfectly inviscid internal waves, in which such dispersion arises only at fourth-order in wave amplitude.

Internal wave‐driven transport of fluid away from the boundary of a lake

A field experiment was conducted to study transport of fluid from the boundary to the interior of a lake. Tracking of a tracer injected into the metalimnion was combined with measurements of meteorological forcing, internal waves, and temperature microstructure. Seiches of vertical mode 2 and horizontal modes 1 and 2 were initiated after a wind event, and the tracer moved 950 m into the interior after 29.2 h. Four potential mechanisms for spreading of the tracer from the boundary to the interior were considered: intrusions from boundary mixing, horizontal dispersion, advection by seiches, and advection and dispersion driven by internal waves. Some evidence of boundary mixing was observed 0.5 h before the dye injection, when the speed of seiche‐driven currents was large, but a model of an intrusion driven by steady input overpredicted the propagation distance by a factor of about two. A one‐dimensional model with only dispersion yielded a dispersion coefficient of 0.8 m2 s−1, and a one‐dimensional model with only advection caused by internal waves predicted the position of the peak concentration and a change in longitudinal variance that was 60% of the measured change. Estimates of dispersion caused by the interaction of vertical diffusion with velocity gradients in the internal wave field are large enough to explain the rest of the spreading and suggest that the transport can be modeled as wave‐driven advection and dispersion.

Aircraft observations and model simulations of concentration and particle size distribution in the Eyjafjallajökull volcanic ash cloud

Abstract. The Eyjafjallajökull volcano in Iceland emitted a cloud of ash into the atmosphere during April and May 2010. Over the UK the ash cloud was observed by the FAAM BAe-146 Atmospheric Research Aircraft which was equipped with in-situ probes measuring the concentration of volcanic ash carried by particles of varying sizes. The UK Met Office Numerical Atmospheric-dispersion Modelling Environment (NAME) has been used to simulate the evolution of the ash cloud emitted by the Eyjafjallajökull volcano during the period 4–18 May 2010. In the NAME simulations the processes controlling the evolution of the concentration and particle size distribution include sedimentation and deposition of particles, horizontal dispersion and vertical wind shear. For travel times between 24 and 72 h, a 1/t relationship describes the evolution of the concentration at the centre of the ash cloud and the particle size distribution remains fairly constant. Although NAME does not represent the effects of microphysical processes, it can capture the observed decrease in concentration with travel time in this period. This suggests that, for this eruption, microphysical processes play a small role in determining the evolution of the distal ash cloud. Quantitative comparison with observations shows that NAME can simulate the observed column-integrated mass if around 4% of the total emitted mass is assumed to be transported as far as the UK by small particles (&lt; 30 μm diameter). NAME can also simulate the observed particle size distribution if a distal particle size distribution that contains a large fraction of &lt; 10 μm diameter particles is used, consistent with the idea that phraetomagmatic volcanoes, such as Eyjafjallajökull, emit very fine particles.

Vertical Eddy Energy Fluxes in the North Atlantic Subtropical and Subpolar Gyres

Abstract Eddy energy generation and energy fluxes are examined in a realistic eddy-resolving model of the North Atlantic. Over 80% of the wind energy input is found to be released by the generation of eddies through baroclinic instability. The eddy energy generation is located near the surface in the subtropical gyre but deeper down in the subpolar gyre. To reconcile the mismatch between the depth of eddy energy production and the vertical structure of the horizontal dispersion of eddy energy, the vertical eddy energy flux is downward in the subtropical gyre and upward in the subpolar gyre.

The Effect of Geometrical Properties of Reservoir Shale Barriers on the Performance of Steam-assisted Gravity Drainage (SAGD)

Many bitumen reservoirs contain shale layers of varying thickness, lateral extent, and frequency. These shale layers, depending on their size, vertical and horizontal locations, and continuity throughout the reservoir, may act as a flow barrier and severely reduce vertical permeability of the pay zone and slow down the steam-assisted gravity drainage steam chamber development. Therefore, to improve productivity in these reservoirs, understanding of the effects of reservoir heterogeneities has become necessary. This work presents numerical investigation of the effects of shale barriers on steam-assisted gravity drainage performance when applied to produce mobile heavy oil. The most concern of the work is upon how different geometrical properties of these shale barriers, such as vertical and horizontal position, density, discontinuity, and dispersion through the reservoir, affect the steam-assisted gravity drainage performance and steam chamber development in above wells region. Simulation results showed that the performance of steam-assisted gravity drainage is lower in the case of presence of continued shale layers, their longer extension, and their higher density (number of shale layers per unit volume). As the vertical distance between the continued shale layer and injector decreases, the recovery factor will be less. Distribution scheme of the layers also affected the recovery performance and production rates, since higher was the produced oil in the case of scattered layers as compared to the case of stacked shale layers on top of each other. It was found that the effect of the aforementioned parameters on the ultimate oil recovery factor and production rates was less in the case of presence of discontinuities in the shale barriers.

New analytical formulations for calculation of dispersion parameters of Gaussian model using parallel CFD

New analytical formulations are presented for calculation of most effective parameters in the Gaussian plume dispersion model; the standard deviations of concentration for horizontal and vertical dispersion in neutral atmosphere conditions. Employing parallel Computational Fluid Dynamics (CFD) as a powerful tool, some well-known analytical generations of Pasquill–Gifford–Turner experimental data are modified. To achieve this aim, CFD simulations are carried out for single stack dispersion on flat terrain surface and ground level concentrations are determined in different distances. An inverse procedure in Gaussian plume dispersion model is then applied and standard deviations of horizontal and vertical dispersions are obtained. The values are compared with those of the well-known methods of Doury, Briggs and Hanna in two cases: the experimental data for release of krypton-85 from 100 m high and pollution dispersion from three 28 m high stacks of Besat power plant near Tehran. The comparison indicates that new formulations for plume dispersion are more accurate than other well-known formulations.