2,001 research outputs found
Ichthyoplankton community structure and comparative trophodynamics in an estuarine transition zone
Surveys were conducted to evaluate and compare assemblage
structure and trophodynamics of ichthyoplankton, and their variability, in an estuarine transition zone.
Environmental gradients in the saltfront region of the Patuxent River subestuary, Chesapeake Bay, were hypothesized to define spatiotemporal distributions and assemblages of ichthyoplankton. Larval fishes, zooplankton,
and hydrographic data were collected during spring through early summer 2000 and 2001. Larvae of 28 fish species were collected and species richness was similar each year. Total
larval abundance was highest in the oligohaline region down-estuary of the salt front in 2000, but highest at the salt front in 2001. Larvae of anadromous fishes were most abundant at or up-estuary of the salt front in both years. Two ichthyoplankton assemblages were distinguished: 1) riverine—characterized predominantly by anadromous species (Moronidae and Alosinae); and 2) estuarine—characterized
predominantly by naked goby (Gobiosoma bosc) (Gobiidae). Temperature, dissolved oxygen, salinity-associated
variables (e.g., salt-front location), and concentrations of larval prey, specifically the calanoid copepod
Eurytemora affinis and the cladoceran Bosmina longirostris, were important indicators of larval fish abundance. In
the tidal freshwater region up-estuary of the salt front, there was substantial diet overlap between congeneric
striped bass (Morone saxatilis) and white perch (M. americana) larvae, and also larvae of alewife (Alosa
pseudoharengus) (overlap= 0.71–0.93). Larval abundance, taxonomic diversity, and dietary overlap were highest
within and up-estuary of the salt front, which serves to both structure the ichthyoplankton community and control trophic relationships in the estuarine transition zone
Damping of MHD turbulence in partially ionized gas and the observed difference of velocities of neutrals and ions
Theoretical and observational studies on the turbulence of the interstellar
medium developed fast in the past decades. The theory of supersonic magnetized
turbulence, as well as the understanding of projection effects of observed
quantities, are still in progress. In this work we explore the characterization
of the turbulent cascade and its damping from observational spectral line
profiles. We address the difference of ion and neutral velocities by clarifying
the nature of the turbulence damping in the partially ionized. We provide
theoretical arguments in favor of the explanation of the larger Doppler
broadening of lines arising from neutral species compared to ions as arising
from the turbulence damping of ions at larger scales. Also, we compute a number
of MHD numerical simulations for different turbulent regimes and explicit
turbulent damping, and compare both the 3-dimensional distributions of velocity
and the synthetic line profile distributions. From the numerical simulations,
we place constraints on the precision with which one can measure the 3D
dispersion depending on the turbulence sonic Mach number. We show that no
universal correspondence between the 3D velocity dispersions measured in the
turbulent volume and minima of the 2D velocity dispersions available through
observations exist. For instance, for subsonic turbulence the correspondence is
poor at scales much smaller than the turbulence injection scale, while for
supersonic turbulence the correspondence is poor for the scales comparable with
the injection scale. We provide a physical explanation of the existence of such
a 2D-3D correspondence and discuss the uncertainties in evaluating the damping
scale of ions that can be obtained from observations. However, we show that the
statistics of velocity dispersion from observed line profiles can provide the
spectral index and the energy transfer rate of turbulence. Also, comparing two
similar simulations with different viscous coefficients it was possible to
constrain the turbulent cut-off scale. This may especially prove useful since
it is believed that ambipolar diffusion may be one of the dominant dissipative
mechanism in star-forming regions. In this case, the determination of the
ambipolar diffusion scale may be used as a complementary method for the
determination of magnetic field intensity in collapsing cores. We discuss the
implications of our findings in terms of a new approach to magnetic field
measurement proposed by Li & Houde (2008).Comment: ApJ accepte
Spatial Development and Energy Consumption
Previous literature has suggested that the urban form (i.e., city size, density, and center distribution pattern) influences urban energy consumption. It has been argued that more dense development is likely to result in more energy-efficient and sustainable cities. However, very little is known about the precise magnitude of possible energy savings from more compact urban form. Moreover, practically no research has been done to investigate which urban policies are likely to be effective in making cities more energy efficient and to quantify those potential energy savings. In this paper we discuss the potential effectiveness of urban policies at improving energy efficiency. First, we analyze several abstract scenarios suggested by the literature to see whether making a previously dispersed city more compact would result in improved energy efficiency. Then we model realistic transportation and land-use policies and examine whether those policies are likely to reduce energy consumption in the urban context.energy consumption, urban form, general equilibrium, land use, transportation, government policy
Washington START Transportation Model
The document describes the Washington START transportation simulation model. In particular, it provides information about the model structure, the equilibrium concept, and the data used to calibrate the model. It also briefly describes the reference scenario and the elasticity analysis. Finally, the document discusses past and potential future applications and possible directions for model extensions.transportation simulation, policy analysis, general equilibrium, travel demand, transportation network, mode of transportation
Marginal Social Cost Pricing on a Transportation Network: Comparison of Second-Best Policies
In this paper we evaluate and compare long-run economic effects of six road-pricing schemes aimed at internalizing social costs of transportation. In order to conduct this analysis, we employ a spatially disaggregated general equilibrium model of a regional economy that incorporates decisions of residents, firms, and developers, integrated with a spatially-disaggregated strategic transportation planning model that features mode, time period, and route choice. The model is calibrated to the greater Washington, DC metropolitan area. We compare two social cost functions - one restricted to congestion alone and another that accounts for other external effects of transportation. We find that when the ultimate policy goal is a reduction in the complete set of motor vehicle externalities, cordon-like policies and variable-toll policies lose some attractiveness compared to policies based primarily on mileage. We also find that full social cost pricing requires very high toll levels and therefore is bound to be controversial.traffic congestion, social cost pricing, land use, welfare analysis, road pricing, general equilibrium, simulation, Washington DC
Topological phase transitions and chiral inelastic transport induced by the squeezing of light
We show how the squeezing of light can lead to the formation of topological
states. Such states are characterized by non-trivial Chern numbers, and exhibit
protected edge modes which give rise to chiral elastic and inelastic photon
transport. These topological bosonic states are not equivalent to their
fermionic (topological superconductor) counterparts and cannot be mapped by a
local transformation onto topological states found in particle-conserving
models. They thus represent a new type of topological system. We study this
physics in detail in the case of a Kagome lattice model, and discuss possible
realizations using nonlinear photonic crystals or superconducting circuits.Comment: 11 pages, 4 figure
Long-Term Consequences of Congestion Pricing: A Small Cordon in the Hand Is Worth Two in the Bush
We evaluate and compare the long-term economic effects of three cordon-based road pricing schemes applied to the Washington, DC, metropolitan area. To conduct this analysis, we employ a spatially disaggregated general equilibrium model of a regional economy that incorporates the decisions of residents, firms, and developers, integrated with a spatially disaggregated strategic transportation planning model that features mode, time period, and route choice. We find that all cordon pricing schemes increase welfare of the residents, as well as lead to GDP growth. At the optimum, the larger cordon and a double cordon lead to higher benefits than the small cordon encompassing downtown core. Nevertheless, the small cordon seems to be a safer bet because when the toll charge is set suboptimally, the net benefits from the small cordon compared to the optimum change negligibly, while the net benefits from the larger cordon decline sharply as the charge deviates from the optimal level.traffic congestion, cordon tolls, land use, welfare analysis, road pricing, general equilibrium, simulation, Washington DC
Congestion Pricing: Long-Term Economic and Land-Use Effects
We employ a spatially disaggregated general equilibrium model of a regional economy that incorporates decisions of residents, firms, and developers integrated with a spatially disaggregated strategic transportation planning (START) model that features mode, time period, and route choice to evaluate economic effects of congestion pricing. First, we evaluate the long-run effects of a road-pricing policy based on the integrated model of land use, strategic transport, and regional economy (LUSTRE) and compare them with the short-term effects obtained from the START model alone. We then look at distributional effects of the policy in question and point out differences and similarities in the short run versus the long run. Finally, we analyze the mechanisms at the source of the economic and land-use effects induced by the road-pricing policy.traffic congestion, welfare analysis, CGE modeling, cordon tolls, distributional effects
A broad survey of spectro-temporal properties from FRB 20121102A
We survey the spectro-temporal properties of fast radio bursts from FRB
20121102A observed by earlier studies across a wide range of frequencies. We
investigate 167 bursts from FRB 20121102A spanning frequencies 1--7.5GHz,
durations of less than 1 ms to approximately 10 ms, with low and high energies,
and with different wait-times. We find from this sample of bursts a strong
agreement with the inverse relationship between sub-burst slope and duration
and with other predictions made by the triggered relativistic dynamical model
(TRDM). Earlier results found agreement with those predictions across three
different repeating FRB sources. For this sample of bursts, we find that the
sub-burst slope as well as the 'sad trombone' drift rate are consistent with
being in a quadratic relationship with frequency and that both these quantities
are inversely proportional to the duration. We also find that the duration
decreases with increasing frequency as well as a statistically significant
correlation between the sub-burst duration and bandwidth (proportional to
) that is unexpected. No distinct group of bursts in this sample
deviated from these relationships, however significant scatter can be seen in
measurements. This study demonstrates the consistent existence of relationships
between the spectro-temporal properties of bursts from a repeating FRB source.
A simple explanation for the inverse relation between the sub-burst slope and
duration is an inherently narrowband emission process. We make all measurements
available as well as a graphical user interface called Frbgui developed and
used to perform measurements of burst waterfalls.Comment: Accepted at MNRAS. 13 pages, 7 figure
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