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121.
A mathematical model was used to simulate monthly responses of the Gulf of Mexico hypoxia to variations in climate and anthropogenic nutrient loading over a 45-year period. We examined six hypothetical future scenarios that are based on observed and projected changes in the Mississippi River discharge, Mississippi River nitrate concentrations, and ambient water temperatures. In particular, we investigated the implications of a 30% decrease in the Mississippi River nitrogen flux, which was recently proposed by the Mississippi River Watershed/Gulf of Mexico Hypoxia Task Force as a measure to reduce the size of the hypoxic zone. Model simulations suggest that the frequency of hypoxia in the northern Gulf of Mexico is highly sensitive to variations in riverine nitrate flux, but also to variations in freshwater discharge and ambient water temperatures. A 30% decrease in the Mississippi River nitrate flux, for example, would reduce the frequency of hypoxia by 37%. Nevertheless, a 20% increase the Mississippi River discharge, which may occur under some climate change scenarios, would produce an increase in the frequency of hypoxia of the same magnitude. Thus, if the potential climatic variations are taken into account, a 30% decrease in the nitrogen flux of the Mississippi River may not be sufficient to accomplish the proposed hypoxia management goal. 相似文献
122.
Lawrence Frank Mark Bradley Sarah Kavage James Chapman T. Keith Lawton 《Transportation》2008,35(1):37-54
The primary purpose of this study was to investigate how relative associations between travel time, costs, and land use patterns
where people live and work impact modal choice and trip chaining patterns in the Central Puget Sound (Seattle) region. By
using a tour-based modeling framework and highly detailed land use and travel data, this study attempts to add detail on the
specific land use changes necessary to address different types of travel, and to develop a comparative framework by which
the relative impact of travel time and urban form changes can be assessed. A discrete choice modeling framework adjusted for
demographic factors and assessed the relative effect of travel time, costs, and urban form on mode choice and trip chaining
characteristics for the three tour types. The tour based modeling approach increased the ability to understand the relative
contribution of urban form, time, and costs in explaining mode choice and tour complexity for home and work related travel.
Urban form at residential and employment locations, and travel time and cost were significant predictors of travel choice.
Travel time was the strongest predictor of mode choice while urban form the strongest predictor of the number of stops within
a tour. Results show that reductions in highway travel time are associated with less transit use and walking. Land use patterns
where respondents work predicted mode choice for mid day and journey to work travel.
Lawrence Frank is an Associate Professor and Bombardier Chair in Sustainable Transportation at the University of British Columbia and a Senior Non-Resident Fellow of the Brookings Institution and Principal of Lawrence Frank and Company. He has a PhD in Urban Design and Planning from the University of Washington. Mark Bradley is Principal, Mark Bradley Research & Consulting, Santa Barbara California. He has a Master of Science in Systems Simulation and Policy Design from the Dartmouth School of Engineering and designs forecasting and simulation models for assessment of market-based policies and strategies. Sarah Kavage is a Senior Transportation Planner and Special Projects Manager at Lawrence Frank and Company. She has a Masters in Urban Design and Planning from the University of Washington and is a writer and an artist based in Seattle. James Chapman is a Principal Transportation Planner and Analyst at Lawrence Frank and Company in Atlanta Georgia. He has a Masters in Engineering from the Georgia Institute of Technology. T. Keith Lawton transport modeling consultant and past Director of Technical services, Metro Planning Department, Portland, OR, has been active in model development for over 40 years. He has a BSc. in Civil Engineering from the University of Natal (South Africa), and an M.S. in Civil and Environmental Engineering from Duke University. He is a member and past Chair of the TRB Committee on Passenger Travel Demand Forecasting. 相似文献
T. Keith LawtonEmail: |
Lawrence Frank is an Associate Professor and Bombardier Chair in Sustainable Transportation at the University of British Columbia and a Senior Non-Resident Fellow of the Brookings Institution and Principal of Lawrence Frank and Company. He has a PhD in Urban Design and Planning from the University of Washington. Mark Bradley is Principal, Mark Bradley Research & Consulting, Santa Barbara California. He has a Master of Science in Systems Simulation and Policy Design from the Dartmouth School of Engineering and designs forecasting and simulation models for assessment of market-based policies and strategies. Sarah Kavage is a Senior Transportation Planner and Special Projects Manager at Lawrence Frank and Company. She has a Masters in Urban Design and Planning from the University of Washington and is a writer and an artist based in Seattle. James Chapman is a Principal Transportation Planner and Analyst at Lawrence Frank and Company in Atlanta Georgia. He has a Masters in Engineering from the Georgia Institute of Technology. T. Keith Lawton transport modeling consultant and past Director of Technical services, Metro Planning Department, Portland, OR, has been active in model development for over 40 years. He has a BSc. in Civil Engineering from the University of Natal (South Africa), and an M.S. in Civil and Environmental Engineering from Duke University. He is a member and past Chair of the TRB Committee on Passenger Travel Demand Forecasting. 相似文献
123.
Between 1990 and 2000, U.S. transit agencies added service and increased ridership, but the ridership increase failed to keep
pace with the service increase. The result was a decline in service effectiveness (or productivity). This marks the continuation
of a long-running and often-studied trend. The scholarly literature attributes this phenomenon, at least in part, to transit
agency decisions to decentralize their service rather than focus on serving the traditional CBD market. Many scholars argue
that a decentralized service orientation is both ineffective and inefficient because it attracts few riders and requires large
per-rider subsidies. This research tests whether a non-traditional, decentralized service orientation, called multidestination
service, results in reduced service productivity. Contrary to what the literature suggests, we find that MSAs whose transit
agencies pursued a multidestination service orientation did not experience lower productivity. These results indicate that
policies that have encouraged the growth of decentralized transit services have not necessarily been detrimental to the industry.
相似文献
Gregory L. ThompsonEmail: |
124.
The dynamics of the coastal ocean along the southeastern coast of Africa is dominated by a strong and intense western boundary current, the Agulhas Current. With a near-uniform, narrow continental shelf and a steep shelf slope that stabilizes this current, the trajectory of the Agulhas exhibits great stability. The only substantial perturbation occurs with the irregular passage of a Natal Pulse, a soliton meander. The initiation of this meander at the Natal Bight is due to a barotropic instability when the intensity of the landward border of the current exceeds a certain threshold value. This may come about with natural fluctuations in the current or with the adsorption of deep-sea eddies onto the current. Under a climate change scenario of altered wind stress curl over the South Indian Ocean it is conceivable that the threshold for the triggering of a Natal Pulse will occur more frequently. This will lead to a situation where the current axis on average lies further offshore. The possible consequences of such a situation on the rainfall of the coast, on the ecology of estuaries and the coastal ocean, and on the socio-economics of the region is discussed. 相似文献
125.
R. Pastres C. Solidoro S. Ciavatta A. Petrizzo G. Cossarini 《Journal of Marine Systems》2004,51(1-4):179
The paper aims at investigating the trends of the concentrations of dissolved inorganic nitrogen (DIN) and phosphorous in the Lagoon of Venice, Italy, in the last 35 years. The trend analysis of ammonia nitrogen, nitrate nitrogen and reactive phosphorus (RP) yielded rather different results. The decrease in ammonia nitrogen concentration, in particular, in the basin influenced by severe industrial discharges, is accurately described by a negative exponential model. The nitrate concentration did not show significant interannual trends, while wide seasonal variations have been detected. Reactive phosphorus concentration increased until the end of the 1970s at the stations close to the industrial area as well as in those influenced by river discharges and by sewages of the city of Venice. The sudden decrease that followed was most likely related with environmental policies, namely the improvement of wastewater purification treatments and the regulation of phosphorus use in detergents. 相似文献
126.
J.F. Shriver H.E. Hurlburt O.M. Smedstad A.J. Wallcraft R.C. Rhodes 《Journal of Marine Systems》2007,65(1-4):3
A 1/32° global ocean nowcast/forecast system has been developed by the Naval Research Laboratory at the Stennis Space Center. It started running at the Naval Oceanographic Office in near real-time on 1 Nov. 2003 and has been running daily in real-time since 1 Mar. 2005. It became an operational system on 6 March 2006, replacing the existing 1/16° system which ceased operation on 12 March 2006. Both systems use the NRL Layered Ocean Model (NLOM) with assimilation of sea surface height from satellite altimeters and sea surface temperature from multi-channel satellite infrared radiometers. Real-time and archived results are available online at http://www.ocean.nrlssc.navy.mil/global_nlom. The 1/32° system has improvements over the earlier system that can be grouped into two categories: (1) better resolution and representation of dynamical processes and (2) design modifications. The design modifications are the result of accrued knowledge since the development of the earlier 1/16° system. The improved horizontal resolution of the 1/32° system has significant dynamical benefits which increase the ability of the model to accurately nowcast and skillfully forecast. At the finer resolution, current pathways and their transports become more accurate, the sea surface height (SSH) variability increases and becomes more realistic and even the global ocean circulation experiences some changes (including inter-basin exchange). These improvements make the 1/32° system a better dynamical interpolator of assimilated satellite altimeter track data, using a one-day model forecast as the first guess. The result is quantitatively more accurate nowcasts, as is illustrated by several model-data comparisons. Based on comparisons with ocean color imagery in the northwestern Arabian Sea and the Gulf of Oman, the 1/32° system has even demonstrated the ability to map small eddies, 25–75 km in diameter, with 70% reliability and a median eddy center location error of 22.5 km, a surprising and unanticipated result from assimilation of altimeter track data. For all of the eddies (50% small eddies), the reliability was 80% and the median eddy center location error was 29 km. The 1/32° system also exhibits improved forecast skill in relation to the 1/16° system. This is due to (a) a more accurate initial condition for the forecast and (b) better resolution and representation of critical dynamical processes (such as upper ocean – topographic coupling via mesoscale flow instabilities) which allow the model to more accurately evolve these features in time while running in forecast mode (forecast atmospheric forcing for the first 5 days, then gradually reverting toward climatology for the remainder of the 30-day forecast period). At 1/32° resolution, forecast SSH generally compares better with unassimilated observations and the anomaly correlation of the forecast SSH exceeds that from persistence by a larger amount than found in the 1/16° system. 相似文献
127.
Integrated Control Methodologies for Road Vehicles 总被引:5,自引:0,他引:5
Tim Gordon Mark Howell Felipe Brandao 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》2003,40(1):157-190
Summary This paper considers the scope, methodologies and architectures for the design and development of interacting control systems in road vehicles. The increasing use of electronic controls leads inevitably to an increase in overall system complexity. Given the time and economic constraints of the modern automotive industry, it is not feasible to synthesise and validate the full set of vehicle controls in the form of a unified and centralized controller. On the other hand a fully decentralized approach to control system development and operation will induce performance limitations from un-modelled or unexpected interactions; at worst, such interactions can cause instability and loss of function. There is now increasing pressure to achieve control coordination whilst maintaining a modular approach to the overall system design. With this in mind, the paper provides a framework to review current practice in integrated vehicle control, assesses recent developments in control integration methodologies that are most relevant to the vehicle application, and formulates an enhanced multi-layer architecture that includes explicit coordination functionality. Overall emphasis is placed on the role of control system architecture, the resulting flow of control information and the implications for control system design. An example from handling dynamics is presented, demonstrating the viability of new and flexible approaches. In conclusion a number of outstanding research problems are highlighted. 相似文献
128.
129.
A. G. Thompson B. R. Davis 《Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility》2003,39(3):245-253
Summary Matrix expressions are developed for the direct computation of rms values for the optimal control forces, front and rear suspension strokes and dynamic tyre deflections in a half-car model on a random road of given roughness. A quadratic performance index is employed with assumed weighting factors and evaluated in the same computation. 相似文献
130.
Numerical artifacts can limit accurate simulation of turbulent particle motion when Lagrangian particle-tracking models are implemented in hydrodynamic models with stratified conditions like fronts. Yet, modeling of individual particle motion in frontal regions is critical for understanding sediment dynamics as well as the transport and retention of planktonic organisms. The objective of this research was to develop a numerical technique to accurately simulate turbulent particle motions in a particle-tracking model embedded within a hydrodynamic model of a frontal zone. A new interpolation scheme, the ‘water column profile’ scheme, was developed and used to implement a random displacement model for turbulent particle motions. A new interpolation scheme was necessary because linear interpolation schemes caused artificial aggregation of particles where abrupt changes in vertical diffusivity occurred. The new ‘water column profile’ scheme was used to fit a continuous function (a tension spline) to a smoothed profile of vertical diffusivities at the x–y particle location. The new implementation scheme was checked for artifacts and compared with a standard random walk model using (1) Well Mixed Condition tests, and (2) dye-release experiments. The Well Mixed Condition tests confirmed that the use of the ‘water column profile’ interpolation scheme for implementing the random displacement model significantly reduced numerical artifacts. In dye-release experiments, high concentrations of Eulerian tracer and Lagrangian particles were released at the same location up-estuary of the salt front and tracked for 4 days. After small differences in initial dispersal rates, tracer and particle distributions remained highly correlated (r = 0.84 to 0.99) when a random displacement model was implemented in the particle-tracking model. In contrast, correlation coefficients were substantially lower (r = 0.07 to 0.58) when a random walk model was implemented. In general, model performance tests indicated that the ‘water column interpolation’ scheme was an effective technique for implementing a random displacement model within a hydrodynamic model, and both could be used to accurately simulate diffusion in a highly baroclinic frontal region. The new implementation scheme has the potential to be a useful tool for investigating the influence of hydrodynamic variability on the transport of sediment particles and planktonic organisms in frontal zones. 相似文献