A comparison of raster-based travel time surfaces against vector-based network calculations as applied in the study of rural food deserts

While the term food insecurity is gaining popularity in contemporary literature, there is debate as to how tenets of this phenomenon can be quantitatively measured. One of these tenets, proximity to food resources, which is used to measure food deserts, can be measured within a digital GIS (Geographic Information System). Metrics such as Euclidean and network distance represent planimetric distance measurements between locations and resources, but do not truly represent the empirical cost that serves as a barrier, most notably time and/or money, to those who must decide to travel to these resources. While the vector data model has been the standard by which these calculations are done within a GIS, raster-based travel time surfaces can serve as a faster, replicable and scalable alternative. However, little research has been done to test the efficacy of these surfaces and their alignment with vector-based network calculations. In this research, we developed two travel-time surfaces for a rural region in southeastern North Carolina. One represented travel times to grocery stores and while the other represented travel time to convenience stores. We found that the travel times derived from this surface were statistically consistent with vector-based counterparts for sample sizes at a 95% confidence. When utilized correctly using an appropriate scale and spatial resolution, these surfaces have the potential to be effective tools in the study of food deserts.     

Time-domain numerical simulation of ocean cable structures

This paper describes the numerical features of WHOI Cable, a computer program for analyzing the statics and dynamics of oceanographic cable structures. The governing equations include the effects of geometric and material nonlinearities, bending stiffness for seamless modeling of slack cables, and a model for the interaction of cable segments with the sea floor. The program uses the generalized-α time integration algorithm, adaptive time stepping, and adaptive spatial gridding to produce accurate, stable solutions for dynamic problems. The nonlinear solver uses adaptive relaxation to improve robustness for both static and dynamic problems. The program solves surface and subsurface single-point mooring problems, multi-leg and branched array systems, and towing and drifting problems. User specified forcing can include waves, currents, wind, and ship speed.     

离散Hopfield神经网络的稳定性

研究离散Ｈｏｐｆｉｅｌｄ神经网络的串行稳定性和并行稳定性，分别提出了条件较弱的串行稳定性和并行稳定性的新判据．     

带乘性噪声系统的多尺度最优滤波融合算法

利用小波变换和多尺度分析的思想，将基于模型的动态系统分析和基于统计特性的多尺度信号变换方法相结合。提出了在线性最小方差意义下的带乘性噪声系统的多尺度最优滤波融合算法。并用计算机仿真说明了融合算法的有效性。     

Transient behavior of towed cable systems during ship turning maneuvers

The dynamic behavior of a towed cable system that results from the tow ship changing course from a straight-tow trajectory to one involving steady circular turning at a constant radius is examined. For large-radius ship turns, the vehicle trajectory and vehicle depth assumed, monotonically and exponentially, the large-radius steady-state turning solution of Chapman [Chapman, D.A., 1984. The towed cable behavior during ship turning manoeuvers. Ocean Engineering 11, 327–361]. For small-radius ship turns, the vehicle trajectory initially followed a corkscrew pattern with the vehicle depth oscillating about and eventually decaying to the steady-state turning solution of Chapman (1984). The change between monotonic and oscillatory behavior in the time history of the vehicle depth was well defined and offered an alternate measure to Chapman's (1984) critical radius for the transition point between large-radius and small-radius behavior. For steady circular turning in the presence of current, there was no longer a steady-state turning solution. Instead, the vehicle depth oscillated with amplitude that was a function of the ship-turning radius and the ship speed. The dynamics of a single 360° turn and a 180° U-turn are discussed in terms of the transients of the steady turning maneuver. For a single 360° large-radius ship turn, the behavior was marked by the vehicle dropping to the steady-state turning depth predicted by Chapman (1984) and then rising back to the initial, straight-tow equilibrium depth once the turn was completed. For small ship-turning radius, the vehicle dropped to a depth corresponding to the first trough of the oscillatory time series of the steady turning maneuver before returning to the straight-tow equilibrium depth once the turn was completed. For some ship-turning radii, this resulted in a maximum vehicle depth that was greater than the steady-state turning depth. For a 180° turn and ship-turning radius less than the length of the tow cable, the vehicle never reached the steady-state turning depth.     

脂类生物标志物在海洋环境中的应用与研究进展

海洋沉积物中的脂类生物标志物可以提供丰富的信息,包括有机物质源,沉积过程的古环境条件等。由于脂类生物标志物提供的信息准确、稳定,成为研究海洋环境的有力工具。本文介绍了其在古气候效应、河口系统的有机物质检测、降解以及污染物溯源的研究应用进展。     

What can flatfish ontogenies tell us about pelagic and benthic lifestyles?

Eye migration, asymmetrical pigmentation, and a 90° rotation in posture are developmental changes that unify all flatfishes and facilitate a transition from a pelagic to a benthic existence. Settlement places the fish in new environmental conditions which may require different types and levels of performance for survival compared to those needed in the water column. Since structure and performance vary as a consequence of ontogeny, it is likely that natural selection has acted on the ontogeny of flatfishes differently from fishes that do not settle (pelagic fishes) to provide different survival skills by the time flatfishes settle. This paper provides examples of quantitative methods for comparing fish ontogenies in the context of three predictions: (1) different flatfish species undergo settlement at a common ontogenetic state; (2) they have a common set of skills at settlement that differ from those of pelagic species of the same ontogenetic state; and (3) skills or features that appear earlier in the ontogenetic program of flatfishes than in pelagic fishes suggest attributes that are important to survival in a benthic habitat. Preliminary comparisons suggest that flatfishes may have accelerated development of their mechanosensory system relative to pelagic species. Firm conclusions about ecologically important differences between the pelagic and benthic habitats derived from analyses of ontogenetic events will require more kinds of data from a broader selection of species.     

N2 fixation variability in the oligotrophic Sulu Sea, western equatorial Pacific region over the past 83 kyr

N₂ fixation is an important biological process that adds new nitrogen to oceans and plays a key role in modulating the oceanic nitrate inventory. However, it is not known how, when, and where N₂ fixation rates have varied in response to past climate changes. This study presents a new record of nitrogen isotopic composition (δ¹⁵N) over the last 83 kyr from a sediment core (KH02-4 SUP8) taken in the Sulu Sea in the western equatorial Pacific region; data allow the N₂ fixation variability in the sea to be reconstructed. Sediments, sinking, and suspended particulate organic matter (POM) all have lighter isotopic values compared to the δ¹⁵N values of substrate nitrate (av. 5.8‰) in North Pacific Intermediate Water. These lighter δ¹⁵N values are regarded as reflecting N₂ fixation in the Sulu Sea surface water. A δ¹⁵N mass balance model shows that N₂ fixation rates were significantly enhanced during 54–34 kyr in MIS-3 and MIS-2. It has been speculated that higher interglacial denitrification rates in the Arabian Sea and the eastern tropical Pacific would have markedly decreased the global oceanic N inventory and contributed to the increase in N₂ fixation in oligotrophic regions, but such a model was not revealed by our study. It is possible that changes in N₂ fixation rates in the Sulu Sea were regional response, and accumulation of phosphate in the surface waters due to enhanced monsoon-driven mixing is thought to have stimulated enhancements of N₂ fixation during MIS-3 and MIS-2.     

一类非线性系统次优控制的灵敏度法

本文研究一类非线性定常系统的次优控制问题。通过在系统中引入 1个灵敏度参数并将系统变量关于灵敏度参数展开 Maclaurin级数 ,使求解最优控制的非线性两点边值问题化为一族线性两点边值问题。利用截取最优控制级数的有限项求得系统的次优控制律。仿真实例表明 ,该方法对非线性系统次优控制律的设计是有效的