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791.
J.C Comiso 《Journal of Marine Systems》1991,2(3-4)
The advent of satellite remote sensing has opened doors for detailed monitoring of the polar regions at a relatively high temporal resolution. Several sensors have been developed over the years, the most noteworthy of which are the visible, the infrared and the microwave systems. The visible and infrared systems are most intelligible to the human eye and provide images that have good spatial resolution. However, because of persistent cloudiness and several months of darkness in the polar regions the most promising sensors appear to be the microwave systems, both passive and active. The passive microwave sensors have been the source of consistently derived ice cover and ice extent data and have provided about fifteen years of global data sets. The Synthetic Aperture Radar (SAR), on the other hand, is noted for very high resolution and a wide range of applications including ice dynamics and ice/wave interaction studies. Nevertheless, there are applications which require the use of the visible and infrared channels, such as the mapping of chlorophyll pigment concentrations in the marginal ice zones which require an ocean color scanner. Also, the infrared sensors are most effective for detecting physical temperatures over cloud-free ice and open water surfaces. Furthermore, altimeters could play an important role in mapping ice surface topography and in evaluating ridge and lead statistics. A review of past, present and future satellite systems, methods and techniques of interpretation and potential applications to the study of the polar oceans is presented. 相似文献
792.
Craig J. Forsyth 《Maritime Policy and Management》1991,18(4):313-319
The major factors influencing tanker safety are discussed. For the discussion information has been gleaned from maritime labour officials, industry experts and a recent United States Coast Guard study on tanker safety. Results of other tanker safety studies and/or accident reports are also utilized. Some of the factors analysed are the size and age of vessel, licence qualifications for mates and engineers and manning levels. Other factors are those related to or attributed to flags of convenience and the pressure to maintain shipping schedules. 相似文献
793.
切尔诺夫·尤里·安东诺维奇 《铁道学报》1998,20(1):110-114
提出了确定铁路牵引供电系统发展最佳途径的课题,并建议采用动态规划法解决这一课题。 相似文献
794.
Primary production events in both the Arctic and the Antarctic are highly localized. Carbon-14 incubations that did not account for this caused antarctic primary production estimates to be revised too far downwards from the historic view of high productivity. The primary production regime in the Arctic is even more heterogeneous than in the Antarctic. Arctic primary production rates are in the process of being revised upwards because of a better spatial and temporal distribution of incubation experiments and a re-awakening of interest in estimating new production from the distribution of chemical variables. Similarly, recent examination of temporal changes in nitrate concentrations and recognition of the importance of ice-edge blooms has caused antarctic primary productivity to be revised upwards. In both the Arctic and the Antarctic, the ratio of “new” to total primary production is high, and neglect of this fact can lead to an underestimation of the potential that these regions have for influencing global cycles of bioactive chemicals. Some recent data on temporal changes in nitrate from Fram Strait emphasize the poor state of our knowledge by suggesting an unexpectedly high “new” production rate of 1 g C m−2 d−1 for a 35 day experiment that encountered an early Phaeocystis bloom. Chemical distributions suggest that new production over the shelf seas that border the Polar Basin is about 50 g Cm−2 yr−1.The shelves in the Arctic Ocean's marginal and adjacent seas comprise 25% of the total global continental shelf. These extensive shallow regions have much higher rates of primary production than the Polar Basin and may be globally significant sites of denitrification. Globally significant silica deposition could occur on these shelves or on the adjacent slopes.Because of the differences in geomorphology and stratification, global warming is likely to increase primary production in the Arctic and will probably decrease antarctic primary production.In addition to sharing high ratios of “new” to total primary production, high ammonium concentrations occur in the Arctic and Antarctic. It is possible that these accumulations arise from a strong repression of nitrification at low temperatures. 相似文献
795.
796.
Michael A. Haughton 《Transportation Research Part B: Methodological》1998,32(8):551-566
A key concern in managing vehicle routing operations under stochastic demands is whether, on the basis of travel distance, route modification yields materially greater logistical efficiency than fixed routes. This research uses statistical calibration as the primary technique to develop a robust and tractable model for estimating this difference in logistical efficiency. Based on features such as the models predictive accuracy and generalizability, it constitutes a substantive improvement over existing models. The present study also expands the range of predictive models relevant to vehicle routing under stochastic demands with models to estimate the transportation and inventory effects of persuading customers to stabilize their ordering patterns. 相似文献
797.
我叫大卫·泽洛斯特里克,是斯泰勒工业有限公司的董事长.斯泰勒工业有限公司是美国第一家拉臂钩制造商,成立于1990年,座落于美国爱荷华州的加纳区. 相似文献
798.
2008年重卡各车型的销量格局比较稳定,牵引车仍旧是重卡市场中份额最大的车型。而载货车和自卸车的市场份额也势均力敌。重卡各吨位销量还是以总质量19~26t为主,但份额稍有下滑,而1~19t和26~32t的市场份额各有一个百分点的增长。今年1—5月是近年来重卡车型格局变化最大的一次,自卸车市场迅速扩张,牵引车市场的萎缩,使原来三足鼎立的格局发生了翻天覆地的变化。目前,我国正处于大规模基础设施建设中。新开工项目比较集中,形成了自卸车的销售高峰。在前5个月销售的重卡中,自卸车占到了相当大的比重,市场份额几乎翻倍。部分企业自卸车的销量占到总销量的近一半甚至更多。预计这种势头还将持续一段时期。 相似文献
799.
Using data on state highway expenditures and employment from 30 Minnesota non-metropolitan counties over a 25-year period, possible interactions between transportation and employment are investigated. While crosssectional analysis suggests no significant interactions, causality tests and time-series analyses indicate that highway expenditures affect manufacturing and retail employment, and employment influences expenditures. Although expenditure increases cause employment improvements in the short-term, long-term effects are less favorable. Highway expenditures respond quickly to increased needs caused by retail improvements. 相似文献
800.
Brian Gaylord Daniel C. Reed Libe Washburn Peter T. Raimondi 《Journal of Marine Systems》2004,49(1-4):19
The physical–biological linkages controlling the dispersal of spores produced by macroalgae that reside in kelp forests are complicated and laced with feedbacks. Here we discuss the fundamental elements of these interactions. Biological considerations include spore swimming and sinking speeds, their periods of viability in the plankton, and the height of spore release above the seafloor, which together determine the durations over which spores can be swept by horizontal currents before they contact the seafloor. Morphologies and material properties of canopy forming kelps may also influence the drag exerted on passing waters by the kelps, the plants' ability to persist in the face of rapid flows, and thereby the degree to which impinging currents are redirected around, or slowed within, kelp forests. Macroalgal life histories, and the size of spore sources as controlled by the dimensions of kelp forests and the density and fecundity of individuals within them, influence effective dispersal distances as well. Physical considerations encompass the mean speed, direction, and timescales of variability of currents relative to spore suspension times, the interaction of surface gravity waves with currents in producing turbulence in the benthic boundary layer, wind-driven surface mixing, water stratification, and shoreline bathymetry and substratum roughness, all of which can affect the interplay of vertical and horizontal transport of macroalgal spores. Intricate within-forest processes may induce attenuation of current speeds and consequent reductions in seabed shear, along with simultaneous production of small-scale turbulence in kelp wakes. Slower mean currents and smaller eddy scales in turn may attenuate vertical mixing within forests, thus extending spore suspension times. Further complexities likely arise due to changes in the relative rates of horizontal and vertical dispersion, modifications to the overall profiles of vertical mixing, and the creation of fine-scale secondary flows around kelp individuals and substratum features. Under conditions of more rapid currents, submergence of the surface canopy and the establishment of skimming flows at the canopy–fluid interface may introduce additional coherent flow structures that alter rates of fluid exchange to and from the forest. Many of these coupled physical–biological processes are just beginning to be examined in a rigorous fashion in kelp forests, but their potential importance is clear. 相似文献