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471.
Mark B. Dyurgerov & Mark F. Meier 《Geografiska Annaler: Series A, Physical Geography》1999,81(4):541-554
Seasonal mass balance components bw (winter balance) and bs (summer balance) as well as ct (total accumulation) and at (total ablation), can be used directly to infer climate variables. In contrast, ac (net balance of the accumulation area) and aa (net balance of the ablation area), and ba or bn (annual or net balance) can not. The traditional Alpine system of observations of ac and aa , however, can be converted to true seasonal values bw and bs if both pairs of components are simultaneously observed for some years, because a correlation between the two pairs of components exists. We analyzed bw and bs data and their mean, standard deviations and ratios of these to the corresponding net or annual balances for 50 glaciers with relatively long records representing different regions in the northern hemisphere. We also investigated correlations between seasonal components. A negative correlation between bw and bs exists at many glaciers. About two-thirds of the glaciers show insignificant correlations (?0.3 < r < 0.3), implying independence of summer and winter balances. In a few unusual cases the correlations are positive. These different correlations, or lack thereof, may offer insight into feedback conditions that must exist in this climate-related system. The correspondence of the bw and ct , and bs and at , appears to depend largely on the relative amounts of summer snowfall, a function of their climatic environment expressed as [α = (bw+bs)/2]. The contribution of variability of bs to the net balance increases markedly with decreasing values of α. The variability of bw and bs , and therefore the net balance, has been increasing with time; whether this is due to an increase in climate variability or to other causes is not clear. It appears that bw has been increasing with time at the highest altitudes, but bs has been increasing more rapidly especially at low altitudes; the many-glacier average net balance is becoming more negative. 相似文献
472.
473.
Mark Lacy Mary Elizabeth Kaiser Gary J. Hill Steve Rawlings Gareth Leyshon 《Monthly notices of the Royal Astronomical Society》1999,308(4):1087-1095
Further imaging observations of a sample of radio sources in the North Ecliptic Cap are presented and a number of new identifications are made. Using redshifts from spectroscopic data presented in a companion paper by Lacy et al., the photometric properties of the galaxies in the sample are discussed. It is shown that: (1) out to at least z ≈0.6 radio galaxies are good standard candles irrespective of radio luminosity; (2) for 0.6≲ z ≲1 a large fraction of the sample has magnitudes and colours consistent with a non-evolving giant elliptical, and (3) at higher redshifts, where the R -band samples the rest-frame UV flux, most objects have less UV luminosity than expected if they form their stellar populations at a constant rate from a high redshift to z ∼1 in unobscured star-forming regions (assuming an Einstein–de Sitter cosmology). The consequences of these observations are briefly discussed. 相似文献
474.
Abstract— Velocities of ejecta from seven impacts of aluminum projectiles into coarse-grained sand have been measured with a laser-based apparatus that produces stroboscopic photographs of individual grains in ballistic flight. Speeds and angles of the majority of the ejecta can then be measured very precisely. There appears to be little effect of impact velocity on the functional relationship between the scaled, radial launch position and either the speed or angle of ejection; the seven experiments covered a range of impact velocities from 0.8 to 1.9 kms—1. The measured ejection speeds follow power-law distributions, as predicted by dimensional analysis, but the angle of ejection is not constant throughout a given event as predicted. Indeed, the angle of ejection declines gradually with increasing radial distance from the impact point, but there are indications that the angle increases again as the position of the final crater's rim is approached. The exponents determined from scaled crater dimensions and ejection-speed distributions are substantially different. Although this might imply that assumptions used in the dimensional analysis are not valid, it is also possible that the coarse sand, whose component grains were comparable in dimension to the diameter of the impactors, instead presented a target that was more of an inhomogeneous aggregate of large fragments than a uniform, continuous medium. 相似文献
475.
476.
Abstract. Four individual dugongs Dugong dugong , MÜLLER, 1776 were tracked with buoyant, tethered. conventional and satellite radio transmitters. The dugongs three adult females and one immature male were encircled with a net at a catch site near Haruku Island, East Indonesia, and tracked for between 41 and 285 days. The animals showed individualistic patterns of movement, moving between 2 and 3 core areas, travelling between 17 and 65 km from the site of capture. One adult female spent most of the time at two distinct inshore seagrass beds separated by about 17 km; she made five trips between the two sites. Two other females made separate trips to two distinct sites, one of them returning to the catch site at Haruku Island. The immature male journeyed between two areas about 65 km apart. completing the journey in four days. The patterns of movement confirmed a practice of regular recropping of restricted grazing swards by small, loose feeding assemblages rather than fixed herds with a strong social bond. Mean home ranges covered 4.1 km2 50% harmonic mean and 43.4 km2 95% harmonic mean. 相似文献
477.
Improving neural network performance on the classification of complex geographic datasets 总被引:4,自引:0,他引:4
Neural Networks are now established computational tools used for search minimisation and data classification. They offer
some highly desirable features for landuse classification problems since they are able to take in a variety of data types,
recorded on different statistical scales, and combine them. As such, neural networks should offer advantages of increased
accuracy. However, a barrier to their general acceptance and use by all but `experts' is the difficulty of configuring the
network initially.
This paper describes the architectural problems of applying neural networks to landcover classification exercises in geography
and details some of the latest developments from an ongoing research project aimed at overcoming these problems. A comprehensive
strategy for the configuration of neural networks is presented, whereby the network is automatically constructed by a process
involving initial analysis of the training data. By careful study of the functioning of each part of the network it is possible
to select the architecture and initial weights on the node connections so the constructed network is `right first time'. Further
adaptations are described to control network behaviour, to optimise functioning from the perspective of landcover classification.
The entire configuration process is encapsulated by a single application which may be treated by the user as a `black box',
allowing the network to the applied in much the same way as a maximum likelihood classifier, with no further effort being
required of the user. 相似文献
478.
Higher education in England has expanded rapidly in the last ten years with the result that currently more than 30% of young
people go on to university. Expansion is likely to continue following the recommendations of a national committee of inquiry
(the Dearing Committee). The participation rate is known to vary substantially among social groups and between geographical
areas. In this paper the participation rate is calculated using a new measure, the Young Entrants Index (YEI), and the extent
of variation by region, gender and residential neighbourhood type established. The Super Profiles geodemographic system is
used to facilitate the latter. This is shown to be a powerful discriminator and to offer great potential as an alternative
analytical approach to the conventional social class categories, based on parental occupation, that have formed the basis
of most participation studies to date.
Received: 20 October 1998/Accepted: 5 May 1999 相似文献
479.
Earle Williams Bob Boldi Anne Matlin Mark Weber Steve Hodanish Dave Sharp Steve Goodman Ravi Raghavan Dennis Buechler 《Atmospheric Research》1999,51(3-4)
The development of a new observational system called LISDAD (Lightning Imaging Sensor Demonstration and Display) has enabled a study of severe weather in central Florida. The total flash rates for storms verified to be severe are found to exceed 60 fpm, with some values reaching 500 fpm. Similar to earlier results for thunderstorm microbursts, the peak flash rate precedes the severe weather at the ground by 5–20 min. A distinguishing feature of severe storms is the presence of lightning ‘jumps' — abrupt increases in flash rate in advance of the maximum rate for the storm. The systematic total lightning precursor to severe weather of all kinds — wind, hail, tornadoes — is interpreted in terms of the updraft that sows the seeds aloft for severe weather at the surface and simultaneously stimulates the ice microphysics that drives the intracloud lightning activity. 相似文献
480.
Mark A. Rathmell Margaret M. Streepey Eric J. Essene Ben A. van der Pluijm 《Contributions to Mineralogy and Petrology》1999,134(2-3):217-231
The Elzevir Terrane of the Grenville Orogen in southern Ontario contains metapelites and abundant graphitic marbles that
were regionally metamorphosed from the upper greenschist to upper amphibolite facies. Comparative thermometry was undertaken
with widely used calibrations for the systems garnet-biotite, calcite-dolomite, and calcite-graphite. Temperatures that are
obtained from matrix biotites paired with prograde garnet near-rim analyses are usually consistent with those determined using
calcite-graphite thermometry. However, calcite-graphite thermometry occasionally yields low temperatures due to lack of equilibration
of anomalously light graphite. Application of calcite-graphite and garnet-biotite systems may yield temperatures up to 70 °C
higher than calcite-dolomite in amphibolite facies rocks. Calcite-dolomite temperatures most closely approach those from calcite-graphite
and garnet-biotite when the samples contain a single generation of dolomite and calcite grains contain no visible dolomite
exsolution lamellae. However, some of these samples yield temperatures considerably lower than temperatures calculated from
calcite-graphite and garnet-biotite thermometry, indicating that the calcite-dolomite thermometer may have been partially
reset during retrogression. Estimated peak metamorphic temperatures of regional metamorphism between Madoc (upper greenschist
facies) and Bancroft (upper amphibolite facies) range from 500 to 650 °C. These results place the chlorite-staurolite isograd
at 540 °C, the kyanite-sillimanite isograd at 590 °C, and the sillimanite-K-feldspar isograd at 650 °C. Although each thermometer
may have an absolute uncertainty of as much as ±50 °C, the 50 to 60 °C temperature differences between the isograds are probably
accurate to 10 to 20 °C. An incomplete picture of the thermal gradients can result from the application of only one thermometer
in a given area. Simultaneous application of several systems allows one to recognize and overcome the inherent limitations
of each thermometer.
Received: 26 March 1997 / Accepted: 15 April 1998 相似文献