Butterfly pitch-angle distributions observed by ISEE-1     

Theodore A. Fritz  Mohamed Alothman  Jyotirmoyee BhattacharjyaDavid L. Matthews  Jiasheng Chen 《Planetary and Space Science》2003,51(3):205-219

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Band Selection for Hyperspectral Image Classification Using Mutual Information   总被引：3，自引：0，他引：3  

Guo  B. Gunn  S.R. Damper  R.I. Nelson  J.D.B. 《Geoscience and Remote Sensing Letters, IEEE》2006,3(4):522-526

Spectral band selection is a fundamental problem in hyperspectral data processing. In this letter, a new band-selection method based on mutual information (MI) is proposed. MI measures the statistical dependence between two random variables and can therefore be used to evaluate the relative utility of each band to classification. A new strategy is described to estimate the MI using a priori knowledge of the scene, reducing reliance on a "ground truth" reference map, by retaining bands with high associated MI values (subject to the so-called "complementary" conditions). Simulations of classification performance on 16 classes of vegetation from the AVIRIS 92AV3C data set show the effectiveness of the method, which outperforms an MI-based method using the associated reference map, an entropy-based method, and a correlation-based method. It is also competitive with the steepest ascent algorithm at much lower computational cost  相似文献   

Snow cover variability across central Canada (1978–2002) derived from satellite passive microwave data     

Michael A. Wulder  Trisalyn A. Nelson  Chris Derksen  David Seemann 《Climatic change》2007,82(1-2):113-130

Twenty-four winter seasons (1978–2002) of mean February snow water equivalent (SWE) values were analyzed in an exploration of the spatial pattern of temporal variability in snow cover across the non-mountainous interior of Canada. The SWE data were derived from space-borne passive microwave brightness temperatures processed with a land cover-sensitive suite of algorithms. Spatial patterns in the frequency and amount of variability were investigated on an annual basis through comparisons with average trends over all 24 years. Changes in temporal variability through time were also investigated by comparing three eight year time periods to general trends. Analyses were synthesized at the ecozone scale in order to link results both to potential land cover influences on algorithm performance and climatological variability in SWE. Prairie and northern ecozones were typically found to be the most variable in terms of SWE magnitude. Analyses indicate that non-treed land cover classes are generally more variable than treed classes. The results also indicate that extreme weather events appear to be occurring with increasing consistency in the Prairie and Arctic regions. Discerning climatologically significant variability in the time series, compared to algorithm-related issues can be a challenge, but in an era of eroding surface observing networks the passive microwave time series represents an important resource for monitoring and detecting trends and variability in terrestrial snow cover.  相似文献   

STAMP: spatial–temporal analysis of moving polygons     

Colin Robertson  Trisalyn A. Nelson  Barry Boots  Michael A. Wulder 《Journal of Geographical Systems》2007,9(3):207-227

Research questions regarding temporal change in spatial patterns are increasingly common in geographical analysis. In this research, we explore and extend an approach to the spatial–temporal analysis of polygons that are spatially distinct and experience discrete changes though time. We present five new movement events for describing spatial processes: displacement, convergence, divergence, fragmentation and concentration. Spatial–temporal measures of events for size and direction are presented for two time periods, and multiple time periods. Size change metrics are based on area overlaps and a modified cone-based model is used for calculating polygon directional relationships. Quantitative directional measures are used to develop application specific metrics, such as an estimation of the concentration parameter for a von Mises distribution, and the directional rate of spread. The utility of the STAMP methods are demonstrated by a case study on the spread of a wildfire in northwestern Montana.   相似文献   

Crustal thickening and crustal extension as controls on the evolution of the drainage network of the central Swiss Alps between 30  Ma and the present: constraints from the stratigraphy of the North Alpine Foreland Basin and the structural evolution of the Alps     

Fritz Schlunegger  Rudy Slingerl  & Albert Matter 《Basin Research》1998,10(2):197-212

The combined information about sedimentary petrography from the North Alpine Foreland Basin and structural geology from the Alps allows a qualitative reconstruction of the drainage network of the central Swiss Alps between 30 Ma and the present. This study suggests that crustal thickening and crustal thinning significantly controlled the location of the drainage divide. It also reveals the possible controls of crustal thickening/thinning on the change of the orientation of the drainage network from across-strike between 30 and 14 Ma to along-strike thereafter. Initial crustal thickening in the rear of the wedge is considered to have formed the drainage divide between north and south at 30 Ma. Because the location of crustal thickening shifted from east to west between ≈30–20 Ma, the catchment areas of the eastern dispersal systems reached further south than those of the western Alpine palaeorivers for the same time slice. Similarly, the same crustal dynamics appear to have controlled two phases of denudation that are reflected in the Molasse Basin by petrographic trends. Uplift in the rear of the wedge caused the Alpine palaeorivers to expand further southward. This is reflected in the foreland basin by increasing admixture of detritus from structurally higher units. However, tectonic quiescence in the rear of the wedge allowed the Alpine palaeorivers to cut down into the Alpine edifice, resulting in an increase of detritus from structurally lower units. Whereas uplift in the rear of the wedge was responsible for initiation of the Alpine drainage systems, underplating of the external massifs some 50 km further north is thought to have caused along-strike deviation of the major Alpine palaeorivers. Besides crustal thickening, extension in the rear of the wedge appears to have significantly controlled the evolution of the drainage network of the western Swiss Alps. Slip along the Simplon detachment fault exposed the core of the Lepontine dome, and caused a 50-km-northward shift of the drainage divide.  相似文献   

Collapse and fragmentation of rotating, prolate clouds     

Richard P. Nelson 《Monthly notices of the Royal Astronomical Society》1998,298(3):657-676

We present the results of collapse calculations for uniformly rotating, prolate clouds performed using the numerical method: smoothed particle hydrodynamics (SPH). The clouds considered are isothermal, prolate spheroids with different axial ratios ( a/b ), and with different values of β, the ratio of the rotational to gravitational energy. Small density perturbations are added to the clouds, and different initial perturbation spectra are studied. All of the clouds considered are strongly unstable to gravitational contraction, and so collapse to form a spindle configuration. Such a linear structure is unstable to fragmentation, so that the clouds break up into a number of subcondensations. The long-term evolution of the system is then determined by the angular momentum possessed by these fragments.
It is found that a number of the calculations performed result in the formation of orbitally stable binary systems, composed of two rotationally supported discs in orbit about their common centre of mass. Tidal interactions during closest approach, close three-body interactions and the continued accretion of material with high specific angular momentum are all found to increase the orbital separation during these calculations, ensuring that the systems do not merge at later times. The calculations are therefore relevant to the problem of binary star formation, though the systems produced tend to have large orbital separations and periods. One of the strong points of the models presented, however, is their ability to produce systems with a range of mass ratios and orbital eccentricities, without the explicit inclusion of biases in the initial conditions.  相似文献   

Pivot poles and plate motion reconstructions     

T.H. Nelson  R.A. Baker  R.W. Murphy 《Tectonophysics》1981,74(1-2)

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Detailed observational study of a weak Echo region     

Stephan P. Nelson  Roscoe R. Braham Jr. 《Pure and Applied Geophysics》1975,113(1):735-746

High resolution 3-cm weather radar was used to observe the three dimensional structure of a weak echo region of a severe convective storm in northern Minnesota, U.S.A. Observations show that a seemingly steady-state updraft in the lower portion of the storm broke into an unsteady structure in the upper levels. This is interpreted as indicating that precipitation loading gradually decelerated the upper reaches of the updraft, resulting in periodic breakout of a new updraft core on the inflow side. This periodic behavior of the updraft may be related to hailstreaks and what has been termed the multi-celled storm.  相似文献   

Die Erschliessung der Numerischen Wetterprognose durch die Hypothese der Lärmfreien Atmosphäre     

Fritz Herbert 《Pure and Applied Geophysics》1980,118(2):880-912

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Radiative Effects on Temperature in the Stable Surface Layer   总被引：2，自引：1，他引：1  

Nelson Dias 《Boundary-Layer Meteorology》1998,89(1):141-159

The interaction between longwave radiation and temperature fluctuations plays a role in the dissipation of temperature variance. This interaction is most easily described by spectral models of atmospheric turbulence and a spectral radiative dissipation function which gives the intensity of the damping at each radiative wavelength and wavenumber k. We have used a Corrsin–Pao closure for the spectral budgets of turbulent kinetic energy and temperature to study the coupling of radiation to turbulence. The spectral radiative dissipation function and a related integral have been fitted by analytical approximations with the correct asymptotic behavior. This resulted in a simple analytical formula for the dimensionless temperature spectrum as a function of Monin-Obukhov stability, and a new dimensionless parameter describing the relative importance of radiation in the temperature spectral budget. The radiative effects both on the temperature spectrum and on the dimensionless temperature variance can then be calculated. Based on typical values of the radiative dimensionless parameters for the surface layer, we conclude that radiative dissipation is probably negligible there.  相似文献