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At least 883 records · Page 49

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NASA NTRS1990

The spatial variation of vegetation changes at very coarse scales

Previous analysis (Townshend and Justice 1988) is extended to examine the spatial variations in images of the normalized difference vegetation index (NDVI) of seven areas. These images were derived by subtracting corresponding pixel values from pairs of registered MSS images. The changes depicted by these derived images were analyzed by scale variance analysis for pixel sizes between 4 km and 64 km. It is shown that for some areas substantial changes are detectable at these very coarse scales, although there is less contrast between the areas than at finer spatial resolutions below 1 km. In all the areas the total spatial variability of the images is contributed at a wide variety of spatial scales.

Townshend, John R. G.↗

NASA NTRS1988

The meridional scale of baroclinic waves with latent heat release

The control on the meridional scale of a class of the baroclinic waves exercised by latent heat release is analyzed. A meridional-scale equation is derived, in which the dry model and the moist model without the meridional variation of the baroclinic waves are revealed. It is shown that, in the dry model the stability analysis cannot determine the meridional scale of the baroclinic waves. When latent heat release is included, the meridional variation of the waves either vanishes or is finite. When the waves have the meridional variation with latent heat release, the growth rate increases as the heating increases for a given Froude number, and there are two modes - the first mode has a small ascending region and a large descending region, while the second mode has a small descending region and a large ascending region.

Tang, Chung-Muh↗

NASA NTRS1989

Analysis of regional-scale vegetation dynamics of Mexico using stratified AVHRR NDVI data

Previous studies have shown that a good relationship exists between AVHRR Normalized Difference Vegetation Index (NDVI) measurements, and both regional-scale patterns of vegetation seasonality and productivity. Most of these studies used known samples of vegetation types. An alternative approach, and the objective was to examine the above relationships by analyzing one year of AVHRR NDVI data that was stratified using a small-scale vegetation map of Mexico. The results show that there is a good relationship between AVHRR NDVI measurements and regional-scale vegetation dynamics of Mexico.

Turcotte, Kevin M.↗

NASA NTRS1990

A theory of forest dynamics: Spatially explicit models and issues of scale

Good progress has been made in the first year of DOE grant (number sign) FG02-90ER60933. The purpose of the project is to develop and investigate models of forest dynamics that apply across a range of spatial scales. The grant is one third of a three-part project. The second third was funded by the NSF this year and is intended to provide the empirical data necessary to calibrate and test small-scale (less than or equal to 1000 ha) models. The final third was also funded this year (NASA), and will provide data to calibrate and test the large-scale features of the models.

Pacala, S.↗

NASA NTRS1989

Small scale variations of abundances of transiently heated grains in molecular clouds

IRAS images of a variety of fragments in nearby molecular clouds show that the energy distribution of their IR emission varies widely from cloud to cloud and from place to place within a given cloud. These variations at small scale are all the more unexpected since the colors of the IR emission of cold material differ very little at large scale: the colors of the cirrus emission above the 3kpc molecular ring are the same as those of the cirrus emission in the solar neighborhood. To quantitatively study these variations, 12, 60, and 100 microns brightnesses were obtained of small areas centered at different positions within the set of clouds and complexes. The range of observed 12/100 micron colors is given for each cloud. Variations by an order of magnitude are found in most clouds. Variations by a factor of 2 to 3 are observed within a cloud on scales as small as 0.5pc, the resolution of this study. It is concluded that large variations of the abundances of small particles with respect to those of the large grains responsible for the 100 micron emission are required to explain the observed color variations and that these abundances have to vary by large factors; an order of magnitude from cloud to cloud.

Boulanger, F.↗

NASA NTRS1991

Light domain walls, massive neutrinos and the large scale structure of the Universe

Domain walls generated through a cosmological phase transition are considered, which interact nongravitationally with light neutrinos. At a redshift z greater than or equal to 10(exp 4), the network grows rapidly and is virtually decoupled from the matter. As the friction with the matter becomes dominant, a comoving network scale close to that of the comoving horizon scale at z of approximately 10(exp 4) gets frozen. During the later phases, the walls produce matter wakes of a thickness d of approximately 10h(exp -1)Mpc, that may become seeds for the formation of the large scale structure observed in the Universe.

Massarotti, Alessandro↗

NASA NTRS1991

Void probability as a function of the void's shape and scale-invariant models

The dependence of counts in cells on the shape of the cell for the large scale galaxy distribution is studied. A very concrete prediction can be done concerning the void distribution for scale invariant models. The prediction is tested on a sample of the CfA catalog, and good agreement is found. It is observed that the probability of a cell to be occupied is bigger for some elongated cells. A phenomenological scale invariant model for the observed distribution of the counts in cells, an extension of the negative binomial distribution, is presented in order to illustrate how this dependence can be quantitatively determined. An original, intuitive derivation of this model is presented.

Elizalde, E.↗

NASA NTRS1990

Small-scale disequilibrium in a magmatic inclusion and its more silicic host

An investigation of small-scale isotopic, compositional, and mineralogical variation across the interface of a basaltic-andesite inclusion and its dacitic host from Cerro-Chascon, a Holocene dome in northern Chile, is discussed. Serial sectioning across the interface of the inclusion and its host dacite, complemented by microdrill sampling and detailed microprobe work, has enabled an examination of the scale of mixing and chemical disequilibrium. The composition of the inclusion is found to be relatively homogeneous; the dacite host is heterogeneous on a small scale; the isotopic composition in the marginal zone shows the highest Sr-87/Sr-86 and lowest Nd-143/Nd-144; the large plagioclase crystals in the inclusions and host are xenocrystic. These differences are reconciled with a model of magma evolution in a crustal magma chamber.

Davidson, Jon P.↗

NASA NTRS1990

Properties of the large- and small-scale flow patterns in and around AR 19824

A series of magnetograms spanning just over 5 days and covering about 4 x 5 arcmin has been used to trace the photospheric motions of 170 concentrations of magnetic flux tubes in and around the decaying active region 19824. A persistent large-scale flow pattern superposed on small-scale random motions of both polarities is noted whose overall pattern resembles the flow of a river around a sand bar, and whose long-term evolution suggests a persistence of several months. The observed motions of concentrations of flux tubes are corrected for the large-scale flow in order to study their random motions.

Schrijver, C. J.↗

NASA NTRS1990

A class of systolizable IIR digital filters and its design for proper scaling and minimum output roundoff noise

A class of infinite impulse response (IIR) digital filters with a systolizable structure is proposed and its synthesis is investigated. The systolizable structure consists of pipelineable regular modules with local connections and is suitable for VLSI implementation. It is capable of achieving high performance as well as high throughput. This class of filter structure provides certain degrees of freedom that can be used to obtain some desirable properties for the filter. Techniques of evaluating the internal signal powers and the output roundoff noise of the proposed filter structure are developed. Based upon these techniques, a well-scaled IIR digital filter with minimum output roundoff noise is designed using a local optimization approach. The internal signals of all the modes of this filter are scaled to unity in the l2-norm sense. Compared to the Rao-Kailath (1984) orthogonal digital filter and the Gray-Markel (1973) normalized-lattice digital filter, this filter has better scaling properties and lower output roundoff noise.

Lei, Shaw-Min↗

NASA NTRS1991

Measures of large-scale structure in the CfA redshift survey slices

Variations of the counts-in-cells with cell size are used here to define two statistical measures of large-scale clustering in three 6 deg slices of the CfA redshift survey. A percolation criterion is used to estimate the filling factor which measures the fraction of the total volume in the survey occupied by the large-scale structures. For the full 18 deg slice of the CfA redshift survey, f is about 0.25 + or - 0.05. After removing groups with more than five members from two of the slices, variations of the counts in occupied cells with cell size have a power-law behavior with a slope beta about 2.2 on scales from 1-10/h Mpc. Application of both this statistic and the percolation analysis to simulations suggests that a network of two-dimensional structures is a better description of the geometry of the clustering in the CfA slices than a network of one-dimensional structures. Counts-in-cells are also used to estimate at 0.3 galaxy h-squared/Mpc the average galaxy surface density in sheets like the Great Wall.

De Lapparent, Valerie↗

NASA NTRS1991

Large-scale structure in a texture-seeded cold dark matter cosmogony

This paper studies the formation of large-scale structure by global texture in a flat universe dominated by cold dark matter. A code for evolution of the texture fields was combined with an N-body code for evolving the dark matter. The results indicate some promising aspects: with only one free parameter, the observed galaxy-galaxy correlation function is reproduced, clusters of galaxies are found to be significantly clustered on a scale of 20-50/h Mpc, and coherent structures of over 50/h Mpc in the galaxy distribution were found. The large-scale streaming motions observed are in good agreement with the observations: the average magnitude of the velocity field smoothed over 30/h Mpc is 430 km/sec. Global texture produces a cosmic Mach number that is compatible with observation. Also, significant evolution of clusters at low redshift was seen. Possible problems for the theory include too high velocity dispersions in clusters, and voids which are not as empty as those observed.

Park, Changbom↗

NASA NTRS1991

Mixing transition and the cascade to small scales in a plane mixing layer

Direct numerical simulations of time-developing plane mixing layers have been performed using a variety of initial conditions. Up to two pairings of the dominant spanwise rollers have been simulated. When the flow is sufficiently three-dimensional, a pairing can cause the mixing layer to undergo a transition to small-scale turbulence. This small-scale transition is accompanied by increased mixing of a passive scalar, similar to observations of the mixing transition in experiments. As part of the transition process, thin vortex sheets are generated by vortex stretching and roll up in a two-dimensional mixing layer. This higher-order rollup is part of the cascade to small-scale turbulence. Estimates are obtained for the degree of three-dimensionality required for the pairing to initiate the transition.

Moser, R. D.↗

NASA NTRS1990

Functional relation among subpixel canopy cover, ground shadow, and illuminated ground at large sampling scales

The functional relation among subpixel canopy cover, illuminated soil, and shadowed soil, which progressively develops with increasing pixel size, is investigated for Poisson distributed plants using a geometric canopy simulation model. An analytical relation among cover components is shown to be applicable when the scale of the pixel is much larger than the scale of the plant and ground shadow. The analysis is facilitated through the use of a nondimensional solar-geometric similarity parameter, eta, equal to the ratio of the area of one plant canopy to its associated ground shadow area, as viewed from nadir. A sampling scale ratio, defined as the ratio of the area of the pixel to the mean area of a single plant shadow, is tested as a quantitative criterion to evaluate when the functional relation among subpixel components occurs. The results of a remote sensing experiment over a natural conifer landscape provide preliminary confirmation of the theoretical analysis.

Jasinski, Michael F.↗

NASA NTRS1990

Small-scale plasma, magnetic, and neutral density fluctuations in the nightside Venus atmosphere

The evolution of the Venus small-scale waves as they propagate into the nightsite is examined, and the small-scale structures are compared with the waves in the three components of the magnetic field, magnetic dip angle, and neutral density. It is demonstrated that the small-scale fluctuations evolve between the transterminator and antisolar regions. It is shown that atmospheric gravity waves may also be producing some of the fluctuations observed at longer wavelengths. The electron temperature and density are shown to be approximately 180 deg out of phase and exhibiting the highest correlation of any pair of variables. Waves in the electron and neutral densities are found to be correlated moderately on most orbits, while the average electron temperature is higher when the average magnetic field is more horizontal.

Hoegy, W. R.↗

NASA NTRS1991

The small-scale clustering properties of dwarf galaxies

Two results on the small-scale clustering properties of dwarf galaxies are reported, which were identified in the vicinity of early-type Shapley-Ames galaxies on high-resolution photographic plates. The first result indicates that dwarf galaxies display the same trend of stronger clustering toward earlier morphological type on small scales as their giant counterparts on larger scales. It is suggested that early-type dwarfs can be used as dynamical probes of dark halos around early-type giant galaxies and as tracers of the dynamical evolution of such halos in dense environments. The second result pertains to the trend of increasing early-type dwarf frequency per early-type giant with environment richness previously established for rich groups. It is found that a minimum value of isolated early-type galaxies is approximately 0.25, as compared to a maximum of approximately 8 in rich environments like the Virgo Cluster.

Vader, J. P.↗

NASA NTRS1990

A non-isotropic multiple-scale turbulence model

A newly developed non-isotropic multiple scale turbulence model (MS/ASM) is described for complex flow calculations. This model focuses on the direct modeling of Reynolds stresses and utilizes split-spectrum concepts for modeling multiple scale effects in turbulence. Validation studies on free shear flows, rotating flows and recirculating flows show that the current model perform significantly better than the single scale k-epsilon model. The present model is relatively inexpensive in terms of CPU time which makes it suitable for broad engineering flow applications.

Chen, C. P.↗

NASA NTRS1991

The scaling of secondary craters

Secondary craters are common features around fresh planetary-scale primary impact craters throughout most of the Solar System. They derive from the ejection phase of crater formation, thus secondary scaling relations provide constraints on parameters affecting ejection processes. Secondary crater fields typically begin at the edge of the continuous ejecta blankets (CEB) and extend out several crater radii. Secondaries tend to have rounded rims and bilateral symmetry about an axis through the primary crater's center. Prominent secondary chains can extend inward across the CEB close to the rim. A simple method for comparing secondary crater fields was employed: averaging the diameters and ranges from the center of the primary crater of the five largest craters in a secondary crater field. While not as much information is obtained about individual crater fields by this method as in more complete secondary field mapping, it facilitates rapid comparison of many secondary fields. Also, by quantifying a few specific aspects of the secondary crater field, this method can be used to construct scaling relations for secondary craters.

Croft, Steven K.↗