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At least 505 records · Page 28
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

An experimental investigation on flame interaction and the existence of negative flame speeds

Downstream interaction between two counterflow premixed flames of different stoichiometries are investigated. Various flame configurations are observed and quantified; these include the binary system of two lean or rich flames, the triplet system of a lean and a rich flame separated by a diffusion flame, and single diffusion flames with some degree of premixedness. Extinction limits are determined for methane/air and butane/air mixtures over the entire range of mixture concentrations. Results show that these extinction limits can be significantly modified in the presence of interaction such that a mixture much beyond the flammability limit can still burn if it is supported by a stronger flame. The experiment also demonstrates the existence of negative flames whose propagation velocity is in the same general direction as that of the bulk convective flow. Implications of the present results on the flammability of stratified mixtures and on the modeling of turbulent flames are discussed.

Sohrab, S. H.↗
NASA NTRS1986

Analysis of forest structure using thematic mapper simulator data

The potential of Thematic Mapper Simulator (TMS) data for sensing forest structure information has been explored by principal components and feature selection techniques. In a survey of forest structural properties conducted for 123 field sites of the Sequoia National Park, the canopy closure could be well estimated (r = 0.62 to 0.69) by a variety of channel bands and band ratios, without reference to the forest type. Estimation of the basal area was less successful (r = 0.51 or less) on the average, but could be improved for certain forest types when data were stratified by floristic composition. To achieve such a stratification, individual sites were ordinated by a detrended correspondence analysis based on the canopy of dominant species. The analysis of forest structure in the Sequoia data suggests that total basal area can be best predicted in stands of lower density, and in younger even-aged managed stands.

Peterson, D. L.↗
NASA NTRS1986

Coniferous forest classification and inventory using Landsat and digital terrain data

Machine-processing techniques were used in a Forest Classification and Inventory System (FOCIS) procedure to extract and process tonal, textural, and terrain information from registered Landsat multispectral and digital terrain data. Using FOCIS as a basis for stratified sampling, the softwood timber volumes of the Klamath National Forest and Eldorado National Forest were estimated within standard errors of 4.8 and 4.0 percent, respectively. The accuracy of these large-area inventories is comparable to the accuracy yielded by use of conventional timber inventory methods, but, because of automation, the FOCIS inventories are more rapid (9-12 months compared to 2-3 years for conventional manual photointerpretation, map compilation and drafting, field sampling, and data processing) and are less costly.

Franklin, J.↗
NASA NTRS1986

Field size distributions for selected agricultural crops in the United States and Canada

Digitized agricultural field boundary taken in the United States and Canada during the LACIE and AgRISTARS programs, in 1977 through 1980, were used to construct histograms showing the distributions of field area, width, and length for crops for which there were data for 700 or more fields per state. The observed distributions of area and width for fields of 10 crops grown in 13 states of the United States and Canada were compared with best-fit inverse Gaussian distributions and with log-normal distributions. For 28 distributions of area and 16 distributions of width there was found to be a probability of greater than .01 of their being inverse Gaussian. There were 10 distributions of area for which there was probability of greater than .005 of their being log-normal. Distributions of area and width stratified by state and crop type appear to be unique. The inverse Gaussian, which represents a wide range of statistical distributions from skewed to almost symmetrical, can provide a useful model for distributions of field area.

Ferguson, M. C.↗
NASA NTRS1986

Transition region of the earth's upper mantle

The chemistry of the earth's mantle is discussed using data from cosmochemistry, geochemistry, petrology, seismology, and mineral physics. The chondritic earth, the upper mantle and the 400-km discontinuity, the transition region, lower mantle mineralogy, and surface wave tomography are examined. Three main issues are addressed: (1) whether the mantle is homogeneous in composition or chemically stratified, (2) whether the major element chemistry of the mantle is more similar to upper mantle peridotites or to chondrites, and (3) the nature of the composition of the source region of basalts erupted at midocean ridges.

Anderson, D. L.↗
NASA NTRS1986

Extension of continental lithosphere - A model for two scales of basin and range deformation

The development of a model for deformation in an extending continental lithosphere that is stratified in density and strength is described. The lithosphere model demonstrates that the necking instabilities at two wavelengths originate due to a strong upper crust, a mantle layer, and a weak lower crust. It is observed that the dominant wavelengths of necking are controlled by layer thickness and the strength of the layers control the amplitude of the instabilities. The model is applied to the Basin and Range Province of the western U.S. where deformations in ranges and tile domains are detected. The relation between the Bouguer gravity anomaly and the deformations is studied. The data reveal that the horizontal scale of short wavelength necking correlates with the spacings of individual basins and ranges, and the longer wavelength corresponds to the width of tilt domains. The control of the Basin and Range deformation by two scales of extensional instability is proposed.

Zuber, M. T.↗
NASA NTRS1986

Transition to unstable ion flow in parallel electric fields

The stability of ionospheric O(+)-H(+) outflows accelerated by a nonambipolar parallel electric field is considered under conditions where the ion motion initially develops adiabatically and the ambient plasma is vertically stratified with an effective temperature that increases with altitude. Such conditions are expected near the bottom of the auroral acceleration region where ion and electron streaming instabilities first develop. It is shown for a particular equilibrium profile that the differentially accelerated ion flows become unstable within about 100 km from their entry point in the acceleration region. At O(+)/H(+) density ratios less than about 9, the instability is dominated by a violent H(+)-O(+) two-stream interaction which couples the O(+) and H(+) acoustic modes, and which mediates a transition to nonadiabatic acceleration. At higher altitudes and/or larger O(+)/H(+) density ratios, a much weaker resonant instability exists, which is driven by the relative drift between electrons and O(+) or H(+) ions. The results suggest that the H(+)-O(+) two-stream instability may be a viable mechanism for heating upflowing auroral ions.

Bergmann, R.↗
NASA NTRS1986

Meridional distribution of stratospheric trace constituents

Vertically stratified tracers such as methane and nitrous oxide tend to have constant mixing ratio surfaces that slope downward toward the poles in the meridional plane. The equilibrium tracer slope results from the competition between the slope steepening effects of advection by the diabatic circulation and the slope flattening effects of quasi-isentropic eddy transport and photochemical loss. The diabatic circulation iself is, however, driven primarily by eddy transports, which maintain the departure of stratospheric temperatures from radiative equilibrium. If the eddy transports are weak, the diabatic circulation is also weak and the slope is small. Using a simple beta-plane channel model and an eddy diffusion parameterization for the eddy potential vorticity and tracer transports, it is shown that the slope is a maximum for a value of eddy diffusion such that the dynamical time scale lies between the radiative and chemical time scales.

Holton, J. R.↗
NASA NTRS1986

Vertical structure of Arctic haze observed by lidar

In the study of the Arctic Haze phenomenon, understanding the vertical structure of the haze aerosol is crucial in defining mechanisms of haze transport. Questions have also arisen concerning the representativeness of surface observations of Arctic Haze. Due to the strongly stratified nature of the Arctic troposphere, the mechanisms which transport aerosol to the surface from the transport altitudes of the lower troposphere are not obvious. In order to examine these questions, a Mie scattering lidar was installed at Alert, NWT, Canada. Lidar observes atmospheric aerosols and hydrymeteors as they appear in nature, unmodified by sampling effects. As such the results obtained are more realistic of the light scattering characteristics of the in situ aerosol than are those obtained by integrating nephelometers, for example, which heat the aerosol and dry it before measurement. With this lidar, a pulse was transmitted vetically through an evacuated tube in the roof of a building at Alert. The receiver consisted of a 20cm diameter Fresnel telescope, neutral density and polarizing filters, and RCA C31000A PMT, Analog Modules LA-90-P logarithmic amplifier and a Lecroy TR8827 32 MHz digitizer. The lidar equation was solved for the backscattering coefficient of the aerosol assuming no two way transmission losses in the signal. The lidar results have shown that intercomparison between lidar obtained visibilities and observer visibilities are in much better agreement than for other optical or aerosol monitors. Three new effects were identified in the lidar profiles which contribute to the vertical transport of haze. These effects are briefly discussed.

Hoff, R. M.↗
NASA NTRS1986

Observations of mesoscale vertical velocities around frontal zones

Vertical velocity and reflectivity data obtained with a VHF Doppler radar over a 15-day period in October and November of 1981 are analyzed. Standard radiosonde data and surface observations were used to locate two occluded fronts, two warm fronts, and a cold front that passed the radar site. These fronts are also evident in the radar reflectivity data. Most studies of the vertical circulation patterns associated with mososcale systems have used precipitation and cloud formations as tracers. Unlike other observational techniques, the VHF radar permits the continuous measurement of the three-dimensional air velocity vector in time and height from a fixed location. With the beam in a vertically pointing position, signals are scattered from turbulent variations in the refractive index with half the scale of the radar wavelength and by regions with sudden changes in the refractive index associated with horizontally stratified layers. Generally, the strongest echoes occur at the maximum in the vertical gradient of refractivity, usually at the base of a temperature inversion, such as the tropopause. VHF radars can also be used to locate atmospheric fronts, which are characterized by static stability, large horizontal temperature gradients, large vorticities, and vertical wind shears. These radars can provide the velocity field data needed to study wave motions associated with fronts and compare the actual vertical circulation to theoretical predictions.

Dennis, T. S.↗
NASA NTRS1986

Canopy reflectance modeling in a tropical wooded grassland

Geometric/optical canopy reflectance modeling and spatial/spectral pattern recognition are used to study the form and structure of savanna in West Africa. An invertible plant canopy reflectance model is tested for its ability to estimate the amount of woody vegetation cover in areas of sparsely wooded grassland from remotely sensed data. Dry woodlands and wooded grasslands, commonly referred to as savannas, are important ecologically and economically in Africa, and cover approximately forty percent of the continent by some estimates. The Sahelian and Sudanian savanna make up the important and sensitive transition zone between the tropical forests and the arid Saharan region. The depletion of woody cover, used for fodder and fuel in these regions, has become a very severe problem for the people living there. LANDSAT Thematic Mapper (TM) data is used to stratify woodland and wooded grassland into areas of relatively homogeneous canopy cover, and then by applying an invertible forest canopy reflectance model to estimate directly the height and spacing of the trees in the stands. Since height and spacing are proportional to biomass in some cases, a successful application of the segmentation/modeling techniques will allow direct estimation of woody biomass, as well as cover density, over significant areas of these valuable and sensitive ecosystems. Sahelian savanna sites in the Gourma area of Mali being used by the NASA/GIMMS project (Global Inventory Modeling and Monitoring System, at Goddard Space Flight Center), in conjunction with CIPEA/Mali (Centre International pour l'Elevage en Afrique) will be used for testing the canopy model. The model will also be tested in a Sudanian zone crop/woodland area in the Region of Segou, Mali.

Simonett, D.↗
NASA NTRS1986

Canopy reflectance modeling in a tropical wooded grassland

Geometric/optical canopy reflectance modeling and spatial/spectral pattern recognition is used to study the form and structure of savanna in West Africa. An invertible plant canopy reflectance model is tested for its ability to estimate the amount of woody vegetation from remotely sensed data in areas of sparsely wooded grassland. Dry woodlands and wooded grasslands, commonly referred to as savannas, are important ecologically and economically in Africa, and cover approximately forty percent of the continent by some estimates. The Sahel and Sudan savannas make up the important and sensitive transition zone between the tropical forests and the arid Sahara region. The depletion of woody cover, used for fodder and fuel in these regions, has become a very severe problem for the people living there. LANDSAT Thematic Mapper (TM) data is used to stratify woodland and wooded grassland into areas of relatively homogeneous canopy cover, and then an invertible forest canopy reflectance model is applied to estimate directly the height and spacing of the trees in the stands. Because height and spacing are proportional to biomass in some cases, a successful application of the segmentation/modeling techniques will allow direct estimation of tree biomass, as well as cover density, over significant areas of these valuable and sensitive ecosystems. The model being tested in sites in two different bioclimatic zones in Mali, West Africa, will be used for testing the canopy model. Sudanian zone crop/woodland test sites were located in the Region of Segou, Mali.

Simonett, David↗
NASA NTRS1986

Convection without eddy viscosity: An attempt to model the interiors of giant planets

In the theory of hydrostatic quasi-geostrophic flow in the Earth's atmosphere the principal results do not depend on the eddy viscosity. This contrasts with published theories of convection in deep rotating fluid spheres, where the wavelength of the fastest growing disturbance varies as E sup 1/3, where E, the Ekman number, is proportional to the eddy viscosity. A new theory of quasi-columnar motions in stably stratified fluid spheres attempts to capture the luck of the meteorologists. The theory allows one to investigate the stability of barotropic and baroclinic zonal flows that extend into the planetary interior. It is hypothesized that the internal heat Jupiter and Saturn comes out not radially but on sloping surfaces defined by the internal entropy distribution. To test the hypothesis one searches for basic states in which the wavelength of the fastest-growing disturbance remains finite as E tends to zero, and is which the heat flux vector is radially outward and poleward.

Ingersoll, A. P.↗
NASA NTRS1986

A model of the spatial and temporal variation of the Uranus thermal structure

Seasonal variability of the temperature structure of Uranus is modeled for all latitudes in the .0004 to 2 bar pressure range in anticipation of the Voyager encounter in January 1986. Atmospheric heating in the model results on the one hand from an internal heat source and, on the other hand, from absorption of solar energy by methane and by non-conservative aerosols located between the 0.5 and 2 bar levels. Various cases for the behavior of the internal heat flux are investigated, such as constant with latitude or constrained to yield a time-averaged thermal emission independent of latitude. Meridional transport of heat in the stably stratified atmosphere is not taken into account. The results indicate that the Voyager encounter time, very small north-south temperature asymmetry should be expected. Moreover, the northern hemisphere, although not illuminated, should emit as much energy (within one percent) as the southern hemisphere at this date. At a given latitude, extreme temperatures are reached at the equinoxes. At the poles, seasonal amplitudes of about 10 K in the upper stratosphere and 6 K at the 0.6 bar level are predicted, and the variation with time of the emission to space is found to be at most 20 percent. The atmosphere of Uranus appears to be characterized by very long radiative response times (mainly due to its cold temperature) which inhibit the large seasonal variations that one could otherwise expect in view of the high obliquity of the planet and its long orbital period.

Bezard, B.↗
NASA NTRS1986

The HD/H2 ratio in the atmosphere of Uranus

High resolution spectra of HD and H2 were brought together to derive the D/H ratio for Uranus. The deuterium concentration in the dominant molecular hydrogen phase is least susceptible to the effects of isotope fractionation in the planetary atmosphere, and the determination of relative abundances of HD and H2 is unambiguous due to nearness and relative weakness of the chosen spectral lines. The HD 5-0 R(0) and R(1) dipole lines and the H2 4-0 S(0), S(1), and S(2) quadrupole lines were obtained with a PEPSIOS instrument at the Palomar 5-meter telescope. The H2 spectra, which resolve the asymmetric line profiles resulting from pressure shifts in the deep stratified Uranus atmosphere, unambiguously define the line-of-sight hydrogen abundance for comparison with the HD spectra. The 5-0 band of HD was chosen to minimize interference from blended CH4 lines. However, weak interfering lines were found in the 5-0 bands from Uranus as well, and some uncertainties remain regarding the intrinsic line strengths in molecular hydrogen, complicating the analysis of the HD/H2 data. Nevertheless, it is established that the D/H ratio in the atmosphere of Uranus is smaller than the Jovian value, and is significantly smaller than recent theoretical predictions for Uranus based on estimates of isotope fractionation in the pre-planetary solar nebulae.

Trauger, J. T.↗
NASA NTRS1987

Bidirectional reflectance spectroscopy

An analytical model is developed for the opposition effects (heiligenshein) in the case of light scattering from a semi-infinite, particulate medium with particles that are large relative to the wavelength. The effect is common for natural materials, and comprises a bright surge in light diffusively reflected from a surface at near zero phase. A generalized expression is devised for the extinction coefficient of a particulate medium. Models are developed for step function and hyperbolic tangent distributions of light scattered from a stratified medium and exhibiting the opposition effect. A maximum brightness amplitude increase of 0.753 is projected for the effect. Greater values must have other causes. To illustrate the theory it is fitted to observations of the Moon, an asteroid, and a satellite of Uranus; Europa is also discussed.

Hapke, Bruce↗
NASA NTRS1987

Atmospheric surface and boundary layers of the Amazon Basin

Three phases of work were performed: design of and preparation for the Amazon Boundary Layer Experiment (ABLE 2-A); execution of the ABLE 2-A field program; and analysis of the ABLE 2-A data. Three areas of experiment design were dealt with: surface based meteorological measurements; aircraft missions; and project meteorological support. The primary goal was to obtain a good description of the structure of the atmosphere immediately above the rain forest canopy (top of canopy to a few thousand meters), to describe this region during the growing daytime phase of the boundary layer; and to examine the nighttime stratified state. A secondary objective was to examine the role that deep convective storms play in the vertical transport of heat, water vapor, and other trace gases. While significant progress was made, much of the analysis remains to be done.

Garstang, Michael↗
NASA NTRS1986

Bidirectional reflectance spectroscopy. IV - The extinction coefficient and the opposition effect

An analytical model is developed for the opposition effect (heiligenshein) in the case of light scattering from a semi-infinite, particulate medium with particles that are large relative to the wavelength. The effect is common for natural materials, and comprises a brightness surge in light diffusively reflected from a surface at near zero phase. A generalized expression is devised for the extinction coefficient of a particulate medium. Models are developed for step function and hyperbolic tangent distributions of light scattered from a stratified medium and exhibiting the opposition effect. A maximum brightness amplitude increase of 0.753 is projected for the effect. Greater values must have other causes.

Hapke, B.↗