SEARCH · Search NASA

Results for “MULTIPLE SCATTERING”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 883 records · Page 49

All sources NASA NTRS DOE OSTI

NASA NTRS1992

Inversion of snow parameters by neural network with iterative inversion

The inversion of snow parameters from passive microwave remote sensing measurements is performed with a neural network trained with a dense media multiple scattering model. A constrained iterative inversion scheme is used. Inversion of four parameters is performed from five brightness temperatures. The four parameters are: mean grain size of ice particles in snow, snow density, snow temperature, and snow depth. The five brightness temperatures are that of 19-GHz vertical polarization, 19-GHz horizontal polarization, 22-GHz vertical polarization, 37-GHz vertical polarization, and 37-GHz horizontal polarization. Based on the neural network constrained iterative inversion algorithm, synthetic mapping of the terrain is performed. The retrieval of synthetic mapping has been achieved. The incorporation of ground truth information is considered.

Chen, Zhengxiao↗

NASA NTRS1992

Effects of temperature-dependent molecular absorption coefficients on the thermal infrared remote sensing of the earth surface

The effect of temperature-dependent molecular absorption coefficients on thermal infrared spectral signatures measured from satellite sensors is investigated by comparing results from the atmospheric transmission and radiance codes LOWTRAN and MODTRAN and the accurate multiple scattering radiative transfer model ATRAD for different atmospheric profiles. The sensors considered include the operational NOAA AVHRR and two research instruments planned for NASA's Earth Observing System (EOS): MODIS-N (Moderate Resolution Imaging Spectrometer-Nadir-Mode) and ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer). The difference in band transmittance is as large as 6 percent for some thermal bands within atmospheric windows and more than 30 percent near the edges of these atmospheric windows. The effect of temperature-dependent molecular absorption coefficients on satellite measurements of sea-surface temperature can exceed 0.6 K. Quantitative comparison and factor analysis indicate that more accurate measurements of molecular absorption coefficients and better radiative transfer simulation methods are needed to achieve SST accuracy of 0.3 K, as required for global numerical models of climate, and to develop land-surface temperature algorithms at the 1-K accuracy level.

Wan, Zhengming↗

NASA NTRS1993

Results from a study of scintillation behavior at 12, 20, and 30 GHz using the results from the Virginia Tech Olympus receivers

Tropospheric scintillations are rapid fluctuations of signal caused by multiple scattering from the small scale turbulent refractive index inhomogeneities in the troposphere. They can strongly impair satellite communications links operating at frequency above 10 GHz. The VA Tech OLYMPUS propagation experiment which includes 12, 20, and 30 GHz beacon receivers at an elevation angle of 14 degrees provides us with valuable multifrequency scintillation data. A long term analysis of tropospheric scintillation results from the VA Tech OLYMPUS experiment is presented. It includes statistics of both the scintillation intensity and the attenuation relative to clear air as well as seasonal, diurnal and meteorological trends. A comparison with the Consultative Committee for International Radio (CCIR) predictive model for scintillation fading is presented.

Pratt, Timothy↗

NASA NTRS1993

Vertical structure models of the 1990 equatorial disturbance on Saturn

In September 1990, an atmospheric disturbance in the form of an abnormally high albedo area developed in the equatorial region of Saturn. Events of this nature are exceedingly rare for this planet as they have been detected in the equatorial region on only two other occasions in over a century. In ongoing monitoring of the atmospheres of the outer planets, CCD imaging observations of Saturn by New Mexico State University's Tortugas Mountain Station were made before, during, and after the disturbance's formation through both broad-band filters and narrow-band visible/near-IR filters centered in methane absorption bands. Also, multispectral Hubble Space Telescope observations were made within weeks of the event and later in 1991. These observations were calibrated and scans of reflectivity at constant latitude are being modeled with a vertically inhomogeneous, multiple scattering model previously used to model Jupiter's South Equatorial Belt brightening event in 1989. In addition, the reflectivity of the disturbance as a function of the scattering angles is being obtained so as to model this feature's vertical structure in particular. A preliminary report of the modeling results will be presented.

Kuehn, D. M.↗

NASA NTRS1993

The effect of dissipative inhomogeneous medium on the statistics of the wave intensity

One of the main theoretical points in the theory of wave propagation in random medium is the derivation of closed form equations to describe the statistics of the propagating waves. In particular, in one dimensional problems, the closed form representation of the multiple scattering effects is important since it contributes in understanding such problems like wave localization, backscattering enhancement, and intensity fluctuations. In this the propagation of plane waves in a layer of one-dimensional dissipative random medium is considered. The medium is modeled by a complex permittivity whose real part is a constant representing the absorption. The one dimensional problem is mathematically equivalent to the analysis of a transmission line with randomly perturbed distributed parameters and a single mode lossy waveguide and the results can be used to study the propagation of radio waves through atmosphere and the remote sensing of geophysical media. It is assumed the scattering medium consists of an ensemble of one-dimensional point scatterers randomly positioned in a layer of thickness L with diffuse boundaries. A Poisson impulse process with density lambda is used to model the position of scatterers in the medium. By employing the Markov properties of this process an exact closed form equation of Kolmogorov-Feller type was obtained for the probability density of the reflection coefficient. This equation was solved by combining two limiting cases: (1) when the density of scatterers is small; and (2) when the medium is weakly dissipative. A two variable perturbation method for small lambda was used to obtain solutions valid for thick layers. These solutions are then asymptotically evaluated for small dissipation. To show the effect of dissipation, the mean and fluctuations of the reflected power are obtained. The results were compared with a lossy homogeneous medium and with a lossless inhomogeneous medium and the regions where the effect of absorption is not essential were discussed.

Saatchi, Sasan S.↗

NASA NTRS1993

Momentum loss in proton-nucleus and nucleus-nucleus collisions

An optical model description, based on multiple scattering theory, of longitudinal momentum loss in proton-nucleus and nucleus-nucleus collisions is presented. The crucial role of the imaginary component of the nucleon-nucleon transition matrix in accounting for longitudinal momentum transfer is demonstrated. Results obtained with this model are compared with Intranuclear Cascade (INC) calculations, as well as with predictions from Vlasov-Uehling-Uhlenbeck (VUU) and quantum molecular dynamics (QMD) simulations. Comparisons are also made with experimental data where available. These indicate that the present model is adequate to account for longitudinal momentum transfer in both proton-nucleus and nucleus-nucleus collisions over a wide range of energies.

Khan, Ferdous↗

NASA NTRS1993

Investigation of passive atmospheric sounding using millimeter and submillimeter wavelength channels

Presented in this study are the results of controlled partially polarimetric measurements of thermal emission at 91.65 GHz from a striated water surface as corroborated by a geometrical optics radiative model. The measurements were obtained outdoors using a precision polarimetric radiometer which directly measured the first three modified Stokes' parameters. Significant variations in these parameters as a function of azimuthal water wave angle were found, with peak-to-peak variations in T(sub u) of up to approximately 10 K. The measurements are well corroborated by the GO model over a range of observations angles from near nadir up to approximately 65 degrees from nadir. The model incorporates both multiple scattering and a realistic downwelling background brightness field.

Gasiewski, Albin J.↗

NASA NTRS1993

The NASA/GISS Mars general circulation model: Preliminary experiments

The NASA/GISS Mars General Circulation Model (GCM) is an adapted version of the GISS Global Climate/Middle Atmosphere Model, specifically developed for the diagnostic validation and objective analysis of measured atmospheric temperatures from the Mars Observer Pressure Modulator Infrared Radiometer (PMIRR) experiment. The GISS Mars GCM has 23 vertical layers extending from the surface to approximately 80 km altitude, representing a vertical resolution of about 0.3 scale heights. The primitive (vertically hydrostatic) equations are solved in finite difference form on the Krakawa B grid, with a horizontal resolution of 8 deg x 10 deg (latitude-longitude). The model includes a diurnal solar cycle, heat transport within a two-layer ground, and a high-order 'slopes-scheme' for the advection of heat in the upper atmosphere. The radiative transfer scheme is based on the correlated k distribution method for the treatment of nongray gaseous absorption thermal emission, and multiple scattering, including options for suspended dust. A special feature of the model of particular importance for Mars is a parameterization of gravity-wave-induced drag incorporating orographic forcing, wind shear, convection, and radiative damping. The implementation of the GISS Mars model includes global maps of topography, roughness, and albedo.

Allison, Michael↗

NASA NTRS1994

The updated statistical inversion technique to the evaluation of Umkehr observations

In the present study the standard retrieval Umkehr method to estimate the vertical distribution of ozone was updated using a statistical approach to the mathematical inversion scheme. The vertical ozone profile covariance matrix was used as a priori information for the inverse problem. A new method of the ozonesonde data organization according to air mass types helped to improve the covariance matrix quality. A retrieval method was developed using eigenvector technique. An optimal vertical ozone profile resolution was determined from the mathematical inversion scheme analysis based on the same technique. The sun radiation transfer was accounted for multiple scattering and atmospheric sphericity in this calculation. The retrievals using actual Umkehr Dobson spectrophotometer observations were also performed to provide the comparison of the standard and updated methods with concurrent ozone sound data at Boulder U.S. The comparison has revealed that the present method has some advantages in both resolution and accuracy, as compared to the standard one, especially for the atmospheric layers below ozone maximum.

Frolov, Alexander D.↗

NASA NTRS1994

Comparison of the seasonal and interannual variability of phytoplankton pigment concentrations in the Peru and California Current systems

Monthly composite images from the global coastal zone color scanner (CZCS) data set are used to provide an initial illustration and comparison of seasonal and interannual variability of phytoplankton pigment concentration along the western coasts of South and North America in the Peru Current system (PCS) and California Current system (CCS). The analysis utilizes the entire time series of available data (November 1978 to June 1986) to form a mean annual cycle and an index of interannual variability for a series of both latitudinal and cross-shelf regions within each current system. Within 100 km of the coast, the strongest seasonal cycles in the CCS are in two regions, one between 34 deg and 45 deg N and the second between 24 deg and 29 deg N, each with maximum concentrations (greater than 3.0 mg m(exp-3)) in May-June. Weaker seasonal variability is present north of 45 deg N and in the Southern California Bight region (32 deg N). Within the PCS, in the same 100-km-wide coastal region, highest (greater than 45 deg S) and lowest (less than 20 deg S) latitude regions have a similar seasonal cycle with maximum concentrations (greater than 1.5 mg m(exp -3)) during the austral spring, summer, and fall, matching that evident throughout the CCS. Between these regions, off northern and central Chile, the seasonal maximum occurs during July-August (austral winter), contrary to the influence of upwelling favorable winds. Within the CCS, the dominant feature of interannual variability in the 8-year time series is a strong negative concentration anomaly in 1983, an El Nino year. The relative value of this negative anomaly is strongest off central California and is followed by an even stronger negative anomaly is strongest off central California and is followed by an even stronger negative anomaly in 1984 off Baja, California. In the PCS, strong negative anomalies during the 1982-1983 El Nino period are evident only off the Peruvian coast and are evident there only in the regions 100 km or more from the coast. Although negative anomalies associated with the El Nino were not present at higher latitudes (more than approximately 20 deg S) in the PCS, the extremely sparse sampling weakens our confidence in the results of the interannual analysis in this region. An upper estimate of the systematic winter bias remaining in the global CZCS data after reprocessing with the multiple scattering algorithm is given in the appendix.

Thomas, A. C.↗

NASA NTRS1994

Airborne CO2 coherent lidar measurements of cloud backscatter and opacity over the ocean surface

The use of an airborne CO2 lidar to obtain cloud backscatter and extinction data at a thermal infrared wavelength is described. The extinction in this spectral region is proportional to the cloud liquid water content. The use of coherent detection results in high sensitivity and narrow field of view, the latter property greatly reducing multiple-scattering effects. Backscatter measurements in absolute units are obtained through a hard target calibration methodology. For clouds of low to moderate optical thickness at the lidar wavelength, both geometric thickness and optical thickness can be measured. The sea surface reflectance signal is used to obtain estimates of the cloud optical thickness. The utility of this technique results from studies that indicate that the spatial scale of variability of the sea surface reflectance is generally large compared with that of cloud optical thickness. Selected results are presented from data taken during flights over the Pacific Ocean.

Menzies, Robert T.↗

NASA NTRS1994

Belt-zone variations in the Jovian cloud structure

Voyager Infrared Interferometer Spectrometer (IRIS) observations of Jupiter in the far infrared (180-1200/cm) are sensitive to emission originating from pressures less than 2 bars, while the 5-micrometer (1800-2300/cm) observations are primarily sensitive to emission originating from pressures greater than 2 bars. We use these differences in the location of the peak emission level to constrain the properties of the upper tropospheric cloud structure from the far-infrared observations and then use the 5-micrometer observations, with the upper tropospheric cloud structure fixed, to constrain the deep cloud structure. The relationship between observed 45- and 5-micrometer brightness temperatures reveals three distinct regions between +/- 25 deg latitude: North Equatorial Belt hot spots, which are 'hot' at both 45 and 5 micrometers; Equatorial Zone spectra, which are 'warm' at 45 micrometers and 'cold' at 5 micrometers; and North Tropical Zone spectra, which are 'cold' at both 45 and 5 micrometers. We find that the hot extreme spectral ensemble is unique to belts, and the cold extreme spectral ensembles are unique to zones, but that all other intermediate spectral ensembles are common to both regions. Analyses of these spectra using an anisotropic multiple scattering radiative transfer model reveal that the primary difference between belts and zones is the increased opacity and vertical extent of clouds in zones relative to their belt counterparts. In addition, we find a shift in the location of the para hydrogen gradient toward lower pressures in zones, with an increase in the 'equilibrated' cloud-top para fraction. We suggest that all of the variations in gas abundance profiles, temperature, and cloud structure are consistent with the effects of a simple mean circulation and large-scale wave motions.

Carlson, Barbara E.↗

NASA NTRS1994

Diffusion model for lightning radiative transfer

A one-speed Boltzmann transport theory, with diffusion approximations, is applied to study the radiative transfer properties of lightning in optically thick thunderclouds. Near-infrared (lambda = 0.7774 micrometers) photons associated with a prominent oxygen emission triplet in the lightning spectrum are considered. Transient and spatially complex lightning radiation sources are placed inside a rectangular parallelepiped thundercloud geometry and the effects of multiple scattering are studied. The cloud is assumed to be composed of a homogeneous collection of identical spherical water droplets, each droplet a nearly conservative, anisotropic scatterer. Conceptually, we treat the thundercloud like a nuclear reactor, with photons replaced by neutrons, and utilize standard one-speed neutron diffusion techniques common in nuclear reactor analyses. Valid analytic results for the intensity distribution (expanded in spherical harmonics) are obtained for regions sufficiently far from sources. Model estimates of the arrival-time delay and pulse width broadening of lightning signals radiated from within the cloud are determined and the results are in good agreement with both experimental data and previous Monte Carlo estimates. Additional model studies of this kind will be used to study the general information content of cloud top lightning radiation signatures.

Koshak, William J.↗

NASA NTRS1995

Systematic effects of randomness in radiative transfer

In this paper, the authors show how the variability of the water content in individual clouds, the complexity of individual cloud structure, and the lateral and vertical heterogeneity of the distribution of individual clouds can produce systematic effects in the inversion of intensity distributions and the inference of source functions and the vertical temperature profile. This is possibly very significant, even in simple applications of radiative transfer theory where multiple scattering is not very important, in light of the randomness in the water vapor content and geometry associated with the microphysics of clouds. A practical procedure is provided to quantify this effect and to obtain, in certain circumstances, an improved estimate of the vertical temperature profile.

Newman, William I.↗

NASA NTRS1995

Discovery and imaging of a Galactic cirrus cloud with the far ultraviolet space telescope

We present new far-ultraviolet (1400-1800 A) data concerning a Galactic cirrus cloud G251.2+73.3 near the north Galactic pole obtained with the space-borne imaging telescope FAUST (Far Ultraviolet Space Telescope). We obtain a good correlation between the far-ultraviolet (FUV) and IRAS 100 micrometers surface brightnesses, their relation being I(sub FUV) = (128 +/- 3) I(sub 100 micrometers) - (264 +/- 9), where the I(sub FUV) flux is given in units of photon/s/sq cm/A/sr and I(sub 100 micrometers) in MJy/sr. Using uvbyH-beta photometry, we get a distance of 120 pc and a visual extinction in the center of the cloud of 0.39 mag corresponding to an extinction of 1.0 mag at 1565 A. We have performed a multiple scattering calculation for the scattered light using the Monte Carlo method. These calculations provide restrictions on the FUV scattering properties of the interstellar dust.

Haikala, Lauri K.↗

NASA NTRS1996

Light ion components of the galactic cosmic rays: Nuclear interactions and transport theory

Light nuclei are present in the primary galactic cosmic rays (GCR) and are produced in thick targets due to projectile or target fragmentation from both nucleon and heavy induced reactions. In the primary GCR, He-4 is the most abundant nucleus after H-1. However, there are also a substantial fluxes of H-2 and He-3. In this paper we describe theoretical models based on quantum multiple scattering theory for the description of light ion nuclear interactions. The energy dependence of the light ion fragmentation cross section is considered with comparisons of inclusive yields and secondary momentum distributions to experiments described. We also analyze the importance of a fast component of lights ions from proton and neutron induced target fragementation. These theoretical models have been incorporated into the cosmic ray transport code HZETRN and will be used to analyze the role of shielding materials in modulating the production and the energy spectrum of light ions.

Cucinotta, F. A.↗

NASA NTRS1995

The correlated k-distribution technique as applied to the AVHRR channels

Correlated k-distributions have been created to account for the molecular absorption found in the spectral ranges of the five Advanced Very High Resolution Radiometer (AVHRR) satellite channels. The production of the k-distributions was based upon an exponential-sum fitting of transmissions (ESFT) technique which was applied to reference line-by-line absorptance calculations. To account for the overlap of spectral features from different molecular species, the present routines made use of the multiplication transmissivity property which allows for considerable flexibility, especially when altering relative mixing ratios of the various molecular species. To determine the accuracy of the correlated k-distribution technique as compared to the line-by-line procedure, atmospheric flux and heating rate calculations were run for a wide variety of atmospheric conditions. For the atmospheric conditions taken into consideration, the correlated k-distribution technique has yielded results within about 0.5% for both the cases where the satellite spectral response functions were applied and where they were not. The correlated k-distribution's principal advantages is that it can be incorporated directly into multiple scattering routines that consider scattering as well as absorption by clouds and aerosol particles.

Kratz, David P.↗

NASA NTRS1995

GHRS observations of cool, low-gravity star. 2: Flow and turbulent velocities in the outer atmosphere of gamma CRU CIS (M3.4 III)

The Goddard High Resoulution Spectrograph (GHRS) on the Hubble Space Telescope (HST) has been used to obtain medium (R = 20,000) and high (R = 85,000) resoultion UV spectra of chromosphere emission features for the M3.4 III star gamma Cru. Small Science Aperture (SSA) G270M and Echelle-B spectra of selected regions in the 2300-2850 A range were obtained to determine the kinematics of the chromosphere using lines of C2), Fe2, Co2, Si1/2), Ni2, Mn2, and Mg2. Profiles of C2) (UV 0.01) lines and fluorescently excited lines of low optical depth indicate average turbulent velocities (Doppler FWHM) of 30.2 +/- 1.3 and 28.8 +/- 1.3 km/s, respectively. The fluorescent emission lines (mean RV = 21.3 +/- 0.9 km/s) and the wings of the emission components of Fe2 lines (mean RV = 22.8 +/- 0.4 km/s) are approximately at rest relative to the radial velocity of the star (21 km/s), while the C2) lines show a modest inflow (mean RV = 23.1 +/- 0.9 km/s). The more opaque lines of Fe2 and Mg2 exhibit complex profiles resulting from line formation in an optically thick, extended expanding atmosphere. The emission wings of these lines are broadened by multiple scattering, and they are centered near the photospheric radial velocity. Closer to line center, these strong lines show a strong blueshifted self-absorption feature (already seen in IUE data), indicative of formation in an expanding chromosphere, and a previously unseen dip in the profiles on the red side of line center. The absorption components, when extracted using simple Gaussian fits, show strong correlations with the relative optical depths of the lines. The derived absorption flow velocities converge to the photospheric velocity as one examines spectra features formed deeper in the atmosphere. The blueward abosrption velocity increases in magnitude from about 7 to 14 km/s with increasing line optical depth - the strong absorptions directly map the acceleration of the outflowing stellar wind, while the interpretation of the weaker redshifted abosrptions is more ambiguous, indicating either an inflow of material or formation in an extended, spherically expanding outflow. The Mg2 and Fe2 profiles, taken together, imply that the wind speed decreases between the atmospheric layers where the Mg2 and Fe2 self-absorption components are formed. Interstellar absorptions are seen in the resonance lines of Mg2 (UV 1) and Fe2 (UV1) with zero-volt lower levels, at about -3 km/s, consistent with models of the interstellar medium in the direction of gamma Cru. Finally, we have detected the Mg2 'satellite lines' seen in solar spectra obtained above the limb. In gamma Cru these lines are probably fluorescently excited by H Ly beta.

Carpenter, Kenneth G.↗