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At least 703 records · Page 39

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NASA NTRS1992

Measurement of Mean or Effective Radius of Cloud Drop Size Distributions with a 10.6-micron Wavelength Lidar

Platt and Takashima (1987) proposed and analytically evaluated a technique to observe the mode radius R sub p of drop size distributions in clouds using the backscatter detected by a CO2 lidar. The scheme depends on fortuitous relationships between the backscatter and extinction properties of common drop size distributions when probed with wavelengths between about 9 and 11 microns, which is the range of CO2 laser transitions. This study extends their work to measurement of mean and effective radius and presents experimental results that demonstrate that the technique is practical.

Eberhard, Wynn L.↗

NASA NTRS1991

The mean zonal flow response to Rossby wave and gravity wave forcing in the equatorial lower stratosphere - Relationship to the QBO

Observations show that the westerly acceleration of the equatorial quasi-biennial oscillation (QBO) can be accounted for by Kelvin waves, but that there is a deficiency in the easterly acceleration due to Rossby-gravity waves. Rossby waves and westward propagating gravity waves have been suggested as alternative sources for the easterly acceleration. The possible role of these two wave modes has been tested in a two-dimensional model of the QBO. When the easterly acceleration is due to Rossby waves, the zonal-mean response is steady; when it is due to gravity waves, an oscillation with some features similar to the QBO occurs, but it is of short period and weak amplitude. A similar result occurs when a standing-wave forcing pattern is imposed. These results suggest that Rossby waves play only a minor role in the QBO, and that while the Rossby-gravity mode is essential, other gravity modes may also be important for the easterly phase.

Takahashi, Masaaki↗

NASA NTRS1992

Development of the mean velocity distribution in rectangular jets

The mean flowfield of 1 x 2 and 1 x 4 aspect ratio rectangular jets has been measured using a laser Doppler anemometer system. The development of the downstream velocity distribution is analyzed with respect to centerline velocity decay, shear layer growth, axis switching, and velocity profile development. Comparisons are made with axisymmetric, planar, and other rectangular jets.

Morrison, G. L.↗

NASA NTRS1992

Simulation of the zonal mean climatology of the middle atmosphere with a three-dimensional spectral model for solstice and equinox conditions

A 3D spectral model was used to simulate the zonal mean state of the middle atmosphere for solstice and equinox conditions. The model incorporates realistic parameterizations of atmospheric infrared cooling and a gravity wave formulation based on a combination of Lindzen's (1981) and Matsuno's (1982) approaches. The temperature distributions for both seasons and the zonal wind distribution for solstice are found to be in satisfactory agreement with the empirical model of Fleming et al. (1988). Net vertical fluxes of horizontal momentum are in good agreement with systematic observations of gravity waves in the middle atmosphere.

Akmaev, R. A.↗

NASA NTRS1992

A global mean sea surface based upon GEOS 3 and Seasat altimeter data

A mean sea surface relative to the International Union of Geodesy 1980 Geodetic Reference System reference ellipsoid has been derived from Seasat and GEOS 3 altimeter measurements. This surface, called MSS-9012, has been computed on a grid of 1/8 deg resolution. The surface covers the global ocean between 70 deg N and 72 deg S. Comparisons are made between MSS-9012 and ocean bathymetry. The eastern extent of the Chain Fracture Zone in the Gulf of Guinea is more apparent in the altimetry than in the bathymetry data, as are many other features. The combination of data from the two satellites has successfully retrieved more information about the sea surface than was previously possible using only Seasat data.

Marsh, J. G.↗

NASA NTRS1990

Annual variation of deseasonalized mean flow acceleration in the equatorial lower stratosphere

The quasi-biennial oscillation (QBO) in the equatorial lower stratosphere appears to be influenced by the seasonal cycle, as phase transitions at 50 mb occur primarily in the northern spring/summer season (April-August). Descent of east wind regimes varies widely from one QBO cycle to another. Most of this variation occurs because easterly shears slow down or 'stall' in their descent sometime between July and February. Minimum mean flow accelerations at 50 mb occur in the northern winter season, slightly before the annual minimum in equatorial tropopause temperature. Although a weak effect of the semiannual oscillation can be detected near 10 mb, the seasonal effect over most of the QBO region is annual. The seasonal cycle apparently modulates the onset of QBO phases, and slightly enhances the ability to predict the QBO, but is of insufficient strength or consistency to exactly synchronize the quasi-biennial oscillation with the seasonal cycle.

Dunkerton, Timothy J.↗

NASA NTRS1991

Simulation and modeling of homogeneous compressible turbulence under isotropic mean compression

Compressible homogeneous turbulence subjected to isotropic mean compression is simulated numerically for high and low turbulent Mach numbers, at various compression rates. It is found that at low Mach numbers, the effects of viscosity variations on the development of the turbulent kinetic energy can be significant. A model consistent with the invariance of the Navier-Stokes equations to spherical compression, that takes into account variable viscosity effects is proposed. At high Mach numbers, the distribution of energy between the acoustic and solenoid fields is found to depend upon initial conditions. The simulation results are also used to evaluate two-equation compressible turbulence models.

Coleman, G. N.↗

NASA NTRS1990

Digital image gathering and minimum mean-square error restoration

Most digital image restoration algorithms are inherently incomplete because they are conditioned on a discrete-input, discrete-output model which only accounts for blurring during image gathering and additive noise. For those restoration applications where sampling and reconstruction are important, the restoration algorithm should be based on a more comprehensive end-to-end model which also accounts for the potentially important noiselike effects of aliasing and the low-pass filtering effects of interpolative reconstruction. It is demonstrated that although the mathematics of this more comprehensive model is more complex, the increase in complexity is not so great as to prevent a complete development and analysis of the associated minimum mean-square error (Wiener) restoration filter.

Park, Stephen K.↗

NASA NTRS1992

A lower limit of the cosmic mean density from the ages of clusters of galaxies

The presence of substructure in clusters of galaxies indicates that they have recently formed or accreted a significant fraction of their mass. The cosmic time at which this occurs is set in large part by the density of that region of space at very large redshift. If the distribution of density perturbations at high redshift is Gaussian, then the frequency of subclustering now is set by the mean density at recombination. Thus, the observed frequency of subclustering can be used to constrain. Surprisingly, plausible values of Lambda have little effect on this result. Unless three independent evaluations of the incidence of subclustering are all overestimates, or substructure lasts significantly longer than a crossing time in clusters, or perturbations at high redshift are not Gaussian, the universe is fairly dense (Omega sub 0 greater than about 0.5). The same line of argument suggests that direct observations (e.g., with Rosat) of evolution of the cluster population with shifts will provide a complementary direct measure of Omega sub 0.

Richstone, D.↗

NASA NTRS1992

Sea surface mean square slope from Ku-band backscatter data

A surface mean-square-slope parameter analysis is conducted for 14-GHz airborne radar altimeter near-nadir, quasi-specular backscatter data, which in raw form obtained by least-squares fitting of an optical scattering model to the return waveform show an approximately linear dependence over the 7-15 m/sec wind speed range. Slope data are used to draw inferences on the structure of the high-wavenumber portion of the spectrum. A directionally-integrated model height spectrum that encompasses wind speed-dependent k exp -5/2 and classical Phillips k exp -3 power laws subranges in the range of gravity waves is supported by the data.

Jackson, F. C.↗

NASA NTRS1992

Spectral synthesis in the ultraviolet. IV - A library of mean stellar groups

A library of mean UV stellar energy distributions is derived from IUE spectrophotometry of 218 stars. The spectra cover 1230-3200 A with a spectral resolution of about 6 A. They have been corrected for interstellar extinction and converted to a common flux and wavelength scale. Individual stars were combined into standard groups according to their continuum colors, observed UV spectral morphology, MK luminosity class, and metal abundance. The library consists of 56 groups: 21 dwarf(V), 8 subgiant(IV), 16 giant(III), and supergiant(I + II) groups, covering O3-M4 spectral types. A metal-poor sequence is included, containing four dwarf and two giant groups, as is a metal-enhanced sequence with a single dwarf, subgiant, and giant group. Spectral indices characterizing the continuum and several strong absorption features are examined as temperature, luminosity, and abundance diagnostics. The library is intended to serve as a basis for interpreting the composite UV spectra of a wide variety of stellar systems, e.g., elliptical galaxies, starburst systems, and high-redshift galaxies.

Fanelli, Michael N.↗

NASA NTRS1992

A plan for accurate estimation of daily area-mean rainfall during the CaPE experiment

The Convection and Precipitation/Electrification (CaPE) experiment took place in east central Florida from 8 July to 18 August, 1991. There were five research themes associated with CaPE. In broad terms they are: investigation of the evolution of the electric field in convective clouds, determination of meteorological and electrical conditions associated with lightning, development of mesoscale numerical forecasts (2-12 hr) and nowcasts (less than 2 hr) of convective initiation and remote estimation of rainfall. It is the last theme coupled with numerous raingage and streamgage measurements, satellite and aircraft remote sensing, radiosondes and other meteorological measurements in the atmospheric boundary layer that provide the basis for determining the hydrologic cycle for the CaPE experiment area. The largest component of the hydrologic cycle in this region is rainfall. An accurate determination of daily area-mean rainfall is important in correctly modeling its apportionment into runoff, infiltration and evapotranspiration. In order to achieve this goal a research plan was devised and initial analysis begun. The overall research plan is discussed with special emphasis placed on the adjustment of radar rainfall estimates to raingage rainfall.

Duchon, Claude E.↗

NASA NTRS1992

The orbital evolution of real asteroids near the 4:1 mean-motion resonance with Jupiter

Numerical integrations of the orbits of ten asteroids with osculating elements near the 4:1 mean-motion resonance with Jupiter have been performed over 200,000 years into the future. A variety of orbital evolutions was found, depending on the start values of the semi-major axis. The orbit of asteroid 1983 RJ4, which lies almost exactly at the resonance center, experiences large variations in eccentricity, evolving into an Earth-crosser on a time-scale of a few 10(exp 4) years. This makes this region a potential source for Apollo objects and meteoritic material, although the width of the resonance region in semi-major axis seems to be very narrow.

Dahlgren, Mats↗

NASA NTRS1992

Toward achieving global sole means radionavigation systems

This paper briefly reviews tentative requirements for global, earth-referenced sole means of navigation systems with emphasis on integrity and availability. These requirements can be allocated to integrated navigation system architectures based on for instance GPS, GLONASS, VOR/DME, TACAN, Omega, Chayka, and Loran-C. Fault detection and isolation techniques (FDI) for integrated radio navigation systems are presented. The FDI algorithm provides a protection radius with a specified confidence level as a function of measurement geometry and algorithm requirements. This is followed by a case study of integrated GPS/Loran-C.

Vangraas, F.↗

NASA NTRS1993

The detection of large amounts of cool, x ray absorbing gas in distant clusters of galaxies. What does this mean?

We present an x-ray spectral study of 12 distant (z = 0.17-0.54) rich clusters of galaxies observed with the Einstein Observatory Imaging Proportional Counter. These x-ray spectral data show evidence for substantial excess absorptions beyond those expected in the galaxy, indicating the presence of large amounts of x-ray absorbing cool gas in these distant clusters. The mean value of the excess absorptions corresponds to an absorbing gas column density approximately greater than 10(exp 21)/sq cm. We calculate the x-ray luminosities of the clusters with observed fluxes only in the 0.8-3.5 keV band where the fluxes are less effected by the absorptions, and use the temperature-to-luminosity correlation (known only for nearby clusters) to estimate the temperatures of the hot intracluster medium (ICM) in the distant clusters. These temperature estimates, together with the spectral fits, provide further constraints on the column densities in the individual clusters. For the cluster CL 0016+16, the lower limit on the column density is found to be 8 x 10(exp 20)/sq cm at the 99 percent confidence limit. We also show that the ratio of the temperature obtained from the spectral fit to the temperature expected from the correlation tends to decrease with increasing look-back time, indicating possible temperature evolution of the hot ICM in the recent past. The inclusion of this evolutionary effect further increases the absorptions required in fitting the spectra.

Wang, Qingde↗

NASA NTRS1992

Deviations of interstellar extinctions from the mean R-dependent extinction law

An analysis is conducted of the deviations from the mean extinction law A(lambda)/A(V) over UV wavelengths emphasizing sightlines through bright nebulosities vs dark clouds. The stellar sample is subdivided into sight lines with defined properties, and different behavior is noted for the deviations in the dark-cloud and bright-nebula sight lines. The deviations are correlated within the same wavenumber interval, but unique shapes for the FUV rise in the deviations are observed. Distributions of silicate grains are considered to account for the extinctions at certain wavenumbers that would explain the observations. The FUV rise in the extinctions is theorized to be related to the UV extinction of polycyclic aromatic hydrocarbons.

Mathis, John S.↗

NASA NTRS1992

The determination of a one year mean sea surface height track from Geosat altimeter data and ocean variability implications

Results are presented for determinations of mean sea surface along a reference Geosat altimeter tracking, using Geosat data from the first year of the exact repeat mission (ERM) and improved orbits. The results were used to study the time variability of the ocean surface, and a map was constructed of the major current structure. The data included 21 complete and one incomplete ERMs, with the sea surface heights being referenced to the single reference track through the application of geoid gradient corrections. This Geosat surface can be used as a reference surface to which other altimeter data could be incorporated; the data could then be used for the recovery of geoid undulations and gravity anomalies in the ocean areas.

Wang, Yan M.↗

NASA NTRS1992

Monitoring global monthly mean surface temperatures

The accuracy of the global surface air temperature (SST) estimates for a particular month over the past decade is assessed using all of the in situ observations available today. The sources of noise in the data, the numbers of observations, and the spatial coverage are appraised for the comparison with the climate signal, and different analyzed results are compared to determine their reproducibility. The data are further evaluated by comparing anomalies of near-global monthly mean surface temperatures with those of global satellite channel 2 microwave sounding unit temperatures for 144 months from 1979 to 1990. The results indicate that the inherent noise level in an SST observation is about 1.0 C, and this is compounded when the observation is made in regions of large temperature gradient.

Trenberth, Kevin E.↗