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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Correlation of interannual length-of-day variation with El Nino/Southern Oscillation, 1972-1986

Correlation between interannual length-of-day (LOD) changes and an index representation of El Nino/Southern Oscillation (ENSO) for the period of 1972-1986 was examined in two ways: as the cross-correlation function in the time domain and as the complex coherence spectrum in the frequency domain. The ENSO representation was derived from barometric measurements from the Tahiti and Darwin, Australia, meteorological stations, as the difference of the sea-level pressure between the two locations. The cross correlation between this ENSO representation and LOD variation on the interannual time scale was found to have a maximum value of 0.68 and the LOD phase lag of 2 months, indicating that most of the interannual LOD variation is caused by ENSO, and that the transfer of ENSO's axial angular momentum to the solid earth lags behind the Tahiti-Darwin pressure variation by about 2 months. The corresponding coherence spectrum showed minimum correlation around biennial periods, indicating an influence of the stratospheric quasi-biennial oscillation on LOD variations.

Chao, B. Fong↗
NASA NTRS1988

Large time scale variation in hydrogen emission from Jupiter and Saturn

The IUE and Voyager spacecraft observations of Jupiter and Saturn were combined to obtain a consistent measurement of temporal variation of the equatorial subsolar hydrogen emission. The outer planets appear to have rather independent behavior over time scales of the order of 10 yr, particularly in emission from the H Ly alpha line. The time interval from 1978 to the present shows variation of mean equatorial H Ly alpha brightness of 2 at Jupiter and 5 at Saturn. The relative magnitudes of the variations is sufficiently different to suggest that response to input from the Sun is at least nonlinear. The brightness of H2 band emission appears to be relatively more stable than H Ly alpha. There is evidence in IUE observations of a moderate increase in H2 band brightness with increasing time at Jupiter, in opposition to the variation in H Ly alpha.

Shemansky, D. E.↗
NASA NTRS1988

Earth orbital variations and vertebrate bioevolution

Cause of the Pleistocene-Holocene transition mammalian extinctions at the end of the last age is the subject of debate between those advocating human predation and climate change. Identification of an ambient air temperature (AAT)-uterine blood flow (UBF) coupling phenomenon supports climate change as a factor in the extinctions, and couples the extinctions to earth orbital variations that drive ice age climatology. The AAT-UBF phenomenon couples mammalian bioevolution directly to climate change via effects of environmental heat upon blood flow to the female uterus and damage to developing embryos. Extinctions were in progress during climatic warming before the Younger Dryas event, and after, at times when the AAT-UBF couple would have been operative; however, impact of a sudden short-term cooling on mammals in the process of adapting to smaller size and relatively larger S/V would have been severe. Variations in earth's orbit, and orbital forcing of atmospheric CO2 concentrations, were causes of the succession of Pleistocene ice ages. Coincidence of mammalian extinctions with terminations of the more intense cold stages links mammalian bioevolution to variations in earth's orbit. Earth orbital variations are a driving source of vertebrate bioevolution.

Mclean, Dewey M.↗
NASA NTRS1989

On the interpretation of seasonal variations of stratospheric ozone

The causes of the annual and semiannual ozone oscillations were investigated. Using the middle-atmosphere model of Garcia and Solomon (1983), the monthly variations of the ozone mixing ratio were computed and Fourier-analyzed, with excellent agreement found between the computed values and those measured with the SBUV instrument. It was found that, at high latitudes and low altitudes, the modeled ozone abundances increased in the winter due to transport and decreased in the summer due to chemical destruction. In the middle stratosphere, the calculated annual ozone variation was found to be largely due to the annual variation in the odd-oxygen production rate, while in the upper stratosphere, the annual ozone variation was found to be caused by the large annual oscillation in temperature.

Perliski, Lori M.↗
NASA NTRS1990

Far-infrared intensity variations caused by 5 minute oscillations

Observations of solar IR intensity variations at 50, 100, and 200 microns were made simultaneously and cospatially with Doppler measurements in the sodium D1 line at 5896 A. Brightness temperature variations of several K in amplitude are highly correlated with five minute Doppler oscillations. The brightness variations are attributed to work done on the chromospheric medium by compression, driven by the five minute oscillations. The Doppler oscillations lead the brightness variations by about 47 deg in phase at 50 and 100 microns and by about 72 deg in phase at 200 microns.

Lindsey, C.↗
NASA NTRS1990

Directional variations in thermal emission from geologic surfaces

The directional emission properties of geologic surfaces were investigated using a ground-based, hand-held infrared radiometer and thermistor probe. Field sites involved surfaces ranging from smooth playa and sand surfaces to a very rough aa lava flow. Large directional variations in thermal emission were found; they result from the presence of surface roughness at large scales producing spatial variations in kinetic temperature and at small scales producing emissivity variations. These variations are important in remotely determining surface structure and understanding surface energy balance and emission spectra.

Jakosky, Bruce M.↗
NASA NTRS1991

Lateral variation in upper mantle temperature and composition beneath mid-ocean ridges inferred from shear-wave propagation, geoid, and bathymetry

Resolution of both the extent and mechanism of lateral heterogeneity in the upper mantle constraints the nature and scales of mantle convection. Oceanic regions are of particular interest as they are likely to provide the closest glimpse at the patterns of temperature anomalies and convective flow in the upper mantle because of their young age and simple crustal structure relative to continental regions. Lateral variations were determined in the seismic velocity and attenuation structure of the lithosphere and astenosphere beneath the oceans, and these seismological observations were combined with the data and theory of geoid and bathymetry anomalies in order to test and improve current models for seafloor spreading and mantle convection. Variations were determined in mantle properties on a scale of about 1000 km, comparable to the thickness of the upper mantle. Seismic velocity, geoid, and bathymetry anomalies are all sensitive to variations in upper mantle density, and inversions were formulated to combine quantitatively these different data and to search for a common origin. Variations in mantle density can be either of thermal or compositional origin and are related to mantle convection or differentiation.

Sheehan, Anne Francis↗
NASA NTRS1990

Global, seasonal cloud variations from satellite radiance measurements. II - Cloud properties and radiative effects

Cloud and surface radiative properties and their effects on the earth and surface radiation budgets are obtained based on global daily visible and IR radiance measurements. The magnitude of cloud property variations and their effects on radiation increase strongly with decreasing space/time scales. Cloud properties are systematically different between land and ocean, with ocean having larger cloud cover with somewhat larger optical thicknesses and lower cloud top altitudes. Although cloud variations appear to be the primary cause of regional radiation budget variability at 5-30 daytime scales, the effects of their seasonal variations at larger spatial scales are less important than the changes associated with changes in solar declination and atmospheric/surface temperatures. The largest seasonal variations in radiation occur in the 30-60 deg latitude band in each hemisphere. Although clouds have a net cooling effect on the global, annual mean radiation balance at both the top of the atmosphere and the surface, their net effect on regional, seasonal balances is much more varied.

Rossow, William B.↗
NASA NTRS1991

Interannual variations of ozone - Interpretation of 4 years of satellite observations of total ozone

Observations show that the monthly zonal mean total ozone between 1978 and 1982 varies by as much as 5 percent with respect to the 4-year average. These variations have been analyzed with the aid of a two-dimensional chemistry transport model. Observed temperatures and meridional stream functions, calculated from data, were used as inputs for the model. Deviations of the model-calculated total ozone from the 4-year mean were compared with the observed variations. It is shown that a substantial part of the observed variability of total ozone is reproduced by the model, despite uncertainties in the heating rates and without any special attempts to obtain consistent eddy diffusion or otherwise to tune the model. Results indicate that the temperature dependence of the ozone gas phase chemistry plays a negligible role in explaining the observed interannual variations in total ozone. A large part of the variability can be explained by the year-to-year differences in the transport circulation. The variability of the diabatic circulation is shown to be largely independent of the ozone fluctuations and to be a consequence of the observed temperature variations.

Schneider, Hans R.↗
NASA NTRS1991

Time-cumulated visible and infrared radiance histograms used as descriptors of surface and cloud variations

The spatial and temporal stability of the distributions of satellite-measured visible and infrared radiances, caused by variations in clouds and surfaces, are investigated using bidimensional and monodimensional histograms and time-composite images. Similar analysis of the histograms of the original and time-composite images provides separation of the contributions of the space and time variations to the total variations. The variability of both the surfaces and clouds is found to be larger at scales much larger than the minimum resolved by satellite imagery. This study shows that the shapes of these histograms are distinctive characteristics of the different climate regimes and that particular attributes of these histograms can be related to several general, though not universal, properties of clouds and surface variations at regional and synoptic scales. There are also significant exceptions to these relationships in particular climate regimes. The characteristics of these radiance histograms provide a stable well defined descriptor of the cloud and surface properties.

Seze, Genevieve↗
NASA NTRS1991

Mean and inter-year variation of growing-season normalized difference vegetation index for the Sahel 1981-1989

Images are presented that show the mean and coefficient of variation of nine years (1981-1989) of NOAA AVHRR normalized difference vegetation index (NDVI) data for the growing season (July-October) in Africa, north of the equator. The variation in the growing season NDVI is represented by the coefficient of variation image that shows the large variation in the Sahelian growing season between years. It is concluded that these images illustrate some aspects of the perspective being brought to regional and continental scale processes by coarse resolution satellite sensors and the potential of these sensors to provide consistent, long-term datasets.

Tucker, C. J.↗
NASA NTRS1991

A Variational Formalism for the Radiative Transfer Equation and a Geostrophic, Hydrostatic Atmosphere: Prelude to Model 3

The second step in development of MODEL III is summarized. It combines the four radiative transfer equations of the first step with the equations for a geostrophic and hydrostatic atmosphere. This step is intended to bring radiance into a three dimensional balance with wind, height, and temperature. The use of the geostrophic approximation in place of the full set of primitive equations allows for an easier evaluation of how the inclusion of the radiative transfer equation increases the complexity of the variational equations. Seven different variational formulations were developed for geostrophic, hydrostatic, and radiative transfer equations. The first derivation was too complex to yield solutions that were physically meaningful. For the remaining six derivations, the variational method gave the same physical interpretation (the observed brightness temperatures could provide no meaningful input to a geostrophic, hydrostatic balance) at least through the problem solving methodology used in these studies. The variational method is presented and the Euler-Lagrange equations rederived for the geostrophic, hydrostatic, and radiative transfer equations.

Achtemeier, Gary L.↗
NASA NTRS1991

Joint inversion of shear wave travel time residuals and geoid and depth anomalies for long-wavelength variations in upper mantle temperature and composition along the Mid-Atlantic Ridge

Measurements were carried out for SS-S differential travel time residuals for nearly 500 paths crossing the northern Mid-Atlantic Ridge, assuming that the residuals are dominated by contributions from the upper mantle near the surface bounce point of the reflected phase SS. Results indicate that the SS-S travel time residuals decrease linearly with square root of age, to an age of 80-100 Ma, in general agreement with the plate cooling model. A joint inversion was formulated of travel time residuals and geoid and bathymetric anomalies for lateral variation in the upper mantle temperature and composition. The preferred inversion solutions were found to have variations in upper mantle temperature along the Mid-Atlantic Ridge of about 100 K. It was calculated that, for a constant bulk composition, such a temperature variation would produce about a 7-km variation in crustal thickness, larger than is generally observed.

Sheehan, Anne F.↗
NASA NTRS1992

Sub-daily resolution of earth rotation variations with Global Positioning System measurements

Data from a worldwide Global Positioning System (GPS) tracking experiment have been used to determine variations in earth rotation (UT1-UTC) over a time period of three weeks. Kalman filtering and smoothing enabled changes in UT1-UTC over intervals of 2 to 24 hrs to be detected with the GPS data. Internal consistency checks and comparisons with other solutions from very long baseline interferometry (VLBI) and satellite laser ranging (SLR) indicate that the GPS UT1-UTC estimates are accurate to about 2 cm. Comparison of GPS-estimated variations in UT1-UTC with 2-hr time resolution over 4 days with predicted variations computed from diurnal and semidiurnal oceanic tidal contributions strongly suggests that the observed periodic sub-daily variations of about 0.1 msec are largely of tidal origin.

Lichten, Stephen M.↗
NASA NTRS1992

Frequency variations of the earth's obliquity and the 100-kyr ice-age cycles

Changes in the earth's climate are induced by variations in the earth's orbital parameters which modulate the seasonal distribution of solar radiation. Periodicities in the geological climate record with cycles of 100, 41, and 23 kyr have been linked with changes in obliquity, eccentricity, and precession of the equinoxes. The effect of variations of eccentricity during a 100 kyr period is weak relative to the signals from obliquity and precession variations and it may therefore be expected that the 100 kyr signal in the climate record would be of low intensity. However, this signal dominates the climate record and internal nonlinear processes within the climate system have previously been proposed to account for this fact. The author shows that variations in the frequency of the obliquity cycle can give rise to strong 100-kyr forcing of climate.

Liu, Han-Shou↗
NASA NTRS1992

Magnetic susceptibility variations in Loess sequences and their relationship to astronomical forcing

The long, well-exposed and often continuous sequences of loess found throughout the world are generally thought to provide an excellent opportunity for studying long-term, large-scale environmental change during the last few million years. In recent years, the most fruitful loess studies have been those involving the deposits of the loess in China. One of the most intriguing results of that work has been the discovery of an apparent correlation between variations in the magnetic susceptibility of the loess sequence and the oxygen isotope record of the deep sea. This correlation implies that magnetic susceptibility variations are being driven by astronomical parameters. However, the basic data have been interpreted in various ways by different authors, most of whom assumed that the magnetic minerals in the loess have not been affected by post-depositional processes. Using a chemical extraction procedure that allows us to separate the contribution of secondary pedogenic magnetic minerals from primary inherited magnetic minerals, we have found that the magnetic susceptibility of the Chinese paleosols is largely due to a pedogenic component which is present to a lesser degree in the loess. We have also found that the smaller inherited component of the magnetic susceptibility is about the same in the paleosols and the loess. These results demonstrate the need for additional study of the processes that create magnetic susceptibility variations in order to interpret properly the role of astronomical forcing in producing these variations.

Verosub, Kenneth L.↗
NASA NTRS1993

Variations of the ultraviolet Fe II and Balmer continuum emission in the Seyfert galaxy NGC 5548

We present measurements of the Balmer continuum/Fe II emission blend between 2160 and 4130 A in the Seyfert galaxy NGC 5548. The measurements are from spectra obtained as part of the combined space-based and ground-based monitoring program of this object in 1988-1989. An iterative scheme is used to determine and subtract the continuum emission underlying the emission blend so as to obtain a light curve sampled once every four days. The small blue bump is an important component of the emission-line cooling, constituting about one third of the line flux in this object. Its flux varies with an amplitude of approximately +/- 20 percent about the mean, similar to the amplitude of the Balmer line variations during the same period. Its light curve resembles that of Ly-alpha, with a lag of about 10 days behind the continuum variations. The bump variation amplitude is independent of the wavelength interval where it is measured, which indicates that both the Balmer continuum and Fe II emission have comparable variation amplitudes. These results suggest that the Fe II UV multiplets and the Balmer continuum are emitted in the same parts of the broad-line region as most other broad emission lines in this object.

Maoz, D.↗
NASA NTRS1993

Radial and latitudinal variations of the magnetic field strength in the outer heliosphere

The radial, latitudinal, and temporal variations of the magnetic field in the outer heliosphere are analyzed on the basis of observations from Voyagers 1 and 2 and Pioneers 10 and 11. The radial variation of the magnetic field strength measured during 1973-1989 from 1 to 19 AU is compared with that predicted by Parker's (1963) spiral field model. Additional Voyager 2 magnetic field data from 1986 through most of 1989 are discussed. Latitudinal variations in the magnetic field strength are examined on Voyager 1 and 2 observations from 1981 to 1989. It is concluded that both the Voyager and the Pioneer observations are consistent with Parker's model for the variation of the strength of the magnetic field as a function of distance from the sun in the region from 1 to 19 AU. The topology of the spiral interplanetary magnetic field, as well as a program for analyzing more general magnetic fields and velocity fields in the heliosphere are also discussed.

Burlaga, L. F.↗