Showing papers by "Goddard Space Flight Center published in 1991"

Extension of the MSIS Thermosphere Model into the middle and lower atmosphere

Abstract: The MSIS-86 empirical model has been revised in the lower thermosphere and extended into the mesosphere and lower atmosphere to provide a single analytic model for calculating temperature and density profiles representative of the climatological average for various geophysical conditions. Tabulations from the Handbook for MAP 16 are the primary guide for the lower atmosphere and are supplemented by historical rocket and incoherent scatter data in the upper mesosphere and lower thermosphere. Low-order spherical harmonics and Fourier series are used to describe the major variations throughout the atmosphere including latitude, annual, semiannual, and simplified local time and longitude variations. While month to month details cannot be completely represented, lower atmosphere temperature data are fit to an overall standard deviation of 3 K and pressure to 2%. Comparison with rocket and other data indicates that the model represents current knowledge of the climatological average reasonably well, although there is some conflict as to details near the mesopause.

Expansion and Contraction of the Sahara Desert from 1980 to 1990

Abstract: Data from polar-orbiting meteorological satellites have been used to determine the extent of the Sahara Desert and to document its interannual variation from 1980 to 1990. The Sahara Desert ranged from 8,633,000 square kilometers in 1980 to 9,982,000 square kilometers in 1984. The greatest annual north-south latitudinal movement of the southern Saharan boundary was 110 kilometers from 1984 to 1985 and resulted in a decrease in desert area of 724,000 square kilometers.

The plasma physics of shock acceleration

Abstract: The notion that plasma shocks in astrophysical settings can and do accelerate charged particles to high energies is not a new one However, in recent years considerable progress has been achieved in understanding the role particle acceleration plays both in astrophysics and in the shock process itself In this paper we briefly review the history and theory of shock acceleration, paying particular attention to theories of parallel shocks which include the backreaction of accelerated particles on the shock structure We discuss in detail the work that computer simulations, both plasma and Monte Carlo, are playing in revealing how thermal ions interact with shocks and how particle acceleration appears to be an inevitable and necessary part of the basic plasma physics that governs collisionless shocks We briefly describe some of the outstanding problems that still confront theorists and observers in this field

The least-squares mixing models to generate fraction images derived from remote sensing multispectral data

Abstract: Constrained-least-squares (CLS) and weighted-least-squares (WLS) mixing models for generating fraction images derived from remote sensing multispectral data are presented. An experiment considering three components within the pixels-eucalyptus, soil (understory), and shade-was performed. The generated fraction images for shade (shade image) derived from these two methods were compared by considering the performance and computer time. The derived shade images are related to the observed variation in forest structure, i.e. the fraction of inferred shade in the pixel is related to different eucalyptus ages. >

Revised global model of thermosphere winds using satellite and ground‐based observations

Abstract: Thermospheric wind data obtained from the Atmosphere Explorer E and Dynamics Explorer 2 satellites have been combined with wind data for the lower and upper thermosphere from ground-based incoherent scatter radar and Fabry-Perot optical interferometers to generate a revision (HWM90) of the HWM87 empirical model and extend its applicability to 100 km. Comparison of the various data sets with the aid of the model shows in general remarkable agreement, particularly at mid and low latitudes. The ground-based data allow modeling of seasonal/diurnal variations, which are most distinct at mid latitudes. While solar activity variations are now included, they are found to be small and not always very clearly delineated by the current data. They are most obvious at the higher latitudes. The model describes the transition from predominately diurnal variations in the upper thermosphere to semidiurnal variations in the lower thermosphere and a transition from summer to winter flow above 140 km to winter to summer flow below. Significant altitude gradients in the wind are found to extend to 300 km at some local times and pose complications for interpretation of Fabry-Perot observations.

Total ozone trends deduced from Nimbus 7 TOMS data

Abstract: The Total Ozone Mapping Spectrometer (TOMS) on the Nimbus 7 satellite has been measuring the total column amount of ozone over the globe for more than 11 years. Recent improvements in the data analysis have led to a technique for determining and removing drift in the calibration such that the data at the end of the record are precise to + or - 1.3 percent (2-sigma) relative to the data at the beginning of the record. A statistical model, including terms for seasonal variation, linear trend, quasi-biennial oscillation, solar cycle and second-order autoregressive noise has been fit to the TOMS time series of total ozone data. The linear trend obtained when this statistical model is fit to the TOMS data averaged between 65 N and 65 S latitudes is -0.26 + or - 0.14 percent/year or -3 percent over the 11.6 year time period from November 1978 to May 1990. The trend is near zero (0.0002 + or - 0.2 percent/year) at the equator and increases toward both poles.

Determination of the Optical Thickness and Effective Particle Radius of Clouds from Reflected Solar Radiation Measurements. Part II: Marine Stratocumulus Observations

Abstract: A multispectral scanning radiometer has been used to obtain measurements of the reflection function of marine stratocumulus clouds at 0.75, 1.65 and 2.16 pm. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP [International Satellite Cloud Climatology Project] Regional Experiment (FIRE), conducted off the coast of southern California during July 1987. Multispectral images of the reflection function were used to derived the optical thickness and effective particle radius of stratiform cloud layers on four days. In addition to the radiation measurements, in situ microphysical measurements were obtained from the University of Washington Convair C- 13 I A aircraft. In this paper we compare remote sensing results with in situ observations, which show a good spatial correlation for both optical thickness and effective radius. These comparisons further show systematic differences between remote sensing and in situ values, with a tendency for remote sensing to overestimate the effective radius by -2-3 pm, independent of particle radius. The optical thickness, in contrast, is somewhat overestimated for small optical thicknesses and underestimated for large optical thicknesses. An introduction of enhanced gaseous absorption at a wavelength of 2.16 pm successfully explains some of these observed discrepancies. Marginal probability density functions of optical thickness, liquid water path and effective radius have been derived from our remote sensing results. The joint probability density function of liquid water path and effective radius shows that the effective radius increases as the liquid water path increases for optically thin clouds, in contrast to optically thick clouds for which the effective radius decreases with increasing liquid water path.

Conditions for accretion-induced collapse of white dwarfs

Abstract: Recent discovery of an unexpectedly large number of low-mass binary pulsars (LMBPs) in globular clusters has instigated active discussions on the evolutionary origin of binary pulsars. Prompted by the possibility that at least some of LMBPs originate from accretion-induced collapse (AIC) of white dwarfs, a reexamination is conducted as to whether or not AIC occurs for the new models of O + Ne + Mg white dwarfs and solid C + O white dwarfs that can ignite explosive nuclear burning at significantly lower central densities than in the previous models. Even with low critical densities, AIC is still much more likely than explosion for both types of white dwarfs. Possible regions for AIC are presented in a diagram of mass accretion rate vs initial mass of the white dwarfs.

High-energy neutrinos from active galactic nuclei.

Abstract: The spectrum and high-energy neutrino background flux from photomeson production in active galactic nuclei (AGN) is calculated using the recent UV and X-ray observations to define the photon fields and an accretion-disk shock-acceleration model for producing high-energy particles. Collectively, AGN produce the dominant isotropic neutrino background between 10,000 and 10 to the 10th GeV, detectable with current instruments. AGN neutrinos should produce a sphere of stellar disruption which may explain the 'broad-line region' seen in AGN.

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. I, an 8 month campaign of monitoring NGC 5548 with IUE

Abstract: This is an electronic version of an article published in The Astrophysical Journal. Clavel, J. et al. Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. I. An 8 month campaign of monitoring NGC 5548 with IUE. The Astrophysical Journal 366 (1991): 64-81

Light limitation of phytoplankton biomass and macronutrient utilization in the Southern Ocean

Abstract: The Antarctic Circumpolar Current (ACC) is unique in that it has continually high concentrations of major plant nutrients but low phytoplankton biomass. This enigmatic phenomenon is the focus of significant speculation that trace nutrients, including Fe, may limit phytoplankton crop size. Global climatologies indicate that the ACC is a region with low surface temperatures, weak density stratification, little summertime surface solar irradiance, and strong wind stress. These physical phenomena act to limit growth rates of the phytoplankton community. Using a photo-physiological description of phytoplankton growth in a simple one-dimensional ecosystem model forced by observations or climatologies of mixing depth and surface irradiance, the authors make an evaluation of the potential for massive, nutrient-exhausting, phytoplankton blooms forming in the ACC. The ACC has persistent mixed layers in excess of 50 m. Literature values and model optimization indicate that the minimal aggregate specific loss rate and typical physical conditions of stratification and surface irradiance, the model predicts that phytoplankton in the ACC would not utilize >10% of the available macronutrients. Without a mechanism for increasing the strength of stratification, the authors predict that massive Fe additions to the Southern Ocean would fail to significantly mitigate the atmospheric CO{sub 2} derived from fossilmore » fuel.« less

Preliminary spectral observations of the Galaxy with a 7 deg beam by the Cosmic Background Explorer (COBE)

Abstract: The FIR absolute spectrophotometer (FIRAS) on the Cosmic Background Explorer (COBE) has carried out the first all-sky spectral line survey in the FIR region, as well as mapping spectra of the Galactic dust distribution at below 100 microns Lines of forbidden C I, C II, and N II, as well as of CO are all clearly detected The mean line intensities are interpreted in terms of the heating and cooling of the multiple phases of the interstellar gas In addition, an average spectrum of the galaxy is constructed and searched for weak lines The spectrum of the galaxy observed by FIRAS has two major components: a continuous spectrum due to interstellar dust heated by starlight, and a line spectrum dominated by the strong 158-micron line from singly ionized carbon, with a spatial distribution similar to the dust distribution, and a luminosity of 03 percent of the dust luminosity There are in addition moderately strong 122- and 2053-micron lines, identified as coming from singly-ionized nitrogen Maps of the emission by dust and forbidden C II and N II are presented

The AE-8 trapped electron model environment

Abstract: The machine sensible version of the AE-8 electron model environment was completed in December 1983. It has been sent to users on the model environment distribution list and is made available to new users by the National Space Science Data Center (NSSDC). AE-8 is the last in a series of terrestrial trapped radiation models that includes eight proton and eight electron versions. With the exception of AE-8, all these models were documented in formal reports as well as being available in a machine sensible form. The purpose of this report is to complete the documentation, finally, for AE-8 so that users can understand its construction and see the comparison of the model with the new data used, as well as with the AE-4 model.

Ultraviolet variability of NGC 5548 : dynamics of the continuum production region and geometry of the broad-line region

Abstract: Data from the 1989-1990 IUE monitoring of the Seyfert galaxy NGC 5548 are used here to analyze the continuum variability properties of the galaxy and to derive the structure or its emission-line region. The mean shape of the UV continuum is well fit by an accretion disk model with a given black hole mass and an additional component required to reproduce the observed soft X-ray flux. The continuum fluctuation power spectrum is very steep, with most of the variance coming from about 1 yr time scales. The entire optical/UV continuum rises and falls almost simultaneously, so that the logarithmic slope of the power spectrum is nearly the same for all bands, but the flux at higher photon frequencies varies with larger amplitude. The emission-line material around the nucleus may best be described by a highly ionized inner zone of high and nearly constant pressure that stretches about 4-14 light-days from the center and an outer, more weakly ionized zone of considerably lower ionization at least 20-30 light-days out.

Retrieval of Monthly Rainfall Indices from Microwave Radiometric Measurements Using Probability Distribution Functions

Abstract: An algorithm for the estimation of monthly rain totals for 5° cells over the oceans from histograms of SSM/I brightness temperatures has been developed. Them are three novel features to this algorithm. First, it uses knowledge of the form of the rainfall intensity probability density function to augment the measurements. Second, a linear combination of the 19.35 and 22.235 GHz channels has been employed to reduce the impact of variability of water vapor. Third, an objective technique has been developed to estimate the rain layer thickness from the 19.35- and 22.235-GHz brightness temperature histograms. Comparison with climatologies and the GATE radar observations suggest that the estimates are reasonable in spite of not having a beam-filling correction. By-products of the retrievals indicate that the SSM/I instrument noise level and calibration stability am quite good.

The effect of snow cover on the climate

Abstract: Large-scale snow cover anomalies are thought to cause significant changes in the diabatic heating of the earth's surface in such a way as to produce substantial local cooling in the surface temperatures. This theory was tested using the GISS 3-D GCM (General Circulation Model). The results of the GCM experiment showed that snow cover caused only a short term local decrease in the surface temperature. In the surface energy budget, reduction in absorbed shortwave radiation and the increased latent heat sink of melting snow contributed to lower temperatures. However, all the remaining heating terms contribute to increasing the net heating over a snow covered surface. The results emphasize the negative feedback which limits the impact of snow cover anomalies over longer time scales.

Comparison of Ice-Phase Microphysical Parameterization Schemes Using Numerical Simulations of Tropical Convection

Abstract: The performance of several ice parameterizations has been evaluated through a numerical cloud model. Ice effects using different schemes are contrasted with each other and with an ice-free control by incorporating them into the cloud model and by applying them to simulations of tropical squall systems. The latter are simulated in 2D so that a large domain can be used to incorporate a complete anvil. Nonsquall-type convective lines are simulated in 3D owing to their smaller horizontal scale. It is concluded that inclusion of ice microphysics in the cloud model enhanced the agreement of the simulated convection with some features of observed convection, including the proportion of surface rainfall in the anvil region and the intensity and structure of the radar brightband near the melting level in the anvil. In the experiments with bulk microphysics, three ice categories produced much better results than two ice categories, which in turn was better than no ice. For the tropical squall-type and nonsquall-type systems the optimal mix was ice, snow, and graupel.

Intermittent turbulence in the solar wind

Abstract: This paper demonstrates the existence of intermittent turbulence in the solar wind at 8.5 AU. The pth-order velocity structure functions show scaling behavior in the range of periods from 0.85 hour to 13.6 hours for p of less than 20. The exponent of the scaling law s(p) is a quadratic function of p. These observations of s(p) for compressible MHD turbulence on a scale of the order of about 1 AU are consistent with laboratory measurements of s(p) for gasdynamic turbulence on a scale of the order of 1 m, indicating the universal character of intermittent turbulence. The observations are not described by the 'constant beta' model of intermittent turbulence. They are marginally consistent with the lognormal model. The observations are consistent with a random beta model prediction which assumes that the turbulence is a mixture of sheets and space-filling eddies.

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. II - An intensive study of NGC 5548 at optical wavelengths

Abstract: A large, international program of ground-based optical spectroscopy and photometry of the variable Seyfert 1 galaxy NGC 5548 undertaken in support of an IUE monitoring campaign is described. This contribution presents the data base and describes the methods used to correct for systematic differences between spectra from different sources. Optical continuum and H-beta emission-line light curves are derived from the spectra. The behavior of the optical continuum is qualitatively the same as the behavior of the ultraviolet continuum. Cross-correlation of the ultraviolet and optical continuum measurements does not reveal any significant lag between them. The h-beta emission-line variations show the same basic pattern as seen in the continuum and ultraviolet emission lines, with H-beta lagging behind the continuum by about 20 days. This is significantly larger than the about 10 day lag deduced for Ly-alpha.

The ionospheric signatures of rapid subauroral ion drifts

Abstract: Subauroral ion drifts (SAID) are latitudinally narrow regions of rapid westward ion drift located in the evening sector and centered on the equatorward edge of the diffuse aurora Observations of SAID, as identified by the ion drift meters on the Atmosphere Explorer C and Dynamics Explorer B spacecraft, are utilized to determine their effect on the F region ion composition, their relationship to the midlatitude trough, and their temporal evolution At altitudes near the F peak, a deep ionization trough is formed in regions of large ion drift where the O(+) concentration is considerably depleted and the NO(+) concentration is enhanced, while at higher altitudes the trough signature is considerably mitigated or even absent SAID have been observed to last longer than 30 min but less than 3 hours, and their latitudinal width often becomes narrower as time progresses The plasma flows westward equatorward of the SAID and becomes more westward as invariant latitude increases Poleward of the SAID, the flow is, on average, westward throughout the auroral zone in the evening, while near midnight it becomes eastward

The seasonal circulation of the upper ocean in the Bay of Bengal

Abstract: The present analysis of results generated by a multilayer, adiabatic, monthly mean wind-driven numerical model of the upper Indian Ocean indicates that its simulated northeastern currents are in general agreement with observations and interpretations Attention is given to the large anticyclonic flow during Northern Hemisphere winter The model's layer-thickness reveals the propagation of Kelvin waves along the coast, travelling the entire perimeter of the Andaman Sea and Bay of Bengal; this wave excites westward-propagating Rossby waves into the bay's interior A time-series analysis of transport calculations yields peaks in the 20-30 day and 50-60 day ranges which are unlikely to have been directly forced by the applied stress

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

Abstract: 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.

Satellite-Image-Derived Velocity Field of an Antarctic Ice Stream

Abstract: The surface velocity of a rapidly moving ice stream has been determined to high accuracy and spatial density with the use of sequential satellite imagery. Variations of ice velocity are spatially related to surface undulations, and transverse velocity variations of up to 30 percent occur. Such large variations negate the concept of plug flow and call into question earlier mass-balance calculations for this and other ice streams where sparse velocity data were used. The coregistration of images with the use of the topographic undulations of the ice stream and the measurement of feature displacement with cross-correlation of image windows provide significant improvements in the use of satellite imagery for ice-flow determination.

Cosmic rays from primordial black holes

Abstract: The quark and gluon emission from primordial black holes (PBHs) which may have formed from initial density perturbations or phase transitions in the early universe are investigated If the PBHs formed from scale-invariant initial density perturbations in the radiation dominated era, it is found that the emission can explain or contribute significantly to the extragalactic photon and interstellar cosmic-ray electron, positron, and antiproton spectra around 01-1 GeV In particular, the PBH emission strongly resembles the cosmic-ray gamma-ray spectrum between 50 and 170 MeV The upper limits on the PBH density today from the gamma-ray, e(+), e(-), and antiproton data are comparable, provided that the PBHs cluster to the same degree as the other matter in the Galactic halo

Aircraft active and passive microwave validation of sea ice concentration from the Defense Meteorological Satellite Program special sensor microwave imager

Abstract: Results are presented of a series of coordinate special sensor microwave imager (SSM/I) underflights that were carried out during March 1988 with NASA and Navy aircraft over portions of the Bering, Beaufort, and Chukchi seas. NASA DC-8 AMMR data from Bering Sea ice edge crossings were used to verify that the ice edge location, defined as the position of the initial ice bands encountered by the aircraft, corresponds to an SSM/I ice concentration of 15 percent. Direct comparison of SSM/I and aircraft ice concentrations for regions having at least 80 percent aircraft coverage reveals that the SSM/I total ice concentration is lower on average by 2.4 +/-2.4 percent. For multiyear ice, NASA and Navy flights across the Beaufort and Chukchi seas show that the SSM/I algorithm correctly maps the large-scale distribution of multiyear ice: the zone of first-year ice off the Alaskan coast, the large areas of mixed first-year and multiyear ice, and the region of predominantly multiyear ice north of the Canadian archipelago.

The Radiative Effects of Clouds and their Impact on Climate

Abstract: Our knowledge of the direct role of clouds in long-term climate change is examined in an overview of key results published over the last 15 or 20 years, along with some relevant unpublished model studies. The focus is on 1) the impact of clouds on the incoming and outgoing radiation at the top of the atmosphere, and 2) the two-way interaction of clouds with other variables of the climate system—i.e., the cloud/climate feedback problem—as revealed by climate model simulations. A common framework is established for comparing results from different investigations. The total effect of clouds on radiative fluxes at the top of the atmosphere—specifically, the difference in flux between average conditions and cloud-free conditions, often called cloud forcing–has been derived from earth radiation budget measurements by several investigators using various data sources and methods. There is general agreement that the annual global mean effect of clouds is to cool the climate system, but there is significant disagre...