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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Radon-222 as a test of convective transport in a general circulation model

A three-dimensional tracer model based on the Goddard Institude of Space Studies GCM is used to simulate the distribution of Rn-222 over North America to test the ability of the model to describe the transport of pollutants in the boundary layer and the exchange of mass between the boundary layer and the free troposphere. The model results are compared with surface observations from five sites in the U.S., showing that Rn-222 concentrations are primarily regulated by dry convection. The simulations show satisfactory agreement with observations although the model underpredicts observations at night and the simulated Rn-222 concentrations over the northeastern U.S. are too high in the spring and too low in the fall.

Jacob, Daniel J.↗
NASA NTRS1989

A comparison of two exceptional snowstorms during the early 20th century

Two East-Coast snowstorms of the early twentieth century, which generated massive accumulations over portions of the Atlantic coastal plain, are described. The 'Christmas Storm' of December 25-26, 1909 produced snowfalls of greater than 25 cm over most of the Northeast Corridor from Maryland to southern New England, while the 'Knickerbocker Storm' of January 27-29, 1922 produced a total of 71 cm at Washington, DC. The storms are examined by constructing surface-weather analyses based on original data; surface observations for the continental United States at 0100 and 1300 UTC are obtained from the National Climatic Data Center (NCDC), ship reports from American vessels are taken from the NCDC archives, and the 1300 UTC data is supplemented by Canadian observations and European ship reports. In spite of the vast differences in the evolutions of the storms, it is found that both produced similarly massive snowfalls, therefore, it is concluded that a wide range of synoptic conditions can give rise to major snowstorms along the East Coast.

Weiss, Alan D.↗
NASA NTRS1991

Large-scale solar and heliospheric magnetic fields

The magnetic structure of the extended solar corona varies with the changing photospheric field during the solar cycle. A simple potential model of the corona using solar surface observations from 1976 to the present shows how the large-scale coronal field evolves over more than a solar activity cycle. These predictions match well with large stable structures inferred from measurements of coronal electron density and the IMF, though dynamic changes are poorly modeled. The sun's polar field was about 25 stronger at solar minimum in 1986 than in 1976; the heliospheric current sheet was also flatter. In Cycle 21 the coronal and photospheric large-scale long-lived field patterns rotated every 26.9 days in the northern hemisphere; the southern field rotated every 28 days. Similar periods have been present in the IMF and in the occurrence of solar flares during the last several solar cycles.

Hoeksema, J. Todd↗
NASA NTRS1992

Further analysis of LDEF FRECOPA micrometeroid remnants

Experiments dedicated to the detection of interplanetary dust particles (IDP's) were exposed within the FRECOPA payload, installed on the face of the LDEF directly opposed to the velocity vector (west facing direction, location B3). We were mainly interested in the analysis of hypervelocity impact features of sizes less than or = 10 microns, found in thick Al targets devoted to the research of impact features. In the 15 craters found in the scanned area (approximately 4 sq. cm), the chemical analysis suggests an extraterrestrial origin for the impacting particles. The main elements we identified are usually refered to as chondrite elements: Na, Mg, Si, S, Ca, and Fe are found in various proportions, intrinsic Al being masked by the Al target; we notice a strong depletion in Ni, never observed in our samples. Furthermore, C and O are present in 90 percent of the cases; the C/O peak height ratio varies from 0.1 to 3. Impactor simulations by light gas gun hypervelocity impact experiments have shown that meaningful biogenic element and compound information maybe obtained from IDP residues below impacts of critical velocities, that are less than or = 4 km/sec for particles larger than 100 microns in diameter. Our results obtained for the smaller size fraction IDP's suggest that at such sizes, the critical velocity could be higher by a factor of 2 or 3, as chemical analysis of the remnants were possible in all the identified impact craters, performed on targets possibly hit at velocities greater than or = 7.5 km/s, which is the spacecraft velocity. These samples are now subjected to an imagery and analytical protocol that includes FESEM (field emission scanning electron microscopy) and LIMS (laser ionization mass spectrometry). The LIMS analyses were performed using the LIMA-ZA instrument. Results are presented, clearly indicating that such small events show crater features analogous to what is observed at larger sizes; our first analytical results, obtained for 2 events (P6 and P10) suggest that N is present in the IDP's remnants in which C and O were identified by EDX analysis. In one case (P6), enrichment in K and P is observed. Surface contamination by NaCl is evident on the FRECOPA surfaces.

Borg, J.↗
NASA NTRS1992

The impact of temperature dependent CO2 cross section measurements: A role for heterogeneous chemistry in the atmosphere of Mars?

Carbon dioxide comprises over 95 percent of the Mars atmosphere, despite continuous photolysis of CO2 by solar ultraviolet (UV) radiation. Since the direct recombination of CO and O is spinforbidden, the chemical stability of CO2 in the Martian atmosphere is thought to be the result of a HO(x)-catalyzed recombination scheme. Thus the rate of CO oxidation is sensitive to the abundance and altitude distribution of OH, H, and HO2. Most Martian atmospheric models assume that HO(x) abundances are governed purely by gas phase chemistry. However, it is well established that reactive HO(x) radical are adsorbed by a wide variety of surfaces. The authors have combined laboratory studies of H, OH, and HO2 adsorption on inorganic surfaces, observational data of aerosol distributions, and an updated photochemical model to demonstrate that adsorption on either dust or ice aerosols is capable of reducing HO(x) abundances significantly, thereby retarding the rate of CO oxidation.

Anbar, A. D.↗
NASA NTRS1991

NASA sea ice validation program for the Defense Meteorological Satellite Program special sensor microwave imager

Attention is given to the prime objective of the NASA validation program, namely, to establish quantitative relationships between the sea ice parameters derived from the special sensor microwave imager (SSM/I) using an algorithm originally developed for the Nimbus 7 SMMR. The underlying philosophy of the validation program is that confidence in the SSM/I algorithm products is achieved not so much by detailed comparison with localized surface observations as by consistency with independent spatially and temporally coincident data sets. The results of the satellite and aircraft comparisons that serve as the basis for the validation of the NASA SSMI/I sea ice algorithm are presented. High-resolution radiometer and C-band SAR imagery from the March 1988 NASA and Navy SSM/I underflights are used to verify the location of the ice edge and to validate the sea ice concentrations as determined by the SSM/I algorithm. These studies are argued to provide the most comprehensive measure to date of the accuracy of sea ice products derived from a spaceborne multichannel microwave imager.

Cavalieri, Donald J.↗
NASA NTRS1990

An evaluation of VAS-derived severe storm parameters

The Visible Infrared Spin Scan Radiometer (VISSR) Atmospheric Sounder (VAS) carried by U.S. meteorological satellites in GEO yield sounding parameters applicable to severe local storm forecasting. Discrepancies between VAS and radiosonde parameters are noted to increase with the number of computational steps required. It is found that VAS-derived parameters incorporating observed surface data yield results superior to those of parameters that do not incorporate them. VAS-derived parameters did not represent consistent improvements over 12-hour limited-area fine-mesh model numerical forecasts.

Olson, Steven R.↗
NASA NTRS1991

Monitoring global snow cover

A snow model that supports the daily, operational analysis of global snow depth and age has been developed. It provides improved spatial interpolation of surface reports by incorporating digital elevation data, and by the application of regionalized variables (kriging) through the use of a global snow depth climatology. Where surface observations are inadequate, the model applies satellite remote sensing. Techniques for extrapolation into data-void mountain areas and a procedure to compute snow melt are also contained in the model.

Armstrong, Richard↗
NASA NTRS1992

Providing relay communications support for the Mars Environmental Survey (MESUR) mission

The purpose of the Mars Environmental Survey (MESUR) mission is to put in place, over several launch opportunities, a constellation of Mars landers to make long-term surface observations of the circulation of the atmosphere and changes in climate, and to record the seismic activity of the planetary crust. Short-term objectives will also be addressed. An orbital communications infrastructure capable of providing regular high-rate data transfer to earth from the landers, which are scattered globally from pole to pole, is key to accomplishing the mission goals. A study is thereby presented of the orbit selection for the orbiter spacecraft, which will provide this support, and the relay communications operation. It is concluded that adequate communications support for the objectives of the MESUR mission can be provided by a single orbiter, provided care is taken in the selection of the size and orientation (i.e., inclination and apse line alignment) of the spacecraft orbit.

Swenson, Byron L.↗
NASA NTRS1992

LDEF atomic oxygen fluence update

The definition of LDEF atomic oxygen exposure involves theoretical prediction of fluxes, modeling of shielding and scattering effects, and comparison of predicted with observed atomic oxygen effects on LDEF experiments. Work is proceeding as follows: atomic oxygen fluxes and fluences have been recalculated using a more detailed orbit prediction program; a micro-environments program is being developed to account for the effects of experiment geometry on atomic oxygen flux; and chemical and physical measurements are being made on copper grounding straps to verify correspondence between predicted exposures and observed surface property variations. These three areas of work are reported briefly.

Bourassa, Roger J.↗
NASA NTRS1992

Comparison of column water vapor measurements using downward-looking near-infrared and infrared imaging systems and upward-looking microwave radiometers

Remote soundings of precipitable water vapor from three systems are compared with each other and with ground truth from radiosondes. Ancillary data from a mesoscale network of surface observing stations and from wind-profiling radars are also used in the analysis. The three remote-sounding techniques are: (a) a reflectance technique using spectral data collected by the Airborne Visible-Infrared Imaging Spectrometer; (b) an emission technique using Visible-Infrared Spin Scan Radiometer Atmospheric Sounder (VAS) data acquired from the NOAA's GOES; and (c) a microwave technique using data from a limited network of three ground-based dual-channel microwave radiometers. The data were taken over the Front Range of eastern Colorado on 22-23 March 1990. The generally small differences between the three types of remote-sounding measurements are consistent with the horizontal and temporal resolutions of the instruments. The microwave and optical reflectance measurements agreed to within 0.1 cm; comparisons of the microwave data with radiosondes were also either as good or explainable. The largest differences between the VAS and the microwave radiometer at Elbert were between 0.4 and 0.5 cm and appear to be due to variable terrain within the satellite footprint.

Gao, Bo-Cai↗
NASA NTRS1992

Intercomparison results for FIFE flux aircraft

Three atmospheric research aircraft were used to explore the atmospheric boundary layer during FIFE: the National Research Council of Canada Twin Otter, the National Center for Atmospheric Research (NCAR) King Air, and the University of Wyoming King Air. The aircraft were used to measure the mean and turbulent structure of the boundary layer and its variation with height, time, and space. These measurements are important to FIFE because they are being used to scale up point surface observations to landscape scales and because they can be used to relate satellite radiance measurements to boundary layer processes. Because the aircraft were used in coordinated flight patterns to investigate changes within and between intensive field campaigns, wing-to-wing intercomparisons were made so that measurements from one aircraft could be related to another. Intercomparisons were flown on 4 days in 1987 and 3 days in 1989. The eddy correlation measurements of the mixed layer fluxes of moisture and sensible heat were of particular interest to FIFE. Sensible heat fluxes agreed within 15 W/sq m and moisture fluxes agreed within 21 W/sq m. Mean wind component differences were within 1.0 m/s, air temperature within 0.3 C, and mixing ratio within 2 g/kg. Standard deviations showed similar good agreement, with mean differences generally less than 0.1 m/s for the wind components and 0.03 C for potential temperature. Intercomparisons between the NCAR King Air and the Twin Otter showed better agreement in 1989 than in 1987. Overall, the results suggest that data from the FIFE boundary layer aircraft will need little correction to account for instrument biases and spurious fluctuations.

Macpherson, J. I.↗
NASA NTRS1992

Three-dimensional simulations of atmospheric methyl chloroform - Effect of an ocean sink

A global three-dimensional chemical tracer model of the distribution and seasonal cycles of the surface concentration of CH3CCl3 is compared with surface observations from the Atmospheric Lifetime Experiment (ALE) for the years 1980-1985. Two-dimensional OH distributions calculated by a photochemical model are empirically adjusted from observed trends in the global average and the interhemispheric ratio of methyl chloroform. The effects of the recently discovered ocean sink for methyl chloroform were investigated. The model simulates the 5-year record of observations made at the five ALE sampling sites to generally within +/- 5 percent of the observed mean. The calculated average global lifetime of methyl chloroform is 5.7 +/- 0.3 years. The estimated global mean OH concentration is 6.5 +/- 0.4 x 10 exp 5/cu cm. However, the inclusion of the ocean sink does not significantly improve the simulation of the observed interhemispheric gradient of methyl chloroform. Atmospheric transport dominates the simulated CH3CCl3 seasonal cycle throughout the Northern Hemisphere but is less important in the Southern Hemisphere.

Tie, X.↗
NASA NTRS1993

SO2-rock interaction on Io - Reaction under highly oxidizing conditions

Laboratory simulations are used here to test the possibility that interactions of SO2 and silicates can produce Na-S compounds which can account for the observed surface enrichment of Na relative to Si on Io. It is concluded that there are three viable hypotheses for the nature and origin of the Na compounds on the surface of Io: (1) Na2SO4 from SO2-silicate interactions, (2) Na-sulfides from elemental S, silicate interactions, and (3) directly erupted volcanic Na. The present studies indicate the elemental Si is not required to form Na compounds in crustal igneous interactions. Thus, the presence of Na compounds provides no constraint on elemental S but igneous formation of Na-S compounds is not tied to models of Io which contain elemental S.

Johnson, M. L.↗
NASA NTRS1993

Detection of CO emission from massive molecular clouds in the inner disk of M31

Faint emission in the CO(1-0) and CO(2-1) spectral lines at mm wavelengths has been discovered from dark dust clouds in the inner disk of M31. These dust clouds are located in regions of the galaxy where there is little evidence of star-forming activity. The CO emission from these clouds is resolved, and comes from roughly the same area covered by the dust clouds. The line profile widths and cloud sizes imply cloud masses typical for giant molecular clouds (GMCs) in the Galaxy, but the observed surface brightnesses are only a few tenths of a K, and the typical CO(1-0) luminosities are a factor of about 10 smaller than those of Galactic GMCs of the same size and velocity line width. The CO(2-1)/CO(1-0) line ratios are very low, suggesting subthermal excitation, and precluding use of this ratio alone to determine the kinetic temperature. However, there are indications that the kinetic temperatures are significantly lower than the values of 10-30 K typical for Galactic GMCs, and may be close to the 2.7 K cosmic background.

Allen, Ronald J.↗
NASA NTRS1994

The evaluation of ASOS for the Kennedy Space Center's Shuttle Landing Facility

This report documents the Applied Meteorology Unit's (AMU) evaluation of the effectiveness and utility of the Automated Surface Observing System (ASOS) in terms of spaceflight operations and user requirements. In particular, the evaluation determines which of the Shuttle Landing Facility (SLF) observation requirements can be satisfied by ASOS. This report also includes a summary of ASOS' background, current configuration and specifications, system performance, and the possible concepts of operations for use of ASOS at the SLF. This evaluation stems from a desire by the Air Force to determine if ASOS units could be used to reduce the cost of SLF meteorological observations.

Yersavich, Ann↗
NASA NTRS1994

Volatiles, rheology, and mantle convection: Comparing Earth, Venus, and Mars

Silicate rheology is controlled in part by volatile content. The variation of viscosity with position in the mantle will influence the nature of mantle convection; hence, modeling mantle convection and its effect on surface observables such as the geoid places constraints on the viscosity structure of a planet's mantle and may indirectly constrain the volatile distribution. Models of viscous mantle flow and the Earth's geoid indicate that there is roughly a two order of magnitude variation in viscosity between the upper and lower mantles, although there is some disagreement over the depth of the viscosity minimum in the upper mantle. Some studies of post-glacial rebound also support such a viscosity contrast between the upper and lower mantles. On Venus, several highland regions appear to be supported by mantle plumes. Modeling of the geoid and topography of these regions indicates that if these features are plume-related, then the mantle of Venus can not have an Earth-like low viscosity zone in its upper mantle. On Mars, the Tharsis volcanic province has alternatively been explained as supported either by mantle convection or by flexure of a thick lithosphere. If the convective model is correct, then the large geoid anomaly requires that Mars can not have a low viscosity zone in its upper mantle.

Kiefer, Walter S.↗
NASA NTRS1993

Analysis of rapidly developing fog at the Kennedy Space Center

Space Shuttle landings at Kennedy Space Center (KSC) are of special concern to NASA's landing community because of Florida's rapidly changing weather conditions. Since a large number of Shuttle landing attempts occur in the morning hours (just after sunrise) fog and stratus development are a problem. The deorbit burn decision for a landing at KSC is typically made 90 minutes before Shuttle touchdown. In that 90 minutes weather conditions can change very rapidly. Fog to the west of KSC an advect in and reduce visibility to less than 7 miles. The most important difference between Shuttle and normal aircraft landings is that the Shuttle has no go-around capability requiring a forecast with little room for error. To help guard against rapidly changing weather conditions, flight rules have been developed as guidelines for all landings. This paper concerns fog development that would affect less than 7-statue mile visibility rule which is in effect for End-Of-Mission (EOM) Shuttle landings at KSC (Rule 4-64(A)). Data used for this analysis included hourly surface observations at the X68 Shuttle Landing Facility (SLF) and upper-air observations form the CCAFS (Cape Canaveral Air Force Station-72794) rawinsonde site for the five year period, 1986 to 1990. This investigation focused on rapidly developing fog or stratus that developed between decision time and landing.

Wheeler, Mark M.↗