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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.

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At least 73 records · Page 4
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NASA NTRS1993

Three dimensional modeling of cirrus during the 1991 FIRE IFO 2: Detailed process study

A three-dimensional model of cirrus cloud formation and evolution, including microphysical, dynamical, and radiative processes, was used to simulate cirrus observed in the FIRE Phase 2 Cirrus field program (13 Nov. - 7 Dec. 1991). Sulfate aerosols, solution drops, ice crystals, and water vapor are all treated as interactive elements in the model. Ice crystal size distributions are fully resolved based on calculations of homogeneous freezing of solution drops, growth by water vapor deposition, evaporation, aggregation, and vertical transport. Visible and infrared radiative fluxes, and radiative heating rates are calculated using the two-stream algorithm described by Toon et al. Wind velocities, diffusion coefficients, and temperatures were taken from the MAPS analyses and the MM4 mesoscale model simulations. Within the model, moisture is transported and converted to liquid or vapor by the microphysical processes. The simulated cloud bulk and microphysical properties are shown in detail for the Nov. 26 and Dec. 5 case studies. Comparisons with lidar, radar, and in situ data are used to determine how well the simulations reproduced the observed cirrus. The roles played by various processes in the model are described in detail. The potential modes of nucleation are evaluated, and the importance of small-scale variations in temperature and humidity are discussed. The importance of competing ice crystal growth mechanisms (water vapor deposition and aggregation) are evaluated based on model simulations. Finally, the importance of ice crystal shape for crystal growth and vertical transport of ice are discussed.

Jensen, Eric J.↗
NASA NTRS1989

Photochemistry of the atmosphere of Uranus

A detailed one-dimensional photochemical model incorporating vertical transport by eddy and molecular diffusion is used to study the importance of chemical and physical processes that control the hydrocarbon abundances in the stratosphere of Uranus. The study includes updated photochemical reaction rates and photolysis branching ratios; explicit calculation of the condensation rate of condensing species; and appropriate boundary conditions at the tropopause. It is found that the basic photochemical process generally produces low net conversion efficiencies of methane to higher hydrocarbons. However, even with these low efficiencies, the total condensation rate, which represents the net rate of conversion of methane to hydrocarbons, is roughly 2 x 10 to the -16th g/cm sec, in agreement with the upper range of the low condensation rate for the production of the stratospheric aerosol layer as determined by Pollack et al. (1987). Furthermore, a range of vertical transport rates is required to separately match the observational constraints on hydrocarbon abundances. The solar reflection data refer to the sunlit polar region of Uranus and require an eddy diffusion coefficient of K approx. = 50 sq cm/sec for globally averaged insolation or K approx = 1500 (Nh/N) exp 1/2 for overhead sun. The Voyager 2 Utraviolet Spectrometer solar occultation data refer to the equatorial regions, thus implying a large latitudinal gradient in stratospheric hydrocarbon abundance.

Summers, Michael E.↗
NASA NTRS2003

Vertical Electron Transport through PbS-EuS Structures

Temperature dependence of current-voltage I-V characteristics and resistivity is studied in ferromagnetic PbS-EuS semiconductor tunnel structures grown on n-PbS (100) substrates. For the structures with a single (2-4 nm thick) ferromagnetic EuS electron barrier we observe strongly non-linear I-V characteristics with an effective tunneling barrier height of 0.3-0.7 eV. The experimentally observed non-monotonic temperature dependence of the (normal to the plane of the structure) electrical resistance of these structures is discussed in terms of the electron tunneling mechanism taking into account the temperature dependent shift of the band offsets at the EuS-PbS heterointerface as well as the exchange splitting of the electronic states at the bottom of the conduction band of EuS.

Wrotek, S.↗
NASA NTRS1996

Chemical Characteristics of Continental Outflow Over the Tropical South Atlantic Ocean from Brazil and Africa

The chemical characteristics of air parcels over the tropical South Atlantic during September - October 1992 are summarized by analysis of aged marine and continental outflow classifications. Positive correlations between CO and CH3CL and minimal enhancements of C2CL40, and various ChloroFluoroCarbon (CFC) species in air parcels recently advected over the South Atlantic basin strongly suggest an impact on tropospheric chemistry from biomass burning on adjacent continental areas of Brazil and Africa. Comparison of the composition of aged Pacific air with aged marine air over the South Atlantic basin from 0.3 to 12.5 km altitude indicates potential accumulation of long-lived species during the local dry season. This may amount to enhancements of up to two-fold for C2H6, 30% for CO, and 10% for CH3Cl. Nitric oxide and NO(x) were significantly enhanced (up to approx. 1 part per billion by volume (ppbv)) above 10 km altitude and poorly correlated with CO and CH3Cl. In addition, median mixing ratios of NO and NO(x) were essentially identical in aged marine and continental outflow air masses. It appears that in addition to biomass burning, lightning or recycled reactive nitrogen may be an important source of NO(x) to the upper troposphere. Methane exhibited a monotonic increase with altitude from approx. 1690 to 1720 ppbv in both aged marine and continental outflow air masses. The largest mixing ratios in the upper troposphere were often anticorrelated with CO, CH3Cl, and CO2, suggesting CH, contributions from natural sources. We also argue, based on CH4/CO ratios and relationships with various hydrocarbon and CFC species, that inputs from biomass burning and the northern hemisphere are unlikely to be the dominant sources of CO, CH4 and C2H6 in aged marine air. Emissions from urban areas would seem to be necessary to account for the distribution of at least CH4 and C2H6. Over the African and South American continents an efficient mechanism of convective vertical transport coupled with large-scale circulations conveys biomass burning, urban, and natural emissions to the upper troposphere over the South Atlantic basin. Slow subsidence over the eastern South Atlantic basin may play an important role in establishing and maintaining the rather uniform vertical distribution of long-lived species over this region. The common occurrence of values greater than 1 for the ratio CH3OOH/H2O2 in the upper troposphere suggests that precipitation scavenging effectively removed highly water soluble gases (H2O2, HNO3, HCOOH, and CH3COOH) and aerosols during vertical convective transport over the continents. However, horizontal injection of biomass burning products over the South Atlantic, particularly water soluble species and aerosol particles, was frequent below 6 km altitude.

Talbot, R. W.↗
NASA NTRSDate not supplied

Cloud Condensation Nuclei in the Early Martian Atmosphere

Explanations for how early Mars supported warmer and wetter conditions 3.5 to 4 billion years ago have long been debated. A water cloud greenhouse has been proposed as a mechanism of warming early Mars, but modeling studies of these environments produce mixed results. Studies show that both H 2 O and CO 2 clouds are capable of influencing atmospheric and surface temperatures, but the amount of warming induced is very sensitive to the cloud physics assumptions made. Producing a strong water cloud greenhouse effect is dependent on forming clouds as high in the atmosphere as possible to maximize warming. However, the majority of studies of H 2 O and CO 2 clouds on early Mars assume that a constant number of seed nuclei are available at all altitudes for clouds to condense onto. Here we test the validity of this assumption in massive CO 2 atmospheres. Explaining how dust is lofted into the middle atmosphere to serve as seed nuclei for cloud formation is a challenge even for present day Mars. This problem may change in an early Mars environment where having a more massive atmosphere could result in differences in the general circulation, vertical transport, and settling time scales of dust. Here, we begin a first-ever study of the early Mars dust cycle and how it might couple to the water cycle. We present early Mars simulations in which dust is injected at the surface and can be transported throughout the atmosphere. We explore the resulting vertical and horizontal distributions of dust particles and their sizes with an emphasis on the maximum height of the dust. We aim to investigate what the distribution of cloud condensation nuclei may have been on early Mars and how this might impact cloud formation.

Kathryn Steakley↗
NASA NTRS1981

Numerical modeling of diffusive-convective physical vapor transport in cylindrical vertical ampoules

Diffusive-convective physical vapor transport (PVT) in cylindrical, vertical ampoules of aspect ratio (length/radius) between 0.5 and 10 was modeled numerically. The transport of a crystal forming component through an inert component that undergoes zero net transport was considered. Systems were treated in which: (1) with unequal molecular weight of the components and with temperature gradients typically employed in PVT, convective flow arises dominantly from solutal density gradients; and (2) with equal molecular weight of the components, convective flow can arise only from thermal expansion. It was found that, due to the diffusion-induced horizontal density gradients, buoyancy-driven convective flows are superimposed on the diffusive-advective fluxes without threshold. Net recirculation sets in adjacent to the growing interface, in contrast to the corresponding monocomponent situation where marginally stable convective modes fill the whole fluid space. Depending on the orientation of the main transport direction with respect to gravity, convection can either reduce or enhance the diffusion-induced radial concentration gradients. Significant enhancement of the net transport rate was found to occur only when the whole vapor space between source and growing crystal is filled by a convective recirculation roll. Solutal and thermal convection results are similar; yet for quantitative discussions, thermal and solutal Rayleigh numbers are not interchangeable in contrast to convective situations that lack net mass transport across the fluid space.

Markham, B. L.↗
NASA NTRS1992

Flexural analysis of uplifted rift flanks on Venus

Knowledge of the thermal structure of a planet is vital to a thorough understanding of its general scheme of tectonics. Since no direct measurements of heat flow or thermal gradient are available for Venus, most estimates have been derived from theoretical considerations or by analog with the Earth. The flexural response of the lithosphere to applied loads is sensitive to regional thermal structure. Under the assumption that the yield strength as a function of depth can be specified, the temperature gradient can be inferred from the effective elastic plate thickness. Previous estimates of the effective elastic plate thickness of Venus range from 11-18 km for the foredeep north of Uorsar Rupes to 30-60 km for the annular troughs around several coronae. Thermal gradients inferred for these regions are 14-23 K km(exp -1) and 4-9 K km(exp -1) respectively. In this study, we apply the same techniques to investigate the uplifted flanks of an extensional rift. Hypotheses for the origin of uplifted rift flanks on Earth include lateral transport of heat from the center of the rift, vertical transport of heat by small-scale convection, differential thinning of the lithosphere, dynamical uplift, and isostatic response to mechanical uploading of the lithosphere. The 1st hypothesis is considered the dominant contributor to terrestrial rift flanks lacking evidence for volcanic activity, particularly for rift structures that are no longer active. In this study, we model the uplifted flanks of a venusian rift as the flexural response to a vertical end load.

Evans, Susan A.↗
NASA NTRS1993

Transport and residence times of tropospheric aerosols inferred from a global three-dimensional simulation of Pb-210

A global three-dimensional model is used to investigate the transport and tropospheric residence time of Pb-210, an aerosol tracer produced in the atmosphere by radioactive decay of Rn-222 emitted from soils. The model uses meteorological input with 4 deg x 5 deg horizontal resolution and 4-hour temporal resolution from the Goddard Institute for Space Studies general circulation model (GCM). It computes aerosol scavenging by convective precipitation as part of the wet convective mass transport operator in order to capture the coupling between vertical transport and rainout. Scavenging in convective precipitation accounts for 74% of the global Pb-210 sink in the model; scavenging in large-scale precipitation accounts for 12%, and scavenging in dry deposition accounts for 14%. The model captures 63% of the variance of yearly mean Pb-210 concentrations measured at 85 sites around the world with negligible mean bias, lending support to the computation of aerosol scavenging. There are, however, a number of regional and seasonal discrepancies that reflect in part anomalies in GCM precipitation. Computed residence times with respect to deposition for Pb-210 aerosol in the tropospheric column are about 5 days at southern midlatitudes and 10-15 days in the tropics; values at northern midlatitudes vary from about 5 days in winter to 10 days in summer. The residence time of Pb-210 produced in the lowest 0.5 km of atmosphere is on average four times shorter than that of Pb-210 produced in the upper atmosphere. Both model and observations indicate a weaker decrease of Pb-210 concentrations between the continental mixed layer and the free troposphere than is observed for total aerosol concentrations; an explanation is that Rn-222 is transported to high altitudes in wet convective updrafts, while aerosols and soluble precursors of aerosols are scavenged by precipitation in the updrafts. Thus Pb-210 is not simply a tracer of aerosols produced in the continental boundary layer, but also of aerosols derived from insoluble precursors emitted from the surface of continents. One may draw an analogy between Pb-210 and nitrate, whose precursor NO(sub x) is sparingly soluble, and explain in this manner the strong correlation observed between nitrate and Pb-210 concentrations over the oceans.

Balkanski, Yves J.↗
NASA NTRS1994

Microphysical modeling of cirrus. 1: Comparison with 1986 FIRE IFO measurements

We have used a one-dimensional model of cirrus formation to study the development of cirrus clouds during the 1986 First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment (FIRE) intensive field observations (IFO). The cirrus model includes microphysical, dynamical, and radiative processes. Sulfate aerosols, solution drops, ice crystals, and water vapor are all treated as interactive elements in the model. Ice crystal size distributions are fully resolved based on calculations of homogeneous freezing nucleation, growth by water vapor deposition, evaporation, coagulation, and vertical transport. We have focused on the cirrus observed on November 1, 1986. Vertical wind speed for the one-dimensional simulation is taken from a mesoscale model simulation for the appropriate time period. The mesoscale model simulation suggested that strong upward motions over Wyoming and subsequent horizontal transport of upper level moisture were responsible for the cirrus observed over Wisconsin on this date. We assumed that our one-dimensional model could be used to represent a vertical column moving from Wyoming to Wisconsin over a period of several hours. Ice crystal nucleation occurs in our model in the 8 to 10-km region as a result of the strong updrafts (and cooling) early in the simulation. Growth, coagulation, and sedimentation of these ice crystals result in a broad cloud region (5-10 km thick) with an optical depth of 1-2 after a few hours, in agreement with the FIRE measurements. Comparison with aircraft microphysical measurements made over Wisconsin indicates that the simulation generated reasonable ice water content, but the predicted ice number densities are too low, especially for radii less than about 50 microns. Sensitivity tests suggest that better agreement between simulated and observed microphysical properties is achieved if the nucleation rate is higher or stronger vertical mixing (perhaps associated with multidimensional motions) is present.

Jensen, Eric J.↗
NASA NTRS1973

Atomic oxygen transport in the thermosphere.

The photodissociation of oxygen in the lower thermosphere is evaluated to obtain its global average value and the hemispheric imbalance. The observed concentrations of atomic oxygen do not reflect this imbalance in production due to the effect of seasonal wind patterns redistributing the atomic oxygen. The wind system necessary to compensate for the imbalance in solar thermal input into the lower thermosphere is found to transport an amount of atomic oxygen sufficient to compensate for the hemispheric imbalance in production. Ionospheric data indicate a winter enhancement in atomic oxygen concentration; to produce this, a higher degree of oxygen dissociation than that normally accepted (i.e., higher than an atomic to molecular oxygen ratio of unity at 120 km) is needed. The concept that the concentrations of atomic oxygen observed over the winter polar region are maintained by transport from lower latitudes requires that eddy diffusion coefficients derived from vertical transport at low latitudes (ignoring horizontal transport) be reduced by about 25%.

Johnson, F. S.↗
NASA NTRS1979

A coupled radiative-convective-photochemical model of the stratosphere

A one-dimensional atmospheric model is described. The neutral photochemical processes of the troposphere and stratosphere are presented by a reaction set of 87 kinetic processes and 28 photolytic reactions, with vertical transport being taken into account by an eddy diffusion parameter. The vertical temperature profiles are coupled with the chemical calculations by means of a radiative-convective code. The model has the capability of including effects of multiple scattering and diurnal variations in species concentrations. The numerical solution methodology is described in detail, and current input data are given along with results obtained in a sample calculation.

Boughner, R. E.↗
NASA NTRS1982

The photochemistry of the paleoatmosphere

Recent progress in the understanding of the chemistry and photochemistry of the paleoatmosphere is reviewed with emphasis on the application of photochemical models to the investigation of the evolution of the atmosphere. Photochemical calculations are presented which show that a primordial highly reducing atmosphere composed of methane and ammonia, if it formed at all, would be short-lived in the presence of solar ultraviolet radiation, giving way rapidly to a more mildly reducing atmosphere of carbon dioxide and nitrogen. Estimations of O2 produced from the photolysis of water vapor prior to the emergence of photosynthesis range from less than 10 to the -14th to 0.1 times the present atmospheric level, indicating the need for further research. A series of photochemical models of increasing complexity has been developed to study the evolution of atmospheric ozone taking into account reactions with O atoms, hydrogen oxides, nitrogen oxides, and chlorine as well as vertical transport, temperature and tropospheric chemistry so that the total content and vertical distribution of O3 may be determined for a specified level of paleoatmospheric O2.

Levine, J. S.↗
NASA NTRS2000

A Composite View of Ozone Evolution in the 1995-1996 Northern Winter Polar Vortex Developed from Airborne Lidar and Satellite Observations

The processes which contribute to the ozone evolution in the high latitude northern lower stratosphere are evaluated using a three dimensional model simulation and ozone observations. The model uses winds and temperatures from the Goddard Earth Observing System Data Assimilation System. The simulation results are compared with ozone observations from three platforms: the differential absorption lidar (DIAL) which was flown on the NASA DC-8 as part of the Vortex Ozone Transport Experiment; the Microwave Limb Sounder (MLS); the Polar Ozone and Aerosol Measurement (POAM II) solar occultation instrument. Time series for the different data sets are consistent with each other, and diverge from model time series during December and January. The model ozone in December and January is shown to be much less sensitive to the model photochemistry than to the model vertical transport, which depends on the model vertical motion as well as the model vertical gradient. We evaluate the dependence of model ozone evolution on the model ozone gradient by comparing simulations with different initial conditions for ozone. The modeled ozone throughout December and January most closely resembles observed ozone when the vertical profiles between 12 and 20 km within the polar vortex closely match December DIAL observations. We make a quantitative estimate of the uncertainty in the vertical advection using diabatic trajectory calculations. The net transport uncertainty is significant, and should be accounted for when comparing observations with model ozone. The observed and modeled ozone time series during December and January are consistent when these transport uncertainties are taken into account.

Douglass, A. R.↗
NASA NTRS2000

A Composite View of Ozone Evolution in the 1995-96 Northern Winter Polar Vortex Developed from Airborne Lidar and Satellite Observations

The processes which contribute to the ozone evolution in the high latitude lower stratosphere are evaluated using a three dimensional model simulation and ozone observations. The model uses winds and temperatures from the Goddard Earth Observing System Data Assimilation System. The simulation results are compared with ozone observations from three platforms: the differential absorption lidar (DIAL) which was flown on the NASA DC-8 as part of the Vortex Ozone Transport Experiment; the Microwave Limb Sounder (MLS) on the Upper Atmosphere Research Satellite; and the Polar Ozone and Aerosol Measurement (POAM II) solar occulation instrument, on board the French Satellite Pour I'Observations de la Terre. Comparisons of the different data sets with the model simulation are shown to provide complementary information and a consistent view of the ozone evolution. The model ozone in December and January is shown to be sensitive to the ozone vertical gradient and the model vertical transport, and only weakly sensitive to the model photochemistry. The most consistent comparison between observed and modeled ozone evolution is found for a simulation where the vertical profiles between 12 and 20 km within the polar vortex closely match December DIAL observations. Diabatic trajectory calculations are used to estimate the uncertainty due to vertical advection quantitatively. The transport uncertainty is significant, and should be accounted for when comparing observations with model ozone. The model ozone evolution during December and January is broadly consistent with the observations when these transport uncertainties are taken into account.

Douglass, Anne R.↗