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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 55 records · Page 3
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

Gravity waves and convection in Colorado during July 1983

The dynamics of gravity-wave/convective-cell interaction is studied using NOAA data collected in NE Colorado during July and August 1983. The pressure fields measured with microbarographs, the tropospheric wind profiles obtained with a UHF wind profiler radar, and precipitation data collected with a 10-cm weather radar for four events (A, B, C, and D) are analyzed. The four disturbances are detected through a substantial depth of the troposphere. It is observed that in event A the wave and convective cells appear to be locked together; in event B, the wave and convective cells commence about the same time, but the wave velocities differ from the cell velocities; and in events C and D, the waves move faster than the maximum wind in the jet and faster than the convective cells. It is suggested that events A and B are generated by wind shear in the jet stream, and the excitation of events C and D depends on mechanisms such as vertical convective motion and acceleration in the jet flow.

Einaudi, F.↗
NASA NTRS2010

Seismic Constraints on Interior Solar Convection

We constrain the velocity spectral distribution of global-scale solar convective cells at depth using techniques of local helioseismology. We calibrate the sensitivity of helioseismic waves to large-scale convective cells in the interior by analyzing simulations of waves propagating through a velocity snapshot of global solar convection via methods of time-distance helioseismology. Applying identical analysis techniques to observations of the Sun, we are able to bound from above the magnitudes of solar convective cells as a function of spatial convective scale. We find that convection at a depth of r/R(solar) = 0.95 with spatial extent l < 30, where l is the spherical harmonic degree, comprise weak flow systems, on the order of 15 m/s or less. Convective features deeper than r/R(solar) = 0.95 are more difficult to image due to the rapidly decreasing sensitivity of helioseismic waves.

Hanasoge, Shravan M.↗
NASA NTRS1975

Vertical velocity structure and geometry of clear air convective elements

The paper discusses observations of individual convective elements with a high-power narrow-beam scanning radar, an FM-CW radar, and an acoustic sounder, including the determination of the vertical air velocity patterns of convective structures with the FM-CW radar and acoustic sounder. Data are presented which link the observed velocity structure and geometrical patterns to previously proposed models of boundary layer convection. It is shown that the high-power radar provides a clear three-dimensional picture of convective cells and fields over a large area with a resolution of 150 m, where the convective cells are roughly spherical. Analysis of time-height records of the FM-CW radar and acoustic sounder confirms the downdraft-entrainment mechanism of the convective cell. The Doppler return of the acoustic sounder and the insect-trail slopes on FM-CW radar records are independent but redundant methods for obtaining the vertical velocity patterns of convective structures.

Rowland, J. R.↗
DOE OSTI2020

The Boreal Summer Madden–Julian Oscillation and Moist Convective Morphology over the Maritime Continent

Abstract While the boreal summer Madden–Julian oscillation (MJO) is commonly defined as a planetary-scale disturbance, the convective elements that constitute its cloud dipole exhibit pronounced variability in their morphology. We therefore investigate the relationship between the intraseasonal cloud anomaly of the MJO and the convective elements that populate its interior by simulating a boreal summer MJO event over the Maritime Continent using a cloud-resolving model. A progressive relationship between convective cell morphology and the MJO within the convectively enhanced region of the MJO was identified and characterized as follows: anomalously long-lasting cells in the initial phases, followed by an increased number of cells in the intermediate phases, progressing into more expansive cells in the terminal phases. A progressive relationship does not seem to exist within the convectively suppressed region of the MJO within the simulated domain, however. Within the convectively enhanced region of the MJO, the progressive relationship is partially explained by the evolution of bulk atmospheric characteristics, such as instability and wind shear. Positive midlevel moisture anomalies coincide with anomalously long-lasting convective cells, which is hypothesized to further cascade into an increase in convective cell volume, although variability in the number of convective cells seems to be related to an unidentified variable. This intraseasonal relationship between convective cell morphology and the boreal summer MJO within the Maritime Continent may have broader implications for the large-scale structure and evolution of the MJO, related to both convective moistening and cloud-radiative feedbacks.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2022

A Machine‐Learning‐Assisted Stochastic Cloud Population Model as a Parameterization of Cumulus Convection

Abstract A machine‐learning‐assisted stochastic cloud population model is coupled with the Advanced Research Weather Research and Forecasting (WRF) model to represent fluctuations in the cloud‐base mass flux associated with the life cycles and interactions among cumulus convection cells. In this cloud population model, the size distribution and the associated cloud‐base mass flux of the convective cells are related to their previous state and to the change in the total convective area via a transition function. The convective area tendency in turn is assumed to depend on the cloud‐base mass flux that is resolved by the host WRF model. The transition function is represented by a single hidden‐layer neural network trained by the evolution of convective cell size distributions in a 1‐km grid‐spacing WRF simulation run over the Australian Monsoon region. At every grid point of the host model, the cloud population model predicts the cell size and cloud‐base mass flux distributions from which a random sample of cells is fed to an entraining parcel model that calculates precipitation as well as the associated liquid water potential temperature and total moisture tendencies. These tendencies are averaged over the cells and provided to the host model. Several regional simulations are performed over tropical and midlatitude domains to test this as a potential approach to scale‐aware parameterization. It is shown that such an approach could be a new promising path to simulating realistic precipitation statistics and propagation of precipitation associated with the Madden‐Julian Oscillation while maintaining realistic depictions of the diurnal cycle over both land and ocean.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1990

Plasma sheet polytropic index as inferred from the FPE measurements

It is shown here that a four-cell convection pattern imposed on the magnetosphere by the solar wind during northward IMF conditions can be distorted into a wrapped two-cell convection pattern which is similar to the empirical distorted two-cell convection patterns deduced by Heppner and Maynard (1987). The convection distortions are caused by the anisotropic reflections of Alfven waves due to the anisotropic and nonuniform ionospheric conductance. The distributions of the resulting field-aligned currents in the model are characteristically consistent with observed current distributions, including the NBZ currents during northward IMF.

Zhu, X. M.↗
DOE OSTI2025

Rapid SACR Observations of Convection at Bankhead National Forest (RAPID) Field Campaign Report

Improving our representation of convective cell processes requires better quantification of convective clouds throughout their entire life cycle. This includes gaining a clearer understanding of the controls on key convective cloud properties, such as updraft intensity, particle size distributions, rainfall rates, and hydrometeor species. Our inability to improve convective cloud process modeling stems, in part, from a limited understanding of convective cell properties, particularly given how rapidly these storms evolve. This lack of detailed observations in the most intense and organized convective storms is especially significant, as large errors remain in representing these clouds, which are critical for severe weather prediction and Earth system model performance. Cloud and precipitation radars are essential tools for studying cloud microphysics and dynamics, particularly in deeper convective clouds. The recent U.S. Department of Energy Atmospheric Radiation Measurement (ARM) User Facility’s third Mobile Facility (AMF3) Bankhead National Forest (BNF) deployment provides a unique opportunity to investigate important land–atmosphere interactions, as well as the environmental controls on deep convective cloud processes, in a location favorable for frequent convection. We operate the X/Ka-band Scanning ARM Cloud Radar (X/Ka SACR) using a scan strategy optimized to capture these rapidly evolving clouds and their properties, thereby improving studies of deep convective cloud processes. This effort is strengthened by a complementary and coordinated partnership with ongoing university and multi-agency radar activities collocated in north Alabama—a unique opportunity to examine clouds and precipitation from a lifetime-centric perspective.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

A Machine Learning Assisted Development of a Model for the Populations of Convective and Stratiform Clouds

Abstract Traditional parameterizations of the interaction between convection and the environment have relied on an assumption that the slowly varying large‐scale environment is in statistical equilibrium with a large number of small and short‐lived convective clouds. They fail to capture nonequilibrium transitions such as the diurnal cycle and the formation of mesoscale convective systems as well as observed precipitation statistics and extremes. Informed by analysis of radar observations, cloud‐permitting model simulation, theory, and machine learning, this work presents a new stochastic cloud population dynamics model for characterizing the interactions between convective and stratiform clouds, with the goal of informing the representation of these interactions in global climate models. Fifteen wet seasons of precipitating cloud observations by a C‐band radar at Darwin, Australia are fed into a machine learning algorithm to obtain transition functions that close a set of coupled equations relating large‐scale forcing, mass flux, the convective cell size distribution, and the stratiform area. Under realistic large‐scale forcing, the derived transition functions show that, on the one hand, interactions with stratiform clouds act to dampen the variability in the size and number of convective cells and therefore in the convective mass flux. On the other, for a given convective area fraction, a larger number of smaller cells is more favorable for the growth of stratiform area than a smaller number of larger cells. The combination of these two factors gives rise to solutions with a few convective cells embedded in a large stratiform area, reminiscent of mesoscale convective systems.

54 ENVIRONMENTAL SCIENCES↗
NASA NTRS1987

Ionospheric convection signatures and magnetic field topology

A statistical study of signatures of the high-latitude ionospheric convection pattern and the simultaneously observed energetic electron precipitation is presented. Most often found are convection cells in which the sunward flowing region contains auroral particle precipitation but the antisunward flowing region does not. However, observations also show the frequent occurrence of convection cells in which neither the antisunward nor the sunward flowing plasma region contains auroral particle precipitation. These findings may appear within the dawnside or duskside convection pattern and strongly suggest that such convection cells may be associated with open magnetic field lines that thread the magnetotail lobes.

Coley, W. R.↗
NASA NTRS2000

Raman Shifting a Tunable ArF Excimer Laser to Wavelengths of 190 to 240 nm With a Forced Convection Raman Cell

Tunable radiation, at ultraviolet wavelengths, is produced by Raman shifting a modified 285-mJ ArF excimer laser. Multiple Stokes outputs are observed in H2, CH4, D2, N2, SF6, and CF4 (20, 22, 53, 21, 2.1, and 0.35 percent, respectively). Numbers in parentheses are the first Stokes energy conversion efficiencies. We can access 70 percent of the frequency range 42000-52000 cm (exp -1) (190-240 nm) with Stokes energies that vary from 0.2 microJoule to 58 mJ inside the Raman cell. By using 110 mJ of pump energy and D 2 , the tunable first Stokes energy varies over the 29-58 mJ range as the wavelength is tuned over the 204-206 nm range. Dependence on input energy, gas pressure, He mixture fraction, and circulation of the gas in the forced convection Raman cell is discussed; Stokes conversion is also discussed for laser repetition rates from 1 to 100 Hz. An empirical equation is given to determine whether forced convection can improve outputs for a given repetition rate.

Balla, R. Jeffrey↗
NASA NTRS1986

Ionospheric convection signatures observed by DE 2 during northward interplanetary magnetic field

Observations of the ionospheric convection signature at high latitudes are examined during periods of prolonged northward interplanetary magnetic field (IMF). The data from Dynamics Explorer 2 show that a four-cell convection pattern can frequently be observed in a region that is displaced to the sunward side of the dawn-dusk meridian regardless of season. In the eclipsed ionosphere, extremely structured or turbulent flow exists with no identifiable connection to a more coherent pattern that may simultaneously exist in the dayside region. The two highest-latitude convection cells that form part of the coherent dayside pattern show a dependence on the y component of the IMF. This dependence is such that a clockwise circulating cell displaced toward dawn dominates the high-latitude region when B(Y) is positive. Anti-clockwise circulation displaced toward dusk dominates the highest latitudes when B(Y) is negative. Examination of the simultaneously observed energetic particle environment suggests that both open and closed field lines may be associated with the high-latitude convection cells. On occasions these entire cells can exist on open field lines. The existence of closed field lines in regions of sunward flow is also apparent in the data.

Heelis, R. A.↗
NASA NTRS1977

Flight IV technical report for experiment 74-37 contained polycrystalline solidification in low-G

Experiments were performed to study the effect of a low-gravity environment on the columnar-to-equiaxed transition (CET) during polycrystalline solidification. Solutions of H2O-30 wt% NH4Cl and H2O-37 wt% NH4Cl were solidified in semicylindrical molds with radial heat extraction. Both solutions were quenched from the same soak temperature (90 C); the respective superheat temperatures were, therefore, approximately 57 and 23 C. The lower superheat resulted in a completely columnar structure, and the higher superheat resulted in a 1/3 columnar - 2/3 equiaxed microstructure; these results were independent of the relationship between heat flow direction and gravity. Grain multiplication mechanisms observed were showering, thermal inversion driven convection cells, and compositionally induced density inversion driven convection cells.

Papazian, J. M.↗
NASA NTRS2012

The Kinematic and Microphysical Control of Storm Integrated Lightning Flash Extent

The objective of this preliminary study is to investigate the kinematic and microphysical control of lightning properties, particularly those that may govern the production of nitrogen oxides (NOx) in thunderstorms, such as flash rate, type and extent. The mixed-phase region is where the noninductive charging (NIC) process is thought to generate most storm electrification during rebounding collisions between ice particles in the presence of supercooled water. As a result, prior radar-based studies have demonstrated that lightning flash rate is well correlated to kinematic and microphysical properties in the mixed-phase region of thunderstorms such as updraft volume, graupel mass, or ice mass flux. There is also some evidence that lightning type is associated with the convective state. Intracloud (IC) lightning tends to dominate during the updraft accumulation of precipitation ice mass while cloud-to-ground (CG) lightning is more numerous during the downdraft-driven descent of radar echo associated with graupel and hail. More study is required to generalize these relationships, especially regarding lightning type, in a wide variety of storm modes and meteorological conditions. Less is known about the co-evolving relationship between storm kinematics, microphysics, morphology and three-dimensional flash extent, despite its importance for lightning NOx production. To address this conceptual gap, the NASA MSFC Lightning Nitrogen Oxides Model (LNOM) is applied to North Alabama Lightning Mapping Array (NALMA) and Vaisala National Lightning Detection NetworkTM (NLDN) observations following ordinary convective cells through their lifecycle. LNOM provides estimates of flash type, channel length distributions, lightning segment altitude distributions (SADs) and lightning NOx production profiles. For this study, LNOM is applied in a Lagrangian sense to well isolated convective cells on 3 April 2007 (single cell and multi-cell hailstorm, non-severe multicell) and 6 July 2007 (non-severe multi-cell) over Northern Alabama. The LNOM lightning characteristics are compared to the evolution of updraft and precipitation properties inferred from dual-Doppler and polarimetric radar analyses applied to observations from a nearby Doppler radar network, including the UA Huntsville Advanced Radar for Meteorological and Operational Research (ARMOR, C-band, polarimetric). The LNOM estimated SAD and lightning NOx production profiles are placed in the context of radar derived profiles of vertical motion, precipitation types and amounts. Finally, these analyses are used to determine if storm integrated flash channel extent is as well correlated to volumetric updraft and precipitation ice characteristics in the mixed phase region as flash rate for these individual convective cells.

Carey, Lawrence↗
NASA NTRS1994

Polar cap potential distributions during periods of positive IMF B(sub y) and B(sub z)

We compare the DE-2 electric field measurements used by HEPPNER and MAYNARD (1987) to illustrate strongly distorted, BC convection patterns for interplanetary magnetic field (IMF) B(sub z) greater than 0 and large absolute value of B(sub y), with simultaneous detections of particle spectra, plasma drifts and magnetic perturbations. Measured potentials greater than 50 keV, driven by the solar wind speeds exceeding 500 km/s, are greater than published correlation analysis predictions by up to 27%. The potential distributions show only two extrema and thus support the basic conclusion that under these conditions the solar wind/IMF drives two-rather than four-cell convection patterns. However, several aspects of the distorted two-cell convection pattern must be revised. In addition to the strong east-west convection in the vicinity of the cusp, indicated by Heppner and Maynard, we also detect comparable components of sunward (equatorward) plasma flow. Combined equipotential and particle precipitation distributions indicate the presence of a lobe cell embedded within the larger, afternoon reconnection cell. Both types rotate in the same sense, with the lobe cell carrying 20-40% of the total afternoon cell potential. We detected no lobe cell within morning convection cell.

Burke, William J.↗
NASA NTRS1993

The interaction of a magnetic cloud with the Earth - Ionospheric convection in the Northern and Southern Hemispheres for a wide range of quasi-steady interplanetary magnetic field conditions

Observations are presented of the ionospheric convection in cross sections of the polar cap and auroral zone as part of the study of the interaction of the Earth's magnetosphere with the magnetic cloud of January 13-15, 1988. For strongly northward IMF, the convection in the Southern Hemisphere is characterized by a two-cell convection pattern comfined to high latitudes with sunward flow over the pole. The strength of the flows is comparable to that later seen under southward IMF. Superimposed on this convection pattern there are clear dawn-dusk asymmetries associated with a one-cell convection component whose sense depends on the polarity of the magnetic cloud's large east-west magnetic field component. When the cloud's magnetic field turns southward, the convection is characterized by a two-cell pattern extending to lower latitude with antisunward flow over the pole. There is no evident interhemispheric difference in the structure and strength of the convection. Superimposed dawn-dusk asymmetries in the flow patterns are observed which are only in part attributable to the east-west component of the magnetic field.

Freeman, M. P.↗
NASA NTRSDate not supplied

Ground Validation Activities for the INCUS Satellite Mission

Over the next decade, there will be several satellite-based radar missions designed to estimate vertical motions and mass flux within deep convection. One of these is the upcoming Investigation of Cumulus Updrafts (INCUS) mission. INCUS will do this using a low-Earth orbit train of three small satellites carrying Ka-band radars to observe the 3-D evolution of radar reflectivity within deep convective cells at time intervals of 30-, 90-, and 120-sec. One approach to validating this novel time-difference (delta-t) approach would include a pair of Doppler radars flown on two different high-altitude aircraft (i.e., satellite simulator), but that solution is not practical within the scope of a cost-capped Earth Venture mission like INCUS. Instead, the ground-based validation (GV) approach for INCUS will consist of using vertical profiling radars, including analysis of existing convective cases, and multiple polarimetric, Doppler radars performing coordinated rapid sampling of the same convective cell. These new observations will be collected in the coming years through local deployments of existing research radar facilities at 1 - 3 sites over multiple seasons to capture a statistically significant sample of storm types (both pre- and post-launch) or as a single post-launch field campaign in a region where convection is very predictable. This presentation will include an overview of INCUS GV, both pre- and post-launch activities, as well as some of our collaborations with the radar community to address some of the challenges in sampling the vertical evolution of convective cells on 120-sec or less time scales.

remote sensing↗
NASA NTRS1994

Theoretical models of polar-cap convection under the influence of a northward interplanetary magnetic field

The unexpected patterns of high-latitude auroral luminosity and ionospheric convection that are observed when the interplanetary magnetic field (IMF) has a northward orientation have inspired a variety of theoretical interpretations. The existing models, all referring to steady-state conditions, can be classified according to the topology of the polar magnetic field lines and of the polar-cap convection streamlines. The classes of model include: (1) a closed magnetosphere model, (2) a conventional open model with a distorted, but topologically unchanged, polar-cap boundary, (3) a conventional open model with distorted, but topologically unchanged, polar-cap convection cells, (4) a modified open model with 'lobe convection cells' contained wholly on open magnetic-field lines, and (5) a modified open model with a bifurcated polar cap. The third and fourth types require significant regions of sunward flow on open polar-cap field lines, a concept that presents serious theoretical difficulties. The other three types appear equally viable from a theoretical point of view, and the comparison against observations is an ongoing enterprise. Outstanding theoretical questions include (a) how do observed structures in the polar ionosphere map along magnetic field lines into the magnetosphere?, (b) what is the mechanism that drives the observed sunward convection at highest latitudes on the day side?, and (c) what role does time dependence play in the observed phenomena?

Hill, T. W.↗
DOE OSTI2022

Reduction in RF sheath rectification with insulating antenna enclosure walls

Abstract Radiofrequency (RF) sheath rectification is one of the most prominent deleterious effects associated with ion cyclotron range of frequencies (ICRF) heating in fusion plasmas. RF sheaths and associated effects, such as impurity generation and convective cell generation, need to be mitigated to ensure that ICRF is a viable option for heating in future fusion devices. Experiments were performed on the Large Plasma Device (LAPD) at UCLA to explore the effects of using electrically-insulating antenna enclosures on RF rectified sheaths. Three different enclosure side-wall materials were used, including copper, MACOR (electrically insulating), and MACOR over copper. In the case of the MACOR–copper side walls, the non-conductive MACOR material was exposed to the bulk plasma but a layer of copper was added below to allow for image currents to flow. All three of the experiments had similar plasma density, temperature, and background magnetic field. In the case of the copper enclosure, RF rectified potentials, many times the local electron temperature, and associated formation of convective cells were observed and reported Martin M. et al (2017 Phys. Rev. Lett. 119 205002). In the experiments with MACOR and MACOR–copper enclosures, RF rectification was significantly reduced. Additionally, these latter two experiments showed no evidence of convective cell formation. Although the results from the MACOR experiment are reminiscent of the results obtained in ASDEX-U with a three-strap antenna optimized to reduce image currents on the antenna limiters Bobkov V. et al (2016 Nucl. Fusion 56 084001), the MACOR–copper experiment seems to suggest that insulating plasma facing materials have at least an equally strong impact on reducing potential rectification.

Physics↗