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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 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Lightning and precipitation history of a microburst-producing storm

Quantitative measurements of the lightning and precipitation life cycle of a microburst-producing storm are discussed. The storm, which occurred on July 20, 1986 at Huntsville, Alabama, was studied using Doppler radar data. The storm produced 116 flashes, 6 of which were discharges to the ground. It is suggested that an abrupt decrease in the total flash rates is associated with storm collapse, and serves as a precursor to the arrival of the maximum microburst outflows at the surface. Ice-phase precipitation is shown to be an important factor in both the formation of the strong downdraft and the electrification of the storm.

Goodman, Steven J.↗
NASA NTRS1988

Numerical simulation of tropical cumulus clouds and their interaction with the subcloud layer

A two-dimensional numerical model, with an emphasis on turbulent processes in the boundary layer and in clouds, was developed for simulating an ensemble of cumulus clouds. The model was used to investigate the response of tropical cumulus clouds to imposed large-scale vertical advection by performing two simulations which were identical except that one was with upward large-scale vertical velocity (the 'disturbed' case) and the other was without any advection (the 'undisturbed' case). The results showed that deep cumulus clouds formed in the disturbed case, while only shallow clouds formed in the undisturbed case. The subcloud layer (SCL) in the disturbed case was strongly affected by cumulus circulations and rain evaporation, with both cumulus updrafts and downdrafts being important contributors to the SCL sensible and latent heat fluxes. Cumulus-scale circulation in the disturbed SCL determined where new clouds formed by creating convergence zones and moisture anomalies.

Krueger, Steven K.↗
NASA NTRS1988

Numerical simulations of an isolated microburst. I - Dynamics and structure

Isolated and stationary microbursts are simulated using a time-dependent, high-resolution, axisymmetric numerical model. A microburst downdraft is initiated by specifying a distribution of precipitation at the top boundary of the model and allowing it to fall into the domain. Two case studies of a hazardous wet (high reflectivity) microburst are presented. The initial and boundary specifications, evolution, microphysics, ring vortex, burst front, vorticity, and structure of the microbursts are discussed.

Proctor, Fred H.↗
NASA NTRS1988

Initial observations of cloud-to-ground lightning activity in microburst producing storms

Preliminary observations of cloud-to-ground lightning activity associated with wet microburst producing storms observed during the summer of 1986 near Huntsville, AL are presented. Reflectivity and velocity measurements obtained from two 10 cm Doppler radars are used to identify and characterize the microburst windshears and their parent clouds. A four-station lightning location network is used to characterize the ground discharges produced by these storms. Results show cloud-to-ground flashes preceding the microburst onset by 0 - 28 minutes and suggest that any storm in a microburst environment that produces a discharge to ground has the potential to generate an intense downdraft.

Buechler, Dennis E.↗
NASA NTRS1989

A proposed microburst nowcasting procedure using single-Doppler radar

Thirty-one microburst-producing storms from northweast Colorado were studied using single and multiple Doppler radar for the purpose of identifying radar signatures that indicated the development of a downdraft capable of producing a microburst. Descending reflectivity cores, increasing radial convergence within cloud, rotation, and reflectivity notches, were found to be microburst precursors, appearing typically 2-6 min prior to initial surface outflow. Three conceptual models have been drawn, based on the 31 events, to summarize the radar signatures of importance in low, moderate, and high-reflectivity microburst-producing storms. A forecaster-computer environment is proposed to allow the forecaster to readily view radar reflectivity and Doppler velocity information in both unprocessed and analyzed form. Use of multiimage radar displays and time-height profiles of quantitative radar estimates of reflectivity and radial shear are suggested to provide an environment where rapid progress can be made in developing techniques to nowcast microbursts.

Roberts, Rita D.↗
NASA NTRS1989

The K-Gill - A twin propeller-vane anemometer for measurements of atmospheric turbulence

A twin propeller-vane anemometer developed at the University of British Columbia was successfully used for unattended measurement of atmospheric turbulence over extended periods of time during moderate to high winds in the presence of sea spray. In this paper, a new design in twin propeller-vane anemometers is introduced. The instrument consists of two Gill anemometers mounted on a vane. The propeller shafts, one pointing up and the other pointing down, make an angle of 45 deg with the horizontal (hence, the name K-Gill). In addition to the desirable characteristics of the earlier design the K-Gill has symmetry, so that updraft and downdraft winds are measured with equal sensitivity. An algorithm for obtaining the vertical and downstream horizontal components of the wind velocity is described. Various sources of error and their magnitudes are discussed.

Atakturk, Serhad S.↗
NASA NTRS1989

Numerical simulations of an isolated microbursts. II - Sensitivity experiments

A time-dependent, high-resolution, axisymmetric numerical model simulating isolated and stationary microbursts is used to evaluate the sensitivity of microbursts to the environment and other factors. It is found that microbursts intensity is sensitive to ther vertical distirbution of ambient temperature and humidity, the horiontal width of the precipitation shaft or downdraft, the magnitutde of precipitation loading, and the type and duration of precipitation. Scenarios for the generation of an intense microburst and the possible applications of the results are examined.

Proctor, Fred H.↗
NASA NTRS1990

On the dynamic and thermodynamic structures of marine stratocumulus

Latent heating effects on stratocumulus circulations were studied successfully with a nine-coefficient spectral model of two-dimensional shallow Boussinesq convection (Laufersweiler and Shirer, 1989). Further, more realistic investigations are being performed currently with a larger, 18-coefficient spectral model, in which the effects of cloud top radiational cooling and in-cloud radiational heating are also being represented. Because assuming a rigid lid at the inversion base may have affected previous results significantly, the domain top was raised to include the lower portion of the capping inversion. As in the previous model, a uniform cloud base is assumed and latent heating effects are included implicitly such that the motions in the sub- and above-cloud regions are dry adiabatic and the motions in the cloud region are moist adiabatic. The effects of forcing by radiational heating profiles that are tied to the cloud layer, such as the one used by Nicholls will be investigated, as will profiles measured during the FIRE experiment. One concern of using truncated spectral models is that the phenomena are so poorly represented that they can change dramatically as the number of spectral coefficients is increased. The efficacy of the nine-coefficient model results is checked by examining the steady state solutions of the 18-coefficient model for parameter values used by Laufersweiler and Shirer (1989), which corresponds to the case of a moderately deep cloud and no capping inversion. The horizontally asymmetric circulation patterns that have narrow downdraft areas and broad updraft areas are virtually the same as those found in the smaller spectral model. Also captured in the case of weaker heating is an elevated circulation centered at cloud base. Thus, the results of the smaller model are substantiated. Since one of the goals of studying the new model is to represent a more realistic domain, the second test of the model is to investigate whether the steady solutions are suppressed in the case of an inversion with no cloud.

Laufersweiler, Mark J.↗
NASA NTRS1990

Turbulence structure in clear and cloudy regions of the 7 July 1987 Electra mission

The 7 July mission of the 1987 FIRE Marine Stratocumulus Intensive Field Observations was chosen for analysis because of a well-defined transition from stratocumulus to clear conditions sampled by the aircraft on this day. It is hoped that by studying this case something can be learned about the processes responsible for the maintenance and breakup of stratocumulus layers, a primary objective of FIRE. The preliminary analysis is based on data from the Electra flight of this day. The properties of turbulence elements, i.e., updrafts and downdrafts, are examined to gain information on the nature of the turbulent exchanges through the boundary layer and across the inversion. Since such exchanges in large measure determine the stability and structure of cloud layers, a study of draft properties should be informative. The results will also be useful in the development of boundary layer models that are based on draft circulations (e.g., Randall, 1988; Hanson, 1988).

Khalsa, Siri Jodha Singh↗
NASA NTRS1989

Uranus deep atmosphere revealed

The present examination of the radio spectrum and latitudinal radio brightness temperature variation of Uranus leads to a ejection of thermochemical equilibrium models and the adoption of an atmospheric model characterized by a low ammonia volume-mixing ratio that is uniformly distributed over a wide altitude range. The elemental ratios derivable from this model support the planetary accretion theory of Pollack and Bodenheimer (1989). It is noted that while the equatorial and midlatitude values of Uranian radio brightness temperature are explainable by condensation theories, the polar value can only be accounted for through the invocation of strong, dry air downdrafts.

De Pater, Imke↗
NASA NTRS1989

Polarization radar and electrical observations of microburst producing storms during Cohmex

The life cycles of two electrified, microburst-producing storms that occurred on July 19 and 20, 1986 near Huntsville, Alabama are described and compared. The kinematic and microphysical development of the storm clouds is examined. Lightning activity prior to the onset of the microburst is studied. It is observed that ice phase precipitation particles are important in the electrification of the storm and in the formation of the strong downdraft, and the vertical distribution and movement of mass have a role in determining the total lightning activity and type of flashes.

Goodman, Steven J.↗
NASA NTRS1989

Planned ER-2 Doppler radar (EDOP) for studying convective storms and mesoscale phenomena

A radar development is presently funded by the NASA Mesoscale Atmospheric Research Program to build an X-band Doppler radar (EDOP) for the ER-2 aircraft for measuring the structure of updrafts and downdrafts in deep convective systems. In this paper, the radar's scientific objectives and characteristics are briefly reviewed. These objectives include documentation of deep isolated thunderstorms, examination of cloud top and anvil structure, structure of microscale convective systems, cloud microphysics, and testing of satellite precipitation estimation algorithms. System specifications for the radar are given.

Heymsfield, Gerald M.↗
NASA NTRS1991

Estimating Atmospheric Turbulence From Flight Records

Method for estimation of atmospheric turbulence encountered by airplanes utilizes wealth of data captured by multichannel digital flight-data recorders and air-traffic-control radar. Developed as part of continuing effort to understand how airplanes respond to such potentially hazardous phenomena as: clear-air turbulence generated by destabilized wind-shear layers above mountains and thunderstorms, and microbursts (intense downdrafts striking ground), associated with thunderstorms. Reconstructed wind fields used to predict and avoid future hazards.

Wingrove, R. C.↗
NASA NTRS1990

Plume mechanics and stratocumulus convection

The First ISCCP Regional Experiment (FIRE) marine stratocumulus Intensive Field Observation (IFO) held in July, 1987 produced a data set that is far more comprehensive than data sets from previous stratocumulus experiments. One exciting new development was the use of the 10.6 micron lidar system for cloud-top mapping that was available on the National Center for Atmospheric Research (NCAR) Electra. This system provided a unique look at the small scales of the turbulence in the clouds, images of the turbulent structures that are quantitatively revealed by conditional sampling (e.g., Khalsa and Greenhut, 1987). The behavior of these updrafts and downdrafts is central to the dynamics of the stratocumulus-topped marine boundary layer. FIRE's objectives of understanding cloud dynamics and how they affect the cloud optical depth - which, in turn, is the crucial factor in determining the clouds' albedo - therefore require the investigation of these drafts. Given here are initial results from a simple model capable of simulating moist, entraining plumes that are subject to water phase changes and radiative heating and cooling. These results correspond, therefore, to the 'dry cloud' case discussed by Lilly and Schubert (1980). The model's simplicity limits the realism of the results - the plumes are assumed not to interact, for example - but the role of radiative processes in influencing the plume dynamics is clear. Also revealed is the role the plumes play is maintaining the cloud-top inversion. The model equations and methodology used, and qualitative results are given.

Hanson, Howard P.↗
NASA NTRS1990

The effect of latent heat release on synoptic-to-planetary wave interactions and its implication for satellite observations: Theoretical modeling

The project objectives are to develop process models to investigate the interaction of planetary and synoptic-scale waves including the effects of latent heat release (precipitation), nonlinear dynamics, physical and boundary-layer processes, and large-scale topography; to determine the importance of latent heat release for temporal variability and time-mean behavior of planetary and synoptic-scale waves; to compare the model results with available observations of planetary and synoptic wave variability; and to assess the implications of the results for monitoring precipitation in oceanic-storm tracks by satellite observing systems. Researchers have utilized two different models for this project: a two-level quasi-geostrophic model to study intraseasonal variability, anomalous circulations and the seasonal cycle, and a 10-level, multi-wave primitive equation model to validate the two-level Q-G model and examine effects of convection, surface processes, and spherical geometry. It explicitly resolves several planetary and synoptic waves and includes specific humidity (as a predicted variable), moist convection, and large-scale precipitation. In the past year researchers have concentrated on experiments with the multi-level primitive equation model. The dynamical part of that model is similar to the spectral model used by the National Meteorological Center for medium-range forecasts. The model includes parameterizations of large-scale condensation and moist convection. To test the validity of results regarding the influence of convective precipitation, researchers can use either one of two different convective schemes in the model, a Kuo convective scheme or a modified Arakawa-Schubert scheme which includes downdrafts. By choosing one or the other scheme, they can evaluate the impact of the convective parameterization on the circulation. In the past year researchers performed a variety of initial-value experiments with the primitive-equation model. Using initial conditions typical of climatological winter conditions, they examined the behavior of synoptic and planetary waves growing in moist and dry environments. Surface conditions were representative of a zonally averaged ocean. They found that moist convection associated with baroclinic wave development was confined to the subtropics.

Branscome, Lee E.↗
NASA NTRS1991

An airborne FLIR detection and warning system for low altitude wind shear

It is shown through some preliminary flight measurement research that a forward looking infrared radiometer (FLIR) system can be used to successfully detect the cool downdraft of downbursts (microbusts/macrobursts) and thunderstorm gust front outflows that are responsible for most of the low altitude wind shear (LAWS) events. The FLIR system provides a much greater safety margin for the pilot than that provided by reactive designs such as inertial air speed systems. Preliminary results indicate that an advanced airborne FLIR system could provide the pilot with remote indication of microburst (MB) hazards along the flight path ahead of the aircraft. Results of a flight test of a prototype FLIR system show that a minimum warning time of one to four minutes (5 to 10 km), depending on aircraft speed, is available to the pilot prior to the microburst encounter.

Sinclair, Peter C.↗
NASA NTRS1991

Charge control experiments on a CH-53E helicopter in a dusty environment

Charge control tests were carried out on a ground based, Marine Corps helicopter to determine if control of the electric fields acting on the engine exhaust gases could be used to reduce the electrification of the helicopter when it operated in a dusty atmosphere. The test aircraft was flown to a dusty, unpaved area and was then isolated electrically from the earth. When the helicopter engines were operated at ground idle with the rotor locked, the isolated aircraft charged positively, as had been observed previously. However, when the rotor brake was released and the turning rotor created a downdraft that raised dust clouds, the aircraft always became charged more positively, to potentials ranging form +30 to +45 kV. The dust clouds raised by the rotor downwash invariably carried negative space charges with concentrations of up to -100 nC/cu m and caused surface electric fields with strengths of up to 10 kV/m immediately down wind of the aircraft. The natural charging of the helicopter operating in these dust clouds was successfully opposed by control of the electric fields acting on the hot, electrically conductive exhaust gases. The control was achieved by placing electrostatic shield around the exhausts.

Moore, C. B.↗
NASA NTRS1991

Electrification in winter storms and the analysis of thunderstorm overflight

The emergence of 24 hr operational lightning detection networks has led to the finding that positive lightning strokes, although still much fewer in number than the normal negative strokes, are present in summer and winter storms. Recent papers address the importance of understanding the meteorological conditions which lead to a dominance of one polarity of stroke over another; the appearance of positive strokes at the end of a storm appeared to presage the end-of-storm downdraft and subsidence leading to downburst activity. It is beginning to appear that positive strokes may be important meteorological indicators. Significant research accomplishments on the following topics are addressed: (1) a study to verify that the black boxes used in the lightning networks to detect both negative and positive strokes to ground were accurate; (2) the use of slow tails to determine the polarity of distant lightning; (3) lightning initiation in winter vs. summer storms; (4) the upgrade of sensors for the measurement of electric field signals associated with lightning; (5) the analysis of lightning flash records from storms between 40 and 125 km from the sensor; and (6) an interesting aspect of the initiation process which involves the physical processes driving the stepped leader. The focus of current research and future research plans are presented.

Brook, Marx↗