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At least 217 records · Page 12

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NASA NTRS1980

Direct effects of lightning on an aircraft during intentional penetrations of thunderstorms

An armored T-28 aircraft was struck by lightning on two different days while participating in the 1979 severe environmental storm and mesoscale experiment in Oklahoma. The T-28, which is specially armored and instrumented, was making intentional penetrations of thunderstorms and was struck twice on 30 May and once on 5 June. Various degrees of damage, mainly in the form of large burn spots and holes, resulted to the aircraft.

Musil, D. J.↗

NASA NTRS1979

Flight through thunderstorm outflows

Computer simulation of aircraft landing through thunderstorm gust fronts is carried out. The 3 degree-of-freedom, nonlinear equations of aircraft motion for the longitudinal variables containing all two-dimensional wind shear terms are solved numerically. The gust front spatial wind field inputs are provided in the form of tabulated experimental data which are coupled with a computer table lookup routine to provide the required wind components and shear at any given position within an approximate 500 m x 1 km vertical plane. The aircraft is considered to enter the wind field at a specified position under trimmed conditions. Both fixed control and automatic control landings are simulated. Flight paths, as well as control inputs necessary to maintain specified trajectories, are presented and discussed for aircraft having characteristics of a DC-8, B-747, and a DHC-6.

Frost, W.↗

NASA NTRS1980

A study of tornadic thunderstorm interactions with thermal boundaries

A study of tornadic thunderstorm interactions with thermal boundaries using a model of subcloud wind profiles is presented. Within a hot, moist, and conditionally unstable air mass, warm thermal advection and surface friction cause the winds to veer and increase with height, while within a cool, moist air mass cool thermal advection and friction combine to produce a wind profile that has maximum speeds near the surface and veers little with height. The spatial distribution of different wind profiles and moisture contents within the boundary layer may act together to maximize mesoscale moisture contents, convergence, and cyclonic vorticity within a narrow mixing zone along the thermal boundary.

Maddox, R. A.↗

NASA NTRS1981

Spiral feature observed at top of rotating thunderstorm

A dark spiral feature is noted in the geosynchronous satellite visible image of the top of a thunderstorm which also has a Doppler radar-observed mesocyclone. Although the evidence is not conclusive, the feature may represent cyclonic rotation at cloud top associated with the pre-tornado mesocyclone.

Adler, R. F.↗

NASA NTRS1980

Doppler radar observation of the evolution of a thunderstorm

Triple-Doppler radar observations of the evolution of the three-dimensional structure of a thunderstorm on May 19, 1978 are analyzed. Continuous data were taken over a long period of the non-severe storm's lifetime as it passed through the radar and the portable automated mesonet network. A fairly low cloud top of 10 km and high reflectivities were observed, and horizontal rotations developed in the middle troposphere, which never reached lower levels. The cyclonic and anticyclonic circulations at mid-levels intensified after the maximum cell height was reached, and a high reflectivity maximum lasted 15-20 min aloft, with the core descending to the surface because of rapid fallout from the largest precipitation particles. Due to the fairly small scale of the updrafts, future use of smaller grid spacings is considered a necessity.

Heymsfield, G. M.↗

NASA NTRS1980

Doppler radar observations of the three-dimensional turbulent structure of a quasi-steady thunderstorm

The spatial patterns and temporal evolution of Doppler radar-inferred turbulence within a quasi-steady, left-moving thunderstorm are reported. An organized downdraft circulation was observed, which maintained a persistent, continuously-propagating, anticyclonic and turbulent updraft. The updrafts attained peak speeds of 25 m/s, and maximum estimated reflectivity factors exhibited peak magnitudes of 50-55 dBZ. Mechanical and buoyant production appeared to be the primary generators of turbulence, although buoyancy generation may have also significantly contributed, since turbulent and dynamic entrainment into the vertically accelerating updraft was appreciable below the strongest turbulent activity at 7 km. Patterns of the radial shears of radial velocity exhibited a significant decrease in strength and areal coverage as the storm weakened, which is attributed to decreased activity of large turbulent eddies and the diminution of large-scale mean wind gradients produced by vertical exchanges of horizontal momentum and strong updraft gradients.

Knupp, K. R.↗

NASA NTRS1981

Evolution of downdrafts and rotation in an Illinois thunderstorm

Multiple-Doppler radar observations of a non-severe Illinois thunderstorm with nearly rectilinear vertical wind shear and storm motion related to the wind at a height of 2.5 km are discussed. The cell examined had a radar top of 10 km, and reflectivities of 60 dB(Z). Attention is given the evolution of updraft, downdrafts, and vorticity in this cell, where typical magnitudes were 12 m/sec, 8 m/sec and 0.005/sec, respectively. The main downdrafts were found to be the upshear downdraft during the growth period of the cell, and the left bank downdraft during the mature and downdraft periods. The location of upshear downdraft at mid to upper levels resembles the Lennon and Doswell (1979) conceptual model, as well as three-dimensional cloud model results of severe storms.

Heymsfield, G. M.↗

NASA NTRS1981

Thunderstorm hazards flight research - Program overview

The NASA thunderstorm hazards research program, designed to study the effects of lightning strikes on the design and operation of aircraft, is described. An all-weather F-106B is instrumented to document the EM characteristics of direct and nearby strikes, measure the field parameters and analyze the ambient atmospheric content, and film the strikes; X-ray detectors are also on board, along with instrumentation for determining the frequency of visible light waveforms. Data is either recorded on-board or sent by telemetry to base, while ground based telemetry is used to direct the pilot and craft into regions of optimal lightning activity. The sensing apparatus is described, and ongoing programs to correlate different storm parameters are reviewed, along with operational procedures and safety precautions. Continued use of the craft through 152 storms and 16 direct hits, with no fatalities or circuit breaker throw, confirms the ability of metal skinned aircraft to withstand lightning strikes; data gathered from flights during 1980 are provided.

Deal, P. L.↗

NASA NTRS1982

An intense, quasi-steady thunderstorm over mountainous terrain. I - Evolution of the storm-initiating mesoscale circulation

The evolution of mesoscale systems that eventually lead to the formation of large quasi-steady storm systems is investigated. The morphological and turbulent structure of the quasi-steady storm is described. Data obtained during the South Park Area Cumulus Experiment from surface meteorological stations, rawinsondes and tethered balloons, conventional and Doppler radars, powered aircraft, and satellites, indicate that on July 19, 1977, a north-south oriented line of intense convective cells formed and remained within South Park. Elevated surface heating created a region of low-level convergence, importing Pacific moisture from west of the Rockies. The mesoscale thunderstorm line formed over this convergence zone, and a single large convective cell was observed to grow on the southern end of the mesoscale line, exhibiting supercell characteristics and substantial modifications of the environmental flow.

Cotton, W. R.↗

NASA NTRS1982

An intense, quasi-steady thunderstorm over mountainous terrain. III - Doppler radar observations of the turbulent structure

The evolution of the turbulent structure of the intense, quasi-steady thunderstorm that occurred on July 19, 1977, over South Park, Colorado, is examined. Doppler radar estimates of turbulent kinetic energy dissipation rates and radial shears of raw radial velocity are used, and a comparison is made between turbulent patterns and mean storm airflow. Turbulent intensity and activity peaked at mid to upper storm levels, and the primary storm updraft was almost turbulence-free at low levels. It is suggested that buoyancy and wind shear acted together to generate turbulent eddies at middle storm levels, and at mid to upper levels, maximum estimated dissipation rate estimates approached 0.15 sq m/cu s.

Knupp, K. R.↗

NASA NTRS1982

The life cycle of thunderstorm gust fronts as viewed with Doppler radar and rawinsonde data

This paper presents the time-dependent analysis of the thunderstorm gust front with the use of Project NIMROD data. RHI cross sections of reflectivity and Doppler velocity are constructed to determine the entire vertical structure. The life cycle of the gust front is divided into four stages: (1) the formative stage; (2) the early mature stage; (3) the late mature stage; and (4) the dissipation stage. A new finding is a horizontal roll detected in the reflectivity pattern resulting from airflow that is deflected upward by the ground, while carrying some of the smaller precipitation ahead of the main echo core of the squall line. This feature is called a 'precipitation roll'. As determined from rawinsonde data, the cold air behind the gust front accounts for the observed surface pressure rise. Calculations confirm that the collision of two fluids produce a nonhydrostatic pressure at the leading edge of the outflow. The equation governing the propagation speed of a density current accurately predicts the movement of the gust front.

Wakimoto, R. M.↗

NASA NTRS1983

The NASA Thunderstorm Overflight Program (TOP): Research in atmospheric electricity from an instrumented U-2 aircraft platform

An overview of the NASA Thunderstorm Overflight Program (TOP) is presented. The various instruments flown on the NASA U-2 aircraft, as well as the ground instrumentation used to collect optical and electronic signature from the lightning events, are discussed. Samples of some of the photographic and electronic signatures are presented. Approximately 6400 electronic data samples of optical pulses were collected and are being analyzed.

Vaughan, O. H., Jr.↗

NASA NTRS1982

Medium-range objective predictions of thunderstorms on the McIDAS/CSIS interactive computer system

Until recently, all operational meteorological data has been made available to forecasters in a variety of different forms. Predictions based upon these different data formats have been complicated by the inability of forecasters to easily assimilate, in real-time, all data to provide an optimum decision regarding future weather occurrences. By March 1980, a joint NASA/NOAA effort had been initiated to develop the Centralized Storm Information System (CSIS). The primary objectives of this joint project are related to an improvement of the overall severe storm forecast and warning procedure and to a demonstration of the operational utility of techniques developed within the applied research community. CSIS is to utilize the Man Computer Interactive Data Access System (McIDAS). The present investigation is concerned with one of the first attempts to employ the CSIS system for the evaluation of a new research technique involving the prediction of thunderstorms over a forecast period of 12-48 hours.

Wilson, G. S.↗

NASA NTRS1983

In situ aircraft measurements of enhanced levels of N2O associated with thunderstorm lightning

A series of measurements of enhanced levels of atmospheric N2O associated with thunderstorm lightning are reported. The data were gathered by instrumentation on-board an aircraft operated as part of the NASA Storm Hazards Project. Air samples were taken both during storms and in clear conditions to have a basis for comparisons; sample bottles were filled at altitudes from 11,000-40,000 ft. Gas chromatography was employed for sample composition analyses, revealing clear air N2O concentrations of about 310 ppbv, while storm concentrations reached, for example, 490, 729, and 393 ppbv. Although the measurements did not precisely characterize the actual lightning contributions, the enhancements being present during electrically active storms did confirm that trace gases are produced by lightning. Calculations are presented to demonstrate that the 3 to greater than 12 keV X rays detected in storm clouds are of sufficient energy to drive the production of N2O from the reaction of metastable nitrogen with molecular oxygen.

Levine, J. S.↗

NASA NTRS1983

Heavy thunderstorms observed over land by the Nimbus 7 scanning multichannel microwave radiometer

Brightness temperatures obtained through examination of microwave data from the Nimbus 7 satellite are noted to be much lower than those expected on the strength of radiation emanating from rain-producing clouds. Very cold brightness temperature cases all coincided with heavy thunderstorm rainfall, with the cold temperatures being attributable to scattering by a layer of ice hydrometeors in the upper parts of the storms. It is accordingly suggested that brightness temperatures observed by satellite microwave radiometers can sometimes distinguish heavy rain over land.

Spencer, R. W.↗

NASA NTRS1982

Mature thunderstorm cloud top structure - Three-dimensional numerical simulation versus satellite observations

Preliminary results of four runs with a three-dimensional model of the effects of vertical wind shear on cloud top height/temperature structure and the internal properties of isolate midlatitude thunderstorms are reported. The model is being developed as an aid to analyses of GEO remote sensing satellite data. The grid is a 27 x 27 x 20 mesh with 2 km horizontal resolution and 0.9 vertical resolution. The total grid is 54 km on a side and 18 km deep. A second-order Crowley scheme for advecting momentum is extended with a third-order correction for spatial truncation error, and the earth-relative horizontal surface wind components are decreased to 50 percent of their values at 0.45 km. A temperature increase with height is included, together with an initial impulse consisting of a nonrotating cylindrical weak buoyant updraft 10 km in radius. The results of the runs are discussed in terms of the time variation of the vertical velocity extrema, the effects of strong and weak shear on a storm, the cloud top height, the Lagrangian dynamics of a thermal couplet, and data from a real storm.

Schlesinger, R. E.↗

NASA NTRS1983

Hy-wire and fast electric field change measurements near an isolated thunderstorm, appendix C

Electric field measurements near an isolated thunderstorm at 6.4 km distance are presented from both a tethered balloon experiment called Hy-wire and also from ground based fast and slow electric field change systems. Simultaneous measurements were made of the electric fields during several lightning flashes at the beginning of the storm which the data clearly indicate were cloud-to-ground flashes. In addition to providing a comparison between the Hy-wire technique for measuring electric fields and more traditional methods, these data are interesting because the lightning flashes occurred prior to changes in the dc electric field, although Hy-wire measured changes in the dc field of up to 750 V/m in the direction opposite to the fair weather field a short time later. Also, the dc electric field was observed to decay back to its preflash value after each flash. The data suggest that Hy-wire was at the field reversal distance from this storm and suggest the charge realignment was taking place in the cloud with a time constant on the order of 20 seconds.

Holzworth, R. H.↗

NASA NTRS1984

Nighttime observations of thunderstorm electrical activity from a high altitude airplane

Photographs from a NASA U-2 airplane flying over nocturnal thunderstorms show frequent lightning activity in the upper part of the cloud. In some cases, unobscured segments of lightning channels 1 km or longer are visible in clear air around and above the cloud. Multiple images of lightning channels indicate multiple discharges in the same channel. Photographs taken through a diffraction grating show that the lightning has a spectrum similar to that observed in the lower troposphere. Lightning spectra obtained with a slitless line-scan spectrometer show strong singly ionized nitrogen emissions at 463.0 and 500.5 nm. Field changes measured with an electric field-change meter correlate with pulses measured with a photocell optical system. Optical signals corresponding to dart leader, return stroke, and continuing current events are readily distinguished in the scattered light emerging from the cloud surface. The variation of light intensity with time in lightning events is consistent with predicted modification of optical lightning signals by clouds. It appears that satellite based optical sensor measurements cannot provide reliable information on current rise times in return strokes. On the other hand, discrimination between cloud-to-ground and intracloud flashes and the counting of ground strokes is possible using the optical pulse pairs which have been identified with leader, return-stroke events in the cloud-to-ground flashes studied.

Brook, M.↗