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At least 73 records · Page 4

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NASA NTRS1978

Initiation of non-tropical thunderstorms by solar activity

A theory of thunderstorm initiation is proposed to account for the statistical correlation between solar activity and thunderstorm occurrence in middle to high latitudes. It is suggested that cosmic ray decreases and/or high-energy solar protons associated with active solar events enhance the electric field at low heights so that, if appropriate meteorological conditions are present during a solar event, the atmospheric electric field enhancement may be sufficient to trigger thunderstorm development. Statistical correlations and atmospheric electric effects are described. The theory could be tested if the possible forcing functions and the responding atmospheric electrical and ionic species' characteristics were measured.

Herman, J. R.↗

NASA NTRS1977

Three dimensional cloud top changes related to thunderstorm severity

Digital infrared data from a geostationary satellite are used to study thunderstorm top growth rates and other parameters in relation to the occurrence of severe weather on the ground. Both the rate of upward growth of the thunderstorm top and the maximum height reached are shown to be useful parameters in the detection of severe thunderstorms.

Adler, R. F.↗

NASA NTRS1979

Thunderstorm intensity as determined from satellite data

Digital infrared data from SMS 2 obtained on May 6, 1975 are used to study thunderstorm vertical growth rates and cloud top structure in relation to the occurrence of severe weather (tornadoes, hail, and high wind) on the ground. All thunderstorms from South Dakota to Texas along a N-S oriented cold front were monitored for a 4 h period with 5 min interval data. Thunderstorm growth rate, as determined by the rate of blackbody temperature isotherm expansion and minimum cloud top temperature, are shown to be correlated with reports of severe weather on the ground.

Adler, R. F.↗

NASA NTRS1980

Solar terrestrial relationships related to thunderstorms and BUV dark current and ozone data

Solar terrestrial interactions as they affect Nimbus 4 BUV dark current and possibly affect thunderstorm occurrence are investigated. A solar wind index is calculated for 1970 to 1971. Dark current enhancements appear to be associated in some way with solar proton events and the solar wind index, but additional investigations by GSFC are required before conclusions can be drawn. Superposed epoch analysis of an index of North American thunderstorm occurrence reveals a discernible increase in the index magnitude on days 1 and 2 following solar proton events. There appears to be little or no 27 day recurrence tendency in thunderstorm occurrence frequency and no association with vorticity area index on a day to day basis.

Herman, J. R.↗

NASA NTRS1980

Gravity waves from thunderstorms

Gravity waves generated by severe thunderstorms in the eastern Ohio-Pennsylvania area were recorded by an array of microbarovariographs at Palisades, New York and by standard microbarographs across northeastern United States. The waves were associated with the cold mesohigh from the outflow of the thunderstorms. Along their path the waves apparently triggered new thunderstorms. The waves were observed to propagate with the velocity of the wind just below the tropopause. The long-distance propagation of the waves is explained by the presence of a dust associated with the critical level (steering level), in agreement with the derivations given by Lindzen and Tung (1976). The dust was directional and waves were absent to the west of the generating area. In the generating area wave-CISK might have been operating. Sharp vertical temperature gradients associated with the passage of the waves were observed by temperature sensors on a tower.

Balachandran, N. K.↗

NASA NTRS1981

Thunderstorm hazards flight research: Storm hazards 1980 overview

A highly instrumented NASA F-106B aircraft, modified for the storm hazards mission and protected against direct lightning strikes, was used in conjunction with various ground based radar and lightning measurement systems to collect data during thunderstorm penetration flights. During 69 thunderstorm penetrations, there were 10 direct lightning strikes to the aircraft. No problems were encountered with any of the aircraft's systems as a result of the strikes and the research instrumentation performed as designed. Electromagnetic characteristics of nine strikes were recorded, and the results of other experiments confirm the theory that X-ray radiation and nitrous oxide gas are being produced by processes associated directly with thunderstorm electric fields and lightning discharges. A better understanding of aircraft lightning attachment mechanisms and strike zones is being accomplished by careful inspection, identification, and documentation of lightning attachment points and swept stroke paths following each strike to the aircraft.

Deal, P. L.↗

NASA NTRS1980

Thunderstorm Overflight Program

The Thunderstorm Overflight Program is being conducted by NASA, NOAA, and universities to evaluate the feasibility of making meaningful measurements of lightning parameters from an orbiting platform above thunderstorms. A NASA instrumented U-2 high-altitude research aircraft was used during the summer of 1979 and spring of 1980 to collect data over the tops of the thunderstorms while ground-based measurements were being made simultaneously. Test sites at Langmuir Laboratory, Socorro, N. Mex., and the National Severe Storms Laboratory, Norman, Okla. were used for this program. Additional flights are planned for the spring and summer of 1981. Data from the NASA U-2 flights will also be used to interpret measurements made during the Nighttime/Daytime Optical Survey Lightning Experiment to be flown on the Space Shuttle in late 1981.

Vaughan, O. H., Jr.↗

NASA NTRS1982

Maxwell currents under thunderstorms

Time variations observed in thunderstorm electric fields may be interpreted in terms of a total Maxwell current density, varying slowly with time in the intervals between lightning discharges, which can be used to estimate and map thunderstorms. Using the quasi-static behavior of the Maxwell current density, an expression is derived for the field-dependent current density under a thunderstorm during the field recovery following a lightning discharge. Values of air conductivity under the small storm which range from 2 to 6 x 10 to the -13th mho/m are inferred. Data are presented which indicate that the area-average Maxwell current is not usually affected by lightning, and instead varies slowly throughout the evolution of the storm. In light of this, it is suggested that cloud electrification processes probably do not depend on the cloud electric field as much as on the more slowly varying storm dynamics and meteorological structure.

Krider, E. P.↗

NASA NTRS1983

Thunderstorm top structure observed by aircraft overflights with an infrared radiometer

Thunderstorm top structure is examined with high spatial resolution radiometric data (visible and infrared) from aircraft overflights together with other storm views, including geosynchronous satellite observations. Results show that overshooting cumuliform towers appear as distinct cold areas in the high resolution, 11-micron IR aircraft images, but that the geosynchronous satellite observations significantly overestimate the thunderstorm-top IR brightness temperature, T(B), due to field of view effects. Profiles of cloud top height and T(B) across overshooting features indicate an adiabatic cloud surface lapse rate. However, one-dimensional cloud model results indicate that when comparing thunderstorm top temperature and height at different times or different storms, a temperature-to-height conversion of about 7 K/km is appropriate. Examination of mature storm evolution indicates that, during periods when the updraft is relatively intense, the satellite IR 'cold point' is aligned with the low-level radar reflectivity maximum, but during periods of updraft weakening and lowering cloud top heights, the satellite T(B) minimum occurs downwind with cirrus anvil debris. The growth period of a relatively weak cumulonimbus cluster is also examined with aircraft and satellite data.

Adler, R. F.↗

NASA NTRS1985

Detection of severe Midwest thunderstorms using geosynchronous satellite data

In the present exploration of the effectiveness of severe thunderstorm detection in the Midwestern region of the U.S. by means of approximately 5-min interval geosynchronous satellite data, thunderstorms are defined in IR data as points of relative minimum in brightness temperature T(B) having good time continuity and exhibiting a period of rapid growth. The four parameters of rate of T(B) decrease in the upper troposphere and stratosphere, isotherm expansion, and storm lifetime minimum T(B), are shown to be statistically related to the occurrence of severe weather on four case study days and are combined into a Thunderstorm Index which varies among values from 1 to 9. Storms rating higher than 6 have a much higher probability of severe weather reports, yielding a warning time lead of 15 min for hail and 30 min for the first tornado report.

Adler, R. F.↗

NASA NTRS1985

The interaction of a dipolar thunderstorm with its global electrical environment

The role of the thundercloud in the global electric circuit has been considered by many researchers. Thus, Holzer and Saxon (1952) have constructed a simple model of a bipolar thunderstorm. The global models considered provide insight into the atmospheric electric circuit but are restricted, both by various analytical mathematical representations and by computer size, to a grid of about five degrees in latitude and longitude. A need exists, therefore, for the development of a numerical regional model capable of resolving small-scale phenomena so that their coupling into the global-scale circuit can be examined. The construction of a two-dimensional quasi-static numerical model of atmospheric electricity is discussed. The model provides a basis for the calculation of the global electric field and current distribution produced by a single thunderstorm generator. In connection with the calculations, the thunderstorm was defined by a quasi-static current source function which generates a dipole charge configuration.

Tzur, I.↗

NASA NTRS1985

Thunderstorm currents and lightning charges at the NASA Kennedy Space Center

The NASA Kennedy Space Center, (KSC) and the U.S. Air Force are currently operating an extensive network of ground-based electric-field mills in order to detect atmospheric electrical hazards to ground operations, launches, and landings of spacecraft. Over the past 10 years, the University of Arizona has utilized data provided by this network, and other instrumentation, to investigate the electrical structure of thunderstorms and lightning in Florida. Recent analyses have included estimates of the Maxwell current density that thunderstorms produce at the ground and computations of the locations and magnitudes of lightning-caused changes in the cloud charge distribution. This work is reviewed, and an example of how thunderstorm current patterns and lightning charges develop in both space and time is presented.

Krider, E. P.↗

NASA NTRS1987

Automated detection of severe thunderstorms using geosynchronous infrared data

A procedure to identify thunderstorms from short-interval (5 min) GOES infrared (IR) data and to estimate their intensity (updraft strength) in an automated fashion is described. Thunderstorms are identified in the IR field by a simple pattern-recognition approach. For mature thunderstorms, two key parameters are used to estimate intensity: the amount of penetration above the neutral point and the occurrence (or not) of a cold-warm couplet in the IR TB field. Based on cloud model results, the amount of penetration is related to the maximum updraft speed, which is converted to an index. Application of the algorithm to a case study is presented.

Adler, Robert F.↗

NASA NTRS1987

Short range forecasting of sea breeze generated thunderstorms at the Kennedy Space Center: A real-time experiment using a primitive equation mesoscale numerical model

The operational efficiency of using guidance from a mesoscale numerical model to improve sea breeze thunderstorm forecasts at and around the Shuttle landing strip was assessed. The Prognostic Three-Dimensional Mesoscale (P3DM) model, developed as a sea breeze model, reveals a strong correlation between regions of mesoscale convergence and the triggering of sea breeze convection thunderstorms. The P3DM was modified to generate stability parameters familiar to the operational forecaster. In addition to the mesoscale fields of wind, vertical motion, moisture, temperature, a stability indicator, a combination of model-predicted K and Lifted Indices and the maximum grid cell vertical motion, were proposed and tested. Results of blind tests indicate that a forecaster, provided with guidance derived from model output, could improve local thunderstorm forecasts.

Lyons, Walter A.↗

NASA NTRS1988

Finite element simulation of thunderstorm electrodynamics in the proximity of the storm

Observations of electric fields, Maxwell current density, and air conductivity over thunderstorms were presented. The measurements were obtained using electric field mils and conductivity probes installed on a U2 aircraft as the aircraft passed approximately directly over an active thunderstorm at an altitude of 18 to 20 km. Accurate electrical observations of this type are rare and provide important information to those involved in numerically modeling a thunderstorm. A preliminary set of computer simulations based on this data were conducted and are described. The simulations show good agreement with measurements and are used to infer the thundercloud's charging current and amount of charge exchanged per flash.

Baginski, Michael Edward↗

NASA NTRS1989

A study of turbulence near thunderstorm tops

It has been known for many years that turbulence significant for aviation often occurs near thunderstorm tops. However, that turbulence is not well-predicted because of an incomplete understanding of the processes which generate it and because of inadequate observations. The current study seeks to alleviate these problems via: (1) a comprehensive review of recent theoretical and experimental studies related to turbulence near thunderstorm tops (TNTT), and (2) three case studies designed to examine the feasibility of using data derived from commercial aircraft to study TNTT. The literature review revealed extensive evidence which showed that convection often produces significant barrier effects; several mesoscale phenomena capable of producing turbulence may occur, depending on wind and stability conditions near the thunderstorm tops. These include two- and three-dimensional lee waves, rotors, Kelvin-Helmholtz instabilities, and Karman vortices. Conventional meteorological data were combined with data derived from the aircraft flight tapes to produce quantitative descriptions of the turbulence and its mesoscale environment for the three cases.

Pantley, Kim Christine↗

NASA NTRS1989

Thunderstorm related variations in stratospheric conductivity measurements

The vector electric field and polar conductivities were measured by zero-pressure balloon-borne payloads launched from Wallops Island, Virgina during the summers of 1987 and 1988. Data were collected over thunderstorms (or electrified clouds) during 6-hour flights at altitudes near 30 km. The vector electric field measurements were made with the double Langmuir probe high-impedance method, and the direct conductivity measurements were obtained with the relaxation technique. Evidence is presented for conductivity variations over thunderstorms (or electrified clouds). It is found that both positive and negative polar conductivity data do show variations of up to a factor of 2 from ambient values associated with the disturbed periods. Some ideas for possible physical mechanisms which may be responsible for the conductivity variations over thunderstorms are also discussed in this paper.

Hu, Hua↗

NASA NTRS1991

Thunderstorm monitoring and lightning warning, operational applications of the Safir system

During the past years a new range of studies have been opened by the application of electromagnetic localization techniques to the field of thunderstorm remote sensing. VHF localization techniques were used in particular for the analysis of lightning discharges and gave access to time resolved 3-D images of lightning discharges within thunderclouds. Detection and localization techniques developed have been applied to the design of the SAFIR system. This development's main objective was the design of an operational system capable of assessing and warning in real time for lightning hazards and potential thunderstorm hazards. The SAFIR system main detection technique is the long range interferometric localization of thunderstorm electromagnetic activity; the system performs the localization of intracloud and cloud to ground lightning discharges and the analysis of the characteristics of the activity.

Richard, Philippe↗