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NASA NTRS1974

Infrasonic waves in the ionosphere during severe thunderstorms

The coupling between the lower and upper atmosphere during severe weather and thunderstorms is investigated from the ionospheric Doppler frequency fluctuations recorded using a three dimensional CW Doppler sounder array. Two events typical of the severe thunderstorms that occurred on May 26, 1973 and March 20, 1974, are chosen for our investigation. The CW Doppler records show a wave-like disturbance with a periodicity of 3 to 5 minutes that persisted for 3 to 4 hours. A possible explanation, based on a theoretical model is presented. There is good agreement between the experimental observations and the model.

Rao, G. L.↗

NASA NTRS1978

Turbulent transport model of wind shear in thunderstorm gust fronts and warm fronts

A model of turbulent flow in the atmospheric boundary layer was used to simulate the low-level wind and turbulence profiles associated with both local thunderstorm gust fronts and synoptic-scale warm fronts. Dimensional analyses of both type fronts provided the physical scaling necessary to permit normalized simulations to represent fronts for any temperature jump. The sensitivity of the thunderstorm gust front to five different dimensionless parameters as well as a change from axisymmetric to planar geometry was examined. The sensitivity of the warm front to variations in the Rossby number was examined. Results of the simulations are discussed in terms of the conditions which lead to wind shears which are likely to be most hazardous for aircraft operations.

Lewellen, W. S.↗

NASA NTRS1978

Investigations of simulated aircraft flight through thunderstorm outflows

The effects of wind shear on aircraft flying through thunderstorm gust fronts were investigated. A computer program was developed to solve the two dimensional, nonlinear equations of aircraft motion, including wind shear. The procedure described and documented accounts for spatial and temporal variations of the aircraft within the flow regime. Analysis of flight paths and control inputs necessary to maintain specified trajectories for aircraft having characteristics of DC-8, B-747, augmentor wing STOL, and DHC-6 aircraft was recorded. From the analysis an attempt was made to find criteria for reduction of the hazards associated with landing through thunderstorm gust fronts.

Frost, W.↗

NASA NTRS1977

Study of stratospheric-ionospheric coupling during thunderstorms and tornadoes

A continuous-wave-spectrum high-frequency Doppler sounder array with three transmitters at each of three sites was used to observe the dynamics of the coupling of energy between the stratosphere and the ionosphere. During times of severe weather activity wavelike disturbances have been detected on ground-based ionospheric sounding records as perturbations in electron densities. Infrasonic waves with wave periods of 3-7 min and with horizontal phase velocities of 600-800 m/s were observed when there was thunderstorm activity; gravity waves with wave periods of 10-15 min and horizontal phase velocities of 100-200 m/s were detected when there was tornado activity. Both triangulations from the cross correlation functions of the Doppler records based on an assumption of no background wind shear and ray-tracing computations including an assumed background wind shear indicate that the waves originated in the vicinity of the thunderstorms and tornadoes. A comparison of the wavelengths of the infrasonic and gravity waves observed at ionospheric heights and those in cloud-top pictures from satellites show that they are all of the order of 100-300 km.

Hung, R. J.↗

NASA NTRS1977

Turbulent transport model of a thunderstorm gust front

A second-order closure turbulent model is adopted to predict complex fluid flows; few a priori assumptions about the internal turbulent and mean flow structures are required. The flow simulations are applied to the analysis of local gust fronts generated by rain-cooled outflows from severe thunderstorms. Results of the simulations provide a means for discussing the behavior of an aircraft passing through a thunderstorm-related downdraft.

Teske, M. E.↗

NASA NTRS1979

Thunderstorm vertical velocities and mass flux estimated from satellite data

Infrared geosynchronous satellite data with an interval of five minutes between images are used to estimate thunderstorm top ascent rates on two case study days. A mean vertical velocity of 3.5/ms for 19 clouds is calculated at a height of 8.7 km. This upward motion is representative of an area of approximately 10km on a side. Thunderstorm mass flux of approximately 2x10 to the 11th power/gs is calculated, which compares favorably with previous estimates. There is a significant difference in the mean calculated vertical velocity between elements associated with severe weather reports (w bar=4.6/ms) and those with no such reports (2.5/ms). Calculations were made using a velocity profile for an axially symmetric jet to estimate the peak updraft velocity. For the largest observed w value of 7.8/ms the calculation indicates a peak updraft of approximately 50/ms.

Adler, R. F.↗

NASA NTRS1979

Thunderstorm vertical velocities estimated from satellite data

Infrared geosynchronous satellite data with an interval of 5 min between images are used to estimate thunderstorm top ascent rates on two case study days. A mean vertical velocity of 3.4 m/sec for 23 clouds is calculated at a height of 8.7 km. This upward motion is representative of an area of approximately 10 km on a side. Thunderstorm mass flux of approximately 2 times 10 to the 8th power kg/sec is calculated, which compares favorably with previous estimates. There is a significant difference in the mean calculated vertical velocity between elements associated with severe weather reports (omega = 4.9 m/sec) and those with no such reports (2.4 m/sec). Calculations were made using a velocity profile for an axially symmetric jet to estimate the peak updraft velocity. For the largest observed omega value of 7.8 m/sec the calculation indicates a peak updraft of approximately 50 m/sec.

Adler, R. F.↗

NASA NTRS1979

Field studies of the electrification of thunderstorms

Many theories have been advanced to explain the development of electric fields in thunderstorms, culminating in lightning, but thorough appraisal of these has been hampered by the lack of reliable and comprehensive observational data on the electrical characteristics, microphysical properties and dynamical behavior of the storms. A major field experiment (the Thunderstorm Research International Project) has been in progress for three years, in an effort to remedy this deficiency, and this paper describes some of this work and the results emanating from it. Major tools in this investigation are: an instrumented aircraft capable of penetrating the clouds; dual-Doppler and fast scanning radars; field-change and precipitation-recording networks; and an acoustic system for reconstructing the location of points on the lightning channels. The early results indicate a strong correlation between updraughts, precipitation and high fields. Circumstantial evidence points towards the presence of ice as being crucial to rapid field growth.

Christian, H.↗

NASA NTRS1982

Global thunderstorm activity research survey

The published literature on the subject of the monitoring of global thunderstorm activity by instrumented satellites was reviewed. A survey of the properties of selected physical parameters of the thunderstorm is presented. The concepts used by satellites to identify and to measure terrestrial lightning pulses are described. The experimental data acquired by satellites are discussed. The scientific achievements of the satellites are evaluated against the needs of scientists and the potential requirements of user agencies. The performances of the satellites are rated according to their scientific and operational achievements.

Coroniti, S. C.↗

NASA NTRS1981

Operational evaluation of thunderstorm penetration test flights during project Storm Hazards '80

The National Aeronautics and Space Administration is conducting a research project called Storm Hazards '80 in order to study the prediction, detectability and avoidance of the hazards of severe storms for aircraft operations. The project, using a highly instrumented NASA F-106B airplane to penetrate thunderstorms, gathered and correlated data from both airborne and ground based instrumentation. The objectives of this project are to determine the effects of lightning on the design and operation of aircraft composite structures and digital electronic systems. The data will be used to determine the correlation of lightning hazards with other severe storm hazards, such as heavy precipitation, hail, turbulence and wind-shear, in order to develop an initial data base for use in design and avoidance. The NASA F-106B was equipped with a weather radar, stormscope, lightning measurement instrumentation and air sampling equipment. This paper will focus on the operational aspects of thunderstorm penetrations and the pilot techniques used to avoid the extremely hazardous portions of the storm such as the tornadoes and hail. It will deal with the effects of the storm elements on the aircraft hardware, avionics and the crew.

Keyser, G. L., Jr.↗

NASA NTRS1982

The structure and dynamics of mesoscale systems influencing severe thunderstorm development during AVE/SESAME 1

Relationships between meso-beta scale systems and thunderstorm formation were examined as part of the NASA atmospheric variability experiment/severe environmental storms and mesoscale experiment 1979. The McIdas program was employed for meso-beta scale analyses of atmospheric structure and dynamics in kinematic computations of the Abilene Triangle on a grid mesh of 100 km for station spacing of 275 km. Mesoscale short wave systems were detected imbedded and propagating cyclonically around upper-level vortex circulation and creating environmental conditions conducive to thunderstorm development. TIROS-N and GOES satellite data served to connect the systems with two convective storms which developed. The necessity to use spaceborne instrumentation carried on the Shuttle or on free-flying satellites for enhancing the data-base on storm development is noted.

Wilson, G. S.↗

NASA NTRS1983

Thunderstorm observations from Space Shuttle

Results of the Nighttime/Daytime Optical Survey of Lightning (NOSL) experiments done on the STS-2 and STS-4 flights are covered. During these two flights of the Space Shuttle Columbia, the astronaut teams of J. Engle and R. Truly, and K. Mattingly II and H. Hartsfield took motion pictures of thunderstorms with a 16 mm cine camera. Film taken during daylight showed interesting thunderstorm cloud formations, where individual frames taken tens of seconds apart, when viewed as stereo pairs, provided information on the three-dimensional structure of the cloud systems. Film taken at night showed clouds illuminated by lightning with discharges that propagated horizontally at speeds of up to 10 to the 5th m/sec and extended for distances on the order of 60 km or more.

Vonnegut, B.↗

NASA NTRS1982

Lightning and surface rainfall during Florida thunderstorms

Lightning and surface rainfall data are presented which were obtained during summer air mass thunderstorms at the NASA Kennedy Space Center. Attention is given to a computer algorithm which employed abrupt changes in the thundercloud electric fields to detect and count flashes. Statistics are given for the occurrence of lightning in 79 storms during the summer seasons of 1976-1980, as well as 28 lightning storms from the summers of 1977 and 1978. The relationship between lightning and rainfall is examined in the case of two thunderstorms whose locations allow a direct comparison of measurements. It is found that when meteorological conditions favor the production of lightning, there is an almost direct proportionality between the total rain volume and the total number of flashes.

Piepgrass, M. V.↗

NASA NTRS1982

Stereoscopic observations of hurricanes and tornadic thunderstorms from geosynchronous satellites

Results are presented which show the application of GOES stereoscopy to the investigation of hurricanes and tornadic thunderstorms. Stereo cloud top height contour maps were constructed to observe the structural evolution of two hurricanes, Frederic on September 12, 1979 and Allen on August 8, 1980, and a tornadic thunderstorm complex over Oklahoma on May 2-3, 1979. Stereo height contours of Hurricane Allen reveal a very intense and symmetric storm with a circular shaped central dense overcast with an average height of 16.5 km. Contours of Hurricane Frederic reveal a preferred region for convection with an explosive exhaust tower attaining a maximum height of 17.8 km. Also presented is a technique for estimating tropical cyclone intensity using GOES stereo height and IR temperature information. Results indicate vertical motions ranging from 4.4 m/s for a moderate storm to 7.7 m/s for an intense storm.

Mack, R.↗

NASA NTRS1982

Estimating the temperature and height of overshooting thunderstorm tops from geostationary satellite infrared data

Information is presented on the extent to which the brightness temperature is overestimated. A revised version of the technique proposed by Adler and Markus (1982) for overcoming this problem is discussed, together with aspects of its application. GOES IR thunderstorm top observations are compared with high-resolution data. The technique described here to estimate the small-scale cloud top temperature from the GOES data corrects for most of the field-of-view effect but requires additional analysis and testing. When applied to a time sequence of a growing thunderstorm, the calculated cloud top temperature time profile is found to agree very well with a similar profile derived from stereo measurements.

Adler, R. F.↗

NASA NTRS1984

Nighttime observations of thunderstorm electrical activity from a high altitude airplane

Nocturnal thunderstorms were observed from above and features of cloud structure and lightning which are not generally visible from the ground are discussed. Most, lightning activity seems to be associated with clouds with strong convective cauliflower tops. In both of the storms lightning channels were visible in the clear air above the cloud. It is shown that substances produced by thunderstorm electrical discharges can be introduced directly into the stratosphere. The cause and nature of the discharges above the cloud are not clear. They may be produced by accumulations of space charge in the clear air above the cloud. The discharges may arise solely because of the intense electric fields produced by charges within the cloud. In the latter case the ions introduced by these discharges will increase the electrical conductivity of the air above the cloud and increase the conduction current that flows from the cloud to the electrosphere. More quantitative data at higher resolution may show significant spectral differences between cloud to ground and intracloud strokes. It is shown that electric field change data taken with an electric field change meter mounted in an airplane provide data on lightning discharges from above that are quite similar to those obtained from the ground in the past. The optical signals from dart leaders, from return strokes, and from continuing currents are recognizable, can be used to provide information on the fine structure of lightning, and can be used to distinguish between cloud to ground and intracloud flashes.

Brook, M.↗

NASA NTRS1984

Conditions for lightning strikes to an airplane in a thunderstorm

Conditions conducive to lightning strikes on aircraft were determined through the 1982 Storm Hazards Program. UHF-radar and SPANDAR were used to guide a NASA F-106B research aircraft through the upper regions of active thunderstorms to facilitate direct lightning strikes to the aircraft. Analysis of radar echoes from lightning at the moments of the strikes suggests that the aircraft itself triggers the lightning, in both stormy and nonstormy clouds, and that the induced flashes are not much different from naturally occurring intra-cloud flashes. The highest risk for a direct strike in the upper portions of a thunderstorm occurred where the ambient temperature was -40 C or colder, where turbulence and precipitation were negligible to light, and where the lightning flash rate was no greater than 5 flashes per minute.

Mazur, V.↗

NASA NTRS1984

Investigations of severe/tornadic thunderstorm development and evolution based on satellite and AVE/SESAME/VAS data

The use of rapid scan satellite imagery to investigate the local environment of severe thunderstorms is discussed. Mesoscale cloud tracking and vertical wind shear as it affects thunderstorm relative flow are mentioned. The role of pre-existing low level cloud cover in the outbreak of tornadoes was investigated. Applying visible atmospheric sounding imagery to mesoscale phenomena is also addressed.

Vonderhaar, T. H.↗