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All sourcesNASA NTRSDOE OSTI
NASA NTRS2013

Reply to Comment by S. E. Yuter et al. on 'Why do Tornados and Hailstorms Rest on Weekends?'

We show here that none of the concerns of Yuter et al. [2013, hereinafter Y2013] have any validity. We use this opportunity to clarify issues that may have been misunderstood by some readers (Y2013 among them) of Rosenfeld and Bell [2011, hereinafter RB2011], elaborate on our explanations there and further substantiate the evidence showing the impacts of aerosols on severe convective storms and the ways by which they are manifested in the weekly cycle. [2] Y2013 raise two general questions concerning the evidence for a weekly cycle of tornadoes given by RB2011: (1) whether the statistical analysis is valid, and (2) whether the discussion of physical mechanisms that explain the weekly cycle is correct. [3] In section 2, we show that there is no basis for the criticisms of Y2013 concerning the physical mechanisms proposed in RB2011 behind the observed changes in tornado activity and show further the mechanism by which aerosols can modulate tornadoes. Most of the comments by Y2013 appear to result from a misunderstanding and misinterpretation of RB2011. In section 3, we show that the comments by Y2013 concerning the statistical analysis are either incorrect or irrelevant. Some of the questions they raised were already answered previously by RB2011, and references therein. We also show that the complaint of Y2013, that the spatial averaging used in RB2011 inappropriately blends tornado behavior of different kinds in different regions, is unfounded.

severe storms↗
NASA NTRS1972

Tornado occurrences related to overshooting cloud-top heights as determined from ATS pictures

A sequence of ATS 3 pictures including the development history of large anvil clouds near Salina, Kansas was enlarged by NASA into 8X negatives which were used to obtain the best quality prints by mixing scan lines in 8 steps to minimize checker-board patterns. These images resulted in the best possible resolution, permitting use to compute the heights of overshooting tops above environmental anvil levels based on cloud shadow relationships along with the techniques of lunar topographic mapping. Of 39 heights computed, 6 were within 15 miles of reported positions of 3 tornadoes. It was found that the tornado proximity tops were mostly less than 5000 ft, with one exception of 7000 ft, suggesting that tornadoes are most likely to occur when overshooting height decreases. In order to simulate surface vortices induced by cloud-scale rotation and updraft fields, a laboratory model was constructed. The model experiment has shown that the rotation or updraft field induces a surface vortex but their combination does prevent the formation of the surface vortex. This research leads to a conclusion that the determination of the cloud-top topography and its time variation is of extreme importance in predicting severe local storms for a period of 0 to 6 hours.

Fujita, T. T.↗
NASA NTRS1974

Ionospheric S-shaped Doppler fluctuations produced by the tornadoes

A three-dimensional nine element HF-CW Doppler sounder array has been used to detect ionospheric disturbances which may be due to tornadoes. The typical events chosen in the present study occurred on November 20 and 27, 1973. Both events are apparently associated with tornadoes sighted in the Huntsville, Alabama area. The Doppler records show S-shaped waves rather than the quasi-sinusoidal waves observed in conjunction with and apparently due to thunderstorms. The wave-periods are in the range of 6 to 8 minutes instead of the 3 to 5 minute periods associated with thunderstorms. Dissipation of waves is mostly due to the evanescent effect and they cannot propagate very far from the path of the tornado center. A theory is presented which is in good agreement with the observations.

Hung, R. J.↗
NASA NTRS1975

Overshooting top behavior of three tornado-producing thunderstorms

The behavior of overshooting tops and jumping cirrus observed in three tornado-producing thunderstorms during the 1974 Learjet Cloud-Truth experiment is discussed. An investigation of temporal changes in the heights of overshooting domes (conglomerations of overshooting tops with diameters less than 1 km) reveals several distinctive features associated with tornadic events. There is a gradual decrease in dome height prior to tornado touchdown. Minimum dome activity occurred 5 min after, 5.5 min before, and at approximately the same time as the tornadic event in the storms observed. In all cases, dramatic dome growth at a rate of 17 to 23 m/sec immediately followed the occurrence of the minimum dome heights. There is evidence that tornado production is insensitive to the pre-touchdown maximum dome heights between 1 and 3 km.

Umenhofer, T. A.↗
NASA NTRS1977

Great Bend tornadoes of August 30, 1974

Photogrammetric analyses of movies and still pictures taken of the Great Bend, Kansas Tornado series have been used to develop design specifications for nuclear power plants and facilities. A maximum tangential velocity of 57 m/sec and a maximum vertical velocity of 27 m/sec are determined for one suction vortex having a translational velocity of 32 m/sec. Three suction vortices with radii in the 20 to 30 m range are noted in the flow field of one tornado; these suction vortices apparently form a local convergence of inflow air inside the outer portion of the tornado core.

Umenhofer, T. A.↗
NASA NTRS1979

Coupling of ionosphere and troposphere during the occurrence of isolated tornadoes on November 20, 1973

The paper examines the coupling between the ionosphere and the troposphere during time periods with isolated tornadoes on the stormy day of November 20, 1973. Observations are made with a high-frequency CW Doppler array system, in which radio receivers located at a central site monitored signals transmitted from three independent remote sites on three sets of frequencies (4.0125, 4.759, 5.734 MHz) and reflected off the ionosphere approximately halfway between the transmitter and receiver sites. It is shown that the sources of the gravity waves associated with tornadoes are always on the squall lines and near the tornado touchdown locations, and that analyses of ionospheric Doppler sounder observations of medium-scale gravity waves can contribute to the understanding of the coupling between the ionosphere and the troposphere during periods of severe storm activity.

Hung, R. J.↗
NASA NTRS1987

US tornadoes. Part 1: 70-year statistics

If tornadoes were counted as the Gross National Product, no other country on the surface of the earth could come even close to the United States. During the recent 70 year period, the United States produced 31,054 tornadoes which left behind a cumulative path of 132,005 miles (212,396 km) which would circle the world 5.3 times along the equator. In completing the book, staff members of the Satellite and Mesometeorlogy Research Project (1961 to the present) played an important role in collecting, evaluating, and archiving the historical tornado data.

Fujita, T. T.↗
NASA NTRS1994

Mars vertical axis wind machines: The design of a tornado vortex machine for use on Mars

Ever since Viking 1 and 2 landed on the surface of Mars in the summer of 1976, man has yearned to go back. But before man steps foot upon the surface of Mars, unmanned missions such as the Martian Soft Lander and Martian Subsurface Penetrator will precede him. Alternative renewable power sources must be developed to supply the next generation of surface exploratory spacecraft, since RTG's, solar cells, and long-life batteries all have their significant drawbacks. One such alternative is to take advantage of the unique Martian atmospheric conditions by designing a small scale, Martian wind power generator, capable of surviving impact and fulfilling the long term (2-5 years), low-level power requirements (1-2 Watts) of an unmanned surface probe. After investigation of several wind machines, a tornado vortex generator was chosen based upon its capability of theoretically augmenting and increasing the available power that may be extracted from average Martian wind speeds of approximately 7.5 m/s. The Martian Tornado Vortex Wind Generator stands 1 meter high and has a diameter of 0.5 m. Martian winds enter the base and shroud of the Tornado Vortex Generator at 7.5 m/s and are increased to an exit velocity of 13.657 m/s due to the vortex that is created. This results in a rapid pressure drop of 4.56 kg/s(exp 2) m across the vortex core which aids in producing a net power output of 1.1765 Watts. The report contains the necessary analysis and requirements needed to feasibly operate a low-level powered, unmanned, Martian surface probe.

Carlin, Daun↗
NASA NTRS1999

Cloud-to-Ground Lightning Characteristics of a Major Tropical Cyclone Tornado Outbreak

A comprehensive analysis has been conducted of the cloud-to-ground lightning activity occurring within a landfalling tropical cyclone that produced an outbreak of strong and damaging tornadoes. Radar data indicate that 12 convective cells were responsible for 29 tornadoes, several of which received an F3 intensity rating, in the southeastern United States on 16 August 1994 within the remnants of Tropical Storm Beryl. Of these 12 tornadic storms, the most active cell produced 315 flashes over a 5.5 hour period, while the other storms were less active. Three tornadic storms failed to produce any CG lightning at all. In general, the tornadic storms were more active electrically than other non-tornadic cells within Beryl's remnants, although the flash rates were rather modest by comparison with significant midlatitude severe storm events. Very few positive polarity flashes were found in the Beryl outbreak. During some of the stronger tornadoes, CG flash rates in the parent storms showed sharp transient decreases. Doppler radar data suggest the stronger tornadic storms were small supercells, and the lightning data indicate these storms exhibited lightning characteristics similar to those found in heavy-precipitation supercell storms.

McCaul, Eugene W., Jr.↗
NASA NTRS1976

Tornado Mechanics

The Nimbus and other weather satellites are helping determine why and how tornadoes form their structure and dynamics and ultimately how they can be prevented or artificially dissipated. NASA's Marshall Space Flight Center is also planning a cooperative research program later this year with the University of Arkansas to investigate how tornado damage occurs, and to develop tornado resistant building designs. Hardware and field-data collection are funded by the Technology Utilization Office while data reduction is being performed by the National Oceanic and Atmospheric Administration.

Source record↗
NASA NTRS1976

Luminous electrical phenomena in Huntsville, Alabama, tornadoes on April 3, 1974

Unusual lightning and varicolored luminous phenomena were observed on the evening of April 3, 1974, when severe tornadoes passed through Madison County, Alabama. Photographs and eyewitness accounts of this electrical activity are related to the trajectories of the tornadoes and the damage areas they produced.

Vaughan, O. H., Jr.↗
NASA NTRS1976

Tornadoes and other atmospheric vortices

The growth of random vortices in an atmosphere with buoyant instability and vertical wind shear is studied along with the velocities in a single gravity-driven vortex; a frictionless adiabatic model which is supported by laboratory experiments is first considered. The effects of axial drag, heat transfer, and precipitation-induced downdrafts are then calculated. Heat transfer and axial drag tend to have stabilizing effects; they reduce the downdrafts of updrafts due to buoyancy. It is found that downdrafts or tornadic magnitude might occur in negatively-buoyant columns. The radial-inflow velocity required to maintain a given maximum tangential velocity in a tornado is determined by using a turbulent vortex model. Conditions under which radial-inflow velocities become sufficiently large to produce tangential velocities of tornadic magnitude are determined. The radial velocities in the outer regions, as well as the tangential velocities in the inner regions may be large enough to cause damage. The surface boundary layer, which is a region where large radial inflows can occur, is studied, and the thickness of the radial-inflow friction layer is estimated. A tornado model which involves a rotating parent cloud, as well as buoyancy and precipitation effects, is discussed.

Deissler, R. G.↗
NASA NTRS1976

Numerical simulation of tornado wind loading on structures

A numerical simulation of a tornado interacting with a building was undertaken in order to compare the pressures due to a rotational unsteady wind with that due to steady straight winds used in design of nuclear facilities. The numerical simulations were performed on a two-dimensional compressible hydrodynamics code. Calculated pressure profiles for a typical building were then subjected to a tornado wind field and the results were compared with current quasisteady design calculations. The analysis indicates that current design practices are conservative.

Maiden, D. E.↗
NASA NTRS1976

Reduction and analysis of data collected during the electromagnetic tornado experiment

Techniques for data processing and analysis are described to support tornado detection by analysis of radio frequency interference in various frequency bands, and sea state determination from short pulse radar measurements. Activities include: strip chart recording of tornado data; the development and implementation of computer programs for digitalization and analysis of the data; data reduction techniques for short pulse radar data, and the simulation of radar returns from the sea surface by computer models.

Davisson, L. D.↗
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 NTRS1978

Observation of gravity waves during the extreme tornado outbreak of 3 April 1974

A continuous wave-spectrum high-frequency radiowave Doppler sounder array was used to observe upper-atmospheric disturbances during an extreme tornado outbreak. The observations indicated that gravity waves with two harmonic wave periods were detected at the F-region ionospheric height. Using a group ray path computational technique, the observed gravity waves were traced in order to locate potential sources. The signals were apparently excited 1-3 hours before tornado touchdown. Reverse ray tracing indicated that the wave source was located at the aurora zone with a Kp index of 6 at the time of wave excitation. The summation of the 24-hour Kp index for the day was 36. The results agree with existing theories (Testud, 1970; Titheridge, 1971; Kato, 1976) for the excitation of large-scale traveling ionospheric disturbances associated with geomagnetic activity in the aurora zone.

Hung, R. J.↗
NASA NTRS1979

Analytic studies on satellite detection of severe, two-cell tornadoes

From funnel-cloud-length interpretation, the severe tornado is characterized by peak swirl speed relative to the axis of rotation of about 90 m/s. Thermohydrodynamic achievement of the pressure deficit from ambient necessary to sustain such swirls requires that a dry, compressionally heated, non-rotating downdraft of initially tropopause-level air lie within an annulus of rapidly swirling, originally low-level air ascending on a near-moist-adiabatic locus of thermodynamic states. The two-cell structure furnishes an observable parameter possibly accessible to a passively instrumented, geosynchronous meteorological satellite with mesoscale resolution, for early detection of a severe tornado. Accordingly, the low-level turnaround region, in which the surface inflow layer separates to become a free ascending layer and for which inviscid modeling suffices, is examined quantitatively. Preliminary results indicate that swirl overshoot, i.e., swirl speeds in the turnaround region in excess of the maximum achieved in the potential vortex, is modest.

Carrier, G. F.↗