Showing papers on "Thunderstorm published in 1979"

Severe Thunderstorm Evolution and Mesocyclone Structure as Related to Tornadogenesis

Abstract: Severe thunderstorm evolution is synthesized, using published and unpublished studies of radar, instrumented aircraft, visual and surface observations. These observations reveal the existence of a downdraft (originating at 7–10 km AGL) on the relative upwind side of the updraft. Air decelerates at the upwind stagnation point, is forced downward and mixes with air below which then reaches the surface through evaporative cooling and precipitation drag. The initially rotating updraft is then transformed into a new mesocyclone with a divided structure, in which the circulation center lies along the zone separating the rear blank downdraft from the updraft. This process appears to result, in part, from tilting of horizontal vorticity into the vertical. It is proposed that the zone of strong vertical velocity gradient across which the mesocyclone comes to be positioned is also characterized by a strong temperature gradient and is the genesis region of strong tornadoes. Although no direct observations a...

Doppler weather radar

Abstract: The Doppler weather radar and its signals are examined from elementary considerations to show the origin and development of useful weather echo properties such as signal-to-noise ratio (SNR), range correlation, signal statistics, etc. We present a form of the weather radar equation which explicitly shows the echo power loss due to finite receiver bandwidth and how it is related to the range weighting function. Echoes at adjacent range samples have a correlation that depends on receiver bandwidth-transmitter pulsewidth product as well as sample spacing. Stochastic Bragg scatter from clouds is examined, but experimental work is required to determine if this echo power is larger than incoherently scattered power. Section III presents the relation between Doppler power spectrum and the distribution of reflectivity and velocity within a resolution volume. A new formula that relates spectrum width to the shear of radial velocities as well as turbulence, signal decorrelation from antenna rotation, and signal processing biases is presented. The estimation of power spectral moments is reviewed and properties of the most commonly used algorithms are discussed. Section V highlights some of the considerations that need to be made for Doppler radar observation of severe thunderstorms. Echo coherency is shown to limit the pulsed Doppler radar's unambiguous range and velocity measurements. Single and dual Doppler-radar techniques for wind measurements are reviewed. Observations of thunderstorms show tornado cyclones, and clear air measurements in the boundary layer reveal turbulence and waves.

A quasi-static model of global atmospheric electricity 1. The lower atmosphere

Abstract: A quasi-steady model of global lower atmospheric electricity is presented. The model considers thunderstorms as dipole electric generators that can be randomly distributed in various regions and that are the only source of atmospheric electricity and includes the effects of orography and electrical coupling along geomagnetic field lines in the ionosphere and magnetosphere. The model is used to calculate the global distribution of electric potential and current for model conductivities and assumed spatial distributions of thunderstorms. Results indicate that large positive electric potentials are generated over thunderstorms and penetrate to ionospheric heights and into the conjugate hemisphere along magnetic field lines. The perturbation of the calculated electric potential and current distributions during solar flares and subsequent Forbush decreases is discussed, and future measurements of atmospheric electrical parameters and modifications of the model which would improve the agreement between calculations and measurements are suggested.

Effects of coronae on electric fields beneath thunderstorms

Abstract: Intense electric fields beneath thunderstorms produce electrical discharges (coronae) at the tips of trees, bushes and other sharp objects attached to the surface of the earth. We find typical corona current densities of about 1 nA m−2 in an 8 kV m−1 field at the ground. The ions released into the air limit the magnitude of the field at the ground to about 10kV m−1. Our measurements beneath thunderstorms with a balloon-borne electric field meter show that the magnitude of the field a hundred metres above the ground is several times larger than at the ground; in one case the field 300 metres above ground was 6 times that at the ground. The substantial thickness of the space charge layer and the speed with which it vanishes when the electric field strength declines imply that the charge carriers have substantial velocities (0.4m s−1) either because their mobilities are high or because they are carried by air motions. Coronae also influence the time behaviour of the electric field at the ground. The field at the ground often changes very rapidly after a lightning flash. The rate of change decreases as the field approaches the value it had prior to the flash. In contrast, the field a hundred metres above the ground, which is often above most of the influence of space charge produced by coronae, increases more uniformly (linearly) during the time interval between lightning flashes. This behaviour is similar to that of the field farther aloft in the interior of the cloud. Our numerical simulations of the shapes of recovery curves indicate that the corona current density is more accurately described by a cubic function than by a quadratic function of the electric field strength at the ground. Despite strong influences of coronae, three properties of the field at the ground accurately reflect what happens above the space charge layer. First, the rapid changes in electric field during a lightning flash are not usually affected by corona space charge. Second, when the field at the ground is nearly constant it usually has the same polarity as the field above the space charge layer. And third, when the field strength at the ground is nearly zero, and when certain other conditions are met, the time rate of change of the field at the ground is the same as that above the space charge layer.

Global Lightning Flash Frequency

Abstract: Lightning flashes recorded in photographs by two satellites in the Defense Meteorological Satellite Program (DMSP) are used to calculate flash frequencies for land and ocean regions in 10° latitude belts between 60°S and 60°N for dusk and midnight local times Data are analyzed for the first week of each month between March 1974 and February 1975 We find that the annual land-ocean ratio of global lightning at dusk ranges from 8 to approximately 20, depending on whether lightning near the coast is judged to be from thunderstorms produced predominantly by ocean effects or land effects A similar analysis for data obtained near midnight yields ratios which range from 4 to 8 The global land-ocean lightning ratio is significantly higher during the northern summer than during the southern summer The dusk lightning flash frequency as a function of latitude peaks at 10–20°N during the northern summer and 0–10°N during the southern summer The midnight flash frequency peaks at 0–10°N throughout the yea

Doppler Radar and Radio Observations of Thunderstorms

Abstract: Three-dimensional motion fields in a thunderstorm are compared to measurements of the radar reflectivity structure of the storm and to the locations of VHF radiation from electrical discharges, in an attempt to determine the physical conditions which prevailed during the electrically active phase of the storm. The observations were obtained from a network of three Doppler radars and a long-baseline radiation source location system during the 1978 Thunderstorm Research International Program (TRIP 78) at Kennedy Space Center, FL. The analysis shows that two cells developed sequentially on the upshear side of the storm. In the second cell, the updraft velocity between 6- and 7-km altitude (-10 to -15°C) increased rapidly to greater than 20 ms-1; and coincided with a substantial increase in the VHF radiation burst rate. Growth of the updraft was accompanied by the simultaneous development of an upper level downdraft on its upshear side. Reflectivity values in excess of 50 dBZ were observed within the vicinity of the updraft, and have been attributed to graupel or hail that was suspended in the updraft and which grew by riming of supercooled water droplets. Initial source locations for each radiation burst formed an umbrella-like pattern above the high-reflectivity core. We interpret the results to be consistent with an ice-ice electrical charging interaction first investigated by Reynolds et al.

Thunderstorm Intensity as Determined from Satellite Data

Abstract: 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.

Automated 12–36 Hour Probability Forecasts of Thunderstorms and Severe Local Storms

Abstract: Probability equations were developed for 12–36 h forecasts of thunderstorms, severe local storms, and major or family tornado outbreaks. The equations were derived by applying screening regression techniques to relate manually digitized radar (MDR) data and severe storm reports to large-scale meteorological predictors obtained from numerical forecast models. The probability forecasts are prepared once daily on the NOAA computer system and are transmitted over facsimile and teletypewriter circuits. One of the more important innovations in the forecast equations was the development of an interactive predictor which takes into account the seasonal variations in thunderstorm occurrence, yet is responsive to the daily synoptic situation. This predictor is formed by combining the K stability index with daily mean thunderstorm relative frequencies estimated from MDR data. Local variations in thunderstorm occurrence over the MDR grid were also included in the generalized equation by incorporating probabi...

Thunderstorms as possible micrometeorological sink for stratospheric water

Abstract: The dryness of the stratosphere has been explained, in general terms, as water condensation from the rising branch of the Hadley cell at the tropical tropopause (Brewer, 1949), but Ellsaesser (1974) suggested that the mean tropical tropopause is not cold enough to account for the observed water vapor mixing ratios. During intense thunderstorms that in part penetrate the tropopause, investigators have observed (1) an increase in local stratospheric water vapor and (2) the temporary presence of air parcels substantially colder than and higher than the tropopause. These cold parcels are calculated to have extremely low water vapor mixing ratios, and their occurrence in the stratosphere suggests a mechanism whereby the effective condensation temperature could be systematically colder than the tropopause. Ice crystals from the cloud, evaporating in the warmer stratosphere, presumably cause the observed increase in water vapor, but mixing of cold desiccated air parcels with lower stratospheric air would tend to decrease its water content. Thus there are opposing factors concerning the role of severe cumulonimbus storms on stratospheric water, and it may require detailed, microphysical analysis to see which effect is larger.

Storm Motion as Related to Boundary-Layer Convergence

Abstract: Data presented indicate that severe thunderstorm areas may become anchored to intense, boundary- layer convergence zones. In such cases. the mean cloud-layer wind vector has little relation to the movement of regions of large hail, wall clouds and tornadoes. Suggestions for improving severe thunderstorm warnings are offered based on this possibility.

Lightning location and lower-ionospheric height determination from two-station magnetic field measurements

Abstract: Wide band magnetic direction finding from two stations is used to determine lightning locations and ionospheric height during two nocturnal maritime thunderstorms near Florida. A comparison of calculated lightning locations with radar reflectivity data of the two storms indicates systematic direction-finding errors to be on the order of 1 deg. From the lightning locations and measured arrival times of the first and second sky waves, ionospheric reflecting heights for both sky waves were determined. The effective heights for the first and second sky waves may differ, leading to large errors in single-station determinations of ionospheric height, such as in the method described by Kinzer (1974). The distribution of peak electric fields for the strokes studied is shown to be in reasonable agreement with other determinations, indicating the reliability of the present measurements.

A numerical study of tornadogenesis in a rotating thunderstorm

Abstract: The structure and dynamics of tornadoes and the mechanisms leading to their formation are explored in the context of an axi-symmetric numerical model of a tornado cyclone. The paper extends a recent study of tornadogenesis by the authors to include explicit representation of moisture effects. The calculations show that the distribution of buoyancy in the updraught of a severe ‘supercell’ thunderstorm can account for the generation and maintenance of an intense tornado when the level of rotation in the storm is within the observed range of values. The simulated vortex extends to high levels in the cloud but maximum swirling velocities occur within the lowest kilometre or two above the ground in concordance with recent Doppler radar observations. For a fixed vertical distribution of ambient temperature and moisture, the vortex strength is sensitive to the imposed circulation: if the latter is too small, the cloud updraught rotates weakly but there is no evidence of a vortex or a funnel cloud; if it is too strong, a vortex and its associated funnel cloud may form but terminate aloft. This explains why only a few apparently suitable thunderstorms spawn damaging tornadoes. The effects of surface friction on vortex strength and structure are studied in some detail and the results suggest that the terminating inflow layer at the ground may have a less important control on the vortex dynamics than has hitherto been supposed.

Some Observations of a Splitting Severe Thunderstorm

Abstract: A severe thunderstorm which formed in an environment not usually associated with severe weather split into right- and left-moving parts near Verden, Oklahoma, on 1 May 1977. The right-moving storm developed a mesocyclone signature and produced large hail. The storm's life is described from data from the NSSL single-Doppler radar, conventional radar, surface mesonetwork, and from intercept observations. The storm-split process and the evolution of couplets of vorticity are, highlighted. This is the first single-Doppler documentation of a splitting storm from first echo. Evidence was found of two separate organized updrafts and two mirror-image cyclonic/anticyclonic couplets 30 min before the low-level radar echo split. We conclude that a downdraft may have formed in between the updrafts, and that the storms behaved like the one simulated by Wilhelmson and Klemp (1978).

Radar Measurement of Precipitation-Related Depolarization in Thunderstorms

Abstract: Observations of thunderstorm regions above 4 km have been made by means of a two-channel polarization diversity radar at 16.5 GHz. There is a brief description of the radar equipment. Computer-generated plots of the radar data show regions of depolarization presumably originating in ice-crystal clouds, and abrupt changes in depolarization induced by lightning strokes. There is a discussion of the physical processes causing the various effects.

Preliminary observations of lightning radar echoes and simultaneous electric field changes

Abstract: Measurements of radar echoes from lightning discharges within intense thunderstorms in Oklahoma have been made with a 10 cm Doppler radar. Also measured was the electric field change associated with the lightning. The lightning echoes were analyzed by displaying echo intensity at a particular range versus time. Temporal comparisons could then be made with the electric field changes. Radar echoes were detected from both intracloud and cloud-to-ground discharges. Relatively abrupt increases in lightning radar echo intensity sometimes appeared related to the occurrence of return strokes, K-type electric field changes, and during continuing currents. The radar antenna beam width was 0.8°, and we undoubtedly often did not receive echoes from the entire discharge process during any given lightning flash. Lightning echoes were detected at ranges of 30-180 km and at altitudes of 0.8-9.8 km in regions of storms with reflectivity factors of up to 58 dBz. Lightning echoes were 10-25 dB stronger than the precipitation echo in which they occurred, and they generally were observed in regions where the precipitation echo was less than the maximum for the storm. The mean duration of the observed lightning echoes is 342 ms with a standard deviation of 270 ms. The longest spatial extent of a lightning echo that we observed was 45 km.

Dynamics of severe storms through the study of thermospheric-tropospheric coupling

Abstract: Atmospheric acoustic-gravity waves associated with severe local thunderstorms, tornadoes, and hurricanes can be studied through the coupling between the thermosphere and the troposphere. Reverse group ray tracing computations of acoustic-gravity waves, observed by an ionospheric Doppler sounder array, show that the wave sources are in the neighborhood of storm systems and the waves are excited prior to the storms. It is suggested that the overshooting and ensuing collapse of convective turrets may be responsible for generating the acoustic-gravity waves observed. The results of this study also show that the study of wave-wave resonant interactions may be a potential tool for investigating the dynamical behavior of severe storm systems using ionospheric observations of atmospheric acoustic-gravity waves associated with severe storms.

Record Torrential Rainstorms on the Island of Hawaii, January–February 1979

Abstract: The heaviest and most extensive rainstorms in the history of weather records on the island of Hawaii occurred during the first two months of 1979. Nearly every rainfall record kept by the National Weather Service from the 45 min total through the monthly and two monthly totals was exceeded during January and February 1979. Only twice in the 90-year history of weather records at Hilo have rainfalls of similar magnitude occurred. The most significant characteristic of the 1979 storms is that the record rainfall was produced from warm clouds; clouds whose temperature is everywhere greater than 0°C. The February 1979 storm produced rainfalls of 78.2 mm in 1 h and 566.4 mm in 24 h at Hilo from storm clouds with tops below the freezing level. Such rainfall amounts and rates are customarily believed to occur only during thunderstorms, where the cloud tops extend considerably above the freezing level. Thunderstorm activity was absent during the 1979 rainstorms. An account of the January and February 1979...

Jet transport performance in thunderstorm wind shear conditions

Abstract: Several hours of three dimensional wind data were collected in the thunderstorm approach-to-landing environment, using an instrumented Queen Air airplane. These data were used as input to a numerical simulation of aircraft response, concentrating on fixed-stick assumptions, while the aircraft simulated an instrument landing systems approach. Output included airspeed, vertical displacement, pitch angle, and a special approach deterioration parameter. Theory and the results of approximately 1000 simulations indicated that about 20 percent of the cases contained serious wind shear conditions capable of causing a critical deterioration of the approach. In particular, the presence of high energy at the airplane's phugoid frequency was found to have a deleterious effect on approach quality. Oscillations of the horizontal wind at the phugoid frequency were found to have a more serious effect than vertical wind. A simulation of Eastern flight 66, which crashed at JFK in 1975, served to illustrate the points of the research. A concept of a real-time wind shear detector was outlined utilizing these results.

Generation of Ice Nuclei in the Surface Outflow of Thunderstorms in Northeast Colorado

Abstract: Measurements of ice-forming nuclei and other components of the atmospheric aerosol (cloud condensation nuclei, Aitken nuclei, giant particles) were made at two ground stations in the operational area of a Colorado field experiment on convection storms. Large increases in the concentration of ice nuclei, sometimes by a factor of 100 and typically tenfold, have been found to coincide with the arrival of cold thunderstorm outflows at the observing sites, confirming some previous reports. A study of a single case shows that the enhanced ice nucleus activity noted at the surface is also detectable aloft. An aircraft equipped with an ice nucelus counter detected ice nucleus concentrations of up to ten times background during a chance encounter with a zone of strong turbulent mixing between the inflow and outflow of a large hailstorm. This suggests that ice nuclei observed in the outflow may be introduced by turbulent mixing into the inflow air and their possible role in precipitation formation processe...

Pulsed Laser Doppler Measurements of Wind Shear

Abstract: A pulsed CO2 laser Doppler system was used to monitor thunderstorm gust fronts. Wind shears associated with the gust fronts were measured and tracked.

A Summertime Tornado Outbreak in Colorado: Mesoscale Environment and Structural Features

Abstract: On 14 August 1977, there was a mini-outbreak of three tornadoes about 40 km cut of Denver, Colorado. There were no significant synoptic-scale disturbances affecting Colorado on that day. Mesoscale analysis is used to establish several smaller scale systems that influenced storm development. The most notable feature of the mesoscale band of parent thunderstorms was the active growth along their northwest flank, in spite of cell movement toward the east. On the convective scale, the situation can be described as discrete propagation of multi-cell storms by new cell development on the left rear flank. Two of the three tornadoes were documented photographically, and post-analysis shows that they were of large size and long duration, but slow moving. Structural features of the largest tornado are analyzed in different portions of the life cycle, and compared with other cases in the literature. This tornado moved on a track curving toward the north-northwest, remaining at least 5–10 km distant from any...

NASA's Participation in the AVE-SESAME ‘79 Program

Abstract: NASA's Marshall Space Flight Center participated with its AVE (Atmospheric Variability Experiment) in a large interagency mesoscale and severe storms experiment identified herein as AVE-SESAME '79 (Atmospheric Variability Experiment-Severe Environmental Storms and Mesoscale Experiment 1979). A primary objective of NASA was to support an effort to acquire carefully edited sets of rawinsonde data during selected severe weather events for use in correlative and diagnostic studies with satellite and radar data obtained at approximately the same times. Data were acquired during six individual 24-h experiments on both the regional and storm scales over a network in the central United States that utilized approximately 20 supplemental rawinsonde sites meshed among 23 standard National Weather Service sites. Included among the six experiments are data obtained between 1200 GMT on April 10 and 1200 GMT on April 11, encompassing the formation and development period for the tornado-producing systems that devastated Wichita Falls, Texas, and other sections of Oklahoma and Texas. The other dates for which data sets are available are April 19-20 and 25-26, May 9-10 and 20-21, and June 7-8, 1979.

Contributions of cloud and precipitation particles to the electrical conductivity and the relaxation time of the air in thunderstorms

Abstract: Electrical conductivity caused by the charged cloud and precipitation particles in thunderstorms has been calculated. It is found that this conductivity may be 1 or perhaps 2 orders of magnitude higher than the clear air conductivity at the same altitude. Since this conductivity increases with the precipitation intensity, liquid water content, electric field, and the electrical charges on the particles, it may show large fluctuations in thunderstorms in both space and time. In spite of this high conductivity, however, it is argued that the corresponding relaxation time may not necessarily be small, being especially so for individual charged particles and for space charge regions of small dimensions. A possible reconciliation of the discrepencies in the earlier studies and the measurements has been discussed on the basis of the present results.

Training Guide for Severe Weather Forecasters

Abstract: : Presented here is a formal and detailed training guide designed for a forecaster's initial exposure to the Air Force Global Weather Central's (AFGWC) severe weather function This TN details the analysis procedures for all charts and prognostic tools available to the severe weather function Significant severe weather parameters are analyzed at the surface 850 mb, 700 mb, and 500 mb levels Additionally, the 850/500 mb thickness and maximum wind charts are examined Also discussed are the severe weather parameters chart and the 12-hour surface pressure change chart A detailed, step-by-step evaluation of a synoptic situation is presented along with appropriate forecasts and verification data Finally, the automated prognoses available at AFGWC are discussed in relation to severe weather forecasting Prior to using this guide, forecasters should become familiar with AWSTR 200 (Rev), 'Note on Analysis and Severe Storm Forecasting of the Air Force Global Weather Central' AWSTR 200 (Rev) is referred to briefly when relating analyzed parameters to those synoptic patterns producing severe thunderstorms and tornadoes This TN will familiarize field users with the techniques used to produce the Military Weather Advisory (MWA)

A thunderstorm gust-front detection system. part i. system operation and significant case studies. part ii. statistical results

Abstract: : The Dulles International Airport wind and wind shear detection system recorded more than 160 events from November 1976 through March 1978. Eighty-two percent of 113 significant events occurred in conjunction with thunderstorms, squall-lines and frontal passages. The chief source of false alarms for anemometers was boundary layer disturbances representing 10% of the total; while gravity shear waves related to the 500mb winds caused the most false alarms for pressure sensors (4% of the total). Using complementary arrays of wind and pressure sensors, total system false alarms can be greatly reduced. These data demonstrate the importance of stable surface layers in determining how well surface temperature and wind measurements represent flow at higher levels and explain underestimates of system severity. Moreover, because we demonstrate the value of wind vector information and since temperature is a scaler quantity, there seems little operational value in using temperature sensors as part of a detection system. Conversely, pressure sensors will not reliably detect relatively thin outflows (100-200 M thick) occurring at a distance from downflows having small dimensions. Again, the wind and pressure sensors combine to provide a total system offering good reliability for detection of thunderstorm outflows. The results suggest an array configuration (composed of wind and pressure jump sensors) for use in airport warning systems. This configuration extending about 10km from the airport provides a warning for Thunderstorm gust-fronts.

The atmospheric electric global circuit

Abstract: The hypothesis that world thunderstorm activity represents the generator for the atmospheric electric current flow in the earth atmosphere between ground and the ionosphere is based on a close correlation between the magnitude and the diurnal variation of the supply current (thunderstorm generator current) and the load current (fair weather air-earth current density integrated over the earth surface). The advantages of using lightning survey satellites to furnish a base for accepting or rejecting the thunderstorm generator hypothesis are discussed.

Time-Dependent Fields and a New Mode of Charge Generation in Severe Thunderstorms

Abstract: This paper shows how the field parameters throughout a thunderstorm are related, and it shows how some of these relations have been adequately tested, while other relations will require more testing. It is shown how precipitation currents can alter the storm's behavior in predictable ways, and, as an example of this type of modification, how the thunderstorm can become a much larger generator of electrical energy.