Showing papers on "Thunderstorm published in 1986"

Characteristics of Isolated Convective Storms

Abstract: Convective storms exist under a wide variety of conditions and evolve in an equally wide variety of ways. As the understanding of convective phenomena has increased, so has appreciation of their complexity. Storm behavior is inherently dependent on the environment in which the storm grows, including thermodynamic stability, vertical wind profiles, and mesoscale forcing influences. To the extent that the important prestorm conditions can be identified (through rawinsonde ascents, surface observations, satellites, vertical profilers, etc.), current knowledge provides valuable guidance on how convection will evolve in a given environment. For example, inferences can be made about storm motion, longevity, and potential severity. Because of the complexity of the problem, however, the knowledge of storm dynamics to date is most applicable to relatively isolated convective events, i.e., individual thunderstorm cells, small groups of cells, or some very simple squall lines. To the extent that larger scale systems such as Mesoscale Convective Complexes (MCCs) are made up of individual convective cells, this knowledge of the properties of isolated convection is still very useful. But as interactions among cells, along with mesoscale and synoptic-scale influences, become important, any inferences regarding storm behavior are made with less certainty.

Early electrification and precipitation development in a small, isolated Montana cumulonimbus

Abstract: Instrumented aircraft and radar were used to investigate the microphysical, electrical, and dynamic evolution of the life cycle of a small thunderstorm which occurred in southeastern Montana. The observations commenced as precipitation development was just beginning, continued through the active stage of growth as the cloud produced graupel up to 8 to 9 mm diameter and reflectivities aloft of 45 dBZ, through the dissipation stage when only an anvil with a trail of light precipitation remained. The largest particles and the primary development of precipitation were found to occur in the fringes of the updraft. The electric fields inside the cloud did not exceed 100 V m−1 until 5 mm graupel, ice particle concentrations of 10 L−1, and reflectivities of 35 dBZ were already present, but then rapidly electrified to produce a single intracloud discharge 8 min later, near the peak of microphysical development. Early in the electrical development of the cloud when observed electric fields were only 200 V m−1, negative charge accumulation was observed near the 7 km (−20°C) level and was associated with the high reflectivity region. In the early stages of precipitation fallout, particle charge measurements near 4.5 km showed primarily negatively charged particles which appeared to be associated with precipitation falling from the cloud. Less than 5 to 10% of the observed particles larger than 100 μm were carrying charges larger than 5 pC, the detection limit of the instrument. These observations are discussed from the point of view of charge generation in thunderstorms, particularly, charge transfer between colliding ice particles.

Diurnal and Spatial Variability of Lightning Activity in Northeastern Colorado and Central Florida during the Summer

Abstract: Lightning location data from northeastern Colorado and central Florida for the summer months of 1983 have been studied to ascertain the diurnal development of spatial distributions of flash frequencies. The data sources are discussed, and for both investigated regions, the regional geographic and climatic characteristics, the day-to-day variability of lightning activity, the diurnal cycle over the entire region, the spatial distribution of lightning activity, the diurnal changes of spatial distribution, and the diurnal variation of lightning at individual sites are described in detail. In both regions, the time and space distributions of lightning are modulated by the topographic features and the contrasts of the terrain. Lightning activity is a relatively rare and variable phenomenon in both regions when day-to-day frequencies are considered. There thus must be meteorological parameters that determine the extent and frequency of lightning occurrence.

Cloud-to-Ground Lightning Activity in Mesoscale Convective Complexes

Abstract: An analysis of the evolution of cloud-to-ground lightning discharges attending convective storms in mesoscale convective systems was conducted in the framework of the mesoscale convective complexes (MCCs) life cycle. The lightning discharge data were acquired by a commercially available lightning detection and location system. Peak rates averaged 42/min for the MCCs analyzed; these rates are comparable to the highest observed rates within other mesoscale storm systems and are greater than 20 times the rates previously observed in isolated thunderstorms. Lightning damage occurs with half of the MCCs and is most frequent between the development and the mature phases of the MCC life cycle. The most active period is also characterized by the greatest average number of discrete strokes and largest fraction of multiple-stroke discharges. The total cloud-to-ground lightning activity and maximum flashing rate do not appear to be directly related to either the size of the cloud shield or total duration of the MCC.

The relationship of weather types to dust storm generation in Arizona (1965–1980)

Abstract: A general classification of dust weather types for Arizona first-order stations was developed using synoptic data derived from surface, 850 mb, and 500 mb pressure and wind characteristics from daily weather maps for 321 observed dust storms during 1965–1980. Four major dust weather types were identified: (1) frontal, (2) thunderstorm/convective, (3) tropical disturbance and (4) upper level/cut-off low. Frontal dust storms (Type 1) occur most frequently during the late autumn, winter and spring and can be divided into two subtypes; pre-frontal dust and post-frontal dust. Dust storms generated by thunderstorm or strong convective activity (Type 2) are most frequent during the summer months and are typified by large scale pressure pattern variability, particularly at the upper levels of the troposphere. Dust storms associated with tropical disturbances (Type 3) are infrequent at all stations but are typically very intense, long lasting events. The upper level/cut-off low type (Type 4) dust storms usually occur during May and June and from September to November and are associated with upper level split flow, where high amplitude ridging to the north and troughing to the south occurs across the western U.S.A. Although dust storms can be traced to specific synoptic events, the interannual variability of dust storms shows a strong correspondence to antecedent winter moisture variability, probably reflecting an overriding control by surface crusting and vegetative effects on dust generation.

Evaluation of Cloud-to-Ground Lightning Data from the Western United States for the 1983–84 Summer Seasons

Abstract: Over two million cloud-to-ground lightning strike locations for the period from mid-June through mid-September for the 1983–84 summer seasons were evaluated to determine the large-scale climatological characteristics of summertime lightning activity over mountainous terrain in the western United States. During the evaluation, manually digitized radar data and GOES satellite observations from the Techniques Development Laboratory's data archives were included for comparison. Generalized statistical relationships were established among the lightning data, radar data, and minimum cloud-top infrared temperature and maximum visible brightness values from the satellite VISSR data. In the climatological analysis, a high correlation between terrain elevation and the hour of maximum frequency of lightning was found along with a pronounced increase in the magnitude of lightning activity over high terrain. Maps of daily lightning frequency were in general agreement with conventional thunderstorm climatologi...

Doppler Radar Study of the Trailing Anvil Region Associated with a Squall Line

Abstract: The kinematic structure and reflectivity distribution within a region of widespread precipitation associated with a summertime midlatitude (Illinois) squall line, as revealed by an analysis of Doppler radar data, are presented and discussed. The squall line moved in a southeasterly direction while active convection forming on its leading edge moved in a northeasterly direction. Decaying thunderstorms and their anvils merged to form the extensive region of stratiform precipitation which trailed the squall line. An extension of the VAD (Velocity Azimuth Display) method, or the EVAD (Extended VAD) method, has been developed for the analysis of single-Doppler radar data. In contrast to the VAD method, which requires knowledge of the particle fall velocities (or assumptions regarding it) to calculate the divergence of the horizontal wind, the EVAD method yields the vertical distributions of both the particle fall speed and the divergence. Also presented are results from a multiple-Doppler (MDOP) analy...

Thunderstorm cloud top dynamics as inferred from satellite observations and a cloud top parcel model

Abstract: Thunderstorm top temperature-height structure and temperature-height conversion relations in the convective overshooting top region are examined using data from GOES IR, AVHRR, and aircraft overflights, and GOES stereoscopic observations. Using a Lagrangian one-dimensional cloud parcel model applied in the overshooting region, three classes of storm tops are described and the causes of their unique temperature-height structures are explored. Although the parcel model used does not fully reproduce the three-dimensional structure and dynamics of an evolving thunderstorm top, it is shown to crudely reproduce the parcel dynamics of updraft core parcels. The locations of cold points, high points, warm points, and the magnitude of cold-high offsets compare favorably between the model and the satellite observations.

Hail in Southwestern France. I: Hailfall Characteristics and Hailstrom Environment

Abstract: In the context of an experiment of hail prevention in the Aquitaine region, 12 869 reports of damaging hail have been compiled during 29 years in an area of 88 980 km2. These observations, together with crop insurance data, have led to a unique hail climatology. The data presented in this paper concern the geographical distribution of hail damage, the yearly, 5-day and hourly frequencies of hailfall, and the distributions of hailstone size and of hailfall duration. Most of these data are well explained by the fact that hail in the surveyed area is the result of almost any rather severe thunderstorm: large hail, however, is produced by a few isolated long-lived hailstorms traveling downwind of the central part of the Pyrenees with the strong upper level winds. Study of the mean characteristics of 30 of the most severe storms which have damaged the Aquitaine in the last three decades leads to the following description: a typical long-traveling hailstorm moves at 15 m s−1 for 1.5 h, dropping a hail ...

Stratospheric conductivity variations over thunderstorms

Abstract: This paper reports the first in-situ observation of variations in the electrical conductivity over thunderstorms at 26 km altitude. The vector electric field, positive and negative polar conductivity, and optical lightning power/flash were measured by payloads on superpressure balloons in the Southern Hemisphere in early 1984. It is found that in 72 percent of the thunderstorm periods observed (or in 23 of 32 periods) there were clear cases of conductivity variations while the balloons were over the thunderstorms. Examples from two separate balloons at widely separated dates and locations showing both daytime and nighttime events are presented. The conductivity measurements are made with the relaxation technique, and the vector field measurements are based on the double Langmuir probe high-impedance method. It is found that the positive and negative conductivity measurements vary independently and have a different temporal profile than the dc electric field. The polar conductivity variations can exceed a factor of 2 at this altitude. In seven of the nine most intense thunderstorm events the total conductivity increased, while in only one of these nine events did it decrease (one event had no change). Implications of these observations for global current patterns are discussed.

Ice Particle Evolution in the Anvil of a Severe Thunderstorm during CCOPE

Abstract: Measurements from the National Center for Atmospheric Research Sabreliner aircraft are combined with a multiple Doppler radar synthesis of the wind field to investigate particle growth processes in the anvil region of a severe thunderstorm. The aircraft measurements, obtained in mid- to lower anvil levels at temperatures from −25° to −36°C, show than the size spectra broaden with decreasing altitude, yet ice water content values and other measurements indicate this to be a zone of evaporation. Aggregation—allowing particles to develop to sizes as 1 cm—accounts for this observed growth. Intensification of the storm over the observational period results in the development of increasingly large aggregates. Growth histories and trajectories of anvil particles are calculated using the wind field measurements and are compared to the hydrometeor measurements. Calculations show that particles forming the outer flanks of the anvil initiate in the upwind (relative to the midlevel wind) portions of the updr...

Relationships for Deriving Thunderstorm Anvil Ice Mass for CCOPE Storm Water Budget Estimates.

Abstract: Relationships between radar reflectivity and ice water content are derived from pendmtions into thundemonn anvils in Montana on seven days during the Cooperative Convective Precipitation Experiment (CCOPE), using aircraft data and radar reflectivity, based upon an approach which minimizes the errors in converting measured crystal size to mass. Other sources of error do exist, particularly when measurements are taken in the vicinity of convective cells. The effects of truncation of the measured size spectrum due to sampling volume limitations are discussed. Ice water content (IWC) values predicted from the curves for most of the cases investigated are about the same for a given value of the radar reflectivity factor. Derived curves differ significantly in some cases from those applied to thunderstorm anvils in the past. A sensitivity study is performed to develop an improved mass-diameter relationship for anvil crystals. The choice of Z-IWC relationship has a major effect upon the estimate of the ...

Tropical convection, ionospheric potentials and global circuit variation

Abstract: The ionospheric potential (VI), a measure of the earth's overall electric field intensity, correlates with the classic 'Carnegie curve' diurnal variation in electric field caused by the global distribution of thunderstorms. A comparison of VI variation with satellite images of high tropical cloud fields indicates that equatorial thunderstorms over the continents are dominant in maintaining the earth's electric field. VI time series obtained at just one location may provide a high-resolution measure of intense tropical convection.

An Intense, Quasi-Steady Thunderstorm over Mountainous Terrain. Part IV: Three-Dimensional Numerical Simulation.

Abstract: A three-dimensional numerical simulation of an intense, quasi-steady left-moving thunderstorm observed over mountainous terrain is presented. The observational analysis of the evolution of convection leading to this storm is presented in Part I, and a detailed analysis of the Doppler radar-observed storm structure is presented in Parts II and III. This storm was particularly interesting because it initially grew in an environment characterized by terrain-induced boundary layer convergence before a massive mesoscale cold front passed underneath. The front cooled and moistened low levels while veering the surface winds to the north, creating a hodograph of winds strongly backing with height. After frontal passage the initial storm cell grew explosively and turned to the left. The observed storm evolution after the frontal passage was reproduced well by the numerical simulation. An observed secondary updraft which was not simulated, was attributed to residual effects of the prefrontal environment, w...

Relations of Kinematics, Microphysics and Electrification in an Isolated Mountain Thunderstorm

Abstract: This paper addresses aspects of the airflow, microphysics, and electrification in a mountain thunderstorm which occurred on 7 August 1979 over the Langmuir Laboratory new Socorro, New Mexico, site of the Thunderstorm Research International Program (TRIP). Single Doppler observations are used to form a conceptual model of the essentially one-dimensional storm updraft which is expressed in simple analytical form. A one-dimensional kinematic numerical cloud model is employed with the analytic updraft profile to diagnose the evolution of temperature, war substance, radar reflectivity, space charge density and axial electric field in the main updraft region. Retrieved thermal, microphysical, and electrical variables are verified with in situ aircraft and balloon observations and measured radar reflectivity. The calculated rate of noninductive charge transfer accompanying collision and separation of ice crystals and riming graupel particles is in direct proportion to cloud and precipitation content, an...

The Cooperative Huntsville Meteorological Experiment (COHMEX)

Abstract: The Satellite Precipitation and Cloud Experiment, the Microburst and Severe Thunderstorm, and the FAA Lincoln Laboratories Operational Weather Studies of the COHMEX are described. The precipitation and cloud experiment focuses on the prestorm period in order to observe the physical processes leading to the formation of small convective systems. Aircraft, remote sensing and rewinsonde data are utilized to determine various storm/environment characteristics. Doppler velocity and reflectivity of microburst clouds are studied to evaluate the three-dimensional structure of microbursts from thunderstorms. The weather studies are designed to develop and test automatic algorithms for wind shear detection using Doppler weather radars. The application of satellite systems to data collection for these experiments is discussed.

An Investigation of the Transition from Multicell to Supercell Storms

Abstract: Nearly 2½ hours of dual-Doppler radar data with high temporal and spatial resolution are used to examine the evolution and morphology of a thunderstorm that evolved from a complex of small cells into a supercell storm. Individual storm cells and updrafts moved east-northeastward, nearly with the mean wind, while the storm complex, which encompassed the individual cells, propagated toward the south–southeast. Cells were first detected at middle levels (5–10 km) on the storm's right flank and dissipated on the left flank. Generally, the storm contained three cells—a forming cell, a mature cell, and a dissipating cell; life stages were apparently dictated by the source of updraft air. During the growth stage, cell inflow had a southerly component. As the cell moved through the storm complex, it started ingesting stable air from the north and soon dissipated. A storm-environment feedback mechanism of updraft–downdraft interactions, in conjunction with increasing environmental vertical wind shear and ...

Lightning strikes to a NASA airplane penetrating thunderstorms at low altitudes

Abstract: The NASA Storm Hazards program was dedicated during the 1984 storm season to a study of lightning strikes on an instrumented F-106B aircraft, during penetrations of thunderstorms at altitudes lower than the 6-8 km center of lightning flash density. These altitudes coincide with the negative charge region of thunderstorms. An analysis of the correlation between the UHF band radar data obtained and TV images of lightning strikes indicates that, with a known aircraft position relative to the radar, the lightning channel motion can be adequately interpreted on the basis of radar echo evolution.

Evolution of lightning flash density and reflectivity structure in a multicell thunderstorm

Abstract: The radar reflectivity structure and the distribution of lightning in a storm cell was investigated using S-band and UHF-band radar data for six storm cells over Wallops Island. The S-band scans were vertical and continuous, while the UHF data were taken in steps of 2.5 deg elevation. The peak in lightning activity during the study corresponded to a merging of two storm cells. A minimum height of 7 km was found necessary for the appearance of a 40 dBZ core with lightning, which first appears in a multicell thunderstorm at the leading edge of the 50 dBZ core of the cell and between a cell and its decaying neighbor. The lightning moves further into the cell during cell decay and decreases in density. Finally, the lightning is offset horizontally from the precipitation core during cell growth but colocates with the precipitation core as the cell dissipates.

Hurricane structure and wind fields from stereoscopic and infrared satellite observations and radar data

Abstract: Hurricane cloud and precipitation structure have been studied by means of IR and stereoscopic visual satellite data from synchronized scanning GOES-East and -West, in combination with ground-based radar data for Hurricane Frederico and time-composited airborne radar data for Hurricane Allen It is noted that stereoscopically measured cloudtop height in these hurricanes is not as closely correlated to radar reflectivity at lower levels as it is in intense thunderstorms over land This and other results obtained imply that satellite precipitation estimation techniques for tropical cyclones that are based on cloudtop measurements will not be accurate with respect to time and place scales that are less than several hours and a few hundred km, respectively

Sferics rate in relation to thunderstorm dimensions

Abstract: On the basis of data from 3 days in 1978 this paper investigates the relation between thunderstorm dimensions and electrical activity to determine the relative importance of thunderstorm size and thunderstorm environment on the sferics rate. Sferics were recorded continuously by a wide-band (100 ± 50 kHz) crossed-loop radio direction finder located at the radar site; precipitation data during the corresponding period came from the radar volume scan. The number of sferics associated with specific storms and recorded in 5-min intervals varied from a few tens to a few thousands, depending on the dimensions of the source storm and on its distance from the sferics receiver. For four thunderstorm situations this variation was reduced to a small scatter, with standard deviations corresponding to factors of 1.17–1.47, by fitting the data with the empirical relation: S ∝ Ax × 10yH × 10−zr, where S is the observed number of sferics per 5 min, A the cross-sectional area, H the height, and r the range of the storm. For three of the four sets of data analyzed, x was just over unity, giving a nearly linear relation between the sferics rate and the storm's area; for a supercell storm the relation was higher than linear. For thunderstorms between 50 and 300 km distant the number of recorded sferics decreased exponentially, by a factor of about 2 every 45 km, or with z = 0.0068 ± 0.0011 per km. Thunderstorm height appeared the dominant parameter determining the rate of electrical activity; values of y for the 3 days were between 0.13 and 0.28 per km, the greater effect of thunderstorm height was found associated with a stronger wind shear in the 7.5- to 12-km layer.

On the occurrence of positive ground flashes

Abstract: Positive ground flashes are a relatively rare occurrence in ordinary summer thunderstorms, but a higher frequency of 30% to 100% is found during winter thunderstorms in Japan and Norway, although both summer and winter thunderclouds have the same polarity of electric dipole with a net positive charge located above a net negative charge. We have computed the fraction of positive ground flashes occurring for various locations of lightning charges in a thundercloud, using the stepped leader model in which a streamer progresses in the direction of maximum potential gradient at its tip. The calculation is in good agreement with observations in that both the vertical wind shear and the height of the lightning charge contribute significantly to the occurrence of positive ground flashes.

Airborne Doppler lidar techniques for observing severe thunderstorms.

Abstract: On 30 June 1981 the wind fields around an Oklahoma severe thunderstorm were observed in detail using an airborne Doppler lidar operated by NASA. Despite uncertainties caused by inertial navigation errors and problems in sampling some of the aircraft attitude and motion parameters, reasonably clear pictures of the distributions of relative reflectivity, horizontal wind velocity, and velocity spectral width near the cloud base have been obtained. Aspects of the design and functioning of the NASA lidar relevant to the collection and analysis of the data are described.

The lightning threat to aerospace vehicles

Abstract: During 1984, the U.S. Air Force Wright Aeronautical Laboratories and the Federal Aviation Administration conducted a lightning measurement program flying a CV-580 aircraft in the vicinity of the Kennedy Space Center, FL. The aircraft was flown in thunderclouds where the regions of precipitation did not exceed 40 dBz between 2000 and 18,000 ft. The aircraft was instrumented to measure the current flow on the wingtips, the displacement and surface current densities on the fuselage and on the wings, the electrostatic field, and the VHF radiation during direct lightning attachment. The transient electromagnetic characteristics of lightning attachments were recorded using six Tektronix 7612D waveform digitizers with sample rates of 5 ns and a Honeywell 101 analog recorder with a 2 MHz frequency response. Data from 21 direct lightning attachments were recorded in the CV-580 aircraft during the program. The maximum displacement and surface current densities measured in the aircraft digital acquisition system during direct lightning attachments were 22 A/m and 2950 T/s, respectively. The maximum charge and action integrals calculated from the analog data collected during direct lightning attachments were about 100 C and 2.9xl0 A-s, respectively. This paper presents the characteristics of the fast transient pulses, the overall characteristics of the lightning flashes, and their relationship with the MIL-STD-1757A test qualification waveform.

Short-period disturbances in the ionosphere observed at the time of typhoons in September 1982 by a network of HF Doppler receivers

Abstract: Short-period disturbances of the ionosphere were observed by a network of HF Doppler receivers in central Japan at the time of typhoons 18 and 19 which crossed the Islands of Japan successively in September 1982.Both dynamic and static spectral analyses on sample records revealed that (1) these waves have the periods ranging between 1.4 and 9.7min that correspond to infrasound at F-layer heights (and partly to gravity wave at E-layer heights), (2) the spectral content varies from hour to hour, from station to station for a given typhoon, and further one typhoon to the other, and (3) the spectral fine structure does not accord with existing theoretical prediction for thunderstorms. It is, therefore, plausible that the observed spectral peaks arose from the characteristics of the radiation sources in the typhoon air mass.Horizontal and vertical phase velocities, and corresponding wavelengths of the ionospheric waves were also measured using cross-correlation method. It is shown that (1) the horizontal phase velocities take similar values to those observed during the severe thunderstorms and tornadoes so far reported, with scatter range of the values being slightly larger than the latter two, and (2) the mean values of the upward vertical phase velocities are several to ten times larger than those of the horizontal ones.

The Austin, Texas, Flash Flood: An Examination from Two Perspectives—Forecasting and Research

Abstract: During the evening of 24 May 1981, rainfall from a slow-moving, multicell thunderstorm exceeded 200 mm (8 in) in just 2 h over western sections of Austin, Texas. This intense precipitation, falling on previously saturated terrain, resulted in record flash flooding. The evolution of the large-scale meteorological setting associated with the flood is briefly examined. The presentation then focuses on the development and movement of several mesoscale convective systems (MCSs) over the southern Plains during the night preceding the flash flood. It is shown that two significant mesoscale convective outflow boundaries stalled and weakened in the Austin vicinity. The flash flood storm appears to have developed along a weak baroclinic zone left behind by these outflows. Additionally, the precursor MCSs primed the hydrologic setting in south central Texas as they produced 75–125 mm (3–5 in) rains. Upper-air soundings are considered and related to the anomalous (relative to other storms over Texas) movemen...

Case studies of thunderstorms initiated by radar-observed convergence lines

Abstract: This study analyzes two of the many convergent windshift lines that occurred during the Convection Initiation Project in eastern Colorado during the summer of 1984. The coincidence of these boundaries with the initiation of convection is examined by means or Doppler radar observations, surface mesonet data, and chase team verifications and measurement. The surface mesonet data and chase team measurements in both cases presented verify that a sensitive Doppler radar can effectively detect windshift lines. It is also shown that in both cases the convergent windshift lines were directly associated with thunderstorm development and were likely a primary forcing mechanism.

Rapidly occurring short duration discharges in thunderstorms, as indicators of a lightning‐triggering mechanism

Abstract: Unusual rapidly occurring and small-scale discharges in a narrow (1-2 km) region inside the most electrically active part of a thunderstorm, observed with a VHF-band (2 m wavelength) radar in East Georgia, USSR, were relatively stationary on the radar display, and had a maximum rate of about 200 per minute, an average duration of 12.5 ms, an estimated radar cross section a fraction of a meter to a few meters square, and an estimated length of a few meters to tens of meters. Occurrence of quasi-stable discharges coincided with period of the highest altitude of the 50 dBZ-reflectivity core. It is proposed that the quasi-stable discharges are long sparks produced by the natural lightning-triggering mechanism inside the storm. This hypothesis appears to support observations on triggered lightning in the United States obtained with an instrumented aircraft and wire-trailing rockets.