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Research on Electrical Properties of Severe Research on Electrical Properties W. David Rust, William L. Taylor, and of Severe Thunderstorms Donald R. MacGorman National Severe Storms Laboratory NOAA/ERL in the Great Plains Norman, Okla. 73069 and Roy T. Arnold Physics Department University of Mississippi Oxford, Miss. 38655 Abstract nosity high within the cloud of a tornadic storm, a phenome- non he calls the tornado pulse generator (Jones, 1965). Gunn In 1978 we began a coordinated effort to study the electrical behav- (1956) concludes from measurements on a large tornado that ior of large and severe thunderstorms that form over the Great passed near an eight-station field mill network that there is Plains of the central United States. Methods of approach include the little evidence of unusual electrical effects near the tornado study of characteristics of individual phenomena and storm case funnel and that the observed electrical behavior is what studies. Our goal is to understand the interrelationships between electrical phenomena and the dynamics and precipitation of storms. would be expected from an ordinary thunderstorm if turbu- Evidence that interrelationships do exist can be seen in the results to lent velocities were increased by an order of magnitude. date. In one squall-line storm we have studied, 44% of all observed Vonnegut and Moore (1957) interpret the smooth electric lightning flashes were cloud-to-ground (CG); the total flashing rate field waveforms obtained closest to the tornado by Gunn averaged 12 min-1 and coarsely followed the changes in Doppler- derived maximum updraft speed. Most of the intracloud (IC) dis- (1956) as possibly due to continuous charge flow, and they charge processes in a supercell severe storm were located predomi- suggest that cloud-to-ground (CG) lightning may be sup- nately around the region of the intense updraft of the mesocyclone pressed in the vicinity of the tornado funnel. Frier (1959) re- and near large gradients in reflectivity and horizontal velocity. ports the presence of a 4-cycle-per-minute oscillation in the Both 10 cm and 23 cm wavelength radars have been used to detect electric field that he attributes to a tornado about 100 km lightning radar echoes. The lightning echoes from the 10 cm radar generally had peak signals 10-25 dB greater than the largest precipi- away. Numerous eyewitness observations of unusual lumin- tation echo in the storm, and they usually were observed where pre- osity and electrical activity near tornadoes have been col- cipitation reflectivities were less than maximum. Comparison of lected by Vonnegut (1968). Scouten et al. (1972), Taylor lightning echoes and electric field changes shows that abrupt in- (1973), Kinzer (1974), and Brown and Hughes (1978) have creases in radar reflectivity often are associated with return strokes and K-type field changes. measured intense sferics from severe storms and conclude CG flashes that lower positive charge to earth have been observed that they are associated, not with the tornado, but with the to emanate from the wall cloud, high on the main storm tower, and parent storm. Davies-Jones and Golden (1975) made day- well out in the downwind anvil of severe storms. The percentage of light movies near eight tornadoes, and they observed and CG flashes that lower positive charge is apparently small. filmed very little electrical activity near the tornado funnels. They found no vegetation scorching (Vonnegut, 1960) along the damage paths of 21 tornadoes. Zrnic' (1976) reports that 1. Introduction no abnormal magnetograms were obtained for 10 tornado passages within 12 km of magnetometer sites. In addition to Until recently, the study of electricity from severe storms has these observations, discussions on possible direct links be- been typified by isolated measurements of sferics, surface tween electricity and tornado genesis have been presented by electric fields, or visual observations. This led the National Vonnegut (1960), Wilkins (1964), Colgate (1967, 1968), and Severe Storms Laboratory (NSSL), which is located in the Watkins et al. (1978). Great Plains, to expand its research on severe storms to in- In the central United States, large and often severe storms clude a study of their electrical behavior. typically occur during the spring and summer. These storms Results from earlier studies by others, e.g., Jones (1951) can be categorized roughly into two groups. In the first group and Vonnegut and Moore (1959), include the finding of high are those triggered by synoptic-scale events such as fronts rates of electrical activity in large storms. Furthermore, there and disturbances in the jet stream. These storms are most have been several isolated studies of electrical phenomena in frequent during the spring. In the second group are those tornadic storms. Jones (1951) discusses tornado tracking triggered by smaller mesoscale processes. These generally using sferics and observations of cyclic pulsations of lumi- occur in the summer and move more slowly than the storms in the first group. Springtime storms are emphasized here. They are typified by large hail (>3 cm in diameter is not un- 0003-0007/81/091286-08$06.00 usual), strong straight-line winds (often >30 m/s), high © 1981 American Meteorological Society cloud tops (often 15 km and above), intense updrafts, and 1286 Vol. 62, No. 9, September 1981 Unauthenticated | Downloaded 10/07/21 08:25 AM UTC Bulletin American Meteorological Society 1287 mesocyclones (e.g., Lemon and Doswell, 1979), and they which have microsecond rise times and exponential decay sometimes produce tornadoes. Storms can occur as individ- time constants of 10 s for the slow antenna and 100 /JLS for the ual cells along a squall line or they can be isolated, and their fast antenna. Other parameters we measure include the at- structure can vary from the single to the multicell type. mospheric electric field at the ground, optical transients from Sometimes an isolated storm develops into a supercell lightning, strike points for CG flashes, and corona current. (Browning, 1977), which is a particularly long-lived severe Movie and television video tape recordings of clouds and storm. The varied severe storm situations make coordinated lightning also are made. study of their electrical behavior challenging. Our goal is to A van has been equipped as a mobile laboratory to meas- develop an understanding of relationships between electrical ure most of the electrical phenomena mentioned previously. phenomena and the dynamics and precipitation of storms. Mobile intercept of severe storms and tornadoes has been ac- We are examining these both in case studies of storms and for complished for several years by NSSL for photographic stud- studies of specially selected events that occur during these ies and verification of Doppler radar signatures (Davies- storms. Jonesand Kessler, 1974; Davies-Jones, 1981), but application The first season of coordinated electrical observations at of the technique to electrical measurements is new (Arnold NSSL was 1978. At first, use of the Doppler radars was dom- and Rust, 1979). The use of a mobile laboratory creates logis- inated by other experimental priorities, and only a few par- tical problems, but placing instrumentation in a relatively tial data sets suitable for our purposes were obtained. During fixed position within the severe storm, in particular beneath the 1980 season, however, radar data for use specifically in the precipitation-free cloud base (Fig. 1), provides the possi- electrical studies were acquired. Initial results from the 1978 bility of making quantitative electrical measurements in this and 1979 seasons and a few comments based on the cursory region. The region is characterized by a strong, inflowing examination of a small amount of data from 1980 are pre- low-level wind that turns upward into the updraft. A meso- sented here. cyclone, with a typical rotation diameter of 5-10 km, often develops in this region, and Doppler radar usually displays a velocity distribution resembling a Rankine combined vortex 2. Instrumentation (Donaldson, 1970). The mesocyclone is sometimes visible beneath cloud base as a lowered rotation called a wall cloud. Radar information on storms is obtained with both conven- If strong tornadoes occur, they usually form under the wall tional and Doppler radars. NSSL operates two 10 cm wave- cloud. Details of mesocyclones and related storm develop- length Doppler radars and one 10 cm conventional radar ment have been given by several investigators, e.g., Lemon (WSR-57). One Doppler (NRO) is located at NSSL and the and Doswell (1979). Using the results of the previous work other (CIM) is 42 km to the northwest. This arrangement on the correlation between visual severe storm cloud features forms a primary dual-Doppler data acquisition area that is and Doppler-derived storm windfields, we can, through shaped approximately as a figure 8, about 200 km in length documentation of the visual features of these storms, infer and 100 km in width, and with the long axis aligned from gross storm dynamics with which to correlate electrical activ- southwest to northeast (Brown et al., 1975), which is gener- ity even when storms occur outside the Doppler data acquisi- ally the direction of movement of springtime storms. Both tion area. Mobility also increases substantially the number of precipitation reflectivity and single Doppler information, severe storms that can be studied. i.e., the radial component of velocity toward or away from the radar, can be obtained to ranges greater than 300 km. Dual Doppler data usually are obtained by coordinating scans up through a selected storm or region of a storm. Each 3. Observations tilt sequence takes about 4-5 min. This scanning technique and dual Doppler data synthesis provide much information a. Squall line, 6 June 1979 on the structure and dynamics of large storms (Ray et al., 1975). Other meteorological data are obtained from atmos- We examine one group of cells (sustained, identifiable re- pheric soundings and NSSL's surface network that measures flectivity cores) within a squall line that developed during wind, pressure, temperature, and humidity.
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