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- Author or Editor: Brenda C. Johnson x
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Abstract
The structure and steadiness of radar-observed supercell thunderstorms are examined in terms of their particular distribution of vorticity. The data confirm that the vorticity vector in supercells points in the direction of the storm-relative velocity vector and that supercell updrafts contain large positive helicity (V·ω). The alignment of vorticity and velocity vectors dictates that low pressure associates not only with vorticity but also with helicity. Accelerating pressure gradients and helicity, both thought important for suppressing small-scale features within supercells, may combine with shear-induced vertical pressure gradient forces to organize and maintain the large-scale persistent background updrafts that characterize supercells.
Rear downdrafts possess weak positive or negative helicity. Thus, the decline of storm circulation may be hastened by turbulent dissipation when the downdraft air eventually mixes into supercell updrafts.
Abstract
The structure and steadiness of radar-observed supercell thunderstorms are examined in terms of their particular distribution of vorticity. The data confirm that the vorticity vector in supercells points in the direction of the storm-relative velocity vector and that supercell updrafts contain large positive helicity (V·ω). The alignment of vorticity and velocity vectors dictates that low pressure associates not only with vorticity but also with helicity. Accelerating pressure gradients and helicity, both thought important for suppressing small-scale features within supercells, may combine with shear-induced vertical pressure gradient forces to organize and maintain the large-scale persistent background updrafts that characterize supercells.
Rear downdrafts possess weak positive or negative helicity. Thus, the decline of storm circulation may be hastened by turbulent dissipation when the downdraft air eventually mixes into supercell updrafts.
Abstract
On 22 May 1981, we acquired lightning and Doppler radar data on two tornadic storms in Oklahoma. Cloud-to-ground lightning flash rates were measured with a magnetic direction-finder network, and total flash rates in the vicinity of the mesocyclone were measured with an L-band radar. In both storms, there was no clear relationship between tornado occurrence and ground flash rates of the storm as a whole, but the stroke rate of each storm was highest after it stopped producing tornadoes. For the second storm, we examined both intracloud and cloud-t-ground lightning rates relative to mesocyclone evolution, analyzing the region within 10 km of the mesocyclone core. Our analysis began during initial stages of the mesocyclone core associated with the fourth and strongest of five tornadoes in the storm and continued until all mesocyclone cores in the storm dissipated. During this period, intracloud lightning flash rates reached a peak of almost 14 min−1 approximately 10 min after the peak in cyclonic shear at the 6 km level and at the same time as the peak in cyclonic shear at the 1.5 km level. The peak in intracloud rates also occurred 5–10 min after the peak in the area within 40 and 45 dBZ contours at the 8 km level and at about the same time as the peak in the area within 50 dBZ contours at 8 km and within 40 dBZ at 6 km. However, ground flash rates in the mesocyclone region were usually less than 1 min−1 during periods when intracloud rates were high and were negatively correlated with cyclonic shear at both 1.5 and 6 km. The ground flash rate was the last parameter to peak, approximately 15 min after intracloud lightning and a few minutes after the latest reflectivity area (the area having >55 dBZ at the 1 km level).
We suggest that intracloud rates were governed, in part, by particle interactions during the growth in reflectivity at 7–9 km and, in part, by some process associated with the evolution of cyclonic shear at low altitudes. Earlier studies of tornado storms indicate that the evolution of updrafts and downdrafts affects the evolution of both reflectivity and low-altitude cyclonic shear and so, as in previous storm studies, updraft evolution will affect intracloud rates. We suggest that the peaks in ground flash rates resulted from increasing the distance between the main positive and negative charge centers, from the sedimentation of negative charge to lower altitudes, or from the generation or advection of positive charge below the main negative charge. Although these data are from only a single day, consideration of sferics data from previous studies suggests that 1) most tornadic storms (80% or more) have an increase in total flash rates near the time of the tornado, and 2) the increase in total flash rates is often dominated by intracloud flashes.
Abstract
On 22 May 1981, we acquired lightning and Doppler radar data on two tornadic storms in Oklahoma. Cloud-to-ground lightning flash rates were measured with a magnetic direction-finder network, and total flash rates in the vicinity of the mesocyclone were measured with an L-band radar. In both storms, there was no clear relationship between tornado occurrence and ground flash rates of the storm as a whole, but the stroke rate of each storm was highest after it stopped producing tornadoes. For the second storm, we examined both intracloud and cloud-t-ground lightning rates relative to mesocyclone evolution, analyzing the region within 10 km of the mesocyclone core. Our analysis began during initial stages of the mesocyclone core associated with the fourth and strongest of five tornadoes in the storm and continued until all mesocyclone cores in the storm dissipated. During this period, intracloud lightning flash rates reached a peak of almost 14 min−1 approximately 10 min after the peak in cyclonic shear at the 6 km level and at the same time as the peak in cyclonic shear at the 1.5 km level. The peak in intracloud rates also occurred 5–10 min after the peak in the area within 40 and 45 dBZ contours at the 8 km level and at about the same time as the peak in the area within 50 dBZ contours at 8 km and within 40 dBZ at 6 km. However, ground flash rates in the mesocyclone region were usually less than 1 min−1 during periods when intracloud rates were high and were negatively correlated with cyclonic shear at both 1.5 and 6 km. The ground flash rate was the last parameter to peak, approximately 15 min after intracloud lightning and a few minutes after the latest reflectivity area (the area having >55 dBZ at the 1 km level).
We suggest that intracloud rates were governed, in part, by particle interactions during the growth in reflectivity at 7–9 km and, in part, by some process associated with the evolution of cyclonic shear at low altitudes. Earlier studies of tornado storms indicate that the evolution of updrafts and downdrafts affects the evolution of both reflectivity and low-altitude cyclonic shear and so, as in previous storm studies, updraft evolution will affect intracloud rates. We suggest that the peaks in ground flash rates resulted from increasing the distance between the main positive and negative charge centers, from the sedimentation of negative charge to lower altitudes, or from the generation or advection of positive charge below the main negative charge. Although these data are from only a single day, consideration of sferics data from previous studies suggests that 1) most tornadic storms (80% or more) have an increase in total flash rates near the time of the tornado, and 2) the increase in total flash rates is often dominated by intracloud flashes.
