Positive Cloud-to-ground Lightning Associated With the Spencer F4 Tornado of 30 May 1998

Lawrence D. Carey, Walter A. Petersen, and Steven A. Rutledge
Department of Atmospheric Science
Colorado State University



On 30 May 1998 (31 May UTC), a tornadic storm devastated Spencer, South Dakota, killing 6 people, injuring more than 150 persons, and destroying nearly 90% of the structures in the community.  The Spencer tornado (rated F4 on the Fujita tornado intensity scale) was the third and most intense of five tornadoes, creating a nearly continuous damage track along 30 miles.  All five tornadoes were produced by a single supercell storm during a 1 hour and 5 minute period.  

Photograph of the Spencer F4 tornado looking W from 5 miles ESE of Spencer, SD.  The tornado is leaving Spencer at this time.  Photo Credit:  1998, Mr. Keith B. Brown (OU)

This supercell produced over 76% positive cloud-to-ground (CG) lightning and a peak positive CG flash rate in excess of 17 min-1  (5-minute average) during a two-hour period surrounding the tornado damage.  Earlier studies have reported anomalous positive CG lightning activity in some supercell storms producing violent tornadoes.  What makes the CG lightning activity in this tornadic storm unique is the timing of the positive ground strokes relative to the F4 tornado.  In previous studies (e.g., MacGorman and Burgess, 1994; Bluestein and MacGorman, 1999), a supercell dominated by positive CG lightning produced its most violent tornado after it attains its maximum positive ground flash rate, whenever the rate is in excess of 1.5 min-1. Often, tornado genesis occurs during a lull in CG lightning activity and sometimes during a reversal from positive to negative polarity CG lightning.  Contrary to these findings, the positive CG lightning flash rate and percentage of positive CG lightning in the Spencer supercell began to increase dramatically while the storm was wreaking F4 damage on Spencer. 

These results have very important implications for the use of CG lightning in the Nowcasting of tornadoes and for the understanding of cloud electrification in these unusual storms.  In order to further explore these issues, we present detailed analyses of storm evolution and structure as seen by the Sioux Falls WSR-88D Doppler radar and the National Lightning Detection Network (NLDN).




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A Regional View of Lightning Ground Strokes


Fig. 1.  A regional depiction of the cloud-to-ground (CG) lightning activity surrounding the lifetime of the Spencer supercell (00-03 UTC, 31 May 1998; Note UTC=CDT + 5 hr).  The + [-] indicates the position of a positive [negative] ground stroke.  The box highlights a 120 km x 120 km area (see Fig. 7) around Spencer (X).


Large (Mesoscale a) convective event dominated (56%) by positive CG lightning.


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A Local View of Lightning Ground Strokes



Fig. 2. A depiction of the cloud-to-ground (CG) lightning activity in a 120 km x 120 km box centered on Spencer, SD from 00:18:30 to 02:36:30 UTC on 31 May 1998.  The + [-] indicates the position of a positive [negative] ground stroke.  The green line () segments indicate the approximate locations of the tornado tracks.


Mesoscale convection in the vicinity of Spencer, SD is dominated (67%) by positive CG lightning

Tornado tracks are approximately collocated with +CG lightning clusters


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National WSR-88D Radar Reflectivity Summary



Fig. 3.  U.S. radar reflectivity (Zh, dBZ) summary at 0130 UTC on 31 May 1998.


 Intense convective line stretching from South Dakota, through Minnesota, into Wisconsin.  Storm data (1998) shows severe weather (tornadoes, large hail, strong winds) associated with much of this line, particularly in SD.


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Spencer Supercell Storm Track and Analyses Rings



Fig. 4. Spencer supercell storm track as depicted by the Sioux Falls (KFSD) WSR-88D high radar reflectivity (>=50dBZ) echo swath from 0021-0234 UTC.  The analyses rings utilized in Figs. 5-10 are shown.  The red line segments () show the approximate location of the tornado tracks. 


Positive CG lightning is collocated with high reflectivity echo swath (c.f. Figs. 2 and 4).


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CG Lightning Statistics and Histogram Within Spencer Supercell Analyses Rings




Fig. 5. Histograms of CG lightning peak current (kA) associated with the Spencer supercell (i.e., within analyses rings of Fig. 4) from 0021 - 0234 UTC.


Interestingly, positive (negative) peak currents were relatively high (low).




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Time Series of CG Lightning Flash Rate and % +CG


Fig. 6.  Time series of the five minute average cloud-to-ground lightning flash rate (min-1) and % of +CG lightning for the Spencer supercell (i.e., within the analyses rings shown in Fig. 4).  Times of tornadoes associated with Spencer supercell are indicated.


Positive CG lightning is anomalously high (>20%) and nearly always dominates

Positive CG lightning rapidly increases during the onset of F4 tornadic damage in Spencer and peaks minutes after the F4 tornado has wreaked havoc on Spencer

The 17.6 min-1 +CG lightning flash rate is one of the highest published values associated with a supercell


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Evolution of the Spencer Supercell


Fig. 7.  Evolution of the KFSD reflectivity (dBZ) at 2 km AGL for (a) 0021 UTC, (b) 0036, (c) 0051, (d) 0106, (e) 0129, (f) 0144, (g) 0159, (h) 0214, and (i) 0229 UTC. The grid is in km and is centered on Spencer, SD. 

Note that there was a gap in the recorded KFSD data from 0111 to 0128 UTC.


Note the merger between the original Spencer supercell and another supercell that developed on the rear-flank downdraft between 0129 0144 UTC.


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Horizontal Supercell Structure Associated with an Intense +CG Lightning Cluster


Fig. 8.  Horizontal cross-section of KFSD reflectivity (dBZ) at 2 km height for 0149 UTC.  Positive/negative polarity lightning ground strike locations (+/-) for a 5-minute period (0146:30 - 0151:30) are overlain.  The lines at x=2 and y=0 highlight the planes of the vertical cross-sections in Figs 9a,b.


+CG (-CG) ground strike locations were generally associated with high (low-to-moderate) reflectivity at the center (periphery) of echo cores.


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Vertical Supercell Structure Over an Intense +CG Lightning Cluster


Fig. 9. (a) east-west (y = 0 km) and (b) north-south (x = 2 km) oriented vertical cross-sections of KFSD reflectivity (dBZ) at 0149 UTC.  See Fig. 8 for a depiction of the planes of the cross-section.


+CG ground strokes were found beneath high reflectivity cores at low-to-mid levels

+CG (-CG) lightning occurred near developing-to-mature (mature-to-dissipating) convection



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Time-Height Cross-Sections of Radar Reflectivity Moments


Fig. 10.  Time-height cross-sections of (a) mean reflectivity (dBZ), (b) maximum reflectivity (dBZ), and (c) high reflectivity (>= 55 dBZ) echo volume (km3) in cylinders defined by the analyses rings in Fig. 4.  There was a 17 minute gap (0111 0128 UTC) in the recorded data at the KFSD WSR-88D radar. Otherwise, the VCP-11 scans repeated every five minutes.  The five minute average positive CG lightning flash rate (min-1) is repeated for comparison.  The start times of tornadoes are indicated.


There is a pulse evident in the mean, maximum, and >=55 dBZ echo volume just before the +CG peak and during the F4 tornado.  The +CG peak is well correlated with the descent of a low-level high reflectivity echo centroid seen in all three plots (and which follow 10-15 minutes after the updraft pulse).

Based on reflectivity alone, it is difficult to ascertain why the +CG pulse occurred at 0144-0154 UTC associated with the third more moderate pulse in the reflectivity and not with the earlier (2nd), more intense pulse from about 0056 to 0110 UTC.  What is the role of hail?

A merger of the original Spencer supercell and a subsequent "supercell" which formed on the rear-flank downdraft of the original cell was nearly coincident with the 3rd pulse and the onset of F4 tornadic damage, and just preceded the flurry in +CG lightning activity (c.f. Figs. 7, 10a-c). 

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Positive CG lightning dominated the severe Spencer storm complex at all scales (mesoscale a to microscale (by cell.)

Small (< 10 kA) positive CG lightning flashes were NOT a factor in this storm.  Positive (negative) CG peak currents were relatively large (small). 

The peak positive CG flash rate (17.6 min-1), which is one of the highest documented +CG lightning flash rates, occurred after most of the F4 tornadic damage in Spencer was complete.

A merger between two supercells occurred at about the time of the F4 tornado and just prior to the peak +CG lightning flash rate.  This merger process likely had a causal role in the updraft pulse, reflectivity intensification, onset of F4 damage, and rapid increase in the +CG lightning flash rate.

Positive (negative) ground strokes were located in regions of moderate-to-high (low-to-moderate) reflectivity, associated with developing-to-mature (mature-to-dissipating) convection.

Time-height cross sections of reflectivity moments reveal that the Spencer F4 tornado and intense +CG lightning cluster were associated with the 3rd pulse in the supercell associated with a merger process.

The 2nd storm pulse was significantly more intense from a reflectivity perspective but produced significantly less +CG lightning and only F1 tornado damage.

The 2nd reflectivity pulse may have been associated with low concentrations of large hail and hence electrical inefficiency and low CG flash rates.

The 3rd pulse in reflectivity was likely associated with a higher concentration of smaller hail in the presence of high liquid water contents from intense updrafts.  According to NIC, this could lead to positive charging of hail, an anomalous low-level positive charge center, and positive CG lightning.

This research was supported by NSF grants ATM 9726464 and 9912051.

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