1. Introduction and background
The field phase of the Severe Thunderstorm Electrification and Precipitation Study (STEPS) took place in the late spring and early summer of 2000. The broad objective of STEPS was to gain a better understanding of the interactions among the kinematics, microphysics, and electrification of severe thunderstorms in the high plains of the United States (Lang et al. 2004). The high plains region of northeast Colorado, southwest Nebraska, and northwest Kansas was chosen for the study because there is a climatological maximum of storms producing predominately positive cloud-to-ground (CG) lightning in this region (Carey et al. 2003). One focus of STEPS was to understand how the charge structures in thunderstorms with mostly positive CG lightning compares with the charge structure in thunderstorms with mostly negative CG lightning. The storm that was in the STEPS domain on 25 June 2000 was chosen for extensive study because most of the CG flashes it produced were positive and because electric field measurements indicated that the vertical polarity of its charge structure was inverted (Rust and MacGorman 2002; Rust et al. 2005). This study focuses on the structure of individual lightning flashes in the storm, the storm’s charge structure, and relationships between the lightning charge structure and the overall storm structure.
a. Models of thunderstorm charge structure
The conceptual model of electrical charge structure of a typical thunderstorm has evolved over time. Wilson (1916, 1925) first proposed that a thunderstorm’s charge structure is that of a positive dipole, with positive charge above negative charge. Later work by Simpson and Scrase (1937) and Simpson and Robinson (1941) prompted the change to a tripole model of thunderstorm charge structure in which there is a smaller positive charge beneath the positive dipole. For many years, the tripole model was the paradigm for thunderstorm charge structure (e.g., Williams 1989), although Rust and Marshall (1996) proposed that it be abandoned because many electric field soundings are more complex than can be explained by a tripole charge structure. In light of their findings, a new paradigm for the charge structure in the convective region of a thunderstorm was offered by Stolzenburg et al. (1998). This new paradigm consists of four charge regions in the updraft and six or more charge regions outside the convective updraft. In both the updraft and downdraft, the lowest level of charge in the cloud is positive, and the uppermost level of charge is negative, with alternating charge polarities in between.
b. Inverted-polarity charge structure and flashes
When the positive dipole paradigm was first presented, some scientists believed that the paradigm should instead be that of a negative dipole. Wormell (1930, 1939) used observations of electric fields at the surface to conclude that although the majority of clouds have positive dipoles, negative dipoles do at times exist within clouds. As the paradigm changed to a tripole model, the idea of an inverted-polarity tripole (a small negative charge beneath a negative dipole) was suggested to exist at times within thunderstorms (Williams 1989; Rutledge et al. 1993). More recently, electric field soundings suggesting that inverted-polarity electrical structure can exist within thunderstorms have been presented (e.g., Marshall et al. 1995; Rust and MacGorman 2002; Rust et al. 2005). Inverted-polarity electrical structure is defined by Rust and MacGorman (2002) as a structure in which the lowest region of charge is negative, the uppermost region (usually a screening layer at the cloud boundary) is positive, and the charge regions in between alternate in polarity with height.
Just as thunderstorm charge structures can be inverted-polarity individual intracloud (IC) flashes, CG flashes can be inverted polarity. Data from lightning mapping systems have been used in the past to examine the structure of normal-polarity IC flashes (e.g., Shao and Krehbiel 1996; Coleman et al. 2003). These flashes move through two layers of charge: positive charge above negative charge. Inverted-polarity IC flashes move through two layers of charge in the opposite configuration: negative charge above positive charge (e.g., Figs. 4b, 7c, 8c,d, and 10b of Rust et al. 2005). Identification of IC flash polarity is important because thunderstorm charge structure can be inferred by examining a composite of the charge structures of many individual flashes within a storm (e.g., see Krehbiel et al. 2000 and Wiens et al. 2005).
2. Instrumentation and data sources
a. Lightning mapping array
The lightning mapping array (LMA) used during STEPS was developed and deployed by the New Mexico Institute of Mining and Technology (Rison et al. 1999; Krehbiel et al. 2000). The LMA uses a time-of-arrival technique to locate sources of very high-frequency (VHF) radiation pulses produced by the electrical breakdown of lightning channels as they propagate. The accuracy of the LMA system used during the STEPS field project is discussed in detail by Thomas et al. (2004). The LMA network used during STEPS consisted of 13 stations, which were deployed in northwest Kansas and northeast Colorado. For the greatest spatial and temporal accuracy, only pulses detected at six or more stations are used in the final analysis (as suggested by Rison et al. 1999). The LMA analysis in this paper is constrained to a range of 100 km to ensure high accuracy in the mapped 3D structure of the lightning.
b. National Lightning Detection Network
The CG lightning data used in this study were recorded by the National Lightning Detection Network (NLDN), which was owned and operated by Global Atmospherics, Inc. (now a division of Vaisala). The NLDN detects CG lightning flashes across the continental United States by using a hybrid system consisting of both time-of-arrival and magnetic direction finder sensors (Cummins et al. 1998). The NLDN data used for this study consist of the time and location of both negative and positive CG lightning strokes with a temporal resolution of 1ms and a spatial accuracy of about 500 m. Cummins et al. (1998) theoretically estimated that the NLDN detects 90% of CG lightning flashes in the STEPS domain.
c. Mobile ballooning facilities
Environmental atmospheric sounding data were obtained with the GPS/Loran Atmospheric Sounding System (GLASS), which was developed by the Atmospheric Technology Division [ATD; now the Earth Observing Laboratory (EOL)] of the National Center for Atmospheric Research (NCAR). Pressure, temperature, and relative humidity measurements were taken by a Vaisala RS80-15GH radiosonde and transmitted to a surface station. Wind and position measurements were made by a type of GPS in the sonde.
In addition to thermodynamic soundings from the mobile GLASS facility, thermodynamic and electric field profiles were obtained by balloon-borne instruments launched into the storm by a team using a mobile laboratory from the National Severe Storms Laboratory (NSSL). Two balloon soundings were obtained and were used in this analysis. Dropsondes developed by NCAR/ATD (Hock and Franklin 1999) were used as balloon-borne radiosondes to measure balloon location, winds, pressure, temperature, and relative humidity with respect to water. Relative humidity with respect to ice was calculated by using an empirical expression developed by Buck (1981). Horizontal balloon locations were measured directly by GPS units inside the radiosondes. While GPS altitudes were available, the hypsometric equation was used to calculate the vertical position of the balloon with better accuracy. All altitudes listed in this research are measured relative to mean sea level (MSL). An electric field meter (EFM), fundamentally like the one originally designed by Winn et al. (1978), was also attached to the balloon. The balloon-borne EFM measured the total electric field vector E. The direction of E is defined as the direction of the electric force on a positive charge. Using E, the horizontal components of the electric field (Ex and Ey) and vertical electric field (Ez) were calculated.
d. Meteorological radars
Data from the NCAR/ATD S-Pol and the Colorado State University–Illinois State Water Survey (CSU–CHILL) S-band Doppler radars were used in this analysis. The S-Pol radar was stationed near Idalia, Colorado, and the CSU–CHILL radar was stationed near Burlington, Colorado. Both radars recorded plan-position indicator (PPI) and range–height indicator (RHI) scans of the storm throughout its lifetime. The operation of these radars during the STEPS project is described in detail by Lang et al. (2004).
3. Charge analysis techniques
Three approaches are used in the analysis of thunderstorm charge structure. The first uses a one-dimensional approximation of Gauss’s law (1D Gauss) to infer charge regions from the electric field soundings. [For a summary of this technique, see MacGorman and Rust (1998, 130–131).] The second approach uses the patterns of three-dimensional (3D) electric field vectors to identify charge regions and to assess the validity of the 1D Gauss results. This approach was used previously by Rust et al. (2005) and others. The third approach uses 3D lightning mapping observations to infer the structure of charge involved in lightning flashes. This technique has already been described extensively by Coleman et al. (2003), Rust et al. (2005), and Wiens et al. (2005). By studying the direction and sequence of channel development and the density of mapped points along an apparent channel, as described by Shao and Krehbiel (1996), one can infer which channels propagated in regions of positive charge and which in regions of negative charge. Not every flash can be analyzed in this way, however, because the structure of some flashes is too small or too complex.
For each 10-min period analyzed for this paper, channel polarity was determined for every flash whose structure was clear enough to be analyzed. Points on positive channels were colored red, points on negative channels were colored blue, and points on channels connecting positive and negative channels were colored green. All flashes that were analyzed in this way were then compiled for each 10-min period to indicate the overall structure of the charge involved in lightning. Spark experiments (Williams et al. 1985) and physical intuition suggest that a larger density of VHF source points indicates a larger charge density, but no quantitative relationship between the two parameters is known. It is also important to note that not all charge regions are necessarily discernable from the LMA data (Coleman et al. 2003).
For the composites of charge structure shown in this paper, pixels with more than one point of a given polarity were coded to indicate the number of points. Because VHF signals from channels in negative regions are typically an order of magnitude smaller than those from channels in positive regions (Rison et al. 1999), there are at least an order of magnitude fewer points in negative regions than in positive regions of a flash. In fact, there rarely is more than one negative mapped point in a pixel on the density plots. Thus, composite maps of density by polarity have a scattering of isolated blue points (negative storm charge) amid the more solid regions of the gold to red used to indicate an increasing density of positive points. This is exacerbated because the color of a given pixel is governed by the last flash that had a mapped point at that location, so some of the already sparse blue points are covered up by the lower densities of gold points. To alleviate this problem, flashes were selected for the plots of lightning charge structure during each period to allow regions of negative charge to be clearer, though these regions still have a much lower density of points.
4. Observations
a. Meteorological conditions and storm structure
The 25 June 2000 storm observed near Haigler, Nebraska, formed from the combination of several storm cells that began in northeastern Colorado. A surface boundary moved westward through the STEPS domain in the morning, bringing easterly, upslope winds into the region. There was a strong east–west dewpoint gradient across eastern Colorado, western Nebraska, and Kansas, but the situation was not that of a typical dryline case, as the surface winds were from the east in both the dry and moist air. An approaching upper-level disturbance with a weak cold front provided enough lift at the surface to initiate the storm.
The environment had moderate instability and shear during the afternoon (Fig. 1). The straight-line hodograph indicated a possibility for splitting storms (e.g., Weisman and Klemp 1982), and one preceded the Haigler storm analyzed in this study. The splitting storm developed in far northwestern Kansas and split just as the Haigler storm became organized. Although the two halves of the splitting storm died rather quickly, they did play a role in the development of the Haigler storm. Radar analysis indicates that outflow from the dying, splitting cells intersected the Haigler storm, causing new cells to develop. The two balloon-borne EFMs flew through the new cells that developed along the outflow.
The Haigler storm did not split and was multicellular throughout its lifetime. Radar reflectivity indicates that this storm was most organized and intense at 0134 UTC. After 0134 UTC, it became more linear in nature, with a strong radar reflectivity gradient at the forward flank of the storm and a developing stratiform region north of the main cores of radar reflectivity. At the time the first balloon was launched into the storm at 0154 UTC, precipitation cells had developed along the old outflow boundaries, and by the time the second balloon was launched at 0212 UTC, the storm was becoming outflow dominated. The changes in storm structure at these times are reflected in both the radar reflectivity data and the lightning data, as will be shown in future sections.
b. Storm charge structure
The charge structure of the storm as inferred from both IC and CG flashes recorded by the LMA differs from one part of the storm to another at any given time. These charge structures range from two to five regions of charge, and there are up to four different structures in the storm at the same time. To demonstrate the changes in charge structure that took place, the storm is divided into four sections (A–D). These sections have been chosen subjectively based on charge structure as inferred from LMA data and storm structure as inferred from radar reflectivity data. The evolution of charge structures in these sections will now be examined for a 1-h period that includes the EFM balloon flights.
1) Storm charge structure at 0154 UTC
At 0154 UTC, the main body of the storm was entering the 3D range of the LMA. The first balloon-borne EFM was launched into the storm at this time. Base scan reflectivity and 7 min of lightning density data, with sections A–D overlaid, are shown in Fig. 2. Although section A does not stand out as a separate region in the radar reflectivity, it has been identified as a separate region in the storm because it was the only place in the storm producing normal-polarity IC flashes. The lightning in section A indicates a charge structure with positive charge over negative charge (Fig. 3a). Most of the lightning in the storm is in section B. The LMA data indicate that the storm had four regions of charge in this section (Fig. 3b). An examination of individual flashes in section B indicates that all of the IC lightning flashes were inverted-polarity flashes that initiated either between the upper two charge regions or the lower two charge regions. Sections C and D contain cells that developed along outflow boundaries from dying storms to the southeast of this storm. The IC lightning flashes in both sections are bilevel, inverted-polarity flashes (Figs. 3c,d).
2) Storm charge structure at 0224 UTC
Figure 4 shows the base scan radar reflectivity and 7 min of lightning density data from 0224 UTC with sections A–D overlaid. Section A contains normal-polarity IC flashes (Fig. 5a): however, the storm-relative location of the flashes was behind the convective cells of the storm at this time, as seen in the radar reflectivity. Also, a new region of negative charge became electrically active above the original charge regions. One hypothesis as to the origin of this new negative charge region is that negatively charged ice particles in the upper levels of the main convective region were advected into section A, in a manner similar to the advection of ice particles observed by Biggerstaff and Houze (1991) in a squall line. The flashes that propagated into this new region of negative charge were inverted-polarity IC flashes that initiated between the positive charge seen previously and the new negative charge above it.
By 0224 UTC, the storm cells in section C had merged with the eastern half of section B. Because this merger caused the lightning structure in the eastern portion of section B to become more complex than the western portion, section C is now defined in Fig. 4 as being the eastern portion of the main core of the storm. Figure 5b shows how the lightning-inferred charge in section B has a less complex structure than the lightning-inferred charge in section C. The charge structure in section B is a tripole structure with a region of positive charge between two regions of negative charge; whereas the charge structure in section C consists of five layers of alternating charge, with negative charge regions at the lowest and highest altitudes in the storm. Initiation of individual IC flashes within section C are complex as well. Some flashes initiate between the positive charge region around 6 km and the negative charge region around 13 km, while other flashes initiate between the positive charge region around 6 km and the negative charge region around 8 km or between the positive charge region at 10 km and the negative charge region around 13 km. This complexity seems due to the merger of storm cells described earlier and the subsequent generation of new convection.
Further evidence of the complexity of the charge structure in section C at this time is seen in the vertical electric field profile from the first EFM balloon flight (Fig. 6). For the purposes of this study, only charge regions that pass the criteria for charge density as defined by Stolzenburg et al. (1994) are considered to be representative of charge found in the thunderstorm. The EFM data and the LMA data indicate a similar pattern of charge structure in section C, but the heights of the charge regions vary depending on which charge analysis techniques are used. The LMA data indicate that the lowest altitude charge region in the storm, centered at 3.5 km, was negative. There is some question as to whether or not the EFM is in the cloud beneath a height of 6 km, but assuming that it is in the cloud where the relative humidity data begin to level off near 3 km (Fig. 6), there is a corresponding region of negative charge indicated by the EFM data near 3.5 km. The region of missing EFM data from 4.5 to 6 km prevents a comparison of charge regions in low altitudes of the storm. Both datasets indicate a region of positive charge in the middle altitudes of the storm, but the EFM data indicate a region of negative charge imbedded between two regions of positive charge rather than one thick region of positive charge. This discrepancy may be explained by an IC lightning flash that propagated very close to the EFM (Fig. 7). Just after this lightning flash, the EFM recorded negative electric fields, which can be seen in Fig. 7 as the change from upward-pointing to downward-pointing 3D electric field vectors near 7.5 km. These 3D electric field vectors point toward the area where the negative lightning channel from this flash propagated, indicating that this negative charge region was deposited by the lightning channel. Similar cases of EFMs recording lightning-deposited charge regions are documented in Coleman et al. (2003). Both LMA and EFM data indicate that there was a negative charge region above this midlevel positive charge region and that there was a positive charge region centered at 10 km (Fig. 5b). The EFM stopped recording data before the balloon was able to reach 13 km, where the LMA data indicate that there was a region of negative charge (Fig. 5b).
The second EFM flight also flew through the storm near section C at this time, but it went through the southern edge of the storm and only skirted the main core of lightning activity. Also, the EFM data have a large gap in the middle altitudes of the flight, so much of the complex charge structure seen in the LMA data cannot be compared to the EFM data. The charge structure indicated by the EFM data that are available corresponds well with the charge structure inferred from the LMA data, particularly at low altitudes and in the uppermost negative charge region. A detailed comparison of the EFM data recorded during flight 21 with the corresponding LMA data is in Rust et al. (2005).
The lightning structure and charge structure in section D remain the same as in the last time period. Bilevel, inverted-polarity IC flashes continue in this section (Fig. 5c), indicating a positive charge region beneath a negative charge region. The lightning cells remained separate from the main core of the storm rather than interacting with it in the way that the lightning in section C did.
3) Storm charge structure at 0244 UTC
The radar and lightning plots for 0244 UTC are shown in Fig. 8. Both radar reflectivity and lightning density data indicate that the storm was dissipating at this time. The lightning flashes in section A are bipolar, normal-polarity IC flashes (Fig. 9a). The majority of the lightning flashes in section B indicate a tripole charge structure with a positive charge region between two negative charge regions (Fig. 9b). However, the charge structure is more complex in a newly intensified radar reflectivity cell in this section that has a higher magnitude and vertical extent of radar reflectivity than the other cells in section B. Within that small cell there are two additional charge regions above the three charge regions found in the rest of section B. These two additional charge regions are within the higher radar reflectivity values found at higher altitudes at this location, indicating that a localized updraft is in this location and is able to support electrification at higher altitudes. The most frequent lightning activity in the storm is still in section C, but the charge structure is less complex there than in the previous time period. There are three charge regions rather then five, with a positive charge region between two negative charge regions (Fig. 9c). Some lightning flashes that initiate in section C propagate out into the developing stratiform region north of the storm. These flashes indicate that there was positive charge in the stratiform region centered at an altitude of 6.5 km. No other charge regions are indicated by the LMA data in the stratiform region, and no lightning flashes were initiated in the stratiform region. The lightning in section D is separated from the rest of the storm’s lightning, and the flashes are bilevel inverted-polarity IC flashes (Fig. 9d). The positive charge region in section D at this time is slanted upward from west to east, with positive charge centered at 6 km on the western edge of the section and centered at 8 km on the eastern edge.
c. IC and CG lightning flash rates and radar reflectivity
The base scan radar reflectivity at 0244 UTC (Fig. 8) shows that the storm had diminished in intensity since 0224 UTC (Fig. 4). This decrease coincided with a decrease in overall lightning flash rate in the storm, as well as an increase in positive CG lightning frequency. Past studies (e.g., Larson and Stansbury 1974; MacGorman et al. 1989) have shown that CG flash rates increase within some storms as the cores of high radar reflectivity descend to low levels, and that was the case in this storm. For example, at 0244 UTC radar reflectivity values greater than 50 dBZ are not found above 6 km anywhere in the storm, and the frequency of CG flashes doubled from a 10-min average of one CG flash per minute from 0220 to 0230 UTC to a 10-min average of two CG flashes per minute from 0240 to 0250 UTC.
d. Flash structures
1) Positive cloud-to-ground flashes
The LMA data indicate that all of the positive ground flashes detected by the NLDN in this storm initiated between a low-altitude region of negative charge and the region of positive charge directly above it. An example of one of these positive ground flashes is shown in Fig. 10. The negative charge region is inferred in part by the fact that the initial propagation of the flash as seen by the LMA (blue points in the figure) is upward and into positive charge. One would expect there to be a region of negative charge below this positive charge region to make the electric field large enough to initiate a flash. A few radiation source points in a lower layer at the end of the flash (orange and red points in the figure) are indicative of the presence of negative charge (e.g., Rust et al. 2005).
2) Flash showing the transition of charge regions
The IC lightning flash shown in Fig. 11 highlights the transition of the charge structure found in section B at 0154 UTC to the structure found in section B at 0224 UTC. At 0154 UTC, all of the individual flashes in this section were bilevel, inverted-polarity IC flashes, and all of the flashes initiated either between the positive charge region centered at 10 km and the negative charge region centered at 12 km or between the positive charge region at 5 km and the negative charge region at 7 km (Fig. 3b).
The transition to the tripole structure seen at 0224 UTC (Fig. 5b) happened in four steps. First, the frequency of lightning discharges that propagated into the region of positive charge centered at 10 km decreased. Next, the horizontal extent of the lighting channel propagating through this positive charge region decreased. An example of a flash with a smaller horizontal extent in this positive charge region is shown in Fig. 11. (The lightning channel of the flash shown in the figure propagates through both regions of positive charge.) Eventually, there was no horizontal propagation of lightning channels into the positive charge region at 10 km at all. Lightning no longer initiated between the negative charge regions at 12 and 7 km, and the two regions of negative charge merged into one region centered near 9 km, with lightning initiating between the negative charge region at 9 km and the positive charge region at 5 km. Finally, positive CG flashes began initiating between the positive charge region at 5 km and a new region of negative charge centered at 3.5 km, completing the transition to the charge structure seen at 0224 UTC (Fig. 5b).
3) Flashes overlapping sections
Lightning flashes in this storm would often initiate in one section and then propagate into another section. Sometimes when this occurred, the two sections had one or more charge regions of the same polarity at similar altitudes. For example, Fig. 12a shows flashes that overlapped between sections A and B at 0159 UTC. At this time, the LMA data indicate that sections A and B both have positive charge centered at 10 km and negative charge centered at 7 km; however, the LMA indicates that section A only has charge at these heights, while section B has two additional charge regions. In other cases, adjacent sections had common charge regions at different heights. For example, Fig. 12b shows flashes that overlap sections A and B at 0244 UTC. The LMA data indicate that the storm had positive charge centered at 9 km and negative charge centered at 5.5 km in section A, while it had positive charge centered at 6 km and negative charge centered at 4 km in section B. Flashes that overlapped these sections were slanted such that the two positive charge regions were connected and the two negative charge regions were connected. Because the charge regions were slanted, there were opposite polarities of charge at the same altitude in different parts of the storm.
4) Radar reflectivity and ic flashes
Almost all of the IC flashes observed by the LMA in the 25 June 2000 storm initiated and propagated within areas of large radar reflectivity values, as can be seen in Figs. 3, 5 and 9. (The main exception is flashes that initiated within cores of high reflectivity but then propagated into the developing stratiform region after 0440 UTC, where the radar reflectivity values are much smaller.) Figure 13 shows examples of three IC flashes that occurred between 0221 and 0222 UTC overlaid on an RHI scan of radar reflectivity from close to the same time. The lightning flash in Fig. 13a is a normal-polarity flash from section A that initiated just above a core of high radar reflectivity, as is consistent with flashes documented in other studies (e.g., Shao and Krehbiel 1996). This lightning flash indicates that the storm had a region of positive charge in an elevated region of high (∼45 dBZ) radar reflectivity. The negative region of charge indicated by this flash propagates across the upper edge of two low-level cores of high radar reflectivity. The easternmost core of radar reflectivity also contains a region of lightning-indicated positive charge in section B, as is seen in Fig. 13b. This positive charge region is just above the core of high radar reflectivity at the same height as the negative charge region indicated by the flash in Fig. 13a, but it is displaced horizontally. Similarly, the inverted-polarity flashes indicate an upper region of negative charge at the same height as the positive charge region indicated by the normal-polarity flash, but the regions are displaced horizontally.
5. Summary and discussion
The charge structures inferred from lightning data for the storm that occurred on 25 June 2000 during the STEPS field project can be split into four sections (A–D). A summary of the inferred charge regions for each section at each of the three analyzed times is shown in Fig. 14. The charge structure of the storm differed by section and changed with time.
The charge regions that contained the most lightning activity in section A were the positive charge region at 9–10 km and the negative charge region below it. Normal-polarity IC flashes initiated between these two charge regions. Charge regions of the same polarity were at the same altitudes in section B at 0154 UTC, but lightning never initiated between them in section B. All of the lightning in section B at that time initiated either in the upper pair of charges (negative charge at 12 km and positive charge at 10 km) or in the lower pair (negative charge at 7 km and positive charge at 5 km). This suggests that it took longer for the electric field to grow between the middle two charge regions than between the charges in the upper pair and the charges in the lower pair. The frequency of lightning flashes involving these two pairs of charges in section B was much higher than the frequency of flashes involving the two charge regions in section A. As a result, the electric field between the middle two charge regions in section B never had the opportunity to build a strong enough electric field to trigger lightning.
The simplest explanation for this behavior that appears likely to us is that, with a comparable amount of charge in each region, the geometry of the charge distribution produced a larger electric field in the upper and lower charge pairs than between the middle two charge regions. There are an infinite number of possible charge geometries that would give rise to this behavior. For example, the middle two charge regions could be much more horizontally extensive than the lowest and highest charge regions, thereby creating larger electric fields in the upper and lower pairs than between the middle two charge regions. Because there are so many possibilities, it is unclear which kinematic and microphysical processes are responsible for the lack of lightning initiation between the middle two charge regions.
This storm had a complex, multicell structure, and its electrical structure was more complex than has been inferred in other studies of the electric structure of multicell storms (e.g., Ray et al. 1987). The fact that the storm had different updrafts in various stages of maturity contributed to the fact that the storm’s charge structure varied by section. The most complex lightning structures corresponded to the parts of the storm with the largest reflectivity values and the deepest reflectivity cores.
The complexity of the storm was also enhanced by the merger of sections B and C around 0224 UTC. The storm cells and lightning in section C originally initiated along an outflow boundary produced by a different storm, and as the outflow boundary and subsequent cells merged with the main part of the storm, vertical cores of radar reflectivity at the point of the merger indicate that new cells developed. Because the part of the storm not affected by the merger (section B) had no new cell development in the upper altitudes of the storm at that time, it is likely that the enhanced surface convergence as the outflow boundary merged with the storm was responsible for these new cells. The lightning in section C became extremely complex at the time of the merger, with a high frequency of very small lightning flashes that changed location rapidly, rather than consistently initiating in and propagating through roughly the same locations. It is possible that turbulent eddies in the newly developing cells inhibited the stratification of charge regions needed for lightning flashes to have large horizontal leader propagation.
Because the lightning in section C at the time of the merger (0224 UTC) became so complex, it is difficult to compare the charge structure indicated by the LMA data with the charge structure indicated by the EFM data. The two datasets indicate similar patterns of charge structure, but there are some differences in the altitudes of the charge regions, probably because the lightning in section C changed location rapidly as the EFM flew through the storm, and the EFM was displaced horizontally from the majority of the lightning.
Almost all of the observed IC flashes were concentrated in areas with high radar reflectivity values. Some IC flashes propagated across more than one core of high reflectivity, and some IC flashes also overlapped different sections. This gave rise to very complex flash structure. Sometimes a single flash propagated through positive charge at markedly different altitudes connected by a predominantly vertical channel, and sometimes a flash in one section propagated through negative charge at the same altitude at which a nearby flash in another section propagated through positive charge.
A decrease in IC lightning flash rates coincided with the weakening of the storm. Cores of large reflectivity decreased in altitude and in overall area. Similar relationships of lightning flash rates with height and area of large reflectivity have been noted previously (e.g., Cherna and Stansbury 1986; Watson et al. 1995).
An increase in CG lightning flash rates coincided with cores of high reflectivity descending to low altitudes. All of the positive CG flashes in the storm initiated between the low-altitude negative charge and a positive charge region directly above it. A similar relationship between a lower negative charge region and the production of positive CG flashes was observed by Wiens et al. (2005) and occurred in numerical storm simulations by Mansell et al. (2002) and Kuhlman et al. (2006).
It is difficult to determine whether this thunderstorm’s overall charge structure was inverted or normal polarity because, at every analyzed time, different sections of the storm had different charge structures. The charge structures also changed over time, and the addition or subtraction of one charge region sometimes was enough to change the polarity classification (as defined by Rust and MacGorman 2002) of the overall charge structure from one polarity to another. Clearly the electrical structure of many of the storms documented recently from the Great Plains is different from the electrical structure of most storms documented from other regions. However, it is not as important to define a storm as having normal- or inverted-polarity charge structure as it is to recognize the wide variety of electrical structures that exist from storm to storm, or even within the same storm, as in the present case.
Acknowledgments
We thank William Rison, Ron Thomas, Tim Hamlin, and Jeremiah Harlin of New Mexico Tech for the use of LMA data and software and Conrad Ziegler for his analysis of environmental soundings. This research was supported in part by the U.S. National Science Foundation (NSF) through Grants ATM 9912562, ATM 0233268, ATM 9912073, and the American Meteorological Society through a graduate fellowship. Funding was also provided by NOAA/Office of Oceanic and Atmospheric Research under NOAA/University of Oklahoma Cooperative Agreement NA17RJ1227. The NSF provides funding for the CSU–CHILL and the S-Pol radars.
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Skew T–logp plot of GLASS environmental sounding near Goodland, KS, at 2032 UTC 24 Jun 2000. The hodograph and wind barbs show ground-relative winds. The gray line is the dewpoint measured on the EFM flight in the 550–750-mb layer at 0154 UTC 25 Jun 2000. Note the homogeneity below 750 mb and the deeper moisture in the near-storm environment.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
(a) PPI sector scan of radar reflectivity factor from the S-Pol radar at 0154 UTC 25 Jun 2000 and 1.2° elevation. The scan covers all but the easternmost portion of the storm. Outflow from the western portion of the storm is seen in a radar fine line oriented southwest–northeast from (−30, 50) to (−15, 60). (Axis coordinates in all figures showing radar and/or LMA data reference the distance from the center of the LMA network.) Sections A, B, C, and D refer to regions of the storm with common electrical structures inferred from LMA data. New cells were developing along another outflow boundary in section C. (b) Vertically integrated density of VHF sources mapped by the LMA from 0150 to 0200 UTC.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
Density of VHF sources of each polarity inferred from LMA data for select flashes from 0150 to 0200 UTC 25 Jun 2000. Sections A, B, C, and D refer to the areas indicated in Fig. 2. Blue indicates negative storm charge; orange indicates positive storm charge. Darker colors represent larger densities from more mapped lightning channels propagating through charge at that location.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
(a) PPI sector scan of radar reflectivity factor from the S-Pol radar at 0226 UTC 25 Jun 2000 and 1.2° elevation. Cells in section C have merged with the main portion of the storm. The radar fine line associated with outflow from the western portion of the storm can still be seen. (b) Vertically integrated density of VHF sources mapped by the LMA from 0220 to 0230 UTC.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
Density of VHF sources of each polarity inferred from LMA data for select flashes from 0220 to 0230 UTC 25 Jun 2000. Blue indicates negative storm charge; orange indicates positive storm charge. Darker colors represent larger densities from more mapped lightning channels propagating through charge at that location. Sections (a) A, (b) B, C, and (c) D refer to the areas indicated in Fig. 4. In (b), section B is to the left of east = 5 km and section C is to the right of east = 10 km. The 5 km between these two coordinates is a region where the two sections overlap due to the projection view of the lightning. The first EFM balloon flight traversed the path indicated by the black and red line in (b). Red indicates the portion of the balloon path traveled during the period of the LMA data in this figure. The times signs denote the location of positive CG strikes indicated by the NLDN.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
Electric field and thermodynamic sounding launched from Haigler, NE, at 0154 UTC 25 Jun 2000. No E data are available from 4.5 to 6 km and from 10.7 km to the top of the sounding. The heights of each charge polarity inferred from LMA (see Fig. 5, storm section C) and EFM data are indicated on the left side of the figure. The negative charge inferred in the EFM data at 7.5 km was likely due to charge deposited by lightning.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
Electric field vectors and LMA data from a flash at 0226:25 UTC. Red LMA source points indicate positive inferred storm charge and blue source points indicate negative inferred charge. LMA source points in green were not classified. Electric field vectors are plotted along the flight path of the balloon and point away from the path in the direction a positive test charge would move if placed on the flight path. Vectors highlighted in a brighter shade of purple correspond to the minute during which the flash occurred. Vectors at 66-km range and 7.5-km altitude point toward the negative lightning leader that traveled through positive storm charge after the flash.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
(a) PPI sector scan of radar reflectivity factor from the S-Pol radar at 0245 UTC 25 Jun 2000 and 1.2° elevation. (b) Vertically integrated density of VHF sources mapped by the LMA from 0240 to 0250 UTC.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
Density of VHF sources of each polarity inferred from LMA data for select flashes from 0240 to 0250 UTC 25 Jun 2000. Blue indicates negative storm charge; orange indicates positive storm charge. Darker colors represent larger densities from more mapped lightning channels propagating through charge at that location. Sections A, B, C, and D refer to the areas indicated in Fig. 8. The times signs denote the location of positive CG strikes indicated by the NLDN.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
LMA data from a positive CG flash detected by the NLDN. (a) LMA sources are colored according to time. (b) The ground strike is shown by an x at 0234:36.116 UTC, 5.0 km east, and 63.7 km north. As depicted by the LMA, the flash initially propagated upward into positive charge. The negative storm charge below this positive storm charge is inferred from the sparse sources, mostly colored orange and red, (c) near 5-km altitude and 67–71 km north and (a) from 0234:36.180 to 0234:36.290 UTC.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
LMA data from an intracloud flash in section C at 0219:38 UTC. Red LMA source points indicate positive inferred storm charge and blue source points indicate negative inferred charge. LMA source points in green were not classified. This flash propagated through two positive charge regions, one between 8 and 10 km and the other between 5 and 7 km. The horizontal extent of the lightning structure in the uppermost positive region is smaller than it was previously.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
Density of VHF sources of each polarity inferred from LMA data for select flashes that span sections A and B at (a) 0154 and (b) 0244 UTC 25 Jun 2000. The boundaries between sections A and B are not east–west or north–south, so the sections overlap in the projections shown. In (a), section A is to the left of east = −29 km, and section B is to the right of east = −27 km. In (b), section A is to the left of north = 73 km, and section B is to the right of north = 69 km. Blue indicates negative storm charge; orange indicates positive storm charge. Darker colors represent larger densities from more mapped lightning channels propagating through charge at that location.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
CSU–CHILL RHI scans of radar reflectivity factor (a) at 0222:29–36 UTC and 3.0° azimuth and (b) at 0222:49–56 UTC and 6.8° azimuth. (c), (d) The azimuths of the radar scans superimposed on the plan projection of the LMA data shown in (a) and (b). The LMA data in (a) and (c) are from a normal-polarity IC flash that initiated at the white triangle at 100-km range and 5.5-km altitude. This flash spans negative charge across two cores of reflectivity. In (b) and (d) this flash is shown along with LMA data from two other flashes. Inferred storm charge polarity associated with the LMA sources is indicated by the + and − symbols used to plot the LMA data in (b). [Note that neutral regions from the flash in (a) are omitted in (b).] These flashes are an example of lightning flashes from adjacent storm sections that have opposite polarity charge regions at the same altitude.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
A summary of the vertical distribution of charge regions in sections A, B, C, and D, at each of the three analyzed times. The number beside each + or − is the approximate center altitude of that charge in kilometers relative to mean sea level. The asterisks in section B at 0244 UTC indicate charges that were only present in a new small cell of large reflectivity.
Citation: Monthly Weather Review 136, 2; 10.1175/2007MWR2023.1
