Lightning Meteorology I: Electrification and Lightning Activity by Storm Scale

LIGHTNING METEOROLOGY I: ELECTRIFICATION AND LIGHTNING ACTIVITY BY STORM SCALE

TALKING POINTS

Slide 1 – Title
Welcome to this VISIT teletraining session on Lightning Meteorology. This session was developed by Bard Zajac and John Weaver at the NOAA/NESDIS branch loated at CSU/CIRA (Colorado State University / Cooperative Institute for Research in the Atmosphere) with contributions from those individuals listed.

During the next 90 minutes, we will discuss electrification and lightning activity in isolated storms and mesoscale convective systems (MCSs) using a mix of theory and AWIPS case studies. At the end of the session, you should have a better understanding of thunderstorms and know how to use lightning data in most convective weather scenarios.

Slide 2 – Teletraining Tips
No comment.

Slide 3 – Introduction
The theoretical concepts presented in this session apply to roughly 80–90% of the isolated storms and MCSs occurring during the warm season. Lightning activity in other storms is examined in Lightning Meteorology II. Storms examined in Part II include severe-positive strike dominated storms and winter storms.

Slide 4 – Objectives
In this teletraining session, the ice-ice collisional charging mechanism is considered the primary charging mechanism in thunderstorms. The requirement of ice for electrification means that thunderstorms have common characteristics that can identified from satellite and radar. We will examine the charge distributions in thunderstorms and see how they control the occurrence of lightning. With an understanding of electrification, charge distributions and lightning, positive and negative cloud-to-ground (CG) lightning can be used to infer precipitation location and intensity and storm lifecycle. Throughout this session, lightning data is integrated with other data sets.

Slide 5 – Overview
No comment.

Slide 6 – Sec 1: introduction to review section
The lifecycle of a typical isolated thunderstorm is separated into four stages: shallow cumulus, towering cumulus, mature cumulonimbus and dissipating cumulonimbus.

Slide 7 – Sec 1: shallow cumulus (16 frames)
Frame 1: Figure shows a shallow cumulus in the process of growing into a towering cumulus. The cloud is mostly updraft with weak vertical motions. Downward motions occur only along the edges of the cloud and are associated with turbulence and entrainment. Most of the cloud lies below the freezing level but a part of the cloud penetrates above the freezing level and even above the –10°C level. The –10°C level is noted since it is an important temperature threshold for ice nucleation, to be discussed later in this slide.

Frames 2-3: Question (given to an office) highlights moist convective processes: lifting of moist air from the boundary layer, adiabatic expansion and cooling, water supersaturation and condensation.

Frames 4-5: Question (taken by the instructor) highlights how CCN provide an active surface for condensation and droplet formation.

Frames 6-7: Question (given to an office) highlights collision-coalescence as the main droplet growth process in warm clouds. This question leads to a more thorough discussion of collision-coalescence in the next frame.

Frame 8: Figure shows the growth of cloud droplets into rain drops. Initially droplets grow by condensation on CCN, but this process is not efficient for droplets larger than 10 mm in radius due to curvature effects. Once droplets reach this size, the main growth process is collision-coalescence. Over the course of many collisions, droplets grow into drizzle drops and eventually rain drops.

Frames 9-10: Question (given to an office) highlights the fact that water does not necessarily freeze when cooled below freezing. The term, supercooled droplets, is important to remember.

Frames 11-12: Question (taken by the instructor) highlights how IN provide an active surface for ice nucleation. The –10°C temperature threshold represents an average temperature at which various IN become active (IN include soil mineral, volcanic ash, etc.).

Frame 13: Figure depicts ice nucleation involving IN. Ice nucleation can occur without IN. Riming and spontaneous nucleation are two important examples discussed later in this section.

Frames 14-15: Question (given to an office) highlights the growth of ice crystals through deposition (i.e., the movement of water molecules directly from vapor to solid under ice supersaturation).

Frame 16: Figure summarizes the dynamics and microphysics of a shallow cumulus that is just starting to develop ice. The figure depicts varying amounts of cloud liquid water below the freezing level (see legend). Variations in cloud liquid water content (CLWC) are caused by variations in updraft strength and associated water supersaturation. The figure also depicts the presence of supercooled liquid water above the freezing level and the presence of small ice particles above the –10°C level.

Slide 8 – Sec. 1: towering cumulus (9 frames)
Frame 1: Figure shows a towering cumulus cloud with a large volume of updraft and a downdraft forming at mid-levels. The tower is tall, growing rapidly and is starting to produce precipitation-sized ice particles at mid-levels. These particles are starting to descend, but have yet to fall out of the cloud. The cloud top is approaching the –40°C level and the tropopause is assumed to be around –60°C.

Frames 2-3: Question (given to an office) highlights the fact that small ice particles of various origins are lofted to upper-levels.

Frames 4-6: Question (taken by the instructor) highlights the formation of graupel through riming (i.e., the immediate freezing of supercooled droplets on contact with larger ice particles).

Frame 5: The three photos depict riming, starting on the left with a stellar plate that has grown large enough to descend with respect to supercooled droplets and accrete a dense coat of rime. The middle photo shows the same stellar plate after accreting even more rime. The photo on the right shows conical graupel. Graupel is the stage of riming when the embryo (a stellar plate in this case) can no longer be discerned. Note that riming is a positive feedback mechanism: as an ice particle accretes rime, its radius and fall speed increases and it collides with a larger number of supercooled droplets per unit time. Graupel becomes hail when supercooled droplets no longer freeze immediately upon collision (due to latent heat of freezing, the graupel surface warms). Hail normally accretes supercooled droplets in both wet and dry growth modes.

Frame 6: The concept of a particle balance level is worth mentioning here. The balance level is the height in the storm where a particle's fall speed equals the updraft speed. For small supercooled droplets and small ice crystals, the balance level is found at upper-levels where the updraft is weak. For large ice particles accreting rime, the balance level found at mid-levels where the updraft is stronger. In this region of the storm, ice particles are near-stationary as they collect upward-moving supercooled droplets. Riming ice particles eventually gain enough mass to fall through the updraft.

This discussion indicates that large ice particles collide frequently with both small supercooled droplets and small ice crystals. Ice-supercooled droplet collisions cause riming. Ice-ice collisions cause electrification. How? Laboratory cloud chamber studies show that electrical charge is transferred between ice particles as they collide in the presence of supercooled droplets. This ice-ice collisional charging mechanism is examined in the next section. mechanism is discussed in the next section on thunderstorm electrification.

Frame 7: The main distinction between the towering cumulus stage and shallow cumulus stage is the formation of graupel at mid-levels due to riming. As graupel grows and eventually descends, a downdraft is initiated by precipitation drag.

Frame 8: The appearance of radar echo > 30 dBZ at mid-levels indicates that graupel has formed and that this graupel may reach the surface as convective precipitation within a short time period (as soon as 10 minutes in some cases).

Slide 9 – Sec. 1: mature cumulonimbus (3 frames)
Frame 1: The sounding is assumed to have sufficient instability and shear to separate the updraft and downdraft. The storm has the potential to maintain itself for at least a short period of time.

Frames 2-3: Question (given to an office) highlights the fact water cannot exist in the liquid phase at temperatures colder than –40°C. Even in the absence of IN, supercooled droplets freeze around –40°C due to spontaneous or homogeneous nucleation.

Frame 4: The main distinctions between the mature cumulonimbus stage and the towering cumulus stage is the extension of precipitation/downdraft to the surface and the formation of an anvil. The storm continues to produce heavy precipitation so long as the updraft provides supercooled droplets for riming.

Frame 5: Radar echo > 30 dBZ extends from mid-levels to the surface. This echo is associated with graupel and hail above the freezing level and graupel and hail in various stages of melting below the freezing level. Some frozen precipitation may reach surface. Range of reflectivities listed is approximate.

Slide 10 – Sec. 1: dissipating cumulonimbus (2 frames)
Frame 1: Figure shows the storm after most heavy precipitation has fallen out.

Frame 2: Above the freezing level, the cloud is mostly glaciated with little to no supercooled droplets present due to weakening updraft. Light to moderate precipitation may still fall out of the storm.

Slide 11 – Sec. 1: review questions (2 frames)
Questions (given to an office) highlight the radar echo associated with graupel as well as the formation of graupel by riming.

Slide 12 – Sec. 2: introduction to electrification
Laboratory cloud chambers can create the microphysical environment of a thunderstorm at mid-levels. This environment comprises supercooled droplets, ice crystals, and ice undergoing riming. Riming ice is reproduced using a rotating metal rod that accretes rime as it collides with supercooled droplets. The charging experiment uses a rotating metal rod attached to sensitive electrical equipment.

Slide 13 – Sec. 2: ice-ice collisional charging mechanism
It is beyond the scope of this session to examine the process (or processes) responsible for charge transfer during ice-ice collisions, especially since charge transfer is not well understood. For this reason, we ask forecasters to take the this information at face value.

Slide 14 – Sec. 2: dipole charge structure and lifecycle (8 frames)
In this slide, graupel-ice crystal charging mechanism is applied to the four-stage thunderstorm lifecycle.

Frames 1-2: No comment.

Frames 3-4: Figures focus on charge generation at mid-levels.

Frames 5-6: Figures focus on charge generation at mid-levels as well as the advection of charge. Positively charged ice crystals are lofted to upper-levels and negatively charged graupel is either suspended at mid-levels by the updraft or descends towards the surface. Normal dipole is an important term to remember.

Frame 7-8: Figures focus on the lifecycle stage when riming and charge generation have ended but charge advection continues. Positively charged ice crystals are carried downshear in the anvil, forming the tilted dipole charge structure. Negatively charged graupel falls out of the storm.

Slide 15 – Sec. 2: review questions (2 frames)
Questions (given to an office) highlight the key points of electrification including the requirement of graupel for charging.

Slide 16 – Sec. 2: introduction to Fort Collins (FCL) case
No comment.

Slide 17 – Sec. 2: FCL sounding (2 frames)
Frame 1: The Denver evening sounding is warm and moist throughout the troposphere. The Denver sounding is similar to an average sounding from the tropical western Pacific (TOGA COARE in green).

Frame 2: The potential for deep convection is low based on the parcel trajectory plotted. Cloud tops will be much lower than the tropopause. The potential for CG lightning is low based on the small positive area (CAPE) above –10°C. The weak CAPE above –10°C suggests that graupel may not form. Without graupel, charging and lightning do not occur.

Note that vertical wind shear may be sufficient to support long-lived storms.

Slide 18 – Sec. 2: FCL IR4 & CGs (20 frame loop)
Loop shows 15-minute GOES-8 IR4 imagery and 15-minute CG lightning data from 23:15 to 05:00 on 28–29 July 1997.

Cloud tops as cold as –70°C and copious lightning around the Denver area indicate that the Denver sounding is not representative of these storms. Post-event analysis indicates that mid-level drying and cooling occurred. However, the Denver sounding appears to be representative of the Fort Collins area with cloud tops between –30°C and –40°C and infrequent lightning.

Slide 19 – Sec. 2: FCL radar time-height analysis
Plot shows a time-height cross-section of maximum reflectivity over the location in southwest Fort Collins where it rained the most. Radar data were collected by the WSR-88D in Cheyenne, WY over the full storm period from 17:25–22:25 MDT on 28 July 1997. Maximum reflectivities were calculated over a cylindrical volume with dimensions of 14 km in height and 10 km in diameter. The O's on the x-axis indicate cloud-to-ground (CG) lightning strikes associated with the storms over Fort Collins. The 0°C and –10°C isotherms are plotted and are estimated from the Denver sounding launched at 00:00 UTC on 29 July (Slide 17). Two black lines are also plotted. One traces the 40 dBZ echo and the other traces rain mass flux. The red arrows indicate four periods of heavy precipitation.

The following points are discussed: 1) the CSU-CHILL dual-polarization radar, located 40 miles east-southeast of Fort Collins, was scanning during the time period plotted. Dual-polarization data was used to diagnose the fraction of ice versus water above the freezing level; 2) during the first two periods of heavy precipitation, the storms were shallow and produced no CG lightning. The CHILL radar indicated that the fraction of ice was less than 25%; 3) during the second two periods of heavy precipitation, the storms were slightly deeper and produced CG lightning. The CHILL radar indicated that the fraction of ice was greater than 75%; 4) a radar reflectivity threshold of 45 dBZ at –10°C is chosen to distinguish the first two periods of heavy precipitation from the second two periods. This threshold appears to identify ice particle sizes and concentrations necessary to produce CG lightning (i.e., high concentrations of millimeter-sized graupel). Note that the 45 dBZ at –10°C threshold does not guarantee CG lightning: no lightning is produced between 200 and 240 minutes after 17:25 MDT, even though the threshold is met.

Slide 20 – Sec. 2: FCL radar four-panel display (5 frames)
The radar time-height cross-section in the previous slide took research meteorologists several days to produce. How can the operational meteorologist assess the potential for CG lightning in real-time? The instructors encourage forecasters to use the AWIPS four-panel radar plot and a proximity sounding.

Frames 1-4: Radar data from the WSR-88D in Denver and temperatures derived from the Denver sounding shown in Slide 17. Five-minute CG lightning data is also plotted. These four frames correspond to the four periods of heavy precipitation discussed with the previous slide. The radar/temperature data highlight the shallow nature of the first two periods of heavy precipitation and the greater depth of the second two periods of heavy precipitation. As in the previous slide, the 45 dBZ at –10°C threshold distinguishes those storms that produce CG lightning and those that do not. However, Frame 4 shows that this threshold does not work at all times. For this reason, we consider the radar threshold a necessary but not sufficient condition for CG lightning.

Frame 5: This four-panel plot shows storms southeast of Denver. Three of the storms meet the 45 dBZ at –10°C threshold, yet not all produce CG lightning. Again, this reflectivity threshold is a necessary but not sufficient condition for CG lightning.

Slide 21 – Sec. 2: FCL summary
Radar and satellite thresholds are approximate, but capture physical characteristics of thunderstorms. The radar threshold represents the minimum ice content necessary for charging and CG lightning. The satellite threshold represents the minimum cloud depth necessary for ice crystals to separate from graupel, thus forming the normal dipole.

We ask the offices to test these thresholds locally. Published research shows that the radar reflectivity threshold may vary between 35-45 dBZ. Informal research at CIRA suggests that satellite threshold works throughout the year and in most locations.

The –30°C cloud top temperature threshold appears to be most useful in cases with low instability such as the Fort Collins case. In cases with greater instability, the threshold may not be as meaningful since clouds are growing rapidly and may penetrate the –30°C level before they start to produce CG lightning. The –30°C cloud top temperature threshold does not work well with small, isolated storms. GOES IR data cannot resolve the cloud tops of these storms and, thus, the IR temperatures can be much warmer than -30°C. This topic will be discussed in Lightning Meteorology II.

To summarize this section, we have identified the minimum ice content and cloud depth associated with thunderstorms. In the next section, we assume that these thresholds are met and examined how the normal and titled dipole charge distributions control the occurrence of CG lightning.

Slide 22 – Sec. 3: introduction to isolated storms
No comment.

Slide 23 – Sec. 3: induction of charge at the earth's surface (9 frames)
Frame 1 - This slide is an exercise for one office. Frame 1 provides the instructions and background necessary to complete the exercise.

Frames 2, 4, 6 and 8: Please draw the location, polarity and amount of charge induced on the surface by the storm overhead.

Frame 3: Shallow cumulus: no comment.

Frame 5: Towering cumulus: no comment.

Frame 7: Mature cumulonimbus: As negatively-charged graupel descends, the electrical potential between cloud and surface builds. Eventually the electrical potential within the cloud exceeds the "breakdown potential" and channels of negatively-ionized air form in the cloud and step towards the surface. These channels are draw towards the surface by the strong cloud-to-surface potential. When negatively-ionized channels come within ~50 meters of the surface, positively-ionized channels extend upward from the surface. A negative CG "return stroke" occurs when negatively- and positively-ionized channels meet and, in this process, negative charge from the cloud and positive charge from the surface are neutralized.

The normal dipole charge distribution is quasi-steady if the storm updraft and charge generation are strong. In this scenario, the amount of charge neutralized during a negative CG strike is small relative to the total charge in the cloud and on the surface. The electrical potential is able to recover quickly, allowing another strike to occur seconds later.

Negative CGs are positive correlated with convective precipitation in space and time since both require a strong updraft and riming. Negative CGs usually strike within or near storm cores and trends in negative CGs usually follow trends in convective precipitation. The tendency for negative CGs to be collocated with heavy precipitation is fairly consistent. The tendency for trends in negative CGs and convective precipitation to be in phase is not as consistent and varies from storm-to-storm.

Frame 9: Dissipating cumulonimbus: Positive CGs behave in a manner similar to negative CGs in terms of breakdown, ionized channel and return stroke processes (with the polarity reversed from the previous description). But positive and negative CGs differ in shielding and longer cloud-to-surface path. The effects of shielding are to favor positive CGs from anvil areas that are 1) exposed to the surface and 2) contain enough positive charge to induce significant amounts of negative charge at the surface. For this reason, positive CGs tend to strike away from storm cores and often strike downshear of storm cores. The effects of longer path are to make positive CGs rare and often more powerful. Positive CGs account for only 10% of CG lightning and can carry more current than negative CGs since they often release more electrical potential (potential is largely a function of path length).

Isolated storms sometimes exhibit a transition from predominantly negative CGs during the mature stage to predominantly positive CGs during the dissipating stage as negative CGs become much less frequent. However, positive CGs can be so rare that this signal is not observed.

Slide 24 – Sec. 3: meteorology of neg & pos CGs (3 frames)
Frame 1: Negative CGs can be used to infer the location and intensity of convective precipitation.

Frame 2: Positive CGs provide information about the formation and orientation of storm anvils, the later being a function of vertical wind shear. Positive CGs also provide information about storm dissipation, in conjunction with infrequent negative CGs.

Frame 3: As stated at the outset of this session, the material presented applies to 80–90% of warm season isolated thunderstorms. Exceptions will be examined in Lightning Meteorology II.

Slide 25 – Sec. 3: Melbourne (MLB) sounding
Shown is a special sounding released from Melbourne, FL at 22:00 UTC on 28 June 2000.

An office is asked to provide brief answers to the questions listed on the slide. The potential for deep convection is favorable based on the parcel trajectory. The potential for CG lightning is also good since the updraft extends well above the –10°C level. Storms should move slowly to the northeast with anvil motion to the southwest. Drier air at mid-levels suggests a potential for outflows but not necessarily strong. The instructor notes the small cap at 950 mb and asks the office "what triggering mechanism is common to Florida and may overcome this cap?" The answer is the sea breeze or outflows from earlier storms.

Slide 26 – Sec. 3: MLB radar & CGs (25 frame loop + overlay)
Loop shows base reflectivity (0.5 degree) from the WSR-88D in Melbourne from 19:19 to 21:23 UTC.

An office is asked to describe the loop using reflectivity alone. Storms form along pre-existing boundaries that move from east to west. Storms are short-lived, producing heavy precipitation and outflows. Distant storms to the northwest of the radar are dissipating.

Five-minute CG lightning data are toggled on using the check box at the bottom-center of the Controls Frame. The office is asked to re-examine the loop and describe what they see. In the three storms that produce lightning, the onset of negative CG lightning is coincident with or lags slightly the onset of heavy precipitation cores at the surface (heavy precipitation = 50+ dBZ; lag = 5-10 minutes). Negative CGs are usually within or near heavy precipitation cores. Soon after negative CG lightning ends, storms dissipate. A positive CG strike is observed away from storm cores at 19:50 UTC. The radar images following this strike show a narrow area of light precipitation area extend from the northern core to the southwest. This area of light precipitation appears to be anvil precipitation and suggests that the positive CG strike was an anvil strike. The distant storms to the northwest produce no CG lightning.

The office is asked what is the "value-added" of CG lightning data. Negative CG lightning indicates the location and intensity of convective precipitation. The lack of negative CG lightning indicates an inactive or decaying state of convection. Positive CG lightning provides information about the orientation of storm anvils and vertical wind shear. Overall, the CG lightning behaved in a manner consistent with the theory presented. The instructor points out the fact that, should the radar fail, CG lightning data can be used to monitor several important aspects of these storms.

Slide 27 – Sec. 3: MLB summary (3 frames)
Frames 1-3: No comment.

Slide 28 – Sec. 4: introduction to MCSs
No comment.

Slide 29 – Sec. 4: dynamics & microphysics of stratiform regions (3 frames)
Frame 1: Schematic depicts a vertical cross-section of a mesoscale convective system (MSC) that is organized as a squall line with trailing stratiform precipitation. An office is asked to determine the height/temperature of the cell forming along the leading edge of the squall line. The answer is the –10°C to –20°C level based on earlier discussions about graupel formation. The same office is asked to determine the height/temperature of the enhanced echo in the stratiform region. This information has not been covered but most offices are able to provide the correct answer: the 0°C level causes melting, aggregation and enhanced radar return. This feature is called a stratiform radar brightband.

Frames 1-2: The characteristics of convective and stratiform regions are listed and include the main ice growth processes. Deposition occurs in both regions due to ice supersaturation, but riming is more extensive in convective regions and aggregation is more extensive in stratiform regions. The difference in ice growth processes is due to variations in vertical motions and associated cloud liquid water content. Strong vertical motions in convective updrafts provides copious supercooled droplets for riming. Weaker vertical motions in the stratiform updraft results in lower concentrations of supercooled droplets and less riming. The relatively quiescent environment of the stratiform region is conducive to aggregation. The term, seeder-feeder, can be applied to stratiform regions since ice advected from convective regions seeds the stratiform updraft. This ice then grows by deposition, feeding off of ice supersaturation within the stratiform updraft.

Frame 3: The main ice growth processes occurring in stratiform regions.

Slide 30 – Sec. 4: ice-ice collisional charging in stratiform regions
The first bullet summarizes electrification in both convective and stratiform regions. Electrification couples two processes: ice-ice collisional charging and differential ice particle motion under gravity and cloud motions. For convective regions, graupel and ice crystals are the main components of charging. For stratiform regions, where graupel is rare, aggregates and ice crystals are the main components.

The second bullet indicates that charging does occur in low CLWC environments. In contrast to high CWLC environments, less charge is transferred during collisions and ice crystals acquire negative charge.

Slide 31 – Sec. 4: MCS charge structure & CGs (3 frames)
Frames 1-2: Summary of charge regions in a typical MCS. Induced charge at the surface is also depicted. Broad layers of negative and positive charge in the stratiform region favor intracloud lightning with long, horizontal channels. This type of intracloud lightning is called spider lightning. Spider lightning can jump from cloud-to-ground, usually as a positive CG strike. Note that the convective and anvil regions of an MCS have the same charge structure as is found in isolated storms.

Frame 3: Summary of CG lightning in MCSs by region and dominant polarity. The frequency of positive CGs is slightly greater in stratiform regions than anvil regions due to local charge generation. The frequency of positive CGs in stratiform regions is far less than the frequency of negative CGs in convective regions.

Slide 32 – Sec. 4: Flagstaff (FGZ) topography
Flagstaff sits on the Mogollon Rim, with a sharp drop off in elevation to the southwest and a more gradual drop off to the northeast. The mountain peak to the north of Flagstaff is Humphreys Peak at an elevation of 12,600 feet above sea level. The Mogollon Rim and Humphreys Peak often act as elevated heat sources and initiate convection.

Slide 33 – Sec. 4: FGZ sounding
Shown is the sounding released from Flagstaff, AZ at 12:00 UTC on 28 June 2000.

One office is asked to analyze this sounding. The potential for deep convection is favorable based on the parcel trajectory shown. The potential for CG lightning is also present since the updraft extends well above the –10°C level.

Storm motion is complicated by a dry layer between 750 and 550 mb. This dry layer appears to be sufficiently dry and deep to support strong thunderstorm outflows. Winds in the outflow layer are from the northeast, suggesting that gust fronts will propagate toward the southwest. The mid-levels are more moist with winds from the west and southwest. This wind flow will advect precipitation at mid-levels toward the east and northeast. Most forecasters conclude that new convection will form on southwestward-moving gust fronts and that stratiform precipitation will be found to the north and northeast of convection. Anvils are expected to blow off to the northeast.

Slide 33 – Sec. 4: FGZ VIS & CGs (24 frame loop)
Loop shows 15-minute GOES-10 visible imagery and 15-minute CG lightning data from 16:30 to 23:00 on 28 June 2000.

An office is asked to describe what they see in the loop. The following points should be noted: 1) around 17:00UTC, cumulus development occurs on the Mogollon Rim near Humphreys Peak; 2) at 18:00 UTC, the first negative CGs are observed, presumably associated with first precipitation at the surface; 3) around 19:15 UTC, a line of negative CGs is observed and the southern cloud edge of the convective line becomes well-defined, evidence for organized outflows; 4) by 20:30 UTC, the line breaks up into negative CG clusters of various sizes as outflows move southwestward; 5) from 19:30 to 20:30 UTC, CG lightning is consistently observed behind the convective line near Humphreys Peak. This CG lightning is predominantly positive.

The office is then asked if precipitation is reaching the surface beneath the negative CG clusters. The office should recognize that precipitation is likely reaching the surface beneath the larger clusters of negative CGs based on the amount of graupel necessary to produce that much lightning. Precipitation is probably not reaching the surface beneath the smaller clusters, since the amount of graupel produced by these storms may not be sufficient to penetrate the dry layer. Surface observations and discussions with WFO Flagstaff confirm these guesses. The office is also asked whether they expect any precipitation behind the convective line. Based on the discussion of stratiform precipitation and lightning, the office should state that stratiform precipitation is likely occurring in association with the positive CGs around Humphreys Peak. Stratiform precipitation is probably not occurring beneath eastern portions of the trailing cloud shield.

Slide 34 – Sec. 4: FGZ summary (3 frames)
Frames 1-2: No comment.

Frames 3-4: Radar-lightning overlay confirms the associated of negative CG clusters with convective precipitation and the association of trailing positive CGs with stratiform precipitation. Over eastern portions of the trailing cloud shield, no lightning is consistent with no stratiform precipitation.

Slide 35 – Sec. 5: introduction to Des Moines (DMX) case
The Des Moines case summarizes much of the material discussed in this session. It also shows how CG lightning data is especially valuable in a warning environment when other data sources are not available.

Slide 36 – Sec. 5: DMX radar & CGs (33 frame loop + overlay)
Loop shows base reflectivity from the WSR-88D in Des Moines, IA from 16:03–19:01 UTC on 29 June 1998. This time period was selected to capture the developing MCS as it moves into radar range. The loop ends when the radar failed just after 1900 UTC due to a lightning strike. The radar was not repaired until after the event.

An office is asked to first describe the loop (reflectivity only) and then discuss the potential for severe weather. The following features should be noted: 1) the evolution of the system from isolated convective cells to a squall line to a squall line with trailing stratiform precipitation; 2) the formation of a vigorous cell ahead of the line at 17:01 UTC and its merger with the line around 17:46 UTC; 3) the formation of a hook along the leading edge of the line as it approached Des Moines. This hook is clearly visible at 18:33 UTC and is associated with the part of the storm that underwent the merger with the vigorous cell just described; 4) the development of a trailing stratiform region beginning around 18:00 UTC; 5) as the loop ends, maximum reflectivities decrease slightly; and 6) in the last few frames of the loop, two notches form along the trailing edge of the deep convection.

The potential for severe weather is high throughout the loop. For most of the loop, the main threats appear to be hail and tornadoes associated with high reflectivities and hook echo, respectively. Towards the end of the loop, the main threat appears to be strong winds as the leading line consolidates and notches form, indicating the possible development of a rear-inflow jet. Localized flooding is also a threat.

The class is informed that large hail and three tornadoes (F1, F2 and F2) were observed including an F2-tornado associated with the hook echo. Strong winds were reported over widespread areas. These reports were more numerous and intense as the line passed over Des Moines. Small stream and urban flooding were reported over isolated locations.

Five-minute CG lightning data is now turned on and the same office is asked to describe the loop. The following points should be mentioned: 1) lightning data indicates storms beyond radar range; 2) lightning is associated with most areas of heavy precipitation (> 50 dBZ); 3) lightning data picks up the formation and merger of the vigorous cell that forms ahead of the line; 4) lightning clusters help to delineate convective elements; 5) isolated lightning strikes, mostly positive CGs, are observed over the stratiform region; 6) lightning data picks up the formation of two notches along the trailing edge of the deep convection; and 7) once the MCS is entirely within the domain, the total CG strike count varies considerably from period-to-period, but maintains a consistent average around 500 strikes per 5 minutes.

The office is asked to assess the value-added of the lightning data. Negative (positive) CG data correspond well with areas of convective (stratiform) precipitation. In addition, negative CG data are able to resolve important features such as the formation and merger of the vigorous cell and the formation of two notches along the trailing edge of the deep convection.

Slide 37 – Sec. 5: DMX IR4 & CGs (14 frames + overlay)
Loop shows GOES-8 IR4 imagery from 17:32 to 21:32 UTC.

The same office is asked to describe the loop with IR4 imagery alone. The forecasters should note that the system has the appearance of a developing mesoscale convective complex (MCC) with circular cloud shield and expanding area of cold cloud tops. These cold cloud tops stay the same temperature or cool during the loop. New storms form on the western side of the system and appear to develop on outflow emanating from the MCC.

Fifteen-minute lightning data are turned on and the same office is asked to describe the loop, keeping in mind the radar failure at 19:00 UTC. From 17:32 to 19:02 UTC, lightning data show the formation of the squall line from more isolated cells, the development of a trailing stratiform region and formation of two notches on the backside of the squall line. From 19:02 to 21:32 UTC, lightning data show the bowing of the line over southeastern Iowa, the expanding area of stratiform precipitation and the development of new storms along the western side of the system.

The class is informed that most counties in southeastern Iowa experienced severe wind damage. A wind gust of 126 MPH was reported in Washington county.

Slide 38 – Sec. 5: DMX summary (3 frames)
No comment.

Slide 39 – Summary of objectives (13 frames)
Frame 1: We discussed two modes of ice-ice collisional charging: a convective mode involving graupel and ice crystals and a stratiform mode involving aggregates and ice crystals.

Frame 2: The Fort Collins case provided thresholds in radar reflectivity and satellite cloud top temperature. The radar threshold was physically linked to a minimum ice content necessary for ice-ice collisional charging and CG lightning. The satellite threshold was linked to a minimum cloud depth necessary for vertical separation of positively charged ice crystals from negatively charged graupel (i.e., the formation of the normal dipole charge distribution).

Frames 3-6: The ice-ice collisional charging mechanism (convective mode) is applied to the four-stage thunderstorm lifecycle. Negative and positive CGs and induced charge at the surface are depicted.

Frame 4: As graupel forms at mid-levels in a towering cumulus, ice-ice collisional charging begins. The normal dipole charge distribution starts to develop and, at the surface, positive charge collects beneath the cloud due to induction.

Frame 5: The normal dipole charge distribution is well established at this stage due to charge generation and charge advection. This charge distribution is quasi-steady so long as a strong updraft, riming and charge generation are maintained. Negative CGs are favored during this stage as descending graupel induces more positive charge at the surface.

Frame 6: The titled dipole charge distribution forms as the storm anvil is blown downshear. This charge distribution favors positive CGs since the anvil is exposed to the surface. Negative CGs are infrequent at this stage since riming and charge generation have ended with the weakening of the updraft. In addition, most graupel has fallen out of the storm.

Frame 7: The ice-ice collisional charging mechanism (convective and stratiform modes) as applied to an MCS / squall line with trailing stratiform region. Induced charge at the surface is depicted.

The charge distributions in the cloud and at the surface show why negative CGs dominate in convective regions and why positive CGs dominate in anvil and stratiform regions.

Frame 8-10: Summary slides for Melbourne, Flagstaff and Des Moines cases showing the correspondence between negative CGs and convective precipitation and between positive CGs and anvil and stratiform regions. CG lightning data also provides information about storm lifecycle.

Frame 11: Sounding from the Flagstaff case highlighting the –10°C level. The amount of updraft above the –10°C level can be used to determine the potential for graupel formation and CG lightning.

Frame 12: Overlays of CG lightning data on visible satellite imagery and base-scan reflectivity from the Flagstaff case. The satellite-lightning overlay is highlighted since it provides information on both cloud and precipitation fields.

Frame 13: We hope this session was a good introduction to CG lightning and its utilization in convective weather forecasting. Please contact Bard Zajac and John Weaver with comments and questions.

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