A ground flash is simply a cloud flash in which one end of the bidirectional leader network connects with the ground or an object attached to the ground. As with a cloud flash, the ground flash initiates between two in cloud charge regions. However, one end of the leader propagates beyond the initial attractive charge region and onward toward the inductively charged ground. As the leader approaches the ground, typically within 50-150 m, the electric field between the leader and ground strengthens to the point that an oppositely charged leader initiates from the taller objects and rises upward toward the descending leader. More than one upward leader may initiate, however, the first to connect with the downward leader will initiate a return stroke.
The following video recorded at 7,200 ips shows the development of an upward positive leader from a TV tower as the negative end of bidirectional leader development that originated in the cloud travels downward toward the ground. The upward positive leader connects with one of the downward propagating negative leader branches causing a return stroke.
The return stroke results from the sudden decrease in resistance along the leader channel path and the difference in electrical potential between the two connecting leaders. Upon connection, electrons rapidly accelerate causing intense heating and brightening (thermalization) of the connected channel. The zone of acceleration travels away from the connection point at approximately 1/3rd the speed of light in both directions along the joined leader paths. During the time that the return stroke traverses the previously formed leader network (typically 10s of microseconds), the channel temperature can rise to 30,000ºC and peak current can achieve 10s to even 100s of kiloamps.
The good way to visualize what happens during a return stroke is to think of the ionized plasma in a leader as a bunch of cars waiting at a red light. When the leader connects with the ground, the light turns green and the cars (electrons) begin moving with those closest to the intersection moving first. The subsequent cars begin moving when there is room for them to move created by the preceding cars accelerating forward and this “zone of acceleration” travels from the intersection down the line of waiting cars.
What is interesting is that lightning only lasts for a fraction of a second, and the distance that an individual electrons and ions move is rather short on the order of a few to 10s of meters. In essence, all the electrons, positive ions and negative ions that are liberated at the tip of the leader during the breakdown of the neutral air basically shifts within the conductive lightning leader in the direction driven by the electric field. After the flash terminates these ions and electrons recombine with oppositely charged ions and the channel returns to its neutral, non-conductive state over a matter of seconds to 10s of seconds. The concept of charge traveling all the way from a charge region up in the cloud into the ground (a distance of several kilometers) is not physically possible. Again it is the neutral air that is ionized along a path, and the ionized atoms, molecules and electrons move along the channel once it becomes conductive. The net result is that a surplus of electrons is shifted toward a region that originally had a deficit of electrons and electrons are shifted away from a region that originally had a surplus of electrons. However, the charge residing on the hydrometers did not move at all except for a small fraction that becomes enveloped within the leader volumes as they propagated through the charge regions.
The following video recorded at 54,000 ips shows the development of a return stroke as the positive end of the bidirectional leader connects with the ground. It is important to note that typically only a small portion of the bidirectional leader network is visible during a ground flash. The portion below cloud is only part of one end of the bidirectional leader network.
Once the return stroke reaches the upper end of the leader network that formed prior to the return stroke, the channel, if still connected to ground, will achieve at or near the electrical potential of the inductively charged ground. The upper extent of the leader network may continue to grow if there is a large enough potential difference between the upper channel tip (at ground potential) and the cloud charge region ahead of it (at cloud potential). Frequently, this post return stroke leader growth / extension is quite intense due to the large potential difference between the ground and cloud region and a bright surge of new leader development occurs. The high-speed videos below show this post return stroke growth.
Negative Ground Flash
A negative ground flash, also known as a negative cloud-to-ground flash or -CG, has the negative end of the bidirectional leader connect with ground, and electrons accelerate downward during the resulting return stroke. On average, 90% of all CG flashes are negative. The importance of a smaller lower positive charge region that underlays the main negative charge region is apparent as it serves as a small positive potential well for the negative end of the leader to propagate toward and through before continuing toward the positively charged ground.
The figure below shows bidirectional leader growth prior to a return stroke. The leader initiates between the main negative charge region and a lower small positive charge region near the bottom of the storm. The positive end of the bidirectional leader propagates upward into the main negative charge region while the negative end propagates down into and beyond the smaller positive charge region and then continues toward the positively charged ground.
The negative end of the leader connects to ground or an upward positive leader that rises up from an object on the ground, and the resulting connection causes electrons to rapidly accelerate downward along the channel path. The zone of electron acceleration (i.e., the return stroke) travels up the entire leader network until reaching the upper extent.
Once the return stroke reaches the upper extent of the leader network the channel may fade ending the flash as shown in the video below recorded at 7,200 ips.
However, if the upper end of the connected channel grows following the return stroke, it will grow via positive breakdown. Specifically, it will grow as an upward propagating positive leader that is connected to ground with luminous current flowing through the entire grounded channel. The video below shows continuing current following the return stroke of a negative ground flash. Brightness in the main channel may vary as the upper part of the leader grows and branches while encountering varying potential difference. A higher potential difference will cause the leader to grow more intensely and brighten.
As we have already shown, positive leaders that branch tend to decay, and then recoil leaders initiate which attempt to reionize the decayed branches. If recoil leaders develop on decayed branches while the main channel is still connected to ground, the recoil leader connections can cause a brightness increase along the main channel path. This brightening has been defined as an m-component or m-event.
If a recoil leader forms in a decayed upper positive branch after the main channel also decays, the negative end of the recoil leader can travel down the entire decayed main channel path and initiate another return stroke upon reaching the ground. The descending recoil leader negative end travels at 1,000s of km/s since the decayed channel path is still somewhat conductive, and has been historically referred to as a dart leader. This return stroke / channel decay / recoil leader (dart leader) / return stroke process can be repeated multiple times during -CG flashes, and defines a multistroke CG flash. The flickering one sees when witnessing a -CG flash in real time is due to this repeated process.
The following video recorded at 10,000 ips, shows recoil leader formation in the upper branches of a decayed leader network following the initial return stroke of a negative ground flash. The upward pointing white arrow at the bottom shows the location of the initial return stroke. The repeated recoil leader development results in a subsequent return stroke and m-event brightening.
The following animation depicts the leader development during a mulitstroke negative ground flash with one subsequent return stroke followed by m-event brightening.
Revisiting the Bolt From The Blue
A type of negative ground flash that is frequently misidentified as a positive ground flash initiates between the two main charge regions of a storm. However, if the upper positive charge region is not as big as the lower main negative charge region, the negative end of the leader can “fill” the upper main positive potential well first. The negative end of the leader may then propagate outside the main updraft of the storm and and travel toward and connect with the ground well outside the storm. This type of flash has been referred to as a “Bolt From The Blue.” Due to the initially incorrect concept that a leader emerges from a charge region with the same charge as that region, it was believed that the channels that extend outside the upper portion of a storm must be positive having emerged from the upper positive charge region. Relatively recent research using 3-dimensional lightning mapping has shown this not to be the case. However, as will be shown next, positive ground flashes can initiate in the upper part of a thunderstorm with the positive end of the bidirectional leader traveling down from the upper region, but this usually occurs in the vault region of a thunderstorm where the anvil extends downwind from the main updraft.
Positive Ground Flash
A positive ground flash, also known as a positive cloud-to-ground flash or +CG, has the positive end of the bidirectional leader network descend toward and connect with the ground. Upon connection, electrons accelerate up the channel away from ground as opposed to down the channel toward ground in the -CG return stroke case. Since the typical storm charge tripole has a main positive over main negative over smaller positive region. The two charge regions that form the basis for the bidirectional leader development must be inverted from that required for a -CG flash. In some cases, storms due develop an inverted tripole charge arrangement, and a bidirectional leader develops between a main lower positive charge region overlaying a small negative charge region. In this case the process would be the same as a -CG flash, but with opposite polarities, as shown in the figures below.
Also, positive ground flashes tend to occur more frequently during the decay phase of normal polarity storms. One suggested mechanism for this behavior involves the complete dissipation of the small lower positive charge region and the lowering and dissipation of the lower main negative charge region as the updraft dies and the storm becomes downdraft dominate. As the lower main negative charge region decreases in size, an imbalance with the main upper positive charge region makes it easier for the downward propagating positive end of the bidirectional leader to continue beyond the lower main charge region and connect with the ground.
In the Northern High Plains of the United States, which includes South Dakota where I primarily film lightning, there is a corridor where positive ground flashes occur at a much higher percentage than the global average. Although the reason for this anomaly has yet to be fully explained, analysis of charge distribution observations using 3-dimensional lightning mapping and weather radar data suggest that storms can develop normal polarity charge regions but then become inverted due to ongoing lightning activity and/or develop inverted charge regions in the lower humidity environment found on the High Plains. In an additional note on the increased occurrence of the positive ground flashes, research has shown that particulate matter and aerosols from wildfires tends to affect the microphysical properties of thunderstorms resulting in an increased percentage of +CG flashes.
I personally observe more +CG flashes than -CG flashes when filming in the South Dakota region and below is a unique high-speed video recorded at 10,000 ips of a positive ground flash where both the positive and negative end of the bidirectional leader development are visible during the flash. After the return stroke, the channel network continues to grow as an upward propagating negative leader with luminous current flowing through the entire connected channel.
We showed in the previous section that positive and negative leaders behave differently. The prolific development of recoil leaders on decaying branched positive leaders frequently leads to multiple return strokes during negative ground flashes. However, negative leader branch redevelopment occurs infrequently and therefore multiple return strokes utilizing the same return stroke channel in positive ground flashes are rarely seen. We have only captured a handful of positive multistroke ground flashes and have yet to optically capture the initiation of negative branch redevelopment that leads to a subsequent return stroke. The following animation shows our best understanding of how subsequent same channel return strokes may occur in positive ground flashes.
Addition to redevelopment on previously formed channels as shown above, completely new development may occur along the upper portion of the channel following the return stroke. This new development would result in a new branch and the corresponding downward propagating reionization along the main channel could also result in a subsequent stroke. Below are some high-speed video examples of same channel multistroke positive ground flashes.
Bipolar Ground Flashes
On rare occasions, a positive ground flash can experience negative return strokes that occur along the same channel path as the initial positive return stroke. This happens when downward propagating positive leader branches do not participate in the return stroke; they are cutoff from the main downward leader at the time of the return stroke. If these “orphaned” positive leader branches then develop recoil leaders following the positive return stroke and after the main return stroke channel has decayed, the negative end of the recoil leaders, upon reaching the branch point, may travel down the decayed main channel (instead of up the channel) and initiate a negative return stroke upon connection with ground. The return stroke will travel up the main channel and out the orphaned branch that initiated the recoil leader. Frequently, this branch will decay and initiate additional recoil leaders some of which will travel down the main channel and cause additional negative return strokes. Therefore, some positive ground flashes can actually be bipolar ground flashes with a positive first return stroke and multiple subsequent negative return strokes in the same return stroke channel. So if you see a +CG that flickers repeatedly after the initial return stroke, chances are it is a bipolar ground flash.
Below is an animation of the process as well as a high-speed video showing a bipolar ground flash.
The rarity of decayed negative leader branch reionization suggests that a bipolar negative ground flash (negative return stroke followed by a same channel positive return stroke) would be quite rare, and not surprisingly, we have yet to capture one on high-speed video. However, researchers in Florida recently observed this type of flash and are continuing analysis to determine the process leading to the subsequent same channel positive return stroke.
Positive Bolt From The Blue Flashes
If a storm has a normal polarity charge arrangement, positive ground flashes can still occur, and this is sometimes seen in a storm’s vault region which is the area underlying the storm anvil downwind of the updraft. In this case, the bidirectional leader will develop between the upper positive charge region and a smaller negative screening layer charge region. Due to smaller potential negative well presented by the screening layer, the positive end of the leader will continue past the screening layer and continue to the negatively charged ground. The following figures illustrate this process.
The following video recorded at 7,200 ips shows a positive ground flash from the vault region of a severe thunderstorm.
Horizontally Extensive Ground Flashes
When storms grow upscale they can become Mesoscale Convective Systems (MCSs) and cover large areas, sometimes the size of half a midwest state. They are driven by convective regions, but when aided by favorable low- and upper-level wind patterns, also develop stratiform precipitation areas that can lead, parallel or trail the convective regions. Stratiform precipitation areas may also have weak convective areas, but primarily encompass widespread precipitation with gradual horizontal gradients of intensity. Charge regions form primarily due to electrification in the convective areas with the regions spreading downwind as storm system grows. These horizontally extensive charge regions frequently slope down to lower altitudes as they become part of the stratiform precipitation area. However, electrification can also occur in areas of weak convection that form in the stratiform precipitation area as well. Furthermore, the melting of frozen hydrometeors as they descend through the melting layer (0ºC) can create large raindrops which can then aerodynamically shed smaller drops as they fall. Since this happens in the presence of a strong electric field, the polarized large rain drops shed positive small drops at their top and continue their descent as negatively charged rain. This inductive charge separation process results in a large horizontally extensive positive charge layer that remains near the melting point.
However they originate, these horizontally extensive layered charge regions of MCSs serve as the conduit for lightning flashes that can travel over 100 km. Negative ground flashes tend to occur in the convective region and positive ground flashes tend to dominate the stratiform precipitation areas. Positive ground flashes can also initiate and cause a +CG return stroke in the convective region, but the post return stroke negative leader development then travels in the positive charge region that extends into the stratiform precipitation area. Positive ground flashes can also initiate in the stratiform precipitation area between the positive charge region at the melting layer and the underlying negative charged rain or negative screening layer that may form at cloud base.
It is the positive ground flashes that travel 10s to 100s of kilometers and frequently contain multiple spatially and temporally separate but related return strokes of both polarities. These horizontally extensive flashes transfer large amounts of charge, create large electric field changes and even emit electromagnetic pulses that influence the ionosphere, and in doing so, can act as the parent flashes for Lightning-Triggered Upward Lightning and Transient Luminous Events such as Sprites and ELVES all of which will be explained later. The multiple spatially separated return strokes frequently act to reenergize the flash allowing the leader network to travel farther through the layered charge regions. How does this happen?
Following the return stroke of a positive ground flash, we have shown that the upper portion of the leader network can grow as a negative leader that is connected to ground. Often, this post return stroke negative leader network will propagate long distances through a layered positive charge region. Often, however, the negative leader decays back toward the ground connection and the forward section becomes cutoff. The still conductive forward leader section, in the presence of an electric field, will become polarized with positive charge collecting toward the “back” cutoff end. A new positive leader may develop from the cutoff end and reenergize the bidirectional growth. If this newly developed positive end of the bidirectional leader propagates toward and connects with ground, another return stroke occurs with a different termination point. The subsequent return stroke frequently leads to further post return stroke negative leader growth if there is still positive charge ahead of the negative leader. This process can be repeated multiple times causing the flash to last multiple seconds and travel well over 100 km. The animation below shows this process.
The following high-speed videos shows a horizontally extensive positive ground flash with two spatially and temporally separate positive return strokes. The post return stroke negative leader growth and extension following the first return stroke initiates weak upward positive leaders from some tall towers. This is followed by a second positive return stroke that occurs when the negative leader extension becomes cutoff and then develops highly branched positive leaders on the cutoff ends that propagate toward the ground. Following the second return stroke you can see additional negative leader growth further extending the length of the leader network and duration of the flash.
The following standard-speed video shows this process as well from a much wider perspective.
Negative CG return strokes also occasionally occur during a horizontally extensive groun flashes, and surprisingly, we have observed it is the recoil leaders on the cloud base crawling positive leaders that tend to lead to these much less common -CG return strokes. Sometimes the positive leader channel path becomes so decayed that the negative end of a recoil leader will begin propagating outside and away from the previously formed channel path. If it travels toward and connects with ground, a negative CG return stroke occurs and further extends the positive leader network from which it originated. Below is a high-speed video recorded at 1,000 ips showing this process. Horizontally propagating branched positive leaders travel from right to left while producing numerous recoil leaders. The negative end of a recoil leader deviates from the previously formed leader path and travels toward and connects with ground causing a -CG return stroke. Following the return stroke that traverses the path of the parent recoil leader, the continued branching and decaying of the positive leader results in additional recoil leader negative ends that travel down the latest return stroke channel path causing additional negative return strokes while the positive end continues to propagate forward.
In the next section, we will look at a unique type of ground flash that develops as an upward propagating leader from a tall object.
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