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Radar and lightning detection

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Lightning detection

Extreme weather phenomena have always been one of humans profound interests and fascinations. This is apparent in the different mythologies impersonating the god of thunder and weather from the time in ancient Egypt (Typhon), China (Tien Mu) and India (Indra) to the ancient Greek times of which Zeus, whose symbol is a lightning flash, is the most famous one. As such, lightning must have been known by many civilisations throughout the history of mankind. Furthermore, fossil evidence confirms that terrestrial lightning has been present for over 250 million years. Thus, electrification is a long-lived phenomenon, whose abundance and longevity has even let to the suggestion that lightning lies at the formation of molecules giving rise to life.

Today, researchers at universities and meteorological institutes still reap the fruits of pioneering work of many researchers, e.g., Franklin, Lemonnier, and Wilson, ... which has eventually led to our present understanding of the electrical nature of thunderstorms. In the following we give a brief introduction into this fascinating field of science. 

 

Lightning formation

The initiation of a lightning discharge is a complex process, involving various factors. Even today, many questions in the different steps towards the buildup of the lightning discharge are still unanswered. However, scientists agree that for lightning to occur negatively and positively charged particles need to become separated within the cloud in order to build up a strong electric field. But how does the cloud acquire charge in the first place? The so-called graupel-ice process is most favored to explain cloud electrification, i.e., the cloud obtains positively and negatively charged particles. In short, this mechanism states that larger riming graupel and smaller ice crystals collide with each other and interchange electric charges. The rebounding ice crystals tend to become positively (negatively) charged, whereas graupel particles gain negative (positive) charge at temperatures below (above) the so-called reversal temperature. These charged particles are subsequently separated by gravity, i.e., the heavier precipitation particles (graupel) are removed from the cloud whereas the lighter ice crystals remain in the cloud due to the updraft. As such, an upper positively charged region and a lower negatively charged region is created; a prerequisit for lightning initiation.

 

Charge transfer by collisions in the graupel-ice mechanism of cloud electrification. Here it is assumed that the reversal temperature is -15C at a height of 6 km. Adapted from Rakov & Uman (2003).

In the same way as we experience a small electric shock when our hand approaches the door handle after acquiring some electric charge, the electric field between the separated positively and negatively charged particles needs to be of some considerable strength in order to bridge the distance between the bottom of the cloud and the Earth's surface. It would be out of the scope to go deeper into the details. However, it is worth mentioning that when the electric field becomes stronger than a critical value, the electric field is able to gather vast amounts of free electrons to form an electron beam; the start of the lightning discharge. For more details about the initiation of lightning, the interested reader is referred to the following article: pdf.

 
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Why study lightning at RMI?

In general, two types of lightning occur within a thunderstorm. The first are the cloud discharges that do not involve contact with the ground. A second type are the cloud-to-ground discharges that strike the Earth's surface. Even though not always visible when in the center of a stormcell, the majority of lightning discharges that occur during extreme weather are cloud discharges. However, cloud-to-ground discharges are the ones that are responsible for serious lightning damage associated with thermal and electromagnetic (induction) effects, such as forest fires and burned holes in buildings, burned-through electric wires and electronic failure.

 

This plot depicts the different kind of lightning discharges that can occur during extreme weather.

It is clear that observing a region of thundery activity is of great importance for instance to (1) aviation since thunderstorms can cause unstable air motions, leading to major difficulties in the take-off and landing of an airplane. Additionally, it is very dangerous to fuel an aircraft while lightning activity is approaching the airport, with the possibility of lightning striking the aircraft. Other sectors also profit from accurate lightning information, such as (2) energy plants (power lines) and (3) public transport services (in particular the train network) whom are vulnerable to direct hits and (4) insurance companies to check damage claims. Other reasons to monitor and analyse accurately lightning activity in great detail include (5) nowcasting the motion and severity of a storm (linked to the storms electric activity). This may lead to the broadcast of a general warning to the public to caution for the approaching hazard. (6) The protection of electrical and electronic systems, buildings, etc., from the deleterious effects of lightning. (7)  And last but not least, the scientific importance to better understand the physics and occurence of lightning, the relationship between lightning, precipitation, and convective state of the thundercloud, ... The latter in turn will have a positive influence on the aformentioned points.

 
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Belgian Lightning Location System

The Royal Meteorological Institute operates since 1992 a so-called BElgian Lightning Location System (BELLS) to detect and locate electrical activity including intra- and intercloud (IC), as well as cloud-to-ground (CG) discharges. BELLS consists of a number of lightning sensors capable of intercepting the electromagnetic radiation emitted by lightning discharges. This, in its turn, is sent to a central processor which provides, after calculation, the time, position, type (IC/CG), polarity (negative/positive) and strength (kA) of the discharge. An exemplary output is shown in the picture below.

 

 

 

An example of the observed electrical activity on 06.06.2010. Each lightning discharge is color-coded as function of its occurrence to visualise the temporal evolution of the storm.

An example of a lightning sensor is depicted in the figure below.

 

Lightning sensor

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Poelman D., On the science of lightning: an overview, RMI 2010/0526/56

Contact person for this topic: Dieter Poelman