Download Lightning Applications in Weather and Climate Research

Surv Geophys
DOI 10.1007/s10712-012-9218-7
Lightning Applications in Weather and Climate
Research
Colin G. Price
Received: 18 October 2012 / Accepted: 26 December 2012
Ó Springer Science+Business Media Dordrecht 2013
Abstract Thunderstorms, and lightning in particular, are a major natural hazard to the
public, aviation, power companies, and wildfire managers. Lightning causes great damage
and death every year but also tells us about the inner working of storms. Since lightning
can be monitored from great distances from the storms themselves, lightning may allow us
to provide early warnings for severe weather phenomena such as hail storms, flash floods,
tornadoes, and even hurricanes. Lightning itself may impact the climate of the Earth by
producing nitrogen oxides (NOx), a precursor of tropospheric ozone, which is a powerful
greenhouse gas. Thunderstorms themselves influence the climate system by the redistribution of heat, moisture, and momentum in the atmosphere. What about future changes in
lightning and thunderstorm activity? Many studies show that higher surface temperatures
produce more lightning, but future changes will depend on what happens to the vertical
temperature profile in the troposphere, as well as changes in water balance, and even
aerosol loading of the atmosphere. Finally, lightning itself may provide a useful tool for
tracking climate change in the future, due to the nonlinear link between lightning, temperature, upper tropospheric water vapor, and cloud cover.
Keywords
Lightning Severe weather Climate Thunderstorms
1 Introduction
There are approximately 50 lightning flashes every second in total around the planet
(Christian et al. 2003). Due to the high currents (*20 kilo Amperes) and high temperatures
(*30,000 K) within the lightning channel, lightning presents a major natural hazard to
power companies, civil aviation, wind farms, high tech industries, forestry managers, and
the public in general. Thousands of people are killed every year by lightning bolts, while
tens of thousands are injured as well (Cooray et al. 2007). Most commercial airliners are
struck about once a year by lightning; however, due to the protective metal skin, generally
C. G. Price (&)
Department of Geophysical, Atmospheric and Planetary Science, Tel Aviv University, Tel Aviv, Israel
e-mail: [email protected]
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little damage is incurred. Tens of thousands of fires are also ignited by lightning every year,
generally in temperate or high latitudes (e.g., Canada, Siberia, etc.) (Stocks et al. 2003). In
such cases, tens of fires can be ignited locally on the same day as a storm passes through,
causing major problems for fire crews and fire management.
The distribution of lightning around the globe is not random, following the general
circulation patterns of the atmosphere, driven by solar heating (Price 2006), while also
being influenced by the continental/oceanic design of our planet. In addition, water vapor,
and particularly the release of latent heat during condensation and freezing, plays a vital
role in thunderstorm development. Since the saturation water vapor concentrations increase
*7 % for every one degree increase in temperature (Clausius–Clapeyron relationship), the
tropical atmosphere has an order of magnitude more water vapor than the polar atmospheres. For the above reasons, the vast majority of lightning occurs in the tropics between
30 N and 30 S, while 90 % of all lightning occurs over continental regions and in the
summer hemisphere (Christian et al. 2003). The planetary distribution of transient luminous events (TLEs) also follows this global distribution since TLEs are only known to
occur above thunderstorms. However, the distribution of certain TLEs (such as halos and
elves) appears to be somewhat different, with elves more prevalent over the oceans than
the continents (Chen et al. 2008).
Lightning activity in convective clouds depends both on the microphysics and dynamics
of the clouds. It is now well known that the electrification process in thunderstorms is
related to the existence of hydrometeors in different phases and sizes interacting with each
other through collisions, freezing, melting, coalescence, and breakup (Williams et al.
1991). In the thunderstorm, there is a layer where we can find liquid water (supercooled),
ice crystals, snow, hail, and graupel (soft hail) all existing together. This is between the
0 °C isotherm and the -40 °C isotherm. At temperatures higher than 0 °C, all ice will start
melting and turn to water drops. At temperatures below -40 °C, all hydrometeors will be
frozen solid. However, water can exist in the liquid form at temperatures below 0 °C and
above -40 °C, called supercooled water. It has been shown in laboratory studies that
collisions between all these particles (especially ice and graupel), in this mixed-phase
region of clouds, is key for the charge transfer between cloud particles (Takahashi 1978;
Saunders et al. 1991). Cloud particle collisions are thought to be the main mechanism for
cloud electrification. Rebounding particles carry away equal and opposite charges. When
we find predominantly ice crystals in the mixed-phase region, with small amounts of
supercooled water and graupel, we get little electrification (Takahashi 2006) and little
lightning. However, it should be noted that in thunderstorm anvils with no liquid water or
graupel, in situ charging has been observed (Kuhlman et al. 2009).
What determines which clouds have hail, graupel, and supercooled water in the mixedphase region of convective clouds? This region in summer thunderstorms can extend from
around 2–10 km altitude and, therefore, we need significant updrafts in these clouds to
carry the heavier particles up above the freezing level. It turns out that the threshold updraft
speed is around 10 m/s. Clouds with little lightning activity (maritime clouds) often have
maximum updrafts less than 10 m/s (Deierling and Petersen 2008), while electrically
active storms have updrafts reaching up to 50 m/s. In addition to the transport of larger
hydrometeors into the mixed-phase region of clouds, the stronger updrafts also enhance the
collisions between different-sized particles. Increased collisions result in increased charge
transfer between particles, leading to rapid charge buildup in clouds.
It should be noted that the updraft speed may influence electrification, but not necessarily the production of rainfall. While weak updrafts cannot carry supercooled droplets
above the freezing level, the collision and coalescence between drops of different sizes can
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very efficiently result in ‘‘warm’’ rain production, with no involvement of the ice phase
(Johnson 1981; Beard and Ochs 1993). Such heavy rainfall is seen in many monsoon
regions, as well as over the equatorial oceans, but with little lightning activity. In fact, the
rainiest regions of the globe paradoxically have the least lightning activity (Price et al.
2009).
Charge transfer between particles is not a sufficient condition to produce lightning. To
build up large-scale electric fields in convective clouds, we need to separate the positively
charged particles from the negatively charged particles in the cloud. If all particles in
clouds randomly got either positively or negatively charged, we would have a cloud filled
with charged particles, but with no net electric field on the large scale (kilometers).
However, it turns out that the smaller ice crystals in clouds, in general, acquire a net
positive charge due to all the collisions, while the larger graupel acquire a net negative
charge (Saunders et al. 2006). Due to their different sizes (and terminal velocities), the
smaller positive crystals are carried aloft to the top of the cloud by the updrafts, while the
larger negative hail stones drift downwards to the base of the cloud. In this way, within
20 min of the formation of the cloud, we can build up regions of positive and negative net
charge, with electric fields approaching the breakdown electric field in air. When we pass
that threshold, we see lightning, and hear thunder.
So the lightning activity in thunderstorms is directly related to the microphysics and
dynamics of thunderstorms, which are intertwined since the dynamics impacts the
microphysics, and vice versa due to latent heat release as the storm develops. However,
thunderstorm cells often follow a regular cycle of birth, development, maturity, decay, and
dissipation (Fig. 1). This cycle can take around 1 h, with new thunderstorm cells forming
on the cold outflow boundary of previous cells. From the birth of the convective cloud to
its dissipation, the cloud is electrified at different levels, while we only see the lightning
activity during the developing, mature, and decaying stage. The lightning activity itself
follows a specific pattern, with the intracloud (IC) lightning normally appearing first
(during developing stage), followed by the cloud-to-ground (CG) lightning starting during
the mature stage (Williams et al. 1989). Both types of lightning can occur during the
decaying stage. In addition to the lightning changes, the mature stage can be associated
with heavy rainfall, hail, and tornadoes, while the dissipating stage is known to be associated with downdrafts, microbursts and wind shear.
2 Lightning and Weather
As mentioned above, lightning in thunderstorms is strongly linked to the microphysics and
dynamics of thunderstorms and, hence, changes in the lightning activity can tell us about
changes in the internal processes within the thunderstorms. Both the amount of lightning in
thunderstorms, as well as the polarity of the lightning discharges, are found to change
associated with specific severe weather phenomenon (Williams 2001; Dotzek and Price
2009). The amount of lightning can be related to the intensity of the updrafts, which affects
the rate of charge transfer and charge separation. As regards to the polarity of CG lightning, this can change by either changing the temperature in the charging region or by
changing the liquid water content (LWC) in the charging zone (Takahashi 1978; Saunders
et al. 1991).
Hailstorms have been studied in many countries due to the damage caused both to
agricultural harvests, as well as the damage to property, cars, etc. Hail size is directly
related to the updraft speed, with pea-sized hail needing updrafts of 35 km/h, while
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Fig. 1 The various stages of development and decay of a thunderstorm cell and the associated lightning and
possible severe weather associated with each stage. Updrafts (black arrows) and downdrafts (white arrows)
are shown relative to their intensity. IC and CG lightning are show in red either in cloud or below cloud
base. The approximate time (minutes) between each stage is shown below
grapefruit-sized hail needs updrafts reaching 160 km/h, vertically upwards! Hail forms by
multiple ascents and descents within the thunderstorm, ascending through the mixed-layer
region, where any supercooled water colliding with the hailstone will freeze and build a
new layer (like an onion) on the hailstone. While descending from the top of the mixedphase region, the hail will collect another layer of water that will add to the mass of the
hailstone. Eventually, the hailstone will be too heavy for the updrafts to support the weight,
or the hailstone will exit the updraft and fall out of the cloud. A number of studies have
shown a link between lightning activity and hail occurrence on the ground (MacGorman
and Burgess 1994; Carey and Rutledge 1998; Emersic et al. 2011). Chagnon (1992) and
Montanya et al. (2009) have shown that lightning activity rapidly increases at the time of
hail occurrence on the ground. Liu et al. (2009) have shown that the polarity of the CG
lightning shifts to being primarily positive (?CG) during the hail portion of the storm.
Generally, non-severe thunderstorms have CG lightning that is predominantly negative
polarity (carries negative charge to ground).
Tornadoes are also associated with certain lightning signatures. Two distinct signals
have been observed. The first is what is called the lightning ‘‘jump’’ in total lightning
(IC ? CG) 10–20 min before the touchdown of a tornado (Kane 1991, Perez et al. 1997;
Weber et al. 1998; Williams et al. 1999; Schultz et al. 2009; Gatlin and Goodman 2010). In
addition, numerous studies also show a shift in the CG polarity to positive lightning around
the time of tornado sightings. Carey et al. (2003) showed that during an episode of 5
tornadoes within 1 h, the ?CG fraction increased to *60 % of all CG lightning. This shift
in lightning polarity is a common feature of tornadic storms. Another type of severe wind
storm with straight-line winds is called a derecho. Such severe thunderstorms can cause
tremendous damage over hundreds of kilometers. Price and Murphy (2002) studied a
derecho that exhibited predominantly positive CG lightning activity during the most
intense part of the storm. As mentioned above, polarity changes in the laboratory have been
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linked with changes in LWC or temperature. Hence, such changes in thunderstorms may
tell us something about the inner workings of these severe storms.
Due to the strong link between lightning, cloud microphysics, and cloud dynamics,
lightning in individual storms is generally positively correlated with rainfall amounts.
However, this relationship is very variable based on location and season (Piepgrass et al.
1982; Petersen and Rutledge 1998; Gungle and Krider 2006; Price and Federmesser 2006).
In particular, rapidly developing storms can produce heavy precipitation in continental
regions that can result in flash floods due to heavy rainfall in short periods of time.
Furthermore, lightning activity has been observed to precede the rainfall by 10–20 min,
allowing for some short-term forecasting skill. For this reason, a number of groups have
attempted to use lightning data to estimate regions of heavy rainfall and possibly flash
flooding (Price et al. 2011; see www.flashproject.org).
Tropical storms, hurricanes, and typhoons have embedded within them thunderstorm
cells that influence the development and intensification of these monster storms (Molinari
et al. 1994; Samsury and Orville 1994; Black and Hallett 1999). In recent years, we have
started to study these oceanic storms using global lightning networks (Price et al. 2009).
Since these storms have lifetimes of 1–2 weeks, migrating thousands of kilometers, we
need global monitoring to track and study them. It has been shown recently that lightning
activity in hurricanes peaks 24 h before the peak intensity of the storm (maximum winds)
(Price et al. 2009). The lightning activity appears to act as the pulse of the storm and may
allow us to better forecast the intensification of these killer storms.
Finally, lightning is also a major cause of forest fires in temperate latitudes, burning
more than 1.5 million hectares of wilderness in Canada alone every year (Stocks et al.
2003). Here too, ?CG lightning appears to be a key in the ignition of fires, since the CG
flashes generally are characterized by the presence of ‘‘continuing current’’ that allows
enough time for the biomass to ignite (Latham and Schleiter 1989). When the current pulse
is too short (\100 ls) the lightning flash is less likely to start a fire. Hence, tracking the
polarity in lightning across regions may supply important information about the likelihood
of forest fires due to lightning.
Due to the radio waves emitted by the lightning discharges, we can monitor lightning
activity in storms from great distances, with information about the time, location, polarity,
peak current, and multiplicity available from ground detection networks (Abreu et al. 2010;
Lagouvardos et al. 2009). By 2017, both the Europeans and the Americans will launch
lightning sensors into geostationary orbit, allowing us hemispheric observations of total
lightning continuously in time and space (Stuhlman et al. 2005; Goodman et al. 2010). This
will allow us to better monitor severe weather and perhaps to provide better information
about the storm severity, intensification, precipitation, etc., allowing us to better warn the
public and other stake holders in real time (Kohn et al. 2010).
3 Lightning and Climate
The distribution of lightning and thunderstorms around the globe is closely linked to the
Earth’s climate. The climate is strongly influenced by the general circulation of the
atmosphere, driven by the Hadley circulation between the equator and the mid-latitudes
(Price 2006). The general circulation also determines the location of the Earth’s two jet
streams (subtropical and polar) that further influence the location of storms. In addition, the
physical size, location, and orientation of the continental land masses further impact the
distribution of lightning and thunderstorms around the globe (Christian et al. 2003). While
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the continental structure will not change significantly in the coming decades to centuries,
the Earth’s climate may significantly change in the near future due to increasing greenhouse gases in the atmosphere (IPCC 2007).
It has been shown by many studies that lightning activity, thunderstorm days, or indices
linked to global lightning activity (ionospheric potential, Schumann resonances, etc.) are
sensitive indicators of surface temperature changes (Williams 1992; Price 1993; Williams
1994; Reeve and Toumi 1999; Markson and Price 1999; Price and Asfur 2006; Markson
2007, Williams 2009). These studies show that, on different temporal and spatial scales,
small changes in surface temperature result in large changes in thunderstorm and lightning
activity (a nonlinear link) (Williams 2005; 2009). But what about the future? Can we use
diurnal, seasonal, or interannual warming as a proxy for future global warming?
The IPCC (2007) report predicts a global warming of 1–5 °C by the end of this century,
depending on the scenario we use for future uses of energy and land use. However, one of
the weaknesses of these predictions is the simulation of convective clouds, which are a subgrid scale process that needs to be parameterized in climate models (Del Genio et al. 2007;
Futyan and Del Genio 2007). While climate models have problems accurately modeling
convective clouds, they are even more problematic modeling lightning activity, although
this has been attempted (Price and Rind 1994a, b; Grenfell et al. 2003; Shindell et al.
2006).
For understanding lightning changes in the future, we need to not only look at surface
temperatures, but also the temperature profile (lapse rate) in the atmosphere as greenhouse
gases increase. There are three possibilities regarding the mean vertical temperature profile
in the troposphere as greenhouse gases increase and surface temperatures warm (Fig. 2). If
the surface warms more than the upper troposphere, the atmosphere will become more
unstable (on average), and we would expect more convection and thunderstorms. If the
surface and the upper troposphere warm at the same rate, then no change will be seen in the
lapse rate and, hence, there will be no change in the mean stability (or instability) of the
troposphere. Finally, if the upper troposphere warms more than the surface temperatures,
this will stabilize the atmosphere, with likely fewer thunderstorms developing in a warmer
world. Climate models of all complexity and sizes show that, as the climate warms at the
surface, the tropical upper troposphere (exactly the location of most of the global thunderstorms) warms even more (IPCC 2007). The reason for this is that increased convection
transports additional water vapor into the upper atmosphere where it acts as a strong
greenhouse gas, absorbing infrared radiation emitted from the surface of the Earth. The
increase in water vapor results in a larger warming in the upper troposphere than at the
surface, resulting in the stabilization of the tropical atmosphere. However, within the
thunderstorms themselves the instability, measured by the convective available potential
energy (CAPE), tends to increase in a warmer climate (Del Genio et al. 2007), especially
for the most intense thunderstorms. And increases in CAPE in the present climate show
clear increases in lightning activity (Williams et al. 1992; Pawar et al. 2011; Siingh et al.
2012). Therefore, when these same climate models are run under a scenario with a doubled-CO2 atmosphere (a situation we will reach by the middle of this century), the models
show increases in lightning activity, of approximately 10 % for every 1 °C global warming
(Price and Rind 1994b; Grenfell et al. 2003). Locally that increase can be much larger. This
appears to present a paradox—more lightning as the global mean atmosphere stabilizes.
However, it appears that the CAPE in thunderstorms actually increases in a warmer climate, while in the fair weather troposphere, the overall instability decreases.
To understand this apparent paradox, we need to return to the link between rainfall and
lightning in the present climate (Price et al. 2009). When we look at global maps of
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Fig. 2 The mean vertical temperature profile in the lower atmosphere (troposphere). The normal dry
adiabatic lapse rate (*10 °C/km) is shown in bold by Cd. Possible mean future lapse rates are shown by C1,
C2, and C3 all having the same surface warming DTs, but different upper troposphere warmings
precipitation and lightning, we can rank the tropical continental regions according to
rainfall: (1) Southeast Asia (most), (2) South America, and (3) Africa (least). The ranking
according to lightning is opposite: (1) Africa (most), (2) South America, and (3) Southeast
Asia (least). In addition, if we look at the very wet maritime regions along the intertropical
convergence zone (ITCZ), there is very little lightning. Hence, on a spatial and temporal
mean (climate scales), lightning is associated with drier regions of the globe. The wet
monsoon regions and maritime convection show little lightning, perhaps due to the warm
rain processes (cloud temperatures above 0 °C) being dominant in those regimes. An
additional example of a short-term climate change is the El Nino phenomenon. The largest
impact occurs in the Pacific region, with the western Pacific (Indonesia, Borneo, northern
Australia) experiencing major drought conditions during El Nino years. Satellite data from
the tropical rain measuring mission (TRMM) have shown us that, during the severe
drought period of 1997/8, the lightning activity increased nearly 60 % (Hamid et al. 2001;
Yoshida et al. 2007). The explanation is that, while there were fewer thunderstorms during
the El Nino dry period, those that did develop were much more explosive, producing much
more lightning activity. Could this be true for the future in climate models as well? Should
we expect fewer thunderstorms in the future, but more explosive storms when they do
occur? The models tend to agree with this prediction (Price and Rind 1994b; Del Genio
et al. 2007).
As the climate changes, and world population expands, we can also expect to see
changes in aerosol loading of the atmosphere (air pollution, biomass burning, dust storms,
etc.). How will this impact lightning activity? Some studies show that lightning can be
impacted by changes in aerosol loading (Lyons et al. 1998; Steiger et al. 2002; Bell et al.
2009; Rudlosky and Fuelberg 2011), showing changes in either polarity or amount of
lightning as the aerosol loading changes. Every cloud drop nucleates on a hygroscopic
particle called a cloud condensation nucleus (CCN). Ice crystals also need nuclei, called ice
nuclei (IN), for the initiation of ice crystal growth in clouds. Changing the concentration,
size, and chemistry of CCN and IN will change the microphysics of thunderstorms. The
number concentrations of CCN have a particular influence on the cloud microphysics and
development of thunderstorms. In a polluted atmosphere, the CCN compete with each
other for moisture and, hence, we get more drops, but smaller drops, in the cloud. The
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result is that the smaller drops can be lifted to higher altitudes in the cloud by the updrafts,
increasing the chances of entering the mixed-phase region of the storm (electrification
region) and therefore increasing the chances of charge transfer between hydrometeors
(Rosenfeld et al. 2008). Furthermore, if the droplets reach the altitude of freezing, the
additional release of latent heat due to freezing will add buoyancy to the cloud, further
enhancing the development of the thunderstorm.
Altaratz et al. (2010) investigated the regional impact of increased aerosol loading in the
Amazon region (due to biomass burning) on lightning activity. They used the MODIS
aerosol optical depth (AOD) index as a measure of the amount of aerosols in the atmosphere, together with the world wide lightning location network (WWLLN) lightning flash
amount from a global VLF network. They found that, as the AOD increased from 0 to 0.25
(relatively clean atmosphere), there indeed was an increase in the regional lightning
activity. However, on days when the AOD increased beyond 0.25 up to values of 0.8, the
lightning activity started to decrease with increasing AOD, giving a ‘‘boomerang’’ effect.
At low AOD values, lightning activity increases with increasing aerosols. At high AOD
values, lightning activity decreases with increasing AOD values. The increasing lightning
at low AOD values is explained above due to microphysical processes. However, what is
happening on days when the AOD is very high? The explanation is possibly related to the
radiative effect of the aerosols when their concentrations are high. The aerosols in the
lower troposphere absorb some of the incoming solar radiation, heating the mid troposphere, while cooling the surface. This stabilizes the local atmosphere, choking the
development of convection and thunderstorm cells. So when the aerosol loading is too
high, the radiative effect outweighs the microphysical effect, and lightning activity
decreases. Hence, as the climate changes in the future, we will also need to understand the
changes in aerosol loading in different regions of the globe in order to understand how
thunderstorms and lightning will change. Long-term aerosol changes have been linked to
‘‘global dimming’’ before 1985 as the global pollution and aerosol loading increased,
followed by a ‘‘global brightening’’ since 1985 as air quality regulations have improved
(Wild 2012). Global increases in aerosol loading act to reduce global warming, cooling the
surface either directly by the backscatter of solar radiation or indirectly through changes in
cloud albedo or lifetime (Lohmann and Feichter 2005). It is interesting to note that precipitation and number of thunder days also tend to follow this long-term trend in global
aerosols (Chagnon 1985; Gorbatenko and Dulzon 2001; Wild 2012).
While future climate change may have direct impacts on the distribution and intensity of
lightning and thunderstorms, lightning itself impacts the Earth’s climate by the production
of nitrogen oxides (NOx) that later influence tropospheric ozone concentrations (Price et al.
1997a, b; Schumann and Huntrieser 2007). Since NOx has a short lifetime of days in the
upper troposphere, it does not have any significant impact on the radiative balance of the
atmosphere. However, ozone has a much longer lifetime of weeks in the upper troposphere
and therefore can act as a strong greenhouse gas. Ozone has infrared absorption bands in
the middle of the ‘‘atmospheric window’’ region of the spectrum (8–11 lm), influencing
the ability of the planet to cool itself. Hence, changes in lightning activity in a warmer
climate may result in positive feedbacks that amplify the initial warming due both to ozone
and water vapor feedbacks (Price 2000). It should be noted that the concentrations of
tropospheric CO2 and water vapor are still much larger than of tropospheric O3, and,
therefore, the overall contribution of lightning NOx and O3 to global climate feedbacks is
small, although the regional impacts may be significant.
Many field campaigns have observed high concentrations of NOx in the anvil regions of
thunderstorms, where the lifetime of NOx is a few days, compared with only hours in the
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boundary layer (Huntrieser et al. 2002, 2007; Skamarock et al. 2003). Lightning is likely
the largest source of NOx in the upper troposphere. When looking at the ozone generated
by NOx from lightning, Martin et al. (2007) showed that in the tropics, where the concentration of tropospheric ozone is quite low, lightning is a significant source of this
greenhouse gas. This was further supported by Labrador et al. (2004) and Grewe (2004)
who used models to separate out the different contributors of NOx to the atmospheric O3
fields. They showed that lightning is the most important source of tropospheric ozone in the
upper tropical atmosphere, as well as in the southern hemisphere mid-latitudes.
Finally, lightning itself may be used as a tool to monitor changes in the climate system.
As discussed above, lightning appears to be a sensitive thermometer of regional and global
temperatures (Williams 1992, 2009). Others have shown lightning to be a sensitive indicator of upper tropospheric water vapor (Price 2000; Price and Asfur 2006), clouds (Sato
and Fukunishi 2005), ice crystal size (Sherwood et al. 2006), and ice water path in
thunderstorms (Petersen et al. 2005). Since lightning activity today can be monitored with
either VLF or ELF networks (Rodger et al. 2006; Betz et al. 2009; Fullekrug and Constable
2000), we now have the opportunity to use lightning as a convenient, cheap, reliable tool to
study the Earth’s climate. However, due to increasing improvements in detection efficiency
of lightning networks, care is needed when trying to detect long-term natural trends in
lightning and thunderstorm activity.
4 Summary and Conclusions
Global lightning and thunderstorm activity is driven first and foremost by the Earth’s
climate, driven by solar insolation that varies with latitude, longitude (land/ocean), season,
and hour. The climate drives circulation patterns that promote thunderstorms in the tropics
and midlatitudes and inhibit thunderstorms in the subtropics and polar regions. Locally,
thunderstorm activity depends on surface temperature, water vapor, the tropospheric lapse
rate, as well as aerosol loading. These parameters can impact the intensity and polarity of
lightning in thunderstorms.
It has been shown by numerous studies that the types of severe weather phenomena
produced by thunderstorms are linked to anomalous lightning signatures. Tornadoes,
derechos, and hail storms often show shifts in lightning polarity (more ?CG), while
sometimes jumps in total lightning activity are also precursors for tornadic activity. In
recent years, hurricanes have also been shown to provide lightning signatures related to
their development, with lightning activity peaking 1 day before the maximum intensity of
tropical storms.
The thunderstorms themselves are directly linked to the production of NOx that leads to
the production of tropospheric ozone, a strong greenhouse gas. Furthermore, thunderstorms
act as huge vacuum cleaners, sucking up large amounts of boundary layer water vapor and
dumping it in the upper troposphere. Water vapor is also a strong greenhouse gas, with
strong feedbacks in the upper troposphere. Hence, thunderstorms themselves can act as a
positive feedback on the climate system if they increase in the future due to global
warming. Although not discussed in depth, lightning can also ignite fires in temperate and
boreal forests, with associated emissions of greenhouse gases and aerosols. Hence lightning, via fire ignition, may provide another means of acting as a climate feedback.
Due to the radio waves emitted by each lightning discharge propagating around the
planet, there are now reliable ways of monitoring regional and global lightning activity in
real time and relatively cheaply. Hence, lightning observations should be considered a
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useful tool to monitor severe weather, while keeping track of the changing climate of
Earth.
Acknowledgments This paper was the result of an invited lecture at the European Science Foundation
TEA-IS Summer School entitled ‘‘Thunderstorm effects on the atmosphere–ionosphere system’’. Support
from ESF is appreciated.
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