Download Selected topics in Atmospheric Electricity

Selected topics in
Atmospheric
Electricity
Charging mechanisms
Prof. Yoav Yair, IDC Herzliya
The updated view – mature thunderstorm
(super-cell) (Stolzenburg et al., JGR, 1998)
Observed relations between cloud parameters and
lightning
• Dierling et al. [JGR, 2008]: a linear relationship between the flash rate (f, in fl.
min−1) and the precipitation ice mass (pm, in kg) for temperatures colder
than −5°C.
• Peterson et al. [JGR, 2005]: The flash density over land (FD in fl km-2 day-1)
can be deduced from the ice water path (IWP in kg m-2).
• Price and Rind [JGR, 1992]: The flash rate in continental storms is
proportional to cloud top height H (in km) and sometimes expressed as a
function of the maximum velocity w in m sec-1 (where k=4.5)
Liu et al. [JGR,
2012] used 13
years of TRMM
data and found a
high correlation
between the flash
rates and the
volumes with
radar reflectivity
greater than 30,
35, or 40 dBZ in
the mixed phase
region.
Radar reflectivity and flash rates
Liu et al. [JGR, 2012]
Charge structure as a function of time
(Emersic et al., MWR, 2011)
Dolan and Rutledge (MWR, 2010)
The LMA maps VHF sources that indicate
IC and CG activity. Note the correlation
between f and the appearance of large
reflective volumes of graupel
Formation of the stepped leader
‫בעת התקדמות המוביל‬
‫המדורג בתוך הענן‪ ,‬הזרם‬
‫פולט קרינה‬
‫אלקטרומגנטית בתחום ה‪-‬‬
‫‪ VHF‬אשר ניתנת לגילוי‬
‫ואיכון במרחב‬
‫כך נוצרת תמונה תלת‬
‫ממדית של התקדמות‬
‫תעלת הברק בזמן אמת‬
‫לא ניתן לחזות היכן יפגע‬
‫הברק אבל אפשר לזהות‬
‫את מרכזי המטען ואת‬
‫נקודת הפגיעה בקרקע‬
‫כמה מובילים מדורגים מתקדמים לעבר הקרקע ויוצרים מספר ענפים‬
‫שלב זה סמוי מהעין וניתן לזיהוי‬
‫במצלמות רגישות בלבד או על ידי‬
‫מערכות לחישה מרחוק בתום גלי רדיו‬
‫‪VHF‬‬
‫‪20:25:40.669‬‬
‫המוביל המדורג מתקדם‬
‫בתוך הענן בצעדים‬
‫שאורכם ‪ 30‬עד ‪90‬‬
‫מטר עם הפוגות של ‪20‬‬
‫עד ‪ 100‬מיקרושניות‪,‬‬
‫ומוריד מטען שלילי‬
‫לעבר הקרקע‬
‫כאשר הקצה של ענף מסוים מהמוביל המדורג מתקרב‬
‫לקרקע‪ ,‬השדה החשמלי בסמוך לקרקע מתחזק מאד‬
‫וכתוצאה מכך נוצרות נקודת פריצה רבות מהן בוקע ניצוץ‬
‫עולה‪ .‬אחד מהם מתחבר למוביל היורד בנקודת צומת‬
‫שבדרך כלל נמצאת כמה עשרות מטרים מעל פני השטח‪.‬‬
‫נקודת פגיעה מרכזית אחת דרכה יש מכת‬
‫החזר‬
‫תעלת הברק מתלהטת ופולטת אור בעת‬
‫מעבר הזרם הגבוה (עשרות אלפי אמפר)‬
‫התלהטות האוויר גורמת להתפשטותו‬
‫המהירה וליצירת גל אקוסטי על‪-‬קולי‬
‫שהוא הרעם‬
‫‪20:25:40.716‬‬
‫נקודת החיבור בין המוביל המדורג לבין ניצוץ שעולה מקצותיהם של עצמים מחודדים‬
‫(עצים‪ ,‬עמודים) יוצרת צומת ‪ Junction‬ש"פותחת את המעגל" לזרימה מסיבית של‬
‫אלקטרונים לקרקע‬
‫לא כל הניצוצות מצליחים להתחבר לתעלה הראשית‬
Observational Constraints (I)
Moore and Vonnegut (in “Lightning, vol.1 : Physics of lightning”, pp 51-98, 1977)
• Cloud depth should be at least 34 km, taller is better
• Strong electrification is not
observed unless the clouds
extends above the freezing level
(mixed-phase region)
• Highly electrified regions usually
coincide with the coexistence of
ice and super-cooled water
• Strong electrification coincides
with strong convective activity
and rapid vertical growth
• The location of charge centers is a
function of temperature
Observational Constraints (II)
• Charge generation and
separation processes are
closely associated with the
development of
precipitation (graupel, hail).
• First flash occurs ~ 12-20
minutes after radardetectable precipitation
particles appear.
• The average duration of
precipitation and electrical
activity is ~ 30 minutes.
Cloud particles sizes and fall speeds
•
•
•
•
•
•
•
Cloud droplets: r < 0.1 mm, V~ 0.01 m/sec, n=1,000,000 per liter
Drizzle: 0.25 mm > r > 0.1 mm, V > 0.3 m/sec
Rain: r > 0.25 mm, V > 0.5 m/sec, n=1 per liter
Cloud Ice, size = F(T), n= F(T)
Snow (aggregates, crystals): V ~ 0.3 – 1.5 m/sec
Graupel (soft hail): V ~ 1-3 m/sec ; Density ~ 0.7 g/cm3
Hail: r > 10 mm ; 50 m/sec > V > 3 m/sec
Water – a polar material
• The electron density distribution for
water shows some higher density
contours around the oxygen atom. For
an isolated molecule, the calculated OH length is 0.9584 Å and the H-O-H
angle is 104.45°
• Polar molecules, where the centers of
positive and negative charge are
separated, possess dipole moment.
This means that in an applied electric
field, polar molecules tend to align
themselves with the field.
The molecular structure of ice
• Each water molecule is
tetrahedrally coordinated to
four other water molecules,
forming a regular array of
Hydrogen bonds.
•
• Water-ice has a very open
structure, that results in a low
density of ice.
• Lattice defects and dislocations
in the ice structure create net
charge that can move within
the ice crystal.
Growth processes in the Ice phase
• Riming: accreted particles freeze
rapidly to the ice/graupel as
distinct particles.
– Fast/Wet growth: latent heat
released during phase change
keeps liquid water on the surface
 fills pores and increases ice
density to 0.9
• Deposition: when supersaturated
with respect to ice, water vapor
condenses directly from the gas
phase to the crystal.
– Sublimation growth regime: if
liquid drops freeze  the rise in T
may cause ice to sublimate
Droplet freezing on a graupel surface
• When a droplet impacts an ice
surface of a graupel particle,
much of the latent heat released
by the riming droplet is
conducted into the ice.
• If the accretion rate is fast
enough (cloud with a large LWC),
the released heat will warm the
ice enough above the ambient
temperature and cause it
sublimate.
• The freezing droplet remains at
0°C throughout the phase change
and so create a source for vapor
to the surrounding annular ice
surface.
• Thus, although the ice is
sublimating, there may be many
spots where growth is taking
place.
Basic Constraints on theories of thunderstorm
electrification
• Charge must be placed on
hydrometeors (cloud
droplets, ice crystals,
graupel, hail, snow…)
– This reduces charge
mobility, and offsets the
influence of electric forces
• Charges of opposite signs
must be isolated or
separated from each
other (gravity/wind)
– This leads to areas on net
charge of either sign
How do we get from the micro-scale separation of charge to
the macro-scale build-up of electric fields?
Field-Dependant (Inductive) Mechanisms
Such mechanisms rely on pre-existing fair-weather electric field
1. Charging by convection (Convective charging)
2. Diffusion / Ion drift
3. Drop breakup
4. Selective ion-capture (Wilson effect)
5. Particle rebound (Inductive charging)
1. Convective charging (Grenet (1947), Vonnegut
(1953)
• Atmospheric ions are carried by cloud updrafts inside the developing
cumulus cloud and by compensating downdrafts outside.
• Since positive ions are more abundant near the ground, they are carried
aloft and attract small negative ions from the clear air above the cloud.
• The negative ions attach to cloud particles and create a screening layer at
the outer cloud boundary, which is carried by downdrafts to the lower part
of the cloud, increasing positive ion production by point discharge and
forming a positive-feedback.
• In experiments by Moore et al. (JGR, 94, 13,127-34,1989) large
amounts of negative ions were released from the ground and
ingested into developing cumulus clouds.
• The expectation was to produce an inverted dipole with positive
charge below the negative charge.
• One case showed the expected result, suggesting that ions in the
boundary layer play a role in cloud charging.
Convective charging - critique
• Both negative and positive ions in updraft
• The concentration of ions is too low
• Corona onset on the ground (800 V m-1) appears
only after significant charge is separated in the
cloud, and thus cannot drive the positive feedback
• Mixing in cloud too dynamic and chaotic
• Dilution and evaporation
• Intra-cloud flashes diminish the operation of this
mechanism.
2. Diffusion charging
•
•
At the early stages of cloud development, cloud droplets are charged due to the
attachment of atmospheric ions (which are generated by cosmic rays).
The ion-transport equation gives the current density as Ji = ρi U + ρi B i E - D i grad ρi
•
Under steady state conditions, and neglecting the flux term and assuming a very
weak initial E that allows to ignore the ion drift term, we get the solution
ρ i = ρ i 0 exp (-4πR Di N t)
•
•
Equilibrium between ion production, recombination and depletion due to
attachment to droplets is achieved within ~ 10 seconds.
The amount of charge on a droplet from a Boltzmann distribution is ~ 20e.
3. Break-Up charging of liquid drops
• Collisions between raindrops
(r=1-6 mm) and drizzle (r=0.1-1
mm) may result in fragmentation
and breakup.
• If we consider the polarization
charge on a spherical drop to be
Q=3πe0 R2 E
we can get the charge that is
separated when a drop is sliced
due to collision.
• The equation gives a conservative
estimate of the charge –
deformation may increase the
effect of polarization by a factor
of 4 (b).
Taken from Beard and Ochs, 1986
The electrical double layer
• An electrical double layer is
defined as a dipolar layer at the
interfaces between two
substances. Such a layer is
assumed to exist at the interface
water-ice, water-air and ice-air.
• Fletcher (1968) showed that at the
interface with air, water molecules
will be oriented such negative
vertices will point outward.
• Double layer can participate or
effect charge separation (for
example by exploding bubbles).
Air
+
+ + + +
+
Water
Iribarne (1972): the potential across the
water-air interface varied from +0.4 to 0.5 V, but most studies show negative
charge outside of positive charge.
Spontaneous break-up of water drops
Liquid water drops that
exceed a critical size may
spontaneously break-up
(without collisions). The
smaller fragments carry
negative charge and the
larger drop becomes
positively charged (this is
known as the Lenard or
Waterfall Effect).
For a sheet breakup (b) occurring in
a downward directed field, the
larger particle will carry positive
charge, and the fragments will carry
an equal charge distributed among
them.
Coulomb fission: Rayleigh jets from levitated
micro-droplets
At the moment of
injection, the droplet
radius is 58 microns
and its charges is ~3.3
pC
•
•
•
The image is observed through a microscope with a long working distance a–f, Microscopic
images taken at t values (in µs) of: a, 140; b, 150; c, 155; d, 160; e, 180, and f, 210.
The droplet changes from a sphere to an ellipsoid (a), tips appear at the poles (b) and a fine
jet of liquid is ejected from each tip (c); the jets disintegrate (d) and the elliptical droplet
reassumes a spherical shape (e, f). Scale bar, 100 µm.
During the jet's disintegration, roughly 100 small daughter droplets are formed, which carry
33% of the total charge and constitute about 0.3% of the mass of the mother droplet. The
diameter of the jet is determined from higher-resolution images to be 1.5 µm
4. Charging by selective ion attachment
(Wilson effect)
• Raindrops polarized by the ambient
electric field when falling, would
preferentially attract negative ions to
the lower part and repel positive ones.
• The drop would gain negative charge
and enhance the field (The Wilson
Effect).
• The maximum charge acquired by
droplets is given by
Q= 3πε0 R2 E,
corresponding to a charge of 36e on a
100μm droplet.
• The charge on a droplet is limited and
lower than observed. Also the
concentration of free ions is insufficient.
5. Polarization of water drops in an external
electric field (Inductive charging)
• (a) A precipitation particle is
polarized by the ambient
vertical electrical field such
that its upper part is negative
and its lower part positive .
• (b) During a collision with a
cloud particle, some of the
positive charge at the bottom
transfers to the small particle
• (c) If no coalescence occurs
(only rebound) it will leave the
larger precipitation particle
with an excess negative charge
equal to δq.
δq = 4πɛ γ1 ΙEΙ r2small ·cosθE, r + A·Qlarge - B·Qsmall
The inductive mechanism: conditions for
effective charge separation (I)
• The colliding particles must separate.
– Jennings (1975): all small liquid cloud droplets coalesce
with raindrops in electric fields strong enough for the
operation of this mechanism (~30 kV/m)
• Contact times must be long enough for charge to flow
between the interacting particles (i.e. the conductivity
must be large so that the charge can be transferred
during contact)
– Numerous laboratory experiments showed that little
charge was transferred in ice-graupel collisions, possibly
because of the low conductivity of natural ice.
The inductive mechanism: the conditions for
effective charge separation (II)
• The inductive mechanism relies on collisions to transfer
positive charge to a particle that will move up and
negative charge to a particle that falls down.
• Thus, the geometry must be such that particles moving
up should collide and separate from the lower surface of
particles falling downward.
– Low and List (JAS, 39, 1591-1606, 1972) show that raindrops
collided, merged and broke apart, with droplets emerging from
the top of the larger drop.
– Latham and Warwicker (QJRMS, 106, 559-568,1972) show that
a raindrop colliding with hail will rotate around it and separate
from its top.
Consensus: inductive charging is a secondary
mechanism
• “The inductive mechanism is attractive because it is simple but in view of
the difficulties it is hard to imagine how it may operate as a viable charge
generation mechanism in thunderstorms”. (Jayarante, 2003; in “The
Lightning Flash” by V. Cooray)
• “Because the inductive mechanism has appeared most likely to have a
significant effect when the preexisting electric field is substantially larger
than the fair weather field, the role usually hypothesized for the
mechanism has been to intensify the electrification initially achieved by
other mechanisms”. (MacGorman and Rust, 1998).
• “The significance of the mechanism to thunderstorm electrification is still
open to question. There are strong doubts about its ability to act as the
primary charging mechanism since it is unable to account for the observed
charges in the early stages of thunderstorms… It seems more likely that it
acts as a contributory mechanism in the later stages of
electrification….”(Brooks and Saunders, 1994).
Non-Inductive Processes (field-independent)
1. Thermoelectric effect in ice (Workman-Reynolds)
2. Contact-potentials effect
3. Grauple-ice collisions
4. Charging during melting
5. Other processes
1. The Thermoelectric Effect in Ice
•
•
•
•
•
In water-ice, the mobility of H+ is an order of magnitude larger than that of OH-.
When a temperature gradient is maintained across an ice crystals, the warmer end
would acquire a net negative charge due to the faster diffusion of H+ ions away
from that end.
Latham and Mason (1961) showed that a potential difference of 2ΔT mV would be
generated across the ice for a steady ΔT. The maximum charge separation occurs
at times of roughly 5-10 ms.
Not only anions are charge carriers, but also lattice defects.
Experimental setting: vapor-grown ice particles or frozen droplets are made to
bounce off an artificially warmed ice target. In all these experiments the warmer
ice particle acquired the negative charge.
The process is too slow to be effective in collisions and the ion concentration in ice
is far too low to account for the observed charge separation in collisions of ice
particles.
The Workman-Reynolds effect
(Phys. Rev., 1950, 78, 254-259).
• Hail particles in “wet growth” regime acquire a thin liquid film (“skin”) on
its surface (due to rise in temperature above 0°C during phase change). If
thick enough, some of this liquid layer may be shed and disperse as
fragments.
• The double layer across the ice-liquid interface will lead to negative charge
on the ice and positive charge on the water droplets.
• The difficulty with this mechanism is that it can only operate in the narrow
range of heights where the temperature is close to 0°C. It cannot account
for the negative charge centers at temperatures colder than ~ -16°C.
• Graupel/hail in wet-growth will charge negatively.
2. Contact Potentials
(Buser and Aufdermaur, 1977; Caranti et al., 1991)
• The contact potential of ice governs the potential difference and current
flow at the interface with the liquid phase.
• Ice formed in different ways has different contact potentials:
– If the ice is sublimating, it will be charged negatively by collisions with
frozen droplets
– If the ice is growing from the vapor by deposition,
it will charge
[
positively
4. Non-inductive graupel-ice interaction
T, LWC
Riming
Graupel
Supercooled
water
Ice particle
Takahashi (JAS, 35, 1536-1548, 1978)
•A 3 mm diameter rod (ice
target, simulating the graupel)
was rotated through a cloud of
super-cooled droplets and
vapor-grown ice crystals at
speeds of 9 m s-1.
•At temperatures warmer than
minus 10°C the rod was
charged positively at all LWCs
•At colder temperatures,
intermediate LWCs produced
negative charge on the
graupel.
Charge (in fC) gained by the rimed graupel as a
function of temperature and liquid water content.
Open circles indicate positive charge, solid ones
indicate negative charge.
Jayaratne et al. (QJRMS, 109, 609-630, 1983)
•A simulated graupel pellet interacted with ice crystals
•Cloud droplets had a mean diameter of 10 μm – 30 μm
•Definition of a “Charge reversal temperature”: at a cloud water content of 1 g
m3, the charging sign was reversed around -20° C
•At warmer temperatures the graupel was charged positively for high LWCs,
while at colder temperatures and low LWCs, the graupel charged negatively.
•Williams et al. (JGR, 99, 10,787-10,792, 1994) showed that both LWC and T
control the physical state of the colliding particles and stated that the “charge
reversal temperature” is a misleading concept: depending on the LWC, any
temperature level can be a charge reversal level.
Saunders et al. (JGR, 96, 11,007-11,017, 1991)
Saunders & Brooks (JGR, 97, 14,671-14,676, 1992)
• Previous experiments by the
UMIST group showed that liquid
droplets not colliding with riming
graupel target had no effect on
the charging process.
• Defined an “Effective LWC” to
compensate for the fact that
some of the droplets are swept
away from the particle due to
aerodynamic forces
• EW is determined by the LWC
multiplied by the fraction of
droplets in the graupel path that
are collided and adhered to the
graupel.
The charging zones as a function of temperature
and effective liquid water content. The velocity
of the graupel target was 3 m s-1 (three times
slower than Takahashi, 1978)
The Integrated View
• Bold dashed lines denote the
charging regions according to
Saunders et al. (1991), as
translated from EW to LWC
(by noting that EW is usually
0.5 of the present LWC)
• It is superimposed on the
charging regions found by
Takahashi (1978)
• The results are in broad
agreement even though the
experimental settings were
different
Non-inductive graupel-ice interaction: Microphysical
explanations in light of observational constraints
• Significant charging occurs
only when the larger particle is
rimed and at least a small
amount of cloud liquid water
are present
• If liquid water contents are
large, graupel becomes
positively charged.
• At small liquid water contents
graupel tends to become
negatively charged
• Near 0°C, graupel charges
positively for most of the
range of liquid water contents
• Though the amount of charge
per collision increases with
size of the ice crystal size, it
approaches a limit
• The results of a single
interaction can differ
substantially from the mean
Gaps in knowledge
• Lack of full understanding of micro-scale
processes, including the details of charge
separation
• The evolution of charge structure and recharge mechanisms after the first lightning is
not fully understood
• Embedding electrification in forecast mesoscale models