Transcript CONVECTIVE UPDRAFT AS DRIVER OF THUNDERSTORM

CONVECTIVE UPDRAFT AS
DRIVER OF THUNDERSTORM
ELECTRIFICATION: POSSIBLE
STUDIES USING WP RASS
P. Pradeep Kumar
Department of Atmospheric &
Space Sciences
University of Pune
1. CHARGE STRUCTURE OF
THUNDERSTORM
• 1.1 What is a thundercloud?
•
Lightning is associated with convective activity.
Thunder (and thus lightning) is used by the
professional weather observer to classify the
severity of convective activity. Cumulonimbus
clouds are the largest form of convective cloud
and typically produce lightning. Cumulonimbus
clouds with lightning activity are generally
referred to as thunderclouds.
1.2
When do Thunderstorms form ?
If the atmosphere is sufficiently unstable, convective
development can result in the thunderstorm, a
cumulonimbus cloud (Cb) with lighting discharge,
possibly hail, and, in extreme cases, accompanied by a
tornado.
Air
mass
thunderstorms
are
thunderstorms not associated with
weather fronts. They form mostly in the
summer as the sun heats the surface
and causes convection.
There are three basic stages in the
lifespan of a thunderstorm:
In the formative cumulative stage, there are updrafts at all points
at the base of the cloud.
In the mature thunderstorm, there will also be a strong downdraft
caused by both (a) the drag of rain and hail falling through the
cloud and (b) evaporative cooling as dry air is entrained into the
cloud.
In the dissipating stage, there are weak downdrafts throughout the
cloud. The typical lifetime of an air mass thunderstorm is one
hour or less.
CHARGE STRUCTURE INSIDE A
THUNDERCLOUD
The classical thundercloud model can be
described as a positive electric dipole with a
positively charged region above a negatively
charged region.
A weaker, positively charged region at the base of
the cloud gives it more of a double-dipole
structure.
The average values taken are p is +10 coulombs (C) at 2 km, N is -40
C at 5 km, and P is +40 C at 10 km. These are representative of values
that can vary considerably with geography and from cloud to cloud.
1.4
What are Super cell Thunderstorms ?
The mesoscale organization is such that the entire storm
behaves as a single entity rather than as a group of cells.
A. Anvil- The Anvil is one of the most
impressive features of a severe storm due
to its aerial coverage and icy texture.
Within a severe storm, moisture is
transported from the lower troposphere to
deep into the upper troposphere.
 B.
Overshooting Top- The core of the updraft
has the strongest convective upward vertical
velocity. This core of rapidly rising air will
only slow down and stop when it encounters a
very stable layer in the atmosphere.
 This very stable layer is the tropopause.
 C.
Mammatus- Mammatus are pouched
shaped clouds that protrude downward
from the thunderstorm's anvil. They form
as negatively buoyant moisture laden air
sinks.
 The cloud remains visible until the air
sinks enough that the relative humidity
falls below 100%.
Charge structure based on charge analyses of 49 soundings through
thunderstorms.(by Stolzenburg, Marshall, and Rust; published in the
Journal of Geophysical Research, Vol. D103, 1998, pp. 14097-14108).
2. THE LIGHTNING FLASH

2.1 Why does lightning occur?
 The charge buildup in thunderclouds are
unstable. When electric fields generated by the
charge buildup become too strong (typically 3-4
kilovolt/cm at the altitude of the negative charge
region of the cloud) electrical breakdown of the
air occurs and charge is exchanged within the
cloud or to the ground. Charge is exchanged by
a lightning flash.
Intracloud (IC) flashes, redistributing the
charge within the cloud, account for over
half the lightning flashes in northern
latitudes.
Cloud-to-cloud and
cloud-to-air flashes are less common.
Aside from aviation, these three types of
flashes have little effect on people.
The negative cloud-to-ground lightning
flash can be broken down into three
stages. The stepped leader, the return
stroke, and the dart leader.
 The
stepped leader is a small packet of
negative charge that descends from the
cloud to the ground along the path of least
resistance.
In
its path, the leader leaves a trail of
ionized gas. It moves in steps, each
typically tens of meters in length and
microseconds in duration.
After
a step, the leader pauses for
about 50 microseconds, then takes its
next step.
The
leader charge packet sometimes
breaks up to follow different paths, giving
lightning its forked appearance.
When the downward moving leader
connects with a surface corona
discharge, a continuous path between
the cloud and the ground is established
and a powerful return stroke is triggered.
The return stroke rapidly moves as a
wave upwards into the cloud following
the ionized trail of the stepped leader,
stripping the electrons from its path.
After the return stroke, the lightning flash
may end, or,
if enough charge in the cloud is collected,
a dart leader may come down from the
cloud following a direct path to the
surface.
In turn, the dart leader triggers a second
return stroke.
A single lightning flash can comprise
several return strokes. The average
number of return strokes in a
lightning flash is 3 or 4, each stroke
typically separated by 40 to 80
milliseconds.
The path of the electrical current is
only a few cm wide but the stroke
heats the air to almost 30,000
degrees C (five times the surface
temperature of the sun). The
resulting shock wave (from the rapid
expansion) of the air) is what we
know as thunder.
2.4 Are lightning flashes positive or
negative?
 Coming from higher altitudes in the cloud,
positive flashes make up about 10% of all
lightning flashes. They are usually
composed of a single stroke, and have
longer, continuing currents. From the
forestry perspective, positive flashes are
of greater concern because the longer
currents are more likely to start fires.
Positive flashes are more
storms. The apparent cause
freezing level, which places
center closer to the ground,
likelihood of a flash.
common in winter
of this is the lower
the positive charge
thus increasing the
A popular theory is that horizontal wind shears
force a tilting of the dipole axis providing a route
for the positive flash, but this has yet to be shown
conclusively.
Red sprites, blue jets and elves are upper
atmospheric
optical
phenomena
associated with thunderstorms that have
only recently been documented by using
low light level television technology.
Red sprites are large but weak luminous
flashes that appear directly above an active
thunderstorm system and are coincident
with powerful positive cloud-to-ground
lightning strokes.
• Their spatial structures range from small single
or multiple vertically elongated spots, to bright
groupings which extend from above the cloud
tops to altitudes up to almost 60 miles (about
95 km.) Sprites are predominantly red and they
usually last no more than a few milliseconds.
Blue jets are a second high altitude optical
phenomenon, distinct from sprites and first
documented in 1994 (although pilots had
earlier reported similar sightings).
Blue jets are optical ejections from the top of
the electrically active core regions of
thunderstorms, but not directly associated with
cloud-to-ground lightning.
Blue starters differ from blue jets in that
the are brighter but shorter (reaching to
only about 12 miles altitude). These were
reported to occur over regions where
large hailstones were falling.
Elves are rapidly expanding (up to 300
miles across) disk-shaped regions of
luminosity, lasting less than a thousandth
of a second, which occur high above
energetic cloud-to-ground lightning of
positive or negative polarity.
Elves most likely result when an
energetic electromagnetic pulse (EMP)
propagates into the ionosphere.
2.6
Is there an association between lightning
activity and radar echoes?
• There is a general association between
radar reflectivity and negatively charged
lightning flashes.
• Lightning discharge sources are located
near, but not necessarily within, the area
of highest reflectivity
3.
CLOUD ELECTRIFICATION
MECHANISMS
There are two general theories to explain the
charge buildup required to electrify a thundercloud.
• (a) The convective theory proposes that free
ions in the atmosphere are captured by cloud
droplets and then are moved by the convective
currents in the cloud to produce the charged
regions.
• (B) The gravitational theory assumes that
negatively charged particles are heavier and are
separated from lighter positively charged
particles
by
gravitational
settling.
For the gravitational theory to work, there must be some
charge exchange process between particles of different sizes.
Charge can be exchanged between particles in various states
by inductive and non-inductive processes.
The most promising is the non inductive exchange between ice
crystals and hailstones, referred to as the ice-ice process.
The effectiveness of the ice-ice process lies in the
thermo-electric properties of ice. The mobility of the
(OH3)+ defect in ice is greater than the (OH)- defect
and the number of defects increase with temperature.
When warm and cold ice particles come in contact,
the positive defect flows faster from the warmer to the
colder particles than the converse, giving the colder
particles a net positive charge.
Therefore in the typical scenario, a warm hailstone or
snow pellet will acquire a net negative charge as it
falls through a region of cold ice crystals.
There are three steps in the process of Ice-Ice
(Graupel or hail) collision in the presence of
supercooled droplets.
• Collisions between hail or graupel and
supercooled water droplets causes
freezing of the previously liquid water.
The release of latent heat maintains the
hail or graupel at a high temperature than
the ice particles.
Collision, but not accretion, between the colder ice
particles and the warmer hail or graupel causes a
charge separation and the transfer of a positive ion
from the warmer object to the colder one.
• Updrafts in the cloud carry the small,
positively charged, ice particles to the
upper part of the cloud while the heavier,
negatively charged, hail and graupel
migrate to lower levels.
WHAT NEEDS TO BE DONE
USING WP/ RASS
• Linking the microphysics and convective
updrafts to charge development and
subsequent lightning discharges needs to
be understood.
• Such an understanding would improve the
way electrical activity is incorporated into
weather forecasting problems.
There is ample evidence that there is link between
convective updrafts and lightning flash rates.
However, their dependence on local regimes
and environmental conditions are still not well
known.
The magnitude and location of
charge destroyed in lightning flash
can be determined by measuring
the electric field change during
lightning flash at spatially separated
locations. Such measurements
have been done over Pune using a
network of electric field stations.
Some observations in other
climatological regions have reported
taller storms that produced no lightning.
• One reason that storm height threshold
sometimes falsely identifies storms as
thunderstorms is that tall storms can have
updrafts with insufficient speeds or spatial
extend to drive electrification mechanisms
to the point of causing lightning.
Some observations show that flash rates
tend to be large on days with large CAPE
(Convective Available Potential Energy).
Others have noted the reverse, that
many storms with low flash rates
occurred on days with large CAPE.
So there are some other factors other
than CAPE which contribute to the
electrification.
The current status shows that
there is considerable uncertainty
in understanding the proper
linkages
to
thunderstorm
electrification.
The role of bulk water properties
like ice/hail/supercooled droplets
have to be properly CAPE,
Convective updrafts and
understood.
Such an understanding would be
helpful in linking the thunderstorm
electrical activity and lightning to
operational weather forecasting.
THANK YOU