Transcript Lect19_thunderstorm

Chapter 10
Thunderstorms and Tornadoes
Thunderstorms
Characterized by strong up- and down- movements, producing
lightning and thunder, and generating gusty winds, heavy rain,
and hail.
Annual number of lighting
Average Number of Thunderstorm Days
~2000 thunderstorms at any given time;
~45,000 per day; ~16 million per year
Air-mass thunderstorms
mT air mass moving northward from the Gulf of Mexico.
Occurring most of the time in the afternoon.
Stages of development of a thunderstorm
Cumulus stage
entrainment
Mature stage
Dissipating stage
Severe Thunderstorm
Definition (NWS), wind: 58 miles/per hour or hailstone: D=1.9 cm or tornado.
~100,000 thunderstorms in US, ~10% reach severe status
Supercell Thunderstorm
Diameter: 20 - 50 km; depth: 20 km
Rotated updraft
~2000 -3000 per year in US
Cluster of Supercell Thunderstorms
Temperature Inversion
Enhances Formation of
Severe Thunderstorms
Squall lines
A squall line is a line of thunderstorms that usually have a common lifting mechanism.
photo
satellite
radar
Mammatus sky preceding squall line
Formation mechanisms
a. Lifting associated with cold front
b. Lifting induced by upper level jet
Jet
divergence
Lifting
Surface strong persistent warm, humidity air from south
c. Lifting created by dryline
Mesoscale Covective Complex (MCC)
Many individual thunderstorms organized into a large oval to
circular cluster
A typical MCC covers an area of at least 100,000 square km
Mostly found in the Great Plains
MCC develops from a group of afternoon air-mass thunderstorms. In the
evening as the local storms decay, MCC starts developing.
It requires a strong low-level flow of very warm and moist air. MCC can remain
self-propagating as the gust fronts lead to the formation of new cells.
Microbursts
Beneath thunderstorms, strong localized
downdrafts, less than 4 km
Formation: downdrafts are accelerated by
a great deal of evaporative cooling.
Can cause a significant atmospheric hazard.
Lightning
The Earth's Electrical Structure
Earth's electrical charge
The Earth is electrically charged. The Earth surface has a net negative
charge, while an equal and positive charge resides in the upper atmosphere.
+ +
++
+
+
+
Averaged fair-weather
electric field near surface
is about 130 V/m
Thunderstorm
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The atmosphere is not a perfect insulator. Negative charge leaks from the
Earth and rises to the electrosphere. So, the Earth's charge would dissipate
in less than an hour if there were no recharge!
What charges the Earth?
Lightning recharges the Earth by delivering negative charges to the surface.
The system can be viewed as an electric circuit in which electrified
clouds are the generator (batteries).
- - -- -- --
--
How do clouds get charged?
1. Convective theory: charged particles are moved by the convective currents to
produce the charged regions.
2. Gravitational theory: charged particles are separated by gravitational settling.
Where do charged particles come from?
Inductive and non-inductive processes
Ice-Ice process (non-inductive): 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 giving the colder particles a net
positive charge. Therefore, a warm hailstone or snow pellet will acquire a net
negative charge as it falls through a region of cold ice crystals.
Collision process (inductive):
Droplet breakup:
E
+
+
- - -
+
+
+
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+
Stages of a Cloud-Ground (CG) Lightning Strike
STEP 1
A CG lightning strike typically initiates inside the thundercloud. When
enough electrons collect in the bottom of the cloud, a very faint, negatively
charged channel, called the step-leader, emerges from the base of the
cloud. The leader propagates towards the ground in a series of luminous
steps (~50 meters in length and 1 microsecond in duration). Between steps
there is a pause of about 50 microseconds, during which the step-leader
"looks" around for an object to strike. If none is "seen", it takes another step,
and repeats the process until it "finds" a target. (a single leader can be
comprised of more than 10,000 steps before reaching ground!) As the
step-leader's channel approaches the ground, it has a VERY strong electric
potential of about 100 million to billion volts with respect to the ground.
STEP 2: When the step-leader approaches the ground, its strong, negative
electric field repels negative charge in the surrounding ground, while
attracting positive charge. This induces an upward moving positive charge
from the ground. When this positive charge collects into a high enough
concentration, they form bolts of ground-to-air lightning known as streamers.
When one of these positively charged streamers contacts the tip of a
negatively charged leader, (~ 30 to 100 meters above the surface), The
leader channel's electric potential is connected to the ground.
STEP 3: All other branches of the leader channel cease further propagation
toward the ground, and all negative charge within these branches starts
flowing to the ground through the newly established ground/cloud connection.
An electric current wave then propagates up the channel as a bright pulse,
known as return stroke, a discharge process taking < 100 microseconds. It
is so intense that illuminates the
Conductive path. The lightning
is traveling FROM the ground
INTO the cloud.
STEP 4: After the current ceased, there is a pause (~20 to 50 milliseconds).
After that, if additional charge is available at the top of the leader channel,
another leader can propagate down the established channel. This leader is
called a dart leader because it is continuous instead of stepped. Dart
leaders are what give lightning its flickering appearance. Not every lightning
flash will produce a dart leader though.
STEP 5: The negatively charged dart leader then will induce a new, positively
charged return stroke. The peak amplitude of the current usually decreases
as additional dart leaders are produced till all the charges are used up. The
combination of each leader (stepped and dart) and the subsequent return
stroke is known collectively as a stroke. All strokes that use the same
cloud-to-ground channel constitute a single cloud-to-ground flash.
A flash can be made up of a single stroke, or as many as tens of strokes.
THUNDER
Thunder is the acoustic shock wave caused by the extreme heat
generated by a lightning flash. When a lightning bolt occurs, the air
surrounding its channel is instantaneously heated to as much as
~33,000 C, a temperature that is five times the surface of the sun! So
air molecules will expand. The faster they are heated, the faster their
rate of expansion. Since air is heated to 33,000 C in a fraction of a
second, its expansion rate exceeds the speed of sound, and a sonic
boom (thunder) results.
This pressure wall is called shock wave
Tornadoes
Tornado development:
The central vortex of a tornado
is typically about 100-600 m in
diameter. The averaged wind
speed is about 96 mph, but can
be as strong as or greater than
220 mph. The central pressure
in a tornado can be lower than
normal atmospheric pressure
by over 100 hPa. The air
around the vortex is pulled into
the low pressure zone where it
expands and cools rapidly to
form clouds to make it visible.
The exact mechanism of tornado formation is still a matter of dispute but it
appears that a tornado grows in a similar fashion to the small vortices that form
in draining bathtubs. When the plug is pulled in a bathtub, water from other
parts of the tub rushes in to replace that going down the drain. If the water has
any swirl in it, the drain soon has a little vortex. Tornadoes appear to be upside
down versions of this phenomenon.
A precondition closely linked to tornado formation is the development of
a mesocyclone with a typical scale of 3 -10 km.
A wide vortex is created, called the mesocyclone.
The mesocyclone begins to build vertically, extending itself upward throughout the
entire height of the cloud. The rapid upward motion causes the air pressure to drop,
pulling more air into growing vortex. The upward motion also causes the incoming
air to cool quickly and form clouds before they rise to the cloud base. This forms
the wall cloud, a curtain-shaped cloud often seen before a tornado forms. As the
rotating wall clouds contract due to strong updrafts, it dips to form the so-called
funnel clouds, indicating this storm is on its way to producing a tornado.
Tornado Descends from Rotating Wall-cloud
Tornado climate
In US, tornadoes are mostly seen in the southern Great Plains, where cold and
dry polar air mass meets warm and moist air mass from the Gulf of Mexico. The
greater the contrast, the more intense the storm tends to be, and thus, the more
tornadoes.
More than 40% tornadoes take place during the spring because of the large contrast
of air masses. During May and June the maximum frequency moves through the
southern Great Plains and then to northern Plains and Great Lakes due to increasing
penetration of warm and moist air.
Tornado Intensity and Destruction
Fujita Intensity Scale:
rating intensity based on the damage produced by a storm
Destruction: 1. sudden drop of pressure; 2. violent winds; 3. debris.
Tornado Forecasting
Tornado watch: issued when there is organized severe weather events
where the tornado threat will affect at 26,000 square km and/or
persist for at least three hours.
Tornado warning: issued when a tornado has actually been signed in an
area or is indicated by weather radar. Warnings are issued for a much
smaller area than watches, usually covering portions of a county or
counties for typically 30 – 60 min.
Predicting Tornadoes
Doppler Radar on wheels
Doppler Radar
Doppler Radar Network
Doppler Effect