Transcript CHAPTER – 3

METEOROLOGY
GEL-1370
Chapter Ten
Thunderstorms and
Tornadoes
Goal for this Chapter
We are going to learn answers to the following questions:
• What atmospheric conditions produce thunderstorms?
• How severe thunderstorms are produced?
• Why severe thunderstorms are not common in polar
latitude?
• How lightning are produced?
• How thunders are produced?
• What are tornadoes and how they are produced?
• What is Fujita scale?
• Major characteristics of a tornadoe?
• Why highest frequency of thunderstorms occur in US?
Thunderstorms
• Thunderstorm: A storm that contains lightning and
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thunder
Birth occurs when warm humid air rises in a
conditionally unstable environment
What can trigger the birth of thunderstorm – unequal
heating of the surface, terrain, lifting of warm air along a
frontal zone
Ordinary thunderstorms (or air-mass thunderstorms):
Develop in warm, humid air masses away from weather
fronts; usually short-lived and rarely produce strong
winds or large hail
Severe thunderstorms may produce high winds, flash
floods, changing hail & tornadoes
Thunderstorms
• Stages of ordinary thunderstorms:
– Cumulus Stage: Humid air rises, cools, & condenses in to
cumulus clouds
– Transformation of water-vapor into liquid or solid cloud
particles releases large quantities of latent heat; this keeps the
air inside the cloud warmer than the surrounding air
– During cumulus stage, insufficient time for precipitation to
form, and the updrafts keep water droplets and ice crystals
suspended within the cloud; no lightning or thunder during this
stage
– As the cloud builds well above the freezing level, cloud
particles grow larger and heavier; drops begin to fall; drier air
around the cloud is being drawn into it; entrainment of drier air
leads to evaporation of raindrops; air becomes colder &
heavier; air begins to descend as a downdraft
Thunderstorms – contd.
• Appearance of the downdraft marks the beginning of the
mature thunderstorms; downdraft & updraft within the
mature thunderstorm constitute a ‘cell’
• In most storms, there are several cells, each of which
may last for an hour or so
• Updrafts & downdrafts reach their greatest strength in
the middle of the cloud, creating severe turbulence
• Overshooting: Intrusion of the updraft above the cloud
top in to the stable atmosphere
• Dissipating stage: When updrafts weaken & downdrafts
tend to dominate throughout much of the cloud
• Three stages: Cumulus stage, maturing thunderstorm
stage, & dissipating stage
Thunderstorms – contd.
• A single ordinary thunderstorm may go through its
three stages in an hour or less
• The cold downdraft may force warm, moist surface air
upward; this air may condense and can gradually build
into a new thunderstorm – multicell thunderstorms
• Most ordinary thunderstorms are multicell storms
• Severe Thunderstorms: Capable of producing large
hail, strong, gusty surface winds, flash floods, and
tornadoes
• Can form from moist air when it is forced to rise into a
conditionally unstable atmosphere; severe
thunderstorms also form in areas with a strong vertical
wind sheer
Air motions associated with thunderstorms; severity
depends on the intensity of the storm’s circulation pattern
Ordinary thunderstorm in its mature stage
A multicell thunderstorm; in the middle is in its mature
stage; to its right of the cell, a thunderstorm is its
cumulus stage
A simplified model describing air motions & other features
associated with a severe thunderstorm; severity depends on
the intensity of the storm’s circulation pattern
Severe Thunderstorms – contd.
• The storm in the previous figure, moves from left to
right & the upper-level winds cause the system to tilt so
that the updrafts move up and over the downdrafts
• The updrafts in a severe thunderstorm may be so strong
that the cloud top is able to intrude well into the stable
atmosphere; top of the cloud may even extend to more
than 18 km above the surface
• Gust Front: The boundary separating the cold
downdraft from the warm surface air
• Along the leading edge of the gust front, the air is
turbulent; strong winds here can pick-up loose dust and
soil and lift them into a huge tumbling cloud
Gust Front & Microburst
• Downbursts: A severe localized downdraft that can be
experienced that fall slowly and reduce visibility more
than light rain
• Microburst: A downburst with winds extending only 4kms or less
• Supercell and Squall-line thunderstorms:
– Supercell Storm: An enormous severe thunderstorm whose
updrafts (can exceed 90 knots) and downdrafts are nearly in
balance, allowing it to maintain itself for several hours. It can
produce large tornadoes & hail (> grapefruit size); most
supercell storms move to the right of the steering winds aloft
– Squall-line storms form as a line of thunderstorms along a
cold front or out ahead of it
The lower half of a severe squall-line type thunderstorms
and some of the features associated with it
Dust clouds rising in response to the outburst
winds of a microburst north of Denver, CO
Doppler radar display showing a line of thunderstorms bent
in the shape of a bow (Red, orange, and yellow)
Supercell near Spearman, TX has a tornado
extending downward from its base
Some of the features of a classic supercell
thunderstorm, viewed from southeast
Diagram of the thunderstorm from above, looking down on
the storm; shaded red: updraft; shaded gray: downdraft
Severe Thunderstorms – contd.
• Dry Line (dew-point fronts): A zone of instability along
which thunderstorms form; dew point temp may drop
along this boundary by as much as 9°C/km
• Mesoscale Convective Complexes: A large organized
convective weather system comprised of a number of
individual thunderstorms; size of an MCC ~ 1000 times
larger than individual thunderstorm
Surface conditions that can produce a dryline with severe
thunderstorms; A developing mid-latitude cyclone with a
cold front, warm front, and three distinct air masses (cP, cT
& mT)
IR image showing a Mesoscale Convective Complex
extending from central Kansas across western
Missouri
Floods & Flash Floods
• Flash floods: Floods that rise rapidly with little or
advance warning; results when thunderstorms stall or
move slowly, causing heavy rainfall over a relatively
small area
• Causes for Flash Floods:
– Thunderstorms stall or move slowly
– Thunderstorms move very quickly but keep passing over the
same area (phenomenon called ‘training’)
– Heavy rain and melting of snow taking place in spring
– Torrential rains from tropical storms
Summer of 1993 rain in the upper Midwest caused the
worst flood 6.5 billion $ crop lost; 43 human lives;
45,000 homes were lost; evacuation of 74,000 people
Distribution of Thunderstorms
• >40,000 thunderstorms/day (14 millions/yr) in the world
• 14 million/year
• Conducive conditions for thunderstorm formation:
Combination of warmth and moisture
• Where thunderstorms are prevalent: i) Southeastern
states along the Gulf Coast with a maximum in Florida
(mainly during summer); ii) Central Rockies; iii) Over
water along the intertropical convergence zone where
the low-level convergence of air helps to initiate uplift
• Where thunderstorms are rare: i) Dry regions such as
polar regions and the desert areas of the subtropical
highs; ii) Pacific coastal and interior valleys
Average number of days each year on which
thunderstorms are observed in US; mountainous west
has sparse data
Average number of days each year hail observed
Thunderstorms and Lightning
• Lightning: A giant spark discharging electricity that
occurs in mature thunderstorms; can take place within a
cloud, from one cloud to another, cloud to surrounding
air or cloud to ground (~20%); 80% within the clouds
• Lightning stroke can heat the air surrounding it to
30,000°C which in turn causes the air to expand, thus
initiating a shock wave that becomes a booming sound
wave-thunder
• Light travels faster than sound (345 m/s @25 °C)
• Time difference between the light and sound can be
utilized to determine how far away the stroke took place
Lightning & Thunder – contd.
• Close distance lightning: Clap sound or crack followed
immediately by a loud bang
• Farther away: rumbling sound due to sound emanating
from different areas of the stroke
• Lightning, but no thunder: Thunder waves were
refracted and the sound waves got attenuated, making
the thunder inaudible
• Sonic boom: Produced when an aircraft exceeds the
speed of sound at the altitude at which it is flying
• Condition for lightning to occur: Separate regions
containing opposite electrical charges must exist within
the cumulonimbus cloud
Electrification of clouds
• Several theories to explain the formation of lightning
• When hail fall through supercooled droplets, the
droplets freeze and release latent heat; this heat warms
the hailstone; contact of warmer hailstone and colder ice
crystal leads to a net transfer of positive ions from the
warmer object to the colder object --- hailstone is
negatively charged and ice crystals +ively charged
• Positively charged ice particles carried to the upper part
of the cloud by updrafts & larger haldstones with –ive
charge fall toward the bottom of the cloud
• Cold, upper part becomes +ively charged & middle of
the cloud becomes –ively charged
Electrification of the Clouds – contd.
• Another school of thought: Regions of separate charge
exist within tiny cloud droplets and larger precipitation
particles during the formation of precipitation --Negative charge in the upper part of these particles &
+ive charge in the lower part of the particles --- when
falling precipitation collides with smaller particles,
larger precipitation particles become negatively
charged and the smaller particles positively charged --updraft sweeps smaller sized particles leading to net
+ive charge
Generalized charge distribution in a mature
thunderstorm
The Lightning Source
• Negative charge at the bottom of the cloud causes a
region of the ground beneath to become +ively charged;
as the thunderstorms move, the positive charge moves
along with it; the positive charge is most dense on
protruding objects; charge separation causes electric
field existence; electric potential difference between
cloud and the ground --- when electric potential builds
up, current flow results and lightning occurs
• Cloud-to-ground lightning begins when the localized
electric potential gradient >3 million volt/m --- leads to
the discharge of electrons toward the cloud base and
then to the ground
Lightning source – contd.
• Stepped Leader: An initial discharge of electrons that
proceeds intermittently toward the ground in a series of
steps in a cloud-to-ground lightning stroke
• Return Stroke: The luminous lightning stroke that
propagates upward from the earth to the base of a cloud
• Dart Leader: Discharge of electrons that proceeds
intermittently toward the ground along the same ionized
channel taken by the initial lightning stroke
• Different types of Lightning: Forked lightning (crooked
or forked in shape), ribbon lightning (ribbon hanging in
the cloud), bead lightning (series of beads tied to a
string), ball lightning (sphere appears to float in the air)
& sheet lightning (cloud appears like a white sheet)
Development of stepped leader: when –ive charge near the
bottom of the cloud becomes large enough to overcome air’s
resistance, flow of electrons rushes to the earth
As the electrons approach the ground, a region of +
charge moves up into the air through any conducting
object, such as trees, buildings
When the downward flow of electrons meets the upward surge
of +ive charge, a strong electric current – a bright return stroke
– carries +ve charge upward into the cloud
Lightning rod extends above the building: when
lightning strikes, it follows an insulated conducting
wire into the ground
Four marks on the road surface represent areas where
lightning, after striking a car entered the roadway; 3 tires
were flattened
Lightning Detection & Suppression
• Heat Lightning: Distant lightning from thunderstorms
that is seen, but not heard
• As the electric potential near the ground increases, a
current of +ive charge moves up pointed objects, such
as antennas
• Lightning rods (made of metal with a pointed tip) are
placed that extend well beyond the height of the
structure
• Lightning Direction-finder: It detects the direction of
lightning by measuring the radiowaves produced by
lightning
Damages by lightning in US
• 10,000 fires/yr in US are started by lightning
• 50 million $ worth of timber is destroyed per yr
• Can we reduce the cloud-to-ground lightning?? Seeding
cumulonimbus clouds with hair-thin pieces of Al wire
(10-cm long) --- metal will produce many tiny sparks
and prevent the electrical potential in the cloud from
building to a point where lightning occurs
Tornadoes
• Tornadoes: A product of thunderstorms; rapidly rotating
winds that blow around a small area of intense low
pressure
• Tornadoe’s circulation is present on the ground either as
a funnel-shaped cloud or as a swirling cloud of dust &
debris; majority rotate counterclockwise
• Other shapes:
–
–
–
–
Twisting ropelike funnels
Cylindrical-shaped funnels
Massive black funnels
Funnels that resemble an elephant’s trunk hanging from a large
cumulonimbus cloud
Tornadoes – Features and stages
• Diameter (most): 100-600 m (few meters – 1,600 m rare)
• Most last only a few minutes & average path length of
~7 km (largest one: 470 km; lasted for 7 hrs) in Illinois
and Indiana in 1917
• Stages of a Tornadoes (most common):
– Dust-Whirl stage: Dust swirling upward from the surface –
damage is light
– Organizing Stage: Tornado increases in intensity with an
overall downward extent of the funnel
– Mature Stage: funnel reaches its greatest width & is almost
vertical; damage is most severe
– Shrinking stage: Overall decrease in the funnel’s width &
increase in the funnel’s tilt; still capable of intense &
Tornadoes – Features and stages
• Sometimes violent damage
– Decay Stage: The final stage, usually finds the tornado
stretched into the shape of a rope
Minor tornadoes may evolve only through certain
stages
Damages:
~ 100 people/year killed (11/10/02 – 37 people died on a
single day)
45% mortalities in mobile homes
March 18, 1925 tornadoes: 695 people died, 7 tornadoes
traveled a total of 703 km across portions of Missouri, Illinois
and Indiana
Tornado outbreak
• Tornado Outbreak: A series of tornadoes that forms
within a particular region, often associated with
widespread damage and destruction; a region may
include several states
• April 3, 1974: 16 hour period, 148 tornadoes cut
through parts of 13 states, 307 people killed, >3700
people injured, damage >600 million $
• Occurrence: Most numbers in US; average: >1,000/yr;
1,424 during 1998
• Tornado alley: Tornado belt, Central Plains, stretches
from central Texas to Nebraska
A mature tornado with winds >150 knots rips
through southern illinois
Tornado incidence by State; upper:number by each
state (25 yrs); lower: average annual number/100,000
square miles; darker: greater frequency
Tornadoes and their impact
• Lifting railroad coach with 117 passengers and dumping
it 25 m away
• Schoolhouse was demolished and 85 students inside
were carried over 100 m without one of them being
killed
• Most tornadoes have winds of less than 125 knots
• Pressure in the center of a tornado may be more than 100
mb lower than the surrounding & there is a momentary
drop in outside pressure when tornado is above a
structure
• When confronted with a tornado, take shelter
immediately (basement, stay away from windows, small
bathroom, closet, interior hallway)
Fujita scale for damaging wind
Category Mi./hr knots Expected damage
•Scale
.
F0
Weak
F1
F2
Strong
F3
F4
F5
40-72
35-62
73-112
63-97
113-157
98-136
158-206
Violent
207-260
261-318
Light; tree branches broken; sign
boards damaged
Moderate; trees snapped;
windows broken
Considerable; large trees
uprooted, weak struc. Destroyed
137-209 Severe trees leveled, cars overturned, walls removed from bldg.
180-226 Devastating frame houses
dstroyed
227-276 Incredible; structure the size of
autos moved over 100 m
Fujita Scale – contd.
• Fujita Scale: Theodore Fujita in late 1960s --classifying tornadoes according to their rotational wind
speed based on the damage done by the storm
• Majority of tornadoes are F0 and F1 (weak ones) and
only a few % are above the F3(violent) with ~ 1 F5/yr
• Tornado Formation: Tornadoes tend to form with intense
thunderstorms and a conditionally unstable atmosphere
is essential for their development
• Most strong and violent tornadoes develop near the right
rear sector of a severe thunderstorm
• In order for a tornado to spawn a tornado, the updraft
must rotate
Total wind speed of a tornado is greater on one side
than on the other
A powerful multi-vortex tornado with three suction
vortices
Conditions leading to the formation of severe
thunderstorms that can spawn tornadoes; red boxed
area: tornadoes are likely to form
Where tornadoes are common
• Greatest tornado activity shifts northward from winter to
summer
• Winter: contrast between warm and cold air masses are
the greatest over the southern Gulf states & tornadoes
are most likely to form in this region
• Spring: humid Gulf air surges northward, jet stream also
moves northward; tornadoes more prevalent from the
southern Atlantic states westward into the southern
Great Plains
• Summer: contrast between air masses lessens & the jet
stream is normally near the Canadian border; tornado
activity tends to be concentrated over the northern plains
Features associated with tornado-bearing
thunderstorm; thunderstorm moves northeast;
tornadoes form in the southwest part
Mesocyclone
• Mesocyclone: A vertical column of cyclonically rotating
air within a severe thunderstorm
• Severe thunderstorms form in a region of strong vertical
wind sheer; most strong and violent tornadoes form
within the mesocyclone
• Existence of the swirling winds of the mesocyclone
inside tornado-producing thunderstorms were observed
1970s (first time) using Doppler Radar
• 30% of all mesocyclones produce tornadoes & 95%
produce severe weather
• Time between mesocyclone identification & tornado
touching the ground is ~20 minutes
Tornadoes – contd.
• Gustnadoes: Tornadoes that form along the gust front
• Wall cloud: An area of rotating clouds that extends
beneath a severe thunderstorm and from which a funnel
cloud may appear
• Tornado Watch: Issued by Storm Prediction Center in
Norman, Oklahoma
• Doppler radar can detect areas of precipitation &
measure rainfall intensity
• Tornado Vertex signature (TVS): An image of a
tornado on the Doppler radar screen that shows up as a
small region of rapidly changing wind directions inside
a mesocyclone
A computer model illustrating motions inside a severe
tornado-generating thunderstorm
Waterspouts
• Doppler Lidar: uses a light beam (instead of
microwaves) to measure the change in frequency of
falling precipitation, cloud particles, and dust
• Waterspout: A rotating column of air over a large body
of water; tend to move slowly than tornadoes; last for
only 10-15 minutes
Doppler radar display of large supercell
thunderstorm that is spawning an F4 tornado
(circled are) near Lula, OK
Average annual number of tornadoes & tornado
deaths by decade
•Decade
.
Tornadoes/year
Deaths/year
1950-1959
480
148
1960-1969
681
94
1970-1979
858
100
1980-1989
819
52
1990-1999
1,220
56
Summary – Chapter - 10
• Stages of a thunderstorm and a tornado
• Air-mass thunderstorm, multicell & supercell
thunderstorm
• Gust front, causes for downdraft, microburst
• Squall line, dry line
• Suitable time for the formation of thunderstorm
• Lightning and thunder – formation and features
• Funneling cloud, mesocyclone, wall cloud
• Fujita scale
• Direction of movement of tornadoes, conditions for its
formation, waterspout