Transcript Slide 1

METEOROLOGY
The Relationship
between Heat
Transfer and
Changes in Density
• Thunderstorms
– A thunderstorm contains thunder and lightning.
• Come in many different shapes and sizes.
– Warm, moist air rises in a conditionally unstable
environment.
• As long as parcel is warmer than environment then it
will continue to rise, it is buoyant.
• Greater the temp difference, the faster the air will rise.
– Rising air must be “triggered”/forcing
mechanism.
• Unequal heating, terrain, lifting of air along shallow
boundaries of converging winds.
• Frontal lifting.
• Large scale divergence aloft.
TYPE OF THUNDERSTORMS
• SINGLE-CELL (ORDINARY)
• MULTICELL
• SUPERCELL
• MESOSCALE CONVECTIVE COMPLEX
Ordinary Single-Cell Thunderstorm
• Few miles in diameter
• Lasts for less than an hour
• Develop in warm, humid air masses away from
weather fronts; usually short-lived and rarely
produce strong winds or large hail
• Three stages of its life cycle:
– Cumulus Stage – all updraft, cloud building
– Mature Stage – precipitation falls at the surface, most
lightning, rain and hail occurs, cloud has an updraft
and a downdraft
– Dissipating Stage – dominated by downdraft
Life cycle of an ordinary thunderstorm cell
Severity depends on the intensity of the storm’s circulation pattern
Fig. 11.7
Ordinary “Air Mass” Thunderstorms
Cumulus Stage
Humid air rises, cools, & condenses in to cumulus clouds
• (Arrows show vertical air currents.
Dashed line represents freezing level, 0°C
isotherm.)
• During this stage there is insufficient
time for precipitation to form, and the
updrafts keep water droplets and ice
crystals suspended in the cloud.
• No lightning in this stage
• As cloud builds above freezing level, the
cloud particles grow larger and become
heavier.
• Entrainment – drier air from around the
cloud is drawn into it, causing some of
the drops to evaporate, chilling the air.
The cooler heavier air descends as a
downdraft
Ordinary “Air Mass” Thunderstorms
Mature Stage
– The most intense stage.
Lightning, thunder, rain, and
occasionally small hail.
– Top of cloud reaches a stable
region (tropopause) and
spreads out into anvil, up to
40,000 ft (12 km).
– Some updrafts are so strong
they penetrate the stable air, a
condition know as
overshooting.
– The cold downdraft forms a
gust front.
– The gust front can act to force
more warm, humid air into the
storm.
An ordinary thunderstorm in its mature stage. Note the distinctive anvil top.
Ordinary “Air Mass” Thunderstorms
Dissipating Stage
• After the storm enters the
mature stage, it begins to
dissipate in about 15 to 30
minutes.
• Dissipating stage occurs when
the updrafts weaken and
downdrafts tend to dominate
throughout much of the cloud.
• Deprived of the rich supply of
warm humid air, cloud droplets
no longer form. Light precip
falls from the cloud, only weak
downdrafts remain.
• Sometimes the lower part of
the storm evaporates leaving
only the cirrus anvil above.
• This entire process cumulus,
mature, dissipating may take
place in an hour or less.
A dissipating thunderstorm. Most of the cloud particles in the lower
half of the storm have evaporated.
A climatology of the average number of thunderstorm days in a year
Fig. 11-3, p. 314
Fig. 11-4, p. 315
Severe Thunderstorms
• Capable of producing large hail, strong,
gusty surface winds, flash floods, and
tornadoes.
• Form in moist air forced to rise into a
conditionally unstable atmosphere. But
also form in areas with strong vertical wind
shear.
• Severe Thunderstorms
– Criteria: A thunderstorm that produces hail
at least ¾” diameter (penny/dime), and/or
surface wind gusts of 50 kts, and/or
produces a tornado.
– The longer a storm survives, the more
likely it will become severe.
A simplified model describing air motions & other features associated with a
severe thunderstorm; severity depends on the intensity of the storm’s
circulation pattern
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
Multicell Thunderstorm
– Composed of several individual single-cell storms,
each at a different stage of development
– Forms in same way as an ordinary storm, only the
environment features vertical wind shear.
• The lack of wind shear in an ordinary thunderstorm
causes the precip to fall into the updraft.
• Increased winds aloft push the precip away from updraft.
• The downdraft can still undercut the updraft without the
precip falling into it. This produces a long-lasting
multicell storm.
Multicell Storms
• A multicell storm complex. This storm is composed of a
series of cells in successive stages of growth. The
thunderstorm in the middle is in its mature stage, with a
well-defined anvil. Heavy rain is falling from its base. To the
right of this cell, a thunderstorm is in its cumulus stage. To
the left, a well-developed cumulus cloud is about ready to
become a mature thunderstorm.
• Severe Thunderstorms (Supercell)
– When the upper level winds are even stronger and
also change direction with height :
• The storm can move in such a way that the downdraft
never undercuts the updraft.
• Can create horizontal spin which may be tilted into a
rotating updraft.
• The result is a supercell.
– A supercell modifies its own environment.
– The vertical wind shear creates a storm structure
that allows the storm to continually move towards
the area of warm moist air.
• Supercells move to the right of the mean flow.
Some of the features of a classic supercell thunderstorm, viewed
from southeast
A supercell thunderstorm with a tornado sweeps over Texas.
Fig. 14-5, p. 372
Lifted Index
• The warmer the parcel, the greater the
buoyancy and the stronger the updraft.
• The warmer the parcel is compared to the
environment, the more negative the LI
• If the lifted index is negative then the atmosphere is
unstable.
• LI of 0 to -3: Air is marginally unstable
• LI -9 or less: Extreme instability
• Severe thunderstorms require a lifting index less than -3
Table 11-1, p. 315
Squall Line
• Is a set of individual intense thunderstorm cells arranged
in a line.
• Thy occur along a boundary of unstable air – e.g. a cold
front.
• Can from as much as 100 to 300 km out ahead of a cold
front in the warm air.
• Strong environmental wind shear causes the updraft to
be tilted and separated from the downdraft.
• The dense cold air of the downdraft forms a ‘gust front’.
Fig. 14-8, p. 374
Squall Line
A model describing air motions and precipitation associated with a squall line that
has a trailing stratiform cloud layer
Pre-frontal Squall line
• Pre-frontal squall-line thunderstorms may form ahead of
an advancing cold front as the upper-air flow develops
waves downwind from the cold front.
• Mesoscale Convective Complex
– A large, convectively driven system that is made up of
many individual thunderstorms. Many times, as much
as 1000 times larger than an individual thunderstorm
cell.
–Within the MCCs,
the individual
thunderstorms
work together to
generate a longlasting weather
system that moves
slowly and quite
often for periods
exceeding 12 hrs.
–Produce hail, high
winds and flash
floods
Fig. 14-11, p. 375
IR image showing a Mesoscale Convective
Complex extending from central Kansas across
western Missouri
Severe Thunderstorms and the
Dryline
• Severe thunderstorms may form along or just
east of a boundary called a dry line.
• The dryline represents a narrow zone where
there is a sharp horizontal change in moisture.
• Because dew-point temperatures may drop
along this boundary by as much as 9°C (16°F)
per km dry lines have been referred to as dewpoint fronts.
• Most frequently observed in western half of
Texas, Oklahoma and Kansas in the spring and
early summer.
Dry Line
• Surface conditions that can produce a dryline
with severe thunderstorms.
Radar image of an outflow boundary
. As cool air from inside the severe thunderstorms (red and orange colors) spreads
outward, away from the storms, it comes in contact with the surrounding warm, humid
air, forming a density boundary (blue line) called an outflow boundary between cool air
and warm air. Along the outflow boundary, new thunderstorms often form.
Fig. 14-16, p. 378
Gust Fronts and Shelf & Roll clouds
• Gust front – the boundary separating the cold
downdraft of a thunderstorm from the warm surface
air. An observer on the ground might mistake the gust
front for the passage of a cold front in that the wind
often shift, gusts can exceed 55 kts and the
temperature drops sharply and pressure often rises.
• Roll cloud – occasionally an elongated ominous
looking cloud forms just behind the gust front. These
clouds, which appear to slowly spin about a horizontal
axis are called roll clouds.
• Shelf Cloud - Prevalent when the atmosphere is very
stable near the base of the thunderstorm
The lower half of an intense thunderstorm and some of the features associated with it.
A dramatic example of a shelf cloud associated with an intense thunderstorm.
The photograph was taken in the Philippines as the thunderstorm approached
from the northwest.
Fig. 14-14, p. 377
A roll cloud forming behind a gust front. (Copyright Howard B. Bluestein.)
Microbursts
• Beneath a severe thunderstorm, the
downdraft may become localized so that it
hits the ground and spreads horizontally in
a radial burst of wind – such downdrafts
are called downbursts.
• A downburst with winds extending only 4
kilometers or less is termed a microburst.
An intense microburst can induce
damaging winds as high as 146 kts.
Dust clouds rising in response to the outburst winds of a microburst north of
Denver, Colorado.
Fig. 14-18, p. 379
Bow echos
• Doppler radar display
showing a line of
thunderstorms (a squall
line) bent in the shape of a
bow (colors red, orange,
and yellow) as they move
eastward across the San
Joaquin Valley of
California. Such bow
echoes often produce
damaging surface winds
near the center of the bow.
Sometimes the left (usually
northern) end of the bow
will develop cyclonic
rotation and produce a
tornado.
Fig. 14-19, p. 379
Flash Floods
• Flash floods –
floods that rise
rapidly with
little or no
advance
warning.
• Often a result
of stalled or
slow moving
thunderstorms
or when
storms “train”
over the same
area for long
periods of time
Lightning and Thunder
• Lightning – simply a discharge of electricity, a giant
spark, which occurs in mature thunderstorms.
• Lightning can take place in cloud, cloud to cloud, cloud
to air, and cloud to ground.
• Thunder – the sound due to rapidly expanding gases
along the channel of a lightning discharge.
• Thunder travels about I mile in 5 seconds. If we see
lightning and then hear thunder 15 seconds later, the
stroke occurred about 3 miles away.
• Sonic Boom – sound produced when aircraft exceeds
the speed of sound at the altitude at which it is flying.
Aircraft compresses the air, forming a shock wave, air
pressure changes rapidly over a short distance, the rapid
change of air pressure causes the sonic boom.
The lightning stroke can travel in a number of directions. It can occur within a
cloud, from one cloud to another cloud, from a cloud to the air, or from a cloud
to the ground. Notice that the cloud-to-ground lightning can travel out away
from the cloud, then turn downward, striking the ground many miles from the
thunderstorm. When lightning behaves in this manner, it is often described as a
“bolt from the blue.”
Fig. 14-24, p. 383
Electrification of Clouds
• The generalized charge
distribution in a mature
thunderstorm.
• Different ideas on why clouds
have charges.
• One theory proposes that
clouds become electrified
when graupel and hail fall
through a region of
supercooled droplets and ice
crystals. When the warmer
hailstone comes in contact
with a colder ice crystal there
is a net transfer of positive ions
from the warmer object to the
colder object. (Hail gets a
negative charge, ice gets
positive charge).
• Top of cloud is generally
positive, middle negative,
lower part negative, with some
positive.
Steps in a Lightning Strike
• Stepped Leader: An initial discharge of
electrons that proceeds intermittently toward
the ground in a series of steps in a cloud-toground 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
The Lightning Stroke
• The development of a lightning
stroke.
(a) When the negative charge near
the bottom of the cloud becomes
large enough to overcome the air's
resistance, a flow of electrons - the
stepped leader - rushes toward the
earth.
• 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
The Lightning Stroke
• As the electrons
approach the ground,
a region of positive
charge moves up into
the air through any
conducting object,
such as trees,
buildings, and even
humans.
The Lightning Stroke
• When the downward flow of
electrons meets the upward
surge of positive charge, a
strong electric current - a
bright return stroke - carries
positive charge upward into
the cloud.
• Return Stroke – the luminous
lightning stroke that
propagates upward from the
earth to the base of a cloud
Time exposure of an evening thunderstorm with an intense lightning display near
Denver, Colorado. The bright flashes are return strokes. The lighter forked flashes
are probably stepped leaders that did not make it to the ground.
A cloud-to-ground
lightning flash hitting a
65-foot sycamore tree.
It should be apparent
why one should not
seek shelter under a
tree during a
thunderstorm.
Fig. 3, p. 389
Fig. 11-33, p. 342
Lightning Rod
• The lightning rod extends
above the building,
increasing the likelihood
that lightning will strike
the rod rather than some
other part of the structure.
After lightning strikes the
metal rod, it follows an
insulated conducting wire
harmlessly into the
ground.
Different types of Lightning:
Heat lightning – distant lightning from thunderstorms that is
seen but not heard. So called because it frequently occurs
on hot summer nights when the sky overhead is clear.
• 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)
• St. Elmo’s Fire – named after the patron saint of sailors. A
bright electric discharge that is projected from objects
(usually pointed) when they are in a strong electric field
such as during a thunderstorm.
Ball Lightning
• This picture of the freak
weather phenomenon of
ball lightning was taken
by a wildlife ranger in
Queensland, Australia, in
1987.
• Ball lightning occurs so
rarely that few
photographs of it exist
and researchers have
had to rely on eyewitness
accounts, some of them
from previous centuries.
•
The four marks on the road surface represent areas where lightning, after
striking a car traveling along south Florida's Sunshine State Parkway,
entered the roadway through the tires. Lightning flattened three of the car's
tires and slightly damaged the radio antenna. The driver and a six-year-old
passenger were taken to a nearby hospital, treated for shock, and released.
Tornadoes
A mature tornado with winds exceeding
150 knots rips through southern Illinois.
• Tornadoes are rapidly rotating
winds that blow around a small
area of intense low pressure.
A tornado’s circulation is
present on the ground either
as a funnel shaped cloud or as
a swirling cloud of dust and
debris.
• Other names – twisters,
cyclones
• Funnel cloud – a tornado
whose circulation has not
reached the ground.
Diameter of most tornadoes (300
– 2000 ft)
Tend to move from southwest
toward northeast
Most last only a few minutes – at
least one lasted
7 hours and moved along a path
of 292 miles
Box 11-1, p. 329
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 &
– Decay (or Rope) Stage
• Tornadoes (Life Cycle)
1. Dust Whirl Stage
2. Organizing Stage
3. Mature Stage
4. Shrinking Stage
5. Decay (Rope) Stage
2.
1.
3.
4.
5.
Fig. 11-23, p. 331
Tornado Outbreak – April 3, 1974
•
•
•
Outbreak – 6 or more tornadoes
over a particular region
Map of 148 Tornadoes that
Occurred on April 3 & 4, 1974
315 people died, 5,000 injured
• Tornadoes (Occurrence)
– Top number: Number of tornadoes in 25 years.
– Bottom number: Number of tornadoes per 10,000 square
miles.
– Tornado alley is from north Texas through Nebraska.
– Can occur in any state.
Geographic distribution of the month of maximum tornado threat
Fig. 11-30, p. 337
Average number
of tornadoes
during each month
in the United
States.
Fig. 14-34, p. 392
Tornado Winds
• The total wind speed of a
tornado is greater on one
side than on the other.
When facing an
onrushing tornado, the
strongest winds will be on
your left side.
• Most tornadoes probably
have winds less than 125
kts, but most powerful
twisters seldom exceed
220 kts.
TR 303c-26
A powerful
multi-vortex
tornado with
three suction
vortices
Fig. 14-36, p. 393
Table 14-1, p. 393
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
Table 11-3, p. 333
• Fujita Scale (F0 and F1)
F0
F1
• Fujita Scale (F2 and F3)
F2
F3
• Fujita Scale (F4 and F5)
F4
F5
A devastating F5 tornado about 200 m (650 ft) wide plows through Hesston,
Kansas, on March 13, 1990, leaving almost 300 people homeless and 13
injured.
Total destruction caused by an F5 tornado that devastated parts of Oklahoma
on May 3, 1999.
Fig. 14-38, p. 395
• Tornadic Thunderstorms
– Not everything is known about the formation of a
tornado.
– It is known that tornadoes tend to form with
intense thunderstorms and that an unstable
atmosphere is essential for their development.
– Tornadoes form with both supercell
thunderstorms and non-supercell thunderstorms.
– The most intense tornadoes form with supercell
thunderstorms. Thunderstorms with a strong,
single rotating updraft that develop in a region of
strong vertical wind shear.
• Tornadic Thunderstorms (Mesocyclones)
– The rotating updraft of a supercell
thunderstorm is called a mesocyclone.
– Change of wind speed and direction with
height is responsible for the rotating updraft.
Thunderstorm Rotation
• Recall that supercells form in a region of strong vertical
wind shear that causes the updraft inside the storm to
rotate.
Thunderstorm Rotation
• The strong updraft in
the thunderstorm
carries the vortex
tube into the
thunderstorm,
producing a rotating
air column that is
oriented in the vertical
plane.
Fig. 11.18
–As air rushes into the low-level core of the mesocyclone, the air
expands, cools, and if moist, condenses into a cloud (funnel cloud).
–As the air beneath the funnel cloud is drawn into its core, the air
cools rapidly and condenses, and the funnel descends to the surface.
–As the funnel reaches the ground, it usually picks up dirt and debris,
making it appear dark.
–While the air along the outside of the funnel is spiraling upward, in
the most violent tornadoes, the air is descending towards the extreme
low pressure at the ground (sometimes 100 mb lower than
surrounding air).
• Hook Echo (Bounded Weak Echo Region)
– The updraft is so strong in a supercell that precipitation
cannot fall through it. Southwesterly winds aloft usually
help to blow the precipitation northeastward.
– If a mesocyclone is strong and persistent, the precip
can be wrapped around the updraft.
– This swirling area of precip
shows up on the radar.
– This is called the bounded
weak echo region, an area that
is bounded by precip. On
radar, it can appear as a hook
on the southern side of the
storm.
Fig. 14-42, p. 396
A classic tornadic supercell thunderstorm showing updrafts and
downdrafts, along with surface air flowing counterclockwise and
in toward the tornado.
Fig. 14-43, p. 397
A computer model illustrating motions inside a severe
tornado-generating thunderstorm
Observing Tornadoes
• Wall cloud – An area of rotating clouds that
extends beneath a severe thunderstorm and
from which a funnel cloud may appear
• Tornado watch – tornadoes are likely to form in
the next few hours – issued by Storm Prediction
Center in Norman
• Tornado warning – Once a tornado is spotted –
either visually or on radar – issued by local NWS
office
A wall cloud photographed southwest of Norman, Oklahoma.
A wall cloud associated with a supercell thunderstorm spins counterclockwise
over the plains of Texas. Beneath the wall cloud, dust rising from the surface
indicates that a tornado is about to form.
Fig. 14-CO, p. 366
A funnel cloud extends downward from the base of a non-supercell
thunderstorm over central California.
Doppler Radar
• Similar to a conventional radar in that is
can detect areas of precipitation and
measure rainfall intensity.
• Doppler can do more – actually measures
the speed at which precipitation is moving
horizontally toward or away from the radar
antenna.
• Doppler radar “peers into severe
thunderstorms and reveals its winds.
Doppler radar display
showing precipitation
inside a large supercell
thunderstorm that is
spawning an F4 tornado
(circled area) near Lula,
Oklahoma. The regions
in orange and red show
the area of heaviest
precipitation. The close
packing of the winds
indicates strong cyclonic
rotation and the
signature of a tornado.
Red and orange indicate
winds blowing away from
the radar. Green and
blue indicate winds
blowing toward the radar
Doppler radar display of winds associated with the supercell storm that
moved through parts of Oklahoma City during the afternoon of May 3,
1999. The close packing of the winds blowing toward the radar (green and
blue shades), and those blowing away from the radar (yellow and red
shades), indicate strong cyclonic rotation and the presence of a tornado.
Fig. 14-47, p. 399
Waterspouts
• A rotating column of air over a
large body of water..
• Fair Weather Waterspouts –
form over waters of Florida
Keys, Caribbean. Much
smaller than an average
tornado, (3 – 100 meters),
wind less than 45 kts, tend to
move slowly.
• A powerful waterspout moves
across Lake Tahoe, California.