Transcript Met10_lecture_11

Chapter 10: Thunderstorms
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Thunderstorms
Tornadoes
Tornadic thunderstorms
Ordinary Cell Thunderstorms
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cumulus stage
mature stage
dissipating stage
• Ordinary cell thunderstorms are sometimes called
‘air mass thunderstorms’, because they form in
conditionally unstable air masses and are not
necessarily associated with fronts or severe
weather.
Ordinary thunderstorms tend to form in regions of
limited wind shear.
Form along shallow zones where surface winds
converge: topographic irregularities, sea-breeze
fronts, outflow boundaries from thunderstorms…
producing more thunderstorms.
Converging wind boundaries are regions of
temperature and humidity contrasts: density
differences.
Life cycle of an ordinary thunderstorm
Cumulus stage
Mature stage
Dissipating stage
Fig. 10-1, p. 265
Cumulus stage:
Is the growth stage, as cumulus cloud builds,
transformation of water vapor to liquid and solid cloud
droplets releases latent heat that keeps the rising air inside
the cloud warmer than environment.
During this stage there is insufficient time for precipitation to
form. Updrafts keep water droplets and ice crystals suspended
within the cloud
There is no lightning or thunder during this stage
The building cumulus takes on the appearance of a ‘tower’
Mature stage:
As the cloud builds above the freezing level, the cloud droplets
grow larger and heavier. Eventually the rising air can no longer
keep the particles suspended and they begin to fall.
Drier air around the cloud begins to be drawn into the cloud in
a processes know as entrainment.
The entrainment of drier air into the cloud causes some of the
raindrops to evaporate, which cools the air.
The air now colder begins to descend as a downdraft.
The appearance of the downdraft marks the beginning of the
mature stage.
Mature stage:
The updraft and downdraft within the mature thunderstorm now
constitute the cell.
During the mature stage, the thunderstorm is most intense.
The top of the cloud, having reached a stable region of the
atmosphere, begins to take on the anvil shape as upper-level
winds spread the cloud’s ice crystals horizontally.
Updrafts and downdrafts are at their greatest strength in the
middle of the cloud, creating severe turbulence.
Lightning and thunder are also present in the mature stage.
Mature stage:
Heavy rain falls from the cloud, and at the surface there is a
downrush of cold air with the onset of precipitation.
Where the cold downdraft reaches the surface, the air spreads
out horizontally in all directions. The boundary between the cold
air and the warm surface air is called a gust front.
The storm begins to dissipate after 15-30 minutes after entering
the mature stage.
Dissipating stage:
Occurs when the updrafts weaken as the gust front moves away
from the storm and no longer enhances the updrafts.
At this stage, downdrafts dominate throughout much of the cloud.
The reason the storm does not last very long is that the
downdrafts inside the cloud tend to cut off the storm’s fuel
supply by destroying the humid updrafts.
Not only do thunderstorms produce summer rainfall for a large
portion of the US, but they also provide momentary cooling after
an oppressively hot day.
Life cycle of an ordinary thunderstorm
Cumulus stage
Mature stage
Dissipating stage
Fig. 10-1, p. 265
The average number of days each year on which thunderstorms are
observed throughout the United States
Fig. 10-18, p. 277
Severe Thunderstorms and
the Supercell
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multicell storms
supercell
Severe Thunderstorms
The likelihood that a thunderstorm will become severe increases
with the length of time the storm survives.
Ordinary thunderstorms tend to from in regions of low wind shear.
Because of this, the storms downdraft and precipitation can fall
into the updraft– which kills the storm.
If there is some moderate wind shear, the precip is pushed
downwind so that it does not fall into the updraft– and thus the
updraft is not suppressed.
If the downdraft undercuts the updraft, then new cells can form,
producing a long-lasting multicell storm. A storm with a cluster
of cells at various stages of their life cycles.
Multicell storm complex
Radar image of an outflow boundary
Fig. 10-13, p. 272
Severe Thunderstorms
In a multicell storm, if convection is strong and updrafts intense,
the storm can become severe.
Updrafts in severe thunderstorms can cause the cloud to reach
into the stratosphere and in some cases extend up to 60,000 ft.
Strong updrafts can keep hailstones suspended in the cloud
long enough for them to grow to considerable size.
Once they become large enough they either fall out the bottom
of the cloud within a downdraft or a strong updraft may toss
them out the side.
Aircraft have encountered hail in clear air several kilometers away
from a storm.
Supercell thunderstorm
When winds become stronger aloft, the wind shear may be
strong enough to create horizontal spin, which when tilted into
the updraft causes it to rotate.
In this case, the thunderstorm may grow into a larger, long-lasting
(longer than an hour), severe storm called a Supercell.
The rotational aspect of supercells can lead to the formation of
tornadoes.
Supercell storms are typically enormous thunderstorms that
consist of primarily of a single violently rotating updraft.
Conditions leading to the
formation of severe
thunderstorms, and
especially supercells
A wall cloud
associated with a
supercell
thunderstorm
spins
counterclockwise
A supercell thunderstorm with a tornado sweeps over Texas
Fig. 10-4, p. 268
Squall Line
A squall line forms as a line of thunderstorms, either right along
a cold front or in the warm air 100-300 km ahead of it.
Pre-frontal squall-line thunderstorms represent the most severe
and largest type of squall lines.
There is still debate as to exactly how pre-frontal squall lines form.
A Doppler radar composite showing a pre-frontal squall line
extending from Indiana southwestward into Arkansas
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 Complexes
Mesoscale Convective Complexes (MCCs) form when a
number of thunderstorms grow into a single, large, circular
convective weather system.
They can be as large as 1000 times that of a ordinary cell
thunderstorm.
Can cover an entire state and move slowly lasting up to 12 hours
Form during summer in regions where upper-level winds are
weak.
Are associated with severe weather, including hail, high winds,
destructive flash floods, and tornadoes.
Mesoscale Convective Complexes
A simplified model describing air motions and other features
associated with an intense thunderstorm that has a tilted
updraft.
Fig. 10-10, p. 271
A dramatic example of a shelf cloud (or arcus cloud) associated
with an intense thunderstorm
Fig. 10-11, p. 271
Summary of today’s lecture
Life cycle of ordinary thunderstorms
Multicell thunderstorms
Severe thunderstorms
Squall lines
Supercell thunderstorms
Mesoscale Convective Complexes.