Thunderstorms and Twisters
Review of last lecture
1. Definition of airmasses? Bergeron classification of
air masses
2. Surface weather analysis: Station model, wind speed
code, present weather
3. Fronts: 6 types. What is a cold front? A warm front?
An occluded front?
4. The developmental stages and vertical structure of
middle latitude cyclones (boundary between
northern cold air and southern warm air, upper level
low to the west of surface low)
5. How upper level longwaves and shortwaves may
enhance cyclonic development at the surface (upper
level low to the west of surface low)
6. The three regions of cyclogenesis and typical tracks
Basic Concepts
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Tornadoes: about 100-600 m, last 1 minute to 1 hour
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Thunderstorms: about 10 Km, last 10 minutes to a
couple of hours. 3 types: ordinary, multicell, supercell
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Mesoscale convective systems (MCSs): A cloud
system that occurs in connection with an ensemble
of thunderstorms and produces a contiguous
precipitation area on the order of 100 Km or more in
at least one direction, and often last for several hours
to a couple of days.
Thunderstorms
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A storm containing lightning & thunder
Convective; form when warm, humid air
rises in conditionally unstable environment
The warmer the rising air parcel is relative
to environment, the more buoyant force is
driving it upwards (stronger convection)
Trigger to start uplift: warming sfc, terrain
(orography), converging sfc winds, frontal
zones, divergence aloft (or combination)
Thunderstorms I. Ordinary Storms
Three stages have been identified in ordinary thunderstorms:
a) an unstable atmosphere and vertical updrafts keep precipitation suspended
b) MATURE: entrainment of dry air that causes cooler air from evaporation,
triggering downdrafts and falling precipitation and gust fronts
c) DISSIPATING: weakening updrafts and loss of the fuel source after 15 to 30
minutes.
Lightning & Thunder
Lightning is a discharge of electricity, a giant spark.
Charge layers in the cloud are formed by the transfer of
positive ions from warmer hailstones to colder ice
crystals.
When the negative charge near the bottom of the cloud is
large enough to overcome the air's resistance, a stepped
leader forms.
A region of positive ions move from the ground toward
this charge, which then forms a return stroke into the
cloud.
Charge differences between the thunderstorm and ground
can cause lightning strokes of 30,000°C, and this rapid
heating of air will creates an explosive shock wave called
thunder, which requires approximately 3 seconds to travel
1 kilometer (5 sec per mile).
Thunderstorms II. Multicell Storm
Cool downdrafts leaving a
mature and dissipating storm
may offer relief from summer
heat, but they may also force
surrounding, low-level moist air
upward.
Hence, dying storms often
trigger new storms, and the
successive stages may be
viewed in the sky.
A Multicell Thunderstorm
Thunderstorm III. Supercell Storm
Storms producing a minimum of
a) 3/4 inch hail and/or
b) wind gusts of 50 knots and/or
c) tornado winds, classify as severe.
In ordinary storms, the downdraft and falling precipitation cut off the updraft.
But in severe storms, winds aloft push the rain ahead and the updraft is not
weakened and the storm can continue maturing and maintain its structure
for hours.
A Supercell Storm in Texas
Tornado
Movie time!
Twisters: Nature’s Deadly Force
Tornadoes
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A rapidly rotating column of air
blowing around intense L pressure
with circulation reaching ground
Various shapes: twisting rope-like
funnels to cylindrical funnels, to
massive black funnels
Various sizes: most are 100-600 m
in diameter; some just a few meters;
some >1 mile
Rotation is almost exclusively
counterclockwise (viewed from
above!)
Most move ahead of cold fronts,
from SW to NE, from 20-40 knots
(some >70 knots)
3 Stages of Tornado Formation
• Before thunderstorms develop, a change in
wind direction and an increase in wind speed
with increasing height creates an invisible,
horizontal spinning effect in the lower
atmosphere.
• Spinning horizontal vortex tubes created by
surface wind shear may be tilted and forced in
a vertical path by updrafts. This rising, spinning,
and often stretching rotating air may then turn
into a mesocyclone.
• Most strong and violent tornadoes form within
this area of strong rotation.
Tornado Breeding Supercell Storm
Supercell thunderstorms may have many of the features illustrated
here, including a mesocyclone of rotating winds formed when
horizontal vortex was tilted upwards.
Life cycle of tornadoes
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Often evolve through a series of stages, from dust-whirl, to
organizing and mature stages, and ending with the shrinking and
decay stages.
Families of tornadoes
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The forms causing the largest damages are families of tornadoes; when
many occur (>6), it is a tornado outbreak
Tornado Occurrence
Record year: 1998
1424 tornadoes!
Tornadoes from T-storms in hurricanes
Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes
annually, but the highest frequency is observed in tornado alley of the Central
Plains. Great setting for potent mixing of air masses.
Nearly 75% of tornadoes form from March to July, when warm humid air is
overlain by cooler drier air to cause strong vertical lift.
Mesoscale Convective Systems
I. Squall Line
• A squall line may
contain several severe
thunderstorms, some
possibly supercells,
extending for more
than 1000 kilometers.
• A squall line always
contains a convective
precipitation region
and a trailing stratiform
precipitation region.
Mesoscale Convective Systems
II. Mesoscale Convective Complex
An organized mass, or collection, of
thunderstorms that extends across a
large region is called a mesoscale
convective complex (MCC). They can
be up to 1000 x larger than individual
storms.
With weak upper level winds, such
MCC's can regenerate new storms and
last for upwards of 12 hours and may
bring hail, tornadoes, and flash floods.
They often form in summer beneath a
ridge of high pressure, with moisture
fed from S by low level jets.
Summary
1. The general size and lifetime of mesoscale convective
systems, thunderstorms and tornadoes
2. 3 types of thunderstorms. 3 stages of the ordinary
thunderstorms.
3. Lightning. The 3 steps of lightning development. How
fast does thunder travel?
4. 3 stages of tornado formation. Which state has the
largest number of tornadoes in U.S.?
5. 2 types of mesoscale convective systems