Tornadoes, MCSs and Downbursts
Review of last lecture
1. The general size and lifetime of mesoscale convective
systems, thunderstorms and tornadoes. 3 types of
2. 3 stages of the ordinary thunderstorms. Downdraft and
falling precipitation cut off the updraft.
3. Formation of multi-cell thunderstorms. Downdrafts initiate
new thunderstorms in nearby regions.
4. 3 stages of the supercell thunderstorms. Winds aloft push
downdraft/precipitation away and the updraft is not
5. Two types of lightning (cloud-to-cloud 80%, cloud-toground 20%)
6. 4 steps of lightning development. How fast does thunder
Convective systems
Tornadoes: about 100-600 m, last 1 minute to 1 hour
Thunderstorms: about 10 Km, last 10 minutes to a
couple of hours. 3 types: ordinary, multicell, supercell
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.
Video: A tornado
A rapidly rotating column of air blowing around
intense low pressure with circulation reaching
Wind speeds between 105 km/hr(65mph) and 450
Rotation is almost exclusively cyclonic; a few spin
in the opposite direction
Various sizes: most are 100-600 m in diameter;
some just a few meters; some >1 mile
Various shapes: twisting rope-like funnels to
cylindrical funnels, to massive black funnels
Usually last only a few minutes, but some lasted
several hours
Most move ahead of cold fronts, from SW to NE;
some move in other directions
Moving speed is about 30 mph (some >70 mph)
Tornado Formation
• Tornadoes can develop in any
situation that produces severe
weather – cold fronts, mesoscale
convective systems, supercells,
and tropical cyclones.
• The processes leading to their
formation are not well understood
• The most intense and destructive
tornadoes come from supercells.
3 Stages of Supercell 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
• Most strong and violent tornadoes form
within this area of strong rotation.
Wind structure of supercell and tornado
The most common atmospheric
circulation structure
or No
Imbalance of heating
 Imbalance of temperature
 Imbalance of pressure
 Wind
Tornado Damage
• Tornadoes are classified by
the magnitude of damage
they cause using the
Enhanced Fujita scale.
• The forms causing the
largest damages are
families of tornadoes; when
many occur (>6), it is a
tornado outbreak
• The largest tornado
outbreak on record was the
April 25–28, 2011 tornado
outbreak, with as many as
358 tornadoes.
Tornado Occurrence (global)
Tornado Occurrence (U.S.)
Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes
annually (75% from March-July), but the highest frequency is observed in
tornado alley of the Central Plains. Great setting for potent mixing of air masses.
Mesoscale Convective Systems
I. 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
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.
Mesoscale Convective Systems
II. 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.
Vertical structure of squall lines
Convective updrafts (controlled by
lower troposphere temperature and
Zipser (1977), modified by Houze (1993)
Downbursts: Introduction
• Downbursts are gusts of wind
that can reach speeds in
excess of 270km/hr (165mph),
and are potentially deadly.
• Three common types:
• Derechos (1000 km)
• Haboobs (10-100 km)
• Microbursts (1 km)
• is a very localized column of
downdraft (sinking air) in a
thunderstorm that is less than 2.5
miles in scale.
• produces damaging divergent and
straight-line winds at the surface
as high as 150mph
• is similar to, but distinguishable
from, tornadoes, which generally
have convergent damage.
• can produce dangerous situations
at airports, as they impede air
• 3 types: dry, wet, hybrid
Dry microburst: visual identification
Very dry boundary layer topped
by a moist layer. Driven by
cooling beneath the cloud base
due to rain evaporation and ice
Virga in the sky: defined as
wisps or streaks of water or ice
particles falling out of a cloud
but evaporating before
reaching the earth's surface as
Blowing dust/dust rings at
Wet microburst: visual identification
A nearly saturated layer topped
by an elevated dry layer. Driven
by entrainment of mid-level dry
air and precipitation loading.
Heavy precipitation at the
A rain foot may be a visible sign
of a wet microburst.
1. 3 stages of supercell tornado formation.
2. Tornado outbreak (number>6)
3. Tornado damage: Enhanced Fujita Scale (EF-0 65-85
mph, EF-5 >200 mph)
4. Tornado occurrence: Global and U.S.. Which country has
the largest number of tornadoes in the world? Which
state has the largest number of tornadoes per unit area
in U.S.? Tornado season in U.S. (March-July)
5. 2 types of mesoscale convective systems. Structure of
squall lines: four components
6. 3 types of downbursts (derechos, haboobs, microbursts)
7. Visual identification of dry microburst (virga in the sky,
blowing dust at the surface)
Works cited
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