Transcript Geography 210:

Mesoscale convective systems
and downbursts
Review of last lecture
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)
Convective systems
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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.
Video: The June 2012 Derecho
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http://www.youtube.com/watch?v=K61qGv
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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
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.
Global distribution of MCCs
From Doswell et al. 2000
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
moisture)
Mesoscale
updrafts
Mesoscale
downdraft
s
Convective
downdrafts
Zipser (1977), modified by Houze (1993)
Developement of a squall line:
Horizontal view
1. Thunderstorm
2. Bow echo
3. Comma echo
Downdraft of a squall line: Derechos
• A derecho (spanish word meaning “straight ahead”) is a widespread,
long-lived wind storm that is associated with a band of rapidly moving
showers or thunderstorms (often a squall line).
• By definition, the wind damage swath extends more than 240 miles and
wind speed > 57 mph. Can produce destruction similar to that of
tornadoes. The damage typically is directed in one direction along a
relatively straight swath.
From NOAA
SPC Derecho
website
Annual number of derecho events
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)
Video: Microburst
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http://www.youtube.com/watch?v=NT3Ybu
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Microburst
• 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
travel.
• 3 types: dry, wet, hybrid
Structure of a microburst
Dry microburst
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Very dry boundary layer topped by a moist layer
Primarily driven by cooling beneath the thunderstorm cloud base
due to rain evaporation and ice sublimation
Little or no rain. Often associated with high-based thunderstorms.
Sometimes associated with fair weather
Dry microburst: visual identification
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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 precipitation. (NOAA 2001)
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Blowing dust/dust rings at
surface
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Very good indicators of dry
microburst potential
Wet microburst
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A nearly saturated layer topped by an elevated dry layer
primarily driven by entrainment of mid-level dry air and
precipitation loading.
accompanied by heavy precipitation at the surface.
Wet microburst: visual identification
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A rain foot may be a visible
sign of a wet microburst.
Causes of atmospheric turbulence
1.
2.
3.
4.
Thermals - Heat from the sun
makes warm air masses rise and
cold ones sink.
Jet streams - Fast, high-altitude air
currents disturb the air nearby.
Mountains - Air passes over
mountains and causes wave and
turbulence on the other side.
Wake turbulence - Near the
ground a passing plane or
helicopter sets up small, chaotic air
currents, or Microbursts – dry,
wet and hybrid.
Summary
1. 2 types of mesoscale convective systems.
Structure of squall lines: four components
1. 3 types of downbursts (derechos, haboobs,
microbursts)
2. 3 types of microbursts (wet, dry, hybrid).
3. 4 causes of atmospheric turbulence
Works cited
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http://www-frd.fsl.noaa.gov/mab/microburst/
http://www.cimms.ou.edu/~doswell/microbursts/Handboo
k.html