Mesoscale convective systems

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Mesoscale convective systems
Review of last lecture
1. 3 stages of supercell tornado formation.
1. Tornado outbreak (number>6)
2. Tornado damage: Enhanced Fujita Scale (EF-0 65-85
mph, EF-5 >200 mph)
3. 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. (MarchJuly)
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.
2 types of Mesoscale Convective Systems
1. Mesoscale Convective Complex
2. Squall line
Video: Mesoscale convective complex

https://www.youtube.com/watch?v=3y7mxqNXYpc
Mesoscale Convective Systems
I. Mesoscale Convective Complex (MCC)
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.
Formation of MCC
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)
Development of a squall line

Thunderstorm stage

Bow echo stage

Comma or line echo stage
Developement of a squall line:
Horizontal view
1. Thunderstorm
2. Bow echo
3. Comma echo
Video: The June 2012 Derecho

http://www.youtube.com/watch?v=K61qGv
KltL8
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
Clouds associated with a derecho
From NOAA SPC Derecho website
"The Line Storm," by John Steuart
Curry, 1897-1946
Different types of derechos




Serial derechos
Progressive derechos
Hybrid derechos
Low dewpoint derechos
Serial derecho



typically is associated with an
extensive squall line or
quasi-linear convective
system with multiple
embedded bow echoes.
usually occur in conjunction
with well-defined, migratory
low pressure system.
Overall convective system
(and, therefore, derecho)
motion is dominated by
advection --- the downstream
movement of individual
thunderstorm cells by the
environmental wind.
From
NOAA
SPC
Derecho
website
Progressive derecho



typically is associated with a
squall line of restricted length
that may involve a single
bow echo or multiple bows.
usually are associated with a
stagnant large scale weather
pattern.
Overall convective system
(and, therefore, derecho)
motion is dominated by
thunderstorm propagation -- that is, the development of
new storm cells downstream
from existing ones.
June 2012 derecho over the
Midwest and Ohio Valley
From
NOAA
SPC
Derecho
website
Annual number of derecho events
Timing of derecho events
From
NOAA
SPC
Derecho
website
Summary
1. 2 types of mesoscale convective systems
(mesoscale convective complex, squall line)
2. Structure of MCCs
3. Structure of squall lines: four components
1. Derechos. Definition (swath wider than 240 miles,
wind speed>57 mph). 4 types (serial, progressive,
hybrid, low dewpoint). Which state has the largest
annual number of derecho events? Most
derechos happen in the three months of May,
June and July
Works cited

http://usatoday30.usatoday.com/weather/wcomform.htm
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