Multicells and Lines

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Transcript Multicells and Lines

Multicells, Lines, and
Mesoscale Convective
Systems
METR 1004: Introduction to Meteorology
Adapted from Materials by Dr. Frank Gallagher III
and Dr. Kelvin Droegemeier
School of Meteorology
University of Oklahoma
Multicellular Thunderstorms
So far we have discussed the structure
of air mass or single cell thunderstorms.
 We can think of these types of storms
as a single “cell” where each cell is:

– Independent
– Has a complete life cycle
– Has a life cycle of 30 minutes to an hour
– Is usually weak
Multicellular Thunderstorms

We know that many thunderstorms can
persist of longer periods of time.
– These storms are made up of many cells.
– Each individual cell goes through a life cycle but
the group persists.
– These storms are called multicellular
thunderstorms, or simply multicells
– Multicellular storms consist of a series of evolving
cells with each one, in turn, becoming the
dominant cell in the group.
Multicellular Thunderstorms
With air mass storms, the outflow boundaries are usually too
weak to trigger additional convection. In multicell storms, the
outflow boundary does trigger new convection.
Cell #1
Mature
Multicellular Thunderstorms
With air mass storms, the outflow boundaries are usually too
weak to trigger additional convection. In multicell storms, the
outflow boundary does trigger new convection.
Cell #1
Mature
Cell #2
Cumulus
Multicellular Thunderstorms
After about 20 minutes or so, the second cell becomes the
dominant cell. Cell #1 is now dissipating, and a new cell (#3)
is starting.
Cell #1
Dissipating
Cell #2
Mature
Cell #3
Cumulus
Multicellular Thunderstorms
After about 20 minutes or so, the third cell becomes the
dominant cell. This process may continue as long as
atmospheric conditions are favorable for new convection.
Cell #2
Dissipating
Cell #3
Mature
Cell #4
Cumulus
Cell #1
Almost
Gone
Multicellular Thunderstorms

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A cluster of short lived single cells.
Cold outflow from each cell combines to form
a much larger and stronger gust front.
Convergence along the gust front tends to
trigger new updraft development. This is the
strongest in the direction of storm motion.
New cell growth often appear disorganized to
the naked eye.
Structure of a Multicell Storm
Multicell Storm on Radar
Multicellular Thunderstorms

Conditions for development
– Moderate to strong conditional instability

Once clouds form, there is a significant amount
of buoyant energy to allow for rapid cloud
growth
– Low to moderate vertical wind shear

Little clockwise turning
Importance of Vertical Wind
Shear

Single cell
– Weak shear - storm is vertically stacked
– Outflow boundary may “outrun” the motion
of the storm cell
– New storms that develop may be too far
from the original to be a part of it

Multicell
– Weak to moderate shear keeps gust front
near the storm updraft – triggers new cells
– New development forms adjacent to the
older cells and connects with the old cell
Cell and Storm System Motion
Multicellular Thunderstorms
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On the previous diagram, there are two
arrows that show the “cell motion” and the
“storm motion.”
Notice that they are different. Why?
New cells tend to form on the side of the
storm where the warm, moist air at the
surface is located.
In the central Plains, this is often on the
south or southeast side.
1
Average Wind
1
2
Warm, Moist Surface
Air (inflow)
Cell and Storm System Motion
Cell and Storm System Motion
Cell and Storm System Motion
Multicellular Thunderstorms
Individual cells typically move with the
mean (average) wind flow
 The storm system moves differently –
by discrete propagation
 Multicell storms may last a long time.
They constantly renew themselves with
new cell growth.

Multicellular Thunderstorm
Hazards

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Heavy rain -- Flooding
Wind damage
Hail
Lightning
Tornadoes -- Usually weak
Multicell storms are notorious for heavy rain
and hail
Types of Thunderstorms

Squall lines
–
–
–
–
Storms form in a line
Can last for several hours
Occur all year long
Can produce strong
winds and hail
– Rarely produce tornadoes,
except at southern end
Mesoscale Convective Systems
Thunderstorm complexes that are larger
in horizontal scale than multicellular or
supercellular storms.
 Examples

– Squall Line Thunderstorms
– Mesoscale Convective Complexes
Squall Line Thunderstorms
Lines of thunderstorms that tend to act
as a single mesoscale system.
 May be a continuous line of storms or
the storms may be broken into
segments.
 We classify squall lines according to
their development.

Squall Line Thunderstorms
© 1993 Oxford University
Press -- From: Bluestein ,
Synoptic-Dynamic
Meteorology in Midlatitudes
Radar Views of Squall Lines
Squall Line on Radar
Indianapolis, Indiana
25 July 1986 -- 1714 CST
© 1993 Oxford University Press -- From: Bluestein , Synoptic-Dynamic
Meteorology in Midlatitudes
Satellite View of a Squall Line
Structure of a Squall Line
Squall Line Passage
Squall Line Cross-section
Arcus Cloud Examples
© 1980 The Pennsylvania State
University Press -- From:
Ludlam , Clouds and Storms
Patrick AFB
Norman, OK -- 27 May 1977
© 1993 Oxford University
Press -- From: Bluestein ,
Synoptic-Dynamic
Meteorology in Midlatitudes
Gustnado Produced by a
Squall Line
Squall Line Cross-section
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Precipitation falls into the dry mid-level
inflow. The evaporational cooling aids in
sustaining a strong downdraft.
Often there is a region of lighter precipitation
that follows the passage of the squall line >
stratiform precipitation.
Stratiform Precip.
Precipitation Types
Convective
(Leading
Edge of
Line
Precipitation Types
Stratiform
(Well
behind the
line)
Stratiform Precipitation
Requires rising motion behind the
convective towers.
 The cloud tops radiate energy to space,
thereby cooling the top of the stratiform
region.
 Radiation from the ground is absorbed
by the cloud bases of the stratiform
region.
 This combination acts to destabilize the
atmosphere in the stratiform region.

Stratiform Precipitation

Updraft
– Convective towers: 20 - 50 m s-1
– Stratiform region: 0.1 - 0.4 m s-1

Much slower ascent in the stratiform
region. The result is much lighter
precipitation.
Mesoscale Pressure Centers

Just behind the gust front is an area of
high pressure. This is called a
“mesohigh.”
Mesohigh
The mesohigh forms just behind the
gust front.
 A rapid pressure jump occurs.
 Caused by the combined effects of:

– Evaporation of precipitation
– Melting of precipitation

These two effects cool the atmosphere
to form a dense pool of cold air just
behind the gust front.
Mesolow

Behind the stratiform precipitation, a weak
low pressure region develops.

Forms probably because of the slowly sinking
air behind the stratiform rain.
This sinking air warms slightly, creating the
weak low pressure.

Conditions Necessary for
Squall Line Development

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Warm, moist air at low levels
Relatively cold air aloft
Mid-level dry air that can enhance the
downdraft and the vigor of the line
Environmental cap
– Prevents “widespread” convection

“Linear” forcing
– Cold fronts, dry line, outflow maxima
– Orography, valley wind currents
Conditions for Long-Lived Squall Lines
No Environmental
Shear – Updraft is
Vertical
Conditions for Long-Lived Squall Lines
Low-Level Shear
Creates a Bias and a
Tilt
Conditions for Long-Lived Squall Lines
No Environmental
Shear – Updraft is
Vertical
Add Surface Cold
Pool – Updraft is
Biased and Tilted
Conditions for Long-Lived Squall Lines
Low-Level Shear
Creates a Bias and a
Tilt
Low-Level Shear
Counters the Cold
Pool Bias – Optimal
Condition
Conditions for Long-Lived Squall Lines
The Squall Line Bow Echo
Bow Echo on Radar
Visual Siting of a Bow Echo
Mesoscale Convective
Complexes

Classification Information
– Infrared cloud-top temperatures are colder
than -32oC and cover an area larger than
100,000 km2
– Interior cloud-top temperatures are colder
than -52oC and cover an area larger than
50,000 km2 for 6 hours or more.
– Eccentricity greater than 0.7 at the time of
the maximum areal extent of the cloud top.
Mesoscale Convective
Complexes
MCC
Mesoscale Convective
Complexes
Often start as a line of thunderstorms
that form along the Rocky Mountains.
 Move off into the Plains and organize
into a much larger system.
 Often reach maximum strength at
night.
 Provide a significant portion of the
summertime (warm season) rainfall to
the Great Plains.

Mesoscale Convective
Complexes

MCC’s may form structures that
resemble smaller scale cyclones. If
given sufficient time, they may rotate.
The figure on the left shows the
remnants of a MCC. The cyclonic
rotation is clearly evident on this
visible satellite image. Note that
the size of the vortex covers almost
all of Kansas and Oklahoma!
Mesoscale Convective
Complexes
As with squall lines, MCCs have strong
convective updrafts and regions of
slowly ascending air that creates the
stratiform precipitation.
 The difference is that in the MCC the
slowly ascending updrafts tend to
surround a central core of convective
updrafts.

Mesoscale Convective Complexes

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MCCs are producers of
– Heavy Rain
– Flash Floods
– Hail
– Frequent Lightning
– Possibly a tornado if the environmental shear
is sufficient
MCC storms may last for 12 or more hours.
They act to restabilzed a conditionally unstable
atmosphere.
They may set up surface boundaries that can
influence weather a day later.