Thunderstorm: a cumulonimbus cloud or collection of
cumulonimbus clouds consisting of vigorous updrafts,
precipitation and lightning
Thunderstorm: a cumulonimbus cloud or collection of
cumulonimbus clouds consisting of vigorous updrafts,
precipitation and lightning
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Thunderstorms are responsible for most of what we refer to as
“severe weather”, including high winds, lightning, tornadoes
and hail
- By official definition, a severe thunderstorm has at least
one of the following:
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Large hail of ¾ inch diameter or greater
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Wind gusts of at least 50 knots
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A tornado
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Keep in mind that a thunderstorm is a form of cumulus cloud, which
means it's basically a plume of warm, cloudy air (referred to as an
updraft) in a conditionally unstable environment.
The plume keeps rising because of condensation in the cloud,
which keeps the cloudy air warmer than the surroundings.
Labeling the Parts
condensation
evaporation
warm air
cold air
A mature thunderstorm cell consists of a warm updraft and a
(relatively) cold downdraft. The updraft is driven by
condensation and latent heating and the downdraft by
evaporative cooling.
Labeling the Parts
condensation
entrainment
evaporation
warm air
cold air
The evaporation occurs through two different processes:
(i) rain falls into the unsaturated air below and evaporates; and
(ii) dry air is mixed into the cloud across the sides of the cloud,
through a process called entrainment
Labeling the Parts
condensation
entrainment
evaporation
warm air
cold air
gust front
cold pool
As the downdraft reaches the ground, it spreads out and forms a
region of relatively cold air called the cold pool.
Labeling the Parts
condensation
entrainment
evaporation
warm air
cold air
gust front
cold pool
The leading edge of the spreading cold pool is called the gust
front, since the winds behind the gust front (on the cold side)
tend to be strong and gusty.
Labeling the Parts
warm air
cold air
gust front
As the gust front spreads along the ground, it forces warm air up
and over the front. This can initiate new updrafts, which is an
important process for maintaining and propagating the system.
Photo of a gust front approaching outside the daycare.
No walk today!
Recipe for a Thunderstorm
The driving force behind a thunderstorm is the latent heating in the
updraft. So to get things going, we'll need:
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A liberal supply of warm, humid air
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A conditionally unstable environment, allowing the air to rise
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A lifting mechanism (the “trigger”) to raise the initially dry air to
saturation
And if it's a severe storm we're after, then we'll also need:
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A change in the background wind speed and direction with
height (called wind shear)
wind shear
Conditions for the April 15, 2011 Tornado Outbreak
a change in wind
speed and/or
direction with height
a trigger mechanism
to provide lifting
(cold front)
a supply of warm
humid air off the Gulf
Thunderstorm Classification
A given thunderstorm can be classified as:
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A single-cell (or ordinary-cell) thunderstorm, consisting of a
single updraft/downdraft system
A multicell thunderstorm, consisting of several thunderstorm
cells evolving together as a whole
A supercell storm, consisting of a single intense, rotating
updraft/downdraft pair (the “king of storms”)
In addition, we also classify large groups of storms, into what are
called mesoscale convective systems (MCS):
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Squall lines consist of a collection of storms arranged in a line
Mesoscale convective complexes consist of a large group of
storms without a well-defined shape
Dependence on shear
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Which kind of storm we get (single-cell, multicell, or supercell)
depends to a large extent on the wind profile in the
environment (i.e., on the environmental shear)
Larger amounts of wind shear tend to produce longer-lived
and more intense storm systems
Dependence on shear
height (km)
environmental wind
profiles used in computer
modeling experiments
(experiments A, B, C, etc)
A
B
C
wind speed (m/s)
the stronger shear profiles
result in longer-lived multicell
and supercell storms
single-cell (A)
supercell (C)
updraft strength
resulting storm strength
(measured by updraft speed) as
a function of time for the
different wind profiles
multicell (B)
time (min)
Single-cell storms:
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A single-cell storm consists of a single updraft / downdraft pair
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Single-cells form in environments with weak wind shear
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The cell evolves through a series of stages, with the whole process
taking roughly 30 to 60 minutes
The motion of the storm follows the mean background wind
Life cycle of a single-cell storm:
Growth stage: air parcels
lifted to saturation by some
trigger process. Clouds
build into towering
cumulus.
Mature stage: evaporation
leads to formation of a
downdraft. Cold pool
forms and begins to spread
along the ground. Most
intense stage.
Dissipating stage: the
spreading cold pool cuts
the system off from warm
air, killing the updraft and
leading to dissipation.
A single-cell thunderstorm in the dissipating stage
Multicell storms:
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A multicell storm consists of a small collection of single cells in
various stages of development
Multicells are found in environments with weak to moderate shear
Individual cells grow and die in 30 to 60 minutes, but the
collection as a whole can last much longer
Maintenance of a multicell storm:
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The key to multicell longevity is the spreading gust front
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As the front spreads it lifts warm air, initiating new updrafts
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The new updrafts form at the leading edge of the system, and
the older cells shift to the back of the system and die
mature cells move to the back of the
system and eventually dissipate
new updrafts are
formed at the gust front
a shelf cloud formed as air
is lifted up and over the
edge of the gust front
a rotating roll cloud just
behind the leading edge
of the gust front
Supercells:
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A supercell consists of a single intense, rotating updraft and
associated downdrafts
Supercells are typically found in high-shear environments
The organization of a supercell is self-reinforcing, and a
single supercell can last several hours
rotating supercell
wall cloud and
associated tornado
Supercells:
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Most of our intense tornadoes in the US are produced by
supercell storms
rotating supercell
wall cloud and
associated tornado
Supercell Structure as Seen by Radar
roughly N-S
cross-section
(A to B)
radar echoes
seen at various
heights above
the ground
roughly E-W
cross-section
(C to D)
B
Bounded Weak Echo Region (BWER):
a hole in the radar signal where the
updraft is so fast that precipitation has
no time to grow
C
hook
echo
A
D
updraft
region
front-flank
downdraft
rear-flank
downdraft
front-flank
gust front
rotating
updraft
rear-flank
gust front
Tornadic supercells usually have two downdrafts, each with an
associated gust front. The single, rotating updraft is located
where the two gust fronts meet.
The Supercell Updraft and Tornadoes
When a tornado forms, it's usually part of the rotating updraft. So
why does the updraft rotate in the first place?
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The rotation of the updraft ultimately comes from the shear in the
the environmental winds
- Remember: supercells form in high-shear environments
shear in the
environment
The shear in the environmental winds provides a source of
background rotation. (Think of rolling a pencil between your two
hands....) When this rotation gets pulled into the storm's updraft,
it spins about a vertical axis.
shear in the
environment
The end result is an environment favorable for tornado formation.
(Roughly 10-15% of supercells produce tornadoes.)
a rotating wall cloud at the base of a supercell updraft
and another, this time with tornado
this one also had a tornado, shortly before the picture
was snapped
Favorable supercell conditions (and storm conditions in general) are when
warm, humid air off the Gulf is drawn northward below an upper-level
disturbance with strong shear, often as part of a developing cyclone.
Lifting is provided by upper-level divergence and/or the surface cold
front, while the shear is provided by the upper-level jet stream.
Conditions for the April 15, 2011 Tornado Outbreak
Where are tornadoes most likely?
Frequency of tornado occurrence
(average number per year by state)
What time of year are tornadoes most likely?
Number of tornadoes
by month in the US
What time of day are tornadoes most likely?
Number of tornadoes by
time of day in the US
Collections of Storms
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Finally, large collections of individual storm cells often organize
themselves into much larger structures, called mesoscale
convective systems (MCS)
The classic MCS types include:
the squall line: a collection
of multicell or (occasionally)
supercell storms arranged
more or less in a line
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Squall lines usually form
form in environments with
significant shear
Collections of Storms
●
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Finally, large collections of individual storm cells often organize
themselves into much larger structures, called mesoscale
convective systems (MCS)
The classic MCS types include:
the meoscale convective
complex (MCC): a
collection of multicell
storms arranged with no
particular shape
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MCCs tend to form in
environments with
weaker shear