Transcript Prof. Paul Sirvatka Severe and Unusual Weather ESAS 1115

Severe and Unusual Weather
ESAS 1115
Spotter Training and
Radar Meteorology
Part 3 – Thunderstorm Varieties
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Thunderstorm Ingredients
 A source of lift – i.e. a front, sea-breeze,
orography, outflows of previous thunderstorms,
etc…
 Moisture – low-level moisture provides the fuel
needed for thunderstorms (dew point
temperatures are the easiest way to assess the
threat of strong to severe thunderstorms)
 Instability – the ability of air to rise on its own
(i.e. buoyancy)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Launching a Radiosonde
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Receiving Data
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ESAS 1115 Severe and Unusual Weather
Thunderstorm Development
Cumulus Humilis
Cumulus Congestus
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Towering Cumulus
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Cumulonimbus (Cb)
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Texture
 A “rock hard” texture
implies that most of the
cloud is still in liquid
phase
 A growing updraft, or
one that is strong, will
exhibit this rock hard
texture
 It is marked by a
“cauliflower”-type look
with strong shadows
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Rock Hard Growth with Anvil
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Texture
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When the updraft
weakens or reaches high
in the troposphere, the
liquid freezes, giving the
cloud a “glaciated”
texture
When the updraft
exhibits mostly a
glaciated texture, the
updraft is considered
fairly weak
The glaciated texture
refers to the updraft, not
the outflow features of
the storm nor falling rain
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Texture
Soft and fuzzy; glaciated
Prof. Paul Sirvatka
Rock hard updraft growth
ESAS 1115 Severe and Unusual Weather
Glaciated Anvil
Crisp vs. Fuzzy
Crisp delineation between anvil and sky
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Vertical Shear
 Increases longevity and organization
 Ultimately, strong shear results in storm-scale rotation
by tilting horizontal vorticity into vertical vorticity
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Supercell and Flanking Line Tower
 Main tower is vertically
erect in a sheared
environment due to its
very strong updraft
 Flanking line tower to
the main cell’s southwest
is tilted due to the
environmental shear
 Notice the rock-hard
overshooting top,
another clue to the
vigorous nature of the
updraft
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Extremely Sheared Updraft
 Surprisingly, this tower maintained itself for some time before
dying out
 Strong localized point convergence helped maintain the inflow
source of unstable air
 If CAPE is too weak and shear too strong, thunderstorms tend
to be sheared apart, resulting in a short-lived thunderstorm
with minimal threat
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Updraft vs. Downdraft
 Updraft strength is
primarily a function of
instability
 Downdraft strength is
primarily a result of dry air
 The top diagram would be
a fairly weak shower or
thunderstorm
 The bottom diagram would
result in primarily strong
down drafts or microbursts
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Updraft vs. Downdraft
 High instability would increase
the threat of severe hail
(although most likely not very
large hail) but would have a
lesser threat for damaging winds
 When dry air and high instability
are both present, the likelihood
of severe weather increases
 Dry air increases the likelihood of
large hail as well as strong winds
 Supercells are the result of strong
vertical shear where buoyancy
and shear are well-balanced
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ESAS 1115 Severe and Unusual Weather
Low Base
 The Lifting
Condensation Level
(LCL) helps indicate the
relative humidity of
the sub-cloud layer
 High LCL’s indicate
lower RH and more
chance for microbursts
 Low LCL’s indicate
more moisture in the
sub-cloud layer
 Lower LCL storms have
a higher tornado
potential
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Weak Storm Still Dangerous
 High base indicates threat for damaging winds
 Forest fires are also a great threat due to lightning, strong
winds and dry conditions
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
High-based Storm
Low LCL’s correlate well to tornado potential. High LCL’s like
in the storm shown here will struggle to produce tornadoes
Prof. Paul Sirvatka
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Storm Evolution
Prof. Paul Sirvatka
Movie
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Thunderstorm Types
 There are three basic types of
thunderstorms: single cells,
multicells and supercells.
 Theses are loosely defined, as
no thunderstorm is completely
a “single” cell
 Multicell storms can be either
clusters or lines
 The most important distinction
in thunderstorm types is the
presence or absence of stormscale rotation
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Buoyancy, Gust Fronts
and Dynamics
thunderstorms are
dominated by
buoyancy processes
 Multicell thunderstorms
are dominated by gust
front processes
 Supercell thunderstorms
are dominated by
dynamic processes
Prof. Paul Sirvatka
Instability
 Single cell
Vertical Wind Shear
ESAS 1115 Severe and Unusual Weather
Single Cell Thunderstorms
 Sometimes called “air
mass” t-storms, these storms
are poorly organized and
pose relatively little threat
to the public (lightning and
hail)
 Typical of afternoon
thunderstorms
 Updrafts form in relatively
random locations
 The dominant forcing
feature is instability since
they form in a low-shear
environment
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Convective Initiation on Radar
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Convective Initiation on Radar
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Single Cell Thunderstorms
 Buoyancy dominates single cell processes
 Without significant vertical wind shear, the cold pool will
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Movie
undercut the updrafts
Lack of vertical shear means that new updrafts will not be
able to be generated along the outflow boundaries
Cells will go through their cycle in about 30-60 minutes
The severe threat is minimal except in cases of strong
instability and then even then the threat is relatively shortlived
Any severe weather occurrence in difficult to predict and even
then, lead time will be minimal
Tornadoes are unlikely but can occur as landspouts
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Pulse Severe Storm
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A single cell storm may still produce large hail or damaging winds in which
case the storm is called a “pulse” severe
A pulse storm produces severe weather, usually in the form of large hail,
although the severe event will be relatively short-lived
Identification of a pulse severe storm is difficult and gives little lead time
for warnings
“Popcorn severe”
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Using VIL for Discriminating Between
Ordinary and Pulse Storms
Warn when the VIL exceeds
the VIL of the day
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Land Spouts
 These are non-supercell
tornadoes
 Spouts form on
boundaries where preexisting vertical
vorticity is already high
 The updraft stretches
the vertical vorticity
into a vortex that
results in tornado
 Conservation of
Angular Momentum
helps explain their
existence
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Land Spouts
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather
Waterspouts
Multiple Waterspouts over Lake Michigan
Prof. Paul Sirvatka
ESAS 1115 Severe and Unusual Weather