Chap. 18
Single Cell Thunderstorms
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Photo of a supercell storm in eastern Colorado.
Learning objectives
1.
2.
3.
4.
5.
6.
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8.
9.
Name the different ways thunderstorms organize
State the NWS criteria for a severe thunderstorm
Be able to list and describe the impacts of severe thunderstorms
Identify the environmental conditions conducive to severe and
non-severe thunderstorm formation
Explain why vertical wind shear is necessary for severe or
organized thunderstorms
Identify the structural characteristics of mesoscale convective
systems (MCSs), squall lines, and supercell thunderstorms.
Identify the types of severe conditions most commonly associated
with MCS, squall lines, and supercell thunderstorms
Identify the type of severe thunderstorm organization using radar
and satellite imagery
Summarize the current understanding of how thunderstorm
frequency and intensity will be affected by global climate change
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Thunderstorm Frequency Map
Fig. 18.1. Locations of severe weather
events during 2009: tornadoes (top), hail
(middle), and damaging winds (bottom).
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Severe weather summaries are available
from http://www.spc.noaa.gov
Thunderstorms
A cumulonimbus cloud that produces lightning (and hence thunder)
Very common (globally)
Transport heat and moisture vertically in the atmosphere
Most are not severe
Severe thunderstorm:
a) Hail > 1 inch diameter (¾ inch prior)
b) Wind gust >50 kt (58 mph)
c) Tornado
Ingredients for thunderstorm
initiation:
a) Water vapor
b) Conditional instability
c) Lifting mechanism
For a severe thunderstorm, a 4th
ingredient is:
d) Strong vertical wind shear
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Thunderstorm Facts &
Climatology
• At any given time there are 2000 thunderstorms
in progress
• About 45,000 thunderstorms take place each
day
• Annually, The U.S. experiences about 100,000
thunderstorms.
• About 16 million thunderstorms occur annually
around the world!
• The lightning from these storms strikes Earth
about 100 times each second
Lifting from convergence zones
(also termed convergent boundary zones, or CBZ)
• Boundaries (narrow zone of contrast in
temperature and/or humidity, and wind)
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–
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Gust front (generated from thunderstorm outflows)
Sea breeze
Cold front
Dry line
Gravity waves
• ABIDE: Atmospheric Boundary Identification and
Delineation Experiment
– UAH experiment to examine how CBZ’s produce
convective initiation in the daytime, nighttime, and
period-in-between boundary layer
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More definitions
• Boundary Layer (BL) – the lowest 1-2 km of
the atmosphere this is affected by the surface
(heating, roughness, etc); the BL is more
turbulent than the free atmosphere (FA)
• Convective initiation (CI) – a process in which
a CBZ produces a rapidly growing cumulus
cloud which evolves to a deep convective
cloud, often a cumulonimbus (Cb) cloud
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Importance of the Boundary Layer
 It contains much of the high humidity (water vapor)
which is the “fuel” for Cumulonimbus clouds (latent
heating). Therefore, it is related to the magnitude
of the CAPE (Convective Available Potential
Energy).
 The strongest and most important layer of wind
shear is located within the BL. I will often use the
terms shear and deep layer shear interchangeably,
while I will often use the terms low-level shear and
storm-relative helicity interchangeably. We will
discuss more about SRH in the Supercells lecture.
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Vertical Wind Shear
Vertical Wind Shear:
Change in wind
speed and/or
direction with height
Severe storms need
strong veering of
wind with height and
strong increase in
speed
Wind Shear Time Lapse Example
Upscale development of thunderstorms
Individual thunderstorms
• Single cell thunderstorm
• Multicell thunderstorm
• Supercell thunderstorm
– will examine this first
Larger-scale systems with embedded showers
and/or thunderstorms:
• Mesoscale Convective System (MCS)
• Squall line (quasi-linear convective system, QLCS)
• Hurricane
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Single-Cell Thunderstorms
Single-Cell Thunderstorm
• Life span of about 20 to 30 minutes
• Usually not strong enough to produce
severe weather
• Single Cell Pulse Storms often produce
severe weather
– Usually strong winds as it collapses
– Sometimes small hail
Life cycle of a single cell (air mass) thunderstorm
3 stages of development
1
2
3
Fig. 18.3
Vertical shear is weak (as in the SE
during summer)
Form along weak boundaries or
convergence zones.
Microbursts (strong downdrafts and
their outflows) may form during first
part of the mature stage.
Small size: <10 km diameter
Short lifetime: <1 h
15
Life Cycle – Cumulus Stage
• Also called Developing
Stage
• **Only rising air (updraft)
• Little to no rain
• Towering billowy white
clouds
• Lasts 10-15 minutes
• Lightning possible but
infrequent
Cumulus Stage In Detail
• Need boundary or trigger to create convergence
– Differential Heating (land vs water, forest vs urban)
– Layer destabilization
• Warm Air Advection in Low-Levels
• Cold Air Advection aloft
– Strong Forcing
– Convergence within Earth’s Boundary Layer
– Converging air in the boundary layer leads to uplift
•
•
•
•
•
Gust front (generated from thunderstorm outflows)
Sea breeze
Cold front
Dry line
Gravity waves
– If the instability is not great enough (only weak rising air),
then T-Storm activity may be quelled at this stage.
Cumulus Stage In Detail
• If convection can’t break the inversion then development is
limited to Cumulus Humulis (flat, pancake cumulus clouds) with
little vertical development
– Cumulus Mediocrus – more vertical development as the capping
temperature inversion begins to erode due to more air parcels rising,
which in turn moisten and cool the inversion layer.
– Cumulus Congestus – towering cumulus clouds with significant vertical
development that form when the capping inversion breaks
• **Cumulus Stage is dominated solely by updrafts for
the majority of this stage**
– The latter part of the Cumulus Stage is marked by the
development of a rain-cooled downdraft
– End of the Cumulus Stage/Beginning of Mature Stage
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•
•
•
The Downdraft becomes well developed
Produced by viscous drag of falling raindrops
Evaporation of falling raindrops leading to cooler, denser air
Evaporation associated with dry air entrainment into the cloud
Towering Cumulus (Cumulus Congestus)
Is the Cumulus Cloud still growing?
Cauliflower-type hard top =
Strong updraft & growing storm
Wispy soft top =
Weaker updraft & dying storm
Bubbling Cumulus Time Lapse
Life Cycle – Mature Stage
• The weather stage
• **Both rising (updraft) &
sinking air (downdraft)**
• The mature phase
represents the peak intensity
of the storm.
• Updrafts and downdrafts are
about equal in strength.
• Precipitation is typically
heavy and may contain small
hail
• Gusty winds result from the
downdraft spreading out on
the ground.
• Anvil at storm top
• Lasts 10-20 minutes
Life Cycle – Mature Stage
Life Cycle – Mature Stage
Mature Stage In Detail
• Begins with either rain reaching the ground or
rain leaving the base of the cloud and the
coexistence of an updraft and a downdraft
– As the rain load is lessened as a result of the
initiation of the downdraft, the cloud/storm may
explode vertically due to less drag
– Hail is possible during the mature stage
• Typically less than 1.25” for a single cell thunderstorm
Mature Stage In Detail
• Characteristics @ the peak of Mature Stage
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Time = First Cloud + 30-40 minutes
Horizontal Length = approximately 10 km (~6miles)
Height = 30,000’ to 60,000’ (~10 to 15km)
Upward Vertical Velocities = approx. 30 m/s or 67 mph
Downdrafts = approximately 12 m/s (~27 mph)
Low-Levels dominated by rain
Lower Mid-Levels contain both rain and snow because some
of this level is above the freezing level
– Upper Mid-Levels contain mostly wet snow
– Upper Levels contain dry snow
Entrainment
• Entrainment is the process by which saturated
air from the growing cumulus cloud mixes with
the surrounding cooler and drier (unsaturated)
air.
• Entrainment causes evaporation of the exterior
of the cloud and tends to reduce the upward
buoyancy there.
• Evaporation is a cooling process, thus air in the
middle portion of the storm becomes cooler &
more dense, thus it wants to sink => downdraft
enhancement
Downdraft
• When the downdraft reaches the ground, it
spreads out in all directions.
• The leading edge of this cold, often gusty
wind is called the outflow boundary or gust
front.
Downdraft and Outflow (Gust Front)
• The outflow boundary behaves like a cold front:
– Strong wind shift (speed and direction)
– Much colder air behind the gust front
– Acts as a location for additional lift for future storm
development.
New Storm will initiate
along the outflow boundary
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Gust Front Shelf Cloud
National
NationalSevere
SevereStorms
StormsLaboratory
Laboratory
Thunderstorm Outflow Time Lapse
Shelf Cloud Time Lapse
Life Cycle – Dissipating Stage
• Rainfall decreases
• Strong wind still possible
from some storms
• Tall billowy updraft
shrinking rapidly
• Mostly descending air (and
lighter rain)
• Lightning can still be quite
frequent
• Often only an “orphan
anvil” remains at cloud top.
Life Cycle – Dissipating (Orphan Anvil)
Life Cycle – All Stages
Dissipating Stage In Detail
• Time = First Cloud + 60-80 minutes
• Surface inflow is discontinued as a result of the cool
downdraft spreading out at the surface and undercutting
the inflow
• Updraft is cut-off due to the discontinuation of the
surface inflow
• This stage dominated by downdrafts
• Small Hail and heavy rain possible at the beginning of
this stage
• Light rain will dominate the entire base of the storm
• Dissipating process takes 20-30 minutes
• Gust Front/cold pool is left over and is often the focus
for new convection
• Mesohigh associated with the cold pool can last up to
24 hours.
Air Mass Thunderstorms Summary
• Usually weak (but can produce heavy rain in a short
period of time).
• Usually not severe
• Usually move slowly (weak winds aloft)
• Often develop and dissipate in less than one hour
• Form in a weakly sheared environment and thus have
a BUILT-IN SELF-DESTRUCT MECHANISM that
guarantees a short lifetime (downdraft collapsing down
on the updraft).
A Pulse Storm
• Severe single-cell thunderstorm
• Pulse storms typically form in a more unstable
environment than “garden variety” air mass storms.
• More unstable = stronger updraft
• Stronger updraft allows the storm to grow taller and live for
a longer lifetime. (40,000ft to 60,000ft)
• Stronger updraft also allows for the production of more
precipitation in the storm (more intense radar echoes)
• When the larger mass of precipitation comes crashing to
the ground in the downdraft, winds are much stronger (4060mph).
• Since the storm grew taller (to a colder temperature level),
hail is more likely to form & make it to the ground.
Radar
Visual
Life Cycle of Single Cell/Airmass Storm vs Pulse Storm
Back to Vertical Wind Shear
Vertical Wind Shear:
Change in wind
speed and/or
direction with height
Severe storms need
strong veering of
wind with height and
strong increase in
speed
Radar
Visual
Impact of Wind Shear on Storm Life Cycle & Severity
Visual
Life Cycle of Single Cell/Airmass Storm vs Pulse Storm
Radar
Height
AGL
Single-Cell Thunderstorm Homework
Using Chapter 18 & Lecture Material:
1.Summarize the average annual impact of thunderstorms on the United States in
terms of injuries, fatalities, and economic losses.
2.List the four necessary elements for severe thunderstorm formation.
3.What are the 3 ways (types) in which severe thunderstorms typically organize?
4.What are the 3 criteria for which the NWS considers a thunderstorm to be severe?
5.What region of the United States has the highest frequency of thunderstorms (be
specific)? Why does this region have the highest frequency of thunderstorms?
(ingredients & triggers)
6.Draw a vertical schematic of the three stages of an ordinary single-cell thunderstorm.
Be sure to include arrows to denote air motions, and include precipitation regions.
7.How can you tell visually if a thunderstorm (Cumulus Congestus) is still growing?
8. What two mechanisms lead to the formation of downdrafts in thunderstorms?
Continued on next slide
Single-Cell Thunderstorm Homework cont.
Using Chapter 18 & Lecture Material:
9.Describe the “self-destruct” mechanism for single-cell air mass thunderstorms.
10. How does a Pulse Storm differ from a “garden variety” air mass thunderstorms. Be
sure to include description of the storm height, environment, and processes that cause
severe wx.
11. How does increased vertical wind shear with height lengthen the life cycle of a
thunderstorm?
12.It is a summertime afternoon at UAH (Temp = 90, Dewpont = 72) and you are
walking from the UAH Fitness Center to SWIRLL for class. A thunderstorm forms over
Research Park and slowly drifts eastward over UAH as it reaches its mature stage.
You are late for class and decide to risk walking through this storm. Describe what you
would experience as you walk from east to west through the storm at the following
locations: a) Outside the Fitness Center before walking through the storm, b) as the
gust front passes you near the Student Center, c) the main core of the storm near the
Shelby Center, d) as the thunderstorm is dissipating and the sun comes out near the
NSSTC.