Single Cell
Thunderstorms
METR 4433
Mesoscale Meteorology
Spring, 2008 Semester
Adapted from Materials by Drs. Frank Gallagher III, Kelvin Droegemeier and
Ming Xue
School of Meteorology
University of Oklahoma
Textbook materials for reading
Bluestein Vol II pp. 444-462
 Houze pp. 35-37
 Cotton and Anthes p. 455-458

Thunderstorm

Definition: By definition, a thunderstorm is a local
storm, invariably produced by a cumulonimbus cloud,
that always is accompanied by lightning and
thunder. It usually contains strong gusts of wind,
heavy rain, and sometimes hail. Meteorologists often
use the word "convection" to describe such storms in
a general manner, though the term convection
specifically refers to the motion of a fluid resulting in
the transport and mixing of properties of the fluid. To
be more precise, a convective cloud is one which
owes its vertical development, and possibly its origin,
to convection (upward air currents).
Thunderstorm Climatology

At any given time there are an estimated 2000
thunderstorms in progress, mostly in tropical and
subtropical latitudes. 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!
Thunderstorm Climatology
(storms per year)
Hail Climatology
Hail Days Climatology
Courtesy H. Brooks, National Severe Storms Laboratory
Lightning Climatology
Wind Days Climatology
Courtesy H. Brooks, National Severe Storms Laboratory
Modes of Convection / Storm
Classification

Although a continuous spectrum of
storms exists, meteorologists find it
convenient to classify storms into
specific categories according to their
structure, intensity, environments in
which they form, and weather
produced.
Basic Classification

Single-cell or air-mass storm Typically lasts 20-30 minutes. Pulse
storms can produce severe weather elements such as downbursts,
hail, some heavy rainfall and occasionally weak tornadoes.

Multicell cluster storm A group of cells moving as a single unit, with
each cell in a different stage of the thunderstorm life cycle. Multicell
storms can produce moderate size hail, flash floods and weak
tornadoes.

Multicell Line (squall line) Storms - consist of a line of storms with a
continuous, well developed gust front at the leading edge of the line.
Also known as squall lines, these storms can produce small to
moderate size hail, occasional flash floods and weak tornadoes.

Supercells Defined as a thunderstorm with a rotating updraft, these
storms can produce strong downbursts, large hail, occasional flash
floods and weak to violent tornadoes.
Convection and Buoyancy

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Convection: transport of fluid properties by
motions within that fluid
Buoyancy: vertically oriented force on a
parcel of air due to density differences
between between the parcel and surrounding
air
– Mathematically, the buoyancy force can be derived
from the vertical equation of motion
– Let’s derive it now!
Perturbation Vertical
Momentum Equation
(we just derived a very similar form)
where L = liquid+ice water content. Note that the water vapor
contributes to the buoyancy of air parcel and the combined effect
can be expressed in terms of virtual potential temperature in the
formulation we derived
Key Ingredients for Thunderstorms
Static Instability
Cold Air
Warm Air
Cold Dry Air
Warm Moist Air
Convective Available Potential Energy
(CAPE)

CAPE measures the amount of convective instability,
or more accurately the potential energy of an
environmental sounding – the energy that can be
converted into kinetic energy when an air parcel rises
from LFC to EL

It is based on simple parcel theory which neglects
the effect of mixing/friction, PGF and sometimes
water loading. (The vertical momentum equation
becomes simply Dw/Dt = B)

From CAPE, we can estimate the maximum vertical
velocity that can be reached by a parcel
CAPE
CAPE on a
Skew-T
diagram


The positive area (where
air parcel is warmer than
environment) is equal to
CAPE
Lifted Index –
temperature excess in
500mb environment over
that of a parcel lifted
from the low ‘moist’ layer
(negative value indicates
instability)
LCL – Lifting
condensation level
LFC – Level of free
convection – level at
which the parcel is
warmer than the
Environment
EL – Equilibrium
Level – level at which
the parcel’s T becomes
the same as the
environment again
Skew-T analysis and Parcel Theory

Skew-T analysis and Parcel Theory typically neglect the effect of
PGF induced by vertical motion, essentially they assume that
the environment is unchanged by the parcel motion. They also
neglect the effect of mixing/friction

Therefore, parcel theory tends to overestimate the intensity of
the updraft. Still, it provides a useful upper limit for the
convection intensity.
CIN

Convective Inhibition
– The “negative area” on a thermodynamic
diagram, or the area between the lifted
surface parcel and ELR curves, in the layer
where the parcel is colder than the
environment.
– It is defined as the amount of energy
beyond the normal work of expansion need
to lift a parcel from the surface to the Level
of Free Convection (LFC).
Example of CIN
How Can CAPE Increase?
How Can CAPE Increase?
Hotter surface temperature
 More low-level moisture
 Cool the mid-levels

Td
T
W(surface) = 11 g/kg
W(surface) = 14 g/kg
W(surface) = 16 g/kg
Key Ingredients for Thunderstorms
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
Environmental Shear

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CAPE and Vertical Environmental Wind Shear
are the two most important factors in
determining storm type
Numerical models have been very effective
tools to aid our understanding of such effects
In general, single cell storms occur in
environments with little vertical shear ( < 15
m/s over lowest 6 km)
Multicell storms occur in environments with
moderate vertical shear (15 – 25 m/s)
Supercell storms occur in environments with
strong vertical shear ( > 25 m/s)
Numerical
Experiments of
Weisman and
Klemp (1982)
Vertical wind profiles
with unidirectional
shear of different
magnitudes
Time series of max w
for 5 experiments
Supercell behavior is
observed with us =25,
35 and 45m/s cases –
quasi-steady updraft is
found
Results from us
= 15m/s and
35 m/s cases
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Multicell case (left) with u =15 m/s and
supercell case (right) with u = 35m/s.
Only the southern half of the
computational domain is shown
because the fields are symmetric about
the central E-W axis
The storm splitting is a result of
rotational storm dynamics (more later).
The member that moves to the right of
vertical shear vector is called right
mover, and the other the left mover.
When the shear is not unidirectional,
i.e., when shear changes direction with
height, one of the members will be
favored, again due to rotational storm
dynamics
Maximum w
as a function
of CAPE and
shear
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The vertical axis is the lowlevel qv – higher value
corresponds to higher CAPE
First cell intensity increases
with CAPE and decreases
with shear
Second cell occurs only with
moderate shear.
Supercell storm occurs in
stronger vertical shear.
Strong updrafts can survive
in supercells because of the
support of pressure
perturbations associated
with vertical rotation which
initially comes from
horizontal vorcitity in the
environment via tilting.
Multicell case
Supercell case
Predicting Thunderstorm Type: The Bulk
Richardson Number

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The Bulk Richardson Number,
BRN, is a measure of the
relative importance of
environmental instability effects
to environmental shear effects
Essentially the ratio of the KE of
the updraft to the KE of the
inflow
Denominator is really the stormrelative inflow kinetic energy
(sometimes called the BRN
Shear)
Large values associated with
single-cell “pulse storms”
Smaller values associated with
supercells (~ 10 – 20)
Must be used with caution. If
CAPE and shear both low, can
still get “supercell” values of R,
but storms will be weak, if they
form at all.
CAPE
BRN 
2
S
1
where S  (u 6000  u 500 ) 2
2
2
Results from Observations and
Models
Key Ingredients for Thunderstorms

Mechanism to trigger the instability
– Front
– Terrain
– Dryline
– Daytime heating
– Landmass inhomogeneities
Types of Thunderstorms

Ordinary single storms
–
–
–
–
–
Most common
Last for less than an hour
Built-in self-destruct mechanism!
Occur all year long, mostly in summer
Can produce strong winds, hail, and lightning
Ordinary Single (Airmass) Storms
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First studied just after World War II
Many commercial and military aircraft
accidents
Newly developed radar was exploited for
weather studies
The Thunderstorm Project
Resulted in first life cycle of a thunderstorm
Air mass thunderstorms are also referred to
as “Garden Variety!”
Photograph by: NSSL
– This is a single cell storm, looking east from about 15 miles. The
storm was moving east (into the photo). Some of the anvil cloud
has been left behind the storm, but the greater portion of the anvil
is blowing off in advance of the storm and is not observable from
this perspective. (May storm in the Texas Panhandle near
Amarillo.)

This late May storm in Oklahoma, looking northeast from about
20 miles, occurred with weak to moderate vertical wind shear.
It did not produce any severe weather.
Conditions of Formation of Air
Mass Thunderstorms
 Conditional
instability (we’ll come
to that later)
 Warm, moist air near the ground
 Localized source of lift (usually
thermally driven)
 Weak or no environmental vertical
wind shear
Three stages of single-cell storm
development
Developing stage
Mature Stage
Dissipating Stage
Example of Single-cell Life Cycle
(a) – Developing
(c) Mature
(b) – Mature
(d) – Dissipating
Cumulus Phase

Development of towering
cumulus
– Region of low level convergence
– Warm moist air
– Updraft driven by latent heating


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Nearby cumulus may merge
to form a much larger cloud
Dominated by updraft
Mixing and entrainment occur
in the updraft
Cumulus Phase
c David Shohami
c William Zender (2001)
Development of a Single-Cell Storm
Step 1
– In the absence of frontal or other forcing, daytime
heating of the PBL causes the convective
temperature to be reached. Thus, there is no
‘negative area’ on the skew-T diagram for an air
parcel rising from the surface – the lid is broken
Development of Single Cell Storm
Step 2
– Updraft forms – once the air reaches the LCL, latent heat is
released due to condensation:
- L dqv = Cp dT
– For every 1g/kg of water condensed, the atmosphere warms
about 3 degrees. This feeds into the buoyancy term through
an increase in q’ (remember earlier vertical momentum
equation?). The saturated air parcel ascends following the
moist adiabat, along which the equivalent potential
temperature qe is conserved.
– Until the ‘Equilibrium level’ is reached, the air parcel is
warmer than the environment, which keeps the buoyancy
positive (without the effect of water loading – see later)
Development of Single Cell Storm
Step 2 – continued …
– When a cloud forms, some of it is carried upward
by the draft and some moves out of the updraft.
The ‘weight’ of this liquid water makes the air
parcel heavier, this ‘water loading’ effect acts to
reduce the positive buoyancy.
gL
B = g(q’/q)
10 x 3/300
~
- 10 x 0.01 kg/kg
Therefore 10 g/kg of cloud or rain water will offset
a 3 K temperature surplus.
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.

Mature Phase
Precipitation, formed
by the Bergeron cold
rain process, begins to
reach the ground.
 The precipitation drags
some of the
surrounding air down
creating the
downdraft.

Mature Phase
• When the cloud grows to a stage that the updraft becomes too
‘heavy’ because of water loading, it will collapse, updraft then
turns into downdraft.
• Another important process that contributes to the collapse is the
evaporative cooling. When a cloud grows, cloud droplets turn into
larger rain drops that fall out of the updraft, reaching the lower
level where the air is sub-saturated. The rain drops will partially
evaporate in this sub-saturated air, producing evaporative cooling.
This cooling enhances the downdraft.
• In the absence of vertical wind shear, the cell is upright, and this
downdraft would then disrupt the low-level updraft, causing the cell
to dissipate. This is the built-in self-destruction mechanism
mentioned earlier
• The cold downdraft sometimes forms a cold pool that propagates
away from the cell above, further removing the lifting underneath
the cell
Mature Phase
Mature Phase
Downdraft
 The
downdraft is the descending
column of air in a thunderstorm.
 Created and maintained by three
processes
– Evaporational cooling of entrained air
– Downward drag caused by falling
precipitation
– Evaporational cooling of the air below
the cloud base
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.
Reflectivity
Radial Velocity
0.5 deg Elevation, 04:28 UTC
LIT
LIT
Reflectivity
Radial Velocity
0.5 deg Elevation, 04:34 UTC
LIT
LIT
Reflectivity
Radial Velocity
0.5 deg Elevation, 04:40 UTC
LIT
LIT
Reflectivity
Radial Velocity
0.5 deg Elevation, 04:34:12 UTC
Main body of
storm (second part)
LIT
LIT
Gust front;
First part of storm
Time = 04:34:12 UTC
Doppler Radial
Velocity
Radar
First part of
the storm
LIT
Reflectivity
Heavy Precip
Radar
Second part of
LIT
the storm
Second part of
the storm
First part of
the storm
“thunderstorm just NW of the airport moving through the area now”
Time = 04:40:02 UTC
First part of
the storm
Doppler Radial
Velocity
Radar
LIT
Reflectivity
Heavy Precip
Radar
LIT
Second part of
the storm
Second part of
the storm
First part of
the storm
At 0442, ATCT advises “second part of the storm moving through”
Gust Front Shelf Cloud
National Severe Storms Laboratory
Downdraft

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
Mature Phase
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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.
The anvil, or cloud top, begins
to turn to ice, or glaciate.
Mature Phase
Dissipating Phase
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Eventually the downdraft
overwhelms the updraft and
convection collapses – because
the cloud is vertically-oriented
Precipitation becomes lighter and
diminishes.
Cloud begins to evaporate from
the bottom up often leaving
behind an “orphan anvil.”
– Cirrus Spissatus
cumulonimbogenitus
Air Mass Thunderstorms
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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
Radar
Visual
Life Cycle of Single Cell/Airmass Storm
Impact of Pressure Gradient Force

When an air parcel rises (due to buoyancy), it has to push through
air above it, creating higher pressure (positive p’) above (imagine
pushing yourself through a crowd – or drafting of race cars)

Below the rising parcel, a void is created, leading to lower pressure
at the cloud base
H
PGF
L
Impact of Pressure Gradient Force

The higher pressure above will push air to the side, making
room for the rising parcel, while the lower pressure below
“attracts” surrounding air to compensate for the displaced
parcel

Such a positive-negative pattern of p perturbation creates a
downward pressure gradient. The PGF force therefore opposes
the buoyancy force, and therefore acts to reduce the net
upward forcing.
Impact of Pressure Gradient Force


The degree of opposition to the buoyancy force depends upon the
aspect ratio of the cloud (L/H), or more accurately of the updraft. This
aspect ratio dependence ties directly into the degree of validity of the
hydrostatic assumption (see Bluestein Vol. II 433-434)
The effect is larger for a wider/large aspect-ratio cloud, and weaker for
a narrower/small aspect ratio cloud, because
– For a narrow cloud, a small amount of air has to be displaced/attracted by
the rising parcel, therefore the p perturbation needed to achieve this is
smaller, so that the opposing pressure gradient is smaller (often <<
buoyancy) so a narrow cloud can grow faster
– PGF is stronger for a wide cloud: as a result, the net upward force
(buoyancy – PGF) is significantly reduced, and the cloud can only grow
slowly. When B and PGF have similar magnitude, the vertical motion
becomes quasi-hydrostatic – this is typical of large scale broad ascent.

Dynamic stability analysis of inviscid flow shows that the infinitely
narrow clouds grow the fastest, but in reality, the presence of turbulent
mixing prevents the cloud from becoming too narrow, hence the typical
aspect ratio of clouds is ~ 1.
Hazards of Air Mass
Thunderstorms
Heavy Rain
 Hail

– Usually not terribly large
– May be numerous

Downbursts or Microbursts
– Exceptionally strong downdrafts that, when
they hit the earth, may have potentially
destructive winds associated with them.
Hail Produced by an Ordinary
Thunderstorm
ARPS
Simulation
of a Single
Cell Storm

May 20, 1977 Del
City Supercell Storm
Sounding – used
without
environmental wind
ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=0
ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=15min
ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=20min
ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=25min
ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref Eq. Pot. Temp.+qw+Ref+wind t=30min
ARPS Simulation of a Single Cell Storm
T-equivalent buoyancy+qw+ref
Eq. Pot. Temp.+qw+Ref+wind t=45min
ARPS Simulation of a Single Cell Storm
Perturbation Pressure +qw+ref +wind
t=0, 15min
ARPS Simulation of a Single Cell Storm
Perturbation Pressure +qw+ref +wind
t=20, 25min
ARPS Simulation of a Single Cell Storm
Perturbation Pressure +qw+ref +wind
t=30, 45min
ARPS Simulation of a Single Cell Storm
Perturbation Pressure +qw+ref +wind
t=45min
High pressure is seen undernearth the cold pool.
Gust front circulation produces strong lifting.
ARPS Simulation of a Single Cell Storm
Animations
http://twister.ou.edu/MM2007/gmeta3d_pt_anim.mov
http://twister.ou.edu/MM2007/gmeta3d_pte_anim.mov
http://twister.ou.edu/MM2007/gmeta3d_pprt_anim.mov
Downbursts and Microbursts

Microburst
– An anomalously strong, concentrated downdraft
that produces a pocket of dangerous wind shear
near the ground over an area of 4 km or less in
horizontal extent.
– Very short lived (last for 3-8 minutes)
– Very small and isolated (city block)

Associated with cumulonimbus clouds
– Can have heavy rain (Wet microbursts)
– Can have vanishing sprinkles (Dry microbursts)
Microburst
Microburst
Microburst
Dry and Wet Microbursts
Dry Microbursts
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A microburst with little or no precipitation.
Very dry air is located beneath the cloud
base.
Hydrometeors falling into the dry air will
evaporate causing a pool of cold air just
below cloud base.
This cold pool descends rapidly forming the
dry microburst.
Often you can’t detect them until it is too
late.
Dry Microburst
Wet Microbursts
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Microbursts associated with moderate or
heavy precipitation.
Some dry air above cloud top gets entrained
in the top of the thunderstorm.
This dry air mixes with cloud air causing
some evaporation of the cloud.
Evaporational cooling will form a pool of cold
air near the top of the cloud.
This cold pool descends and adds to the
downdraft to form a microburst.
Often there is a “rain gush” coincident with
the microburst.
Wet Microburst
Wet Microburst
Wet Microburst
Microburst
Damage
Visual Detection
Detection of Microbursts

Doppler Radar (Airport and Aircraft)
– Best when precipitation is present
– Terminal Doppler Weather Radar (TDWR)
Integrated Terminal Weather System
(ITWS)
Integrated Terminal Weather System
(ITWS)
Detection of Microbursts

LLWAS
– Low level wind shear alert system
– A network of wind sensors positioned
around the airport.
– Does not detect elevated microbursts or
microbursts that are between sensors.
Microbursts and Aviation

Microbursts are extremely hazardous to
low-flying aircraft because of
– Low airpseed
– Proximity to the ground
– “Dirty” aerodynamic configuration (flaps
out, gear down)
– Difficulty of visual microburst detection
– Rapid onset and short duration
Microburst
Glide Slope
Runway
Microburst
Glide Slope
Runway
Microburst
Glide Slope
Runway
Microburst
Glide Slope
Runway
Microburst
Glide Slope
Runway
Flight of Eastern 902
Flight of Eastern 66
Number of Fatalities
Fatalities Associated with Aviation
Wind Shear Accidents
154
150
136
136
115
Wind Shear R&D
100
Pilot Training
38
50
1
TDWR
0
0
'65-'69 '70-'74 '75-'79 '80-'84 '85-'89 '90-'94 '95-'98
Year of Accident