SO441 Lesson 2: More on thunderstorms
Facts about thunderstorms
• Common world-wide, especially in tropical
and middle latitudes
• Redistribute heat and moisture
– Transport from the surface to upper-levels
• Most (95%) are non-severe
– “Severe” criteria for USA: 1” or larger hail,
50+ kt (58+ mph) wind, OR tornado
Types of thunderstorms
•
Four primary types of organization, listed
in order from least to most organized:
1.
2.
3.
4.
Airmass
Squall line
Multi-cell
Supercell
Elements required for formation
• Source of moisture
• Conditionally unstable atmosphere
• Mechanism to “trigger” an updraft
– Lifting from an advancing frontal boundary or
air flow over a mountain
– Convective heating at the surface (from solar
radiation)
– Convergence of air at the surface
Airmass Thunderstorms
• Occurs away from any frontal boundary
– In fact, typically found in the middle of an
airmass
• “Trigger” mechanism:
– Strong solar heating at the surface
• Formation: typically late afternoon and
evening
– After sun heats the mT airmass for 10+ hours
Airmass Thunderstorms
• Last about 1 hour
• Rain covers maybe a 10 to 15 km area
• Are self-destructive
– Rain/precipitation falls back into the updraft
• Usually form in region of weak upper-level winds
– i.e., little/no vertical wind shear
– Remember the “tropical disturbance”? Simply a large collection
of airmass thunderstorms
• Are not known for most types of severe weather (hail,
straight-line winds, or tornadoes)
– We will see later that air mass thunderstorms are responsible for
microbursts
Parts of airmass thunderstorm
Anvil part of
the cloud
Tropopause
Main “cell”
updraft
LCL (point where
condensation
occurs)
Airmass Thunderstorm:
stages of development
Airmass Thunderstorm:
stages of development
1. Cumulus stage:
– Cloud consists of warm, buoyant plume of
rising air
– Cloud consists of mostly small cloud
droplets; there are only a few raindrops or
ice crystals
Airmass Thunderstorm:
stages of development
2. Mature stage:
–
–
As storm updraft rises to regions well below
freezing, ice crystals form
Graupel forms
•
–
–
•
Graupel: small (a few millimeters) ice particles with
consistency of a snowball
Downdrafts begin to form as raindrops fall back to
earth
Light rain is noticed at the ground
Key point in “mature” stage: Because there is
no vertical wind shear, precipitation must fall
back down through the main updraft.
Airmass Thunderstorm:
stages of development
3. Dissipation stage
– Downdrafts formed by rain falling back down
into the updraft
– Downdrafts overwhelm the main updraft
– Heavy rain falls out of the base of the
thunderstorm
– Dissipation occurs
Dangers from air mass
thunderstorms: microbursts
Not easily detected
because
1. the ambient
thunderstorm (or
even cumuliform
cloud) is usually
considered
benign
2. The scale is
typically very
small (perhaps 1
or 2 km across)
Two primary types of microbursts:
1. Dry microburst. Occurs when surface layer is very dry (low relative
humidity). Rain evaporates and accelerates downward through the
warm, dry surface layer
2. Wet microburst. Occurs when the surface layer is very moist and
upper-levels are very dry. Dry downdraft entrained (mixed) from
above the cloud penetrates through the cloud, evaporatively-cooling
as it mixes with rainwater
** Both types of microbursts are associated with evaporating rainwater **
Danger comes from two sources:
1. Rush of cool, stable air out from
the microburst center once it
reaches the surface
2. Turbulence associated with the
“rotor cloud” – the leading edge of
the microburst
Photos of microbursts
More photos of microbursts
Microbursts can be deadly
• Eastern Airlines flight 66
– June 24, 1975, John F.
Kennedy, New York
– 112 fatalities (12 survivors)
• Pan-Am flight 759
– July 9, 1982, New Orleans,
Louisiana
– 153 fatalities (0 survivors)
• Delta Airlines flight 191
– August 2, 1985, Dallas-Fort
Worth, Texas
– 135 fatalities (29 survivors)
• US Airways flight 1016
– July 2, 1994, Charlotte,
North Carolina
– 37 fatalities (25 survivors)
Squall Line
• Long line of thunderstorms
– individual “cells” are so close together the heavy
precipitation forms a long continuous line
• Typically form along an advancing cold front
– Sometimes associated with a cold front aloft
• Can be hundreds of miles long
• Most commonly associated with strong straightline winds
– Can produce hail and/or tornadoes, too
• Called “squall” because of the abrupt wind
changes
Squall line thunderstorms
Squall line thunderstorms
L
A squall line
approaching
Memphis, TN.
Note the heaviest
precip is along the
leading (eastern)
edge of the line,
with moderate –
but still continuous
– rainfall occurring
100+ km behind
(to the west) of the
“line”
Structure of a squall line
• Already noted the “trigger” is typically an advancing
(cold) frontal boundary
• The squall line will sustain itself by producing its own lift
due to outflow boundaries
• Again, tropopause acts as a “lid” to the thunderstorm
updraft
– Thus, anvil clouds also form in squall lines
• Heavy rain / strong winds occur beneath the convective
region
– Strongest updrafts occur in the convective region
• As long as instability and moisture remain present out
ahead of the squall line, the squall line will continue to
propagate
Structure of a squall line
Looking THROUGH the line … i.e., the “line” is
coming out of / going into the page
Squall line “gust front”
Also called a “bow echo”
Squall line
• Self-propagating (not self-destructive like
airmass thunderstorm)
• Evaporatively-cooled air pushes out
slightly ahead of the squall line
– Acts as the “trigger” mechanism
• i.e., lifts the warm air up and into the squall line
– Easily noticed as a “shelf cloud”
Squall line photos
More photos of a squall line
More photos of a squall line
The threat from a squall line: derecho
Definition of a derecho:
“A widespread convectively induced straightline windstorm.” (AMS Glossary of
Meteorology)
Conditions for a calling an event a “derecho”:
1. There must be a concentrated area of reports
consisting of convectively-induced wind damage
or convective gusts of more than 26 ms-1 (50
kt).
2. The reports within this area must also exhibit a
nonrandom pattern of occurrence. That is, the
reports must show a pattern of chronological
progression, either as a singular swath
(progressive) or as a series of swaths (serial).
3. Within the area there must be at least three
reports, separated by 64 km or more, of either
F1 damage or convective gusts of 33 ms-1 (65
kt) or greater.
4. No more than 3 h can elapse between
successive wind damage (gust) events.
Trajectories and annual frequency
of derechos in the US
Multi-cell Thunderstorms
• Third mode of development
• What happens when storms occur in
clusters rather than lines?
– Call them “multi-cell” thunderstorms
Multi-cell Thunderstorms
• Very common in late spring/summer in plains
states
– Thunderstorms form late afternoon and organize into
large (300-mile wide) region
– Entire complex moves south/southeast during the
night, producing strong winds and hail over a wide
area
• sometimes from Kansas/Nebraska  Gulf Coast!!
– 1500 km
– Called a “mesoscale convective complex”
Multi-cell Thunderstorms
•
Two points:
1. Vertical wind shear keeps downdrafts /
precipitation from falling back into the
updraft
2. Rain-cooled air advances (“gusts”) out
ahead of first thunderstorm, forming a “gust
front”
•
Cool air of the gust front acts as the lifting
mechanism (i.e., the “trigger”) for new
thunderstorm development
Environment of multi-cell
thunderstorm
Multi-cell development
Multi-cell storm propagation
Rain-cooled air pushes out ahead of the
storm: forms the GUST
FRONT
Multi-cell development
Multi-cell storm propagation
Gust front acts as the “trigger” mechanism for new
thunderstorm cell development
Multi-cell development
Multi-cell storm propagation
Note old storms decay as they are “cut off”
from the feed of warm, moist air
Two-dimensional view
(x-z plane), as
pictured on radar
(darker shading
represents higher
radar reflectivities
[and higher
reflectivities imply
both larger particles
and greater particle
density])
Storm propagation is
from right to left (  )
Satellite image
corresponding to Panel H.
Notice the gust front has
propagated far in advance
of the multicell storm
cluster.
Multi-cell complex as shown by
radar
Disorganized collection / grouping
of initial thunderstorms near
Kansas City
Four hours later, notice the
thunderstorms have organized and
have progressed south into central
Missouri
Shelf cloud: found along the
leading edge of the gust front
The supercell
Supercell Thunderstorms
• Most intense type of thunderstorm
– Most tornadoes, and almost ALL strong (F3, F4, and
F5) tornadoes
– Almost all large hail (2” diameter on up)
• Defining characteristic:
– Supercell thunderstorm updrafts ALWAYS rotate
• Strong vertical wind shear induces rotation of the supercell
thunderstorm
– Usually examine magnitude of vertical wind shear in lowest 6
km of the atmosphere (the “zero to 6 kilometer wind shear”)
Environment of a supercell
• Strong vertical wind shear
– A quick note on vertical wind shear
One type of vertical wind shear is called “Speed Shear”,
where wind does not change direction but instead
changes SPEED as you up in the atmosphere.
Environment of a supercell
• Strong vertical wind shear
– A quick note on vertical wind shear
West
West-southwest
South-southwest
South-southeast
Another type of vertical wind shear is called “directional
shear”, where wind changes DIRECTION with height
Environment of a supercell
• Supercell thunderstorms grow in an
environment with BOTH speed AND
directional shear!
Environment of a supercell
• Warm, moist conditionally unstable air at
surface
– Transported northward from Gulf of Mexico by a
strong “low-level jet”
• Strong winds extending from 20 meters to 2 kilometers
– Southeast @ surface  southerly  southwesterly
as you go up in the atmosphere
• This change in wind direction (coupled with a change in
speed) provides LOW LEVEL vertical wind shear
– Low level wind shear is CRITICAL to tornado formation!
Vertical wind profiles for airmass (single-cell), multicell
/ squall line, and supercell thunderstorms
Environment of a supercell
• Strong vertical wind shear: both
directional and speed
• Conditionally unstable atmosphere
– Warm, moist air at the surface
– Cooler, drier air aloft
• At the interface of the warm/moist and cooler/drier
air: a temperature inversion exists
• Examine atmospheric soundings to
identify conditions for supercell
development
Environment of a supercell
Morning sounding:
Afternoon sounding:
capping temperature inversion is
strong (prevents air parcels from
rising convectively)
capping inversion is gone due to
solar heating at the surface
Supercell
thunderstorms
typically form
along the dry
line and warm
front.
If an upperlevel front is
present (often
it is not),
supercell
thunderstorms
can form along
its boundary,
too
Parts of a supercell thunderstorm
Main parts
1. Body of the thunderstorm, which contains:
–
Rotating updraft
•
–
Precipitation core (also contains the downdraft)
•
–
Called a “mesocyclone”
Heaviest rain (“heavy rain curtain”)
Hail core
•
surrounds the main updraft
2. Wall cloud
–
Cloud lowering beneath the main updraft
•
–
Still attached to the body of the supercell
Can be rotating or non-rotating (of course the
rotating type is of waaaaay more interest!)
3. Anvil cloud
–
–
–
–
As updraft reaches tropopause, cloud particulate
spreads out
B/c of strong winds aloft, cloud matter is blown
down-wind (sometimes 100 to 200 miles!)
Overshooting top: updraft is so strong that it
penetrates up to 3000 feet into the stratosphere!
“Backsheared anvil”: part of the anvil cloud that
extends back into
Parts of a supercell thunderstorm
Supercell structure
(view from above)
Supercell
structure
(view from
the east)
Supercell
structure
(view from
the east)
Supercell structure
(viewed on radar)
Supercell structure
(viewed on radar)
“Hook”
echo
plainly
visible on
radar
Supercell structure
(viewed on radar)
Tornado
location
marked
by “T”
Tornado
found in
the “hook”
echo
T
“Backsheared”
part of the
Anvil cloud
Main part of
the Anvil cloud
(blowing
downstream)
Supercell view from the ground
Anvil (not the
backsheared part …
that would be coming
out of the page at us)
Wall cloud
Flanking line (part of the
updraft; new cloud
development!)
Main thunderstorm body
Heavy rain curtain
Hail shaft
Life cycle of supercell thunderstorm
A: Updraft and downdraft are separate; rear-flank downdraft (RFD) begins to form
B: RFD reaches surface, begins interacting with mid-level inflow
C: RFD and forward-flank downdraft (FFD) collectively interact; tornado may form
D: RFD overpowers updraft and pushes (gusts) ahead of the body of the storm. Leading
edge of RFD / FFD intersection acts to promote new updraft formation
Parts of a supercell: wall cloud
Parts of a supercell: main updraft
Parts of a supercell: anvil
(mammatus)
Three types of supercell thunderstorms:
classic, low precipitation (LP), high precipitation (HP)
Compare low-precipitation (LP) with high-precipitation (HP)
Some factors that determine supercell storm mode:
(1) Quantity of water vapor in the atmosphere (mixing ratio)
(2) Degree of storm-scale interaction (precipitation from one storm’s anvil falling
into another storm’s updraft)
HP supercell
HP supercell
HP supercell
LP supercell