Transcript lecture 16 thunderstorms and tornadoes

THUNDERSTORMS
(Hailstorms and Tornadoes)
Thunderstorm Facts and Figures
Thunderstorms are narrow, towering storms, 5-14 miles (8 to 22 km) high and only
about 5 to 25 miles (8 to 40 km) wide. Driven by the large scale winds aloft, they
seldom last long. Typically, the storm’s intensity rises to a quick peak, with cold, gusty
winds, intense lightning and potentially torrential precipitation, which lasts about 10-15
minutes before gradually subsiding and usually ending within an hour.
Most thunderstorms pass by without causing significant damage or death. Violent
downbursts, large hail, and tornadoes are almost exclusively confined to severe
thunderstorms. Thunderstorms mete out death by three principal means--lightning,
tornadoes, and floods. Lightning kills about 100 people annually in the United States.
Tornadoes, despite their extreme violence, now claim an average of only 50 lives
annually in the United States due to the high level of public awareness and the National
Weather Service’s outstanding warning system. Flash floods, resulting from torrential
downpours in normally dry or placid streams, catch more people by surprise and cause
almost 200 deaths annually in the United States.
Thunderstorms are as common as popcorn. At any moment, about 2000 are exploding
worldwide. A day's tally is about 45,000; a year’s total, 16 million. Their energy is
immense--an average thunderstorm releases enough energy (1 trillion watts) to power
40 million homes during the half hour it lasts. Unfortunately, there is no way to tap this
energy.
Thunderstorms occur most frequently in the tropics because sunlight is intense all year.
They also prefer land, which heats up more rapidly than water. Thus the saying,
“Convection craves continents and lightning loves land!” Africa, which straddles the
equator, is the world thunderstorm capital, and their electrical effects are conducted
around the world. Very few places outside the tropics get more than 100 thunderstorms
a year, but at Bogor on the island of Java thunder is heard on 322 days a year.
In most of the middle latitudes, thunderstorms are most common in late spring and
summer and least common in winter. In the United States the two areas most favored for
thunderstorms are Florida, where humid sea breezes converge from two sides, and the
high plains of Colorado and New Mexico. Thunderstorms are rare in polar latitudes and
almost never form over the ice caps.
In many inland locations thunderstorms peak in late afternoon, shortly after the hottest
time of the day. But as the land cools at night, thunderstorms often either move
downwind from mountain ranges, following the convergence of mountain-valley breezes,
or form over warm coastal waters.
All thunderstorms contain strong updrafts of buoyant, humid air and share three
common features,
1. A thick layer with a conditionally unstable lapse rate (SALR < LR < DALR).
2. Adequate moisture at low levels.
3. A lifting mechanism (provided by outflow boundaries from other thunderstorms, large
scale winds, sea breeze fronts, etc.).
Average Annual Number of Days with Thunderstorms
Average Annual Number of Days with Hail
World Map of Lightning Strike Frequency
http://science.nasa.gov/headlines/y2001/ast05dec_1.htm
TYPES OF THUNDERSTORMS
There are several types of thunderstorms. In order of increasing severity, they are
1. Single cell, air mass thunderstorms.
2. Multicell, air mass thunderstorms.
3. Squall-line thunderstorms.
4. Supercell thunderstorms.
Severe storms usually form in highly unstable atmospheres with favored settings (such
as mountains, islands, or east of cold fronts and troughs in the jet stream), and
significant vertical wind shear. Vertical wind shear adds to intensity by tilting or moving
the updraft so that hydrometeors do not fall back into it and weigh it down or choke it on
its own precipitation. Air Mass storms form when there is little vertical wind shear. Squall
line storms form if there is significant shear, but the direction of the wind does not
change with height. Supercell storms form when the wind speed both increases and
turns with height (helicity), so that the updraft moves like a helix or corkscrew.
The next slide relates the type of thunderstorm to the shear and degree of instability.
The 3 slides after that show the 3 stages in the life cycle of an air mass thunderstorm
cell.
1. Cumulus (Youth) (No rain yet – all updrafts)
2. Mature – the most intense and violent weather
3. Dissipating (Old Age)
Thunderstorm Type as a Function of Instability and Shear
Wind Shear
5000
JET
CAPE
4000
3000
2000
1000
0
Bow
Echoes
SuperCell
MultiCell
Air Mass
Derecho
Single Cell
Air Mass
Squall Line
0
40
80
SHEAR (knots) below 2.5 km AGL
(Above Ground Level)
Shear = Change of wind speed with height
Helicity = Screw-like motion (shear + turning)
Air Mass Thunderstorm
Life Cycle
The typical lifespan of a cell
of an air mass thunderstorm
lasts about half an hour. It
consists of 3 stages:
1. Cumulus (Youth)
2. Mature
3. Dissipating (Old Age)
The Cumulus Stage
The cumulus stage consists
entirely of updrafts and all
air near ground level is hot.
There is no precipitation at
this point in the young storm
because cloud droplets and
crystals are tiny but growing
and accumulating. The top
fringe of the cloud is distinct
and closely resembles a
cauliflower.
The Mature Stage
This is the stage with the
most violent weather. The
weight of precipitation and
cooling by evaporation as
the rain falls into the dry
air surrounding the updraft
cools and weighs the air,
causing strong downdrafts
of cold, rain-laden air. The
downdraft splays out at
the ground and shoves the
warm air near the ground
upward, intensifying the
updraft. But the cold air
outflow and shading of the
ground
due
to
the
spreading anvil combine
to eliminate the nearby
source of hot air, so the
storm produces the seeds
of its own destruction.
The Dissipating Stage
This resembles old age.
The nearby source of hot
air has been entirely used
up so that the storm
consists largely of weak
downdrafts and light
precipitation. The only
updraft is a remnant
confined near the top of
the cloud. The downdrafts
help evaporate the cloud
from bottom upward, so
that the last thing left is
the anvil, which has a
fuzzy appearance.
The sun may come out
again and start the
process over if it is early
enough in the afternoon.
Squall Line Thunderstorm Structure
As the storm advances toward the right, mammatus are seen first under the anvil. Wind
becomes strong as temperature falls and the sky darkens as an arcus or roll cloud passes
overhead and darkens. If wind shear is large enough the violent outflow forms a long line
called a derecho. Heavy rain, possibly with small hail follow. Tornadoes are uncommon.
23 July 2010 near Vivian, South Dakota Record Hailstone storm
ChaseTheStorms.com
See also Youtube Shelf Cloud Comes Ashore and Gust Front Videos
1900 UTC
03 May
2009
Radar echoes of an
advancing squall line.
Note, that the most
intense precipitation and
violent weather occur
first. The storms often
finish with a long period
of gentler winds and
stratiform rain and slowly
rising temperature.
2000 UTC
03 May
2009
SQUALL LINE
THUNDERSTORM
Thunderstorms (often in Squall Lines) form in Unstable Air ahead of Cold Fronts
The advancing cold air mass acts as a wedge, forcing the warm, unstable air aloft. The
pulse of rising air propagates to the east like a wave, ahead of the cold front, producing
a squall line. Sometimes, the southernmost thunderstorm becomes a Supercell.
Supercell Thunderstorms
[email protected]
Image © Jorn Olsen
Mammatus
When dry air surrounds thunderstorm anvils, ice crystals and droplets in
the cloudy air at the base of the anvil begins to evaporate. This cools the
air, which sinks in breast-like pouches called mamma or mammatus.
Supercell thunderstorms often rotate and give birth to tornadoes
Chuck Doswell
www.cimms.ou.edu/~doswell/photo.html
Supercell Structure
Cold, dry
air from W
Hot, dry air from
SW (Mexico)
http://www.nssl.noaa.gov/primer/tornado/tor_basics.html
Warm, moist air from S (Gulf of Mexico)
UPDRAFT
CORE
Sorting of Rain and Hail
Updrafts in severe thunderstorms are tilted up to
the North because the warm, moist air blows
from the South. The small, light raindrops are
lifted higher than the large, heavy hailstones, so
they are carried and fall further north.
Rain
Large Hail
The tornado then crossed the Canadian River, passing into far southern Oklahoma
City. As it passed over Bridge Creek, Oklahoma, around 6:54 p.m., a Doppler On
Wheels (DOW: Wurman et al. 1997, Wurman 2001) mobile Doppler weather radar
detected winds of 301 mph (484 km/h), +/- 20 mph inside the tornado at a height of
32 m AGL (Wurman et al. 2007)[1] (The old record was a 257-268 mph wind
measurement from a Doppler weather radar near Red Rock, Oklahoma, as reported
in a formal publication by Bluestein et al. (1993)). These winds, however, occurred
above the ground, and winds at the surface may not have been quite this intense.
22 June 2007 F5 tornado Elie Manitoba
©www.extremeinstability.com
F4 Mile wide Binger OK, 22 May 1981 tornado
http://www.spc.noaa.gov/faq/tornado/torscans.htm
http://www.photolib.noaa.gov/nssl/index.html
Tornadoes
Tornadoes are intense whirlwinds that
form below severe thunderstorms
when instability and corkscrew motion
called helicity are both large. They
range in width from about 50 meters
to over a mile and are generally more
severe the wider they are. Wind
speeds range up to about 300 mph at
ground level (with updrafts up to 100
mph) but such wind speeds have only
been measured by radar several
hundred meters above the ground.
Tornadoes usually live only a matter of
minutes and have a life cycle that
ends in the rope stage, where the
tornado looks strung out. Like
hurricanes, tornadoes have a
relatively calm, clear eye which only a
few people have seen and lived to tell.
Eric Nguyen, 13 June 2004 Rain,
tornado and hail streaks from a
Kansas Thunderstorm. The tornado
hangs down from a wall cloud.
World Distribution of Tornadoes
Tornado Outbreaks
This diagram shows the super
outbreak of tornadoes on 3-4 April,
1964. The tornadoes, rated on the
Fujita scale up to F5 (the most
severe) all moved from southwest
to northeast under the Supercell
thunderstorms that spawned them.
The most devastating tornado
struck Xenia, Ohio and was
videotaped by a Boy Scout who
ran at the last moment. Several
hundred people died that day.
The tornadoes were produced in
an ideal environment, consisting of
extremely unstable air with warm,
humid air blowing from the south at
low levels and cold, dry air blowing
from the west at the jet stream
level. A low pressure area with an
approaching cold front helped
force the warm air aloft. At the
same time, the low produced snow
over the Northern Great Plains, a
common situation in Springtime
storms.
Downdrafts from this mature thunderstorm move air and rain down to the ground and
then outward (Photo © 2004 David Jenkins; Source: PhysicalGeography.net)
Downburst over an
Airport Runway
When strong downbursts near the ground they spread outward and curl
up at the leading edge. Approaching planes first encounter the updraft
and headwinds. Both lift the plane. But then the plane encounters the
downdraft and tailwinds. Both act to slam the plane into the ground.
Microburst and Arcus beneath a Thunderstorm
Jean-Francoise Millet Storm Approaching 1867-8
The artist surely saw this storm
As the Storm Departs
Iridescent Anvil edge from Kuching, Sarawak, Malaysia. © Randy Yit. cldappsoc
Large Scale Environment of Supercell Thunderstorms
Supercell thunderstorms form in a large scale environment that can be predicted several
days in advance even if the precise location and timing of the storms cannot be
predicted more than a few hours in advance. The typical environment in the United
States consists of three levels in the troposphere.
1. An advancing “tongue” of warm, moist air in the lowest 1-2 km from the South
terminated by a dry line with much drier air to the west. The thunderstorm will often form
along the dry line.
2. A warm, dry low level jet stream at about 2-3 km from the Southwest that caps and
delays the moist air from rising until enough daytime heating or general uplift has
occurred.
3. Cold, dry fast jet stream winds from the West from about 5 – 15 km.
The turning (veering) of the large scale wind with height (from South near the ground to
West at Jet Stream Levels) constitutes the screw-like or helical motion that is
transformed into the sometimes noticeable rotation of the thunderstorm and
concentrated into the much more intense rotation of the tornado.
This situation is illustrated on the next slide
Cold, Dry
Upper-Level Jet
Warm, Dry
Low-Level Jet
Warm, Moist
Tongue