Transcript Thunderstorms and Tornadoes

SEVERE WEATHER
Lecture 9
Definition of a Severe Thunderstorm
• The National Weather Service defines a
severe thunderstorm as a thunderstorm
that produces
– Hail of 1 inch diameter or larger
And/or
– Wind gusts 58 mph or greater
– A tornado
• Note: this does not include lightning or heavy
rain!
When do thunderstorms occur?
• During the spring and summer, certain
atmospheric conditions can be set up
which drive severe convective storms:
• Conditionally unstable atmosphere
• Moisture
• Upward vertical motion (“Lifting”)
• Wind shear
Conditional Instability
• Recall from a last week…the
atmosphere is conditionally
unstable if the environmental
lapse rate is less than the dry
adiabatic lapse rate and greater
than the moist adiabatic lapse
rate
• A “dry” (unsaturated) air parcel
will cool at the DALR when
lifted. It will always be colder
than its environment, and sink
back to its original level –
STABLE
• A “moist” (saturated) air parcel
will cool at the MALR when
lifted. It will always be warmer
than its environment, and
continue to rise - UNSTABLE
Moisture
• Since a conditionally unstable atmosphere is
only unstable with respect to an saturated air
parcel, some moisture source is required to
create severe weather
Cyclones can advect
warm, moist air from
the Gulf of Mexico into
the central plains
Other sources of
moisture include the
Pacific and Atlantic
oceans
http://www.srh.noaa.gov/jetstream//tstorms/images/moisture.jpg
Warm, moist air is
being advected from
the Gulf into the Plains
Vertical Lifting
• An unstable atmosphere will only generate
severe weather when it is given a “push”
• Unstable air parcels can be lifted by the
following mechanisms:
– Convection
– Convergence
– Frontal forcing
– Topography
Wind Shear
• Wind shear is a term which describes how
the speed and direction of the wind
change with height
• It is critically important for the formation of
severe weather, because wind shear will
keep upward motion and downward
motion in the storm separated, allowing
the storm to survive
Directional Wind Shear
• Wind changes direction with height
• Wind is backing if the wind direction
rotates counterclockwise with height
• Wind is veering if the wind direction
rotates clockwise with height
• Generally, severe weather will only be
found if the wind is veering with height
Directional Shear
Speed shear often causes severe weather with
strong straight-line winds
Life Cycle of a Thunderstorm
• Building block for any thunderstorm is a
thunderstorm cell
• Typical thunderstorm lasts approx. 30
minutes
• Three stages
– Developing stage
– Mature Cumulus stage
– Dissipating Stage
Developing Stage
• Warm, humid air rises
and develops an
updraft
• Air parcels saturate and
form a towering
cumulus cloud
• Little or no rainfall
• Lasts near 10 minutes
• No severe weather yet
Mature Stage
• Precipitation begins to
fall, creating downward
motion (downdraft)
• When downdraft hits the
ground, it spreads out
and creates a “gust front”
• Storm develops
overshooting (“anvil”) top
• Lasts an average of 1020 minutes.
• Most likely time for
severe weather (large
hail, gusty winds,
tornadoes)
• The change in the wind with height forces
the downdraft to be separate from the
updraft, which allows the storm to live
longer and become severe
Mature Thunderstorm
•
http://www.geography.hunter.cuny.edu/~tbw/wc.notes/10.thunderstorms.tornadoes/thunder
storm.jpg
Roll cloud in Dodge County
-not attached to parent thunderstorm cloud
-associated with thunderstorm gust front. Sign of
possible microburst activity
Shelf Cloud
- Associated with a thunderstorm gust front
- Attached to parent thunderstorm cloud
- Rising cloud motion often can be seen in the leading (outer)
part of the shelf cloud, while the underside often appears
turbulent, boiling, and wind-torn.
Dissipating Stage
• Downdraft dominates
and shuts off the updraft
• Gust front moves out
ahead of the storm and
cuts off inflow of warm,
moist air
• Severe weather threat
diminished
• Lightning still a threat
• Other storms may
develop along outflow
boundary
Gust Front / Outflow Boundary
on Radar
Types of Thunderstorms
•
•
•
•
Ordinary Cell
Multi-cell Cluster
Multi-cell Line (aka Squall line)
Supercell thunderstorm
Tornadoes
• A tornado is defined as “a violently rotating
column of air descending from a
thunderstorm and IN CONTACT with the
ground.” -NWS
• Can sometimes last for more than an hour
and travel several miles
• Most violent tornadoes are capable of
tremendous destruction with wind speeds
of 250 mph or more
What causes tornadoes?
• Tornadoes in the winter and early spring
are often associated with strong frontal
systems that form in the Central US and
move east
• During the spring in the Central Plains,
thunderstorms develop along a dryline –
separates very warm, moist area to the
east from hot, dry air to the west.
• Tornadoes may form as thunderstorms fire
along the dryline during the afternoon
hours
How Do Tornadoes Form?
• Before thunderstorms develop, a change
in wind shear creates an invisible,
horizontal spinning effect near the surface.
How Do Tornadoes Form?
• Rising air within the thunderstorm updraft
tilts the rotating air from horizontal to
vertical
http://www.nssl.noaa.gov/edu/safety/tornadoguide.html
How Do Tornadoes Form?
• An area of rotation, 2-6 miles wide, now
extends through much of the storm.
• Most strong and violent tornadoes form
within this area of strong rotation
http://www.nssl.noaa.gov/edu/safety/tornadoguide.html
Tornado Facts
• Some tornadoes may form during the early
stages of rapidly developing
thunderstorms.
– Most common along range of Rocky
Mountains, the Plains, and the western US
• Tornadoes may appear nearly transparent
until dust and debris are picked up
• Two or more tornadoes may occur at the
same time from the same storm.
Tornado Facts
• The average tornado moves SW to NE but
tornadoes have been known to move in
every direction.
• The average forward speed of a tornado is
30 mph
• Tornadoes can accompany tropical storms
and hurricanes as they move over land
• Tornadoes are most likely to occur
between 3 pm and 9 pm
Tornado on Radar
Wedge Tornado
http://greek.wunderground.com/data/wximagenew/m/MikeTheiss/453.jpg
Rope Tornado
http://www.outdoor.com/wp-content/uploads/2008/01/ropetornado.jpg
Tornadoes in Wisconsin
Stoughton, 8/2005 Tornado - Storm Chaser Scott A. Kampas
Cool Tornado Video
• http://www.weather.com/multimedia/videop
layer.html?clip=14517&from=tv_program_
vortex
Tropical Cyclones
• A tropical cyclone is a low pressure
system that develops over tropical or
subtropical waters (between 30º N and 30º
S)
• Develop in areas without a horizontal
temperature gradient
• Does not have fronts
• Has a warm core center
• Has an organized circulation
Hurricane Katrina- 2005
Tropical Cyclone Names
• Depending upon location, tropical
cyclones have different names around the
world
• In the
– Atlantic/Eastern Pacific Oceans: hurricane
– Western Pacific: typhoon
– Indian Ocean: tropical cyclone
Hurricane
Typhoon
Cyclone
• Do not typically form within 5° latitude of the
equator due to the lack of sufficient Coriolis Force
Tropical Cyclone Names
• Since 1953, Atlantic tropical storms have
been named from lists made by the
National Hurricane Center (NHC)
• In 1979, men’s names were introduced
and they alternate with the women’s
names
• Six lists are used in rotation
Tropical Cyclone Names
• The only time that there is a change in the
list is if a storm is so deadly or costly that
future use of its name would be
inappropriate
– Essentially the storm name is “retired”
– For example, Andrew 1992, Katrina 2005,
Gustav and Ike 2008
• In the event that more than 21 tropical
cyclones occur in a season, additional
storms will take names from the Greek
alphabet
– Alpha, Beta, Gamma, Delta, etc
How Do Tropical Cyclones Develop?
• Several favorable environmental conditions
must be in place before a tropical cyclone
can form:
– Some initial disturbance such as a thunderstorm
complex, which may slowly develop
– Warm ocean waters (at least 80°F)
– Potentially unstable atmosphere favorable to
convection
– Moist air near the middle of the troposphere
– Low values of vertical wind shear between the
surface and upper troposphere
• If these conditions persist for several days, a
tropical cyclone may form
How Do Tropical Cyclones Strengthen?
• Tropical cyclones strengthen and maintain
themselves through latent heat release
• As water vapor rises, it cools and condenses
• Large quantities of latent heat release warm
the air, causing it to be less dense than its
surroundings
• The ideal gas law tells us that the pressure
should then decrease
• The storm must be over warm ocean waters
to supply enough moisture
Stages of Hurricane Development
• When these disturbances first appear, they
are called tropical depressions
– Not named yet
• Once the disturbance has developed with
surface wind speeds stronger than 39 mph,
the storm is classified as a tropical storm
• After further strengthening and surface wind
speeds greater than 74 mph, the system is
upgraded to a hurricane
• Atlantic Hurricane season is June 1 –
November 30
Air converges in the center in a cyclonic pattern and diverges at the top.
In the center of the storm, called the “eye”, the storm is mostly cloud-free
http://www.srh.noaa.gov/jetstream//tropics/tc_structure.htm
Tropical Cyclone Structure
• The main parts of a tropical cyclone are
the eye, the eyewall, and the rainbands
• The eye
– Strong rotation of the cyclone causes a
vacuum of sinking air at the center which
suppresses cloud formation
– This creates a pocket of generally clear, calm
conditions in the center
– Typically 20-40 miles across
– Will usually develop when the winds exceed
74 mph but cause of eye formation still not
fully understood
Tropical Cyclone Structure
• The eyewall
– Consists of a ring of thunderstorms that
produce heavy rains
– Usually produces the strongest winds of the
storm
– Changes in the structure of the eyewall and
eye can cause changes in wind speed
– Double eyewalls can form if the eye changes
size
Tropical Cyclone Structure
• The rainbands
– Curved bands of clouds and thunderstorms that
trail away from the eyewall in a sprial fashion
– Capable of producing heavy bursts of rain and
wind, as well as tornadoes
– Sometimes gaps in the spiral bands where no
rain or wind is found
– if one were to travel between the outer edge of a
hurricane to its center, one would progress from
light rain and wind, to dry and weak breeze, then
back to increasingly heavier rainfall and stronger
wind, over and over again
RAINBAND
EYEWALL
EYE
HURRICANE KATRINA
What is storm surge?
• As the surface winds converge toward the
center of the storm, they “pile up” ocean
water
• Also, the decreased surface pressure raises
the sea level slightly
• Strong waves also can increase the water
height
What causes the most damage?
• A hurricane can cause damage by many
different aspects
– Very strong winds can damage structures
– Heavy rainfall can cause flooding
– Storm surge can inundate low-lying areas
– Occasionally, lightning strikes and/or
tornadoes can cause damage
• Storm surge generally causes the most
damage and deaths, particularly in strong
hurricanes
Damage from Hurricane Ivan
2005 Atlantic Hurricane Season
• Was the most active Atlantic hurricane
season in recorded history
• Forecasts fell far short of the actual activity
• Most tropical storms, hurricanes, and
Category 5 hurricanes recorded in one
season
• 27 named tropical storms, 17 became
hurricanes
• 7 major hurricanes
• 4 Category 5 hurricanes – Emily, Katrina,
Rita, Wilma
• Widespread economical effects
Hurricane Katrina - 2005
• Katrina was the 6th strongest hurricane on
record for the Atlantic Ocean
• After entering the Gulf, rapidly intensified
from Cat. 3 to Cat. 5 in 9 hours on August
28
• Lowest pressure as Cat. 5 storm was 902
mb with max sustained winds of 175 mph
• Weakened slightly before landfall to a
Category 3
• Hurricane force winds extended 120 miles
from the center
Katrina’s Path
Made landfall August
29, 2005 as Cat. 3
Katrina eye
viewed from
NOAA
Hurricane
Hunters
August 28,
2005
Katrina’s Impact
• Largest natural disaster in the history of
the U.S.
• Attributed to over 1830 deaths and 700
missing people, mostly in Louisiana and
Mississippi
• Damages well in excess of $100 billion
• Federal disaster declarations covered
90,000 sq miles (size of United Kingdom)
• 8-16 inches of rain
• Breaching of levees caused 80% of New
Orleans to be flooded
Time Lapse of Hurricane Katrina
• http://www.youtube.com/watch?v=_SLXY
RJnYm0&feature=related
• http://www.youtube.com/watch?v=wFQyX
73eeOA&NR=1