Transcript Thunderstorms and Tornadoes

Thunderstorms
and Tornadoes
Lecture 10
November 11, 2009
Review from last week
At upper levels…
Ridge
Trough
Ridge
Review
Vorticity < 0
Vorticity < 0
Vorticity > 0
Vorticity is simply a measure of how much the air rotates on
a horizontal surface
Positive vorticity is a counterclockwise (i.e. cyclonic) rotation
Negative vorticity is a clockwise (i.e. anticyclonic) rotation
Negative
vorticity
advection
Positive
vorticity
advection
Review
*Positive vorticity advection (PVA) results in divergence at the level of
advection… So downstream of an upper tropospheric trough, there is
divergence, resulting in ascent (upward motion).
* Negative vorticity advection (NVA) results in convergence at the level of
advection…So downstream of an upper tropospheric ridge, there is
convergence, resulting in subsidence (downward motion).
Upper Tropospheric Flow and Convergence/Divergence
• Downstream of an upper tropospheric ridge axis is a
favored location for a surface high pressure, and of
course, downstream of an upper tropospheric trough axis
is a favored location for a surface low pressure center.
Review
How do we know if the surface cyclone will intensify or
weaken?
• If upper tropospheric divergence > surface
convergence, the cyclone will intensify (the low pressure
will become lower)
• If surface convergence > upper tropospheric
divergence, the cyclone will weaken, or “fill.”
• Think of an intensifying cyclone as exporting mass, and
a weakening cyclone as importing mass.
Definition of a Severe Thunderstorm
• The National Weather Service defines a
severe thunderstorm as a thunderstorm
that produces
– Hail of 1 inch diameter (in central US) or
larger
And/or
– Wind gusts 58 mph or great
– 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 few weeks
ago…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
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