Geography 103
Lecture No 8 : Severe Storms (Quiz 2)
1. Thunderstorm
(1). Cumulus stage: updraft
only (warmer air inside the cloud).
(2). Mature stage: both
updraft and downdraft (precipitation)
(3). Decaying stage:
downdraft only.
2. Definition of Severe Thunderstorm
(1). Having 3/4 inch hail
and/or surface gusts of 50 knots.
(2). Capable of producing
large hail, strong gusty surface wind, flash floods, and tornadoes.
3. Types of severe thunderstorms
(1). Tilted updrafts over
downdrafts (not necessary to have tilted updraft)
A. Rain drops or ice crystals fall into downdrafts.
B. Entrainment of dry air.
C. Evaporation into dry air in downdrafts intensifies the downdrafts.
D. Strong updrafts for an extended period of time.
E. Overshooting: penetration of cloud tops into the stable stratosphere (18 km
or 60,000 feet high).
F. Hailstone may be tossed out the side of cloud (the base of anvil).
G. Downdrafts within the anvil may produce mammatus
clouds.
(2). Nontilted
Updrafts (air mass thunderstorm)
A. Strong updrafts and a strong vertical wind shear cause the storm to rotate.
B. Formation of tornadoes.
4. Downdrafts help to maintain updrafts.
5. Gust Front
(1). The boundary separating
the cold downdraft from the warm surface air (updraft).
(2). Resembles the cold
front.
(3). Gusty wind exceeds 55
knots.
(4). Mesohigh
(Mesoscale High)
A. Cold downdraft surface High.
B. Increase of air pressure by several mb.
(5). The cold air may linger
close to the ground for several hours.
(6). Leading edge of the
gust front
A. Haboob: turbulent updraft soils (dust storm).
B. Shelf cloud (Arcus cloud):
C. Roll cloud
An elongated ominous-looking cloud behind the gust front.
6. Downburst
(1). Downdrafts hit the
ground and spread horizontally in a radial burst of winds, much like the water
splashes on the ground.
(2). Types
A. Microburst
Downdraft with winds extending only 4 km or less.
B. Macroburst
A large downdraft with winds extending more than 4 km.
7. Microburst (Wind shear)
(1) Wind speed may reach 146
knots (75 m/sec).
(2). Gust front expands
outward.
(3). Airline crashes
A. Wind shear: rapid changes in wind speeds or directions.
B. Head wind generates extra lift of an airplane.
C. Tail wind causes sudden loss of lift of an airplane.
D. New Orlean’s
145 deaths.
E.
(4). LLWAS (Low-level Wind
Shear Alert System)
A. Detect microburst only after it hits the ground and only where the
instrument is located.
B. Doppler Radar: better instrument.
(5).
(6). 1970s near Chicago
(NIMROD, Northern Illinois Meteorological Research on Downburst).
A. The leading edge of a microburst:
An intense horizontally rotating vortex with dust in a relatively dry region.
B. At Denver, many microbursts emanate from virga-rain falling from a cloud base but evaporating
before reaching ground. Cool dry air (heavier) plunges into lighter moist air
below.
(7). Weather System
A. Thunderstorms
B. Clouds with occasional showers without lightning or thunder.
8. Supercell and
squall-line thunderstorm
(1). An enormous rotating
thunderstorm whose updrafts and downdrafts are structured so that it is able to
maintain itself as a single entity for hours.
(2). Updraft exceeds 90
knots.
(3). Hail size of
grape-fruit.
(4). Damaging surface winds
and/or tornadoes.
9. Squall line
(1). A line of thunderstorms
or Cb clouds.
(2). Along the cold front.
(3). 100-300 km ahead of a
cold front (pre-frontal squall line)
A. Extends for 1000 km (600 miles).
B. Large supercell.
C. Air aloft develops into waves (gravity waves).
(4). Ordinary squall line
A. Weaker updrafts and downdrafts.
B. Similar to air mass thunderstorm.
C. Develops in stationary front, cyclone, or no cyclonic activity.
D. Similar to tropical squall line.
10. Dryline (Dew
point front)
(1). The
boundary between dry and moist air.
(2). A sharp dew
point gradient along the boundary: 9 to 16 oF/km.
(3). Texas,
Oklahoma, Kansas, North Dakota to Texas-Louisiana.
(4). 3 air
masses: cP/cT/mT.
11. Flash floods
(1). Floods that
rise rapidly with little or no advance warning.
(2). Causes
A. Severe thunderstorm stalls or moves very slowly.
B. Many thunderstorms pass the same area.
(3). USA: 100+
deaths/year
(4). Spring
floods: heavy rain + melting snow.
(5). The great
flood of June 1993
A. Mid June: Thunderstorms repeat along a stationary front.
B. An unusual south location of a polar front jet in this month.
C. Warm moist air from the Gulf of Mexico.
D. South Dakota to Minnesota, Wisconsin, Iowa, Illinois and Missouri.
E. Thunderstorm continued into July: twice normal rainfall.
F. Water began to recede in August: more than 60% of levees along the
Mississippi River had been
destroyed.
G. 6.5 billion dollars in crops were lost.
45 deaths, 45,000 homes destroyed or damaged, evacuation of 74,000 people.
12. Mesoscale
Convective Complexes (MCCs)
(1). A large
convective weather system in excess of 100,000 km2 (larger than a
state) and lasts for a period
of about 12 hours.
(2). Beneficial
rainfall over the corn and wheat belts.
13. Thunderstorm movement
(1). New Cb forms to the right of an upper-level wind or to the
right of the old Cb.
(2). The
downdraft within a storm generates a new cell to the south.
(3). The
new cell grows into a mature thunderstorm, cuts off the supply of moisture to
the old cells.
(4). The
new Cell drifts toward the northeast.
(5). The
whole thunderstorm moves slightly to the right of the upper-level wind.
14. Distribution of thunderstorms
(1). 40,000
thunderstorms/day worldwide.
(2). The
equatorial land mass and ITCZ:
A. occurs 1/3 of the days(100 days+).
B. Heat balance.
(3). USA
A. Maximum frequency: Florida.
B. Second maximum frequency: Colorado or the central Rockies.
(4). California:
A. Central and south valley.
B. Winter and spring maximum.
C. Summer: Subsidence inversion prohibits the occurrences of thunderstorms.
D. Desert (Mojave): summer.
(5). The central
plains
Night maximum
A. Low-level southerly jet.
B. Moist air from south: uplifting.
C. Downslope of air from the mountains: Surface
convergence and uplifting.
15. Tornadoes
(1). Tornado
outbreak
Tornado families in a squall line.
(2). Death
tolls: 100/year in USA.
(3). April 3 and
4, 1974
A. 148 tornadoes cut through part of 13 states (4181 km) for a period of 16
hours.
B. 307 deaths, 6000 injuries, 600 million dollar damage.
(4). March 18,
1925
A. Missouri, Illinois, and Indiana.
B. 7 tornadoes(703 km or 437 miles), 695 deaths.
(5). USA:
A. 800+/year tornadoes.
B. 1992: 1293 tornadoes.
(6). The tornado
belt of the Central Plains
(7).
Spatiotemporal variation
A. Winter maximum along the
Jet stream at this location.
B. Spring maximum in the southeast
(A). Unstable air: cold dry air aloft, warm moist air below.
(B). Most violent.
C. May maximum (5/day)in the Central Plains.
D. March to July: 3/4 of the US tornadoes.
Jet stream at this location
E. More frequently in late afternoon (
(8). Tornado
winds
Rarely exceed 220 knots; mostly 125 knots or lower.
(9). Suction
vortices
A. Smaller whirls in a tornado 10 m in diameter).
B. very fast rotation and most damaging.
(10). Fujita scale
A. F0 to F5
(A). Weak: F0 and F1.
(B). Strong: F2 and F3.
(C). Violent: F4 and F5.
B. F5 Tornadoes
(A).
(B). Heaston, Kansas, March 13, 1990, 300 homeless,
13 injuries.
(11). Astonishing feats
A. Lifted a railroad coach with its 117 passengers and dumped it in a ditch 25
meters away.
B. Showers of toads and frogs have poured out of a cloud nearby a pond.
C. Chickens lost all of their feathers.
D. A piece of straw being driven into a metal pipe.
E. A school house was demolished and the 85 students inside were carried over
100 meters without
one of them being killed.
(12). Special tornadoes
A. Los Angeles: March 1, 1983, 5 km path through downtown Los Angeles,
100 + home damage, 33 injuries.
B. Oklahoma City: 32 strikes in 90 years.
C. Codell,
May 20.
(13). Tornado formation:
Unstable air within a severe thunderstorm.
A. Synoptic weather patterns:
(A). Surface: a wave cyclone
a. Behind the cold front: a relatively cold and dry air from the Pacific or
continental air mass.
b. Ahead of the cold front: warm and humid tropical air pushes northward from
the
Mexico
(B). 850-mb height (5000 ft)
Warm and moist air associated with a low-level jet.
(C). 700 mb to 500 mb
a. A cold and dry air(dry tongue) moves in from the southwest.
b. Unstable air: warm air below and cold air aloft.
(D). 500 mb: A trough exists to the west of the
surface low (wave cyclone)..
(E). 300 mb: A polar front jet that provides an area
of upper-level divergence that initiate surface
convergence and rising air.
B. Sounding:
(A). Surface to 800 mb
a. Conditionally unstable air: Lapse rate between dry and wet adiabatic lapse
rates
(1o C/100m to 0.6 oC/100m).
b. warm and very humid.
(B). A shallow inversion at the top of 800 mb.
a. Cold, dry, unstable air sitting above a warm, humid layer
b. Convective instability: The atmosphere will become more
unstable if a layer of air is uplifted.
c. Prevents many small thunderstorms from forming.
d. Upper-level jet rapidly draws upward the moist air that puncture the
inversion to develop into
severe thunderstorm.
e. Overshoot: Clouds penetrate upward 2 to 4 km into the lower stratosphere.
C. Mesocyclone formation
(A). A rising, spinning column of air, 5 to 10 km across.
(B). Large vertical wind shear (low-level southerly wind and upper-level
westerly wind).
(C). Stretches vertically and shrinks horizontally.
(D). Tornado cyclone: Rotation begins in the middle of a Cb
and works downward to the cloud base.
(E). Funnel:
a. Visible cloud caused by the expansion cooling (100 mb
pressure difference between the tornado
cyclone and the surrounding air within a short distance).
b. Funnel core: descending air (cloud free).
(F). Wall cloud (collar cloud)
a. A rotating cloud at the base of mesocyclone.
b. Funnel extends from the wall cloud downward and may touch the ground.
(G). Invisible tornado
a. Air is too dry to form visible cloud.
b. Falling rain, cloud of dust, or darkness obscure the view of the funnel.
(14). Warning
A. Tornado watch
(A). Tornadoes may develop within a specific area during a certain time period.
(B). Issued by the National severe Storm Forecast Center in Kansas City,
Missouri.
B. Tornado warning
(A). A tornado is spotted, either visually or on a radar screen.
(B). By local National Weather Service Office.
(15). Tornado vortex
signature (TVS)
A. A region of rapidly changing wind directions within the mesocyclone
shown on a Doppler radar.
B. The purple shade: The winds blowing toward the radar.
C. The orange shade: The winds blowing away from the radar.
.