Transcript THUNDERSTORMS, TORNADOES, AND HURRICANES

THUNDERSTORMS,
TORNADOES, AND
HURRICANES
THUNDERSTORMS
 ASSOCIATED WITH:
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Strong winds
Gust fronts
Hail
Lightning and thunder
Tornadoes
Extreme precipitation events
 MAY BE ISOLATED EVENT, A CLUSTER OF STORMS
OR A SQUALL LINE OF CUMULONIMBUS CLOUDS
EXTENDING 100S OF KILOMETERS
 NEED:
– Warm humid air rising in an unstable environment
– Divergence of upper-level winds (jet stream) to enhance
growth
TYPES OF THUNDERSTORMS
 AIR MASS THUNDERSTORM
– Short-lived afternoon summer thunderstorms
– Form away from fronts
– Usually not associated with tornado activities
 SEVERE THUNDERSTORM
– Long-lived thunderstorms
– Form in strong vertical, wind shears along cold
fronts
– Associated with high winds, gust fronts,
microbursts, hail and tornadoes
LIFE CYCLE OF
THUNDERSTORMS
 THE LIFECYCLE STAGES FOR AIR MASS AND
SEVERE THUNDERSTORMS ARE THE SAME
– AIR MASS LIFE CYCLE: APPROXIMATELY ONE
HOUR
– SEVERE LIFE CYCLES: MANY HOURS (SELFSUSTAINING)
 THREE STAGES:
– CUMULUS EVENT
– MATURE
– DISSIPATING
CUMULUS EVENT
 WARM, HUMID AIR RISES, COOLS,
CONDENSES
 RELEASE OF LATENT HEAT PROVIDES
 ENERGY FOR CONTINUED GROWTH
 USUALLY NO PRECIPITATION –
UPDRAFTS TOO STRONG
 SOMETIMES NO LIGHTNING OR
THUNDER AT THIS STAGE
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MATURE STAGE
 UPDRAFTS STILL DOMINATE AS WARM
HUMID AIR CONTINUES TO BE LIFTED
 RELEASE OF LATENT HEAT PROVIDES
ENERGY FOR GROWTH
 DOWNDRAFTS BEGIN AS COOLER AIR SINKS
THROUGH CLOUD
 PRECIPITATION BEGINS
 ANVIL SHAPE AS TOP OF CLOUD HITS
STABLE TROPOSPAUSE
 TORNADO FORMATION POSSIBLE
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DISSIPATING STAGE
 DOWNDRAFTS DOMINATE OVER
UPDRAFTS
 NO MORE WARM HUMID RISING AIR =
NO MORE RELEASE OF LATENT HEAT
 PRECIPITATION CONTINUES ALONG
WITH STRONG WINDS
 TORNADO FORMATION POSSIBLE
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MAJOR DIFFERENCE BETWEEN
AIR MASS AND SEVERE
THUNDERSTORMS
 Air Mass Thunderstorms go through
three lifecycle stages in approximately
one hour
– Updrafts cut off by downdrafts
 Severe Thunderstorms last for hours
– Tilted updrafts and downdrafts.
– Updrafts and downdrafts do not interfere
with each other
TILTED UPDRAFTS IN A SEVERE
THUNDERSTORM
A. Anvil Head
B. Overshooting top
C. Mammatus clouds
D. Flanking line
E. Precipitation (rain/hail)
F.
Funnel Cloud/tornado
G. Rain-free downdrafts
H. Gust Front
CHARACTERISTICS OF SEVERE
THUNDERSTORMS
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GUST FRONTS
MICROBURSTS
LIGHTNING AND THUNDER
HAIL
WIND SHEAR
MAMMATUS CLOUDS
OVERSHOOTING TOP
MESOCYCLONE
TORNADO
GUST FRONTS AND
MICROBURSTS
 Gust Fronts are a boundary that separates the cold
downdrafts from the warm updrafts.
– Act as a ‘wedge’ to push up warm air – producing more growth
for storms
 Microbursts are localized downdrafts of air (about 4 km
wide) that hit the ground and spread out horizontally
– Windspeeds of 75 m/sec (~168 mph)
– May evolve into gust fronts
– Responsible for knocking down trees and associated with
damage usually attributed to tornadoes
– Responsible for several airline crashes on approach to
landing:
 1982 727 in New Orleans, Louisiana
 1985 L1011 in Dallas-Forth Worth
DOPPLER RADAR AND MICROBURSTS
 Terminal Doppler
Weather Radar (TDWR) is
located in 47 areas in the
United States.
 A big feature of TDWR is
that it can actually look
inside storms and measure
dangerous wind shifts,
such as those linked to
wind shear and tornadoes,
which pose a threat to
aircraft during take-off and
landing.
LIGHTNING
 Lightning
– Produces temperatures ~54,0000F
– A response to Electric Potential Gradient within Cb
clouds: 3 million volts/meter along 50 meter path
 100,000 amperes
– Warmer cloud bases (with more liquid droplets) is
slightly negatively charged.
– Colder cloud tops (with more solid ‘crystals’) is
slightly positively charged.
– Particles falling through cloud become ‘electrically
charged’
– Cloud-to-cloud lightning more common that cloudto-ground
LIGHTNING
 Electrons rush to cloud base along Electric
Potential Gradient
 Stepped Leader: electrons descending from
base of cloud – hesitant – 50 -100 meters at a
time (seeking ‘fastest’ path to surface)
 Return Stroke: positive charges rush up to
meet descending electrons – ‘FLASH!!’ – path
is complete and energy flows up to the cloud
 Dart Leaders: subsequent lightning strokes
generally follow same original path – only
faster now that there is less resistance!
THUNDER
 Superheated air (54,0000F) rapidly expands the
lightning channel (‘path) at 1100 feet/sec (~700
mph) – SPEED OF SOUND!
 Sound travels approximately 1 mile in 5
seconds
 Closer to lightning: thunder is a ‘cracking’ or
‘clapping’ sound
 Farther from lightning: thunder is a ‘rumbling’
sound
HAIL
 Hail is a multi-layered ice ball thrown from top,
side, bottom of Cb clouds.
 Layers form when a very tiny object (leaf, ice
crystal, insect) is caught in a strong updraft
 Freezing on the way to top of cloud, melting
and colliding with liquid droplets on the way
down.
 Multiple ‘elevator rides’ builds many layers of
ice until Hail is too heavy to be lifted by
updrafts
WIND SHEAR
 Wind Shear is the
change in wind
speed or wind
direction with
increasing height
above the Earth’s
surface.
 This is most
commonly seen
in Cb clouds
OVERSHOOTING TOP AND
MAMMATUS CLOUDS
 Overshooting Tops: develop if the
energy within the Cb is extremely strong
and updrafts can push up above the
anvil head.
 Mammatus clouds: pouch-like structures
that hang inverted from a cloud base, or
the base of an anvil head
– Associated with Cb that produce tornadoes
Overshooting Top in Cb
Mammatus clouds
OVERSHOOTING TOPS
MESOCYCLONE – TORNADO
BEGINNINGS
 Mesocyclones are rising and spinning
columns of air within a Cb cloud
 They can be 5-10 km across and extend
to the top of the Cb – sometimes
producing the overshooting top.
 Precursor to a funnel cloud dropping
from base of cloud to become a tornado
TORNADOES
 Tornadoes are rapidly rotating winds with intense
central low air pressure
 Wind rotation speeds up to 230 mph
 Counterclockwise rotation (Northern or Southern
Hemisphere)
– Coriolis Force not a factor in rotation
 First indicators:
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Overshooting top (indicative of possible mesocyclone)
Wall cloud descends below base of cloud and slowly rotates
Funnel cloud descends from wall cloud
Tornado: not termed tornado until funnel cloud touches
ground
Tornado in the United States
Tornado Alley
 Tornadoes in United States most common across the
Midwest region from Texas to North Dakota – Tornado
Alley
 Average transit speed for tornadoes in this region is
20-40 knots
 Average direction from southwest to northeast
following collision zone between mT air mass from
Gulf of Mexico and cP air mass from Canada.
– Collision zone shifts seasonally
 Largest Occurrence: Spring
 Lowest Occurrence: Winter
COLLISION ZONE BETWEEN mT AND cP
AIR MASSES
Average number of
tornadoes per 26,000 square
km (10,000 sq miles)
Seasonal march of peak
tornado activity
TORNADO LIFECYCLE STAGES
 Stage 1: Dust whirl
– Circulation of air on ground with funnel cloud extension
 Stage 2: Organizing Stage
– Funnel cloud continues dropping
– Wind rotation speed increasing
 Stage 3: Mature Stage
– Funnel cloud at greatest width
– Wind rotation speed increasing
– Funnel cloud on ground
 Stage 4: Shrinking Stage
– Funnel cloud ‘shrinks’ in diameter – increasing wind rotation
speed to maximum
– Most dangerous stage
 Stage 5: Decay Stage
– Funnel cloud takes on a ‘ropey’ look – no longer vertical below
cloud base
– Final stage for tornado: dissipates or is pulled back into cloud
Wall cloud
3rd Stage: Mature
4th Stage: Shrinking
1st Stage Dust Whirl
5th Stage: Decay Stage
2nd Stage: Organizing
Fujita Scale – Measuring tornadoes
 Measuring intensities of tornadoes based on
a scale developed by Dr. Theodore Fujita,
University of Chicago (1960s)
 F0 – F5: F0 is the weakest and F5 is the
most violent possible
HURRICANE KATRINA, AUGUST 2005
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HURRICANES
Largest of mid-latitude cyclones
– A collection of large tropical thunderstorms rotating about a central low air
pressure
– Cyclones in Indian Ocean, Typhoons in Eastern Pacific
– Last several weeks over open oceans
 Most destructive of all storm systems
– Winds of 120 km/hour and higher
 Size can be 220-700 km in diameter
 Northern Hemisphere Hurricane Season: June through November
 Organization
– Energy: Release of latent heat from transfer of sensible heat from warm
tropical ocean to atmosphere
– Unstable air aloft: cold upper air trough with diverging air
– Upper air divergence greater than lower atmosphere air convergence
 Northern Hemisphere: surface convergence of air results in counterclockwise
rotation of air about Low
 Diverging air aloft enhances surface Low pressure development
 Formation
– Over warm tropical water (sea surface temp of 250C)
– In regions with little to no surface winds: “doldrums”
– Latitudinal range: 50 – 250 North or South of equator
 not at equator – insufficient Coriolis Force to start rotation
HURRICANES NEED WARM
TROPICAL WATERS
Hurricanes track with global winds and feed off
Coriolis Force effects on ocean currents
Hurricane Season in Northern
Hemisphere
CHARACTERISTICS OF
HURRICANES
 Inflow and Outflow
– Updrafts so strong, more air is escaping the top of the clouds than can be
brought in
 Eye Wall and Eye Wall Clouds
– Ring of violent super-cell Thunderstorms at the center of hurricane
– Tornadoes can be spawned here
 Eye
– Central low air pressure of hurricane
– Region of relatively clear skies and calm winds
– Air descending: compressional heating of air even though central air
pressure continues to drop
 Storm Surge
– Hurricane pulls up ocean water (“lifts the surface”) an average of 1cm for
each 1mb drop in pressure
– Regions along shores and coasts are inundated with water being pushed
ahead of the hurricane, by intense precipitation and, sometimes, even
excessively high tides.
outflow
outflow
Eyewall clouds
inflow
Eyewall clouds
EYE
inflow
Storm Surge
Storm Surge: Hurricane Katrina
HURRICANE DAMAGE
 High winds are a primary cause of
hurricane-inflicted loss of life and property
damage.
 A second cause is the flooding resulting
from the coastal storm surge of the ocean
and the torrential rains, both of which
accompany the storm.
Wind Damage: Hurricane Katrina
Flood Damage: Hurricane Katrina
HURRICANE DEVELOPMENT
 Tropical Disturbance
– Mass of thunderstorms beginning to organize
– Light wind circulation
 Tropical Depression
– Wind speed 20-34 knots (1 knot = 1.15mph)
– Central low pressure developing with rotation of thunderstorms
 Tropical Storm
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35-64 knots
Strong central low pressure
Increasing wind speeds
Forward movement across oceans
 Hurricane
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64 knots
Well-development central low pressure: eye may be visible
Moving to west along with global winds: NE or SE Trades, usually
Can move up to 50knots over open ocean
Highest wind speed on the ‘forward’ traveling side
MEASURING HURRICANES
INTENSITY
 Saffir-Simpson Scale
 Measures the drop in central low air
pressure and the corresponding
increase in wind speeds.
NAMING HURRICANES
 Initial tracking of hurricane was by latitude and
longitude of origination.
– Too complicated since hurricanes move!
 World War II – use of military terms to identify
individual hurricanes
– Alpha, Bravo, Tango, etc
 Early 1950s – use of female names
 Late 1970s (post Women’s Lib movement) –
Male and Female names are used
– Alternate male/female name for Pacific or Atlantic
hurricanes
HURRICANE NAMES: 2011
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2011 Hurricane Names
Arlene
Bret
Cindy
Don
Emily
Franklin
Gert
Harvey
Irene
Jose
Katia
Lee
Maria
Nate
Ophelia
Philippe
Rina
Sean
Tammy
Vince
Whitney
HURRICANE DISSIPATION
 Hurricanes need a constant source of warm,
humid air
 If the hurricane travels over colder water, it
will lose its source of energy: latent heat
 If the hurricane travels over land, it will lose
its source of energy and encounter friction
DIFFERENCES BETWEEN
HURRICANES AND OTHER MIDLATITUDE STORMS
 Energy sources
– Hurricanes need warm ocean water and release of latent heat
– Mid-latitude storms depend on temperature differences
between contrasting air masses
 Intensity
– Hurricanes actually become weaker with higher altitudes
above the surface
– Mid-latitude storms become stronger with higher altitudes
above the surface
 Centers
– Hurricanes have a central column of warm descending air
(even with intensely low air pressure) which creates an ‘eye’
– Mid-latitudes storms tend to have central columns of cool to
cold air being forced to rise by dynamic low air pressure
processes in the upper atmosphere and they do not produce
an “eye”