Transcript 1022 Lecture, Severe, Tropical, Thunderstorms

Severe Storms:
Tropical Cyclones
Thunderstorms
Tornadoes
Typhoon Lupit, which
means "fierce" in
Filipino, is taking dead
aim at the already
storm battered
Philippines, which will
make it the 18th typhoon
to hit the country this
season…Over the last
month two storms,
Ketsana and Parma,
struck the country,
causing flooding and
landslides. Over 800
people were killed –
ABC World News, Oct.
22, 2009.
t
Tropical Cyclones
Violent, long-lived cyclonic storms with sustained winds in
excess of 74 mph that originate in tropical latitudes,
usually in late summer and early autumn.
Hurricane -- W. Hemisphere, named for Mayan sky god
Hurican (Sp. Huricán, Fr. Ouragan)
Typhoon -- W. Pacific, named for great wind (Cantonese)
Cyclones -- Indian Ocean and Australia
Saffir-Simpson Scale
NAME
WINDS (mph) DAMAGE
Tropical depression
Tropical storm
Hurricane
CAT I
CAT II
CAT III
CAT IV
CAT V
23-39
40-74
74-95
96-110
111-130
131-155
> 155
minor
from rains
trees
roofs
light structures
extensive
devastating
Hurricane Structure
Computer
model of
hurricane
wind
structure
Hurricane Origins and Tracks
Formed by disturbance in trade winds near ITCZ.
Needs warm ocean water T > 26 oC
Needs Coriolis force to support rotation, so never found in latitudes less
than 5o North or South
Needs conditions with no wind shear aloft to get organized.
Sustained by massive release of latent heat of condensation.
Please Read
Hurricane Mechanics:
Nuts and Bolts of Heat Engines
Hurricane Engine: Basic Operating Manual
http://www.ems.psu.edu/~nese/ch11sec3.htm
Hurricane’s Demise
Over land hurricanes are:
1) Cut off from warm ocean moisture
2) Slowed down by increased surface friction
3) Torn apart by increased wind shear aloft in the
westerlies
4) Accelerated northward by mid-latitude winds
Energy Generated by a Hurricane
1. Rain formation: An average hurricane produces 1.5 cm/day of rain
inside a circle of radius 665 km. Converting this to a volume of rain
gives 2.1 x 1016 cm3/day. A cubic cm of rain weighs 1 gm. Using
the latent heat of condensation, this amount of rain produced gives
5.2 x 1019 Joules/day or 6.0 x 1014 Watts. This is equivalent to 200
times the world-wide electrical generating capacity - an incredible
amount of energy produced! And the storm can last for 10 days or
so! This power would illuminate 6 x 1012 lightbulbs!
2. Wind energy: Proportional to the the wind speed cubed. Using 40
m/s for an inner radius of 60 km yields 1.5 x 1012 Watts.
So a hurricane produces an amazing amount of energy. 400 times
more by rain formation than by wind energy.
Hazards of Tropical Cyclones
Torrential rains (10 inches is common),
Storm surges of as much as 25 feet (from low pressure,
onshore winds, and high tides),
Flooding can be widespread.
Mud Slides in hilly terrain
Wind gusts of up to twice the average wind speed
Tornadoes and Thunderstorms embedded in the tropical
storms
Storm track is somewhat erratic and is determined by
the interaction of the storm with its environment.
Hurricane Andrew 1992
Last CAT 5 to hit U.S., 43 deaths, $30B damage.
Past Dealiest Hurricanes
1900 Galveston hurricane, Isaac’s Storm (6,000 dead)
1938 New England Hurricane (Miller video 1687)
The hurricane had no name, it was not tracked, and it came with
little or no warning. There were no emergency services available.
Killer storm surge caused by extreme low pressure, offshore winds,
and the timing of the high tide. Great loss of life.
1969 Hurricane Camille
200 mph wind gusts, 25 foot storm surge, 256 deaths, $4 Billion
damage.
1970 Bengal cyclone (500,000 dead)
2005 Katrina (over 1800 deaths, mainly from flooding)
Hurricane Mitch (22 Oct.-5 Nov. 1998)
Mitch was responsible for over 9,000 deaths
predominately from rain-induced flooding in
portions of Central America, mainly in Honduras
and Nicaragua. This makes Mitch one of the
deadliest Atlantic tropical cyclones in history.
The 905 mb minimum central pressure and
estimated maximum sustained wind speed of 155
knots (Category V) over the western Caribbean
make Mitch the strongest October hurricane since
records began in 1886.
Katrina, Aug. 29, 2005
For more information
National Hurricane Center
http://www.nhc.noaa.gov
Tropical Meteorology Project
at Colorado State University
http://typhoon.atmos.colostate.edu/
Thunderstorms, Lightning,
Tornadoes
Anthes Ch. 5, pp. 124-32
Thunderstorms
http://www.nssl.noaa.gov
A thunderstorm is a deep convective cloud (Cb) reaching as high
as 10 to 18 km, that produces lightning, thunder, heavy rain,
downbursts, possibly hail, flash floods, and on rare occasions,
tornadoes.
It is a thermodynamic heat engine driven by the latent heat of
condensation (vapor to liquid) and fusion (liquid to ice).
Can be caused by solar heating, nighttime cooling, sea breeze
convergence, mountains, cold fronts.
An average of 1800 thunderstorms are active globally at this
moment.
Mesoscale convective complex contains dozens of Cbs and is
1000 times larger than a thunderstorm.
Lifecycle of a Thunderstorm
Supercell
Thunderstorm
Climatology
Hail Climatology
Lightning
Lightning is an electrical
discharge.
Most flashes originate in clouds.
Clouds are “most commonly in a
negative state of electricity,
but sometimes in a positive
state.”
Grounded metallic rods will
protect most structures from
damage.
Source:
Benjamin Franklin (1750s)
Lightning
Lightning is a giant transient high-current electrical discharge
(or spark) of static electricity with a path length of
kilometers.
A typical strike is 100 million volts, 100,000 amps, and
generates temperatures of 30,000 K (five times hotter than
the sun’s surface).
Around the world, there are 1800 thunder storms going on at
any given time with 100 lightning strikes to Earth each
second.
That means 9 million lightning bolts per day!
Most lighting is within a cloud or between clouds.
Only about 20% is cloud to ground.
Lightning
strike
details
Lightning strike is very fast and very complex:
1. Cloud to ground stepped leader moves along a branching path in
20 to 100 meter steps at 10 to 20 km/sec.
2. Ground to cloud return stroke covers the final 100 meter spark
gap to a high spot (tree, tower, golf club).
3. Subsequent (3-40) strokes called dart leaders give lightning its
forked appearance, completing the strike.
Lightning formation
Necessary condition: Separation of charge.
Cb is a static electricity generator.
Charge separation caused by collisions of water and ice.
Electrons (-) and ions (+ -) transported to different levels via
updrafts and downdrafts.
Air is a good insulator.
Cloud can reach 100 million volts, 100,000 amps.
Car battery is 12 volts, 8 amps.
House current is 120 volts, 20 amps.
Sufficient condition: Voltage difference > insulating
ability of the air.
Thunder
Thunder = sound wave
Long tube of air, 6 km x 15 cm diameter heated almost
instantaneously to 12-30,000 K expands air violently.
Shock wave = thunder (both audible and inaudible).
Speed of light = 300,000 km / sec
Speed of sound ~ 0.33 km / sec
3 second delay = 3 km; 5 second delay = 1 mile
Lightning fatalities
About 60 fatalities per year in the US, 360 severe injuries
Benefits of lightning
May have played a role in the pre-biotic formation of amino acids.
Fixes nitrogen, creates natural fertilizer.
Ignites forest fires -- forest ecology.
May have been the source of fire for early humans.
E-M fields of lightning used to study the atmosphere.
Global lightning strikes represent a global “thermometer.”
Lighting strikes on other planets (Jupiter) allow remote study.