Transcript Slide 1

TROPICALCYCLONES
CYCLONES
TROPICAL
HURRICANES
(Hurricanes
and Typhoons)
TYPHOONS
Katrina aiming for New
Orleans 27 August 2005
Tropical Cyclones: Names and Nature
Tropical cyclones, the deadliest of all storms, rage over all the world’s tropical and
subtropical oceans except for the South Atlantic. In each ocean basin they assume
different names but are essentially the same. They are called hurricanes In the Atlantic
and eastern Pacific Oceans, typhoons, in the western Pacific, baguios in the
Philippines, and cyclones in the north Indian Ocean.
Tropical cyclones are intense low pressure areas roughly 600 km (375 miles) in diameter
that form over warm waters (T > 26.5C = 80F) and have wind speeds that reach or
surpass 74 mph. To form, hurricanes need,
Conditions for Formation of Tropical Cyclones
1. Warm ocean waters at least 78°F (26°C) to provide the heat.
2. Coriolis force to provide the rotation.
3. A seedling or cyclonic weather disturbance with extra cloud cover.
4. Almost uniform large scale wind with little vertical shear to allow heat to accumulate.
5. An exhaust system consisting of a divergent upper level disturbance.
Hurricanes develop when regions of disturbed, cloudy weather pass over warm water.
The disturbances include 1: fronts from higher latitudes, 2: poleward bulges of waves in
the ITCZ, 3: squall lines from Tropical West Africa, and 4: reincarnations of Atlantic
hurricanes that pass over Mexico or Central America.
Hurricanes are spinning engines that need the Coriolis force to start the rotation. Only
then can the ice skater effect increase the spin to a violent whirlwind as the air
converges toward the center of the storm. Therefore, no hurricane ever forms within
about 3° latitude of the equator and none ever crosses the equator as a hurricane.
Hurricanes have warm cores (centers), but their seedlings may not. To develop a warm
core, the large scale wind must be almost the same at all heights. Any significant vertical
shear of the wind will tear it apart. Thus, the strong tropical easterly jet shuts off all
Cyclones around India at the height of the summer monsoon in July and August, while
persistent upper level westerly winds virtually rule out hurricanes over the South Atlantic.
The thermal wind implies that hurricanes form where the large scale temperature field is
horizontally uniform. Thus, hurricanes do not have fronts and differ profoundly from
extratropical cyclones or winter lows, which have fronts, and develop where the largest
horizontal temperature gradients occur – directly below the jet stream.
Finally, hurricanes, like any engine, need an exhaust valve to flush out the excess heat
and air at the top of the troposphere. This is provided by disturbances with divergence
over the storm including outflow regions 1: of deceased hurricanes, 2: to the sides of jet
stream troughs that have invaded the tropics from higher latitudes. Unfortunately, these
disturbances are often too weak to identify.
Structure of Tropical Cyclones
Earth’s rotation organizes the hurricane’s clouds into a unmistakable pinwheel pattern
with a relatively calm and clear central region called an eye. The eye ranges from 15 to
as much as 100 km wide. It tends to be clear when the hurricane is intensifying –
otherwise a canopy of high clouds may cover it. There are also frequently bands of low
stratocumulus or cumulus clouds lined into giant spirals.
The eye is surrounded by the eye wall, which gives the appearance of a cosmic stadium,
towering to over 10 miles, 15 km or 50,000 feet. The eye wall, also called the central
dense overcast is a donut-shaped region of the most intense rain and fastest winds.
Further out the clouds are lined into spiral bands, which contain gusty winds and heavy
showers, possibly with thunderstorms. When the hurricanes come over land, some of
these thunderstorms spawn tornadoes. The spiral bands and central dense overcast
make the tropical cyclone resemble spiral galaxies, in which the stars of the galaxy’s
spiral arms correspond to the thunderstorms of the hurricane’s spiral band, and the star
clusters of the galaxy’s core correspond to the hurricane’s central dense overcast.
Winds and Sea State
Near the ground the air spirals inward and is accelerated by the ice skater effect
(conservation of angular momentum) despite loss of energy to friction. Thus, the fastest
winds are found at the inner edge of the eye wall. These winds shred the sea surface,
filling the air with long trails of spray and spume, and in extreme cases making it hard to
tell the boundary between sea and air. But even in large, category 5 storms, hurricane
force winds seldom extend more than 100 miles (160 km) from the center.
As the air converges toward the center some of it rises in the spiral bands to form heavy
showers or thunderstorms. But most air approaches the inner edge of the eye wall,
where it spins too fast to converge any further. It then turns sharply up and tilts out to
leave a distinct eye in the center and form the eye wall. The rapidly rising air of the eye
wall yields the storm’s most torrential rain and its thickest clouds, with temperatures
sometimes dropping below -90°C (-130°F).
Death
Hurricanes die when they leave tropical waters or when they draw in cold or dry
air. Thus, they always die once they come over land in the sense that the winds
weaken rapidly, but they may produce torrential rains and flooding for several
days. When hurricanes draw in cold air they often transform into and are reborn
as extratropical cyclones. This happened with the so-called Perfect Storm, that
was made into a movie, and which generated ocean waves up to 100 feet high!
Hurricane Season
Warm waters make hurricanes storms of summer and early fall. In the Atlantic,
peak hurricane season begins in late July and ends in October. The Pacific
Ocean warms earlier and stays warm longer, so hurricane and typhoon season
there begins in June and continues into November. The North Indian Ocean
warms earliest of all and remains warm the longest, so Cyclone season begins
in April and runs into November. Ironically, no hurricanes form in the Indian
Ocean at the height of the summer monsoon during July and August. South of
the equator, peak season runs from January to April. The South Atlantic Ocean
is rarely warm enough to produce hurricanes although in the past few years two
South Atlantic Hurricanes were recorded after a gap of more than a century.
Seedlings for Hurricanes
Squall
Line
2005/240 28 Aug,
1700 UTC
Hurricane Katrina
approaching the
Gulf Coast
Satellite: Terra Pixel size: 2km
Next slide has
Pixel size: 0.5 km
Features include
Open Eye
Vortices in Eye
Spiral Bands
Central Dense
Overcast
Cumulus rows
aligned with wind
over land
Thunderstorm
overshooting tops
in Spiral Bands
Wind Speed Around
Katrina
This is a typical wind profile
around a moving hurricane.
Speed is small in the eye
decreasing toward the
center. The fastest winds
surround the center but are
greatest on to the right of
the storm. Wind speed is
greater to the right of the
storm’s motion because
the wind is close to the
sum of the storm’s motion
and rotation. These add on
the right hand side and
subtract on the left hand
side.
Radar Image
of Katrina.
The
most
intense rainfall
(orange and red
colors) appears
in spiral bands
and in the eye
wall around the
eye wall (black
circle at bottom
where there is
no rain. This
radar image is
from Slidell, LA.
When the eye
wall hit Slidell,
the radar unit
was destroyed.
Computer Simulation of Katrina’s Wind, Rain, and Clouds
Eyewall of Katrina
Hurricane Radar Cross Section
EYE
EYE
WALL
stratiform
stratiform
rainband
eyewall
As air spirals in toward the center, it conserves angular momentum and speeds up
much like an ice skater who spins faster when she stretches her arms over her head.
Hurricane Tracks
Hurricanes follow well established tracks because they are herded gently by the larger
scale winds around the large subtropical high pressure areas. At first they move from
east to west in the tropics with a typical forward speed about 10-15 mph. They often
slow down and move erratically when they approach the western sides of the oceans,
where the large scale winds often become weak and variable. Some hurricanes continue
moving toward the west until they smash into land, while others turn poleward and
recurve toward the east when they pass beyond about 30° latitude and enter the region
of the westerlies, where they tend to accelerate, and can reach forward speeds up to 60
mph. This happened to the Hurricane of 1938, which occurred before satellites and
radar. The result was horrendous. The hurricane claimed over 600 lives when it
smashed into the south shores of Long Island and New England.
Destruction
The main loss of lives and property occur along the shorelines because hurricanes are
low pressure areas that suck the sea surface up, while its winds push the waters toward
shore. Typically, the sea surface rises about 3 m or 10 feet in what is called the storm
surge but can rise up to about10 meters in extreme cases, such as Hurricanes Camille
(1969) and Katrina. But the winds and torrential, flooding rains also wreak havoc inland,
while tornadoes add insult to injury. Loss of life is now much reduced in the United
States and the Caribbean because of excellent forecasting (the population is
evacuated), but occasional disasters such as in Bangladesh and Burma still occur
because of poverty, ignorance, faulty communications, and terrible governments.
Storm Track for Hurricane
Katrina 23-30 August 2005
6-hour Positions
Birth
TROPICAL CYCLONE TRACKS FOR THE WORLD
Tropical cyclones form over tropical waters at least 80°F but not near the the equator.
They move around the large subtropical high pressure areas and can leave the tropics.
Rainfall from Hurricanes
Hurricanes produce torrential rains.
This can be both good and bad.
Hurricane rains sometimes relieve
droughts but can provide too much
water in too short a time. During
Hurricane Floyd, for example the
Tar River in North Carolina rose up
to 40 feet in places and put much of
the state’s highway system under
water. An enormous number of
animals, many trapped in pens,
drowned leaving a terrible stench of
decay. In 1998, Hurricane Mitch
brought 70” of rain in 5 days (NYC
gets 42” per year) to Honduras in 5
days and Nicaragua, leading to
mudslides and floods that claimed
10,000 lives and destroyed 75% of
the infrastructure of Honduras.
HURRICANE STORM SURGES
The storm surge is a potentially catastrophic
rise of sea level (accompanied with towering
waves) when a storm passes onshore.
Storms raise sea level at least 2 ways. 1: The
inverted barometer effect. Sea water is forced
from the higher air pressure outside a storm
to the storm center. For every inch air
pressure is lowered, sea level rises 13 inches.
2: Winds blowing toward shore force sea
water against the coast. The maximum rise
occurs when the wind forces the water into a
shallow, narrowing bay or channel. The
combination of these two effects can raise
sea level up to 10 m in extreme hurricanes.
Drowning during storm surges has caused the
major loss of human life from hurricanes.
LAND
When a hurricane approaches land, the
winds to its right in the North Hemisphere
blow toward shore, so the maximum
storm surge occurs to the right of the eye.
OCEAN
New Orleans, below Sea Level was a Disaster Waiting to Happen (and it will
again). Katrina lifted Lake Ponchartrain over the levees, flooding the city
Hurricane Katrina’s Storm Surge – up to 27 feet
Katrina
Storm Track
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