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Tropical Cyclones, Hurricanes & Typhoons
10-10-17 4:06 PM
World’s Deadliest Tropical Cyclones:
- 14/20 of the deadliest tropical cyclones occurred in Southern Asia (India,
Bangladesh)
- Deadliest was Great Bhola cyclone, hit Bangladesh in 1970, ~500,000 dead
- Deadliest storm in Atlantic Basin (area occupied by the Atlantic Ocean) was
in 1780: 22,000 dead
- Deadliest U.S storm was Galveston Hurricane; 1900, ~8000 dead
- Only 1/10 of the deadliest storms in the U.S has occurred since 1957;
Katrina, 1900 dead
- ~4 Hurricanes hit Atlantic Canada/year, deadliest was 1775, ~4000 dead
along Newfoundland coast
Definition:
- A tropical cyclone is a warm-core, low pressure system without any "front"
attached, that develops over the tropical or subtropical waters, and has an
organized circulation with winds of at least 120 kph (74 mph)
- Depending upon location tropical cyclones have different names but they’re
all still tropical cyclones:
- Atlantic/Eastern Pacific Oceans - hurricanes
- Western Pacific - typhoons
- Indian Ocean - cyclones
- Warm core = the warmest air in the hurricane is located at its center
- Warm core is generated by two different mechanisms:
1) Latent Heat Release: Heat released by a substance during a change of
state
- Warm most air moves over the ocean where the water vapor eventually
rises into the atmosphere. As the water vapor rises it cools and condenses
into liquid droplets. This change of state (gasliquid = condensation) which
releases heat into the atmosphere. The warm air continues to rise with moist
air from the ocean taking it’s place thus creating more wind.
2) Subsidence: As the air in the eye of the cyclone sinks it’s warmed
because it is being compressed into a smaller space and thus it’s volume
stays the same but the pressure on it increases therefore causing the
temperature to also increase
Where and When:
- Tropical cyclones do not form near the equator due to the lack of the
coriolis effect, also storms tend to curve to the north and east as they
interact with the westerlies
Coriolis Effect
- The Coriolis effect is the apparent deflection of air due to the rotation of
the earth.
- Air, rather than flowing directly from areas of high pressure to low
pressure, rotate to the right of this direction in the Northern Hemisphere.
- For the Northern Hemisphere Coriolis force deflects air to the right (Sense
of rotation about a low pressure center where the air is converging inwards
on the surface is counter-clockwise, high pressure center where the air is
diverging outwards on the surface is clockwise in Northern Hemisphere)
- Worldwide tropical cyclone activity peaks in late Summer when the
temperatures are warmest (late summer b/c ocean takes a long time to
absorb heat and thus takes all summer to warm up to the surface ocean
temperature required for a hurricane to form)
- In general, sea surface temperatures are warmer along eastern coasts
than western coasts and are warmest near Indonesia accounting for the
strongest and most frequent activity.
-There are at least four main requirements for tropical cyclogenesis :
1) Enough Coriolis force to develop a low pressure center,
2) A preexisting low-level focus or disturbance
3) Sufficiently warm sea surface temperatures approximately 27° C at least
60 m deep.
4) Low vertical wind shear
- These conditions are necessary but NOT sufficient conditions for the
formation of tropical cyclones.
- A low-level disturbance is necessary to start and concentrate convection
locally.
- Most commonly in the Atlantic basin, the disturbances are either fronts,
easterly waves or the intertropical convergence zone. Storms that develop
near the coast of Africa from easterly waves are referred to as Cape Verde
storms.
- Why is it that most easterly waves do not develop into hurricanes?
- Strong descending air associated with Azores high (large subtropical semipermanent centre of high atmospheric pressure found near the Azores in the
Atlantic Ocean) produces an inversion inhibiting convection.
- Vertical wind shear is usually too strong over the tropical Atlantic for the
cloud systems to stay together.
- The middle layers of the atmosphere are usually too dry.
- The Intertropical Convergence Zone (ITCZ), also known as the
Intertropical Front, Monsoon trough, Doldrums or the Equatorial
Convergence Zone, is a belt of low pressure girdling Earth at the equator.
Air, converges at the surface towards this low pressure and then rises
forming a band of clouds and convection which can sometimes form tropical
cyclones.
- The ITCZ, migrates with the Sun towards the summer hemisphere. Note
that the ITCZ is slightly displaced towards the Northern Hemisphere since
there is more land mass.
- ITCZ is the site where the trade winds converge
Intensity
- Predicted maximum intensity of a hurricane based upon average ocean
surface temperatures. The agreement between theory and observations is
excellent, showing the importance of a warm ocean for hurricane formation.
- However, we not only need to know how warm the ocean surface is, but
how deep is the warm water. That is because the storm itself can cool the
sea surface by bringing up cooler water through upwelling. (wind-driven
motion of dense, cooler, and usually nutrient-rich water towards the ocean
surface, replacing the warmer, usually nutrient-depleted surface water)
- Weak wind shear:
- Energy is derived from release of latent heat in the eyewall.
- A concentration of energy is necessary for development
- If the clouds are carried away by the high winds aloft, then the energy is
no longer concentrated sufficiently for the hurricane to develop.
- Vertical wind shear of less than 10 m/s (20 kts, 22 mph) between the
surface and the tropopause is required for tropical cyclone development.
- Strong wind shear can "blow" the tropical cyclone apart, as it displaces the
mid-level warm core from the surface circulation and dries out the midlevels of the troposphere, halting development.
Naming
- Storms reaching tropical storm strength were initially given names to
eliminate confusion when there are multiple systems in any individual basin
at the same time, which assists in warning people of the coming storm.
- Naming of Atlantic tropical storms has occurred since 1953
- Lists included only women’s names until 1979
- Since 1979, men’s and women’s names have been alternated
- Six lists are used
- The 2005 list will be used again in 2011 (minus Dennis, Katrina,
Rita, Stan, and Wilma), these are retired
Long Term Trends
- While the number of storms in the Atlantic has increased since 1995, there
is no obvious global trend; the annual number of tropical cyclones worldwide
remains about 87 ± 10.
- In spite of that, there is some evidence that the intensity of hurricanes is
increasing.
Economic Impact
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- Most obvious economic impact is the rebuilding of damaged infrastructure
- Other less often considered costs include:
Cost of evacuation
Impact on energy production
Cost of severe coastal erosion
Money lost due to absence of tourism
- Katrina (2005) cost ~81 billion dollars
- Andrew (1992) cost ~40 billion dollars
Damage + Hazards
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Damage (Wind)
- Andrew for example, caused extreme wind damage with speeds on land
measured at 270 kph
- Wind damage is greater when the winds are coming off the ocean as
opposed to the coast because there’s no resistance on the land
Damage (Water)
- Katrina was an extremely strong hurricane while offshore but most of the
damage was associated with storm surge and flooding
- Storm Surge = An offshore rise of water associated with a low pressure
weather system, typically a tropical cyclone. Storm surges are caused
primarily by high winds pushing on the ocean's surface. The wind causes the
water to pile up higher than the ordinary sea level
- Damage from storm surge and flooding physically resembles wind damage
Erosion
- Weathering and damage of the land on the coast, causing it to get
progressively smaller over time
Hurricane Hazards
A)Storm Surge (8-160 km wide dome of water that sweeps over the
coastline during landfall.)
B)Wind Damage
C)Heavy rains (flooding)
D)Associated tornados
-About 90% of fatalities are caused by coastal and inland storm surge
- Strong onshore winds and relatively low air pressure are
responsible for a storm surge
- A sea-level rise of about .5 m for every 50 mb of pressure loss.
- A surge is superimposed on the normal tidal oscillation, so that the danger
is greatest at high tide
Plus wind-driven waves up to 3 m
- The level of surge in a particular area is also determined by the slope of
the continental shelf. A shallow slope off the coast (top picture) will allow a
greater surge to inundate coastal communities. Communities with a steeper
continental shelf ( bottom picture) will not see as much surge inundation,
although large breaking waves can still present major problems.
- Even a Category 1 hurricane can create significant surges
- Katrina’s landfall was at a category 4, but with a comparable storm
surge to that of Camille (cat.5).
- Katrina had weakened to a Category 4 hurricane with maximum sustained
winds estimated at 145 mph as it made landfall early Monday.
- However, the storm surge of 8.4 m, generally found in category 5 storms,
was maintained during Katrina’s weakening to a category 4.
Warnings + Forecasting
- Warnings existed even in 1900:
- Sailors arrived in port talking of stormy seas
- Though the residents knew there was a storm in the Gulf of Mexico, there
was no hint of where landfall would occur
- There are several elements to tropical cyclone forecasting: track
forecasting, intensity forecasting, rainfall forecasting, storm surge, and
tornado forecasting.
- The large-scale synoptic flow determines 70 to 90 percent of a tropical
cyclone's motion. The deep-layer mean flow is considered to be the best tool
in determining track direction and speed.
- The 1-2-3 rule (mariners' 1-2-3 rule or danger area) is a guideline
commonly taught to mariners for hurricane and tropical storm tracking and
prediction. It refers to the rounded long-term NHC/TPC forecast errors of
100-200-300 nautical miles at 24-48-72 hours, respectively.
- Because of the inherent uncertainty in the exact track forecast, the
national Hurricane Center issues forecasts that include an ever expanding
envelope of threat area.
Wind Damage
-The two storms causing the most widespread wind damage in the US were
Hurricanes Camille (1969) and Andrew (1992)
Flooding
- Hurricane Floyd produced torrential rainfall in eastern North Carolina,
adding more rain to an area hit by Hurricane Dennis just weeks earlier. The
rains caused widespread flooding over a period of several weeks; nearly
every river basin in the eastern part of the state exceeded 500-year flood
levels. In total, Floyd was responsible for 57 fatalities and $4.5 billion
($5.7 billion in 2006 U.S. dollars) in damage, mostly in North Carolina
- Floyd dropped nearly 17 inches (430 mm) of rain during the hours of its
passage and many residents weren’t aware of the flooding until the water
came into their homes. The National Guard and the Coast Guard performed
nearly 1700 fresh water rescues of people trapped on the roofs of their
homes due to the rapid rise of the water. By contrast, many of the worst
affected areas did not reach peak flood levels for several weeks after the
storm, as the water accumulated in rivers and moved downstream.
-The extensive flooding resulted in significant crop damage.
- Around 31,000 jobs were lost from over 60,000 businesses through the
storm, causing nearly $4 billion (1999 USD, $4.7 billion 2006 USD) in lost
business revenue.
- Freshwater runoff, sediment, and decomposing organic matter caused
salinity and oxygen levels in Pamlico Sound and its tributary rivers to drop to
nearly zero.
Life Cycle
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- A tropical depression is designated when the first appearance of a lowered
pressure and organized circulation in the center of the thunderstorm
complex occurs.
- Winds near the center are constantly between 20 (37 kph) and 34 knots
(23 - 39 mph).
- Once a tropical depression has intensified to the point where its maximum
sustained winds are between 35 (63 kph)-64 knots (39-73 mph), it becomes
a tropical storm. It is at this time that it is assigned a name.
- As surface pressures continue to drop, a tropical storm becomes a
hurricane when sustained wind speeds reach 64 knots (74 mph or 120 kph).
- A pronounced rotation develops around the central core.
Large bands of clouds and precipitation spiral from the eye wall and are
thusly called spiral rain bands
Saffir- Simpson Hurricane Intensity Scale
Category
One
Two
Three
Four
Five
Definition
Winds 74-95 mph
96-110 mph
111-130 mph
131-155 mph
>155 mph
- Cape-Verde type hurricanes usually travel slowly westward (10-20 km per
hr) and may take a week to cross the Atlantic.
- Once hurricanes have reached the Caribbean or the Gulf of Mexico, they
recurve to the north and generally speed up.
- Hurricane winds usually diminish very quickly once landfall occurs
- The hurricane has lost its energy source (warm water is the fuel for the
latent heating).
- The increased surface roughness weakens the system with surface
pressures rising, with winds decreasing.
Storms rarely remain hurricanes for more than 12-24 h after landfall.
Storm Structure
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- The main parts of a tropical cyclone are the rainbands, the eye, and the
eyewall. Air spirals in toward the center in a counter-clockwise pattern in the
northern hemisphere (clockwise in the southern hemisphere), and out the
top in the opposite direction. In the very center of the storm, air sinks,
forming an "eye" that is mostly cloud-free.
Storm Components:
The Eye (center)
The hurricane's center is a relatively calm, generally clear area of sinking air
and light winds that usually doesn't exceed 15 mph (24 kph) and is typically
20-40 miles (32-64 km) across. An eye will usually develop when the
maximum sustained wind speeds go above 74 mph (119 kph) and is the
calmest part of the storm
The eyewall (Circular area surrounding the eye)
Consists of a ring of tall thunderstorms that produce heavy rains and usually
the strongest winds. Changes in the structure of the eye and eyewall can
cause changes in the wind speed, which is an indicator of the storm's
intensity. The eye can grow or shrink in size, and double (concentric)
eyewalls can form.
Rainbands
Curved bands of clouds and thunderstorms that trail away from the eye wall
in a spiral fashion. These bands are capable of producing heavy bursts of
rain and wind, as well as tornadoes. There are sometimes gaps in between
spiral rain bands where no rain or wind is found.
- Typical hurricane strength tropical cyclones are about 300 miles (483 km)
wide although they can vary considerably.