Tropical Cyclones, Hurricanes and TyphoonsMotivation (Why do we care)DefinitionWhere and when do they occurFormation and intensityStructureHazards

World’s Deadliest Tropical CyclonesOf the 20 deadliest

tropical cyclones, 14 have occurred in South Asia (India, Bangladesh).

The deadliest was the great Bhola Cyclone which hit Bangladesh in 1970 resulting in app. 500,000 deaths.

World’s Deadliest Tropical CyclonesThe deadliest storm in

the Atlantic Basin occurred in 1780. (22,000 deaths)

Deadliest US storm was the Galveston Hurricane in 1900 which killed app. 8,000 people.

Of the 10 deadliest storms in the US, only 1 has occurred since 1957 (Katrina - 1900 dead)

What About Canada?On average, about 4

hurricanes impact Atlantic Canada in any given year.

Deadliest hurricane in Canada occurred in 1775 when 4000 people died along the Newfoundland coast.

What about the 2nd deadliest?

Economic ImpactThe most obvious impact economically

speaking is the rebuilding of damaged infrastructure.

However, other costs that are less often considered include:The cost of evacuationThe impact on energy productionThe cost of severe coastal erosion.

Costliest U.S. HurricanesHurricane Katrina

(2005) est. 81 billion dollars.

Hurricane Andrew (1992) est. 40 billion dollars.If Andrew had made

landfall 20 miles to the north, loss of life and property would have easily doubled.

Damage (wind)

Wind damage with Hurricane Andrew was extreme with wind speeds on land measured at 270 kph.

What if Andrew hit Miami?

Damage (water)

While Katrina was an extremely strong hurricane while offshore, most of the damage was associated with storm surge and flooding.

Damage (water)New Orleans is

particularly vulnerable to flooding because the city itself is below sea level.

With the storm center passing to the east, most of the flooding came from Lake Pontchartrain.

Damage (water)This is a picture of flooding near Venice, Louisiana that resulted from levee failure.

Damage (water)This is a picture of damage from near Gulfport, Mississippi where the damage was from storm surge. Note that the damage resembles wind damage.

Erosion

DefinitionA 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 around the world. In the:Atlantic/Eastern Pacific Oceans - hurricanesWestern Pacific - typhoonsIndian Ocean - cyclones

DefinitionWhat do we mean by

warm core?Literally that the

warmest air is located at the center of the storm.

This warm air is generated by a couple of different mechanisms.

Latent Heat Release

DefinitionWhat do we mean by

warm core?Another mechanism

is subsidence. As the air in the eye of the cyclone sinks, it warms due to compression.

Where and WhenNote that 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.

Where and WhenThe 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.

At least 4 degrees from the equator:Coriolis force can be large enough to produce rotation(deflecting to right in the Northern Hemisphere)

Before: After:

Where and When

Worldwide, tropical cyclone activity peaks in late summer when water temperatures are warmest.

Where and WhenIn 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.

Where and WhenIn 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.

Formation and intensityThere are at least four main requirements for

tropical cyclogenesis :enough Coriolis force to develop a low pressure

center, a preexisting low-level focus or disturbancesufficiently warm sea surface temperatures

approximately 27° C at least 60 m deep.low vertical wind shear

These conditions are necessary but NOT sufficient conditions for the formation of tropical cyclones.

FormationA 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.

FormationSince easterly waves account for about 60% of all

Atlantic basin tropical cyclones, there is a relationship between West African rain and cyclone frequency.

FormationWhy is it that most easterly waves do not

develop into hurricanes?Strong descending air associated with Azores

high 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.

FormationThe 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, convergences at the surface towards this low pressure and then rises forming a band of clouds and convection which can sometimes form tropical cyclones.

FormationThe Intertropical Convergence Zone (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.

IntensitySo why do we need

warm sea surface temperatures?

Consider the same schematic that we saw earlier:

IntensityThere is a direct relationship between

the intensity (central pressure) of the most intense hurricanes and the temperature of the sea-surface over which the storms are moving.

Note from the attached chart that the six strongest hurricanes all occurred in the western Pacific where the sea-surface temperatures are warmest.

IntensityNote that the 5 lowest pressures ever recorded in tropical cyclones all have occurred in the Pacific basin.

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.

IntensityHowever, 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.

IntensityWeak 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.

IntensityVertical wind

shear of less than 10 m/s (20 kts, 22 mph) between the surface and the tropopause is required for tropical cyclone development.

IntensityStrong wind shear

can "blow" the tropical cyclone apart, as it displaces the mid-level warm core from the surface circulation and dries out the mid-levels of the troposphere, halting development.

In what sense is the wind blowing in the vicinity of the Azores High?

1. Counterclockwise2. Clockwise3. Directly towards

the center4. Directly away from

the center

What is the cause of the relative warmth in the

center (eye) of a hurricane? 1. Strong winds2. Warm oceans3. Condensation

heating4. Convergence

Where would you expect the most significant wind damage?1. A2. B3. C4. D5. E

A

E

B

C

D

Life CycleA 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).

Life CycleOnce 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.

Tropical Storm Fay (2008)

Note that you can see some banding and symmetry

Life CycleAs 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.

The Saffir-Simpson Intensity Scale

Life Cycle (North Atlantic)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.

Life CycleHurricane winds usually diminish

very quickly once landfall occursThe 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 StructureThe 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 norther 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 StructureThe Eye

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. 

Storm StructureThe eyewall

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.

Storm StructureRainbands

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.

Storm StructureTropical Cyclone

SizeTypical hurricane

strength tropical cyclones are about 300 miles (483 km) wide although they can vary considerably.

The relative sizes of the largest and smallest tropical cyclones on record as compared to the United States.

Hurricane HazardsA)Storm SurgeB)Wind DamageC)Heavy rains (flooding)D)Associated tornados

About 90% of fatalities are caused by coastal and inland storm surge

Hurricane HazardsWhat is a storm surge?

It is an 8-160 km wide dome of water that sweeps over the coastline during landfall.

Hurricane HazardsStrong onshore winds and relatively low

air pressure are responsible for a storm surgeA 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

Hurricane HazardsThe level of surge in a

particular area is also determined by the slope of the continental shelf. A shallow slope off the coast (right, top picture) will allow a greater surge to inundate coastal communities. Communities with a steeper continental shelf (right, bottom picture) will not see as much surge inundation, although large breaking waves can still present major problems.

Hurricane HazardsSLOSH (Sea, Lake and

Overland Surges from Hurricanes) is a computerized model run by the National Hurricane Center (NHC) to estimate storm surge heights and winds resulting from historical, hypothetical, or predicted hurricanes by taking into account Pressure Size Forward speed Track Winds

Hurricane HazardsOne of the areas most

vulnerable to storm surge is Tampa Bay, FL. This is of major concern, because over 3 million people live the region, and it is highly vulnerable to storm surge-- particularly for a storm moving northeast or north-northeast at landfall,

Hurricane HazardsEven a Category 1

hurricane can create significant surges--up to 7' in Hillsborough County, 6' in Manatee County, 7' in Pinellas County, and 9' in Pasco County. An extreme Category 5 hurricane can create a storm surge of 28' in Hillsborough County.

Hurricane HazardsDiscussion of the

vulnerability of New Orleans from a 2004 Weather calendar:

The conservative estimates of loss of life from a direct hit from a category 5 hurricane is 25,000 deaths (recall that the case of Katrina was a category 3 tracking to the east of downtown New Orleans).

Photograph (courtesy of NASA)

Hurricane HazardsExamples of hurricanes with noteworthy

storm surge Galveston, Texas (Sept. 8, 1900)

6,000 perished Camille (Aug. 17, 1969)

Cat. 5 system with 186 mile per hour winds; 7.3 metre surge

Bangladesh (Bay of Bengal); November 13, 197o; 7 metre surge; 500,000 perished

Hurricane Hazards

Hurricane Hazards

Coastal effects of Camille (1969)

When Warnings are Ignored

The same apt. building destroyed with 22 lives lost

36 Years LaterKatrina’s landfall was

at a category 4, but with a comparable storm surge to that of Camille;

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.

Galveston Hurricane (1900)

Hurricane Hazards6,000 to 12,000 killed

of the total population of 37,000

75% of the city was destroyed

Most of the fatalities were drownings in a 7 m storm surge

The “Wall Street” of the Southwest‘High ground was only

2.5 m’Top winds were

probably 125 mph though the island’s only anemometer was swept away after recording 102 mph

Hurricane HazardsWarnings 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

Saturday, Sept 8, 1900Atmospheric

pressures plunged in the morning

Winds increasedA steamship was

torn from its moorings and promptly smashed through the three bridges to the mainland

Saturday, Sept 8, 1900By evening, the winds

had shifted to easterly, bringing in the waters of the Gulf of Mexico

The island was flooded, and residents flocked to ‘higher ground’ (8 ft.) as ‘rats clinging to the sinking mast of a ship’

Sunday MorningNearly everyone

in the city lost some family and friends

Flood waters had drained back to seas exposing bodies and ruins of the city

Isaac ClineContributed to the city’s complacency by dismissing

the notion that a hurricane could destroy GalvestonThis attitude contributed to the lack of any seawall

construction

Isaac ClineSent observations and warnings throughout the hurricane’s fury

Endured personal tragedy (wife died) during the storm

Lessons learned

The existence of the new sea wall prevented major loss of life during a 1915 hurricane in which eight people were killed

Wind DamageThe two storms

causing the most widespread wind damage in the US were Hurricanes Camille (1969) and Andrew (1992)

An entire neighborhood leveled by Andrew (1992)

Flooding-Floyd (199)The hurricane 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

Flooding-Floyd (1999)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.

Flooding-Floyd (1999)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.

Flooding-Floyd (1999)Runoff from the

hurricane created significant problems for the ecology of North Carolina's rivers and sounds.

Freshwater runoff, sediment, and decomposing organic matter caused salinity and oxygen levels in Pamlico Sound and its tributary rivers to drop to nearly zero.

ForecastingThere 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.

ForecastingThe 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.

ForecastingBecause of the

inherit uncertainty in the exact track forecast, the national Hurricane Center issues forecasts that include an ever expanding envelope of threat area.

ForecastingSome forecasts

however, have less confidence than others.

Consider the spread in the various track forecasts from different models for Hurricane Katrina when Katrina was crossing Florida.

ForecastingHowever, once the

storm moved into the Gulf of Mexico and intensified, forecast models came into better agreement concerning the track of Katrina.

ForecastingForecasters are less

skillful at predicting the intensity of tropical cyclones than cyclone track.

The lack of improvement in intensity forecasting is due to the complexity of tropical systems and an incomplete understanding of their internal dynamics.

ForecastingHurricane Hunters

are aircraft that fly into tropical cyclones in the North Atlantic Ocean and Northeastern Pacific Ocean for the specific purpose of directly measuring weather data in and around those storms.

1) Need to know how intense the storm is.

2) Need adequate data to determine where the storm will track.

3) Need to know the details of the hurricane to verify computer models of hurricanes.

ForecastingNamingStorms 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 usedThe 2005 list will be used again in 2011 (minus

Dennis, Katrina, Rita, Stan, and Wilma)

ForecastingNamingStorms 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 usedThe 2005 list will be used again in 2011 (minus

Dennis, Katrina, Rita, Stan, and Wilma)

Long Term TrendsWhile 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.

References Gray, W. M., C. W. Landsea, P. W. Mielke, Jr., and K. J. Berry, 1994:

Predicting Atlantic Basin seasonal tropical storm activity by June 1. Weather Forecasting, 9, 103-115.

Larson, E., 2000: Isaac’s storm: A man, a time, and the deadliest hurricane in history. Vintage.

Organization of American States: Primer on Natural Hazard Management in Integrated Regional Development Planning. Available at http://www.oas.org/osde/publications/Unit/oea66e/begin.htm#Contents

Toomey, D., 2002: Storm chasers: The hurricane hunters and their fateful flight into Hurricane Janet. W. W. Norton and Co.

Web Siteshttp://www.srh.noaa.gov/jetstream//tropics/http://en.wikipedia.org/wiki/Hurricanehttp://en.wikipedia.org/wiki/Hurricane_Floydhttp://en.wikipedia.org/wiki/Hurricane_Andrewwww.nhc.noaa.govwww.aoml.noaa.gov/hrd