1. Tropical Cyclones, Hurricanes and Typhoons
Motivation (Why do we care)
Definition
Where and when do they occur
Formation and intensity
Structure
Hazards

2. World’s Deadliest Tropical Cyclones
Of 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.

3. World’s Deadliest Tropical Cyclones
The deadliest storm in the Atlantic Basin occurred in (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 (Katrina dead)

4. 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?

5. Economic Impact
The most obvious impact economically speaking is the rebuilding of damaged infrastructure.
However, other costs that are less often considered include:
The cost of evacuation
The impact on energy production
The cost of severe coastal erosion.

6. Costliest U.S. Hurricanes
Hurricane 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.

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

8. What if Andrew hit Miami?

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

10. 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.

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

12. 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.

13. Erosion

14. 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 around the world. In the:
Atlantic/Eastern Pacific Oceans - hurricanes
Western Pacific - typhoons
Indian Ocean - cyclones

15. Definition What 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

16. Definition What 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.

17. Where and When
Note 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.

18. Where and When
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.

19. 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:

20. Where and When
Worldwide, tropical cyclone activity peaks in late summer when water temperatures are warmest.

21. Where and When
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.

22. Where and When
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.

23. Formation and intensity
There are at least four main requirements for tropical cyclogenesis :
enough Coriolis force to develop a low pressure center,
a preexisting low-level focus or disturbance
sufficiently 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.

24. Formation
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.

25. Formation
Since easterly waves account for about 60% of all Atlantic basin tropical cyclones, there is a relationship between West African rain and cyclone frequency.

26. Formation
Why 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.

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

28. Formation
The 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.

29. Intensity So why do we need warm sea surface temperatures?
Consider the same schematic that we saw earlier:

30. Intensity
There 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.

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

32. 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.

33. Intensity
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.

34. Intensity 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.

35. Intensity
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.

36. Intensity
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 mid-levels of the troposphere, halting development.

37. In what sense is the wind blowing in the vicinity of the Azores High?
Counterclockwise
Clockwise
Directly towards the center
Directly away from the center

38. What is the cause of the relative warmth in the center (eye) of a hurricane?
Strong winds
Warm oceans
Condensation heating
Convergence

39. Where would you expect the most significant wind damage?B C D E A B C D E

40. Life Cycle
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 ( mph).

41. Life Cycle
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.
Tropical Storm Fay (2008)
Note that you can see some banding and symmetry

42. Life Cycle
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.

43. The Saffir-Simpson Intensity Scale

44. 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.

45. Life Cycle
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 h after landfall.

46. Storm Structure
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 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.

47. Storm Structure The 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 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. 

48. Storm Structure The 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.

49. Storm Structure 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.

50. Storm Structure Tropical Cyclone Size
Typical 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.

51. Hurricane Hazards A)Storm Surge B)Wind Damage C)Heavy rains (flooding)
D)Associated tornados
About 90% of fatalities are caused by coastal and inland storm surge

52. Hurricane Hazards
What is a storm surge? It is an km wide dome of water that sweeps over the coastline during landfall.

53. Hurricane Hazards
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

54. Hurricane Hazards
The 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.

55. Hurricane Hazards
SLOSH (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

56. Hurricane Hazards
One 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,

57. Hurricane Hazards
Even 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.

58. Hurricane Hazards
Discussion 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)

59. Hurricane Hazards Galveston, Texas (Sept. 8, 1900) 6,000 perished
Examples 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

60. Hurricane Hazards

61. Hurricane Hazards
Coastal effects of Camille (1969)

62. When Warnings are Ignored

63. The same apt. building destroyed with 22 lives lost

64. 36 Years Later
Katrina’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.

65. Galveston Hurricane (1900)

66. Hurricane Hazards
6,000 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

67. 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

68. Hurricane Hazards 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

69. Saturday, Sept 8, 1900 Atmospheric pressures plunged in the morning
Winds increased
A steamship was torn from its moorings and promptly smashed through the three bridges to the mainland

70. Saturday, Sept 8, 1900
By 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’

71. Sunday Morning
Nearly everyone in the city lost some family and friends
Flood waters had drained back to seas exposing bodies and ruins of the city

72. Isaac Cline
Contributed to the city’s complacency by dismissing the notion that a hurricane could destroy Galveston
This attitude contributed to the lack of any seawall construction

73. Isaac Cline
Sent observations and warnings throughout the hurricane’s fury
Endured personal tragedy (wife died) during the storm

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

75. Wind Damage
The 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)

76. 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

77. 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.

78. 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.

79. 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.

80. Forecasting
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.

81. Forecasting
The rule (mariners' 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 nautical miles at  hours, respectively.

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

83. Forecasting Some 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.

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

85. Forecasting
Forecasters 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.

86. Forecasting
Hurricane 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.

87. Forecasting Naming of Atlantic tropical storms has occurred since 1953
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)

88. Forecasting Naming of Atlantic tropical storms has occurred since 1953
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)

89. 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.

90. References Web Sites http://www.srh.noaa.gov/jetstream//tropics/
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,
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
Toomey, D., 2002: Storm chasers: The hurricane hunters and their fateful flight into Hurricane Janet. W. W. Norton and Co.
Web Sites