1. Tropical Cyclones Lecture 11 November 18, 2009

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5. Review from last week A severe thunderstorm is defined as a thunderstorm that produces - Hail of 1 inch diameter (in central US) or larger and/or wind gusts 58 mph or greater or a tornado Occurs most frequently during the spring and summer when there are the following atmospheric conditions: –Conditionally unstable atmosphere –Moisture –Upward vertical motion (“Lifting”) –Wind shear

7. Review from last week A tornado is defined as “a violently rotating column of air descending from a thunderstorm and IN CONTACT with the ground.” –NWS Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation

9. Tropical Cyclones A tropical cyclone is a low pressure system that develops over tropical or subtropical waters (between 30º N and 30º S) Develop in areas without a horizontal temperature gradient Does not have fronts Has a warm core center Has an organized circulation

10. Hurricane Katrina- 2005

11. Tropical Cyclone Names Depending upon location, tropical cyclones have different names around the world In the –Atlantic/Eastern Pacific Oceans: hurricane –Western Pacific: typhoon –Indian Ocean: tropical cyclone

12. Do not typically form within 5° latitude of the equator due to the lack of sufficient Coriolis Force Hurricane Cyclone Typhoon

13. Tropical Cyclone Names Since 1953, Atlantic tropical storms have been named from lists made by the National Hurricane Center (NHC) In 1979, men’s names were introduced and they alternate with the women’s names Six lists are used in rotation

14. Tropical Cyclone Names The only time that there is a change in the list is if a storm is so deadly or costly that future use of its name would be inappropriate –Essentially the storm name is “retired” –For example, Andrew 1992, Katrina 2005, Gustav and Ike 2008 In the event that more than 21 tropical cyclones occur in a season, additional storms will take names from the Greek alphabet –Alpha, Beta, Gamma, Delta, etc

15. How Do Tropical Cyclones Develop? Several favorable environmental conditions must be in place before a tropical cyclone can form: –Warm ocean waters (at least 80°F) –Potentially unstable atmosphere favorable to convection –Moist air near the middle of the troposphere –Some initial disturbance such as a thunderstorm complex, which may slowly develop –Low values of vertical wind shear between the surface and upper troposphere If these conditions persist for several days, a tropical cyclone may form

16. How Do Tropical Cyclones Strengthen? Tropical cyclones strengthen and maintain themselves through latent heat release As water vapor rises, it cools and condenses Large quantities of latent heat release warm the air, causing it to be less dense than its surroundings The ideal gas law tells us that the pressure should then decrease The storm must be over warm ocean waters to supply enough moisture

17. Stages of Hurricane Development However, when these disturbances first appear, they are called tropical depressions –Not named yet Once the disturbance has developed with surface wind speeds stronger than 39 mph, the storm is classified as a tropical storm After further strengthening and surface wind speeds greater than 74 mph, the system is upgraded to a hurricane Atlantic Hurricane season is June 1 – November 30

19. http://www.srh.noaa.gov/jetstream//tropics/tc_structure.htm Air converges in the center in a cyclonic pattern and diverges at the top. In the center of the storm, called the “eye”, the storm is mostly cloud-free

22. Tropical Cyclone Structure The main parts of a tropical cyclone are the eye, the eyewall, and the rainbands The eye –Strong rotation of the cyclone causes a vacuum of sinking air at the center which suppresses cloud formation –This creates a pocket of generally clear, calm conditions in the center –Typically 20-40 miles across –Will usually develop when the winds exceed 74 mph but cause of eye formation still not fully understood

23. Tropical Cyclone Structure The eyewall –Consists of a ring of thunderstorms that produce heavy rains –Usually produces the strongest winds of the storm –Changes in the structure of the eyewall and eye can cause changes in wind speed –Double eyewalls can form if the eye changes size

25. Tropical Cyclone Structure The rainbands –Curved bands of clouds and thunderstorms that trail away from the eyewall in a sprial fashion –Capable of producing heavy bursts of rain and wind, as well as tornadoes –Sometimes gaps in the spiral bands where no rain or wind is found –if one were to travel between the outer edge of a hurricane to its center, one would progress from light rain and wind, to dry and weak breeze, then back to increasingly heavier rainfall and stronger wind, over and over again

26. EYE EYEWALL RAINBAND HURRICANE KATRINA

27. What is storm surge? As the surface winds converge toward the center of the storm, they “pile up” ocean water Also, the decreased surface pressure raises the sea level slightly Strong waves also can increase the water height

28. What causes the most damage? A hurricane can cause damage by many different aspects –Very strong winds can damage structures –Heavy rainfall can cause flooding –Storm surge can inundate low-lying areas –Occasionally, lightning strikes and/or tornadoes can cause damage Storm surge generally causes the most damage and deaths, particularly in strong hurricanes

29. Damage from Hurricane Ivan

30. 2005 Atlantic Hurricane Season Was the most active Atlantic hurricane season in recorded history Forecasts fell far short of the actual activity Most tropical storms, hurricanes, and Category 5 hurricanes recorded in one season 27 named tropical storms, 17 became hurricanes 7 major hurricanes 4 Category 5 hurricanes – Emily, Katrina, Rita, Wilma Widespread economical effects

31. Hurricane Katrina - 2005 Katrina was the 6 th strongest hurricane on record for the Atlantic Ocean After entering the Gulf, rapidly intensified from Cat. 3 to Cat. 5 in 9 hours on August 28 Lowest pressure as Category 5 storm was 902 mb with max sustained winds of 175 mph Weakened slightly before landfall to a Category 3 Hurricane force winds extended 120 miles from the center

32. Katrina’s Path Made landfall August 29, 2005 as Cat. 3

33. Time Lapse of Hurricane Katrina http://www.youtube.com/watch?v=_SLXY RJnYm0&feature=relatedhttp://www.youtube.com/watch?v=_SLXY RJnYm0&feature=related

34. Katrina eye viewed from NOAA Hurricane Hunters August 28, 2005

35. Katrina’s Impact Largest natural disaster in the history of the U.S. Attributed to over 1830 deaths and 700 missing people, mostly in Louisiana and Mississippi Damages well in excess of $100 billion Federal disaster declarations covered 90,000 sq miles (size of United Kingdom) 8-16 inches of rain Breaching of levees caused 80% of New Orleans to be flooded