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Tropical Cyclones (i.e. Hurricanes) AOS 100 Nick Bassill April 28th 2009 A Review of Extratropical Cyclones • Recall that extratropical cyclones: - Frequently form in the mid-latitudes - Develop from pre-existing horizontal temperature gradients - Have fronts - Are not very symmetric - Have sizes up to 1000 or more kilometers in any direction An Extratropical Cyclone Different Airmasses An Extratropical Cyclone Different Airmasses Fronts (temperature gradients) An Extratropical Cyclone Different Airmasses Fronts Asymmetric An Extratropical Cyclone Different Airmasses Fronts Asymmetric Very Large An Extratropical Cyclone ~ 50º N Different Airmasses Fronts Asymmetric Very Large Mid-latitudes ~ 30º N Then What Are Tropical Cyclones? • In many ways, tropical cyclones have the opposite characteristics of extratropical cyclones: - They mostly form between 30º N and 30º S - They develop in areas without a horizontal temperature gradient - They do not have fronts - They are quite symmetric - They typically can only strengthen when over warm water, and weaken over land • However, like extratropical cyclones, tropical cyclones are also areas of low pressure Hurricane Isabel - 2003 The Storm Center, or “Eye” How Do Tropical Cyclones Develop? • First, tropical cyclones require warm water (typically greater than 80º F) • The environment must have little vertical wind shear (change in wind speed or direction with height) • Some initial disturbance, such as a complex of thunderstorms, which may slowly develop • If these conditions persist for several days, a tropical cyclone may form Common Development Regions Are In Orange How Do Tropical Cyclones Strengthen? • Most simplistically, tropical cyclones strengthen and maintain themselves through latent heat release • Large quantities of latent heat release warm the air, causing it to be less dense than surrounding air • The ideal gas law tells us that the pressure should then decrease • The storms must be over a warm ocean surface to supply enough moisture A 3-D Look Air near the center is heated much more than air at a distance Thunderstorms Air diverges at the top Air converges at the surface Comparison of extratropical vs. tropical pressure distributions Stages Of Hurricane Development • So far, only Hurricanes have been shown • However, when these disturbances first appear, they are called “Tropical Depressions” and do not receive a name • Once the disturbance is sufficiently strong, it is deemed a “Tropical Storm” and given a name, which occurs when the surface winds become stronger than 39 mph • After further strengthening, the system is upgraded to a “Hurricane” after the surface winds strengthen to greater than 74 mph From: http://www.pennwellblogs.com/tropics_watch/wp-content/uploads/2008/10/saffir-simpson.gif Storm Surge Explained • As the winds converge toward the storm center, they help “pile up” ocean water • Also, the decreased surface pressure raises the sea level slightly • Strong waves also can increase the water height What Causes The Most Damage? • Many different aspects of a hurricane can cause damage: - Very strong winds can damage structures - Heavy rainfall can cause flooding - Storm surge associated with rising ocean waters can inundate low-lying areas - Occasionally, lightning strikes and/or tornados can also cause damage The Answer Is: Storm Surge • Generally storm surge causes the most damage and deaths, particularly in strong hurricanes Damage from Hurricane Ivan Hurricane Katrina (2005) • At its most intense, Hurricane Katrina was the 6th strongest on record for the Atlantic Ocean • Its lowest pressure was 902 mb, and it’s strongest sustained winds were 175 mph • Luckily, Katrina weakened slightly before landfall • However, approximately 2500 dead and missing people are attributed to Katrina • Damages of roughly $100 billion were caused by Katrina To learn more, visit http://en.wikipedia.org/wiki/Hurricane_Katrina Hurricane Katrina’s Track http://en.wikipedia.org/wiki/Hurricane_Katrina A Story: Even “Smart” People Make Mistakes • Myself, my advisor, and a fellow graduate student wanted to experience a hurricane, so we (un)luckily chose Katrina to be our first • http://aurora.aos.wisc.edu/~bassill/katrina.html This is the point we left Madison A Map Slidell Pascagoula Actual hotel eviction notice from Slidell Next Stop: Pascagoula Specifically About Two Miles