Chapter 24 Tropical Cyclones Tropical Cyclones Most destructive storms on the planet Originate over tropical waters, but their paths often take them over land and into midlatitudes Names Hurricane (Atlantic and eastern Pacific) Typhoon (western Pacific) Cyclones (Indian Ocean) Tropical Cyclone Classification Classified according to the maximum average sustained wind speed. What is this in the U.S.? Rest of the world? 1
Tropical Cyclone Phases Tropical Wave* Tropical Disturbance Tropical Depression Tropical Storm Hurricane Tropical Wave Elongated area of relatively low air pressure, oriented north to south, which moves east to west across the tropics causing areas of cloudiness and thunderstorms. May serve as the precursor to tropical cyclone development Tropical Disturbance Loosely organized thunderstorms 250-600km in diameter that maintains it s identity for at least 24 hours No noticeable closed circulation 2
Tropical Depression Occurs when a cluster of thunderstorms has an identifiable surface pressure drop and closed wind circulation Winds do not exceed 34kts Assigned a number to identify it (ie Tropical depression #5) Tropical Storm System is given an actual name when winds exceed 34 kts Maintains tropical storm status as long as winds are sustained between 34 and 64 kts Hurricane Winds must exceed 64kts for a storm to become a hurricane 3
Alphabetical Skip letters Q,U,X,Y,Z Hurricane Names Alternate male/female Use names from English, Spanish and French languages, repeats every six years What happens when the list of names is exhausted? Has it ever happened and when? Saffir-Simpson Scale Rates hurricanes on scale of 1 (weakest) to 5 (strongest) Was based on combination of wind speed, central pressure and storm surge Revised in 2010 to be based solely on max sustained wind speed How many storms have been rated a category 5 in the Atlantic since 1924 through 2010? Hurricane Records When was the last Category 5 to hit the U.S.? Has more than 1 Category 5 struck land in one season? If so, when? What hurricane holds the record for the lowest pressure in the Atlantic basin? What area of the world is most prone to Hurricane damage? 4
Hurricane Tracks Hurricane Stats (1/3, ½, 2/3) of all tropical cyclones occur in the northern hemisphere Never originate within 5 degrees of equator and never cross it Rarely originate north (south) of 25 degrees Average of 80 tropical storms develop globally each year, 50% -70% reach hurricane strength Western Pacific produces highest number Form over all tropical oceans except south Atlantic and southeast Pacific Hurricane Tracks Easterly surface (Trade) winds in the tropics guide hurricane movement Coriolis can also affect hurricane movement. How? Eventually northward movement pushes them into the westerlies 5
4/19/17 Erratic Tracks Erratic Tracks Primary Hurricane Structures Eye Eyewall Spiral Rainbands 6
Cross-Sectional Structure Double Eyewalls Develops typically only in strongest of hurricanes Forms from outer rainbands contracting into closed ring around the inner rainband Eyewall Replacement Cycle Inner eyewall eventually weakens and dissipates as the outer eyewall contracts and intensifies A new outer eyewall may form again Hurricane usually a peak intensity when the inner eyewall contracts before dissipating 7
Storm Surge Abnormal rise in sea level associated with the movement of a hurricane over a coastal region Surge varies from one side of hurricane to the other depending on whether winds blow towards or away from shore Low pressure center also causes a rise Biggest right is typically just to the right of the eyewall as it makes landfall Tropical Cyclone Formation Surface convergence is required as a trigger for thunderstorms Develop in three ways ITCZ Tropical Waves Occluded systems from mid-latitudes ITCZ Intersection of the north and south Hadley cells If it intrudes far enough north/south, coriolis can begin to spin the thunderstorms 8
4/19/17 Tropical Waves Embedded in easterly flow at low levels High pressure ridges bulge southward, low pressure troughs bulge northward Flow around the trough is counterclockwise and is less than the geostrophic value Air then decelerates from the ridge crest to trough base and undergoes convergence Air must then rise Tropical Waves Tropical Waves 9
Mid-Latitude Origins Occluded systems at mid-latitudes may intrude far enough south that they move into the tropics Strong vertical shear is offset by latent heat release, which can reduce wind shear Most common in the fall, why? Tropical Cyclone Development Environments Sea surface temperature must exceed (60, 70, 80) Fahrenheit Surface layer of warm water must be a minimum of (100, 200, 300) feet deep Weak vertical shear (speed and direction) Mid-troposphere must be sufficiently moist Location must be > 5 degrees north of equator Sea Surface Temperatures Heat needed to maintain deep convection North Atlantic only maintains warm temps from July Nov Western Pacific temperature is typically always above 80 10
Depth of Warm Water Hurricanes stir the ocean, requiring a deep layer of warm water to maintain ocean temperatures to counter any cold upwelling Weak Wind Shear Strong wind shear carries clouds downstream, dispersing the latent heat that was released in their formation Vortex cannot form in upper-level flow in the presence of strong winds Moist Mid-Levels Dry air aloft forms evaporative-cooled downdrafts that can suppress the rate of thunderstorm growth 11
Range from Equator Further away = more coriolis for spin Thunderstorm Organization Not fully understood how thunderstorms can organize into a vortex and rapidly develop an eyewall Strong easterly waves can provide this rotation Rotation can be understood by examining the conservation of angular momentum Wave Pouch Initially, individual thunderstorms develop within the wave Oftentimes the cold air produced by the precipitation can inhibit additional thunderstorms Large-scale rotation is necessary to help overcome this Wave pouches develop where motions at certain points in the pouch that matches wave propagation 12
Wave Pouch Serves two important roles: Thunderstorms within the pouch continually moisten the mid-levels, which reduces evaporation. What effect does this have on the downdrafts? Promotes the formation and intensification of the early vortex Wave Pouch Wave Pouch 13
Storm Surge Enhancements Tidal Cycle Waves Shoreline shape Other Effects Heavy rains (sometimes up to 30-40 inches of rain in a matter of days) Inland Flooding High Winds Tornadoes Forecasting Data primarily collected from aircraft observations Dropsondes Numerical models Erratic behavior typically not forecast because of poor model initialization data 14
Hurricane Watches Issued for tropical storms and hurricanes that pose a threat within 48 hours Hurricane Warnings Issued for areas when tropical storm or hurricane conditions will be present in 36 hours Other Warnings Coastal Flood Watches/Warnings Heavy Surf Advisories 15
Impact of Climate Change on Hurricanes 16