2 Introduction To Tropical Cyclones

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  • 1. Tropical Low Pressure Systems
  • 2. What is a Tropical Low Pressure System?
      • An intense low pressure system with a closed circulation. Also known as a hurricane, typhoon, cyclone and willy willies.
      • Violent storms between 200 and 650km in diameter which can last for 2-3 days
      • The most significant atmospheric hazard in terms of damage to property and loss of life
  • 3. Hurricane Mitch (visible)
  • 4. What are they like?
      • The eye is very calm. It is the center of rotation and contains cool sinking air , so clouds can’t form. The lowest pressure is found here.
      • Deep convection is concentrated near the center in the eyewall . Warm, rising air with very violent thunderstorms are found here. The strongest winds are found here.
      • Spiral rainbands branch out from the eyewall and also contain strong thunderstorms, heavy rain, and high winds.
  • 5.  
  • 6. The Birth of a Hurricane
      • Steps to becoming a hurricane:
        • Wave/Disturbance (disorganized low pressure)
        • Tropical Depression (closed circulation)
        • Tropical Storm (winds higher than 39mph)
        • Cyclones (winds higher than 74mph)
  • 7. So how does a tropical disturbance develop into a full blown cyclone, hurricane or typhoon?
  • 8. 7 Conditions (after Musk 1988)
    • Ocean with surface temperature over 26 °C with warm water to a depth of 70m
    • A disturbance in lower atmospheric circulation
    • A large enough Coriolis Force to produce circular pattern of wind (7-20° N and S of the equator)
    • Winds at all levels in the same direction ( little or no vertical wind shear)
    • Excess heat is transported away from the storm by winds in the upper troposphere
    • Rapid outflow of air in the upper troposphere is replaced by an inflow at ground level
    • Unstable air with high humidity
  • 9. The Role of Latent Heat
  • 10. Spatial Distribution (Where?)
  • 11.  
  • 12. Temporal Distribution (When?)
    • In the northern hemisphere storms can occur between June and late November
    • The vast majority of activity is concentrated in September and October. Why is this the case?
  • 13. Saffir-Simpson Intensity Scale
      • A scale used to label the strength of a tropical low pressure event by its pressure and winds.
      • CAT 1 = minimal damage, CAT 3 extensive; CAT 5 catastrophic
  • 14. Why do hurricanes, typhoons and cyclones decay and die? Under what conditions will this take place?
  • 15. Real Hurricanes Hurricane Mitch (visible) Hurricane Mitch (infrared) Hurricane Georges (radar)
  • 16. What hazards are associated with tropical low pressure systems
  • 17.  
  • 18. How do humans respond to the hazard? How can the hazard be managed?
  • 19. Modifying Vulnerability vs. Modifying the Event
  • 20. Monitoring, Forecasting and Warnings
    • Mostly done by use of geotsationary satellites, doppler radar and aircraft .
  • 21. Satellite Image Cyclone Sidr, Bay of Bengal. November 2007
  • 22. Radar Image Radar image of Hurricane Katrina, August 2005
  • 23. Aircraft Reconnaissance
  • 24. The End Product - Evacuation
    • In the US for example, The National Hurricane Center (NHC Miami, FL) then inputs this evidence into complex computer models which generate storm track and strength predictions.
    • These are used to issue watches and warnings for the areas that are likely to be affected.
    • Accurate warnings can save lives because people have time to evacuate .
    • Timing is however very important. Evacuating the Florida Keys along the only highway would take 31 hours. Accurate Hurricane warnings are only really possible 12-18 hours in advance
  • 25.  
  • 26.  
  • 27.  
  • 28. Other ways to modify vulnerability
    • Community preparedness
    • Land-use planning
  • 29. Modifying the Event
    • Environmental control
    • Hazard resistant design
    • Modifying the loss; recovery and reconstruction
  • 30. Spatial and Temporal Variations in the Human Response – Case Studies Bangladesh Katrina