Hurricanes review


Tropical weather
  • Lots of solar heating which favors development of showers, weak pressure gradients and regular trade winds
  • Little seasonal change, plus Hadley cells make weather very regular
  • ITCZ movement & monsoons cause seasonal variations in some places
  • Occasionally tropical (easterly) wave of lower pressure develops, looking like inverted trough
  • This provides a region of convergence from which hurricane might form



    • What is a hurricane?
  • Different names in different regions - Most general is tropical cyclone
  • Called "hurricane" in Atlantic and eastern Pacific, "typhoon" in western Pacific, "cyclone" in Indian Ocean region
  • By any name, still an organized rotating storm system of tropical origin
  • Structure shown in EOM Fig. 11.3 - eye at center with little cloudiness and lowest pressure, eye wall of most intense thunderstorms around center, spiral bands of thunderstorms wrapping around system



    • Hurricane formation
  • Form only at lower latitudes since they require a deep (~200m) layer of warm (80°F+) water
  • Hurricane season is summer and fall since that is when ocean is warmest
  • Rotation of storm needs some Coriolis force so they cannot form at the Equator
  • May form at point of surface convergence - possibilities include easterly waves, ITCZ, organized mass of thunderstorms, old front from middle latitudes
  • Certain conditions aloft hinder development - sinking air suppresses convection and strong winds aloft shear apart storms
  • Still debate about exact mechanism that drives storm - but certain is that warm ocean surface is source of energy
  • Condensation of water vapor (which originated from the surface) up in storm provides heat that powers it
  • Thus storm lasts as long as over warm water and other weather conditions are favorable
  • Dissipates rapidly when moving over land since energy source is lost and greater friction at surface slows winds



    • Hurricane development
  • Stages that are somewhat arbitrarily defined
  • Tropical disturbance/tropical wave is just a mass of thunderstorms with evidence of weak circulation
  • Tropical depression when winds are 23-39mph and central pressure a few millibars lower than surrounding pressure
  • Tropical storm when winds are 40-73mph - this is the point where name is given
  • Called hurricane when winds 74+mph



    • Hurricane movement
  • They form in tropics and tend to move westward in the easterly winds that prevail at low latitudes
  • Some tendency to go poleward since the Coriolis force is slightly stronger on the poleward side of the storm
  • Cyclones generally move around the subtropical high pressure centers, eventually turning poleward around them
  • Any given path may be very meandering and unpredictable as circulation of storm and its interaction with the large-scale flow steer the system



    • Hurricane destruction
  • Wind speed at surface it faster on the side of the storm where the winds are blowing the same direction as the storm is moving because you add the rotational speed of the winds around the storm to the translational speed of the storm movement
  • Strong winds may cause substantial damage, some hurricanes spawn tornadoes, heavy precipitation may occur from the bands of thunderstorms
  • But the part of the storm that generally causes the most death and destruction is the storm surge because of the rise is water levels
  • Storm surge may be 10-20 feet, but no more than 2-3 feet due to the low pressure at the storm center
  • The biggest contributor to the storm surge is the winds piling of the water as it moves toward shore



    • Compare/contrast extratropical and tropical cyclones
  • Winds - both cyclonically rotating with low pressure center
  • Environment for development
  • Energy source
  • Lifetime
  • Vertical structure
  • Center