Lecture April 17

Hurricanes

• Definition: A hurricane is a system characterized by a low-pressure center surrounded by organized thunderstorms that produce strong winds and heavy rain.
• Key Structures: Eye, eyewall, and rainbands.

Eye of the Hurricane

• Characteristics:

  • The eye is the calmest part of the hurricane and has light, variable winds.
  • Maximum winds and precipitation occur outside the eye in the eyewall.
  • As you move from the eye outwards, wind speeds increase due to the steep pressure gradient.

• Size of the Eye:

  • A larger eye may be associated with a larger hurricane, but this is not a definitive rule; intensity determines size.

Forces in a Hurricane

• Pressure Gradient Force:

  • Located steepest right outside the eyewall, where max wind speeds and precipitation are found.
  • Maximum convergence (air rising) occurs in the eyewall due to latent heat release from condensation of water vapor.

• Rotation Mechanics:

  • Conservation of angular momentum leads to winds moving faster nearer the eye and slower on the outer bands due to friction and drag.

Requirements for Hurricane Formation

1. Warm Sea Surface Temperatures: Essential for energy; hurricane formation typically occurs where sea surface temperature is above 26.5^{ ext{°C}}.
2. Coriolis Effect: Necessary for rotation; generally formed between 5^{ ext{°}} to 15^{ ext{°}} latitude.
3. Upper Air Flow Support: Allows for divergence of air, promoting convection.

Areas of Hurricane Formation

• Dominant Areas:
  • Tropical Western Pacific has the highest frequency of strong cyclones due to larger warm water regions.
  • Hurricanes are less common near South America due to cold upwelling and deeper ocean depths.

Impact of El Niño and La Niña

• El Niño:
  • Associated with increased wind shear and turbulence, leading to fewer hurricanes.
• La Niña:
  • Generally results in a favorable environment for hurricane formation, leading to higher frequency.

Stages of Hurricane Development

1. Tropical Disturbance: Random convective activity; no organization.
2. Tropical Depression: Begins organizing with a weak low-pressure center and slight rotation.
3. Tropical Storm: Consistent wind speeds above 39 mph, further organization.
4. Hurricane: Wind speeds exceed 73 mph; significant rotation and organized structure develop.

Factors Influencing Hurricane Strength

• Central Pressure: Inversely correlated with wind speed; the lower the central pressure, the stronger the winds.
• Movement Speed: Hurricanes that move faster typically cause less severe impacts due to shorter exposure times.

Storm Surge and Rainfall

• Storm Surge:

  • Caused by high winds pushing water onshore; can significantly increase water levels during high tide.
  • Areas to the east/southeast of the hurricane's track are generally worst affected due to winds pushing water into shore.

• Rainfall Distribution: Determined by interaction of outer bands with land; significant regional impacts can vary even small distances apart.

Hurricane Case Studies

• Hurricane Mitch (1998): Major impacts in Central America; resulted in widespread fatalities due to flooding and landslides.
• Hurricane Katrina (2005): Infamous for its catastrophic flooding in New Orleans; highlighted infrastructure vulnerabilities during major storm events.
• Hurricane Sandy (2012): Fast-moving storm that caused significant flooding in New York City despite hitting during low tide; emphasized the need for infrastructural improvements in urban areas.

Summary of Categories (Saffir-Simpson Scale)

• Category 1: Winds 74-95 mph; minimal damage.
• Category 2: Winds 96-110 mph; moderate damage.
• Category 3: Winds 111-129 mph; significant damage (major hurricane).
• Category 4: Winds 130-156 mph; devastating damage (major hurricane).
• Category 5: Winds 157 mph or greater; catastrophic damage (major hurricane).