11.2 Hurricane structure and intensity classification

Hurricanes are nature's most powerful storms, with complex structures that dictate their strength. The eye, eyewall, and rainbands work together to create a massive weather system capable of causing widespread destruction. Understanding these components is crucial for predicting hurricane behavior.

Intensity classification helps meteorologists and the public gauge a hurricane's potential impact. The Saffir-Simpson scale, based on wind speeds, provides a quick reference. However, other factors like central pressure, size, and storm surge also play vital roles in determining a hurricane's destructive power.

Hurricane Structure

Eye and Eyewall Characteristics

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• Eye forms calm center of hurricane with light winds, clear skies, and low surface pressure
  • Typically ranges from 20 to 40 miles in diameter
  • Acts as the storm's "center of rotation"
• Eyewall surrounds eye with ring of intense thunderstorms
  • Contains strongest winds and heaviest precipitation
  • Extends 5 to 30 miles outward from eye
  • Drives hurricane's energy and circulation

Rainbands and Overall Structure

• Rainbands spiral inward toward hurricane center
  • Curved bands of clouds and thunderstorms
  • Extend outward for hundreds of miles (sometimes over 300 miles)
  • Produce heavy rainfall and occasional tornadoes
• Hurricane shape roughly circular or oval
  • Vertical extent reaches up to 50,000 feet in troposphere
  • Diameter can range from 100 to over 1000 miles
• Rotation direction determined by Coriolis effect
  • Counterclockwise in Northern Hemisphere
  • Clockwise in Southern Hemisphere

Circulation Components

• Inflow layer near surface crucial for hurricane maintenance
  • Draws warm, moist air into storm's core
  • Typically extends from surface to about 3000 feet altitude
• Outflow layer at upper levels essential for intensity
  • Allows rising air to exit storm at high altitudes
  • Usually occurs above 40,000 feet
• Circulation creates pressure gradient
  • Low pressure at center, higher pressure outside
  • Drives wind flow and energy transport within storm

Hurricane Intensity Categories

Saffir-Simpson Hurricane Wind Scale

• Categorizes hurricanes from 1 to 5 based on sustained wind speeds
  • Category 1: 74-95 mph
  • Category 2: 96-110 mph
  • Category 3: 111-129 mph
  • Category 4: 130-156 mph
  • Category 5: 157 mph or higher
• Wind gusts can exceed sustained speeds by 20-30%
  • Category 3 hurricane with 120 mph sustained winds might have gusts to 150 mph
• Scale focuses solely on wind speeds
  • Does not account for storm surge, rainfall, or tornado potential
  • Limitations have led to discussions about revising or replacing the scale

Central Pressure and Intensity Relationship

• Central pressure generally decreases as hurricane intensity increases
  • Category 1: typically 980-989 millibars
  • Category 5: often below 920 millibars
• Relationship between wind speed and central pressure not linear
  • Significant pressure drops can occur between categories
  • Example: Category 4 hurricane might have 935 mb pressure, while Category 5 could reach 915 mb
• Lowest recorded pressure in Atlantic basin: 882 mb (Hurricane Wilma, 2005)
• Pressure gradient between center and outer edges drives wind speeds
  • Steeper gradient generally results in stronger winds

Hurricane Intensity and Damage

Wind-Related Damage

• Higher intensity hurricanes cause more extensive wind damage
  • Can destroy buildings, especially mobile homes and older structures
  • Uproots trees and creates dangerous projectiles
  • Example: Category 5 Hurricane Andrew (1992) leveled entire neighborhoods in Florida
• Size of hurricane affects damage extent independent of intensity
  • Larger storms impact wider areas
  • Prolong duration of hazardous conditions
  • Example: Hurricane Sandy (2012) caused widespread damage despite lower intensity due to its massive size

Storm Surge and Flooding

• Storm surge potential increases with hurricane intensity
  • Stronger winds push more water onshore
  • Can lead to severe coastal flooding and erosion
  • Example: Hurricane Katrina (2005) produced a 28-foot storm surge, devastating the Gulf Coast
• Rainfall totals and inland flooding often correlate with intensity
  • Stronger storms typically have more moisture
  • Better-organized precipitation patterns
  • Example: Hurricane Harvey (2017) dropped over 60 inches of rain in parts of Texas

Additional Hazards and Economic Impact

• Tornado formation likelihood increases with hurricane intensity
  • Stronger hurricanes generally produce more tornadoes
  • Example: Hurricane Ivan (2004) spawned 120 tornadoes across the southeastern United States
• Economic losses tend to increase exponentially with intensity
  • Critical infrastructure becomes more vulnerable
  • Large population centers at greater risk
  • Example: Hurricane Irma (2017) caused an estimated $50 billion in damages
• Damage potential influenced by factors beyond intensity
  • Population density
  • Building codes and construction quality
  • Local topography and natural barriers

Tracking Hurricane Intensity

Satellite-Based Techniques

• Satellite imagery analysis estimates intensity based on cloud patterns and temperatures
  • Dvorak technique widely used since the 1970s
  • Assigns T-numbers to estimate maximum wind speeds
• Microwave imagery penetrates cloud tops
  • Reveals internal structure of hurricanes
  • Helps assess intensity by showing eyewall definition and rainband organization
• Scatterometers measure ocean surface wind speeds and directions
  • Provide data on hurricane wind fields over open water
  • Example: Advanced Scatterometer (ASCAT) on MetOp satellites

In-Situ Measurements

• Reconnaissance aircraft fly directly into hurricanes
  • NOAA Hurricane Hunters and Air Force Reserve Hurricane Hunters
  • Measure wind speeds, pressure, and other parameters
  • Use dropsondes to collect vertical profiles of atmosphere
• Surface observations provide valuable data
  • Weather stations on land
  • Buoys and ships at sea
  • Measure wind speeds, pressure, and wave heights
  • Example: National Data Buoy Center operates network of buoys in hurricane-prone areas

Remote Sensing and Modeling

• Doppler weather radar measures wind speeds and precipitation intensity
  • Used as hurricanes approach land
  • Provides detailed look at storm structure and intensity
• Advanced computer models integrate observational data
  • Create intensity forecasts and track predictions
  • Examples include HWRF (Hurricane Weather Research and Forecasting) model and ECMWF (European Centre for Medium-Range Weather Forecasts) model
• Ensemble forecasting combines multiple model runs
  • Accounts for uncertainties in initial conditions and model physics
  • Provides probabilistic intensity forecasts