🌦️Atmospheric Science Unit 11 – Tropical Weather and Hurricane Systems

Key Concepts and Definitions

• Tropical cyclone: a low-pressure system that forms over warm tropical oceans and has a closed low-level atmospheric circulation
• Eye: the calm, clear center of a tropical cyclone surrounded by the eyewall
• Eyewall: the ring of thunderstorms surrounding the eye where the strongest winds and heaviest rainfall occur
• Tropical disturbance: an area of thunderstorms in the tropics that maintains its identity for 24 hours or more
  • Considered the first stage of tropical cyclone development
• Tropical depression: a tropical cyclone with maximum sustained winds of 38 mph (33 knots) or less
• Tropical storm: a tropical cyclone with maximum sustained winds between 39 mph (34 knots) and 73 mph (63 knots)
• Hurricane: a tropical cyclone with maximum sustained winds of 74 mph (64 knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and the eastern North Pacific Ocean
  • Called typhoons in the western North Pacific Ocean and tropical cyclones in the Indian Ocean and South Pacific Ocean
• Subtropical cyclone: a low-pressure system that has characteristics of both tropical and extratropical cyclones

Tropical Climate Characteristics

• Warm sea surface temperatures (SSTs) of at least 26.5°C (80°F) to a depth of 50 meters are necessary for tropical cyclone development
• Weak vertical wind shear allows for the vertical development of thunderstorms and the organization of the tropical cyclone
• High relative humidity in the mid-troposphere (around 5 km or 3 miles) is essential for the development and maintenance of deep convection
• Sufficient Coriolis force is required for the low-pressure system to develop and maintain a closed circulation
  • Generally, tropical cyclones do not form within 5° of the equator due to the weak Coriolis force
• Instability in the lower atmosphere allows for the development of deep convection and thunderstorms
• Divergence aloft helps to remove air from the top of the system, allowing for continued rising motion and development
• Monsoon troughs and easterly waves can provide the initial disturbance necessary for tropical cyclone formation

Formation of Tropical Cyclones

• Pre-existing disturbance: Tropical cyclones often develop from pre-existing areas of convection, such as tropical waves or monsoon troughs
• Warm core development: As the disturbance organizes and convection intensifies, latent heat release warms the core of the system, causing air pressure to drop
• Convergence and rising motion: The low-pressure center causes air to converge and rise, leading to increased thunderstorm activity
• Closed circulation: As the system strengthens, a closed low-level circulation develops, marking the formation of a tropical depression
• Continued intensification: If conditions remain favorable (warm SSTs, low wind shear, high humidity), the tropical depression can intensify into a tropical storm and eventually a hurricane
• Eye formation: As the hurricane strengthens, an eye develops at the center due to subsiding air, surrounded by the eyewall where the strongest winds and heaviest precipitation occur
• Secondary circulation: The primary circulation (rotational wind) is accompanied by a secondary circulation, characterized by inflow near the surface, rising motion in the eyewall, and outflow aloft

Hurricane Anatomy and Structure

• Eye: the calm, clear center of the hurricane with subsiding air and light winds
  • Typically 20-50 km (12-30 miles) in diameter
• Eyewall: the ring of intense thunderstorms surrounding the eye
  • Contains the strongest winds and heaviest rainfall
  • Primary energy source for the hurricane
• Rainbands: spiral bands of showers and thunderstorms that extend outward from the eyewall
  • Can cause heavy rainfall, strong winds, and tornadoes
• Outflow: high-altitude winds that flow radially outward from the top of the hurricane
  • Helps to remove air from the top of the system, allowing for continued rising motion
• Inflow: low-level winds that flow radially inward towards the center of the hurricane
  • Supplies moisture and energy to the system
• Radius of maximum winds: the distance from the center of the hurricane to the location of the strongest winds, typically found in the eyewall
• Size: hurricanes can vary greatly in size, with the radius of tropical storm-force winds ranging from 50-1000 km (30-620 miles)

Hurricane Classification and Intensity Scales

• Saffir-Simpson Hurricane Wind Scale: categorizes hurricanes based on their maximum sustained wind speeds
  • Category 1: 74-95 mph (64-82 knots)
  • Category 2: 96-110 mph (83-95 knots)
  • Category 3: 111-129 mph (96-112 knots)
  • Category 4: 130-156 mph (113-136 knots)
  • Category 5: 157 mph (137 knots) or higher
• Accumulated Cyclone Energy (ACE): a measure of the total energy generated by a tropical cyclone over its lifetime
  • Calculated by summing the squares of the maximum sustained wind speed at 6-hour intervals
• Integrated Kinetic Energy (IKE): a measure of the destructive potential of a tropical cyclone based on its size and wind speed
  • Accounts for the distribution of wind speeds within the storm, not just the maximum sustained winds
• Central pressure: the atmospheric pressure at the center of the hurricane
  • Lower central pressure generally indicates a more intense hurricane
• Potential intensity: the theoretical maximum intensity a tropical cyclone can achieve given the environmental conditions (SST, atmospheric temperature profile, and moisture content)

Forecasting and Tracking Methods

• Numerical weather prediction models: computer models that simulate the atmosphere and ocean to predict the track and intensity of tropical cyclones
  • Examples: Global Forecast System (GFS), European Centre for Medium-Range Weather Forecasts (ECMWF), and Hurricane Weather Research and Forecasting (HWRF) models
• Statistical models: use historical relationships between storm characteristics and environmental factors to predict tropical cyclone behavior
  • Example: Statistical Hurricane Intensity Prediction Scheme (SHIPS)
• Consensus forecasts: combine the forecasts from multiple models to create a single, more reliable prediction
• Satellite imagery: used to monitor the location, structure, and intensity of tropical cyclones
  • Visible, infrared, and microwave imagery provide information on cloud cover, temperature, and precipitation
• Aircraft reconnaissance: specially equipped aircraft that fly into hurricanes to measure wind speed, pressure, temperature, and humidity
  • Provides valuable data for initializing and verifying forecast models
• Radar: land-based and aircraft-mounted radar systems provide detailed information on the precipitation structure and wind speeds within the hurricane
• Buoys and surface observations: measure wind speed, pressure, and wave heights at the ocean surface, providing ground-truth data for forecast models

Impacts and Hazards

• Storm surge: the abnormal rise in sea level caused by the hurricane's wind and low pressure
  • Can cause severe coastal flooding and damage
• Heavy rainfall: hurricanes can produce extreme rainfall rates and totals, leading to inland flooding
  • Rainfall amounts can exceed 10 inches (250 mm) in a single day
• Strong winds: hurricane-force winds can cause extensive damage to structures, infrastructure, and vegetation
  • Wind damage is often most severe in the eyewall and inner rainbands
• Tornadoes: hurricanes can spawn tornadoes, particularly in the outer rainbands and during landfall
• Rip currents: strong, localized currents that can pose a drowning risk to swimmers and surfers
• Economic losses: hurricanes can cause billions of dollars in damage to property, infrastructure, and agriculture
  • Disruptions to transportation, power supply, and other critical services can have long-lasting economic impacts
• Human health and safety: hurricanes can cause injuries, fatalities, and mental health impacts due to the direct effects of the storm and the challenges of the recovery process
  • Displacement, power outages, and water contamination can pose additional risks to human health

Climate Change and Tropical Weather Systems

• Warmer sea surface temperatures: as the climate warms, SSTs are increasing, providing more energy for tropical cyclone formation and intensification
• Slower storm motion: some studies suggest that tropical cyclones may move more slowly in a warmer climate, increasing the duration of impacts such as heavy rainfall and storm surge
• Heavier precipitation: warmer air can hold more moisture, leading to an increase in the amount of rainfall associated with tropical cyclones
• Rapid intensification: the frequency of rapidly intensifying hurricanes (those that strengthen by 35 mph or more in 24 hours) may increase in a warmer climate
• Poleward shift: the location of peak tropical cyclone intensity may shift poleward in response to the expansion of the tropics due to climate change
• Uncertain frequency changes: while the overall frequency of tropical cyclones may not change significantly, some studies suggest a possible decrease in the total number of storms but an increase in the proportion of intense hurricanes
• Regional variations: the response of tropical cyclones to climate change may vary by region, with some areas experiencing more significant changes than others
• Coastal vulnerability: sea-level rise due to climate change can exacerbate the impacts of storm surge and coastal flooding, increasing the vulnerability of coastal communities to hurricane impacts