☁️Meteorology Unit 11 – Tropical Weather Systems and Hurricanes

Key Concepts and Definitions

• Tropical cyclone: a low-pressure system that forms over warm tropical oceans and has a closed low-level atmospheric circulation
• Hurricane: a tropical cyclone with sustained winds of 74 mph (119 km/h) or higher in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and eastern North Pacific Ocean
  • Known as typhoons in the western North Pacific and cyclones in the Indian Ocean and South Pacific
• Eye: the calm, clear center of a hurricane characterized by low pressure, light winds, and descending air
• Eyewall: the ring of thunderstorms surrounding the eye where the strongest winds and heaviest rainfall occur
• Saffir-Simpson Hurricane Wind Scale: categorizes hurricanes based on their maximum sustained wind speeds from Category 1 (74-95 mph) to Category 5 (157 mph or higher)
• Storm surge: an abnormal rise in sea level generated by a storm, potentially causing severe coastal flooding
• Latent heat: the energy released or absorbed during a phase change (evaporation or condensation) without a change in temperature, crucial for hurricane development

Tropical Weather Patterns

• Intertropical Convergence Zone (ITCZ): a low-pressure belt near the equator where the Northeast and Southeast Trade Winds converge, creating favorable conditions for thunderstorm development
• Easterly waves: areas of low pressure that move westward across the tropics, often serving as seedlings for tropical cyclone formation
• Monsoon troughs: elongated areas of low pressure associated with the seasonal shift of the ITCZ, contributing to enhanced rainfall and potential cyclone development in regions like the Indian Ocean and western Pacific
• Sea surface temperatures (SSTs): warm ocean waters (typically above 26.5°C or 80°F) that provide the necessary energy for tropical cyclone formation and intensification
• Wind shear: the change in wind speed or direction with height, with high wind shear disrupting tropical cyclone development and low wind shear favoring intensification
• Saharan Air Layer (SAL): a dry, dusty air mass that originates over the Sahara Desert and can suppress Atlantic hurricane activity by introducing dry air and increasing wind shear

Hurricane Formation and Structure

• Pre-existing disturbance: hurricanes typically develop from pre-existing areas of low pressure, such as easterly waves or monsoon troughs
• Warm ocean waters: SSTs above 26.5°C (80°F) provide the necessary heat and moisture for hurricane development
• Coriolis force: the apparent force caused by the Earth's rotation that helps to create the cyclonic (counterclockwise in the Northern Hemisphere) rotation of hurricanes
• Convergence: the inflow of air towards the center of the storm, causing rising motion and thunderstorm development
• Divergence: the outflow of air at the top of the storm, allowing for continued rising motion and intensification
• Eyewall replacement cycle: a process in which a new outer eyewall forms and gradually replaces the original inner eyewall, often leading to temporary weakening followed by re-intensification
• Rainbands: spiral-shaped bands of showers and thunderstorms that extend outward from the center of the hurricane

Hurricane Classification and Intensity

• Saffir-Simpson Hurricane Wind Scale: categorizes hurricanes based on their maximum sustained wind speeds
  • Category 1: 74-95 mph
  • Category 2: 96-110 mph
  • Category 3: 111-129 mph (considered a major hurricane)
  • Category 4: 130-156 mph (considered a major hurricane)
  • Category 5: 157 mph or higher (considered a major hurricane)
• Maximum sustained winds: the highest one-minute average wind speed measured at a height of 10 meters above the surface
• Central pressure: the atmospheric pressure at the center of the hurricane, with lower pressures generally indicating a more intense storm
• Rapid intensification: a significant increase in maximum sustained winds (typically 35 mph or more) within a 24-hour period, often associated with more dangerous and unpredictable hurricanes
• Weakening factors: include increasing wind shear, dry air intrusion, movement over cooler waters, and interaction with land

Tracking and Forecasting Methods

• Satellite imagery: provides a visual representation of the storm's structure, intensity, and movement
  • Visible satellite imagery shows the storm's cloud patterns during daylight hours
  • Infrared satellite imagery detects the temperature of cloud tops, with colder temperatures indicating higher, more developed clouds
• Weather radar: detects precipitation and wind speeds within the storm, helping to identify the eye, eyewall, and rainbands
• Reconnaissance aircraft: specially equipped planes that fly into hurricanes to directly measure wind speeds, pressure, and other variables
• Computer models: numerical weather prediction models that simulate the atmosphere and ocean to forecast hurricane track and intensity
  • Dynamical models: solve physical equations to predict storm behavior (e.g., GFS, ECMWF, HWRF)
  • Statistical models: use historical relationships between storm characteristics and environmental factors to forecast intensity (e.g., SHIPS, LGEM)
• Ensemble forecasting: running multiple forecasts with slightly different initial conditions or model configurations to assess uncertainty and improve overall forecast accuracy
• National Hurricane Center (NHC): the U.S. government agency responsible for issuing official hurricane forecasts, watches, and warnings for the Atlantic and eastern Pacific basins

Impacts and Hazards

• Storm surge: the abnormal rise in sea level generated by a hurricane, often the deadliest and most destructive hazard
  • Factors influencing storm surge include storm intensity, size, forward speed, and coastal bathymetry
• Heavy rainfall: can cause inland flooding, particularly when a storm moves slowly or stalls over an area
• Strong winds: can cause structural damage, power outages, and tree damage
• Tornadoes: can form in the outer rainbands of hurricanes, adding to the storm's destructive potential
• Rip currents: strong, narrow channels of fast-moving water that can pose a drowning risk to swimmers along the coast
• Economic losses: hurricanes can disrupt businesses, agriculture, and infrastructure, leading to significant economic impacts
• Environmental impacts: storms can cause erosion, saltwater intrusion, and damage to coastal ecosystems such as coral reefs and mangroves

Preparedness and Response Strategies

• Emergency planning: developing and practicing hurricane response plans at the individual, community, and government levels
• Evacuation: moving people away from areas threatened by storm surge, flooding, or other hazards
  • Evacuation orders: issued by local officials based on the storm's projected impact and the vulnerability of the area
  • Evacuation routes: designated roads and highways that facilitate the safe and efficient movement of people away from danger
• Sheltering: providing safe locations for people to stay during the storm, such as schools, community centers, or hotels
• Communication: disseminating timely and accurate information to the public through various channels (e.g., news media, social media, emergency alerts)
• Supplies and provisions: ensuring adequate food, water, medicine, and other essentials are available for the duration of the storm and its aftermath
• Post-storm assessment: evaluating the storm's impact, identifying areas of need, and coordinating relief efforts
• Resilience and mitigation: implementing strategies to reduce the long-term risk and vulnerability to hurricanes, such as building codes, infrastructure improvements, and coastal restoration projects

Case Studies and Notable Hurricanes

• Hurricane Katrina (2005): one of the deadliest and costliest hurricanes in U.S. history, causing widespread damage and over 1,800 deaths along the Gulf Coast, particularly in New Orleans
• Hurricane Harvey (2017): a slow-moving storm that brought record-breaking rainfall and catastrophic flooding to the Houston metropolitan area
• Hurricane Maria (2017): a Category 5 hurricane that devastated Puerto Rico, causing widespread damage to infrastructure and thousands of deaths
• Hurricane Dorian (2019): a powerful Category 5 storm that stalled over the Bahamas, causing unprecedented damage and loss of life
• Hurricane Ida (2021): a Category 4 hurricane that made landfall in Louisiana on the 16th anniversary of Hurricane Katrina, causing significant wind damage and storm surge flooding
• Hurricane Ian (2022): a Category 4 storm that caused severe damage and numerous deaths in southwestern Florida, particularly in the Fort Myers area
• Hurricane Camille (1969): a compact but powerful Category 5 hurricane that caused catastrophic damage along the Mississippi coast
• Hurricane Andrew (1992): a Category 5 storm that caused extensive damage in South Florida and prompted significant changes to building codes and hurricane preparedness strategies