11.1 Tropical cyclone formation and development
4 min read•july 31, 2024
Tropical cyclones form in specific ocean regions with warm waters and . They start as disturbances, grow into storms, and can become powerful hurricanes. Understanding their formation is key to predicting their paths and potential impacts.
As cyclones develop, they intensify through stages, from tropical depressions to full-blown hurricanes. Warm oceans fuel their growth, while atmospheric conditions like low wind shear allow them to strengthen. The gives them their distinctive spin.
Tropical Cyclone Formation
Ocean and Atmospheric Conditions
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- exceeding 26.5°C (80°F) to a depth of at least 50 meters fuel tropical cyclone formation
- Low vertical wind shear less than 10 m/s (20 knots) between surface and upper troposphere enables cyclone development
- Moist mid-troposphere with relative humidity values of at least 50-60% supports and cyclone growth
- Pre-existing atmospheric disturbance or weak low-pressure system provides starting point for cyclone formation
- Distance of at least 5° latitude from equator supplies sufficient Coriolis force for cyclone rotation
- Upper-level divergence and lower-level convergence in atmosphere create favorable environment for cyclone intensification
- Sea surface temperatures warmer than overlying air promote instability and convection
Examples of Favorable Formation Regions
- Western Pacific Ocean (South China Sea, Philippine Sea)
- Eastern Pacific Ocean (off the coast of Mexico and Central America)
- North Atlantic Ocean (Caribbean Sea, Gulf of Mexico)
- Bay of Bengal and Arabian Sea in the Indian Ocean
- Southwest Indian Ocean (east of Madagascar)
Stages of Tropical Cyclone Development
Early Stages
- Tropical disturbance forms as cluster of thunderstorms with little or no circulation and winds below 20 knots (23 mph)
- develops with organized system of clouds and thunderstorms, defined circulation, and maximum sustained winds up to 33 knots (38 mph)
- emerges as organized system with defined circular rotation and maximum sustained winds between 34-63 knots (39-73 mph)
- Tropical storms receive names from predetermined lists for each basin
Hurricane Stage
- (or /cyclone) intensifies into intense tropical weather system with well-defined circulation and maximum sustained winds of 64 knots (74 mph) or higher
- Hurricanes categorized on Saffir-Simpson Hurricane Wind Scale
- 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
- and structure typically form during hurricane stage
- Most intense winds and heaviest precipitation occur in eyewall
- Central pressure drops as system intensifies
- Wind field expands, increasing overall size and potential impact of hurricane
Examples of Notable Hurricanes
- Hurricane Katrina (2005): Category 5, devastated New Orleans and Gulf Coast
- Hurricane Patricia (2015): Strongest recorded hurricane in Western Hemisphere with 215 mph winds
- Typhoon Tip (1979): Largest tropical cyclone on record with a diameter of 1,380 miles
Conditions for Intensification
Oceanic Factors
- Warm ocean waters act as primary energy source for tropical cyclones
- Sea surface temperatures above 26.5°C (80°F) allow sufficient evaporation and latent heat release
- Oceanic heat content determines available energy supply for sustained intensification
- Deeper warm water layer supports longer-lasting and more intense cyclones
- Examples of high oceanic heat content regions
- Loop Current in Gulf of Mexico
- Western Pacific Warm Pool
Atmospheric Factors
- Low vertical wind shear allows vertical alignment of cyclone's structure
- Wind shear below 10 m/s (20 knots) prevents disruption of cyclone's warm core
- Low shear enables development of deep convection around center
- Combination of warm waters and low shear creates positive feedback loop
- Increased convection leads to lower surface pressure
- Enhanced surface winds and evaporation further fuel storm
- Rapid intensification occurs with increase in maximum sustained winds of at least 30 knots (35 mph) in 24 hours
- More likely when both warm waters and low shear present
Examples of Rapid Intensification
- Hurricane Wilma (2005): Pressure dropped 97 mb in 24 hours, fastest intensification on record
- Hurricane Patricia (2015): Intensified from Category 1 to Category 5 in 24 hours
Coriolis Effect and Rotation
Fundamentals of Coriolis Effect
- Coriolis effect causes apparent deflection of moving objects relative to Earth's surface
- Results from planet's rotation
- Northern Hemisphere cyclones rotate counterclockwise
- Southern Hemisphere cyclones rotate clockwise
- Coriolis force negligible near equator (within about 5° latitude)
- Strength increases towards poles
Impact on Tropical Cyclones
- Coriolis effect initiates and maintains rotation of tropical cyclones
- Deflects air parcels as they flow towards low-pressure center
- Balance between pressure gradient force and Coriolis force leads to gradient wind balance
- Crucial for cyclone intensification
- Coriolis effect becomes more pronounced as tropical cyclone intensifies
- Contributes to tightening of pressure gradient and increased wind speeds
- Explains why tropical cyclones rarely form near equator
- Influences cyclone track, causing poleward curve as they move away from tropics
Examples of Coriolis Effect on Cyclone Tracks
- Hurricane Irma (2017): Curved northward from Caribbean towards Florida
- Typhoon Hagibis (2019): Recurved from tropical Pacific towards Japan
Key Terms to Review (19)
Atmospheric instability: Atmospheric instability refers to the tendency of air parcels to rise or fall in the atmosphere, depending on their temperature and moisture content compared to the surrounding air. This condition is critical for the development of storms and severe weather events, as it determines whether rising air can continue to ascend, leading to the formation of clouds and precipitation. In the context of tropical cyclones, atmospheric instability plays a vital role in their formation, intensification, and overall dynamics.
Convection: Convection is the process of heat transfer through the movement of fluids (liquids or gases) due to differences in temperature and density. This natural phenomenon plays a key role in various atmospheric processes, influencing everything from weather patterns to cloud formation and storm development.
Coriolis effect: The Coriolis effect is the apparent deflection of moving objects, such as air or water, due to the rotation of the Earth. This phenomenon influences global wind patterns, storm systems, and ocean currents, leading to the characteristic rotation of weather systems and variations in local wind behavior.
El Niño: El Niño is a climate pattern characterized by the periodic warming of ocean surface temperatures in the central and eastern Pacific Ocean, significantly impacting global weather patterns. This phenomenon leads to changes in atmospheric conditions, affecting rainfall, temperature, and storm activities around the world, particularly influencing tropical cyclones and various climate zones.
Eye: The eye is the calm center of a tropical cyclone, characterized by clear skies and low wind speeds, surrounded by the intense storm structure of the eyewall. This unique feature plays a crucial role in the formation and development of tropical cyclones, as well as in understanding their structure and intensity classification. The eye can vary in size and shape, influencing the overall behavior and impact of the storm.
Eyewall: The eyewall is the area of intense thunderstorms that surrounds the eye of a tropical cyclone, acting as the storm's most destructive part. It is characterized by strong winds, heavy rainfall, and the most severe weather conditions associated with the storm. The eyewall plays a crucial role in the formation and intensity of hurricanes, influencing both their development and structure.
High humidity: High humidity refers to the condition when the amount of moisture in the air is significantly elevated, which can affect weather patterns and atmospheric phenomena. This level of moisture is crucial for the development and intensification of tropical cyclones, as it promotes the condensation process that fuels storm systems. In addition, high humidity can lead to the formation of clouds, precipitation, and increased wind activity associated with these powerful storms.
Hurricane: A hurricane is a type of tropical cyclone characterized by its organized structure, strong winds, and heavy rainfall. These powerful storms form over warm ocean waters and can cause extensive damage to coastal areas through high winds, storm surges, and flooding. Understanding hurricanes involves exploring their development and the significant impacts they can have on communities and ecosystems.
Low wind shear: Low wind shear refers to a small change in wind speed and direction with height in the atmosphere. In the context of tropical cyclones, low wind shear is crucial as it allows for the vertical alignment of the storm, facilitating its development and intensification. When wind shear is low, the storm can maintain its structure, allowing warm, moist air to rise efficiently and fuel the cyclone's growth.
Madden-Julian Oscillation: The Madden-Julian Oscillation (MJO) is a significant climate phenomenon characterized by a pattern of tropical rainfall and atmospheric circulation that travels eastward around the globe, typically taking about 30 to 60 days to complete one cycle. It plays a crucial role in influencing weather patterns, particularly in the tropics, and affects the formation and development of tropical cyclones by altering wind patterns and moisture distribution in the atmosphere.
Numerical weather prediction models: Numerical weather prediction models are advanced mathematical algorithms used to simulate and forecast atmospheric conditions by solving complex equations related to fluid dynamics and thermodynamics. These models rely on observational data to provide a detailed representation of the atmosphere, enabling meteorologists to predict weather patterns, track storms, and analyze phenomena such as frontogenesis and tropical cyclones. By using high-performance computing, these models can process vast amounts of data from various sources, including satellite and radar observations, leading to more accurate forecasts.
Radar observations: Radar observations are measurements and data collected using radar technology to detect and monitor atmospheric phenomena, including precipitation, wind patterns, and storm structures. This technology is essential in meteorology, as it allows for real-time tracking of weather systems, particularly during the formation and development of tropical cyclones. By analyzing radar data, meteorologists can gain insights into storm intensity, structure, and movement, which are crucial for issuing warnings and forecasts.
Satellite imagery: Satellite imagery refers to the images of the Earth taken from satellites orbiting the planet. These images are crucial for observing weather patterns, tracking storms, and understanding various atmospheric phenomena, as well as for studying climate trends and environmental changes over time.
Storm surge: Storm surge is the abnormal rise in sea level generated by a storm, primarily caused by the wind and atmospheric pressure changes associated with tropical cyclones. This phenomenon can lead to devastating flooding in coastal areas and is often one of the most dangerous aspects of hurricanes, directly impacting both coastal infrastructure and communities.
Track forecasting: Track forecasting refers to the process of predicting the path that a tropical cyclone will take as it moves over the ocean and toward land. This involves analyzing various atmospheric and oceanic conditions, such as wind patterns, sea surface temperatures, and pressure systems, to create accurate predictions about the cyclone's trajectory. Effective track forecasting is essential for timely warnings and preparations for impacted areas.
Tropical Depression: A tropical depression is a weather system characterized by a cluster of thunderstorms organized around a defined center of low pressure, with sustained winds ranging from 23 to 39 miles per hour. It is an early stage in the development of a tropical cyclone, serving as a crucial step toward becoming a more intense storm like a tropical storm or hurricane. Understanding this term is vital in grasping the entire lifecycle of tropical cyclones and their potential impacts and hazards.
Tropical storm: A tropical storm is a type of organized system of clouds and thunderstorms that forms over warm tropical waters and produces sustained winds ranging from 39 to 73 miles per hour. These storms are a crucial stage in the development of tropical cyclones, which can intensify into hurricanes. Understanding the characteristics and behaviors of tropical storms helps in predicting their impact and the potential for further strengthening into more severe weather systems.
Typhoon: A typhoon is a powerful tropical cyclone that forms over warm ocean waters in the Northwest Pacific region, characterized by strong winds and heavy rainfall. Typhoons develop when warm, moist air rises and creates a low-pressure system, which can lead to the formation of organized storm systems that can become extremely destructive. Understanding typhoons is essential in grasping their formation processes and the severe impacts they can have on communities.
Warm ocean waters: Warm ocean waters refer to surface water temperatures that typically exceed 26.5 degrees Celsius (about 80 degrees Fahrenheit), creating favorable conditions for the development and intensification of tropical cyclones. These temperatures are critical because they provide the necessary heat and moisture that fuels storm systems, allowing them to gain strength and organization as they move over the ocean.