Notes on Tropical Cyclones

Tropical Cyclones

Influence of Oceans and Seas

• Oceans greatly influence continental weather due to solar energy being expended on evaporation.

• Water evaporates, forms clouds, and results in precipitation.

• Intense cyclonic circulations are called tropical cyclones.

Definition and Formation

• Warm-core low-pressure systems with spiral inflow at the bottom and outflow at the top.

• Form over oceans with sea surface and air temperatures greater than $26°C$.

• Air accumulates sensible and latent heat when spiraling towards the center.

• Energy derived from the liberation of latent heat of condensation.

Nomenclature

• Non-frontal, synoptic-scale cyclones over tropical and subtropical waters with organized circulation.

• Called hurricanes in the North Atlantic, Eastern North Pacific, and South Pacific Ocean.

• Called cyclonic in the Bay of Bengal, Arabian Sea, and Western South Indian Ocean.

• Called willy-willy in the eastern part of the Southern Indian Ocean.

• Called typhoons in the Western North Pacific Ocean.

Location and Development

• Form over oceans, except in the South Atlantic and southeastern Pacific due to cooler temperatures and higher vertical wind shears.

• Develop at latitudes usually greater than $5°$ from the equator.

• Reach greatest intensity over warm tropical waters.

• Weaken upon moving inland, often causing great destruction.

Impact on the Philippines

• Prone to tropical cyclones due to geographical location.

• Experience heavy rains, flooding, and strong winds.

• Results in casualties and destruction to crops and properties.

• Requires sufficient knowledge for beneficial purposes.

Formation Requirements

• Large ocean areas with surface temperatures exceeding $26°C$ or $27°C$.

• Air lifted from lowest layers remains significantly warmer than surroundings up to 40,000 feet.

• Initial disturbances within $5°$ of the equator do not intensify until more than $5°$ away due to the Coriolis parameter.

• Weak vertical wind shear in the basic current.

Theories of Formation

1. Convective Theory:

   • Large mass of air becomes convectively unstable and moist.

   • Upward motion of air occurs.

   • Air from surroundings tends toward the low-pressure area, forming cyclonic circulation.

2. Frontal Theory:

   • Many tropical cyclones form along the front between trade winds and equatorial air in the doldrums.

   • Winds develop along this front and form tropical cyclones when conditions are favorable.

Development Conditions

• Sea surfaces must have a temperature of at least $26°C$.

• Heat of condensation supports the process once started.

• Generated in disturbances along the intertropical convergence zone, on traverse waves, or under superimposed upper disturbances.

• Upper divergence must exceed low-level convergence to decrease surface pressures (deepening).

• Development requires proper combination of circulation, divergence, and convergence maintained over time on a proper scale.

Areas of Formation

• Tropical North Atlantic Ocean:

  • East of the Lesser Antilles and the Caribbean, east of $70°W$ during July to October.

  • North of the West Indies in June to October.

  • Western Caribbean during June and late September to early November.

  • Gulf of Mexico during June to November.

• Western North Pacific Ocean (including the Philippines) during May to November, but storms can occur in all months.

• North Pacific off the West Coast of Central America during June to October.

• Bay of Bengal and Arabian Sea from May to June and October to November.

• South Pacific Ocean, West of $140°W$ from December to April.

• South Indian Ocean from December to April, Northwestern Coast of Australia during November to April, and West of $90°E$ from November to May.

Restrictions on Formation

• Tropical cyclones do not form in the South Atlantic Ocean and southeastern Pacific Ocean.

• During the Southern Hemisphere summer, the intertropical front moves only slightly south of the equator.

• Tropical cyclones are rare within $5°$ latitude of the equator.

Frequency of Tropical Cyclones (per 10 years)

• North Atlantic Ocean - 73

• North Pacific, off West Coast of Mexico - 57

• North Pacific Ocean, west of $170°E$ - 211

• North Indian Ocean, Bay of Bengal - 60

• North Indian Ocean, Arabian Sea - 15

• South Indian Ocean, west of $90°E$ - 61

• South Indian Ocean, Northwestern Australia - 9

Life History Stages

• Tropical cyclones undergo metamorphosis from birth to decay, lasting about six days on average.

Four Stages:

1. Formative Stage:

   • Incipient stage forming in waves and shear lines of pre-existing disturbances.

   • Winds usually remain below typhoon force.

2. Immature Stage:

   • Deepening stage until the lowest central pressure and maximum wind intensity are reached.

   • Intensification may not always occur; some die down even at typhoon force.

3. Mature Stage:

   • Circulation expands, surface pressure stabilizes, and maximum wind speed plateaus.

   • This stage can last for about a week.

4. Decaying Stage:

   • Surface pressure rises, and the affected area diminishes.

   • Dissipation occurs due to friction, lack of moisture over continents, or when colder, drier air enters.

Classification of Tropical Cyclones

• Derive energy from latent heat of condensation, existing only over oceans and dying out rapidly on land.

• Central sea-level pressure of 900 mb or lower, with surface winds often exceeding 100 knots.

• Intensity varies, and they are classified based on wind strength.

PAGASA Classification (as of March 23, 2022):

• Tropical Depression (TD): Maximum sustained winds up to 62 kph (less than 34 knots).

• Tropical Storm (TS): Maximum wind speed of 62 to 88 kph (34 - 47 knots).

• Severe Tropical Storm (STS): Maximum wind speed of 87 to 117 kph (48 - 63 knots).

• Typhoon (TY): Maximum wind speed of 118 to 184 kph (64 - 99 knots).

• Super Typhoon (STY): Maximum wind speed exceeding 185 kph (more than 100 knots).

Features of a Tropical Cyclone

1. Atmospheric pressure decreases towards the center. As a result, the surrounding air is drawn in, leading to the development of strong rotational winds and distinct weather patterns. This rotation is due to the Coriolis effect, which influences the movement of air masses in the atmosphere, contributing to the cyclone's characteristic spiral shape.

2. Rainfall: Intense rainfall accompanies tropical cyclones, often leading to flooding and landslides in affected areas.

3. Storm Surge: A rise in sea level that results from intense winds and low pressure, causing coastal flooding.

4. Wind Speeds: Tropical cyclones are known for their extreme wind speeds, which can exceed 155 miles per hour in the most severe hurricanes, resulting in significant damage to structures and infrastructure. They can also result in widespread power outages and disrupt transportation systems, making recovery efforts challenging. As a result, preparedness and timely evacuation are critical to minimizing the risks associated with these powerful storms.

5. Destruction: The destructive nature of tropical cyclones can lead to loss of life, damage to homes and businesses, and long-term economic impacts on communities. Additionally, the aftermath of a tropical cyclone often includes numerous challenges such as debris removal, restoring essential services, and providing aid to affected populations, which can take months or even years to fully recover.

6. Flooding: One of the primary hazards associated with tropical cyclones is severe flooding, which can occur due to heavy rainfall and storm surges. This flooding can inundate residential areas, disrupt drinking water supplies, and pose serious health risks to survivors. Furthermore, government agencies and non-profit organizations must work collaboratively to develop and implement effective response strategies that address both immediate needs and long-term reconstruction efforts.

7. Wind Damage: The intense winds generated by tropical cyclones can cause catastrophic damage to infrastructure, uprooting trees, toppling power lines, and severely impacting transportation systems. This wind damage often exacerbates the situation by hindering rescue and recovery operations, complicating efforts to restore normalcy within the affected regions.

8. Storm Surges: In addition to flooding and wind damage, storm surges pose a significant threat during tropical cyclones. These rapid increases in sea level can lead to extreme coastal flooding, overtopping barriers, and causing extensive damage to coastal communities. Adequate preparation, including the establishment of early warning systems and evacuation plans, is critical to minimizing the impacts of storm surges on vulnerable populations.

9. Rainfall and Flooding: Tropical cyclones also bring heavy rainfall, which can result in widespread flooding, particularly in low-lying areas. The resultant flash floods can wash away homes, overwhelm drainage systems, and displace thousands of residents, highlighting the urgent need for effective flood management strategies and community preparedness.