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1.4 Tropical Cyclones (Structure (Eye Wall
Zone of most intense storm…
1.4 Tropical Cyclones
- intense cyclonic storms that originate over warm tropical seas, averaging about 650km in diameter, 8-10km high
- an area of low atmospheric pressure with converging and ascending air
- self-sustaining systems that represents both vertical and horizontal transfer of heat and energy
- Most are formed between 5° and 20° N/S , especially the Western part of the North Pacific Ocean (beyond that SSTs are too low)
- In the N hemi, cyclones occur between June-Dec in the Atlantic/Pacific/Indian Oceans
- In the S hemi, cyclones occur between Jan-March in the Pacific/Indian Oceans
- All travels from EAST to WEST , might curve polewards
Hurricanes: Atlantic and the Eastern Pacific
Cyclones: Indian Ocean
Typhoons: Western Pacific
Willy Willies: Australia
Formation of Cyclones
Formation
- Cyclones begin as weaker systems, as tropical disturbances which originate from easterly waves that are low pressure systems in the normal wind pattern.
- A minority, undergo a lowering of pressure and begin to rotate cyclonically.
- Surface pressure falls as air heated by the warm sea below starts to rise.
- Due to horizontal pressure gradient force, winds that are drawn in towards this low pressure area then converges with one another, and the Coriolis Force causes this rising column of air to twist.
- As the air rises, it cools adiabetically and the condensation process releases latent heat and warms the atmosphere, triggering further instability and fueling the rising air mass.
- Once formed, cyclones are self-sustaining and will constantly release latent heat and rise as long as all the conditions are met.
Conditions Required
Atmospheric Conditions
Coriolis Force
- prevents filling of the central low pressure
- Absence of CF at 5°N and 20°S inhibits formation
Atmospheric Instability
- requires highly unstable air masses to promote continual uplift to draw in more air masses at the surface.
Absence of Wind Shear
- There must be no great vertical wind shear which can disrupt the vertical transport of latent heat
Surface Conditions
High Surface Water Temperature >27°C
- The primary heat source fueling the cyclone is the release of latent heat supplied by evaporation from the ocean surface
Deep Surface Layers >60m
- prevents the upwelling of cold water so high sea surface temperatures (SST) are maintained
Structure
Eye Wall
- Zone of most intense storm activity, of high rainfall and strong winds
- Rapid uplift of unstable air masses forming bands of cumulus and cumulonimbus clouds that rise o 16km with rainfall 2500mm/day
Convective Cells
- Separated by areas of weaker uplift and less intense precipitation (downdrafts)
- Thickness of cloud bands corresponds well with the intensity of rainfall, ↑ towards the centre and max at eye wall
Eye :eye:
- Center of the storm
- Calm and cloud free, a region of clear skies and light winds, little rainfall
- Air is warmer than outside due to adiabatic warming of the air mass
- Change in the size of the eye indicates a strengthening/weakening of the hurricane (↓ eye, intensifying hurricane)
Pressure
- Lowest in the centre with a steep pressure gradient outward from the centre, steepest and most rapid in the vicinity of the eye wall
Life Cycle
Disturbance → depression → storm → hurricane
- Initially moves westward due to easterlies but when intensity of the storm increases, it would move towards warmer seas.
- Once it is fully developed, the storms become more likely to move polewards, reversing the direction of the cyclone to an eastward path
- Often moves in erratic ways so it makes prediction of their irregular paths difficult
Cyclone Dissipates when
- Moves over cooler water (loss of heat energy that drives the cyclone)
- Moves over land resulting in loss of supply of warm moist air
- Encounters strong vertical wind shear which disrupts the cyclone
Types of Hazards
Strong Winds :wind_blowing_face:
- Causes damage to trees and crops where forest can be flattened and tall buildings are collapsed/implode.
- Death and injury caused by flying debris
Storm Surge :ocean:
- Rise in water levels due to hurricane's strong winds and strong rainfall.
- When the cyclone makes landfall, it can produce a storm surge of water as much as 7.5m above normal tide level which can cause low-lying coastal areas to be severely inundated.
- This leads to flooding, damage to properties and loss of lives
- Intrusion of sea water on agriculture can cause lands and cash crops to turn useless
eg: 90% of hurricane induced deaths are by drowning. In 2008, flooding by Cyclone Nargis in Myanmar killed at least 130,000 people
Intense Rainfall and Flooding
- Drastically increase amount of rainfall at areas where they make landfall, causing flooding by swollen rivers and mass movement on slopes in the form of landslides and debris/mud flows.