1. Tropical Cyclones
EAS 309
10 December 2018

2. Review: Extratropical Cyclones
Low pressure center
Strong temperature gradients at surface (fronts)
Strong winds at fronts
Asymmetric precipitation and cloud fields
Fueled primarily by the equator-to-pole temperature gradient
Occur throughout year, largely in the midlatitudes

4. Tropical Cyclones Low pressure center
Rapidly rotating organized systems of thunderstorms
Closed surface circulation around a well-defined center
Originates over tropical or subtropical waters

5. Tropical Cyclone Stages
If environmental conditions promote further intensification  hurricane
Sustained wind speeds > 74 mph
Unorganized cluster of thunderstorms
Over time, it can strengthen into a tropical storm, at which point the storm is named
If the thunderstorms become more organized  tropical depression
Sustained wind speeds of 39 – 73 mph
Sustained wind speeds < 39 mph

6. Does not address other hurricane-related hazards
Saffir Simpson Scale
Major
hurricane
Does not address other hurricane-related hazards

7. Tropical Cyclone Formation Regions

8. Hurricane Ingredients
Formation conditions:
Pre-existing disturbance
Located away from equator
Sea surface temperatures of at least 80°F
Low vertical wind shear
High relative humidity
Decay conditions:
Cool sea surface temperatures
Strong vertical wind shear
Landfall

9. Hurricane Ingredients

10. Tropical Cyclone Structure
Eye: center of the storm where winds are light and skies are mainly clear
Eye wall: area surrounding the eye where winds and strongest and rain is heaviest
Rain bands: bands of clouds and rain that stretch for hundreds of miles and contain thunderstorms and even tornadoes
Eye wall
Rain bands
Rain bands
Eye
Latent heat release fuels tropical cyclones

11. Hurricane as Carnot Engine
A  B: Isothermal expansion
Acquire heat through evaporation
B  C: Adiabatic expansion
C  D: Isothermal compression
Lose heat through radiative cooling
D  A: Adiabatic compression

12. Wind Circulation Slower wind speeds on the left side of hurricane
Storm movement and wind circulation are in opposite direction
Faster wind speeds on the right side of hurricane Storm movement and wind circulation are in same direction

13. Factors that influence the magnitude of storm surge
Rise of water generated by a storm over and above the predicted astronomical tide due to strong winds associated with the storm’s circulation
Factors that influence the magnitude of storm surge
Extent of strong winds
Depth of near-shore waters
Speed of hurricane
Angle track of hurricane takes to coastline
Timing of natural tides

14. Tropical Cyclones vs. Extratropical Cyclones
What are some similarities and differences among tropical cyclones and extratropical cyclones?

15. Extratropical Cyclones
Fronts
No fronts
Powered by equator-to-pole temp gradient
Powered by latent heat release
Cold core
Warm core
No eye at center
Eye at center
Much larger spatially
Much smaller in size
Most intense in winter
Most intense in summer and fall
Vertical wind shear
Vertical wind shear is bad
Similarities:
Low pressure centers
Heating through condensation
Strengthened through divergence aloft

16. Cyclone Spectrum

17. Extratropical Transition
Gradual process in which a tropical cyclone loses its tropical characteristics (warm-core, symmetric) and becomes more extratropical in nature (cold-core, asymmetric) L L

18. Extratropical Transition of Irene

19. Extratropical Transition – Tropical Phase
Wind
Rain

20. Extratropical Transition – Transition Phase
Wind
Rain

21. Extratropical Transition – Extratropical Phase
Wind
Rain

22. Extratropical Transition Climatology
Most tracks DO NOT travel through the Caribbean and Gulf of Mexico
Greatly impact eastern United States
Jones et al. 2003

23. Extratropical Transition Climatology
Areas closer to east coast have higher likelihood of experiencing a tropical cyclone that undergoes extratropical transition
Hart and Evans (2004)

24. Extratropical Transition Climatology
All TCs
Solid line: % of tropical cyclones that transition each month
Peak in number of both sets in September
Likelihood of extratropical transition increases throughout tropical season
ET TCs
Hart and Evans 2001

25. Extratropical Transition Climatology
All TCs
Short-dashed line is % of TCs that made landfall north of 25N
% of landfalling tropical cyclones decreases as tropical season goes on
Long-dashed line is % of TCs that made landfall north of 25N and transitioned
While % is lower, if a storm DOES make landfall, it is more likely to undergo ET
TC land
ET land
Hart and Evans 2001