1. Where Do the Hurricanes Come From?

2. Review of last lecture
Forces acting on a cloud/rain droplet. Terminal velocity. How does it change with cloud drop radius?
Growth mechanisms for rain and snow

3. Summary of Precipitation processes:
Condensation
Warm
clouds
Cool/cold
clouds
Collision-
coalescence
Bergeron
Process
Riming/
Aggregation
Rain
Snow
(can change to rain, sleet, or any other type of precipitation depending on underlying atmosphere

4. Observation of tropical cyclones: Aircrafts (The hurricane hunters)

5. Observation of tropical cyclones: Satellite (1960s-Now)

6. Genesis and tracks of tropical cyclones
5.4
2.5
8.9 16
4.4
3.4
4.3
Different names: hurricanes, typhoons, cyclones
Genesis region generally in the tropics, especially between 5N-15N and 5S-15S.

7. Necessary environmental conditions for tropical cyclone formation
SST > 27 oC (Poleward of about 20o SST too cold for formation. Highest frequency in late summer to early autumn when water is warmest.)
Warm ocean mixed layer is thick enough to supply energy (this is why they weaken quickly upon landfall)
Unstable atmosphere with a moist lower/middle troposphere (central and western ocean basins)
Low vertical windshear (Otherwise upward transfer of latent heat disrupted)
Coriolis force (do not form between 5N-5S where Coriolis force is too weak)
Pre-existing low-level rotating circulations (tropical waves and other disturbances)

8. Tropical waves often cause tropical cyclone formation (e. g
Tropical waves often cause tropical cyclone formation (e.g. African easterly waves, Madden-Julian Oscillation)

9. Four stages of tropical cyclone formation
Vortex w/ max wind >74 mph
Vortex w/ max wind mph
Vortex w/ max wind <38 mph
Organized convection >200 km, >24 hours

10. Hurricane Intensity Scale
Saffir-Simpson scale classifies hurricanes into five categories based on:
- central pressures (ended in 1990s)
- maximum sustained wind speeds (used now)
Categories 3, 4, 5 are collectively called major hurricanes. They account for 21% of hurricane landfall in U.S., but cause 83% of the damage.

11. Hurricane names Named alphabetically 6 lists in rotation
When there is a hurricane that is too devastating, the name will retire and be replaced by a new name
Andrea Arthur Ana Alex Arlene Alberto
Barry Bertha Bill Bonnie Bret Beryl
Chantal Cristobal Claudette Colin Cindy Chris
Dean Dolly Danny Danielle Don Debby
Erin Edouard Erika Earl Emily Ernesto
Felix Fay Fred Fiona Franklin Florence
Gabrielle Gustav Grace Gaston Gert Gordon
Humberto Hanna Henri Hermine Harvey Helene
Ingrid Ike Ida Igor Irene Isaac
Jerry Josephine Joaquin Julia Jose Joyce
Karen Kyle Kate Karl Katia Kirk
Lorenzo Laura Larry Lisa Lee Leslie
Melissa Marco Mindy Matthew Maria Michael
Noel Nana Nicholas Nicole Nate Nadine
Olga Omar Odette Otto Ophelia Oscar
Pablo Paloma Peter Paula Philippe Patty
Rebekah Rene Rose Richard Rina Rafael
Sebastien Sally Sam Shary Sean Sandy
Tanya Teddy Teresa Tomas Tammy Tony
Van Vicky Victor Virginie Vince Valerie
Wendy Wilfred Wanda Walter Whitney William

12. Movie time! How do the meteorologists study and predict hurricanes?

13. Horizontal Structure of tropical cyclones
Total diameter: about 600km
Lifetime: up to a week or more
Make up: many thunderstorms arranged in pinwheel formation
Three components:
1. Central eye - clear skies, light winds (25 km diameter)
2. Eye wall - strongest winds (can be 200 mph), max precip. (up to 2500 mm/day)
3. Spiral rainbands
Cylonic inflow, anticyclonic outflow. Outflow creates cirrus shield that often obscures pinwheel structure

14. Vertical Structure: Wind flow

15. Vertical Structure: Thermal
Warm core - fueled by thermal energy - core warmer than outside at upper levels
Minimum surface pressure occurs in the region of central eye
Maximum winds and rainfall occur in the region of eye wall

16. Vertical Structure: Feedbacks
Sea surface temperature
Convergence-Convection (CISK): positive
Surface flux-Convection (WISHE): positive
Air-sea interaction: negative

17. Tropical cyclone Destruction and Fatalities
Destruction caused by:
Hurricane-scale winds
Rainfall
Storm surge (winds blowing coast-ward + lower atmosphere pressure)
Fine-scale Tornadoes
Destruction most intense on right side of cyclone (wind + storm speed)

18. Hurricane-spawned tornadoes
More than half of the landfall hurricanes produce at least one tornado
Hurricane Buelah (1967) produced 141 tornadoes
Most likely occur in the right-front quadrant
Generally less intense than those that occur in the great plains

19. Tropical cyclone forecast model By Tim Marchok of NOAA/GFDL

20. Tropical cyclone forecast skill: Track

21. Tropical cyclone forecast skill: Intensity

22. Summary Tropical cyclone genesis: 6 necessary conditions, 4 stages
Tropical cyclone tracks
Tropical cyclone structure: 3 major components, rotation direction of inflow and outflow, location of maximum wind and rainfall, 3 feedbacks
Tropical cyclone destruction: 4 reasons? Which side has the most intense destruction?
Tropical cyclone forecast: track and intensity Currently which skill is better?