1. Tropical Cyclone Forecasting

2. Hadley Cell Circulation and the ITCZ

3. Classifications:
Tropical disturbance—a disorganized group of thunderstorms with weak pressure gradients and little or no rotation.
90% (or more) die out before becoming organizing into more powerful systems.
Tropical Depression—more organized, wind speeds less than 60 km/hr
Tropical Storm—wind speed b/w 60 and 120 km/hr
Hurricane—wind speeds exceed 120 km/hr.

4. Lower tropospheric easterly wave in the trade winds

5. Formation of TC:
Tropical storms results from easterly waves that are troughs of low pressure initiating CON off the coast of Africa.
Tropical wave may or may not develop into a tropical storm.
When a hurricane poses a direct threat to an area, a hurricane watch is issued, typically hours before the storm arrives.
When it appears that the storm will strike an area w/in 24 h, a hurricane warning is issued.

6. Typical Summertime Wind Pattern over the North Atlantic

8. Different names for the same thing.
Hurricane—Atlantic and Eastern Pacific
Typhoon—Extreme western Pacific (typically larger and stronger than Atlantic hurricanes.
Cyclone—Indian Ocean and near Australia

9. Necessary Ingredients:
Warm water SST at least 81ºF
Outside of area b/w 20N and 20S the water is usually too cold.
Tropical water is warmest in late spring to early fall explaining the seasonal variation of hurricane occurrence
Hurricane Season—June to Novmember
Coriolis force
Must be strong enough to prevent filling of the central low pressure center.
Lack of coriolis force near the equator prevents hurricane formation b/w 0 and 5 degrees latitude.
2/3 hurricanes form b/w 10 and 20 degrees from the equator.

10. Necessary Ingredients:
Unstable conditions throughout the troposphere
Along the eastern margins of the oceans, upwelling and cold currents lead to a statically stable environment.
As you move west, water temps increase to make hurricane formation more prevalent.
Weak Vertical Wind Shear
Strong Vertical Wind Shear disrupts the vertical transport of LH, the primary source of hurricane’s energy, which is supplied by evaporation from the ocean’s sfc.

11. Triggering Mechanisms for Hurricane Formation

12. Hurricane Characteristics:
Size—several 100 km (avg 600 km in diameter)
Shape—generally circular with cyclonic bands (in NH)
Lifespan—several days to slightly more than a week.
Structure:
Very low central pressure (very strong pressure gradient)
Consists of large number of T-storms arranged in spiral or pinwheel formation
Bands of thick clouds and heavy t-storms spiral CCW around the center of the storm (in NH)
Bands are separated by areas of weaker uplift and less intense precip.
Wind speed and rainfall rate both inc. toward the center, w/ max km away from

13. Hurricane Characteristics:
The eye:
Avg 25 km in diameter can get up to as much as 60 km
Relatively clear skies, scattered showers
Slowly descending air
Calm winds
Tornadoes in the right forward quadrant as they make landfall
Most damage is due to storm surge.
Atlantic Hurricanes—El Nino years
Hurricanes are named using alternating Male and Female names, once a storm reaches Cat. 3 or higher, its name is retired.

14. 2 main theories on why hurricanes can exist for long periods of time.
Conditional Instability of the Second Kind (CISK)
Hurricanes are maintained by a positive feedback:
Warm, moist air rises, then forms an anvil where it begins to sink. This sinking air CON near the sfc due to the lower pressure created by the LH released from condensation. This air then rises again to perpetuate the cycle.
Wind Induced Surface Heat Energy (WISHE)
Basically the same process as above, only described in terms of heat.
The hurricane removes heat from the ocean surface, converts it to kinetic energy (wind), and then loses the heat above the clouds due to radiational cooling.

15. Illustration of Wind-Induced Surface Heat Exchange

17. Primary Factors Contributing to Height of Storm Surge

18. Example of Storm Surge During TC Landfall

19. Trajectories of air parcels moving through a modeled hurricane – over eight days

20. Typical Cross Section through a Hurricane

21. Tracks of Some Typical Atlantic Hurricanes

22. Tracks of Some Unusual Atlantic Hurricanes

25. Profile of a Hurricane

27. Conservation of Angular Momentum in a Hurricane

28. Hurricane Seasons

29. Satellite Image of Hurricane Georges (1998)

30. Radar View of Hurricane Georges (1998) off Florida Keys

31. Radar Estimate Rainfall from Hurricane Georges near Mobile, Alabama

32. Double Eye Wall Structure: Hurricane Gilbert (1988)

33. Dropsondes