What are hurricanes?
Large tropical cyclones that generate winds over 240kph and push massive amounts of water onshore as surges, up to 6m over sea lvl
- Heat engines that convert heat of tropical ocean into winds and waves
- Tornadoes can form from their clouds
- Transporters for excess tropical heat to mid-lats
Tropical storm vs hurricane
Storm: >63kph
Hurricane: >118kph and centrifugal acceleration causes clear, central eye
Strength: depends on the efficiency of energy transfer, winds flowing to core then upward (no wind shear aloft)
Requirements for development of hurricane:
- Upper 60m of seawater >27C
- Warm, humid, unstable air
- High enough latitude for Coriolis effect to spin it -> 5º+
- Weak upper lvl winds, good if blowing in same direction as developing storm is moving
Begins with tropical disturbance:
Low pressure zone draws in cluster of thunderstorms
- Weak surface winds
Coriolis effect creates tropical depression
- Cyclonic rotation
- Receives identifying #
Core sends warm, moist air into stratosphere
Rising moist air cools to dewpoint temp, then condenses and releases latent heat
- Stronger updrafts
How a hurricane works
- Beyond the hurricane air mass, descending air warms adiabatically (density decreases) and clears area outside of the hurricane boundary
- Large latent heat release from rising winds in core = warms eye
- Low pressure core relative to surroundings
Eyewall and Eye
- Inward-flowing air rotates faster closer to core
- when speed can’t increase further, converging winds rise sharply
- Eyewall = cylinder-shaped area of spiraling upward thunderstorms around eye with heaviest rainfall and high winds in storm
- Cool air sinks into eye and warms/absorbs moisture -> clear core forming eye into atmosphere
- Eyes can be 8-200km in diameter
Tornadoes within hurricanes
Form predominately from outer rain bands in hurricanes that are:
- Large, intense, strongly curving
- Moving as system at 12-30kph
- Interacting with old, weakened fronts
- Over land
Friction from movement on land = slows surface wind, higher winds keep up momentum (provides vertical shear for tornado rotation)
Energy flow in hurricane
- Warm, moist surface winds pick up heat from ocean as converge on eyewall
- Rising eyewall winds release latent heat and cool = increased buoyancy and velocity
- Flow outward and lose long-wave energy
- Turbulent waves from surface winds
Hurricane energy release
Transfers heat: tropical seas to hurricane core
Huge amounts of latent heat released as air rises
Generates energy 200x than worldwide electricity generating capacity
Cloud formation and rain
Differences from high-lat storms:
- Latent heat from condensation = main energy source
- Moves onto land and weakens rapidly
- Not associated with fronts
- Weaker high-alt winds = stronger hurricane
- Hurricane winds weaken with height
- Air in center of eye sinks
- Hurricane centers are warmer than surroundings
Different names in different parts of the world:
Cyclones = Indian Ocean
Typhoons = Western Pacific Ocean
Hurricanes = Atlantic Ocean
Hurricane Origins
Form on west sides of oceans where warm water is concentrated or off Pacific coast of Mexico (Not off coast of Brazil, Atlantic too narrow, not enough warm water)
Form b/w 5-20º lat (not along equator due to zero Coriolis effect, cannot cross equator once formed -> would lose rotation)
Saffir-Simpson scale for damage, categories:
1) Some wind damage
2) Some trees, roofs, mobile homes blown down
3) Larger trees down, destroy mobile homes
4) Signs, windows, roof, damaged, inland flooding
5) Severe building damages
North Atlantic Ocean Hurricanes
- Large, mobile, and long-lasting hurricanes, 4-28/yr
- Most form in late summer when sea surface temps are warmest
- Most hit US from Aug-Oct
Cade Verde-type Hurricanes
- Begin as easterly waves moving W from Africa
- May gain typical cyclone status in warm waters near Cape Verde Islands
- Blow W with trade winds, gaining energy over great distances of warm seawater
- Movement curves N due to Coriolis effect when reach western Atlantic
Hurricane paths are difficult to track because:
- Adjust to other high- and low- pressure systems
- Trade winds blow them W
- Coriolis effect add curve to right
- Bermuda high (high-pressure zone) commonly sits above North Atlantic Ocean
Bermuda High influences paths:
If small and E = hurricane curves around it
If strong and large = hurricane pushed along US E coast
If SW = hurricanes pushed into Caribbean Sea and Gulf of Mexico
Caribbean sea and Gulf of Mexico-type hurricanes
- Form at ITCZ, where trade winds meet
- Daily thunderstorms, add more rotation to cyclone
- Location of ITCZ moves with migration of Sun, avgs 5ºN
- ITCZ S of equator in Northern hemi winter and N of equator in summer
- Low-pressure area forms where air flows converge = thunderstorms = hurricanes
How Hurricanes are Named
- North Atlantic basin tropical storms and hurricanes given names, alternating male and female
- Premade lists for next 6yrs
- Begins with 21 letters (skip Q, U, X, Y, Z)
- Names of especially destructive storms are retired
- If >21 storms, start over with Greek alphabet
Is the increase of hurricane due to human activity or natural variation?
Total energy release by hurricanes is higher, probably due to increasing sea-surface temps and increased H2O vapour in lower atmosphere, possibly due to global warming
Atlantic Multidecadal Variability
- Alternating pattern in North Atlantic Ocean surface temps
- Involves ocean, atmosphere, natural, and human changes
- Since 1870, each multidecadal stretch when sea-surface temps were higher = more hurricane activity
- Since 1870, each multidecadal stretch when sea-surface temps were lower = less hurricane activity
Atlantic Cyclone Energy
- Index compiles energy expended in each tropical cyclone in each season, calculated every 6hrs and added cumulatively
Hurricane Damages
Hurricane deaths are down in recent decades
- Better advanced warnings
Hurricane dmgs are up
- More ppl living on coastline
- Larger, more expensive homes on coastlines
- Damages calculated in constant dollars suggests the future will be even more costly
Storm Surge Hazards
Most deaths associated with sea surges
Cubic yard of water is 1685 pounds, incompressible -> water hits with a lot of force
Rise in sea lvl under storm due to:
- Winds push water ashore to pile up above normal lvls, blow inalnd on right of eyewall, out from land on left of eye
- Low atmospheric pressure causes water to mound up under eye of hurricane
- 95% of surge is wind-driven
- Surge greatest on Eastern side of hurricane
