Lecture 21: Hurricanes

• Typhoons (Western Pacific)

• Tropical Cyclones (anywhere)

• Cyclones (Indian Ocean + others)

• Hurricanes (near N. America)

4/22/03

Cool Video

• http://www.open-video.org/results.php?VideoTitle=Anatomy+of+a+Hurricane

• (50 MB!!!)

Cool site:

• http://www.nhc.noaa.gov/index.shtml

Anatomy—Spiral bands and eye

Spiral bandsEye wall

Surface flow(convergence)

Subsidence

Eye

Divergence aloft

Anatomy—Spiral bands and eye

Spiral bandsEye wall

Surface flow(convergence)

Subsidence

Eye

Divergence aloft

Anatomy

1. Outflow aloft

2. Feeder bands (spiral bands)

3. Eyewall

4. Eye

Anatomy

Anatomy

Eye• Hurricanes only• Strongest ones have very tight eyes (10 mi?)• Centrifuge effect of sorts keeps stuff out of eye• Slack winds• Lowest pressure• Subsidence• Birds!! • Surrounded by eye wall

Eye wall movie

Eye Wall

Eye Wall• Strongest winds

• Heaviest rains

• Strongest lift

• Very little lightning! (why?)

Eye Wall (radar—Andrew)

Spiral Bands

• Outer part of the storm

• Many tornadoes, thunderstorms

• Winds may not be hurricane strength

• Not very well understood—why bands of something and then nothing?

• Lift in bands, subsidence in gaps

Hurricane formation

Sea surface temperatures for NH Summer

Hurricane Steering

L

H

Hurricane Frequency• 90% between August-October (N. Atlantic)

• “Season” is June 1 – November 30

Life cycle of a hurricane

• Required for hurricane formation:– Surface temperature > 78ºF (25.5ºC) to at least

60 m depth– At least 5º away from the equator to get started,

more (8-9º) to become a hurricane– Weak, uniform winds, no wind shear– Upper level support

Warm water

• Supplies moisture• More importantly, it supplies LATENT

HEAT• Latent heat of condensation is THE source

of energy for a hurricane. • Must be deep (60 m) or the hurricane will

stir up some colder water

Coriolis force

• Must be outside of 5º to get started or 8º -9º to really get going.

• Otherwise, there’s not enough spin, and air travels directly from high to low pressure

Weak, uniform winds

• Wind shear will tear apart a hurricane• To get good vertical development, it all needs to

occur in the same spot—can’t have it getting all slanted on us!

• This is why El Nino means few hurricanes (strong shear—Westerlies aloft) while La Nina means many hurricanes in the N. Atlantic (weak shear)

Upper level support

• Must have divergence aloft or the hurricane fountain can’t get going nor will it keep going.

Beginnings• Easterly wave

– Tropics are RIPE!!!• No thermal

differences

• No pressure differences

– Look for convergence in winds!!!

• Or a castoff from the ITCZ

Streamlines

Tropical Disturbances• 90% of these die out• If, at any time, wind

shear, cold water, or land are encountered, the whole thing stops.

• Some form a closed circulation, and are able to continue to strengthen—a tropical depression

Spinning up

• Circulation begins in a weak tropical disturbance• A WARM CORE develops from all the latent

heat. • Winds converge toward the center…

– Accumulating more moisture– Which creates more storms & more intense storms– Pressure near tropopause starts to increase—divergence

aloft—in response to latent heat release.

Hurricane transition

• Some tropical depressions intensify to become tropical storms

• About half of those go on to become hurricanes

Fates

• Hurricane can:– Go over land (fizzle)– Lose upper level support (fizzle)– Encounter wind shear (fizzle)– Wander over cooler waters (fizzle)– Turn into an extratropical cyclone w/ fronts– Get sucked up into an extratropical cyclone

Landfall!

What the heck is that?

• A subtropical storm

• Has some features of tropical, some features of extratropical storms