Lecture 21: Hurricanes Typhoons (Western Pacific) Tropical Cyclones (anywhere) Cyclones (Indian Ocean + others) Hurricanes (near N. America) 4/22/03

Cool Video video.org/results.php?VideoTitle=Anatomy +of+a+Hurricanehttp:// video.org/results.php?VideoTitle=Anatomy +of+a+Hurricane (50 MB!!!) Cool site:

Anatomy—Spiral bands and eye Spiral bands Eye wall Surface flow (convergence) Subsidence Eye Divergence aloft

Anatomy—Spiral bands and eye Spiral bands Eye wall Surface flow (convergence) Subsidence Eye Divergence aloft

Anatomy 1.Outflow aloft 2.Feeder bands (spiral bands) 3.Eyewall 4.Eye

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

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