The Science of Hurricanes

1
The Science of Hurricanes

• Dr. John A. Knox
• University of Georgia
• Masters of Disaster Workshop
• June 16, 2009

2
What is a hurricane?

• A very organized nearly circular system of
  thunderstorms around a center of very low air
  pressure that originates in the tropics
• Hurricane name given to intense tropical
  cyclones in Gulf, Atlantic, Caribbean and eastern
  Pacific (but Ill use hurricane generically in
  this talk)
• Generic name tropical cyclone
• Typhoon in western Pacific
• Cyclone in Indian Ocean, etc. (e.g., Nargis in
  Myanmar)

3
Where/when can hurricanes form?

• Necessary ingredients
• Warm, deep ocean waters (energy source for storm)
• Location off of the Equator (else low cant be
  sustained)
• Thunderstorm seedling (e.g., multicell cluster
  off of Africa)
• Relatively gentle high-level winds (dont disrupt
  circulation)
• Ideal location northwest Pacific Ocean

4
How are they structured?From space
Katrina over the Gulf, 2005 (visible satellite)
Mitch over the Caribbean, 1998 (infrared
satellite)

• Hurricanes look like a doughnut in satellite
  pictures
• Why? Clear, nearly circular eye at center
• Thunderstorms in the eye wall etc. make up the
  doughnut
• Outside of eye, spiraling-in rain bands feed
  into the center

5
How are hurricanes structured?Fromradar
INTENSE EYE WALL
TINYEYE
SPIRAL RAIN BAND
Record-setting Wilma over Cozumel, 2005
(radar) http//www.jamesspann.com/bmachine/wxtalk.
php?print2427
6
How are hurricanes structured?Cross-section
schematic

• Low pressure at surface
• Air spiraling into low picks up heat and
  moisture
• Air rises in eye wall, converges aloft
• High pressure in upper troposphere
• Like a fireplace air up near low, then out the
  high-pressure chimney

7
How are hurricanes classified? Saffir-Simpson
Scale

• Invented by engineer Herbert Saffir (1917-2007)
  and National Hurricane Center director Robert
  Simpson (1912-present)
• Scale 1-5 3 and above is a major hurricane
• Like Fujita scale, damage-based
• Unlike Fujita scale, can be estimated in real
  time
• Why the difference? Hurricanes are
  larger-scale, so they can be better observed and
  their winds follow better-understood scientific
  relationships than tornadoes
• Result can estimate Saffir-Simpson scale from
  real-time observations, dont have to dig through
  rubble (usually)

8
http//www.srh.noaa.gov/mob/tropical.shtml
See also http//www.npr.org/news/specials/hurrica
ne/ap/
9
Typical hurricane life cycle

• Seedling a thunderstorm cluster moves west,
  often off the coast of Africa. Most dont get
  any stronger.
• Easterly Wave wiggle in winds. 85 of major
  Atlantic hurricanes start as easterly waves, but
  most waves dont become hurricanes.
• Tropical Depression weak low-pressure area with
  sustained (constant) winds below 39 mph.
• Tropical Storm strong low-pressure center with
  sustained winds between 39-73 mph. Gets a name!
  About 50 of tropical storms intensify further
  to
• Hurricane intense very circular low-pressure
  center with sustained winds at or above 74 mph.
  Often develops an eye, especially as it
  intensifies beyond minimal hurricane status.
• The End hurricane usually either dies over
  land (away from warm water) or becomes part of an
  extratropical weather system.

10
Hurricane life cycle from satellite
CAT 2
CAT 5
CAT 3
CAT 1
11
Hurricane evolution from radarKatrina at Miami
http//orca.rsmas.miami.edu/johnc/katrina.gif
See also http//www.recmod.com/hurricane/charley/
movies/charley-florida-radarloop2.mov
12
What factors control the strength of a hurricane?
Ocean temperatures (above 80F, good for
growth) At left Sea height anomaly (higher
warmer water) during Katrina
Vertical wind shear (jet streams bad for growth,
unlike thunderstorms/tornadoes)
13
What factors control the strength and motion of
a hurricane?Another applet for you to use
http//cimss.ssec.wisc.edu/wxfest/hurricane/hurr.h
tml
Upper-level winds (steer storm in direction of
winds)
Ocean temperatures (above 80F, good for growth)
14
Global hurricane climatology
http//en.wikipedia.org/wiki/Hurricane
15
U.S. hurricane climatology
Cape Verde Islands
http//www.nhc.noaa.gov/gifs/1851_2007_mjrhurr.jpg

16
GA major hurricane climatology
Aug. 1893 2500 dead
Oct. 1898 179 dead, similar to Hugo
http//www.aoml.noaa.gov/hrd/hurdat/images/
http//www.aoml.noaa.gov/hrd/Landsea/history/index
.html
17
Atlantic hurricane season climatology
http//www.nhc.noaa.gov/gifs/peakofseason.gif
18
Typical Atlantic hurricane season
http//www.nhc.noaa.gov/pastprofile.shtml
19
The record-setting 2005 Atlantic hurricane season
20
Hurricane hazards wind

• Extreme pressure gradient causes sustained winds
  of up to 200 mph in eye wall, with higher gusts
• Inside the eye, however, winds are light and
  variable
• Eye halftime even though weather improves
  temporarily, the second half is about to begin.
  Take cover!

21
Hurricane hazards wind, cont.

• Hurricane winds sum of cyclone winds and
  storms forward motion
• When storm is moving quickly, this means winds
  are worst on the right-hand side of the
  hurricanes path
• Sept. 1938 Long Island hurricane hit New
  England at forward speed of 60 mph, causing
  catastrophic damage just to right of the eyes
  path

The winds in Andrew were also strongest to the
right of the eyes path (the storm moved due west
across Miami) http//www.usatoday.com/weather/wan
drew.htm
22
Hurricane hazards tornadoes

• Some hurricanes cause many tornadoes after
  landfall, particularly to the north/east of the
  hurricanes eye (right front quadrant)
• Why there? Maximum low-level vertical wind
  shear
• The biggest risk the first day or two after
  landfall, during daytime
• Over 100 tornadoes have been caused by a few
  hurricanes, including Ivan (below, from Virginia)
  and Frances in 2004

Note the low cloud base due to the very moist
air. This is what a hurricane-caused tornado
looks like.
http//www.recmod.com/hurricane/ivan/carolinecount
y-va-1a.jpg
23
Hurricane hazards storm surge

• High winds push ocean water ahead of the storm,
  causing a wall of water as high as three
  stories
• Not primarily due to low pressure sucking water
  up as in a straw!
• Due to its weight and momentum, this water wipes
  out everything in its path. Most deaths from
  hurricanes are due to storm surge.
• U.S. record storm surge Katrina in Mississippi,
  August 29, 2005, 28 feet (Gulfport, MS, below,
  from http//ngs.woc.noaa.gov/katrina/ )

24
Hurricane hazards storm surge, cont.
Hurricane Camille (24-foot surge) at Richelieu
Apartments, Pass Christian, MS, August 17,
1969 http//sciencepolicy.colorado.edu/about_us/m
eet_us/roger_pielke/camille/gallery.html
25
Hurricane hazards storm surgeHistory keeps
repeating itself

• Lake Okeechobee, Sept. 1928 Category 5
  hurricane hits Puerto Rico, then weakens some
  before hitting south Florida. As it moves over
  Lake Okeechobee, 140-mph winds blow lake water
  over poorly constructed dike, killing 1,836
  people, mostly black migrant farm workers. Story
  chronicled in Zora Neale Hurstons Their Eyes
  Were Watching God.
• Hurricane Katrina (Cat 5, then weakened) death
  toll as of 5/19/2006 1,836, plus 705 missing
  majority of deaths due to levee failures in New
  Orleans, killing primarily African-American
  residents. Story chronicled in Spike Lees When
  the Levees Broke.

http//en.wikipedia.org/wiki/Okeechobee_Hurricane
26
Hurricane hazards rainwater floods
Weakening hurricanes over land can produce
enormous amounts of rain, especially if they
interact with mid-latitude weather systems or
cross over mountains Camille over 30 of rain
in James River Valley of Virginia overnight, 153
dead, damage close to 1 billion in todays
dollars
http//en.wikipedia.org/wiki/Hurricane_Camille
27
Hurricane hazards more rainwater floods
Left Flooding in Greenville, North Carolina due
to Hurricane Floyd, 1999
Right some of the wettest tropical systems are
not the strongest. See this radar image of total
precipitation from (barely) Tropical Storm
Allison in 2001 (gt 25 in downtown Houston)
28
Early history of hurricane detection and
forecasting

• Galveston, TX, Sept. 1900 Category 4 hurricane
  hits, kills 6000 (only white deaths counted).
  U.S. Weather Bureau thought storm in Atlantic
  Ocean! Cuban meteorologists knew better by
  watching the skies, but key individuals in U.S.
  Weather Bureau were anti-Cuban, racist. Terrible
  forecast cost thousands of lives. Read Erik
  Larsons book Isaacs Storm.
• Great Hurricane of 1938 Hurricane warnings
  were posted for Long Island as the hurricane sped
  over it! No warning, hundreds of deaths.
• Texas, July 1943 Lt. Col. Duckworth makes 1st
  aircraft penetration of hurricane on a dare!
  Beginning of U.S. military hurricane hunting,
  which continues to be an indispensable source of
  real-time information on hurricanes for
  forecasters.
• 1960s-present weather satellites go into space,
  for 1st time global surveillance of tropical
  cyclones. No more surprises! Since 1980s, water
  vapor images help forecasters see steering
  upper-level winds.

29
Modern hurricane forecasting

• Forecasts made using computer models of
  individual hurricanes
• Not statistical/climatological, but instead use
  equations of atmosphere to predict particular
  storms at particular times
• In beginning, models were terrible, much better
  now (see below)
• Models still much better at location than
  intensity

30
Modern hurricane forecasting the untold Katrina
story

• Federal governments National Hurricane Center
  6 hurricane specialists, 100 years of forecast
  experience, 5 Ph.D.s.
• All of NWS (including NHC) 3 in taxes per
  American per year
• NHC made spot-on forecast of track 60 hrs before
  landfall
• Key change westward shift of forecast track from
  Apalachicola, FL to MS coast by 11 am on Friday
  before Monday landfall
• But many apparently werent listening to NHC!
• Their source of information private-sector
• AccuWeather tropical guru Joe Bastardi (right)
• Bastardi kept track over Florida until Friday PM
• One consequence oil markets in London
• didnt think Katrina would affect offshore rigs
• What about U.S. government?
• Did forecast confusion contribute to slow
  reactions?

31
Modern hurricane forecasting NHCs spot-on
Katrina forecast
FORECAST TRACK IN BLACK
ACTUAL TRACK IN RED
32
Modern hurricane forecasting Bastardi (left)
vs. NHC (right) late Friday aft.
http//www.stormeyes.org/tornado/blog/archives/acc
usux.jpg
33
Long-range hurricane forecasting

• Developed by Prof. Bill Gray, Colorado State
  University
• Idea forecast aggregate numbers for next years
  hurricane season for an entire basin, based on
  statistical relationships
• Key relationship between El Niño and hurricanes
• El Niño unusually warm water off Peru in winter
• Fewer Atlantic hurricanes than normal
• Why? El Niño -gt stronger subtropical jet stream
  -gt more vertical wind shear -gt disrupts hurricane
  circulation
• La Niña more Atlantic hurricanes than normal
• Forecast issued in December for next hurricane
  season, tweaked during year

34
Long-range hurricane forecasting results

• Method usually correct on whether or not a years
  hurricane season will be MORE or LESS active than
  normal (correlation is 0.57 over 24 years)
• 2005 and 2006 were tough, leading to complaints
• Overall, though, its highly useful information!

35
Long-range hurricane forecasting What will the
2009 season be like?WORSE THAN AVERAGE
ATLANTIC BASIN SEASONAL HURRICANE FORECAST FOR
2009 10 December 2008 Forecast Parameter
Forecast for 2009 and 1950-2000 Climatology (in
parentheses) Named Storms (NS) (9.6)
14 Named Storm Days (NSD) (49.1) 70
Hurricanes (H) (5.9) 7 Hurricane Days
(HD) (24.5) 30 Intense Hurricanes (IH)
(2.3) 3 Intense Hurricane Days (IHD) (5.0)
7 Accumulated Cyclone Energy (ACE) (96.1)
125 Net Tropical Cyclone Activity (NTC)
(100) 135
http//hurricane.atmos.colostate.edu/Forecasts/200
8/dec2008/dec2008.pdf
36
Hurricane safety

• Advance preparation
• Trim trees, clear gutters
• Get supplies
• Food and water
• Plywood, flashlights, batteries and/or generator
• Weather radio
• Make plan of action
• Evacuation routes
• What to do with pets, etc.
• Hurricane watch Winds of 74 mph or more possible
  within about 36 hours
• Hurricane warning Winds of 74 mph or more
  expected within 24 hours

37
In event of hurricane warning

• Do what the authorities say
• Hurricanes can kill via wind, tornadoes, seawater
  surge or rain-fed floods
• Dont get creative or clever
• In event of massive evacuation, interstates may
  be one-way
• In event of major hurricane, interstates could be
  underwater
• If you cant evacuate
• As with tornadoes, avoid mobile homes and windows
• Go to lowest floor unless at risk of flooding
• (this is where it gets complicated)
• Dont be fooled by the calm eye (if it passes
  over)
• Inland, be ready for high winds and flooding
• Fast-moving hurricanes can kill 100s of miles
  from coast

38
Main sources of information

• Ackerman, S.A., and J.A. Knox, Meteorology
  Understanding the Atmosphere (2nd edition),
  Brooks/Cole, 2007.
• http//www.nhc.noaa.gov/pastprofile.shtml
• http//www.nws.noaa.gov/om/hurricane/pdfs/hurrican
  e-safety_flyer.pdf

39
Questions?