Download hurricanes - Jacob Wall

Hurricanes
Chapter 15
Tropical Weather
• The tropics: an area 23.5° N and S of
equator
Tropical Weather
• Tropical areas have different
weather than the midlatitudes.
• Noon sun is always high,
seasonal temperature
changes small
• Daily heating and humidity =
cumulus clouds and
afternoon thunderstorms
• Seasons defined by
precipitation as opposed to
temperature
Streamlines
• Thunderstorms can align
into non-squall clusters or
tropical squall line
• Sudden wind gust with
heavy downpour
• Instead of using isobars on
maps, streamlines are
used to show wind flow.
• A tropical wave is a weak
trough of low pressure, wind
traveling east to west.
A tropical wave (also called an easterly wave) moving off the coast of
Africa over the Atlantic. The wave is shown by the bending of
streamlines — lines that show wind-flow patterns.
Figure 15-1 p426
Anatomy of a Hurricane
• Hurricane: intense storm of tropical origin
with winds greater than 64kts over north
Atlantic and eastern North Pacific.
• Western North Pacific, is called a
typhoon,
• India: cyclone
• Australia: tropical cyclone
Figure 15-11 p433
Anatomy of a Hurricane
• Eye – area of broken clouds at the center
– Clouds align into spiral rain bands that swirl towards
storm’s center, wrap around eye.
• Eye wall – ring of intense thunderstorms around
storm’s center
– Heaviest precipitation and strongest winds
• Anticyclonic divergence
– dry air aloft flows outward from center, clockwise aloft
• Latent heat: Rises and evaporates, sinks and
warms, decrease of clouds in eye
Because this storm is situated north of the equator, surface winds
are blowing counterclockwise about its center (eye).
Figure 15-2 p427
A model that shows a vertical view of air motions and clouds in a
typical hurricane in the Northern Hemisphere.
Figure 15-3 p428
The cloud mass is Hurricane Katrina’s eyewall, and the clear area
is Katrina’s eye photographed inside the eye on August 28, 2005,
from a NOAA reconnaissance (hurricane hunter) aircraft.
Figure 15-4 p428
Hurricane Katrina,
3-d satellite view
from TRMM
satellite. Areas of
heavy rain and tall
clouds show storm
is intensifying.
Figure 15-5 p429
Hurricane Formation and
Dissipation
• The Right Environment
– Tropical waters with light wind
– 26.5°C warm sea surface temperatures
(June-November is hurricane season)
– Needs a ‘trigger’ to start: surface winds
converge and spin counterclockwise around
low (N.H.) as a tropical wave. (intertropical
convergence zone ITCZ)
– Will not form at equator because no Coriolis
effect. Usually at 5-20º latitude in tropics.
The total number of hurricanes and tropical storms (red shade)
and hurricanes only (yellow shade) that have formed during the
past 100 years in the Atlantic Basin — the Atlantic Ocean, the
Caribbean Sea, and the Gulf of Mexico.
Figure 15-6 p429
Hurricane Formation and
Dissipation
• The Developing Storm
– Trade-wind inversion: sinking air with
subtropical high pressure.
– Cluster of thunderstorms around a rotating low
pressure
– Strong winds inhibit the formation of intense
storms
–
With El Nino there are fewer hurricanes over Atlantic
(stronger winds) and more over warm water of northern
Pacific.
– Energy comes from transfer of sensible and latent
heat from warm ocean surface.
Hurricane Formation and
Dissipation
• As latent heat is released inside the clouds,
warming air aloft creates an area of high
pressure, and air moves outward, away from the
high.
• Warmed air lowers air density, which in turn
lowers surface air pressure.
• Air flows towards the low inward
counterclockwise, picking up heat from warm
ocean
• Warm, moist air pushes up into clouds of eyewall,
releases latent heat, warms more, surface
pressure lowers more, winds blow faster.
Figure 15-7 p430
Hurricane formation
• Wind speed increases, picks up more heat from
warm ocean, transferring more energy.
• Greater evaporation rate, air becomes saturated
and more thunderstorms form.
• Heat is lost at top of clouds by radiation of
infrared energy to space, and by conversion to
kinetic energy, similar to a heat engine.
• Warmer ocean means lower pressure and
higher winds, maximum of 230mi/hr.
• https://www.youtube.com/watch?v=4f45jA5UxB0
3-D satellite
image of
Hurricane
Karl over the
Bay of
Campeche in
Sept. 2010,
with
precipitation
intensity
from the
satellite
CloudSat
Figure 15-9 p432
Arrows show surface
winds spinning
counterclockwise
around Hurricane Dora
situated over the
eastern tropical Pacific
during August, 1999.
Colors indicate surface
wind speeds.
Figure 15-10 p432
Hurricane Dissipation
• Most hurricanes last around a week, as
long as the storm stays over warm water
(energy source)
• They lose energy over cold water or large
land mass (friction) or in a region of strong
vertical wind shear.
• Cooling the eyewall by 2.5°C will cause a
storm to dissipate.
Hurricane Stages of Development
Stages of Development:
1. Tropical Disturbance
– Mass of thunderstorms with slight winds
2. Tropical Depression (22-34kts)
o Isobars close together around a ‘L’ center
3. Tropical Storm (35-64kts)
o Isobars packed very closely together, is named.
4. Hurricane (> 65kts)
Figure 15-8 p431
Investigating the Storm
• Visible, IR and surface wind satellites
– Can have 3-D image
• Aircraft ‘hurricane hunters’ carry
instruments on them:
– Dropsonde- dropped into storm, measuring
temp, humidity, and pressure, GPS for winds
– Bathythermograph – measures ocean
temp.
– https://www.youtube.com/watch?v=a-SnxCBkPo
Hurricane movement around the world
Figure 15-12 p433
Hurricane movement: much variation in N. America
Figure 15-13 p435
Hurricane formation
• Eastern Pacific hurricanes:
– About 9 each year, mostly move westward
away from coast.
– Only hurricane to reach west coast of U.S.
was in 1858, when a hurricane hit near San
Diego in California.
– Hawaii usually does not get hit, except in
1982 (Iwa) and 1992 (Iniki)- $1.8 billion in
damage to crops and homes in Kauai.
Hurricane formation
• North Atlantic Hurricanes
– Move westward and northward, around 3 per
year reach land.
– August, most into Gulf of Mexico or Florida,
September can move northward to Atlantic
coast.
– Atlantic hurricanes will survive longer than in
Pacific, because of warmer water.
Naming Hurricane and Tropical
Storms
• Process has changed over the years:
– Began as latitude and longitude
– Then letters of the alphabet (WWII)
– Alphabetical female names (1953-1977)
– Alphabetical, alternating female and male
names (1979+)
– Great damage, cat. 3 or higher, name is
retired for 10 years - (Katrina, Camille)
– p. 437 in book gives names, recycled every 6
years.
Devastating Wind, Storm Surge,
and Flooding
• Highest winds on the eastern side of storm
(winds push storm on right side and subtract
from winds on left side)
• Storm surge on east or north side of storm
(tide)
• Movement of water on top layer moves to the
right (counterclockwise), with net transport
called Ekman transport.
• Large waves, swells, move outward from
storm, hit coasts, can preempt storm
• Storm surge- rise of several meters in ocean
level due to net Ekman transport
Figure 15-16 p438
When a storm surge moves in at high tide, it can inundate
and destroy a wide swath of coastal lowlands.
Figure 15-17 p439
Hurricane Irene’s
path (dark arrows)
and estimated rainfall
totals over the
eastern United States
from August 26
through August 29,
2011.
Figure 15-18 p439
Devastating Wind, Storm Surge,
and Flooding
• Saffir-Simpson scale
– 1 weakest, 5 strongest, numbers can change
as storm intensifies or weakens
– Major hurricanes are Cat. 3 and above
– Typhoon over 150mi/hr is super typhoon
– Prior to 2010, measured central pressure and
storm surge, now measures wind speed
The number of hurricanes (by each category) that made landfall along
the coastline of the United States from 1901 through 2011. All of the
hurricanes struck the Gulf or Atlantic coasts.
Figure 15-20 p441
Figure 15-19 p440
Devastating Wind, Storm Surge,
and Flooding
• Hurricane spawned tornadoes
• One-fourth of hurricanes that hit the U.S.
produce tornadoes
• 300 produced in 2004 alone
• Tornadoes can also form in front of advancing
hurricane, strong updrafts stretch vortices
• Also can have microburst downdrafts that
have high damage similar to tornado
Hurricane-spawned tornadoes