1.
Review of Tropical
Meteorology (Tropic of Capricorn
23.5 S to Tropic of Cancer 23.5 N).
·
Some key
points about tropical meteorology that
influence hurricane formation:
·
Region
includes the ITCZ (ascending
branch of Hadley cell) (Fig. 7.21).
·
Easterly
trade winds (Fig. 7.21).
·
No real
seasons because midday Sun is
always high in the sky.
·
Instead
they have a “rainy” season when
ITCZ heads north and a “dry” season when it heads south.
·
Warm water
in the oceans (except S.E.
Pacific,
·
Rain is
from normal cumulus clouds
(rarely from severe storms).
·
Extremely
weak pressure gradient on
surface WX maps makes isobars nearly useless for WX forecasting in the
tropics. Instead they observe wind
direction and plot
streamlines (Fig. 11.1).
·
NWS
National Hurricane Center and Tropical Prediction Center.
·
Sometimes
these streamlines show a “kink”
called an “easterly wave” or “tropical wave”, often beginning off W.
African
coast. This is where Atlantic hurricanes
seem to
be spawned (Fig. 11.1).
2.
Some Key Facts About
Hurricane Structure, Formation
and Dissipation
·
Called a
“hurricane”, a “typhoon”, a
“cyclone”, or a “tropical cyclone”.
·
Requires huge
energy input from warm ocean water (> about 82F).
·
Requires
Coriolis force to rotate, i.e.,
cannot from on or very near equator.
·
These two
requirements constrain hurricane
formation to tropical oceans (5-20 degrees N or S), and to late
summer/early fall
(Fig. 11.7).
·
Also, first
requirement leads to
dissipation when hurricane moves over land or over colder water.
·
An extremely
large (e.g., 300 miles
diameter) organized system of severe thunderstorms that slowly rotates
counterclockwise (N. Hemisphere) (Fig. 11.3).
·
Spiral bands
of rain clouds rotate around
“eye wall” (Fig. 11.3).
·
Center or
“eye” is region of extremely low
pressure (e.g., 900-950 mb), and is cloud free (curiously) because of sinking air within the eye, that warms
by compression (Fig. 11.3).
·
The resulting
pressure gradient force can
yield winds of 75-150 mph (i.e., fast but not as fast as a tornado).
·
Strong winds
generate huge waves (up to 50
ft).
·
Low pressure
in eye lifts ocean level (1-2
ft)
·
Heavy rains
(e.g., 10 inches per hour)
·
Storm surge
(waves, higher water level, and
heavy rain) causes flooding (the real danger) and loss of life.
·
Typical life
history is: tropical
disturbance >
tropical depression > tropical
storm >
hurricane ( >75
mph), but few make it through all stages (average 6/y
·
Hurricane
paths are hard to predict (no
strong steering winds) and can be erratic (Fig. 11.11, Fig. 4, pg. 319).
·
Saffir-Simpson
scale (Table 11.2):
·
Category 1
(>980 mb, 75 mi/h) to
Category 5 (<920 mb; >155 mi/h).