Download Tornado - metrofacultydevelopment

Chapter 10 part 3
• Tornado: a rapidly rotating column of air that blows around a small area of
intense low pressure with a circulation that reaches the ground, aka cyclone or
twister
• Funnel cloud: a tornado whose circulation has not reached the ground
• Apparently the clockwise rotating tornadoes are more rare than I thought and are
usually in the form of waterspouts (except in the SH where most are clockwise
rotating due to the opposite Coriolis force).
Tornado life cycle
• Dust-whirl stage– where dust swirling upward from
the surface marks the tornado’s circulation on the
ground and a short funnel often extends downward
from the thunderstorm’s base
•
•
•
•
Organizing stage– the tornado increases in intensity
with an overall downward extent of the funnel
Mature stage– damage normally is most severe as the
funnel reaches its greatest width and is almost vertical
Shrinking stage– there is an overall decrease in the
funnel’s width, an increase in the funnel’s tilt, and a
narrowing of the damage swath at the surface
Decay stage– tornado stretched into the shape of a
rope
Tornado Outbreak: When a large number of tornadoes forms over a particular region
• In May 2003, 516 tornadoes touched down in the US—the most in any month
ever.
Tornado Occurrances
• The US has the most tornadoes in the world with 1000 on average per year with a
record of 1424 in 1998
• Most occur in the central plains because it provides the proper atmospheric setting
for the thunderstorms that spawn tornadoes
• Most occur from March to July, where May averages about 6 per day!
Tornado Winds
FIGURE 10.27 The total wind speed of a
tornado is greater on one side than on the
other. When facing an on-rushing tornado,
the strongest winds will be on your left
side.
suction vortices: small rapidly rotating
whirls perhaps 10m in diameter inside a
tornado.
Seeking Shelter
• The pressure in the center of a tornado may be more than 100mb lower than its
surroundings
• There is a momentary drop in outside pressure when the tornado is above the
structure.
• Inside the house there is generally higher pressure and this can cause the roof to
lift off of a house or a general explosion of the house.
• Tornado watch: tornadoes may develop within a specific area during a certain
time period
• Tornado warning: a tornado has been spotted visually or on a radar screen. This
warning is issued by the National Weather Service (NWS)
• Where should you take shelter during a tornado warning?
• Fujita scale: classifies tornadoes according to their rotational wind speed based
on the damage done by the storm
Tornadic Thunderstorms
• Favorable Atmospheric Conditions
– Where the polar front jet stream and the cold, dry air cross the warm,
humid surface air
• Supercell tornadoes: tornadoes that form with supercell thunderstorms (storms
with strong vertical wind shear)
• FIGURE 10.31 Spinning
vortex tubes created by wind
shear.
FIGURE 10.32 The strong updraft in the
thunderstorm carries the vortex tube into the
thunderstorm, producing a rotating air column
that is oriented in the vertical plane.
Mesocyclone: a vertical column of cyclonically rotating air within a severe thunderstorm
FIGURE 10.34 A tornado-spawning
thunderstorm over Oklahoma City on May
3, 1999, shows a hook echo: (a hook
shape on a radar screen indicating the
position of the mesocyclone) in its rainfall
pattern on a Doppler radar screen. The
colors red and orange represent the heaviest
precipitation.
Wall cloud: the area of rotating clouds that lowers from the cumulonimbus from which
a funnel cloud may come out of
Nonsupercell tornadoes: tornadoes that do not occur in association with a pre-existing
wall cloud of a supercell
• Gustnadoes: a relatively weak tornado associated with a thunderstorm’s outflow.
It most often forms along the gust front
• Landspouts: relatively weak nonsupercell tornado that originates with a
cumliform cloud in its growth stage and with a cloud that does not contain a midlevel mesocyclone. Its spin originates near the surface, they look like waterspouts
but over land
FIGURE 10.37 (a) Along the boundary of converging winds, the air rises and condenses
into a cumulus congestus cloud. At the surface the converging winds along the boundary
create a region of counterclockwise spin. (b) As the cloud moves over the area of
rotation, the updraft draws the spinning air up into the cloud, producing a nonsupercell
tornado, or landspout. (Modified after Wakimoto and Wilson)
Severe Weather and Doppler Radar
• Doppler radar: a radar transmitter sends out microwave pulses and when this
energy strikes an object, a small fraction is scattered back to the antenna.
– 1. Precipitation particles are detected
– 2. It can measure the speed at which precipitation is moving horizontally
toward or away from the radar antenna to reveal the winds in a storm
• The TVS or Tornado vortex signature shows up as a region of
rapidly changing wind directions toward or away from the radar
•
•
•
Doppler lidar: the use of light beams to determine the velocity of objects such as
dust and falling rain by taking into account the doppler shift
– Sometimes storm chasers put these on their cars (doppler on wheels) to
peer into storms.
NEXRAD: NEXt generation weather RADar, there is a network of 150 of these
in the US, they read in the radar data and can help detect tornadoes, gust fronts,
derechos, microbursts, etc.
Waterspout: a rotating column of air over a
large body of water.
– It may be a tornado that formed over
land and traveled over water, then
called a tornadic waterspout.
– Fair weather waterspouts form the same
way landspouts do (see figure 10.37),
and the warm humid air near the water
helps create atmospheric instability
– The waterspout does not draw water up
into its core, but it does spray water at
the surface