Transcript H - University of Maryland - Department of Atmospheric and

Lectures on Thunderstorms and Tornadoes
Chanh Q. Kieu
Department of Atmospheric and Oceanic Science
University of Maryland
AOSC400, Fall 2008
Definition of thunderstorms
Thunderstorms are rain showers with accompanied thunder. As
thunder is a result of lighting, you can also define thunderstorms
as rain showers with lighting (NOAA/NWS)
Thunderstorm lifecycle
The typical lifecycle of a thunderstorm consists of three stages: towering,
mature cumulus, and dissipating stage
• Towering cumulus cloud
indicates lifting air. Such
lifting may be due to the
converging air associated
with anomaly heating of the
ground, or due to the
upstream terrain
• Usually little if any rain
during this stage.
• Lasts about 10 minutes.
• Occasional lightning
Thunderstorm lifecycle (cont’d)
• The air continues to rise up
to the tropopause. This upper
lid forces the air to be spread
out, resulting in a “devil”
shape.
• Most likely time for hail,
heavy rain, frequent
lightning, strong winds, and
tornadoes.
• Storm occasionally has a
black or dark green
appearance.
• Lasts an average of 10 to
20 minutes but may last
much longer in some storms
in the absence of wind shear.
Thunderstorm lifecycle (cont’d)
• This stage signifies the
dissipation of thunderstorms,
and it dominates by the
downdraft.
• Rainfall decreases in
intensity.
• Can still produce a burst of
strong downdraft near the
surface, thus cutting off the
inflows at lower levels.
• Lightning remains a danger.
Thunderstorm formation
Three important ingredients for thunderstorm formation are:
• Moisture,
• Instability, and
• a lifting mechanism.
Moisture sources:
• In North America, mostly
transported by large scale flows
from the Atlantic, Pacific, and
Gulf of Mexico.
• SST plays an important role in
controlling the moisture source
• Southeast US can access both
sources of moisture. This partly
explains for the large number of
thunderstorms here
Thunderstorm formation (cont’d)
Instability:
• Occurs when the air near the
surface is heated, or when cold
air aloft is migrating to some
area with warm surface.
• Cumulus cloud is generated as
the warm moist air rises,
condensed under such instability
• Note that “cold” and “warm”
here are in the context of
potential temperature. The naive
use of temperature will tell
nothing as the air above will be
always colder than below (why?)
Thunderstorm formation (cont’d)
Lifting:
• Fronts: the boundary between
cold air mass and warm air mass is
an ideal place for the lifting to
occur, as this is place where
instability is met (CAPE?)
• Dry lines: boundaries between
two air masses of different
moisture.
• Differential heating. Heating
efficiency over grass is not the
same as that over paved streets.
• Terrains: wind blows over terrain
will be forced to rise upstream.
Thunderstorm statistics
Thunderstorm classification
Thunderstorms can be classified based on the numbers of principle updraft
cores. The formation of different types of thunderstorm is caused by
various combinations of above three conditions
Single-cell: There is one updraft core, so-called “cell”. Such single cell
is usually formed in isolation with the other cells. Very often, there are
more than just one cell if the environment is favorable for the
development
singlecell
Thunderstorm classification
Multi-cell cluster: Has more than one cell, formed by mergers of
several storms. The multi-cell clusters are often associated with
convective updrafts near mountain ranges or strong cold fronts.
Sometimes, there is a “back building” storm upstream.
multicell
Thunderstorm classification
Multi-cell line (squall-line): multiple cells merge and form a line
instead of a cluster. May be as long as several hundred miles, move
very fast.
squalline
Thunderstorm classification
Supercell: a special kind of singlecell thunderstorm but stay as long
as few hours. Have very organized and strong updrafts (100 mph)
and downdraft cores. Extremely dangerous and responsible for
nearly all of tornadoes
supercell
Thunderstorm hazards
Thunderstorms are particular dangerous because of their products
during their developments including hails, strong wind, heavy rain,
tornadoes. Hails are produced by strong updrafts, which carry the
raindrops continuously back into the cloud.
Definition of tornadoes
The most severe products of thunderstorms are tornadoes. By
definition, a tornado is a violently rotating column of air extending
from a thunderstorm to the ground.
Tornado characteristics
• Wind speeds between 40 mph
and 110 mph
• Radius varies from few to
hundreds feet. Some stretch
more than a mile across, and
stay on the ground for dozens of
miles.[
• Some attain wind speeds of
more than 300 mph (480 km/h),
• Often travel a few miles before
dissipating.
• Rotate counterclockwise in NH.
Opposite in SN (Is Coriolis a
good explanation?)
Tornado scales
The Fujita (F) Scale is the most commonly used scale for rating the
damage of tornadoes, which is based on damage left behind by a
tornado. There are some disadvantages of the F scale, and a recently
revised scale, so-called Enhanced Fujita (EF) Scale seem to make
improvements to the original F scale. This EF Scale has replaced the
original F scale, which has been used to assign tornado ratings since
1971.
Tornado formation
Before thunderstorms
develop, vertical wind
shear creates an
invisible, horizontal
spinning effect in the
lower atmosphere
Updraft within the
thunderstorm tilts the
rotating tube from
horizontal to vertical.
An area of rotation of
few miles wide
(mesocyclone) is formed
within the storm. Most
tornadoes form within
this area of strong
rotation
Tornado classification
There are four basic types of tornadoes:
Multiple vortex tornado: a type of tornado in which two or more
columns of spinning air rotate around a common center, very often
observed in intense tornadoes. These vortices often create small areas
of heavier damage along the main tornado path.
Tornado classification
Satellite tornado: a weaker tornado which forms very near a large,
strong tornado. The satellite tornado may appear to rotate around a
larger tornado.
Tornado classification
Waterspout: simply as a tornado over water. Can be divided into two
smaller types: fair weather waterspout and tornado waterspout
Tornado classification
Landspout: a tornado over the ground that is not associated with a
mesocyclone. Share many defining characteristics, including relative
weakness, short lifespan, and a small, smooth condensation funnel
with fair weather waterspout
Tornado statistics
References
1. www.nws.noaa.gov
2. http://en.wikipedia.org/wiki/Tornado
3. http://www.nssl.noaa.gov/edu/safety/tornadoguide.html
4. http://en.wikipedia.org/wiki/Thunderstorm#Severe_thunderstorm
5. http://www.physicalgeography.net/fundamentals/7t.html
6. http://www.noaawatch.gov/themes/severe.php
7. http://www.scchealth.org/docs/ems/docs/prepare/tstorms.html