Types of Thunderstorms
A typical thunderstorm is made up of a single cumulonimbus cloud.
This cloud is formed by a strong, vertical updraft tower (a column of
rapidly rising air) and is the building block of all other types of
storms. Side-effects of thunderstorms can include flooding rains, hail,
lightning, straight-line winds, downbursts, and tornadoes. Each will be
briefly described before discussing the different types of thunderstorms.
Flash flooding caused by heavy rain is the number one cause of
weather-related deaths in the United States. On June 9, 1972, 38cm (15")
of rain fell in Rapid City, ND within five hours. The result was 238
fatalities and $168 million in property damage (NWS). More recently, on
July 28, 1997, 25cm (10") of rain fell in Fort Collins, CO within six
hours. The result in this case was 5 fatalities and $200 million in
property damage (Petersen et al, 1999). Several factors contribute to
flash flooding. The two key elements are rainfall intensity and duration.
Intensity is the rate of rainfall, and duration is how long the rain
lasts. Topography, soil conditions (including previous rainfall amounts
and water table status), and ground cover play important roles.
Hail is yet another danger in a thunderstorm resulting in over $1
billion in damaged crops annually and countless dollars of personal
property damage. If hail becomes large enough, it can kill animals or
people, and severely damage roofs, motor vehicles, and other personal
property. For additional information about hail, see MESO's article
entitled "Hail Formation and Effects".
Lightning is always a danger. In the United States, there are
approximately 7,700,000 cloud-to-ground lightning strikes per year (15 per
minute) responsible for an average of 90 deaths and 272 injuries annually.
The majority of lightning casualties occurs in eastern states (FL, MI, PA,
NC, NY, OH, TN, GA), with the exception of Colorado and Texas (Curran et
al, 1997). For additional information about lightning, see MESO's article
entitled "Introduction to Lightning and
Lightning Safety".
A frequently forgotten aspect of thunderstorms is straight-line wind.
It is different than the downburst and tornado that will be discussed
shortly, yet still causes significant property damage because of its
frequency. All types of thunderstorms are capable of producing "severe"
winds (greater than 50 knots). Winds of this strength can damage roofs
and chimneys, break tree limbs, topple small trees, and destroy street
signs, then toss these missiles into buildings and cars. These winds are
created by the downdraft, which in turn is created by cold air rushing out
of the thunderstorm as precipitation inside of it evaporates and cools the
air. If the environment outside of the thunderstorm is particularly dry,
evaporation will be enhanced, and so will the threat for severe winds.
A downburst is defined as an area of strong winds produced by a
downdraft. Once it comes in contact with the earth, it spreads out
laterally and causes damage at ground level much like how water spreads
out from a wall
when you aim a hose at it. There are two types of downbursts, a
macroburst and a microburst. The macroburst covers an area larger than 4
kilometers in diameter, and the microburst is smaller than 4 kilometers in
diameter, as defined by the late Ted Fujita (1985). Both are a serious
threat to structures on the ground and to planes in the air. Damage
caused by downbursts is frequently mistaken for tornado damage. People
who have experienced this phenomenon describe it as a loud (sometimes
dark) cloud rushing toward them and when it reaches them, the winds become
very violent.
One of the most feared thunderstorm offshoots is the tornado, a
concentrated tube of rotating air that forms in a severe thunderstorm and
makes contact with the ground. There have been a multitude of studies
done on tornadoes over the years yet they remain one of Nature's greatest
mysteries. Tornado formation is always a threat associated with
thunderstorm activity; that is, there will not be a tornado in the absence
of a thunderstorm. Tornado intensity is varied as is the resulting
damage. They can last for seconds or hours and cause damage that ranges
from broken tree branches to buildings being lifted off their foundation
and large trees being uprooted.
Four primary types of storms are the single cell (ordinary), multicell
line (squall line), multicell cluster, and supercell. Different
lifetimes, structures, and necessary environmental parameters characterize
each one.
The Single Cell Thunderstorm
The ordinary or single cell thunderstorm is nothing more than
ephemeral
bursts of convection. Each burst creates a towering cumulus or "cell" of
convection. Once the cell gets large (and tall) enough, it is classified
as a cumulonimbus, or thunderstorm. The updraft and downdraft of this
small cloud interfere with each other, resulting in a fairly short-lived
cell, but the downdraft from one single cell may give birth to a new cell
nearby. This "parent-daughter" effect allows an ordinary thunderstorm to
pulse with convection for up to one hour.
When a single cell thunderstorm does form and produces severe weather,
it
is called a "pulse" severe storm. These often form in a more slightly
unstable environment, and tend to have a more intense updraft. If you
ever get caught under a pulse severe storm, one can expect lightning,
marginally severe hail and brief microbursts. Winds can be completely
storm dependent, but a drier sub-layer would increase confidence of wind
potential.
Because of the poor organization of these storms the forecaster may
find
it very difficult to forecast when and where they may occur. Most storms
of this type occur when CAPE (Convective Available Potential Energy)
values are between 1500-2000 J/kg and the vertical shear in the atmosphere
is less than 30 knots.
The Multicell Cluster
This is the most common type of thunderstorm. The name multicell
simply
means that there are many cells that grow to form a group of cells that
move together as one unit. A multicell cluster thunderstorm results from
a vigorous parent-daughter effect. In this case, an ordinary thunderstorm
creates neighboring storms via the downdraft and gust front. If the
surrounding atmosphere is unstable and moist enough (and with adequate
low-level vertical shear), new cells will be created around the old one,
giving birth to a cluster of active thunderstorms.
Each cell will be in a different phase of the life cycle. Each
individual
cell will have its turn to be the dominant one. New cells tend to form on
the western or southwestern edge of the cluster, while the more mature
cells are usually found in the center. The dissipating cells are often
found on the eastern or northeastern edge of the cluster. The multicell
cluster storms last a bit longer than ordinary, or single cell, storms and
cover a larger area. Each cell in a multicell cluster storm usually lasts
about 20 minutes, but the cluster of storms itself can persist for several
hours.
An MCC, or mesoscale convective complex, is a run-away multicell
thunderstorm. It lasts at least 6 hours (sometimes much longer) and cover
over 100,000 square kilometers: an area the size of Indiana (Bluestein,
1993). An MCC is large enough to actually alter the mesoscale or even
synoptic scale environment. By far, the greatest threat posed by an MCC
is the rain... counties or states under the MCC typically experience both
volume and flash flooding.
Because of the storm-storm interaction, it is rare to get extreme
weather
(large hail, tornadoes) from a multicell cluster, but heavy rain, strong
winds, and small to medium sized hail are still threats.
They can also produce flooding rains if they mature over the same
area...
a phenomenon called training. Winds of 70 knots are common, and hail up
to the size of golf balls may occur. A weak tornado may also form,
usually in the region where updrafts and downdrafts are close to each
other. The multicell cluster storm can occur in a variety of atmospheric
conditions, but mostly in areas where CAPE ranges from 500-1500 J/kg and
wind shear is less than 30 knots.
The Multicell Line or Squall Line
These types of storms tend to form in long lines with a well-developed
gust front at the leading edge of the line. Oftentimes there will be gaps
or breaks in this line. Typical lines of thunderstorms tend to have the
strongest most, mature storms in the south or southwest end of the line.
Moderate storms still not fully mature typically are found in the middle
of the line and the dying variety is found in the north or northeast
section of the line. On RADAR, bow echoes can be found denoting areas of
strong straight-line winds.
As the gust front moves forward, the cold outflow forces warm unstable
air
into the leading edge (usually on the eastern side) into the updraft edge
of the storm, with the heaviest rain and largest hail just behind (west)
the updraft. As these cells die they will produce lighter rains behind
the main squall line, so one often sees a broad region of stratiform rain
behind it.
Squall lines are prolific downburst producers. Occasionally an
extremely
strong downburst will accelerate a portion of the line forward causing it
to form what is called a bow. On the leading edge of this bow formation
is where you will find the most damaging winds. Tornadoes will sometimes
form just to the north of this bow, or on the line's southernmost cell,
which is called the anchor cell. Cells that form on the southern ends of
squall lines tend to inherit supercell characteristics since there is
nothing to the south of them to disrupt air flow being entrained into
them.
The Supercell
Finally, the epitome of thunderstorms is the supercell. This type of
storm is the classic cumulonimbus tower with the anvil top and on occasion
the overshooting updraft tower. These storms are notorious for producing
damaging straight-line winds, frequent lightning, flash floods, large
hail, and violent tornadoes.
This is a relatively small stand-alone entity that is so well
organized it
actually supports itself and enhances its own growth. The base of the
storm may be only 20-50 km across, but the anvil aloft can stretch for
many hundreds of kilometers. Conditions have to be just right in the
atmosphere in order for supercells to form. This may make them somewhat
easier to forecast than others. This storm type exist with CAPE values of
2500+ J/kg and vertical wind shear values in excess of 50 knots.
The supercell is unique because the updraft actually rotates; this
rotating updraft is called a mesocyclone. As mentioned earlier,
organization within the storm is incredible. The updraft and downdraft
regions do not interfere with each other like in the ordinary
thunderstorm; instead, they collaborate to prolong the life of the storm.
The inflow, outflow, updrafts, and downdrafts all work together like a
living, breathing creature. A supercell can exist in a quasi-steady state
for hours, advecting along the ground with the ambient wind (unlike an
ordinary storm that was a result of regenerating cells).
In order to form a supercell, three ingredients are necessary: high
thermal instability, strong winds in the middle and upper troposphere, and
veering of the wind with height in the lowest kilometer. As precipitation
is produced in the updraft, the strong upper-level winds blow the
precipitation away from the updraft, allowing little or none of it to fall
back down through the updraft core. In weaker wind environments, the
precipitation actually "chokes" off the updraft, causing the cell to die
quickly. Keeping the precipitation away from the updraft is what allows
the supercell to thrive for many hours, capable of producing violent
weather throughout its lifetime.
Light rain will begin as the supercell approaches. As the updraft
area comes closer, the areas to the north and east will see an increase in
rain and hail, then near the updraft itself (typically toward the rear) is
where most severe weather (tornadoes) will occur.
References
Bluestein, Howard B., 1993: Synoptic-Dynamics Meteorology in Midlatitudes: Volume II. Oxford University Press, 594pp.
CIMSS (Cooperative Institute for Meteorological Satellite
Studies): 08 July 1997 – Mesoscale ConvectiveComplex and Mesoscale
Vorticity Center. URL: http://cimss.ssec.wisc.edu/goes/misc/970708.html.
Curran, Brian E., Ronald L. Holle, and Raúl E. López, 1997:
Lightning Fatalities, Injuries, and Damage Reports in the United States
from 1959-1994. NOAA Technical Memorandum NWS SR-193, National
Weather Service Forecast Office, Fort Worth, TX.
Fujita, Theodore T., 1985: The Downburst – Microburst and Macroburst. Report of Projects NIMROD and JAWS. University of Chicago Press, 122pp.
Houze, Robert A., Jr., 1993: Cloud Dynamics. Academic Press, 573pp.
NWS (National Weather Service): The Rapid City Flood of 1972.
URL: http://www.crh.noaa.gov/unr/iwe/1972_Flood/index.htm.
Petersen, Walter A., Lawrence D. Carey, Steven A. Rutledge, Jason C. Knievel, Nolan J. Doesken, Richard H. Johnson, Thomas B. McKee, Thomas H. Vonder Haar, and John F. Weaver, 1999: Mesoscale and Radar Observations of the Fort Collins Flash Flood o
f 28 July 1997. Bulletin of the American Meteorological Society, 80, 191-216.
MESO, March 2001
http://www.mcwar.org
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