Potter T.D., Colman B.R. (co-chief editors). The handbook of weather, climate, and water: dynamics, climate physical meteorology, weather systems, and measurements
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such as tornadoes, large hail (usually of diameter at least approximately 2 cm),
strong convective wind gusts (usually approximately 90 km=h or more), and extre-
mely heavy precipitation associated with flash floods (frequently 50 mm=hata
single location). Criteria associated with heavy precipitation are the most variable
from country to country and, in some places, even within one country. For instance,
in the United States, flash flooding is not considered a severe thunde rstorm event,
and in Canada the objective definition of heavy precipitation is different in different
geographical regions.
Tornadoes
Tornadoes have been observed on every continent except Antarctica, although they
are most common in North America, particularly the Great Plains of the United
States. Increased efforts to collect information about tornadoes in North America
have led to an increase in the number of reports, with an average of about 1200
tornadoes reported annually in the United States in recent years, compared to only
600 just 50 years ago. The increase has been particularly apparent in the number of
weak tornadoes (classified F0 or F1 on the Fujita damage scale that goes from F0 to
F5). Similar efforts in other countries have also led to large increases in the reported
number of tornadoes there, such as in Germany where the average prior to 1950 was
about 2 per decade, but in the 1990s was 7 per decade, with more than 20 reported in
the year 2000 alone. Climatologies of tornado occurrence in the United States have
identified the temporal and spatial structure of the threat. The strongest tornadoes
(F2 to F5 on the Fujita scale) are most often found in the Great Plains of the United
States (Fig. 1). This is a result of the frequent production of favorable environments
with warm, moist air near the ground, dry, relatively cool air aloft, and strong vertical
speed and direction al shear of the horizontal winds. The Gulf of Mexico acts as a
source region for warm, moist low-level air flowing north, and the Rocky Mountains
act as a source region for the dry, relatively cool air aloft flowing eastward toward the
Plains. The presence of these two fixed geographic features appears to be the domi-
nant reason for the frequency of tornadoes in the Plains.
The rarity of tor nadoes in other regions of the world does not mean that, when
events occur there, the effects are small. Landfalling tropical cyclones often produce
tornadoes. Historical ly, devastating tornadoes have struck Europe approximately
once every 20 years. Since 1984, individual tornadoes with hundreds of fatalities
have occurred in Russia, northeast of Moscow, and in Bangladesh. While it seems
that strong and violent tornadoes are much less common in other parts of the world
compared to the United States, it also appears likely that tornadoes are vastly under-
reported in the rest of the world. The prime evidence for this is that the majority of
reported tornadoes in many parts of the world are fatality-producing events or are
especially newsworthy (such as the 1998 tornado in Umtata, South Africa, while the
South African president was visiting the town). This situation is similar to that in the
United States in the middle part of nineteenth century, when only approximately 25
tornadoes per year were reported. Recent studies have indicated that probability of a
576 SEVERE THUNDERSTORMS AND TORNADOES
particular reported tornado being strong or violent is approximately the same over
most of the world (Fig. 2).
Hail
The d efinition of exactly what is severe hail is troublesome. For some agricultural
interests during some times of the year, even 1 cm in diameter hail may be devastat-
ing. For urban areas, it may take much larger hail, say 4 cm in diameter, to cause
problems. The distribution of regions prone to these levels of threat is very different.
The smaller limit occurs in much of the temperate world during the warm season.
Larger hail is typically limited to the central part of North America and regions near
major mountain ranges in the rest of the world (e.g., the Himalayas and Alps). It has
been suggested that extremely large hail is much more likely in supercell thunder-
storms than in ‘‘ordinary’’ thunderstorms. This is consistent with the observed
distribution of tornadoes, presumably associated with supercells, in the central
part of the United States. The lack of a relationship when hail of any size is
considered has been pointed out for China by showing that the freque ncy of hail
is maximized in the high plateau regions of western China while tornadoes are more
common in the eastern part of the country, particularly the Yangtze River valley.
Figure 1 (see color insert) Number of days per century an F2 or greater strength tornado
might occur within 25 miles of a point. For example, in southcentral Oklahoma one would
expect an F2 tornado within 25 miles about once every 3 years. Calculations based on data
from 1921 to 1995. See ftp site for color image.
2 CLI MATOLOGY OF SEVERE THUNDERSTORMS 577
The high plateaus and other regions downwind of mountains may produce large
hail because of the steep tropospheric lapse rates that develop as air comes over
mountains.
Unfortunately, observations of hail have not been consistent through the years. In
the United States, the number of reports of severe hail (approximately 2 cm or larger)
have increased by an order of magnitude in the past 30 years. Most of the increase
has been in the smaller end of the severe range. As a result, attempts to develop a
climatology based on the reports face significant challenges. Researchers are faced
with the dilemma of having small sample sizes or an inhomogeneous record. Efforts
to use insurance losses are complicated by the issue of what causes the losses
(agricultural versus urban interests) and the temporal inhomogeneity of the insured
base. Nevertheless, extremely large losses (greater than $500 million) have been
associated with hailstorms in areas such as Munich (Germany), Denver, Colorado,
and the Dallas–Fort Worth, Texas, region in the last 15 years.
Damaging Convective Wind Gusts
Strong winds associated with thunderstorms are a common feature. Little has been
done to document their climatological occurrence until recently, although they are
almost certainly the most common severe weather event. Damaging straight-line
thunderstorm wind gusts are usually associated with cold air outflow as the down-
Figure 2 Distribution of F-scale rating for tornadoes in various countries is roughly the
same, particularly for the violent (F4 to F5) tornado. See ftp site for color image.
578
SEVERE THUNDERSTORMS AND TORNADOES
draft of the storm reaches the ground, producing what has been termed a downburst.
Factor s that influence the generation of damaging wind gusts at the surface include:
negative buoyancy enhanced by evaporative cooling within unsaturated air, precipi-
tation loading within the downdraft, and downward transfer of horizontal momentum
by the downdraft. Again, aspects of these processes are dependent on storm-scale
microphysics including drop size distribution and liquid water content per unit
volume, neither of which can be determined from standard observing systems.
Strong winds can occur in a variety of situations. They can be associated with
small, short-lived downdrafts and even when they are relatively weak (say less than
25 m=s), they can be a significant hazard to aviation. Considerable effort has been
expended in the last 20 years to decrease commercial aircraft accidents due to
thunderstorm downdrafts. Radar detection and education of the aviation industry
about the threats seems to have limited the number of accidents in the last decade,
after several occurred from the early 1970s through the mid-1980s.
Larger areas can be affected by high winds when organized systems of thunder-
storms occur. In the United States, widespread convective wind events are some-
times referred to as derechos. They occur in association with mesoscale convective
systems, which are composed of a number of individual thunderstorms. Often, they
are arranged as a squall line, producing a wide area of high winds with new convec-
tive cells initiated on the leading edge of the outflow from earlier cells. The system
may maintain itself for many hours, provided sufficient low-level moisture and
midlevel unstable air can be found as the system moves along.
Flash Floods
Flash floods are the most widespread severe local storm phenomena associated with
large loss of life. They occur all over the world, especially in regions of complex
terrain. They are the most difficult to forecast, in part because they involve both
meteorological and hydrological aspects. Determining their effects is complicated
further by interactions with people and buildings. If a flash flood occurs in a location
where it does not impact life or property, it is unlikely to be reported. On the other
hand, relatively minor precipitation events may produce significant flooding if ante-
cedent condition s exacerbate the flooding as occurred in the Sh adyside, Ohio, flood
of 1990 with saturated soils or in the Buffalo Creek, Colorado, flood of 1996 when a
forest fire cleared vegetation from the area a couple of months before the rain event.
Great loss of life has been associated with flash flooding, even in developed
nations. Recently, a campground in Biescas in the Spanish Pyrenees was flooded
with more than 80 deaths. In 1998, 11 hikers were killed by a flash flood in a ‘‘slot
canyon’’ in northern Arizona when rain-generated runoff from a storm tens of kilo-
meters away was funneled into the canyon. (This storm, by the way, was accurately
located by NWS radar in southern Utah, and its potential impacts relayed to Park
Service personnel. Unfortunately those who died chose to ignore the warning.) The
three biggest convective-weather death toll single events in the United States (with
the exception of aircraft crashes) since 1970 have all been flash floods. Death tolls in
developing countries are frequently difficult to estimate.
2 CLI MATOLOGY OF SEVERE THUNDERSTORMS 579
Flash floods are distinguished from main-stem river floods by the extremely rapid
rate of rise of water levels. While main-stem river floods may have water stages
rising by tens of centimeters per day, flash floods are associated with water stages
rising by tens of centimeters per hour or, in extreme cases, per minute. Small streams
may carry 100 times their normal capacity and, often, it is very small basins that
produce flash floods. (In operational practice, even in the United States, this can
cause problems, since these small basins may not be mapped as well as larger basins,
particularly for comparison to radar estimates of precipitation. As a result, fore-
casters may be unaware of the nature of the threat even if accurate estimates of
rainfall are available.)
The threat from flash floods has increased in some regions because of the
increased use of mountainous regions for recreation. Excellent examples include
the Big Thompson (Colorado) and Biescas (Spain) floods. Public response to flash
flood forecasts is frequently poorer than for other weather hazards forecasts, such as
for tornadoes. Most people have experienced heavy rain events and fail to realize
until it is too late that the particular event underway is more dangerous. Further,
heavy rain often washes out roads and bridges, which makes escape and rescue
difficult.
Lightning
The frequency and location of cloud-to-ground lightning in the United States are
now known quite well. The deployment and operation during the last decade of
automatic real-time lightning detection sensors have made this possible. An average
of about 25 million cloud-to-ground flashes are detected by the Nationa l Lightning
Detection Network (NLDN) in the United States every year.
The map of network-detected flashes in Figure 3 shows that much of peninsular
Florida has the g reatest frequency of flashes per a rea over a year. Flash density
decreases to the north and west from there. Important local variations occur along
the coast of the Gulf of Mexico, where sea breezes and urban areas enhance light-
ning frequency. Other maxima and minima are located in and around the western
United States where there are mountains and large slopes in terrain.
Lightning is most common in summer (Fig. 4a); about two-thirds of the flashes
occur in June, July, and August. In the southeastern states, lightning is more
common throughout the year, since there often is a significant amount of moisture
in the lower and middle levels of the atmosphere. Air needs to be lifted strongly to
form lightning; coastlines and large mountains provide persistent updrafts that result
in lightning-producing thunderstorms.
During the course of the day, lightning is most common in the afternoon (Fig. 4b).
Nearly half of all lightning occurs from 1500 to 1800 Local Standard Time (LST).
Flashes are most common in the afternoon because the updrafts needed for thunder-
storm formation are strongest during the hours of the day when surface temperatures
typically are the highest, which results in the greatest vertical instability.
The primary information for lightning deaths and injuries in the United States is
the National Oceanic and Atmospheric Administration (NOAA) publication Storm
580 SEVERE THUNDERSTORMS AND TORNADOES
Data. Since most lightning casualties occur to one person at a time, and are more
dispersed in time and space than other severe weather phenomena, lightning
casualties are underreported.
The spatial distribution of deaths and injuries in Figure 5 shows the largest
absolute number to be in Florida, whose total is twice that of any other state.
Most of the other states with large numbers of casualties are among the most
populous in the country. However, when population is taken into account, the high-
est rates of lightning casualties per million people are found in the Rocky Mountain
states, Florida, and other states in the southeast. The time distributions of lightning
deaths and injuries by month and day in Figure 4 show a close resemblance to the
distribution of the actual lightning flashes in the same figure.
Over the last 100 years, the rate of lightning deaths has dropped significantly. The
decrease parallels a major shift from rural to urban settings for much of the U.S.
population and is apparent in similar records in Europe and Australia. The most
common activity of lightning casualty victims has also changed during this period
from agricultural to recreational.
Around the world, data from lightning sensors on satellites show that lightning
occurs most often over land in the tropical and subtropical areas of Africa, South
America, and Southeast Asia. The annual rates of lightning per area often exceed
those in Figure 3 for Florida.
Figure 3 (see color insert) Number of cloud-to-ground lightning flashes per year for the
United States based on data from 1996 to 2000. (Courtesy Global Atmospherics, Inc.) See ftp
site for color image.
2 CLI MATOLOGY OF SEVERE THUNDERSTORMS 581
About 100 years ago, the annual U.S. death rate from lightning was 6 per million
people, which is an order of magnitude higher than now. The United States then was
a more agriculturally oriented society, living and working in ungrounded and less
substantial buildings than are now common. This rate may continue to be appro-
priate for populous tropical and subtropical areas where lightning is freque nt.
There are now about 100 lightning deaths per year in the United States, but this
total could be around 1000 if the population was still rural, practiced labor-intensive
agriculture, and lived in less substantial dwellings. So, the worldwide death total
could be expected to be at least 10,000 per year, since many people continue to live
in such situations. A ratio of 10 injuries per death gives a global total of 100,000
injuries a year from lightning.
3 CLOUD PROCESSES AND STORM MORPHOLOGY
Ingredients of Deep Moist Convection
Deep moi st convection requires three ingredients to occur: instability, lift, and
moisture. Interesting weather can occur in the a bsence of these ingredients, but it
will not be deep moist convection. For example, in the absence of instability, forced
Figure 4 Lightning histograms showing flash rates by (a) month of year and (b) time of day.
(Courtesy of National Severe Storms Laboratory.)
582
SEVERE THUNDERSTORMS AND TORNADOES
ascent of moist air over topography or over a frontal surface can produce heavy
precipitation; but, without insta bility, it is not generally considered convection.
Sometimes the term forced convection is applied to this situation. In this section,
however, we will focus mainly on free convection, where all three ingredients are
present.
Before discussing the ingredients of convection individually, it is necessary to
introduce several important concepts. First is the concept of hydrostatic stability. In a
hydrostatically stable atmosphere the downward gravitational force associated with
the weight of the air is exactly balanced by the upward vertical pressure gradient
force (the pressure near the surface of Earth is greater than the pressure aloft,
Figure 5 Spatial distribution of deaths and injuries by U.S. state. (Courtesy of National
Severe Storms Laboratory.)
3 CLOUD PROCESSES AND STORM MORPHOLOGY 583
resulting in a pressure gradient force that acts upward). Generally, the atm osphere is
very close to hydrostatic stability at most times and locations. Another important
concept is that of the lapse rate, a measure of how the temperature in a column of air
changes with height. In the troposphere, the temperature almost always cools with
increasing height, so a positive lapse rate indicates decreasing temperature with
respect to height. As will be explained below, large lapse rates indicate rapidly
decreasing temperatures with height, and this is a favorable environment for the
development of convection.
Buoyant stability is a measure of the atmosphere’s resistance to vertical motions.
The primary way meteorologists measure stability is through parcel theory, a peda-
gogic tool in which idealized bubbles of air called air parcels are employed. An air
parcel is a small bubble of air that does not exchange heat, moisture, or mass with its
environment, i.e., the thermodynamic process is adiabatic. As air parcels sink in the
atmosphere, the increasing pressure they encounter causes them to compress and
warm. In fact, this rate of warming is a constant 9.8
C=km and is termed the dry
adiabatic lapse rate. Likewise, when air parcels rise in the atmosphere, the decreas-
ing pressure allows them to expand and cool at the dry adiabatic lapse rate. The dry
adiabatic cooling rate occurs as long as the parcel’s relative humidity is less than
100%. Saturated parcels (i.e., the parcel relative humidity is 100%) cool less as they
rise because cooling causes condensation of water vapor. A process that releases
latent heat and lessens the rate of cooling with height to a value ranging from 4 to
7
C=km, dep ending on the temperature and pressure of the parcel. The cooling rate
for a saturated parcel is termed the moist adiabatic lapse rate.
Parcel buoyancy is defined by comparing the parcel’s temperature to the
surrounding environment’s temperature. If an air parcel is warmer than its environ-
ment, it is said to be positively buoyant and the parcel will accelerate upward. If an
air parcel is colder than its environment, it is said to be negatively buoyant and the
parcel will accelerate downward. If an air parcel is the same temperature as its
environment, it is said to be neutrally buoyant and there is no net force on the
parcel. If the parcel buoyancy is large, the accelerations are significant and cause
the atmosphere to deviate significantly from hydrostatic ba lance. These nonhydro-
static forces are often large and an important mechanism for the development and
maintenance of severe thunderstorms.
Consider a moist, but unsaturated, air parcel near the surface of Earth. If the
actual lapse rate of the atmosphere is between the moist and dry adiabatic lapse rates,
then a rising air parcel will cool at the dry adiabatic lapse rate, a rate larger than the
environmental lapse rate. Thus, this parcel will be negatively buoyant and will need
to be forcibly lifted to continue to rise. As the air parcel cools and expands, it may
eventually reach 100% relative humidity, or saturation. The height of this point is
called the lifting condensation level (LCL). Further forced lifting will result in the air
parcel cooling at the moist adiabatic lapse rate. Eventually, the temperature of the
rising air parcel may become warmer than the environmental air at the same height,
becoming positively buoyant. The height of this point is called the level of free
convection (LFC). Typically, the positive buoyancy continues with height until
the air parcel rises near the tropopause, where the stability becomes larger, to its
584 SEVERE THUNDERSTORMS AND TORNADOES
equilibrium level, the point of neutral buoyancy. The integrated positive buoyancy
from the LFC to the equilibrium level can be computed and is termed the convective
available potential energy (CAPE), which is related to the maximum updraft possible
under parcel theory. The integrated negative buoyancy needin g to be overcome
during forced parcel lifting near the surface is called the convective inhibition
(CIN), and the layer over which the CIN occurs is called the cap or lid. When the
environmental lapse rate lies between the moist and dry adiabatic lapse rates, condi-
tional instability is said to exist. Conditional instability indicates that the atmosphere
will form convection if enough air parcels are lifted to the LFC by some mechanism.
CAPE is an important measure of the integrated instability in the atmosphere.
One way to generate a high CAPE environment is to get dry air with large lapse rates
on top of warm, moist air. During the spring in the central United States, this so-
called loaded-gun sounding frequently occurs when low-level warm moist air from
the Gulf of Mexico is overrun by dry air at midlevels from the Mexican Plateau or
the Rocky Mountains. This stratification can produce high-CAPE soundings, loaded
for a classic Central Plains outbreak of severe weather. A certain degree of CIN is
also required so that convection does not break out spontaneously and so that the
CAPE can build to large values as the surface temperatures warm during the day.
The second ingredient for c onvection to occur is that sufficient lift for the air to
reach its level of free convection is required. Often this arises due to surface airflow
boundaries where convergence occurs, forcing low-level ascent. These surface
boundaries can be fronts, drylines, sea-breeze convergence lines, horizontal convec-
tive rolls in the boundary layer, or even outflow boundaries from previous convec-
tion. These thunderstorms may occur due to buoyant thermals in the warm boundary
layer air near the surface of Ear th. Sufficiently strong thermals may have enough
penetration out of the boundary layer that their LFC may be reached, thus initiating
convection.
The final ingredient for convection is an adequate supply of moisture. For most
environments, such as the loaded-gun sounding, an increase in low-level moisture
results in an increase in CAPE. The dry air aloft is also important since it can quickly
evaporate the warm moist thermals bubbling up from the boundary layer. Evidence
suggests that it may take several generations of ever-deepening towering cumulus
congestus to sufficiently moisten the lower part of the dry layer in the loaded-gun
sounding. Witho ut sustained vertical motion and moisture, deep moist convection
may never develop, even from a well-primed loaded-gun sounding.
Although not essential for convection to occur, a factor that is important for
controlling the type of convective system that may develop is the vertical change
in the horizontal wind direction and speed. This is called vertical wind shear. (Read-
ers may be familiar with the term wind shear as it relates to aircraft accidents. In that
case, wind shear refers to the horizontal change in the horizontal wind speed and
direction. It is important to distinguish between the two different types of wind shear.)
The vertical wind shear is important to deep moist convection for several reasons.
Too much wind shear tears apart cloud elements, not allowing updrafts to develop
deeply and nearly vertical. Too little wind shear results in the downdrafts suppres-
sing the updrafts, shutting off the flow of warm moist air to the storm. The proper
3 CLOUD PROCESSES AND STORM MORPHOLOGY 585