Thunderstorms and Tornadoes

Thunderstorm: a mesoscale weather system produced by vigorous convection that reach to great altitudes within the troposphere. It effects a relatively small area and is short-lived. A thunderstorm consists of one or more convection cells accompanied by lightning and thunder and, often, locally heavy rainfall (or snowfall), and gusty surface winds.

A thunderstorm cell typically completes its life cycle in 30 minutes to an hour, but sometimes lightning, thunder, and bursts of heavy rain persists many hours. This is because each thunderstorm cell may be at different stage in its life cycle. A thunderstorm may track in a different direction than the individual cells.

The life cycle of a convective cell is divided into three stages: cumulus, mature and dissipating.

Cumulus stage is the initial stage of a thunderstorm in which a cumulus cloud undergoes vertical and horizontal development reaching up to 8 to 10 km (5 to 6 mi.) height as they become a cumulus congestus and cumulonimbus, with a lateral dimension of 10 to 15 km (6 to 9 mi.) over a period of 10 to 15 minutes under favorable atmospheric conditions i.e., warm, humid and unstable air parcel located at surface. Cumulus stage is characterized by updraft throughout the entire system without precipitation, lightning and thunder as well.

Figure 14.2, page #373 (Ahrens)

Mature stage is the middle and the most intense stage of a thunderstorm in which a convective cloud undergoes further vertical development with tops exceeding 18 km (11 mi.) over a period of 15 to 30 minutes with precipitation reaching to ground. Mature stage is characterized by both updrafts and downdrafts with the gusty surface winds, heaviest rainfall, lightning and thunder. Strong winds at high altitudes distort the cloud top into a anvil cloud which has a flat top under extremely stable conditions.

Gust front (mesoscale cold front): the leading edge of a mass of relatively cool air that flow out of base of a thunderstorm and spreads along the ground well in advance of the parent thunderstorm cell. A gust frot passage is noticed with calm winds, then gusty winds with temperature drop and finally precipitation. An uplift along the gust front may trigger formation of secondary thunderstorm cell tens of kilometers ahead of the parent cell. The boundary of the gust front where the air is forced upward, often generating thunderstorms is called outflow boundary.

Figure 14.5, page #375 (Ahrens)

Figure 14.9, page #377 (Ahrens)

Mesohigh: an area of high pressure at the surface created by teh cold heavy pool of air associated with the downdraft.

Shelf cloud: a low, wedge-shaped, and elongated cloud that has a flat base occurring along a gust front and beneath and attached to a cumulonimbus cloud. It sometimes associated with a severe thunderstorms in the presence of strong damaging surface winds.

Figure 14.7, page #376 (Ahrens)

Roll cloud: a low, cylindrically shaped, and elongated cloud that occurs behind the gust front and beneath, but detached from a cumulonimbus cloud. It is seldom accompanied by severe weather.

Figure 14.8, page #377 (Ahrens)

Dissipating stage is the last stage of a thunderstorm in which precipitation spreads throughout the convective cell over a period of 30 minutes. It is characterized by weak downdrafts throughout the entire system with light-to-moderate rainfall.

Entrainment: the mixing of saturated (cloudy) air with unsaturated air that surrounds the cloud. It is one of the continuous process thoughout the life cycle of a thunderstorm.

Most thunderstorms develop within warm, humid and usually conditionally unstable maritime tropical air as a consequence of uplift i) along fronts, i) on mountain slopes, iii) via convergence of surface winds, or iv) through intense solar heating of the Earth's surface.

Outside the tropics, thunderstorms are classified as air mass thunderstorms, frontal thunderstorms, or mesoscale convective complexes, depending on the specific triggering mechanisms.

Air mass thunderstorm develops almost randomly within a mass of maritime tropical air. It is usually relatively weak system that is driven by the intense solar heating mostly during the warmest hours of the day, however, at some locations such as upper Mississippi Valley, air mass thunderstorms are more frequent at night.

Frontal thunderstorm is associated with lifting of air along the surface of a front, usually along or ahead of a cold front. In winter, it sometimes produce snow in northern regions. Frontal thunderstorms are generally more energetic than air mass thunderstorms and may develop at any time of day or night, since a frontal activity persists from hours to days.

Squall line: a line of intense thunderstorms occurring parallel to and ahead of a fast-moving, well-defined cold front. The squall line typically extends 100 to 300 km (60 to 80 mi.) ahead of the front, but it sometimes may extend for over 1000 km (600 mi.), with a huge supercell storms causing severe weather over much its length.

Figure 14.14, page #379 (Ahrens)

Derecho: strong, damaging, straight-line winds associated with a cluster of severe thunderstorms thatmost often form in the evening or at night. It occurs mainly on May, June, and July over the US.

Bow echo: a line of thunderstorms on a radar screen that has a shape of a bow. It associates wtih damaging straight-line winds and small tornadoes.

Figure 14.12, page #379 (Ahrens)

Figure 14.16, page # 380 (Ahrens)

Mesoscale convective complex (MCC) is a nearly circular cluster of many interacting thunderstorms covering an area of many thousands of square kilometers. The size of a MCC can be a thousand times larger than of an individual air mass thunderstorms.

A mesoscale convective complex is not associated with a front and usually develops under conditions of weak synoptic-scale flow. The life time of a MCC is at least 6 hours and often 12 to 24 hours depending on the speed of the system which is generally slow (15 to 30 km/hr) resulting widespread and substantial rainfall.

Mesoscale convective complexes account for a substantial portion of growing season rainfall over the Great Plains and Midwest. They are primarily warm season (March through September) phenomena that generally develop at night. MCCs occur mostly over the eastern two-thirds of the US, where more than 50 may be expected in a single season.

Figure 14.17, pae #381 (Ahrens)

It is estimated that more than 40,000 thunderstorms occur each day throughout the world. Thunderstorms are most common over the continental interiors of tropical latitudes such as steamy Amazon Basin of Brazil, the Congo Basin of equatorial Africa, and the islands of Indonesia that experience thunderstorm activity at least 100 days.

Central Florida, which experiences more 90 thunderstorm days due to sea-breeze circulation, is the most frequent thunderstorm site in North America. The regions east of the Rocky mountains that subject to intense temperature gradient arising from variations in topography, have more than 60 thunderstorm days on average.

Figure 14.26, page #388 (Ahrens)

Forced convection: the convection aided by topographic uplift. It is responsible for high thunderstorm percentage at the regions east of Rocky mountains.

Free convection: the convection triggered by intense solar heating of the Earth's surface. It is responsible for high thunderstorm percentage at Central Florida.

Severe thunderstorm: a thunderstorm that is accompanied by locally damaging surface winds i.e. winds stronger than 93 km/hr (58 mi./hr); frequent lightning; large hail i.e. 1.9 cm (0.75 in.) or larger in diameter; or tornadoes or funnel cloud. When the updraft is tilted as a result of strong vertical shear in the horizontal wind, precipitation in a thunderstorm falls alongside rather than against the updraft. Hence, the updraft maintains its strength and continues to build the cell to great altitudes.

Severe thunderstorm cells usually form along a squall line within the cyclone's warm sector, ahead of and parallel to a fast-moving, well-defined cold front. A midlatitude jetstream tilts the updraft, thereby favoring the vertical development of the cell. It also causes dry air to subside from its left-front quadrant over a surface layer of maritime tropical air which is surged particularly at 3 km (9800 ft) altitude by a low-level jet. This produces a layering of that can lead to explosive convection and the development of severe thunderstorms.

Lightning: a flash of light by an electrical discharge of about 100 million volts in response to buildup of an electrical potential between cloud and ground, between clouds, or within a cloud.

On a clear day, the Earth's surface is negatively charged, and the upper troposphere is positively charged. In the presence of a cumulonimbus cloud, a positive changed is developed on the ground directly under the cloud. Within the cloud, a pancake-zone of negative charge region of a few hundred meters thick and several kilometers in diameter forms between the two positive charged regions at the upper and much narrower lower portions of the cloud.

Figure 14.35, page #404 (Ahrens)

Cloud-to-ground lightning, which is usually initiated from mountain tops or tall structures such as antenna towers, consists of a very rapid sequence of events involving stepped leaders, return strokes, and dart leaders. Each discharge covers 50 to 100 m long from the cloud base toward the ground.

Stepped leaders are the initial electrical discharges in a lightning, consisting of negative electrical charge that travel from the cloud base to within 100 m of the ground. It is usually invisible to the human eye.

Return strokes are the positively charged electrical current that emanates from the ground and meets a downward-moving stepped leader. It is much larger, several centimeters in diameter, and is more luminous than the stepped leader.

Dart leaders are the surges of negative electrical charge that follow the conductive path formed by the initial stepped leaders and return stroke of a lightning bolt. It proceeds more rapidly due to less electrical resistance at the path.

Figure 14.36, page #406 (Ahrens)

Typically, a single lightning discharge consists of two to four dart leaders each followed by a return strokes, negative cloud-to-ground lightning . A positively charged leader emanates from the cloud and initiates a lightning discharge in less than 10% of the cases. The latter, positive cloud-to-ground lightning , is more common in spuercell thunderstorms and had potential to cause more damage due to higher current level and longer flashes.

Electricity flows nearly 50,000 km/sec (31,000 mi/sec); and hence, the entire lightning sequence takes place in less than two-tenths of a second. Sheet lightning consists of bright flashes across the sky indicating cloud-to-cloud lightning.

Heat lightning is simply light reflected by clouds from distant thunderstorms that occur beyond the horizon.

Ball lightning lookslike a luminous sphere. Dry lightning is the ground-to-cloud lighning with no rain and often starts the forest fires. Lightning heats the air along the narrow conducting path to temperatures that may exceed 25,000C (45,000 F).

Figure 14.38, page #407 (Ahrens)

St. Elmo's Fire:Luminous greenish or bluish light above antennas, power lines, wings of planes, or masts of ships during a thunderstorm.

Figure 14.39, page #408 (Ahrens)

Light travels with a speed of 300,000 km/sec (186,000 mi./sec), which is a million times faster than speed of sound. One can determine the distance between the observer and a thunderstorm by counting the time in seconds between the lightning flash and the thunder and dividing by 5. For example, if the thunder is heard 10 sec. after lightning is seen, the lightning strike occurred about 2 miles from the observer's location.

Lightning is deadly! On average, it kills about 80 people and injures 300 per year. It also damages more than $400 million value of electrical equipment each year.

Lightning can strike more than 10 miles away from any rainfall. More than 50% of the lightning fatalitites occur after the storm passed. A national lightning detection network (NLDN) provides real-time information on the location and severity of cloud-to-ground lightning strokes.

Figure 14.42, page #410 (Ahrens)

Downburst: downward directed strong and potentially destructive winds that diverge horizontally as they strike the Earth surface. It is associated with a thunderstorm and with or without rainfall. Downbursts blow down trees, flatten crops, and wreck buildings. Based on size, a downburst is classified as either a macroburst or a microburst.

Macroburst is a downburst that affects a path of longer than 4 km (2.5 mi) with surface winds topping 210 km/hr (130 mi/hr).

Microburst is a downburst that affects a path of 4 km (2.5 mi) or shorter with surface winds topping 270 km/hr (167 mi/hr). It is accompanied by rapid changes in wind speed and direction, wind shear.

Microburst is particularly dangerous for airplanes during takeoff and landing. Federal Aviation Administration (FAA) requires airlines to install an approved mircoburt detection system on their aircraft. A microburst is shorter-lived, but more destructive than a macroburst.

Figure 14.11, page #390 (Ahrens)

Figure 14.12, page #391 (Ahrens)

Flash flood: a sudden rise in river or stream levels causing flooding. It is generally associated with a stationary or slow-moving intense thunderstorm. This stage of a thunderstorm results from weak winds aloft (less than 35 km/hr above 3 km) and/or the presence of persistent flow of humid air up a mountain slope.

Flash flooding is a special hazard in mountainous terrain, where excess water runs off to creeks, streams, rivers, or sewers, or collects in other low-lying areas. Because of their design and composition, urban areas are prone to flash floods during intense rainfall.

Figure 2, page #387 (Ahrens)

Synoptic-scale flood: a gradual raise of river and/or lake level due to continuous rain for several weeks or longer period.

Figure 14.28, page #401 (Ahrens)

Hail: the precipitation particles in the form of conical or lump shape ice crystals with or without a thin surface of liquid water. It develops in intense thunderstorm cells characterized by strong updrafts, great vertical development and an abundant supply of supercooled water droplets.

Hailstones range from pea size to the size golfball or even larger. The largest hailstone ever recorded in the United States was in Aurura, Nebraska (June 2003) and it had a diameter of 17.8 cm (7 in.) and circumference of 47.6 cm (18.7 in.) - about the size of a softball.

Figure 7.28, page #185 (Ahrens)

A hailstone forms when an ice pellet grows by accretion (addition) of freezing water droplets. In general, stronger the updraft, the larger will the ice crystal grow producing larger hailstones. A hailstone results from an alternating layers of clear (graze) and opaque (rime) surfaces. As many as 25 layers have been counted in a single large hailstone.

Figure 7.29, page #185 (Ahrens)

Hailsreak is the accumulation of hail in a long, narrow path along the ground. A typical hailstreak may be 2 km (1.2 mi) wide and 10 km (6.2 mi) long, and a single thunderstorm may produce a severe hailstreak. It forms following hail formation, weakening updraft and hailstones reaching the ground. It takes about 15 to 20 minutes from the formation of hail to the depositing hail at surface.

Hail can break windows and dent cars, batter roofs of homes, but the most costly damage is to crops. It usually falls during the growing season (spring) wiping out the agricultural products. Hail damage is around hundred millions of dollars annually in the United Sates.

Hail frequency is not necessarily related to thunderstorm frequency. For example, Central Florida, experiences the highest frequency of thunderstorms in the US, is almost hail free region. In North America, hail is most frequent on the High Plains just east of the Rocky Mountains, where it can be expected to fall from 10% of all thunderstorms.

Figure 14.27, page #389 (Ahrens)

Tornado: a small mass of air that whirls rapidly about a nearly vertical axis around a small area of intense low pressure. It is made visible by clouds and by dust and debris sucked into the system. Tornadoes can have variety of shapes ranging from cylindrical-shaped funnels to long, slender, ropelike pendants. A funnel cloud is a tornado whose circulation has not reached the ground.

A typical tornado on the ground has diameters of between 100 and 600 m with winds less than 182 km/hr (113 mi/hr) and a lifetime of only one to three minutes. An intense tornado, on the other hand, can have diameters exceeding 1600 m with winds up to 513 km/hr (318 mi/hr) and a lifetime of more than 2 hours.

Tornadoes move in circles with their the average forward speed of around 55 km/hr (34 mi/hr), but sometimes approaching 240 km/hr (149 mi/hr). Most tornadoes travel with a severe thunderstorm cells, supercell, usually (about 90% of time) from southwest to northeast.

Most tornadoes usually evolve through a series of stages: dust-whirl stage, organizing stage, mature stage, shrinking stage and decay stage. Some minor tornadoes, however, may evolve only through the organizing stage, some others may skip the mature and shrinking stages and go directly into the decay stage.

An extremely steep horizontal air pressure gradient between the tornado center and outer edge that is responsible for a tornado's violence, occurs during mature stage. The air pressure drops by about 10% over a distance of only 100 m. The Coriolis effect is also present, but the system is so small that its influence is negligible. Thus, winds in a tornado may rotate in either clockwise or counter-clockwise, but the latter dominates by far in the Northern Hemisphere. In a violent tornado (> 180 knots), small whilrs called suction vortices rotate themselves causing damage.

Figure 14.47, page #409 (Ahrens)

Tornadoes take about 100 lives each year, even though over 100 may die in a single day (553 fatalities in 2011). Hazards of tornadoes are i) extremely high winds, ii) strong updraft, iii) subsidiary vortices, and iv) an abrupt air pressure drop. Tornadic winds blow down trees, power poles, buildings, and other structures. A sequence of tornadoes, non-supercell tornadoes, over distances of 100 km or more are relatively more frequent, but less destructive than a single supercell tornado.

Figure 14.42, page #406 (Ahrens)

Figure 14.45, page #408 (Ahrens)

Tornadoes occur in many parts of the world, but no country experience more tornadoes than the US, which averages 700 to 1100 annually and experienced 1691 tornadoes during 2011. Although tornadoes have occurred in every states, most occur in tornado alley, a region that stretches from central Texas to Nebraska. Central Oklahoma has the highest annual incidence of tornadoes, whereas local tornado maxima occur in central Illinois and Indiana,in southern Mississippi, and Central Florida.

Figure 14.43, page #407 (Ahrens)

Tornadoes slightly above 50% develop during the warmest hours of the day (10 a.m. to 6 p.m.). The peak months for the tornado activity are April (13%), May (22%), and June (21%). The factors that contribute to the spring peak in tornado frequency are i) the relative instability of the lower atmosphere, and ii) ideal synoptic weather conditions.

Figure 14.44, page #408 (Ahrens)

The maximum tornado frequency shifts northward in late Spring and southward in early Autumn following the sun. The chance that one would experience a tornado is very little since less than 1% of all thunderstorms produce a tornado.

Tornadoes are classified in six intensity scale, called F- scale following Professor T. Fujita of the University of Chicago, based on the estimated wind speed from property damage. The American Meteorological Society (AMS) reports that 79% of all tornadoes are weak, 20% are strong, and only about 1% are violent in a typical year.

Tables 14.2,14.3 page #410 (Ahrens)

The most intense tornadoes usually appear on the rain-free rear portions of a severe thunderstorms. A typical sounding of temperature and dew point in the warm sector before a tornado occurs shows the conditionally stable, warm and humid air between the surface and 800 mb followed by a shallow inversion that acts a cap on the moist layer below. A cold, dry and conditionally stable air existing above the inversion produces convective instability, meaning favorable condition for the tornado formation.

Figure 14.52, page #413 (Ahrens)

Figure 14.24, page #393 (Ahrens)

Wall cloud: an area of rotating clouds that extends beneath a supercell thunderstorm and from which a funnel cloud may appear.

Figure 14.59, page #418 (Ahrens)

A tornadic circulation raises from an interaction between the updraft in the thunderstorm and the larger scale horizontal wind. The horizontal wind exhibits a storng vertical shear in both direction and speed as wind speed increases with height and wind direction veers (turns clockwise) with altitude as it is from southeast at the surface and soutwest or west aloft. The shear in the horizontal wind initiates the cyclonic rotation in the updraft. The rising, spinning column of air of 10 to 20 km in diameter is known as mesocyclone. It represents the organizing stage of a tornado and is evolved as one or two into a tornado.

Figure 14.55, page #415 (Ahrens)

The mesocyclone circulation begins in the mid-troposphere followed by upward and downward motion. During the mature stage of a tornado, mesocyclone stretches vertically and shrinks horizontally as the spinning intensifies and extends downward to the cloud base, known as tornado cyclone.

The intensity of a mesocyclone can vary on that F-scale from weak to devastating. Most of the North American tornadoes are linked to thunderstorms associated with midlatitude cyclones. Most of the others are the product of convective instability triggered by hurricanes. Tornadoes often develop on the northeast sector of a hurricane, after the system has curved toward the north and northeast. Most hurricanes that strike the southeastern United States are accompanied by tornadoes.

Waterspout: a rotating column of air over a large body of water. It is usually less energetic, smaller, and short-lived than a tornado. It is associated with a cumulus congestus or cumulonimbus cloud.

Figure 14.63, page #421 (Ahrens)

Radar (Radio detection and ranging): an instrument that sends and receives microwaves to determine the location, movement and intensity of precipitation. Radar waves are scattered by precipitation but not scattered by the very small droplets and ice crystal that compose the clouds. Thus, weather radar detects the precipitation particles but not the parent clouds.

Radar echo: a portion of the radar waves that is scattered back to receiving unit and is displayed as bright spots on the radar screen. The intensity of a radar echo depends on the size, phase and concentration (lesser extent) of the precipitation and is greatest for large raindrops and hail. Therefore, thunderstorms exhibit strong echo intensity.

Radar signals are sent out and received hundred of times each second as the radar continuously scans a 360 circle. Since the speed of the radar pulse is known, the time interval between emission and reception of the radar signal can be calibrated to give the distance of the precipitation.

Hook echo: a distinct radar pattern that often indicates the presence of a severe thunderstorm and perhaps tornado. It usually appears on the south side of a severe thunderstorm cell in the presence of a mesocyclone.

Figure 14.54, page #414 (Ahrens)

Bow echo: a line of thunderstorms on a radar screen that appears that appears in the shape of a bow. Bow echoes are often associated with damaging straight-line winds and small tornadoes.

Figure 14.17, page #389 (Ahrens)

Figure 14.18, page #389 (Ahrens)

Ground clutter: the reflection of radar signals by fixed objects on the Earth's surface. It appears all the time and is readily distinguished from precipitation echoes.

Doppler radar: a conventional radar that can also detect the detailed motion of precipitation toward and away from the radar. As precipitation particles move away from or toward the radar unit, the frequency of the radar signal shifts slightly between emission and reception. The frequency shift, Doppler effect named after Johann Christrian Doppler, is calibrated in terms of motion of the precipitation. Multiple Doppler radar units provide a 3-D image of air circulation.

Doppler radars detects gust fronts, wind shears, mesocyclones and can provide more advance warning (typically 20 minutes) of severe weather than does conventional radar (typically 2 minutes).

Figure 14.60, page #419 (Ahrens)

Figure 14.61, page #419 (Ahrens)

Today, a network of more than 150 dual polarized Doppler radars, called Weather Surveillance Radar - 88 Doppler (WSR-88D) operates across the United States. The WSR -88D was built by NEXRAD (next generation weather radar), a joint effort of the National Weather Service (NWS), US Air Force, and FAA, will provide data for civil, military, and aviation needs. The dual polarized radars provide information on hydrometeor identification and improve precipitation estimation.

Wind profiler: a vertically pointed Doppler radar that measures the wind speed and direction at 65 different altitudes up to 16.25 km at its location. A network of 35 wind profilers are currently operated in 18 midwestern states, and Alaska by the National Atmospheric and Oceanic Administration (NOAA).

Precipitation profiler: is a vertically pointed radar. It operates at 915 MHz or higher frequencies. It measures the reflectivity, Doppler velocity, and spectra width of falling hydrometeors. Doppler velocity is the sum of air and hydrometeor fall velocity and it cannot be readily distinguished.

Sodar (Sonic detection and ranging): is a wind profiler which measures the scattering of the sound waves by atmospheric turbulence. They are used to measure the wind speeds at various heights and thermodynamic structure of the lower at mosphere.

Lidar (Light detection and ranging): an instrument that uses light to generate intense pulses that are reflected from atmospheric particles of dust and smoke. Lidars are used to determine the amount of particles in the atmosphere as well as particles movement that has been converted into the wind speed.