nas 125: meteorology thunderstorms and tornadoes

NAS 125: Meteorology

Thunderstorms and Tornadoes

Rev. 30 April 2006 Thunderstorms and Tornadoes 2

Tornado outbreak, part 1

• On 3 May 1999, more than 70 tornadoes struck an area stretching from northern Texas to south-central Kansas.– Twenty-six tornadoes struck in or near Oklahoma City.

• The most intense tornado, F5 on the Fujita scale, tore through Moore and Bridge Creek, claiming 28 lives.– An F4 tornado killed six at Haysville, Kansas, a few hours

later.



• Rawinsonde data from Norman, Okla., indicated a shallow layer of warm, humid air near the surface with dewpoints near 21 °C.

• The warm, humid layer was capped by a temperature inversion at about 1,500 m with much drier air above.– The inversion layer kept convection currents in check, but

as the temperature and humidity contrast grew during the day, deep convection began, leading to the developmen tof severe thunderstorms.



• The rawinsonde indicated a vertical wind shear, with southerly winds at the surface to westerly or west-southwest winds aloft.– The wind shear contributed to the development of severe

weather.

• The arrival of a jet streak in the afternoon lifted the capping inversion, allowing the development of massive supercell thunderstorms.


Thunderstorms, part 1

• Thunderstorms are violent convective storms accompanied by thunder and lightning; they are usually localized and short-lived.– Vertical air motion, considerable humidity, and instability

combine to create towering cumulonimbus clouds, so thunderstorms are always associated with this combination.

– They frequently occur in conjunction with other kinds of storms (hurricanes, tornadoes, fronts [especially cold fronts]) in midlatitude cyclones, and orographic lifting.

– They are associated with other mechanisms that can trigger unstable uplift.


Thunderstorms, part 2

• Thunderstorms (continued):– Thunderstorms are virtually unknown poleward of 60˚ of

latitude.

– Thunderstorms often occur in clusters.


Life cycle, part 1

• Thunderstorms have a life cycle that consists of there stages: cumulus stage, mature stage, and dissipating stage.

• In the cumulus stage, updrafts prevail and clouds grow.– Cumulus clouds quickly surge to altitudes of more than

8,000 m.

– Cumulus clouds also merge horizontally, such that the lateral size may reach 15 km.


Life cycle, part 2

• Cumulus stage (continued):– Two types of convection drive storm development: free

confection and forced convection.• Free convection is caused by intense heating of the Earth’s surface,

but is not usually adequate to generate a thunderstorm.

• Forced convection is driven by frontal or orographic lifting or by converging surface winds; most thunderstorms result from forced convection.

– Latent heat is released by condensation of water; this extra heat adds to the buoyancy of saturated air parcels, which rise and cool at the moist adiabatic rate.


Life cycle, part 3

• Cumulus stage (continued):– Two types of cumuliform clouds form:

• Clouds with significant vertical development that resemble a huge cauliflower are called cumulus congestus.

• Cumulus congestus clouds that continue to develop vertically such that precipitation, lightning, and thunder are produced, are called thunderstorm clouds – cumulonimbus clouds.

– No precipitation falls during the cumulus stage because updrafts are strong enough to keep water droplets and ice crystals suspended in the upper reaches of the cloud.


Life cycle, part 4

• The mature stage begins when precipitation reaches the ground.– It is the most active and largest stage, with cloud tops

reaching 18,000 m; they often have an anvil shape where high-altitude winds distort cloud shape.

– The weight of water droplets and ice crystals becomes too great for them to remain suspended by the force of the updrafts.

– As the water droplets and ice crystals fall, they drag adjacent air with them to form a downdraft, which descends along the updrafts building the storm.


Life cycle, part 5

• Mature stage (continued):– Unsaturated air at the edge of the cloud is dragged into the

cloud in a process called entrainment.• Entrainment causes some of the water mixes with the saturated air,

causing water droplets and ice crystals to vaporize, which in turn weakens the updrafts and strengthens the downdrafts.

– The downdraft exits the base of the cloud, hits the ground and spreads out across the surface in a strong, cool wind resembling a mini cold front called a gust front.


Life cycle, part 6

• The mature stage begins when precipitation reaches the ground.– Unusual clouds are associated with the gust front.

• Roll clouds are tube-shaped clouds, not connected to the cumulonimbus cloud, that form behind the gust front; they appear to roll horizontally.

• Shelf clouds – or arcus clouds – are low, elongated, wedge-shaped clouds that are attached to the cumulonimbus cloud; they form at the edge of the gust front.

• In the dissipating state, downdrafts dominate, and turbulence ceases.

• Adiabatic warming in the downdrafts contributes to the drying up of moisture that fuels the storm.


Classification, part 1

• Single-celled thunderstorms are weak, isolated systems that form anywhere in warm, humid air masses.– They usually form long some boundary within the air mass.

• Multi-cellular thunderstorms consist of more than one storm cell, each of which may be at different stages of the thunderstorm life cycle.– A squall line is an elongated cluster of thunderstorm cells.

• They typically form along frontal boundaries, thus individual storms are called frontal thunderstorms.


Classification, part 2

• A mesoscale convective complex (MCC) is a nearly circular cluster of interacting thunderstorm cells.– MCCs may be 1000 times larger than individual storms.

– MCCs occur mostly in the warm season, and are not associated with a front.

• Supercell thunderstorms are long, lived, large, intense storms, composed of a single cell, with an updraft that may exceed 240 km per hour.


Distribution, part 1

• Thunderstorm formations require three conditions:– Humid air in the low- to mid-troposphere;

– Atmospheric instability;

– A source of uplift.

• Most form in maritime tropical (mT) air masses.• Most form when mT air is lifted:

– Along fronts;

– Up mountain slopes;

– Via horizontal convergence of surface winds.



• Thunderstorm day: A statistic used to express thunderstorm frequency; defined as a day in which thunder is heard.– Thunderstorm days/month

– Thunderstorm days/year

• Globally, thunderstorm days are highest:– In humid tropics;

– Along ITCZ.



• In North America, thunderstorm days are highest:– In Central Florida;

– Along Colorado Front Range.

• Thunderstorms are common along coastal areas.


Severe thunderstorms, part 1

• Severe thunderstorms are (by convention) accompanied by locally damaging winds, frequent lightning, or large hail.– According to the National Weather Service, Severe

thunderstorms must meet one or more of the following criteria:

• Surface winds stronger than 93 km/hr (58 mph);

• Tornadoes or funnel clouds;

• Hailstones 1.9 cm (0.75 in) or more in diameter.



• The taller the storm, the greater the potential for it becoming severe.– Vertical wind shear (change in horizontal wind speed or

direction) is key to the development of severe thunderstorms.

• Most severe storms develop over the Great Plains.– They are usually associated with synoptic-scale cyclones.

– They usually form ahead of a squall line within the warm sector, ahead of and parallel to a fast-moving cold front.



• Feature of typical Southern Plains storm outbreaks:– Mature cyclone centered over Western Kansas;

– Well defined cold front trailing into West Texas;

– Interaction between the westerly polar front jet aloft and a southerly low-level jet blowing up from Gulf of Mexico create vertical wind shear and instability; effect is enhanced if subtropical jet is near

– Jet streak induced horizontal divergence and convergence aloft, which in turn triggers cyclogenesis under the left-front quadrant of the streak and subsidence under right-front quadrant;



• Southern Plains storm outbreaks (continued):– Compressional warming in area of subsidence leads to

formation of a capping inversion;

– Persistent capping inversion intensifies differences between air masses;

– Updrafts eventually break through capping inversion, allowing explosive convection;

– Updrafts may penetrate the stratosphere, thus creating a severe thunderstorm.



• Southern Plains storm outbreaks (continued):– Dry lines typically form between eastern edge of cold front

and western edge of mT air;

– Mammatus clouds, formed from blobs of cold, cloudy air, often appear at the base of mature cumulonimbus clouds.


Thunderstorm hazards

• Lightning• Downbursts• Flash floods• Hail


Lightning, part 1

• There are more than 8.5 million lightning bolts daily in world.

• Lightning most frequently occurs as exchanges between adjacent clouds or between the upper and lower portions of the same cloud; it also occurs as an electrical connection of ionized air from cloud to ground.


Lightning, part 2

• The sequence that leads to lightning discharge is known, but the mechanism for electrification is not.– Large cumulonimbus cloud experiences a separation of

electrical charges:• Positively charged particles are mostly high in cloud, while

negatively charged particles tend to concentrate in base.

– A growing negative charge in base attracts a growing positive charge on Earth’s surface immediately below cloud.

• An insulating barrier lies between cloud base and surface.


Lightning, part 3

• Sequence (continued):– Growing negative charge (continued):

• The contrast between cloud base and surface builds to tens of million volts and overcomes the insulating barrier.

• A finger of negative current flicks down from cloud and meets a positively charge darting upward from the ground, causing lightning.

• The cause is unknown; theories differ.– Most popular theory is that updrafts carry positively

charged particles to the top of the clouds, while falling ice pellets gather negative charges and transport them downward.


Lightning, part 4

• Cause (continued):– Scientists are investigating if cosmic rays from deep space

may be involved.

• Thunder is an instantaneous expansion of air caused by the abrupt heating that lightning bolt produces. This expansion creates a shock wave that becomes a sound wave.– One can time the distance that lightning is away because of

the different rates thunder and lightning travel at (speed of sound vs. speed of light).

• A three-second interval equals about a kilometer.


Downbursts

• Downbursts are exceptionally strong downdrafts that diverge horizontally after they reach the ground.

• Macrobursts cause destruction over distances of more than 4 km with winds that may exceed 210 km/hr.– They last up to 30 minutes.

• Microbursts are smaller and of shorter duration, but wind speeds may reach 270 km/hr.– Microbursts are hazardous to aircraft.

• Derechos are straight-line downburst winds that may span hundreds of kilometers.


Flash floods, part 1

• Flash floods are short-term, localized, difficult to predict rises in stream level, usually forming in response to high precipitation amounts in watershed areas.– Flash floods typically rise and fall within 6 hours of rain

event.

– Often form under stationary or slow-moving thunderstorms.

– Typically form under conditions of weak vertical wind shear.


Flash floods, part 2

• Big Thompson Canyon flood, 31 July 1976:– Canyon located in Colorado Front Range below Rocky

Mountain National Park.

– Persistent thunderstorm over headwaters of Big Thompson River dropped 25 to 30 cm of rain; about 20 cm fell in 2 hours.

– Floodwaters poured into narrow canyon, raising the river as much as 6 m.

– 139 killed; 418 houses and 197 motor vehicles destroyed; $35.5 million in property damage.


Hail, part 1

• Hail is precipitation in the form of balls or chunks of ice more than 5 mm in diameter.

• Hail almost always falls from tall cumulonimbus clouds with strong updrafts and abundant supercooled water droplets.

• Largest hailstone recorded near Coffeyville, Kansas, on 3 September 1970: 766 g, 44.5 cm.

• The form when ice pellets fall, get picked in updrafts, and grown as additional layers of water freeze on the surface.


Hail, part 2

• Hail falls to the ground when updrafts are no longer able to keep it suspended.

• Hail may cover ground in long, narrow stripes called hailstreaks.– Hailstreaks may reach widths of 2 km and lengths of 10

km.


Tornadoes, part 1

• Tornadoes are localized, typically cyclonic, low-pressure cell surrounded by a whirling cylinder of wind spinning so violently that partial vacuum develops within the funnel.– They have the most extreme pressure gradients known (as

much as 100-millibar difference between tornado center and air immediately outside funnel).

– Extreme pressure differences produce winds of extraordinary speed.


Tornadoes, part 2

• Tornadoes (continued):– How fast are the winds? No one knows, because tornadoes

blow to bits the anemometer (and instrument for measuring wind speed). Maximum estimates range from 320 to 800 kilometers.

– The exact mechanism of formation is unknown.

– They usually develop in warm, moist, unstable air associated with midlatitude cyclones.

– They most often develop along a squall line that preceded a rapidly advance cold front, or along the cold front.


Tornadoes, part 3

• Tornadoes (continued):– The spring and early summer are favorable for

development because there are considerable air-mass contrasts present and greater instability in the lower troposphere at those times.

– They most frequently occur in midafternoon, at the time of maximum heating.

– Less than 1 percent of thunderstorms produce tornadoes.

– Waterspouts occur over ocean and lakes; they have less pressure gradient, gentler winds, and reduced destructive capability.


Tornadoes, part 4

• Tornadoes (continued):– More than 90 percent of all reported tornadoes occur in the

United States. This reflects optimum environmental conditions:

• The relatively flat terrain of the central and southeastern U.S. provides uninhibited interaction of Canadian cP and Gulf mT air masses.

• Tornado alley runs from east Texas and Texas Panhandle north into southeastern South Dakota.

• The United States can expect between 700 and 1100 tornadoes, with the record being 1389 in 1998.


Tornadoes, part 5

• The deadliest tornado was the Tri-State tornado of 18 March 1925, which claimed 695 lives.– The tornado lasted 3.5 hours, moved at a speed of 118

km/hr, and wreaked havoc along a path 350 km in length.

• Tornado hazards:– Extremely high winds

– Strong updrafts

– Subsidiary vortices

– Abrupt drops in air pressure


Tornadoes, part 6

• Tornado intensity is measured using the Fujita scale.– The Fujita scale is a six-point scale rating tornado strength

and damage to structures.– Weak tornadoes

• F0: estimated wind speed 65-118 km/hr; 40-73 mph• F1: estimated wind speed 119-181 km/hr; 74-112 mph

– Strong tornadoes• F2: estimated wind speed 182-253 km/hr; 113-157 mph• F3: estimated wind speed 254-332 km/hr; 158-206 mph

– Violent tornadoes• F4: estimated wind speed 333-419 km/hr; 207-260 mph• F5: estimated wind speed 420-513 km/hr; 261-318 mph


Tornadoes, part 7

• Tornado intensity (continued):– Tornado intensity can fluctuate over the course of its life

cycle.• The Wichita-Andover (Kansas) tornado of 26 April 1991 began as

an F1, worked up to an F5 (as it struck a mobile home park in Andover, Kansas, where it destroyed 241 mobile homes), then decreased to an F1 before it ended.

• Most tornadoes are spawned from supercell thunderstorms.– Supercell thunderstorms have updrafts with speeds

exceeding 240 km/hr.


Tornadoes, part 8

• Tornadoes and supercell thunderstorms (continued):– Tornadoes develop in the interaction between supercell

updrafts and horizontal winds.• Horizontal winds exhibit strong vertical shear in terms of both

speed and direction.– Wind strengthens and turns clockwise with altitude.

• Horizontal cylinder of rolling air is tilted upright by the updraft; and vertical shear within cloud intensified rotation – typically in the counterclockwise direction.

– The spinning thunderstorm cloud is called a mesocyclone.• Wall clouds typically accompany mesocyclones, forming in the

zone of the strongest uplift.• Wall clouds do not normally form tornadoes.


Tornadoes, part 9

• Tornadoes and supercell thunderstorms (continued):– Mesocyclones (continued):

• Tornadic wall clouds are generally long-lived, with strong surface winds blowing in toward the center.

• Mesocyclone circulation is most intense at about 6,100 km.

• Updrafts strengthen as more air converges toward the base of the supercell.

• The mesocyclone narrows (with an accompanying increase in wind speed) and builds toward the ground.

• As interior air pressures drop, water vapor condenses to form a funnel cloud.


Tornadoes, part 9

• Tornadoes and supercell thunderstorms (continued):– Mesocyclones (continued):

• A strong downdraft develops near the back of the supercell, and the funnel follows the downdraft to the ground.

• Squall lines and mesoscale clusters also spawn very productive thunderstorms.


Monitoring tornadic storms, part 1

• In the early 1980s, storm chasers tried to place instruments in the path of tornadoes, without much success.– Remote platforms, such as portable Doppler radar, are

much more effective tools.

• Radar signs– Hook echoes, which are detected by reflectivity radars,

often indicate possible tornadoes.

– A tornado vortex signature on Doppler radar signfies the presence of a possible tornado.

nas 125: meteorology thunderstorms and tornadoes

Documents

cumulus stage

forced convection

free convection

cumulus clouds

convection currents

deep convection

surface winds

mature stage