Thunderstorms and Tornadoes Chapter 14

Thunderstorms • A storm containing lightning and thunder – convective storms • Severe thunderstorms (NWS def) one of following: – large hail - 1 inch dia – Surface wind gusts greater than or equal to 50kts(58mph) – or produces a tornado

• Ordinary Cell Thunderstorms, usually simple, “pop –up” – Air-mass thunderstorms: limited wind sheer – May form at sea breeze fronts, topographic irregularities, outflow boundaries – Stages: cumulus, mature, dissipating

Stages of Development Simplified model depicting the life cycle of an ordinary cell thunderstorm that is nearly stationary as it forms in a region of low wind shear. a) Cumulus Stage – growth of a single or group of cumulus clouds, latent heat released, it grows due instability. Develops into cumulus congestus. b) Mature Stage – Downdraft occurs. As cloud droplets are carried above freezing level, the particles become larger and heavier and begin to fall . The process of entrainment of drier air allows evaporation of some droplets which chills air and creates downdrafts…..may be enhanced by precipitation. c) Dissipating Stage – updrafts have weakened and downdrafts dominate…no longer “fueled”

• Multi-cell Thunderstorms – Thunderstorms that contain a number of convection cells, each in a different stage of development, moderate to strong wind shear – Form when there is moderate to severe vertical wind shear – Causes the convection cell to tilt, with updraft riding over downdraft – Precip does not fall into updraft, so, fuel not cut off, and it lasts much longer – Gust Front forms out ahead, with possible a shelf cloud

When a thunderstorm's downdraft reaches the ground, the air spreads out forming a gust front.

A swirling mass of dust forms along the leading edge of a gust front as it moves across western Nebraska.

A dramatic example of a shelf cloud (or arcus cloud) associated with an intense thunderstorm. The photograph was taken in the Philippines as the thunderstorm approached from the northwest.

A roll cloud forming behind a gust front.

Radar image of an outflow boundary. As cool (more-dense) air from inside the severe thunderstorms (red and orange colors) spreads outward, away from the storms, it comes in contact with the surrounding warm, humid (less-dense) air, forming a density boundary (blue line) called an outflow boundary between cool air and warm air. Along the outflow boundary, new thunderstorms often form.

 Micro-bursts: localized downdraft(downburst) that hits the

ground and spreads horizontally in a radial burst of wind 4km or less outward  generate wind shear - rapid change in wind speed and direction

Flying into a microburst. At position (a), the pilot encounters a headwind; at position (b), a strong downdraft; and at position (c), a tailwind that reduces lift and causes the aircraft to lose altitude.

Stepped Art Fig. 14-11, p. 378

– Squall-line thunderstorms • line of multi-cell thunderstorms • pre-frontal squall-line, form out ahead of advancing cold front

Pre-frontal squall-line thunderstorms may form ahead of an advancing cold front as the upper-air flow develops gravity waves downwind from the cold front.

Doppler radar display superimposed on a map shows a prefrontal squall line extending from Texas into Oklahoma and Arkansas during February, 2011. Some of the thunderstorms embedded within the squall line (dark red and orange color) produced high winds, heavy rain, and large hail.

A model describing air motions and precipitation associated with a squall line that has a trailing stratiform cloud layer.

A side view of the lower half of a squall-line thunderstorm with the rear-inflow jet carrying strong winds from high altitudes down to the surface. These strong winds push forward along the surface, causing damaging straight-line winds that may reach 100 knots. If the high winds extend horizontally for a considerable distance, the wind storm is called a derecho.

A Doppler radar image showing an intense squall line in the shape of a bow—called a bow echo—moving eastward across Missouri on the morning of May 8, 2009. The strong thunderstorms (red and orange in the image) are producing damaging straightline winds over a wide area. Damag-ng straight-line winds that extend for a good distance along a squall line is called a derecho.

Doppler radar image showing that the bow echo has developed into a mesoscale convective vortex by noon on May 8, 2009. Strong, straight-line winds are still occurring with severe thunderstorms, as Carbondale, Illinois, reported an unofficial wind gust of 106 mi/hr.

 Meso-scale Convective Complex( MCC)  a number of individual multi-cell thunderstorms grow in size and organize

into a large circular convective weather system  Tend to form in summertime, with weak upper level winds  Low level jet brings in moisture, and is at max late at night, early morning  May cover entire states - 100,000km2

An enhanced infrared satellite image showing the cold cloud tops (dark red and orange colors) of a Mesoscale Convective Complex extending from central Kansas across western Missouri. This organized mass of multicell thunderstorms brought hail, heavy rain, and flooding to this area.

• Supercell thunderstorms – Large, long-lasting thunderstorm with a single violent rotating updraft – Strong vertical wind shear – Outflow never undercuts updraft, updrafts may exceed 90kts and can cause large sized hail – 3 types:  Classic - CL  high precipitation, HP  low precipitation, LP

Some of the features associated with a classic tornado-breeding supercell thunderstorm as viewed from the southeast. The storm is moving to the northeast.

A wall cloud photographed southwest of Norman, Oklahoma.

Conditions leading to the formation of severe thunderstorms, and especially supercells. The area in yellow shows where supercell thunderstorms are likely to form. Why? - position of cold air above warm creates conditionally unstable atmosphere - Strong vertical wind shear induces rotation Creates a rotating updraft and sets stage for a tornado

A typical sounding of air temperature and dew point that frequently precedes the development of supercell thunderstorms. cap on instability at 800mb cold dry air above means – convective instability

• Thunderstorms and the Dryline – Sharp, horizontal change in moisture – Thunderstorms form just east of dryline

cP

 cT mT

The heavy arrow represents the average position of the upper-level jet stream from midJune through July, 1993. The jet stream helped fuel thunderstorms that developed in association with a stationary front that seemed to oscillate back and forth over the region as an alternating cold front and warm front. The “L” marks the center of a frontal wave that is moving along the front. Many of the thunderstorms that formed in conjunction with this pattern were severe, and over a period of weeks produced “The Great Flood of 1993.” Most of the counties within the blue-shaded area were declared “disaster areas” due to flooding.

Flooding during the summer of 1993 covered a vast area of the upper Midwest. Here, floodwaters near down-town Des Moines, Iowa, during July, 1993, inundate buildings of the Des Moines water-works facility. Flood-contaminated water left 250,000 people without drinking water.

Intense thunderstorms often can create flash flood conditions especially if storms are “training”

Big Thompson Canyon – July 31, 1976, 12 inches of rain in 4 hours created a flash flood associated with $35.5million in damage and 135 deaths

• Distribution of Thunderstorms – Most frequent Florida, Gulf Coast, Central Plains – Fewest Pacific coast and Interior valleys – Most frequent hail Central Plains

The average number of days each year on which thunderstorms are observed throughout the United States. (Due to the scarcity of data, the number of thunderstorms is underestimated in the mountainous far west.)

The average number of days each year on which hail is observed throughout the United States.

• Lightning and Thunder – Causes of electrification of clouds – graupel and hail fail into region of supercooled water , water freezes, releasing latent heat and keeping the hailstone warmer than surrounding ice crystal nuclei • Net transfer ….+ ions from warmer to colder, this leaves larger hail stones negatively charged and smaller ice crystals positively charged

When the tiny colder ice crystals come in contact with the much larger and warmer hailstone (or graupel), the ice crystal becomes positively charged and the hailstone negatively charged. Updrafts carry the tiny positively charged ice crystal into the upper reaches of the cloud, while the heavier hailstone falls through the updraft toward the lower region of the cloud

The lightning stroke can travel in a number of directions. It can occur within a cloud, from one cloud to another cloud, from a cloud to the air, or from a cloud to the ground. Notice that the cloud-to-ground lightning can travel out away from the cloud, then turn downward, striking the ground many miles from the thunderstorm. When lightning behaves in this manner, it is often described as a “bolt from the blue.”

The generalized charge distribution in a mature thunderstorm.

• The Lightning Stroke – A discharge of static electricity – Positive charge on ground, cloud to ground lightning • Thunder – – – –

Lightning heats air to 54,000deg F – hotter than Sun’s surface Explosive expansion of air - shock wave Sound travels at 330m/s or 1100 ft/s, so delay… about 5 sec per mile Sound is refracted upward in unstable atm and we do not hear lightning at approximately 15km away … Heat Lightning

Thunder travels outward from the lightning stroke in the form of waves. If the sound waves from the lower part of the stroke reach an observer before the waves from the upper part of the stroke, the thunder ap-pears to rumble. If the sound waves bend upward away from an observer, the lightning stroke may be seen, but the thunder will not be heard.

The development of a lightning stroke. (a) When the negative charge near the bottom of the cloud becomes large enough to overcome the air’s resistance, a flow of electrons—the stepped leader —rushes toward the earth. (b) As the electrons approach the ground, a region of positive charge moves up into the air through any conducting object, such as trees, buildings, and even humans. (c) When the downward flow of electrons meets the upward surge of positive charge, a strong electric current— a bright return stroke —carries positive charge upward into the cloud.

The lightning rod extends above the building, increasing the likelihood that lightning will strike the rod rather than some other part of the structure. After lightning strikes the metal rod, it follows an insulated conducting wire harmlessly into the ground.

The four marks on the road surface represent areas where lightning, after striking a car traveling along south Florida’s Sunshine State Parkway, entered the roadway through the tires. Lightning flattened three of the car’s tires and slightly damaged the radio antenna. The driver and a six-year-old passenger were taken to a nearby hospital, treated for shock, and released.

Cloud-to-ground lightning strikes in the vicinity of Chicago, Illinois, as detected by the National Lightning Detection Network

Average lightning flash density per square kilometer per year from 1997 to 2010. Notice that in the United States, Florida is the most lightning-prone state. (Data from the North American Lightning Detection Network. Courtesy of Vaisala.)

Tornadoes • Rapidly rotating column of air that blows around a small area of intense low pressure with a circulation that reaches the ground. • Funnel cloud – tornado not on ground

• Tornado life cycle – Dust whirl, organizing, mature, shrinking, decay stage

• Tornado outbreaks – families of T. usually due to a long lived supercell – outbreak • usually 6 or more

 Tornado Occurrence  US experiences most tornadoes – all 50 states  Tornado Alley (warm, humid surface; cold dry air aloft) - Central Plains  Dixie Alley – MS, AL

 Highest occurrences - spring, lowest winter

 Tornado winds  Measurement based upon damage after storm or Doppler radar  For southwest approaching storms, winds strongest in the northeast of the storm, 220 kts maximum - most less than 125 kts

Tornado incidence by state. The upper figure shows the average annual number of tornadoes observed in each state from 1953–2004. The lower figure is the average annual number of tornadoes per 10,000 square miles in each state during the same period. The darker the shading, the greater the frequency of tornadoes. (NOAA)

Average number of tornadoes during each month in the United States. (NOAA)

The total wind speed of a tornado is greater on one side than on the other. When facing an onrushing tornado, the strongest winds will be on your left side. Example forward motion – 50kts Rotational speed – 100 kts

A powerful multi-vortex tornado with three suction vortices.

• Dr. T. Theodore Fujita • The Fujita scale (1971)was revised in 2007 as the EFscale (Enhanced F-Scale) – The EF-scale is based on rotational wind speeds estimated from property damage – Ranges from EF0 to EF5 • EF5 tornadoes are rare • About 77% of tornadoes in the U.S. are considered weak (EF0 to EF1) and 95% are below EF3

Tornadic Formation • Basic requirements are an intense thunderstorm, conditional instability, and strong vertical wind shear

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Supercell Tornadoes  Wind sheer causes spinning vortex tube that is

pulled into thunderstorm by the updraft (a) A spinning vortex tube created by wind shear. (b) The strong updraft in the developing thunderstorm carries the vortex tube into the thunderstorm producing a rotating air column that is oriented in the vertical plane.

A tornado-spawning supercell thunderstorm over Oklahoma City on May 3, 1999,

shows a hook echo in its rainfall pattern on a Doppler radar screen. The colors red and orange represent the heaviest precipitation.

Stepped Art Fig. 14-46, p. 402

Nonsupercell Tornadoes Gustnadoes – form along a gust front Land spout - form out of cumulus congestus clouds and have a similar look to a water spout (a) Along the boundary of converging winds, the air rises and condenses into a cumulus congestus cloud. At the surface the converging winds along the boundary create a region of counterclockwise spin. (b) As the cloud moves over the area of rotation, the updraft draws the spinning air up into the cloud producing a nonsupercell tornado, or landspout.

A well-developed landspout moves over eastern Colorado.

Waterspouts • Rotating column of air that is connected to a cumuliform cloud over a large body of water • Tornadic waterspout – one that formed over land • “fair weather” waterspouts from under cumulus congestus clouds ,45kts • Does not pull up water more than a few meters…what you see is a cloud of condensation

• Special Topic: Forecasting Severe Thunderstorms & Tornadoes – Convective Available Potential Energy (CAPE) • Energy available for updrafts

– Deep layer shear • favorable vertical wind shear

– Storm Relative Helicity (SRH) • Degree to which the surface wind possesses ability to rotate

Severe Weather and Doppler Radar

• Doppler radar measures the speed of precipitation toward and away radar unit • Two Doppler radars can provide a 3D view

Doppler radar display of winds associated with the supercell storm that moved through parts of Oklahoma City during the afternoon of May 3, 1999. The close packing of the horizontal winds blowing toward the radar (green and blue shades), and those blowing away from the radar (yellow and red shades), indicate strong cyclonic rotation and the presence of a tornado.

Doppler radar display showing precipitation inside a large supercell that takes on the shape of a hook. This hook echo is associated with a violent multi-vortex tornado that is moving through Tuscaloosa, Alabama, just south of the University of Alabama, on April 27, 2011. The purple area in the center of the hook is debris that, having been picked up by the tornado, is now swirling counterclockwise around it. Debris on a radar screen such as this is referred to as a debris ball.

• Doppler Lidar • 150 NEXRAD units - WSR-88D and computer system – Also portable Graduate students from the University of Oklahoma use a portable Doppler radar to probe a tornado near Hodges, Oklahoma.

Monitoring Tornadic Thunderstorms

The annual number of reports of tornadoes in the U.S. 1950-2008