Chapter 10: Thunderstorms and Tornadoes Tornadic thunderstorms Severe weather and doppler radar

Thunderstorms Convective storms that form with rising air in a conditionally unstable environment The trigger needed to start air moving upward may be surface heating; topographic lift; convergence zone (e.g., sea breeze leading edge); frontal lift; divergence aloft

Ordinary Cell Thunderstorms cumulus stage mature stage dissipating stage Ordinary cell thunderstorms are sometimes called ‘air mass thunderstorms’, because they form in conditionally unstable air masses and are not necessarily associated with fronts or severe weather Shortlived (<1 hr), less than 1 km wide, low wind shear, rarely produce strong wind or large hail On 7/13/1999 in California, a strong downdraft from a mature thunderstorm dropped the air temperature from 97F to a chilly 57F in one hour!

Moisten environment latent heat warms the downdraft cuts off humid cloud layer; updraft; entrained dry air causes gust front is too far away evaporative cooling and to enhance updraft downdraft and gust front Figure 10.1: Simplified model depicting the life cycle of an ordinary thunderstorm that is nearly stationary. (Arrows show vertical air currents. Dashed line represents freezing level, 0°C isotherm.) Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. Fig. 10-1, p. 265

Severe Thunderstorms and the Supercell Severe thunderstorm is defined as a thunderstorm with at least one of the following: large hail with a diameter > ¾ inch, surface wind gusts >50 knots (58 mi/hr), and/or produces a tornado multicell storms moderate wind shear

Supercell: Strong wind shear in speed and direction; Shallow inversion above warm and humid layer acts as a lid; Long-lasting (hours); Larger than 1 km in diameter; Single violently rotating updraft; Produces tornado, large hail, strong gusts Figure 10.5: Conditions leading to the formation of severe thunderstorms, and especially supercells. The area in green shows where severe thunderstorms are likely to form. Fig. 10-5, p. 268

Squall Lines and Mesoscale Convective Complexes squall line: multicell storms as a line of thunderstorms extending for many kilometers (up to 1000 km) pre-frontal squall line mesoscale convective complex: multicell storms as a large circular cluster of storms; tend to form in summer in regions where the upper-level winds are weak; large size (100,000 square km)

Dryline Thunderstorms These storms occur frequently in the southern Great Plains of the US.

Gust Fronts, Microbursts and Derechoes gust front: leading edge of cold air originating inside a thunderstorm shelf cloud and roll cloud outflow boundary: merging several gust fronts downbursts (intense downdraft) and microbursts (< 4km; caused aircraft crash) Derecho (day-ray-sho): straight-line (not associated with rotation) strong wind extending for 100’s km produced by strong downdrafts; producing large hail and sometimes tornado

Figure 10.11: 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. Fig. 10-11, p. 271

Several airports have installed microburst detection instruments. Microbursts present a severe hazard to aircraft, especially during takeoff and landing. Several airports have installed microburst detection instruments. Figure 10.13: Radar image of an outflow boundary. As cool 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. Fig. 10-13, p. 272

A squall line thunderstorms appear in the shape of a bow (or bow echo) on a radar screen and produce a straight-line wind (derecho) Figure 10.16: The red and orange on this Doppler radar image show a line of intense thunderstorms (a squall line) that is moving south southeastward into Kentucky. The thunderstorms are producing strong straight-line winds called a derecho. Notice that the line of storms is in the shape of a bow. Such bow echos are an indicator of strong, damaging surface winds near the center of the bow. Sometimes the left (usually northern) side of the bow will develop cyclonic rotation and produce a tornado. Fig. 10-16, p. 273

Floods and Flash Floods flash floods: floods that rise rapidly with little or no advance warning 1976 Big Thompson flash flood (12 inch rain in 4 hours) The Great Flood of 1993 over the upper midwest

Distribution of Thunderstorms combination of warmth and moisture geographical placement Great Plains are more favorable for hails partly because evaporation of hail surface water cools the dry air

Lightning and Thunder Lightning: a discharge of electricity, a giant spark, which usually occurs in mature thunderstorms (and may also occur in snowstorms and dust storms) majority of lightning strokes within clouds with only 20% between clouds and surface a lightning stroke can heat the air through it travels to 30,000C, 5 times hotter than the Sun’s surface Thunder: The extreme heating causes the air to expand explosively, thus initiating a shock wave that becomes a booming sound wave Do you see the lightning or hear the thunder first?

Electrification of Clouds For normal fair weather, the atmosphere is usually characterized by a negatively charged surface and a positively charged upper atmosphere electrical charges in clouds relationships of updrafts and downdrafts to electrical charges There is a net transfer of positive ions (charged molecules) from the warmer (and larger) hailstone to the colder (and smaller) ice crystal or supercooled droplets which are lifted to the upper layer of clouds

The Lightning Stroke cloud-to-ground lightning stepped leader return stroke dart leader -subsequent leader

Types of Lightning forked lightning ribbon lightning: hanging from clouds due to winds dry lightning: cause forest fire heat lightning: seen but not heard St. Elmo’s fire: a corona discharge or sparks, can cause the top of a ship’s mast to glow; also seen over power lines and aircraft wings

Lightning Detection and Suppression lightning direction-finder Detecting the radio waves produced by lightning

Where do you stay under thunderstorm? Not under trees; Avoid elevated places; Keep your head as low as possible but not touch ground Inside a building; Inside a car Figure 2: A cloud-to-ground lightning flash hitting a 65-foot sycamore tree. It should be apparent why one should not seek shelter under a tree during a thunderstorm. Figure 2, p. 282

Tornado Life Cycles tornado or twister: typically 100-600 m, may >1 mi, usually at 20-40 knots funnel cloud: not reach the ground dust-whirl stage mature stage decay stage tornado families: different tornados spawned by the same thunderstorm tornado outbreaks: 6 or more tornados over a particular region

Tornado Occurrence tornado alley time of day: most frequent at 4-6pm LT times of year Annual number per state; Annual number per 100 mi by 100 mi

Tornado Winds multi-vortex tornadoes suction vortices

Seeking Shelter tornado watch: likely to form tornado warning: spotted visually or by radar It’s always a good idea to know what to do if a tornado watch or warning is issued for your area. Take shelter in the basement or small room in the middle of the house at a lower level and cover your head Lie flat on the ground in a ditch Don’t stay under a highway overpass Don’t stay near window or wall

The Fujita Scale tornado classification based on damage The “F-scale” was named after Prof. Ted Fujita. Wind damage is proportional to the square of wind speed

Tornadic Thunderstorms Supercell Tornadoes mesocyclones bounded weak echo region hook echo rotating clouds wall cloud A rotating wall cloud is an unforgettable sight - just ask a successful storm chaser.

Figure 10.35: Some of the features associated with a tornado-breeding supercell thunderstorm as viewed from the southeast. The storm is moving to the northeast. Fig. 10-35, p. 290

Nonsupercell Tornadoes Gustnadoes: along a gust front Landspouts: weak and short-lived, from congestus cold air funnels: cold air aloft

Severe Weather and Doppler Radar Doppler shift tornado vortex signature rapidly changing wind direction Doppler lidar use light beam, higher resolution NEXRAD waterspout