Unformatted text preview: Thunderstorms & Tornadoes Thunderstorms This chapter discusses: 1. The different types and development stages of The thunderstorms thunderstorms 1. The life-threatening storm components of lightning The and tornadoes and Ordinary Thunderstorms Ordinary
Three stages have been identified in Three ordinary thunderstorms: ordinary a) an unstable atmosphere and vertical a) an updrafts keep precipitation suspended updrafts a) entrainment of dry air that causes cooler entrainment air from evaporation, triggering downdrafts and falling precipitation and gust fronts gust a) weakening updrafts and loss of the fuel weakening source after 15 to 30 minutes. source
Figure 15.1A Mature Stage Thunderstorm Mature Figure 15.2 During the mature stage, updrafts may stop at the troposphere During where the cloud ice crystals are pushed horizontally by winds and form an anvil top, or they may overshoot further into the tropopause. tropopause. Dissipating Stage of Thunderstorm Dissipating
Once downdrafts Once dominate updrafts, the storm ends as precipitation leaves the cloud faster than it is replenished by rising, condensing air. air. Often, lower level Often, cloud particles will evaporate leaving an isolate cirrus anvil top section. section. Figure 15.3 Multicell Storms Multicell Figure 15.4 Cool downdrafts leaving a mature and dissipating storm may offer Cool relief from summer heat, but they may also force surrounding, lowrelief level moist air upward. Hence, dying storms often trigger new storms, and the successive Hence, stages may be viewed in the sky. stages Severe Thunderstorms Severe Figure 15.5 Storms producing a minimum of a) 3/4 inch hail and/or b) wind gusts of 50 knots and/or c) tornado winds, classify as severe. c) In ordinary storms, the downdraft and falling In precipitation cut off the updraft. precipitation In severe storms, winds aloft push the rain ahead and In the updraft is not weakened and the storm can continue maturing. maturing. The single supercell storm shown here maintained its The structure for hours. structure Gust Front & Microburst Gust
Turbulent air Turbulent forms along the leading edge of the gust front, which can generate tumbling dust clouds. clouds. Such gust fronts Such and associated cold dense air often feel like a passing cold front, and may cause a 1 to 3 mb local rise in pressure, called a mesohigh. mesohigh. Figure 15.6 Gust Front Shelf Cloud Gust When unstable air When is prevalent near the base of the thunderstorm, the warm rising air along the forward edge of the gust front is likely to generate a shelf, or arcus, cloud. or
Figure 15.6 Gust Front Roll Clouds Gust Figure 15.7 Turbulence in the fast moving gust front will spawn eddies and Turbulence possibly roll clouds beneath the shelf cloud. possibly These clouds spin about a horizontal axis near the ground. Microbursts from Dense Air Microbursts Figure 15.9 Dry air entrained into the thunderstorm will evaporate and cool the Dry falling mix of precipitation and air, which may create dry, and in humid areas wet, microbursts of strong winds. humid Flying into a Microburst Flying Figure 15.10 A pilot flying into a microburst must anticipate sudden and strong pilot changes in wind direction and speed. changes Initially a headwind is encountered that lifts the plane, followed by a Initially strong downdraft, and when leaving the storm a tailwind causes a loss of altitude. loss Storm Radar Bow Echo Storm
Derecho, or straight-line Derecho, winds, may form ahead of a several hundred kilometer cluster of storms, known as a squall line or mesoscale convective system, often formed a few hundred kilometers ahead of a cold front. cold This image shows a This squall line within the bow shaped radar echo. bow
Figure 15.11 Pre-Frontal Squall Line Storms Pre-Frontal
Pre-frontal squall lines Pre-frontal identify major storms triggered by a cold front that may contain several severe thunderstorms, some possibly supercells, extending for more than 1000 kilometers. than This 1989 storm This spawned 25 tornadoes, the worst killing 25 people. people.
Figure 15.12 Gravity Waves Gravity Figure 15.13 Pre-frontal squall line formation is not fully understood. One theory suggests that a surging cold front may initiate "gravity One waves" aloft, where the rising motion of the wave causes cumulus cloud development. cloud Trailing Stratified Clouds Trailing Figure 15.14 An extensive region of stratified clouds may follow behind a squall An line. line. This figure shows a loop of rising and falling air that supplies the This moisture to the stratiform clouds and associated light precipitation. moisture Mesoscale Convective Complex Mesoscale Figure 15.15 An organized mass, or collection, of thunderstorms that extends across a large region is called a mesoscale convective complex (MCC). convective With weak upper level winds, such MCC's can With regenerate new storms and last for upwards of 12 hours and may bring hail, tornadoes, and flash floods. and They often form beneath a ridge of high pressure. Dryline Thunderstorms Dryline Figure 15.16 Abrupt geographic changes from moist to dry dewAbrupt point temperature, called drylines, form in western TX, point OK, and KS in the spring and summer. OK, The diagram illustrates how cool cP air pushes hot and The dry cT air, at the height of the central plains, over the warm moist mT air. warm Such mixing causes large scale instabilities and the Such birth of many supercell storms. birth Thunderstorm Movement Thunderstorm Figure 15.17 Middle troposphere winds often direct individual cells of a Middle thunderstorm movement, but due to dying storm downdrafts spawning new storms, the storm system tends to be right-moving relative to the upper level winds. relative In this figure, upper level winds move storms to the northeast, but In downdrafts generate new cells to the south, which eventually cuts off moisture to the old cell. off Flash & Great Floods Flash Figure 15.18 Figure 15.19 Thunderstorms frequently generate severe local flooding, but in Thunderstorms the summer of 1993 a stationary front beneath the unusually southerly polar jet triggered several days of thunderstorms and rain. rain. The jet caused weak surface waves and provided uplift of warm, The moist Gulf air for thunderstorm growth throughout the northern Mississippi region. Mississippi Floods took 45 human lives and 74,000 were evacuated. Average Thunderstorm & Hail Days Average Figure 15.20 Figure 15.21 Observed frequency in the pattern and occurrence of Observed thunderstorms does not overlap with hail frequency, possibly because hail falling into the thick layer of warm Gulf air will melt before reaching the ground. before Lightning & Thunder Lightning Charge differences between Charge the thunderstorm and ground can cause lightning strokes of 30,000°C, and this rapid heating of air will creates an explosive shock wave called thunder, which requires approximately 3 seconds to travel 1 kilometer. seconds Figure 15.22 Lightning Stroke Development Lightning
Charge layers in the cloud are formed Charge by the transfer of positive ions from warmer hailstones to colder ice crystals. crystals. When the negative charge near the When bottom of the cloud is large enough to overcome the air's resistance, a stepped leader forms. stepped A region of positive ions move from region the ground toward this charge, which then forms a return stroke into the cloud. cloud.
Figure 15.23A Types of Lightning Types Figure 15.24 Nearly 90% of lightning is the negative cloud-toNearly ground type described earlier, but positive cloud-toground lightning can generate more current and more ground damage. damage. Several names, such as forked, bead, ball, and sheet Several lightning describe forms of the flash. lightning Distant, unseen lightning is often called heat lightning. Lightning Rods & Fulgurite Lightning Figure 15.26 Figure 15.25 Metal rods that are grounded by wires provide a low resistance Metal path for lightning into the earth, which is a poor conductor. path The fusion of sand particles into root like tubes, called fulgurite, The may result. may Lightning Detection & Suppression Lightning Figure 15.27 Figure 15.28 When lightning is nearby, trees are not safe because they may generate a When return stroke, but a car may provide protection by transferring the charge through its body to the tires. through Lightning is more often the cause for forest fires, triggering nearly 10,000 Lightning yearly in the U.S. yearly A National Lightning Detection Network helps monitor this storm activity. Tornado Tornado
A rapidly rapidly rotating column of air often evolve through a series of stages, from dust-whirl, to organizing and mature stages, and ending with the shrinking and decay stages. decay Winds in this Winds southern Illinois twister exceeded 150 knots. 150 Figure 15.29 Tornado Occurrence Tornado Figure 15.30 Tornadoes from all 50 states of the U.S. add up to more than 1000 Tornadoes tornadoes annually, but the highest frequency is observed in tornado alley of the Central Plains. tornado Nearly 75% of tornadoes form from March to July, and are more Nearly likely when warm humid air is overlain by cooler dryer air to cause strong vertical lift. strong Tornado Wind Speed Tornado
As the tornado moves As along a path, the circular tornado winds blowing opposite the path of movement will have less speed. have For example, if the For storm rotational speed is 100 knots, and its path is 50 knots, it will have a maximum wind of 150 knots on its forward rotation side. forward
Figure 15.31 Suction Vortices & Damage Suction
A system of tornadoes with smaller system whirls, or suction vortices, contained within the tornado is called a multi-vortex tornado. called Damage from tornadoes may Damage include its low pressure centers causing buildings to explode out and the lifting of structures. and Human protection may be greatest Human in internal and basement rooms of a house. house.
Figure 15.32 Fujita Tornado Scale Fujita Figure 15.33 Tornado watches are issued when tornadoes are likely, while a Tornado warning is issued when a tornado has been spotted. warning Once the storm is observed, or has passed, the Fujita scale is used Once to classify tornadoes according to their rotational speed based on damage done by the storm. damage Atmospheric Conditions for Tornadoes Atmospheric
A specific pattern of events often specific coincide during the formation of tornadoes and severe thunderstorms. tornadoes This may include when an open-wave This mid-latitude cyclone mixes together cold dry air with warm moist air at the surface, and 850 mb warm moist and 700 mb cold dry air aloft flow north and north east, as shown in this figure. and Further, at the 500 mb level a trough of Further, low pressure pressure forms to the west of the surface low, and the 300 mb polar jet swings over the region. polar
Figure 15.34 Thunderstorm Sounding Thunderstorm Figure 15.35 Temperature and dew point have typical vertical profile in the Temperature warm sector before a tornado occurs, including the shallow inversion at 800 mb that acts like a cap on the moist air below. inversion The cold dry air above warm humid air produces convective The instability and lifting. instability Vorticity from Horizontal to Vertical Vorticity Figure 15.36 Figure 15.37 Spinning horizontal vortex tubes created by surface wind shear Spinning may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into a tornado. tornado. Tornado Breeding Supercell Storm Tornado Figure 15.38 Supercell thunderstorms may have many of the features illustrated Supercell here, including a mesocyclone of rotating winds formed when horizontal vorticity was tilted upwards. horizontal Radar Image of Supercell Radar
The area of The precipitation and winds in the mesocyclone is known as the bounded weak echo region (BWER) which the radar is unable to detect and displays as a black core to this storm. core The cyclonic flow of The precipitation on the radar screen is often shaped like a hook echo. echo. Figure 15.39 Rear Flank Downdraft Rear Supercell thunderstorm Supercell development may create an area where the updraft and counterclockwise swirl of upper winds converge into a rear flank downdraft. downdraft. This downdraft can then interact This with lower level inflow winds and spawn a tornado. spawn Figure 15.40 Rotating Clouds as Tornado Signal Rotating
The first The sign that a supercell may form a tornado is the sight of rotating clouds at the base of the storm, which may lower and form a wall cloud, shown in this picture. picture. Figure 15.41 NonSupercell Tornadoes NonSupercell
If a preexisting wall existing cloud was not present, than any tornado formed is not from a supercell storm, and is often called a funnel cloud, or may be a gustnado if the form along a gust front. front. Figure 15.42 Landspout Formation Landspout Figure 15.43A Landspouts, which form over land but look like waterspouts, form when surface winds converge along a boundary where opposite blowing wind creates a horizontal rotational spin. horizontal If a storm passes above, its updraft may lift and If stretch the horizontal spinning air, causing it to narrow and increase in rotational speed due to the conservation of angular momentum. conservation Doppler Radar Analysis Doppler
A single Doppler radar unit can uncover single many features of thunderstorm rotation and movement, but cannot detect winds parallel to the antenna. parallel As such, data from two or more units might be combined to provide a complete view of the storm. view Dopplar lidar (light beam rather than microwave beam) provides more details on the storm features, and will help measure wind speeds in smaller tornadoes. tornadoes.
Figure 15.44 NEXRAD Wind Analysis NEXRAD
NEXt Generation NEXt Weather RADar (NEXRAD) is operated by the National Weather Service and uses Doppler measurement to detect winds moving toward (green) and away (blue) from the antenna, which indicates areas of rotation and strong shear. strong Figure 15.45 Portable Radar Units Portable Thunderstorm chasers may carry Thunderstorm portable radar to image finer details of a storm as it moves along the flat lands of Tornado Alley. the Figure 15.46 Waterspout Funnel Waterspout
Warm, shallow coastal water is Warm, often home to waterspouts, which are much smaller than an average tornado, but similar in shape and appearance. in The waterspout does not draw The water into its core, but is a condensed cloud of vapor. condensed A waterspout may, however, lift waterspout swirling spray from the water as it touches the water surface. as
Figure 15.47 ...
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This note was uploaded on 03/15/2010 for the course ATOC ATOC 250 taught by Professor Gyakum during the Spring '09 term at McGill.
- Spring '09