Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
METEOROLOGY GEL-1370 Chapter Ten Thunderstorms and Tornadoes Goal for this Chapter We are going to learn answers to the following questions: • What atmospheric conditions produce thunderstorms? • How severe thunderstorms are produced? • Why severe thunderstorms are not common in polar latitude? • How lightning are produced? • How thunders are produced? • What are tornadoes and how they are produced? • What is Fujita scale? • Major characteristics of a tornadoe? • Why highest frequency of thunderstorms occur in US? Thunderstorms • Thunderstorm: A storm that contains lightning and • • • • thunder Birth occurs when warm humid air rises in a conditionally unstable environment What can trigger the birth of thunderstorm – unequal heating of the surface, terrain, lifting of warm air along a frontal zone Ordinary thunderstorms (or air-mass thunderstorms): Develop in warm, humid air masses away from weather fronts; usually short-lived and rarely produce strong winds or large hail Severe thunderstorms may produce high winds, flash floods, changing hail & tornadoes Thunderstorms • Stages of ordinary thunderstorms: – Cumulus Stage: Humid air rises, cools, & condenses in to cumulus clouds – Transformation of water-vapor into liquid or solid cloud particles releases large quantities of latent heat; this keeps the air inside the cloud warmer than the surrounding air – During cumulus stage, insufficient time for precipitation to form, and the updrafts keep water droplets and ice crystals suspended within the cloud; no lightning or thunder during this stage – As the cloud builds well above the freezing level, cloud particles grow larger and heavier; drops begin to fall; drier air around the cloud is being drawn into it; entrainment of drier air leads to evaporation of raindrops; air becomes colder & heavier; air begins to descend as a downdraft Thunderstorms – contd. • Appearance of the downdraft marks the beginning of the mature thunderstorms; downdraft & updraft within the mature thunderstorm constitute a ‘cell’ • In most storms, there are several cells, each of which may last for an hour or so • Updrafts & downdrafts reach their greatest strength in the middle of the cloud, creating severe turbulence • Overshooting: Intrusion of the updraft above the cloud top in to the stable atmosphere • Dissipating stage: When updrafts weaken & downdrafts tend to dominate throughout much of the cloud • Three stages: Cumulus stage, maturing thunderstorm stage, & dissipating stage Thunderstorms – contd. • A single ordinary thunderstorm may go through its three stages in an hour or less • The cold downdraft may force warm, moist surface air upward; this air may condense and can gradually build into a new thunderstorm – multicell thunderstorms • Most ordinary thunderstorms are multicell storms • Severe Thunderstorms: Capable of producing large hail, strong, gusty surface winds, flash floods, and tornadoes • Can form from moist air when it is forced to rise into a conditionally unstable atmosphere; severe thunderstorms also form in areas with a strong vertical wind sheer Air motions associated with thunderstorms; severity depends on the intensity of the storm’s circulation pattern Ordinary thunderstorm in its mature stage A multicell thunderstorm; in the middle is in its mature stage; to its right of the cell, a thunderstorm is its cumulus stage A simplified model describing air motions & other features associated with a severe thunderstorm; severity depends on the intensity of the storm’s circulation pattern Severe Thunderstorms – contd. • The storm in the previous figure, moves from left to right & the upper-level winds cause the system to tilt so that the updrafts move up and over the downdrafts • The updrafts in a severe thunderstorm may be so strong that the cloud top is able to intrude well into the stable atmosphere; top of the cloud may even extend to more than 18 km above the surface • Gust Front: The boundary separating the cold downdraft from the warm surface air • Along the leading edge of the gust front, the air is turbulent; strong winds here can pick-up loose dust and soil and lift them into a huge tumbling cloud Gust Front & Microburst • Downbursts: A severe localized downdraft that can be experienced that fall slowly and reduce visibility more than light rain • Microburst: A downburst with winds extending only 4kms or less • Supercell and Squall-line thunderstorms: – Supercell Storm: An enormous severe thunderstorm whose updrafts (can exceed 90 knots) and downdrafts are nearly in balance, allowing it to maintain itself for several hours. It can produce large tornadoes & hail (> grapefruit size); most supercell storms move to the right of the steering winds aloft – Squall-line storms form as a line of thunderstorms along a cold front or out ahead of it The lower half of a severe squall-line type thunderstorms and some of the features associated with it Dust clouds rising in response to the outburst winds of a microburst north of Denver, CO Doppler radar display showing a line of thunderstorms bent in the shape of a bow (Red, orange, and yellow) Supercell near Spearman, TX has a tornado extending downward from its base Some of the features of a classic supercell thunderstorm, viewed from southeast Diagram of the thunderstorm from above, looking down on the storm; shaded red: updraft; shaded gray: downdraft Severe Thunderstorms – contd. • Dry Line (dew-point fronts): A zone of instability along which thunderstorms form; dew point temp may drop along this boundary by as much as 9°C/km • Mesoscale Convective Complexes: A large organized convective weather system comprised of a number of individual thunderstorms; size of an MCC ~ 1000 times larger than individual thunderstorm Surface conditions that can produce a dryline with severe thunderstorms; A developing mid-latitude cyclone with a cold front, warm front, and three distinct air masses (cP, cT & mT) IR image showing a Mesoscale Convective Complex extending from central Kansas across western Missouri Floods & Flash Floods • Flash floods: Floods that rise rapidly with little or advance warning; results when thunderstorms stall or move slowly, causing heavy rainfall over a relatively small area • Causes for Flash Floods: – Thunderstorms stall or move slowly – Thunderstorms move very quickly but keep passing over the same area (phenomenon called ‘training’) – Heavy rain and melting of snow taking place in spring – Torrential rains from tropical storms Summer of 1993 rain in the upper Midwest caused the worst flood 6.5 billion $ crop lost; 43 human lives; 45,000 homes were lost; evacuation of 74,000 people Distribution of Thunderstorms • >40,000 thunderstorms/day (14 millions/yr) in the world • 14 million/year • Conducive conditions for thunderstorm formation: Combination of warmth and moisture • Where thunderstorms are prevalent: i) Southeastern states along the Gulf Coast with a maximum in Florida (mainly during summer); ii) Central Rockies; iii) Over water along the intertropical convergence zone where the low-level convergence of air helps to initiate uplift • Where thunderstorms are rare: i) Dry regions such as polar regions and the desert areas of the subtropical highs; ii) Pacific coastal and interior valleys Average number of days each year on which thunderstorms are observed in US; mountainous west has sparse data Average number of days each year hail observed Thunderstorms and Lightning • Lightning: A giant spark discharging electricity that occurs in mature thunderstorms; can take place within a cloud, from one cloud to another, cloud to surrounding air or cloud to ground (~20%); 80% within the clouds • Lightning stroke can heat the air surrounding it to 30,000°C which in turn causes the air to expand, thus initiating a shock wave that becomes a booming sound wave-thunder • Light travels faster than sound (345 m/s @25 °C) • Time difference between the light and sound can be utilized to determine how far away the stroke took place Lightning & Thunder – contd. • Close distance lightning: Clap sound or crack followed immediately by a loud bang • Farther away: rumbling sound due to sound emanating from different areas of the stroke • Lightning, but no thunder: Thunder waves were refracted and the sound waves got attenuated, making the thunder inaudible • Sonic boom: Produced when an aircraft exceeds the speed of sound at the altitude at which it is flying • Condition for lightning to occur: Separate regions containing opposite electrical charges must exist within the cumulonimbus cloud Electrification of clouds • Several theories to explain the formation of lightning • When hail fall through supercooled droplets, the droplets freeze and release latent heat; this heat warms the hailstone; contact of warmer hailstone and colder ice crystal leads to a net transfer of positive ions from the warmer object to the colder object --- hailstone is negatively charged and ice crystals +ively charged • Positively charged ice particles carried to the upper part of the cloud by updrafts & larger haldstones with –ive charge fall toward the bottom of the cloud • Cold, upper part becomes +ively charged & middle of the cloud becomes –ively charged Electrification of the Clouds – contd. • Another school of thought: Regions of separate charge exist within tiny cloud droplets and larger precipitation particles during the formation of precipitation --Negative charge in the upper part of these particles & +ive charge in the lower part of the particles --- when falling precipitation collides with smaller particles, larger precipitation particles become negatively charged and the smaller particles positively charged --updraft sweeps smaller sized particles leading to net +ive charge Generalized charge distribution in a mature thunderstorm The Lightning Source • Negative charge at the bottom of the cloud causes a region of the ground beneath to become +ively charged; as the thunderstorms move, the positive charge moves along with it; the positive charge is most dense on protruding objects; charge separation causes electric field existence; electric potential difference between cloud and the ground --- when electric potential builds up, current flow results and lightning occurs • Cloud-to-ground lightning begins when the localized electric potential gradient >3 million volt/m --- leads to the discharge of electrons toward the cloud base and then to the ground Lightning source – contd. • Stepped Leader: An initial discharge of electrons that proceeds intermittently toward the ground in a series of steps in a cloud-to-ground lightning stroke • Return Stroke: The luminous lightning stroke that propagates upward from the earth to the base of a cloud • Dart Leader: Discharge of electrons that proceeds intermittently toward the ground along the same ionized channel taken by the initial lightning stroke • Different types of Lightning: Forked lightning (crooked or forked in shape), ribbon lightning (ribbon hanging in the cloud), bead lightning (series of beads tied to a string), ball lightning (sphere appears to float in the air) & sheet lightning (cloud appears like a white sheet) Development of stepped leader: when –ive charge near the bottom of the cloud becomes large enough to overcome air’s resistance, flow of electrons rushes to the earth As the electrons approach the ground, a region of + charge moves up into the air through any conducting object, such as trees, buildings When the downward flow of electrons meets the upward surge of +ive charge, a strong electric current – a bright return stroke – carries +ve charge upward into the cloud Lightning rod extends above the building: when lightning strikes, it follows an insulated conducting wire into the ground Four marks on the road surface represent areas where lightning, after striking a car entered the roadway; 3 tires were flattened Lightning Detection & Suppression • Heat Lightning: Distant lightning from thunderstorms that is seen, but not heard • As the electric potential near the ground increases, a current of +ive charge moves up pointed objects, such as antennas • Lightning rods (made of metal with a pointed tip) are placed that extend well beyond the height of the structure • Lightning Direction-finder: It detects the direction of lightning by measuring the radiowaves produced by lightning Damages by lightning in US • 10,000 fires/yr in US are started by lightning • 50 million $ worth of timber is destroyed per yr • Can we reduce the cloud-to-ground lightning?? Seeding cumulonimbus clouds with hair-thin pieces of Al wire (10-cm long) --- metal will produce many tiny sparks and prevent the electrical potential in the cloud from building to a point where lightning occurs Tornadoes • Tornadoes: A product of thunderstorms; rapidly rotating winds that blow around a small area of intense low pressure • Tornadoe’s circulation is present on the ground either as a funnel-shaped cloud or as a swirling cloud of dust & debris; majority rotate counterclockwise • Other shapes: – – – – Twisting ropelike funnels Cylindrical-shaped funnels Massive black funnels Funnels that resemble an elephant’s trunk hanging from a large cumulonimbus cloud Tornadoes – Features and stages • Diameter (most): 100-600 m (few meters – 1,600 m rare) • Most last only a few minutes & average path length of ~7 km (largest one: 470 km; lasted for 7 hrs) in Illinois and Indiana in 1917 • Stages of a Tornadoes (most common): – Dust-Whirl stage: Dust swirling upward from the surface – damage is light – Organizing Stage: Tornado increases in intensity with an overall downward extent of the funnel – Mature Stage: funnel reaches its greatest width & is almost vertical; damage is most severe – Shrinking stage: Overall decrease in the funnel’s width & increase in the funnel’s tilt; still capable of intense & Tornadoes – Features and stages • Sometimes violent damage – Decay Stage: The final stage, usually finds the tornado stretched into the shape of a rope Minor tornadoes may evolve only through certain stages Damages: ~ 100 people/year killed (11/10/02 – 37 people died on a single day) 45% mortalities in mobile homes March 18, 1925 tornadoes: 695 people died, 7 tornadoes traveled a total of 703 km across portions of Missouri, Illinois and Indiana Tornado outbreak • Tornado Outbreak: A series of tornadoes that forms within a particular region, often associated with widespread damage and destruction; a region may include several states • April 3, 1974: 16 hour period, 148 tornadoes cut through parts of 13 states, 307 people killed, >3700 people injured, damage >600 million $ • Occurrence: Most numbers in US; average: >1,000/yr; 1,424 during 1998 • Tornado alley: Tornado belt, Central Plains, stretches from central Texas to Nebraska A mature tornado with winds >150 knots rips through southern illinois Tornado incidence by State; upper:number by each state (25 yrs); lower: average annual number/100,000 square miles; darker: greater frequency Tornadoes and their impact • Lifting railroad coach with 117 passengers and dumping it 25 m away • Schoolhouse was demolished and 85 students inside were carried over 100 m without one of them being killed • Most tornadoes have winds of less than 125 knots • Pressure in the center of a tornado may be more than 100 mb lower than the surrounding & there is a momentary drop in outside pressure when tornado is above a structure • When confronted with a tornado, take shelter immediately (basement, stay away from windows, small bathroom, closet, interior hallway) Fujita scale for damaging wind Category Mi./hr knots Expected damage •Scale . F0 Weak F1 F2 Strong F3 F4 F5 40-72 35-62 73-112 63-97 113-157 98-136 158-206 Violent 207-260 261-318 Light; tree branches broken; sign boards damaged Moderate; trees snapped; windows broken Considerable; large trees uprooted, weak struc. Destroyed 137-209 Severe trees leveled, cars overturned, walls removed from bldg. 180-226 Devastating frame houses dstroyed 227-276 Incredible; structure the size of autos moved over 100 m Fujita Scale – contd. • Fujita Scale: Theodore Fujita in late 1960s --classifying tornadoes according to their rotational wind speed based on the damage done by the storm • Majority of tornadoes are F0 and F1 (weak ones) and only a few % are above the F3(violent) with ~ 1 F5/yr • Tornado Formation: Tornadoes tend to form with intense thunderstorms and a conditionally unstable atmosphere is essential for their development • Most strong and violent tornadoes develop near the right rear sector of a severe thunderstorm • In order for a tornado to spawn a tornado, the updraft must rotate Total wind speed of a tornado is greater on one side than on the other A powerful multi-vortex tornado with three suction vortices Conditions leading to the formation of severe thunderstorms that can spawn tornadoes; red boxed area: tornadoes are likely to form Where tornadoes are common • Greatest tornado activity shifts northward from winter to summer • Winter: contrast between warm and cold air masses are the greatest over the southern Gulf states & tornadoes are most likely to form in this region • Spring: humid Gulf air surges northward, jet stream also moves northward; tornadoes more prevalent from the southern Atlantic states westward into the southern Great Plains • Summer: contrast between air masses lessens & the jet stream is normally near the Canadian border; tornado activity tends to be concentrated over the northern plains Features associated with tornado-bearing thunderstorm; thunderstorm moves northeast; tornadoes form in the southwest part Mesocyclone • Mesocyclone: A vertical column of cyclonically rotating air within a severe thunderstorm • Severe thunderstorms form in a region of strong vertical wind sheer; most strong and violent tornadoes form within the mesocyclone • Existence of the swirling winds of the mesocyclone inside tornado-producing thunderstorms were observed 1970s (first time) using Doppler Radar • 30% of all mesocyclones produce tornadoes & 95% produce severe weather • Time between mesocyclone identification & tornado touching the ground is ~20 minutes Tornadoes – contd. • Gustnadoes: Tornadoes that form along the gust front • Wall cloud: An area of rotating clouds that extends beneath a severe thunderstorm and from which a funnel cloud may appear • Tornado Watch: Issued by Storm Prediction Center in Norman, Oklahoma • Doppler radar can detect areas of precipitation & measure rainfall intensity • Tornado Vertex signature (TVS): An image of a tornado on the Doppler radar screen that shows up as a small region of rapidly changing wind directions inside a mesocyclone A computer model illustrating motions inside a severe tornado-generating thunderstorm Waterspouts • Doppler Lidar: uses a light beam (instead of microwaves) to measure the change in frequency of falling precipitation, cloud particles, and dust • Waterspout: A rotating column of air over a large body of water; tend to move slowly than tornadoes; last for only 10-15 minutes Doppler radar display of large supercell thunderstorm that is spawning an F4 tornado (circled are) near Lula, OK Average annual number of tornadoes & tornado deaths by decade •Decade . Tornadoes/year Deaths/year 1950-1959 480 148 1960-1969 681 94 1970-1979 858 100 1980-1989 819 52 1990-1999 1,220 56 Summary – Chapter - 10 • Stages of a thunderstorm and a tornado • Air-mass thunderstorm, multicell & supercell thunderstorm • Gust front, causes for downdraft, microburst • Squall line, dry line • Suitable time for the formation of thunderstorm • Lightning and thunder – formation and features • Funneling cloud, mesocyclone, wall cloud • Fujita scale • Direction of movement of tornadoes, conditions for its formation, waterspout