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METEOROLOGY The Relationship between Heat Transfer and Changes in Density • Thunderstorms – A thunderstorm contains thunder and lightning. • Come in many different shapes and sizes. – Warm, moist air rises in a conditionally unstable environment. • As long as parcel is warmer than environment then it will continue to rise, it is buoyant. • Greater the temp difference, the faster the air will rise. – Rising air must be “triggered”/forcing mechanism. • Unequal heating, terrain, lifting of air along shallow boundaries of converging winds. • Frontal lifting. • Large scale divergence aloft. TYPE OF THUNDERSTORMS • SINGLE-CELL (ORDINARY) • MULTICELL • SUPERCELL • MESOSCALE CONVECTIVE COMPLEX Ordinary Single-Cell Thunderstorm • Few miles in diameter • Lasts for less than an hour • Develop in warm, humid air masses away from weather fronts; usually short-lived and rarely produce strong winds or large hail • Three stages of its life cycle: – Cumulus Stage – all updraft, cloud building – Mature Stage – precipitation falls at the surface, most lightning, rain and hail occurs, cloud has an updraft and a downdraft – Dissipating Stage – dominated by downdraft Life cycle of an ordinary thunderstorm cell Severity depends on the intensity of the storm’s circulation pattern Fig. 11.7 Ordinary “Air Mass” Thunderstorms Cumulus Stage Humid air rises, cools, & condenses in to cumulus clouds • (Arrows show vertical air currents. Dashed line represents freezing level, 0°C isotherm.) • During this stage there is insufficient time for precipitation to form, and the updrafts keep water droplets and ice crystals suspended in the cloud. • No lightning in this stage • As cloud builds above freezing level, the cloud particles grow larger and become heavier. • Entrainment – drier air from around the cloud is drawn into it, causing some of the drops to evaporate, chilling the air. The cooler heavier air descends as a downdraft Ordinary “Air Mass” Thunderstorms Mature Stage – The most intense stage. Lightning, thunder, rain, and occasionally small hail. – Top of cloud reaches a stable region (tropopause) and spreads out into anvil, up to 40,000 ft (12 km). – Some updrafts are so strong they penetrate the stable air, a condition know as overshooting. – The cold downdraft forms a gust front. – The gust front can act to force more warm, humid air into the storm. An ordinary thunderstorm in its mature stage. Note the distinctive anvil top. Ordinary “Air Mass” Thunderstorms Dissipating Stage • After the storm enters the mature stage, it begins to dissipate in about 15 to 30 minutes. • Dissipating stage occurs when the updrafts weaken and downdrafts tend to dominate throughout much of the cloud. • Deprived of the rich supply of warm humid air, cloud droplets no longer form. Light precip falls from the cloud, only weak downdrafts remain. • Sometimes the lower part of the storm evaporates leaving only the cirrus anvil above. • This entire process cumulus, mature, dissipating may take place in an hour or less. A dissipating thunderstorm. Most of the cloud particles in the lower half of the storm have evaporated. A climatology of the average number of thunderstorm days in a year Fig. 11-3, p. 314 Fig. 11-4, p. 315 Severe Thunderstorms • Capable of producing large hail, strong, gusty surface winds, flash floods, and tornadoes. • Form in moist air forced to rise into a conditionally unstable atmosphere. But also form in areas with strong vertical wind shear. • Severe Thunderstorms – Criteria: A thunderstorm that produces hail at least ¾” diameter (penny/dime), and/or surface wind gusts of 50 kts, and/or produces a tornado. – The longer a storm survives, the more likely it will become severe. A simplified model describing air motions & other features associated with a severe thunderstorm; severity depends on the intensity of the storm’s circulation pattern 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 Multicell Thunderstorm – Composed of several individual single-cell storms, each at a different stage of development – Forms in same way as an ordinary storm, only the environment features vertical wind shear. • The lack of wind shear in an ordinary thunderstorm causes the precip to fall into the updraft. • Increased winds aloft push the precip away from updraft. • The downdraft can still undercut the updraft without the precip falling into it. This produces a long-lasting multicell storm. Multicell Storms • A multicell storm complex. This storm is composed of a series of cells in successive stages of growth. The thunderstorm in the middle is in its mature stage, with a well-defined anvil. Heavy rain is falling from its base. To the right of this cell, a thunderstorm is in its cumulus stage. To the left, a well-developed cumulus cloud is about ready to become a mature thunderstorm. • Severe Thunderstorms (Supercell) – When the upper level winds are even stronger and also change direction with height : • The storm can move in such a way that the downdraft never undercuts the updraft. • Can create horizontal spin which may be tilted into a rotating updraft. • The result is a supercell. – A supercell modifies its own environment. – The vertical wind shear creates a storm structure that allows the storm to continually move towards the area of warm moist air. • Supercells move to the right of the mean flow. Some of the features of a classic supercell thunderstorm, viewed from southeast A supercell thunderstorm with a tornado sweeps over Texas. Fig. 14-5, p. 372 Lifted Index • The warmer the parcel, the greater the buoyancy and the stronger the updraft. • The warmer the parcel is compared to the environment, the more negative the LI • If the lifted index is negative then the atmosphere is unstable. • LI of 0 to -3: Air is marginally unstable • LI -9 or less: Extreme instability • Severe thunderstorms require a lifting index less than -3 Table 11-1, p. 315 Squall Line • Is a set of individual intense thunderstorm cells arranged in a line. • Thy occur along a boundary of unstable air – e.g. a cold front. • Can from as much as 100 to 300 km out ahead of a cold front in the warm air. • Strong environmental wind shear causes the updraft to be tilted and separated from the downdraft. • The dense cold air of the downdraft forms a ‘gust front’. Fig. 14-8, p. 374 Squall Line A model describing air motions and precipitation associated with a squall line that has a trailing stratiform cloud layer Pre-frontal Squall line • Pre-frontal squall-line thunderstorms may form ahead of an advancing cold front as the upper-air flow develops waves downwind from the cold front. • Mesoscale Convective Complex – A large, convectively driven system that is made up of many individual thunderstorms. Many times, as much as 1000 times larger than an individual thunderstorm cell. –Within the MCCs, the individual thunderstorms work together to generate a longlasting weather system that moves slowly and quite often for periods exceeding 12 hrs. –Produce hail, high winds and flash floods Fig. 14-11, p. 375 IR image showing a Mesoscale Convective Complex extending from central Kansas across western Missouri Severe Thunderstorms and the Dryline • Severe thunderstorms may form along or just east of a boundary called a dry line. • The dryline represents a narrow zone where there is a sharp horizontal change in moisture. • Because dew-point temperatures may drop along this boundary by as much as 9°C (16°F) per km dry lines have been referred to as dewpoint fronts. • Most frequently observed in western half of Texas, Oklahoma and Kansas in the spring and early summer. Dry Line • Surface conditions that can produce a dryline with severe thunderstorms. Radar image of an outflow boundary . As cool 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 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. 14-16, p. 378 Gust Fronts and Shelf & Roll clouds • Gust front – the boundary separating the cold downdraft of a thunderstorm from the warm surface air. An observer on the ground might mistake the gust front for the passage of a cold front in that the wind often shift, gusts can exceed 55 kts and the temperature drops sharply and pressure often rises. • Roll cloud – occasionally an elongated ominous looking cloud forms just behind the gust front. These clouds, which appear to slowly spin about a horizontal axis are called roll clouds. • Shelf Cloud - Prevalent when the atmosphere is very stable near the base of the thunderstorm The lower half of an intense thunderstorm and some of the features associated with it. A dramatic example of a shelf cloud associated with an intense thunderstorm. The photograph was taken in the Philippines as the thunderstorm approached from the northwest. Fig. 14-14, p. 377 A roll cloud forming behind a gust front. (Copyright Howard B. Bluestein.) Microbursts • Beneath a severe thunderstorm, the downdraft may become localized so that it hits the ground and spreads horizontally in a radial burst of wind – such downdrafts are called downbursts. • A downburst with winds extending only 4 kilometers or less is termed a microburst. An intense microburst can induce damaging winds as high as 146 kts. Dust clouds rising in response to the outburst winds of a microburst north of Denver, Colorado. Fig. 14-18, p. 379 Bow echos • Doppler radar display showing a line of thunderstorms (a squall line) bent in the shape of a bow (colors red, orange, and yellow) as they move eastward across the San Joaquin Valley of California. Such bow echoes often produce damaging surface winds near the center of the bow. Sometimes the left (usually northern) end of the bow will develop cyclonic rotation and produce a tornado. Fig. 14-19, p. 379 Flash Floods • Flash floods – floods that rise rapidly with little or no advance warning. • Often a result of stalled or slow moving thunderstorms or when storms “train” over the same area for long periods of time Lightning and Thunder • Lightning – simply a discharge of electricity, a giant spark, which occurs in mature thunderstorms. • Lightning can take place in cloud, cloud to cloud, cloud to air, and cloud to ground. • Thunder – the sound due to rapidly expanding gases along the channel of a lightning discharge. • Thunder travels about I mile in 5 seconds. If we see lightning and then hear thunder 15 seconds later, the stroke occurred about 3 miles away. • Sonic Boom – sound produced when aircraft exceeds the speed of sound at the altitude at which it is flying. Aircraft compresses the air, forming a shock wave, air pressure changes rapidly over a short distance, the rapid change of air pressure causes the sonic boom. 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.” Fig. 14-24, p. 383 Electrification of Clouds • The generalized charge distribution in a mature thunderstorm. • Different ideas on why clouds have charges. • One theory proposes that clouds become electrified when graupel and hail fall through a region of supercooled droplets and ice crystals. When the warmer hailstone comes in contact with a colder ice crystal there is a net transfer of positive ions from the warmer object to the colder object. (Hail gets a negative charge, ice gets positive charge). • Top of cloud is generally positive, middle negative, lower part negative, with some positive. Steps in a Lightning Strike • Stepped Leader: An initial discharge of electrons that proceeds intermittently toward the ground in a series of steps in a cloud-toground 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 The Lightning Stroke • 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. • 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 The Lightning Stroke • 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. The Lightning Stroke • 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. • Return Stroke – the luminous lightning stroke that propagates upward from the earth to the base of a cloud Time exposure of an evening thunderstorm with an intense lightning display near Denver, Colorado. The bright flashes are return strokes. The lighter forked flashes are probably stepped leaders that did not make it to the ground. 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. Fig. 3, p. 389 Fig. 11-33, p. 342 Lightning Rod • 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. Different types of Lightning: Heat lightning – distant lightning from thunderstorms that is seen but not heard. So called because it frequently occurs on hot summer nights when the sky overhead is clear. • 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) • St. Elmo’s Fire – named after the patron saint of sailors. A bright electric discharge that is projected from objects (usually pointed) when they are in a strong electric field such as during a thunderstorm. Ball Lightning • This picture of the freak weather phenomenon of ball lightning was taken by a wildlife ranger in Queensland, Australia, in 1987. • Ball lightning occurs so rarely that few photographs of it exist and researchers have had to rely on eyewitness accounts, some of them from previous centuries. • 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. Tornadoes A mature tornado with winds exceeding 150 knots rips through southern Illinois. • Tornadoes are rapidly rotating winds that blow around a small area of intense low pressure. A tornado’s circulation is present on the ground either as a funnel shaped cloud or as a swirling cloud of dust and debris. • Other names – twisters, cyclones • Funnel cloud – a tornado whose circulation has not reached the ground. Diameter of most tornadoes (300 – 2000 ft) Tend to move from southwest toward northeast Most last only a few minutes – at least one lasted 7 hours and moved along a path of 292 miles Box 11-1, p. 329 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 & – Decay (or Rope) Stage • Tornadoes (Life Cycle) 1. Dust Whirl Stage 2. Organizing Stage 3. Mature Stage 4. Shrinking Stage 5. Decay (Rope) Stage 2. 1. 3. 4. 5. Fig. 11-23, p. 331 Tornado Outbreak – April 3, 1974 • • • Outbreak – 6 or more tornadoes over a particular region Map of 148 Tornadoes that Occurred on April 3 & 4, 1974 315 people died, 5,000 injured • Tornadoes (Occurrence) – Top number: Number of tornadoes in 25 years. – Bottom number: Number of tornadoes per 10,000 square miles. – Tornado alley is from north Texas through Nebraska. – Can occur in any state. Geographic distribution of the month of maximum tornado threat Fig. 11-30, p. 337 Average number of tornadoes during each month in the United States. Fig. 14-34, p. 392 Tornado Winds • 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. • Most tornadoes probably have winds less than 125 kts, but most powerful twisters seldom exceed 220 kts. TR 303c-26 A powerful multi-vortex tornado with three suction vortices Fig. 14-36, p. 393 Table 14-1, p. 393 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 Table 11-3, p. 333 • Fujita Scale (F0 and F1) F0 F1 • Fujita Scale (F2 and F3) F2 F3 • Fujita Scale (F4 and F5) F4 F5 A devastating F5 tornado about 200 m (650 ft) wide plows through Hesston, Kansas, on March 13, 1990, leaving almost 300 people homeless and 13 injured. Total destruction caused by an F5 tornado that devastated parts of Oklahoma on May 3, 1999. Fig. 14-38, p. 395 • Tornadic Thunderstorms – Not everything is known about the formation of a tornado. – It is known that tornadoes tend to form with intense thunderstorms and that an unstable atmosphere is essential for their development. – Tornadoes form with both supercell thunderstorms and non-supercell thunderstorms. – The most intense tornadoes form with supercell thunderstorms. Thunderstorms with a strong, single rotating updraft that develop in a region of strong vertical wind shear. • Tornadic Thunderstorms (Mesocyclones) – The rotating updraft of a supercell thunderstorm is called a mesocyclone. – Change of wind speed and direction with height is responsible for the rotating updraft. Thunderstorm Rotation • Recall that supercells form in a region of strong vertical wind shear that causes the updraft inside the storm to rotate. Thunderstorm Rotation • The strong updraft in the thunderstorm carries the vortex tube into the thunderstorm, producing a rotating air column that is oriented in the vertical plane. Fig. 11.18 –As air rushes into the low-level core of the mesocyclone, the air expands, cools, and if moist, condenses into a cloud (funnel cloud). –As the air beneath the funnel cloud is drawn into its core, the air cools rapidly and condenses, and the funnel descends to the surface. –As the funnel reaches the ground, it usually picks up dirt and debris, making it appear dark. –While the air along the outside of the funnel is spiraling upward, in the most violent tornadoes, the air is descending towards the extreme low pressure at the ground (sometimes 100 mb lower than surrounding air). • Hook Echo (Bounded Weak Echo Region) – The updraft is so strong in a supercell that precipitation cannot fall through it. Southwesterly winds aloft usually help to blow the precipitation northeastward. – If a mesocyclone is strong and persistent, the precip can be wrapped around the updraft. – This swirling area of precip shows up on the radar. – This is called the bounded weak echo region, an area that is bounded by precip. On radar, it can appear as a hook on the southern side of the storm. Fig. 14-42, p. 396 A classic tornadic supercell thunderstorm showing updrafts and downdrafts, along with surface air flowing counterclockwise and in toward the tornado. Fig. 14-43, p. 397 A computer model illustrating motions inside a severe tornado-generating thunderstorm Observing Tornadoes • Wall cloud – An area of rotating clouds that extends beneath a severe thunderstorm and from which a funnel cloud may appear • Tornado watch – tornadoes are likely to form in the next few hours – issued by Storm Prediction Center in Norman • Tornado warning – Once a tornado is spotted – either visually or on radar – issued by local NWS office A wall cloud photographed southwest of Norman, Oklahoma. A wall cloud associated with a supercell thunderstorm spins counterclockwise over the plains of Texas. Beneath the wall cloud, dust rising from the surface indicates that a tornado is about to form. Fig. 14-CO, p. 366 A funnel cloud extends downward from the base of a non-supercell thunderstorm over central California. Doppler Radar • Similar to a conventional radar in that is can detect areas of precipitation and measure rainfall intensity. • Doppler can do more – actually measures the speed at which precipitation is moving horizontally toward or away from the radar antenna. • Doppler radar “peers into severe thunderstorms and reveals its winds. Doppler radar display showing precipitation inside a large supercell thunderstorm that is spawning an F4 tornado (circled area) near Lula, Oklahoma. The regions in orange and red show the area of heaviest precipitation. The close packing of the winds indicates strong cyclonic rotation and the signature of a tornado. Red and orange indicate winds blowing away from the radar. Green and blue indicate winds blowing toward the radar 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 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. Fig. 14-47, p. 399 Waterspouts • A rotating column of air over a large body of water.. • Fair Weather Waterspouts – form over waters of Florida Keys, Caribbean. Much smaller than an average tornado, (3 – 100 meters), wind less than 45 kts, tend to move slowly. • A powerful waterspout moves across Lake Tahoe, California.