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Severe Storms Readings: A&B Ch. 11 (308-326, 330-344) Topics 1. Severe weather d. Severe T-storms i. Squall lines ii.Mesoscale Convective Complexes iii.Supercells a. Scale b. Motion 2. Thunderstorms a. Thunder & Lightning b. Factors in T-storm development c. Air-mass T-storms i. Cumulus stage ii.Mature stage iii.Dissipating stage 3. Tornadoes a. Development b. Location and Timing c. Prediction d. Fujita Intensity Scale G109: Weather and Climate Severe Storms • Thunderstorms, tornadoes, and hurricanes Each have things in common and differences Scale Diameter Mid-lat. Cyclone 1600 km Hurricane Tornado * Thunderstorm 600 km 0.25 km A few km Rarely up to 50 km Smaller & more intense than a mid-latitude cyclone * Tornadoes are too small to appear on a weather map G109: Weather and Climate 13: Severe Storms Severe Storms Motion Mid-latitude Cyclone Hurricane Tornado Thunderstorm Inward, spiral Inward, spiral Inward, spiral Strong upward & downward motion Very variable & gusty winds G109: Weather and Climate 13: Severe Storms Thunderstorms • Associated with cumulonimbus clouds Generate heavy rainfall, thunder & lightning, occasional hail • Unlike hurricanes and tornadoes Different air motion – updrafts, downdrafts, variable winds Can form: • “On their own”: air mass thunderstorms • In conjunction with cyclones (Low pressure) Mid-latitude cyclones: t-storms frequently form along the cold front Hurricanes: generate widespread thunderstorm activity On rare occasions: tornado may descend from thunderstorm (cumulonimbus tower) G109: Weather and Climate 13: Severe Storms Thunder and Lightning • Lightning: 80% of lightning is cloud-to-cloud lightning 20% of lightning is cloud-to-ground • During the formation of large cumulonimbus cloud separation of charge occurs Mechanism not known in detail But, lightning only occurs in clouds which extend above the freezing level and which form precipitation • Charge separation: related to Bergeron process G109: Weather and Climate 13: Severe Storms Thunder and Lightning Part of the cloud has an excessive negative charge and another part excessive positive charge Lightning: attempt to equalize these electrical differences Air is a poor conductor, therefore the electrical potential (charge differences) must be very high before lightning will occur G109: Weather and Climate 13: Severe Storms Thunder and Lightning Stepped leader: advancing, branching shaft of negative charges Approaches the ground When stepped leader approached the ground, a spark of positive charges also surges upward from the ground Stepped leader and upward surge from ground create a path for flow of current G109: Weather and Climate 13: Severe Storms Thunder and Lightning Negative current flows downward from cloud Return stroke: positive current flow upward from ground A single flash of lightning is a sequence of strokes and return strokes • Usually 2-3 strokes; sometimes up to 20 strokes G109: Weather and Climate 13: Severe Storms Thunder and Lightning • Thunder Produced by lightning Electrical discharge of lightning heats the air and causes it to expand explosively • Expansion produces sound waves → hear thunder Thunder travels at speed of sound (~300 m/s), while the light seen from a flash of lightning travels at the speed of light (3x108 m/s) • Lag between them • Count the seconds between lightning and thunder; divide by 5, to determine how many miles the storm is from you G109: Weather and Climate 13: Severe Storms Thunderstorm Development • Thunder and lightning are associated with cumulonimbus clouds Generate heavy rainfall, thunder, lightning, occasional hail • Factors in development of cumulonimbus cloud and thunderstorm: Lifting: surface heating, presence of a front, or other mechanism Instability: tendency of warm air to lift Moisture: release of latent heat provides energy and buoyancy for additional lift Wind shear: causes t-storm cells to tilt → more severe storms G109: Weather and Climate 13: Severe Storms Air Mass Thunderstorms • Air mass (or isolated thunderstorms) Occur within a warm humid (mT) air mass Most frequent type of t-storms Limited lifetime (~1-2 hours) Very localized – limited in size Most likely to occur: • Spring & summer – when air is warmed sufficiently from below • Mid-afternoon when surface temperatures are highest • Sometimes after sunset as growth of immature t-storm cells re-stimulated by cloud top cooling → Heavy rain, small hail, high winds, downbursts, possibly weak tornadoes G109: Weather and Climate 13: Severe Storms Air Mass Thunderstorm Development • Three stages Cumulus (developing) stage Mature stage Dissipating stage G109: Weather and Climate 13: Severe Storms Cumulus (Developing) Stage • Need continuous warm moist air • Unstable air caused to rise by local heating or other mechanism • Develop cumulonimbus clouds 10-20 km tall • Dominated by updrafts (up to speeds of 160 km/h) • Once cloud passes freezing level – Bergeron process Usually produces precipitation within an hour G109: Weather and Climate 13: Severe Storms Mature Stage • Precipitation starts to fall Falling precipitation causes drag on the air → downdrafts Some falling precipitation evaporates which is a cooling process • Influx of cool dry air surrounding the cloud: entrainment Intensifies the downdraft (because the air is cool and dense) Downdraft – cooling process • When downdraft leaves the bottom of cloud: Precipitation falls to the surface Cold air spreads laterally G109: Weather and Climate 13: Severe Storms Mature Stage → T-storm cell composed of updrafts and • downdrafts, existing side-by-side When cloud grows to top of unstable region (often at the tropopause): Updrafts spread laterally → Anvil top of cirrus clouds • Most active stage of the T-storm Gusty winds Heavy precipitation Sometimes hail Intense thunder and lightning G109: Weather and Climate 13: Severe Storms Dissipating Stage • Cooling downdrafts cut off supply of warm moist air which caused T-storm • Downdrafts dominate → End of T/storm: without supply of moisture the cloud soon evaporates • Single cumulonimbus cell has a life of 1-2 h However, as storm moves to fresh supplies of warm water laden air, new cells may be generated G109: Weather and Climate 13: Severe Storms Severe Thunderstorms • • Severe t-storms develop by a similar process BUT, strong wind shear → tilt of updraft Updraft and downdraft are not coincident Downdraft does not cut off moisture supply to updraft Updrafts and downdrafts reinforce each other and intensify the storm → Longer life: often 12 hours, sometimes several days → Larger area → Severe storms have: stronger winds, heavier rain, larger hail, more downbursts, more likelihood of tornadoes G109: Weather and Climate 13: Severe Storms Severe Thunderstorms • Frequently form along or ahead of a cold front in the wave cyclone Cold air advances into a region of warm air Warm air less dense - displaced upwards • Tend to appear in groups or clusters of storm cells, 10-1000 km across Squall line: linear band of t-storm cells Mesoscale convective complex: oval or circular cluster of t-storm cells • Can also occur with a single very large, very intense updraft zone: supercell storm G109: Weather and Climate 13: Severe Storms Severe Thunderstorms • Squall line: A group of thunderstorm cells, arranged in a linear band Approximately 500 km long, less than 100 km wide Typically last ~10 hours Common in southern U.S. during spring / summer Usually form immediately ahead of a cold front G109: Weather and Climate 13: Severe Storms Severe Thunderstorms • Squall line: Wind shear pushes updrafts ahead of downdrafts Air spreads laterally ahead of downdraft: gust front • Causes uplift of warm air into cloud G109: Weather and Climate 13: Severe Storms Severe Thunderstorms • Mesoscale Convective Complex (MCC) A group of thunderstorm cells, arranged in an oval or circular cluster Several 100 km across, typically last ~12 hours Self-propagating group of storms: • Downdraft from one cell converges with surface flow → Updraft → Formation of a new cell • New cell draws in moisture from downdraft of old cell G109: Weather and Climate 13: Severe Storms Severe Thunderstorms • Mesoscale Convective Complex (MCC) Frequently form “flanking line”, moving to NE As storm at the north end of line dissipates, a new one is formed at the south of the line G109: Weather and Climate 13: Severe Storms Severe Thunderstorms • Supercell storms A single very large cell • 20-50 km across • Typical lifetime: 2-4 hours Very intense updraft More violent than MCC or squall line Most likely setting for large tornadoes Have large scale rotation, not observed in storm cells in MCCs or squall lines G109: Weather and Climate 13: Severe Storms Severe Thunderstorms • Supercell storms Large scale rotation → intense wind shear → Complex structure with updraft and downdraft wrapping around each other Downdrafts amplify updrafts G109: Weather and Climate 13: Severe Storms Severe Thunderstorms • • • • Supercell storms Track supercell storms on radar, which shows intensity of precipitation Vault: inflow of warm air to the updraft Cloud droplets formed Too small for rain droplets Vault may appear on radar as hook echo Hook echo indicates that tornado formation is imminent G109: Weather and Climate 13: Severe Storms Severe Thunderstorms • Doppler Radar Shows wind toward (green) and away from (red) the radar Highlight region of intense rotation: mesocyclone – often precedes tornado formation G109: Weather and Climate 13: Severe Storms Tornadoes • Tornadoes are: Local violent storm of short duration and sporadic occurrence Very destructive – among nature’s most destructive forces Rotating column of air – vortex – extends downward from a cumulonimbus cloud Also referred to as a twister or a cyclone In northern hemisphere, most tornadoes rotate cyclonically (counter-clockwise), but a few spin in the opposite direction (clockwise) G109: Weather and Climate 13: Severe Storms Tornadoes • Tornadoes can appear as a very thin ropeshaped column or as a funnel shape • Regardless of shape tornadoes are extremely dangerous G109: Weather and Climate 13: Severe Storms Tornadoes • Strong winds within a tornado occur due to extraordinarily large differences in pressure over short distances Pressures inside tornadoes have been estimated to be as much as 10% (100 mb) lower than immediately outside the storm Pressure gradient force → Draws winds into the low pressure center → Generates winds of up to 480 km/h Air near the ground rushes inward, spirals upward around the core and merges with the airflow of the parent thunderstorm deep in the cumulonimbus cloud G109: Weather and Climate 13: Severe Storms Tornadoes • Tornado is visible: Air sucked into the storm expands and cools adiabatically • May cool below dew point temperature → Condensation → Pale ominous cloud Dust and debris picked up by the ground G109: Weather and Climate 13: Severe Storms Profile of a Tornado Width Life Length of path Max. wind spd Frequency of Occurrence • Average Weak Violent Tornado Tornado Tornado 150-600 m 100 m 1000 m ~5 minutes < 3 minutes > 3 hours 3-4 km 1 km 150 km 250-450 km/h < 150 km/h > 480 km/h >50% of all <2% of all tornadoes tornadoes Tornadoes typically travel across the ground: At a speed of ~50 km/h From SW to NE (because they occur ahead of a cold front, where prevailing winds are from SW) G109: Weather and Climate 13: Severe Storms Profile of a Tornado • • Most tornadoes rotate around a single central vortex Some stronger tornadoes have suction vortices Multiple smaller intense whirls, that rotate within the main vortex Suction vortices have diameters of ~10m and rotate very rapidly G109: Weather and Climate 13: Severe Storms Development of a Tornado • • • • Form in association with severe t-storms: < 1% of t-storms produce tornadoes, but most severe storms must be monitored as potential tornado producers Can develop in any situation that produces severe weather Along cold front or squall line Mesoscale convective complex Hurricane Supercell t-storms Not certain exactly what triggers formation Product of the interaction between strong updrafts in T/storm and winds in troposphere G109: Weather and Climate 13: Severe Storms Development of a Tornado • In supercell t-storms, a precondition to tornado formation is a mesocyclone: vertical cylinder of rotating air, ~3-10 km across, that develops in updraft Begins as slow, horizontal rotation of large segment of cloud deep within cloud interior, several km above surface G109: Weather and Climate 13: Severe Storms Development of a Tornado • Mesocyclone formation: Under right conditions, strong updrafts in the tstorm tilt the horizontally rotating vortex to nearly vertical, within the cloud interior • Mesocyclone often precedes tornado by ~30 minutes G109: Weather and Climate 13: Severe Storms Development of a Tornado • Mesocyclone intensifies: At first, mesocyclone is wide, short, and rotating slowly Then, it narrows horizontally and stretches vertically, causing winds to accelerate Narrowing column of rotating air stretches downward, protrudes below the cloud base to produce a very dark, slowly rotating wall cloud G109: Weather and Climate 13: Severe Storms Development of a Tornado • • Funnel cloud: A slender rapidly spinning vortex, which emerges from the base of the wall cloud When a funnel cloud makes contacts with the surface, then it is classified as a tornado Formation of mesocyclone does not necessarily mean tornado formation will follow Approximately 20% of mesocyclones produce tornadoes G109: Weather and Climate 13: Severe Storms Location and Timing of Tornadoes • Predominantly a N. American phenomenon Many tornadoes touch down along wide strip running SW to NE between the southern Plains and the lower Great Lakes region: Tornado Alley Florida: many Tornado density tornadoes embedded in passing hurricanes & tropical storms On average, 770 tornadoes reported annually in U.S. G109: Weather and Climate 13: Severe Storms Location and Timing of Tornadoes • • • 54% of all tornadoes occur during spring Most frequent in April to June May: ~5 per day in US Air masses have greatest contrast cP – still cold and dry mT – warm, humid, and unstable Greater contrast → more intense storms → more likelihood of tornadoes G109: Weather and Climate 13: Severe Storms Tornado Prediction • Very difficult because: Severe t-storms & tornadoes are short lived Observation network sparse • Surface station: ~160 km apart, hourly obs. • Upper air stations: ~320 km apart, 12 hour obs. Doppler radar • Look for characteristic precipitation pattern (hook echo), and wind patterns (rotation of mesocyclone) G109: Weather and Climate 13: Severe Storms Watches and Warning • • • Severe storm watches and tornado watches Issued by National Weather Service Storm Prediction Center (Norman Oklahoma) If a particular region of country is vulnerable to impending storm activity, a watch is issued Severe thunderstorm warning Issued by local weather forecasting office If a severe thunderstorm has developed Tornado warning Issued by local weather forecasting office If an actual tornado has been observed (by trained spotter) Or: if mesocyclone and hook echo are detected by Doppler radar G109: Weather and Climate 13: Severe Storms Fujita Tornado Intensity Scale • • • Tornado winds cannot be measured directly Fujita Intensity Scale Determined by assessing worst damage produced by storm 7 levels of intensity: F0 to F6 • F0 and F1: “weak storms” (69% of all tornadoes) – moderate damage • F2 and F3: “strong storms” (29% of all tornadoes) – major structural damage • F4 and F5: “violent storms” (<2% of all tornadoes) – incredible devastation • F6: never documented in nature Most damage is caused by high winds; most injuries and death result from flying debris G109: Weather and Climate 13: Severe Storms