Severe Storms

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Information about Severe Storms

Published on October 20, 2008

Author: aSGuest1444


Slide 1: Severe Storms: Tropical Cyclones Thunderstorms Tornadoes Typhoon Krosa (CAT II) from JMA (today) : Typhoon Krosa (CAT II) from JMA (today) Tropical Cyclones : Tropical Cyclones Hurricane (typhoon, cyclone) is a violent, long-lived cyclonic storm with sustained winds in excess of 74 mph that originates in tropical latitudes, usually in late summer and early autumn. Hazards: Torrential rains (10 inches is common), Storm surges of as much as 25 feet (from low pressure, onshore winds, and high tides), Flooding can be widespread. Wind gusts of up to twice the average wind speed, and Tornadoes and Thunderstorms Storm track is somewhat erratic and is determined by the interaction of the storm with its environment. Saffir-Simpson Scale : Saffir-Simpson Scale NAME WINDS (mph) DAMAGE Tropical depression 23-39 minor Tropical storm 40-74 from rains Hurricane CAT I 74-95 trees CAT II 96-110 roofs CAT III 111-130 light structures CAT IV 131-155 extensive CAT V > 155 devastating Tropical Cyclones : Tropical Cyclones Hurricane -- W. Hemisphere, named for Mayan sky god Hurican (Sp. Huricán, Fr. Ouragan) Typhoon -- W. Pacific, named for great wind (Cantonese) Cyclones -- Indian Ocean and Australia Hurricane Origins and Tracks : Hurricane Origins and Tracks Formed by disturbance in trade winds near ITCZ. Needs warm ocean water T > 26 oC Needs Coriolis force to support rotation, so never found in latitudes less than 5o North or South Needs conditions with no wind shear aloft to get organized. Sustained by massive release of latent heat of condensation. One Possible Track : One Possible Track Latent Heat Release and Energy in a Hurricane : Latent Heat Release and Energy in a Hurricane 25 cm of precipitation (average) Storm width, 400 km = 4 x 107 cm Storm track, 3000 km = 3 x 108 cm Latent heat release = 3 x 1017 grams x 600 cal/gm = 2 x 1020 cal World energy use = 2 x1017 cal/day So latent heat released by one hurricane could run the world for 3 years! Hurricane Structure : Hurricane Structure W ------------------------------------------------------------------> E CiSt P \ Spiral rain Eye Spiral rain P / Clear winds / bands clear bands winds \ N, NE winds S, SE winds Hurricane’s Demise : Hurricane’s Demise Over land hurricanes are: 1) Cut off from warm ocean moisture 2) Slowed down by increased surface friction 3) Torn apart by increased wind shear aloft in the westerlies 4) Accelerated northward by mid-latitude winds Hurricane Andrew 1992 : Hurricane Andrew 1992 Last CAT 5 to hit U.S., 43 deaths, $30B damage. Past Dealiest Hurricanes : Past Dealiest Hurricanes 1900 Galveston hurricane, Isaac’s Storm (6,000 dead) 1938 New England Hurricane (Miller video 1687) The hurricane had no name, it was not tracked, and it came with little or no warning. There were no emergency services available. Killer storm surge caused by extreme low pressure, offshore winds, and the timing of the high tide. Great loss of life. 1969 Hurricane Camille 200 mph wind gusts, 25 foot storm surge, 256 deaths, $4 Billion damage. 1970 Bengal cyclone (500,000 dead) Hurricane Mitch (22 Oct.-5 Nov. 1998) : Hurricane Mitch (22 Oct.-5 Nov. 1998) Mitch was responsible for over 9,000 deaths predominately from rain-induced flooding in portions of Central America, mainly in Honduras and Nicaragua. This makes Mitch one of the deadliest Atlantic tropical cyclones in history. The 905 mb minimum central pressure and estimated maximum sustained wind speed of 155 knots (Category V) over the western Caribbean make Mitch the strongest October hurricane since records began in 1886. Katrina, Aug. 29, 2005 : Katrina, Aug. 29, 2005 For more information : For more information National Hurricane Center Tropical Meteorology Project at Colorado State University Thunderstorms, Lightning,Tornadoes : Thunderstorms, Lightning,Tornadoes Anthes Ch. 5, pp. 124-32 Thunderstorms : Thunderstorms A thunderstorm is a deep convective cloud (Cb) reaching as high as 10 to 18 km, that produces lightning, thunder, heavy rain, downbursts, possibly hail, flash floods, and on rare occasions, tornadoes. It is a thermodynamic heat engine driven by the latent heat of condensation (vapor to liquid) and fusion (liquid to ice). Can be caused by solar heating, nighttime cooling, sea breeze convergence, mountains, cold fronts. An average of 1800 thunderstorms are active globally at this moment. Mesoscale convective complex contains dozens of Cbs and is 1000 times larger than a thunderstorm. Lifecycle of a Thunderstorm : Lifecycle of a Thunderstorm Supercell : Supercell Thunderstorm Climatology : Thunderstorm Climatology Hail Climatology Lightning : Lightning Lightning is an electrical discharge. Most flashes originate in clouds. Clouds are “most commonly in a negative state of electricity, but sometimes in a positive state.” Grounded metallic rods will protect most structures from damage. Source: Benjamin Franklin (1750s) Lightning : Lightning Lightning is a giant transient high-current electrical discharge (or spark) of static electricity with a path length of kilometers. A typical strike is 100 million volts, 100,000 amps, and generates temperatures of 30,000 K (five times hotter than the sun’s surface). Around the world, there are 1800 thunder storms going on at any given time with 100 lightning strikes to Earth each second. That means 9 million lightning bolts per day! Most lighting is within a cloud or between clouds. Only about 20% is cloud to ground. Lightning strike details : Lightning strike details Lightning strike is very fast and very complex: 1. Cloud to ground stepped leader moves along a branching path in 20 to 100 meter steps at 10 to 20 km/sec. 2. Ground to cloud return stroke covers the final 100 meter spark gap to a high spot (tree, tower, golf club). 3. Subsequent (3-40) strokes called dart leaders give lightning its forked appearance, completing the strike. Lightning formation : Lightning formation Necessary condition: Separation of charge. Cb is a static electricity generator. Charge separation caused by collisions of water and ice. Electrons (-) and ions (+ -) transported to different levels via updrafts and downdrafts. Air is a good insulator. Cloud can reach 100 million volts, 100,000 amps. Car battery is 12 volts, 8 amps. House current is 120 volts, 20 amps. Sufficient condition: Voltage difference > insulating ability of the air. Thunder : Thunder Thunder = sound wave Long tube of air, 6 km x 15 cm diameter heated almost instantaneously to 12-30,000 K expands air violently. Shock wave = thunder (both audible and inaudible). Speed of light = 300,000 km / sec Speed of sound ~ 0.33 km / sec 3 second delay = 3 km; 5 second delay = 1 mile Lightning fatalities : Lightning fatalities About 60 fatalities per year in the US, 360 severe injuries Benefits of lightning : Benefits of lightning May have played a role in the pre-biotic formation of amino acids. Fixes nitrogen, creates natural fertilizer. Ignites forest fires -- forest ecology. May have been the source of fire for early humans. E-M fields of lightning used to study the atmosphere. Global lightning strikes represent a global “thermometer.” Lighting strikes on other planets (Jupiter) allow remote study. Tornado : Tornado A small, funnel shaped, rapidly rotating vertical column of air. Generated in severe thunderstorms (supercells) in a rotating mesocyclone. Extremely low central pressure with winds of up to 300 mph and updrafts of 100 mph. Made visible by dust and debris sucked into the vortex. Typically 100-600 meters in diameter. Travel at about 30-40 mph, usually in a NE direction. May travel in groups, alternately skipping and touching down. Above the ground they are called funnel clouds. Slide 34: Waterspouts (tornadoes over water) are visible due to condensation of water vapor. Tornado Formation : Tornado Formation Supercell, Mesocyclone and Tornado : Supercell, Mesocyclone and Tornado Before thunderstorms develop, wind shear, a change in wind direction and an increase in wind speed with height, creates an invisible, horizontal vortex in the lower atmosphere. Thunderstorm updrafts tilt the vortex from horizontal to vertical. An area of rotation, 2-6 miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation. Enhanced Fujita Damage Scale for Tornadoes : Enhanced Fujita Damage Scale for Tornadoes Scale Winds Damage and frequency EF0 65-85 Light, tree branches broken, 38.9% EF1 86-110 Moderate, trees snapped, 35.6% EF2 111-135 Considerable, trees uprooted, 19.2% EF3 136-165 Severe, cars overturned, 4.9% EF4 166-200 Devastating, homes destroyed, 1.1% EF5 > 200 Incredible, steel structures highly damaged, 0.1% Tornado hazards : Tornado hazards 39 Fatalities in 2005, 74 so far in 2007 Vast majority from F3 tornado hitting mobile homes 700 killed, 11,000 homeless in Ohio, 18 March 1925. 335 killed, $600 million damage in 148 tornado outbreak from Alabama to Ohio, April 1974. Aviation hazards from thunderstorms : Aviation hazards from thunderstorms Impaired visibility Turbulence Downbursts Wind shear Icing conditions Rapid pressure changes (can cause altimeter errors) Rain on runways (can cause hydroplaning) Slide 40: Flash floods: 130 US fatalities per year

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