Transcript Tornadoes
Tornadoes, MCSs and Downbursts Review of last lecture 1. The general size and lifetime of mesoscale convective systems, thunderstorms and tornadoes. 3 types of thunderstorms. 2. 3 stages of the ordinary thunderstorms. Downdraft and falling precipitation cut off the updraft. 3. Formation of multi-cell thunderstorms. Downdrafts initiate new thunderstorms in nearby regions. 4. 3 stages of the supercell thunderstorms. Winds aloft push downdraft/precipitation away and the updraft is not weakened. 5. Two types of lightning (cloud-to-cloud 80%, cloud-toground 20%) 6. 4 steps of lightning development. How fast does thunder travel? Convective systems Tornadoes: about 100-600 m, last 1 minute to 1 hour Thunderstorms: about 10 Km, last 10 minutes to a couple of hours. 3 types: ordinary, multicell, supercell Mesoscale convective systems (MCSs): A cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 Km or more in at least one direction, and often last for several hours to a couple of days. Video: A tornado http://www.youtube.com/watch?v=xCI1u05KD_s Tornadoes A rapidly rotating column of air blowing around intense low pressure with circulation reaching ground Wind speeds between 105 km/hr(65mph) and 450 km/hr(280mph) Rotation is almost exclusively cyclonic; a few spin in the opposite direction Various sizes: most are 100-600 m in diameter; some just a few meters; some >1 mile Various shapes: twisting rope-like funnels to cylindrical funnels, to massive black funnels Usually last only a few minutes, but some lasted several hours Most move ahead of cold fronts, from SW to NE; some move in other directions Moving speed is about 30 mph (some >70 mph) Tornado Formation • Tornadoes can develop in any situation that produces severe weather – cold fronts, mesoscale convective systems, supercells, and tropical cyclones. • The processes leading to their formation are not well understood • The most intense and destructive tornadoes come from supercells. 3 Stages of Supercell Tornado Formation • Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height creates an invisible, horizontal spinning effect in the lower atmosphere. • Spinning horizontal vortex tubes created by surface wind shear may be tilted and forced in a vertical path by updrafts. This rising, spinning, and often stretching rotating air may then turn into a mesocyclone. • Most strong and violent tornadoes form within this area of strong rotation. Wind structure of supercell and tornado H L The most common atmospheric circulation structure H L Radiation Cooling or No Heating Convection Heating Latent/Sensible Conduction H L Imbalance of heating Imbalance of temperature Imbalance of pressure Wind Tornado Damage • Tornadoes are classified by the magnitude of damage they cause using the Enhanced Fujita scale. • The forms causing the largest damages are families of tornadoes; when many occur (>6), it is a tornado outbreak • The largest tornado outbreak on record was the April 25–28, 2011 tornado outbreak, with as many as 358 tornadoes. Tornado Occurrence (global) Tornado Occurrence (U.S.) Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes annually (75% from March-July), but the highest frequency is observed in tornado alley of the Central Plains. Great setting for potent mixing of air masses. Mesoscale Convective Systems I. Mesoscale Convective Complex An organized mass, or collection, of thunderstorms that extends across a large region is called a mesoscale convective complex (MCC). They can be up to 1000 x larger than individual storms. With weak upper level winds, such MCC's can regenerate new storms and last for upwards of 12 hours and may bring hail, tornadoes, and flash floods. They often form in summer beneath a ridge of high pressure, with moisture fed from S by low level jets. Mesoscale Convective Systems II. Squall Line • A squall line may contain several severe thunderstorms, some possibly supercells, extending for more than 1000 kilometers. • A squall line always contains a convective precipitation region and a trailing stratiform precipitation region. Vertical structure of squall lines Convective updrafts (controlled by lower troposphere temperature and moisture) Mesoscale updrafts Mesoscale downdraft s Convective downdrafts Zipser (1977), modified by Houze (1993) Downbursts: Introduction • Downbursts are gusts of wind that can reach speeds in excess of 270km/hr (165mph), and are potentially deadly. • Three common types: • Derechos (1000 km) • Haboobs (10-100 km) • Microbursts (1 km) Microburst • is a very localized column of downdraft (sinking air) in a thunderstorm that is less than 2.5 miles in scale. • produces damaging divergent and straight-line winds at the surface as high as 150mph • is similar to, but distinguishable from, tornadoes, which generally have convergent damage. • can produce dangerous situations at airports, as they impede air travel. • 3 types: dry, wet, hybrid Dry microburst: visual identification Very dry boundary layer topped by a moist layer. Driven by cooling beneath the cloud base due to rain evaporation and ice sublimation Virga in the sky: defined as wisps or streaks of water or ice particles falling out of a cloud but evaporating before reaching the earth's surface as precipitation. Blowing dust/dust rings at surface Wet microburst: visual identification A nearly saturated layer topped by an elevated dry layer. Driven by entrainment of mid-level dry air and precipitation loading. Heavy precipitation at the surface. A rain foot may be a visible sign of a wet microburst. Summary 1. 3 stages of supercell tornado formation. 2. Tornado outbreak (number>6) 3. Tornado damage: Enhanced Fujita Scale (EF-0 65-85 mph, EF-5 >200 mph) 4. Tornado occurrence: Global and U.S.. Which country has the largest number of tornadoes in the world? Which state has the largest number of tornadoes per unit area in U.S.? Tornado season in U.S. (March-July) 5. 2 types of mesoscale convective systems. Structure of squall lines: four components 6. 3 types of downbursts (derechos, haboobs, microbursts) 7. Visual identification of dry microburst (virga in the sky, blowing dust at the surface) Works cited http://www.lakeeriewx.com/Meteo361/ResearchTopicFo ur/Synoptic.html http://www.britannica.com/EBchecked/media/19397/Torn adic-thunderstorm-The-rotating-updraft-that-producesthe-tornado-extends http://www.geog.ucsb.edu/~joel/g266_s10/lecture_notes/ chapt08/oh10_8_3/oh10_08_3.html http://www.crh.noaa.gov/lmk/soo/docu/supercell.php