Tornadoes, MCSs and DownburstsTornadoes, MCSs and Downbursts

Review of last lectureReview of last lecture1.1. The general size and lifetime of mesoscale convective The general size and lifetime of mesoscale convective

systems, thunderstorms and tornadoes. 3 types of systems, thunderstorms and tornadoes. 3 types of thunderstorms. thunderstorms.

2.2. 3 stages of the ordinary thunderstorms. 3 stages of the ordinary thunderstorms. Downdraft and falling precipitation cut off the updraft.

3.3. Formation of multi-cell thunderstorms. Downdrafts initiate Formation of multi-cell thunderstorms. Downdrafts initiate new thunderstorms in nearby regions.new thunderstorms in nearby regions.

4.4. 3 stages of the supercell thunderstorms. 3 stages of the supercell thunderstorms. Winds aloft push downdraft/precipitation away and the updraft is not weakened.

5.5. Two types of lightning (cloud-to-cloud 80%, cloud-to-Two types of lightning (cloud-to-cloud 80%, cloud-to-ground 20%)ground 20%)

6.6. 4 steps of lightning development. How fast does thunder 4 steps of lightning development. How fast does thunder travel? travel?

Convective systemsConvective systems

TornadoesTornadoes: about : about 100-600 m100-600 m, last , last 1 minute to 1 hour1 minute to 1 hour

ThunderstormsThunderstorms: about : about 10 Km10 Km, last , last 10 minutes to a 10 minutes to a couple of hours. couple of hours. 3 types: ordinary, multicell, supercell3 types: ordinary, multicell, supercell

Mesoscale convective systems (MCSs)Mesoscale convective systems (MCSs): A cloud : A cloud system that occurs in connection with an ensemble system that occurs in connection with an ensemble of thunderstorms and produces a contiguous of thunderstorms and produces a contiguous precipitation area on the order of precipitation area on the order of 100 Km100 Km or more in or more in at least one directionat least one direction, and often last for , and often last for several hours several hours to a couple of daysto a couple of days..

Video: A tornadoVideo: A tornado

http://www.youtube.com/watch?v=xCI1u05KD_s

TornadoesTornadoes A rapidly rotating column of air blowing around A rapidly rotating column of air blowing around

intense low pressure with circulation reaching intense low pressure with circulation reaching groundground

Wind speeds between 105 km/hr(65mph) and Wind speeds between 105 km/hr(65mph) and 450 km/hr(280mph)450 km/hr(280mph)

Rotation is almost exclusively cyclonic; a few spin Rotation is almost exclusively cyclonic; a few spin in the opposite directionin the opposite direction

Various sizes: most are 100-600 m in diameter; Various sizes: most are 100-600 m in diameter; some just a few meters; some >1 milesome just a few meters; some >1 mile

Various shapes: twisting rope-like funnels to Various shapes: twisting rope-like funnels to cylindrical funnels, to massive black funnelscylindrical funnels, to massive black funnels

Usually last only a few minutes, but some lasted Usually last only a few minutes, but some lasted several hoursseveral hours

Most move ahead of cold fronts, from SW to NE; Most move ahead of cold fronts, from SW to NE; some move in other directions some move in other directions

Moving speed is about 30 mph (some >70 mph)Moving speed is about 30 mph (some >70 mph)

Tornado FormationTornado 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 Formation3 Stages of Supercell Tornado Formation

• Before thunderstorms develop, a change Before thunderstorms develop, a change in wind direction and an increase in wind in wind direction and an increase in wind speed with increasing height creates an speed with increasing height creates an invisible, horizontal spinning effect in the invisible, horizontal spinning effect in the lower atmosphere. lower atmosphere.

• Spinning horizontal vortex tubes created Spinning horizontal vortex tubes created by surface by surface wind shearwind shear may be may be tiltedtilted and and forced in a vertical path by updrafts. This forced in a vertical path by updrafts. This rising, spinning, and often stretching rising, spinning, and often stretching rotating air may then turn into a rotating air may then turn into a mesocyclonemesocyclone..

• Most strong and violent tornadoes form Most strong and violent tornadoes form within this area of strong rotation.within this area of strong rotation.

Wind structure of supercell and tornadoWind structure of supercell and tornado

L

H

The most common atmospheric The most common atmospheric circulation structurecirculation structure

L

H

H

L

HeatingCoolingor No Heating

Imbalance of heating Imbalance of temperature Imbalance of pressure Wind

Radiation Convection

Latent/Sensible

Conduction

Tornado DamageTornado 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 (global)

Tornado Occurrence (U.S.)Tornado Occurrence (U.S.)

Tornadoes from all 50 states of the U.S. add up to more than 1000 tornadoes 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 annually (75% from March-July), but the highest frequency is observed in tornado alley of the Central Plainstornado alley of the Central Plains. Great setting for potent mixing of air masses.. Great setting for potent mixing of air masses.

An organized mass, or collection, of An organized mass, or collection, of thunderstorms that extends across a thunderstorms that extends across a large region is called a large region is called a mesoscale mesoscale convective complex (MCC).convective complex (MCC). They can They can be up to 1000 x larger than individual be up to 1000 x larger than individual storms.storms.

With weak upper level winds, such With weak upper level winds, such MCC's can regenerate new storms and MCC's can regenerate new storms and last for upwards of 12 hours and may last for upwards of 12 hours and may bring hail, tornadoes, and flash floods.bring hail, tornadoes, and flash floods.

They often form in summer beneath a They often form in summer beneath a ridge of high pressure, with moisture ridge of high pressure, with moisture fed from S by low level jets.fed from S by low level jets.

Mesoscale Convective Systems Mesoscale Convective Systems I. Mesoscale Convective ComplexI. Mesoscale Convective Complex

• A squall line may A squall line may contain several severe contain several severe thunderstorms, some thunderstorms, some possibly supercells, possibly supercells, extending for more extending for more than 1000 kilometers.than 1000 kilometers.

• A squall line always A squall line always contains a convective contains a convective precipitation region precipitation region and a trailing stratiform and a trailing stratiform precipitation region.precipitation region.

Mesoscale Convective SystemsMesoscale Convective SystemsII. Squall LineII. Squall Line

Vertical structure of squall linesVertical structure of squall lines

Zipser (1977), modified by Houze (1993)

Convective updrafts (controlled by lower troposphere temperature and moisture)

Mesoscale updrafts

Convective downdrafts

Mesoscale downdrafts

Downbursts: IntroductionDownbursts: 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)

MicroburstMicroburst

• 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 identificationDry microburst: visual identification

Very dry boundary layer topped Very dry boundary layer topped by a moist layer. Driven by by a moist layer. Driven by cooling beneath the cloud base cooling beneath the cloud base due to rain evaporation and ice due to rain evaporation and ice sublimation sublimation

Virga in the skyVirga in the sky: defined as : defined as wisps or streaks of water or ice wisps or streaks of water or ice particles falling out of a cloud particles falling out of a cloud but evaporating before but evaporating before reaching the earth's surface as reaching the earth's surface as precipitation. precipitation.

Blowing dust/dust rings at Blowing dust/dust rings at surfacesurface

Wet microburst: visual identificationWet microburst: visual identification

A nearly saturated layer topped A nearly saturated layer topped by by an elevated dry layeran elevated dry layer. Driven . Driven by entrainment of mid-level dry by entrainment of mid-level dry air and precipitation loading. air and precipitation loading. Heavy precipitation at the Heavy precipitation at the surface.surface.

A rain foot A rain foot may be a visible sign may be a visible sign of a wet microburst. of a wet microburst.

SummarySummary

1.1. 3 stages of supercell tornado formation. 3 stages of supercell tornado formation.

2.2. Tornado outbreak (number>6) Tornado outbreak (number>6)

3.3. Tornado damage: Enhanced Fujita Scale (EF-0 65-85 Tornado damage: Enhanced Fujita Scale (EF-0 65-85 mph, EF-5 >200 mph)mph, EF-5 >200 mph)

4.4. Tornado occurrence: Global and U.S.. Which country Tornado occurrence: Global and U.S.. Which country has the largest number of tornadoes in the world? Which has the largest number of tornadoes in the world? Which state has the largest number of tornadoes per unit area state has the largest number of tornadoes per unit area in U.S.? Tornado season in U.S. (March-July)in U.S.? Tornado season in U.S. (March-July)

5.5. 2 types of mesoscale convective systems. Structure of 2 types of mesoscale convective systems. Structure of squall lines: four componentssquall lines: four components

6.6. 3 types of downbursts (derechos, haboobs, microbursts)3 types of downbursts (derechos, haboobs, microbursts)

7.7. Visual identification of dry microburst (virga in the sky, Visual identification of dry microburst (virga in the sky, blowing dust at the surface) blowing dust at the surface)

Works citedWorks cited

http://www.lakeeriewx.com/Meteo361/ResearchTopicFour/Synoptic.html

http://www.britannica.com/EBchecked/media/19397/http://www.britannica.com/EBchecked/media/19397/Tornadic-thunderstorm-The-rotating-updraft-that-Tornadic-thunderstorm-The-rotating-updraft-that-produces-the-tornado-extends produces-the-tornado-extends

http://www.geog.ucsb.edu/~joel/g266_s10/http://www.geog.ucsb.edu/~joel/g266_s10/lecture_notes/chapt08/oh10_8_3/oh10_08_3.html lecture_notes/chapt08/oh10_8_3/oh10_08_3.html

http://www.crh.noaa.gov/lmk/soo/docu/supercell.php http://www.crh.noaa.gov/lmk/soo/docu/supercell.php