Mesoscale convective systems Mesoscale convective systems and downburstsand downbursts

Review of last lectureReview of last lecture

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

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

2.2. 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)

3.3. 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? has the largest number of tornadoes in the world? Which state has the largest number of tornadoes per Which state has the largest number of tornadoes per unit area in U.S.? Tornado season in U.S. (March-unit area in U.S.? Tornado season in U.S. (March-July)July)

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..

1. Mesoscale Convective Complex1. Mesoscale Convective Complex

2. Squall line

2 types of Mesoscale Convective Systems 2 types of Mesoscale Convective Systems

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 Complex (MCC)I. Mesoscale Convective Complex (MCC)

Global distribution of MCCsGlobal distribution of MCCs

From Doswell et al. 2000

• 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)

Video: The June 2012 DerechoVideo: The June 2012 Derecho

http://www.youtube.com/watch?v=K61qGvKltL8

DerechosDerechos• A derecho (spanish word meaning “straight ahead”) is a widespread,

long-lived wind storm that is associated with a band of rapidly moving showers or thunderstorms (often a squall line).

• By definition, the wind damage swath extends more than 240 miles and wind speed > 57 mph. Can produce destruction similar to that of tornadoes. The damage typically is directed in one direction along a relatively straight swath.

From NOAA SPC Derecho website

Clouds associated with a derechoClouds associated with a derecho

From NOAA SPC Derecho website

Different types of derechosDifferent types of derechos Serial derechosSerial derechos Progressive derechosProgressive derechos Hybrid derechosHybrid derechos Low dewpoint derechosLow dewpoint derechos

Serial (drifting) Progressive (development)

Annual number of derecho eventsAnnual number of derecho events

Timing of derecho eventsTiming of derecho events

From NOAA SPC Derecho website

HaboobsHaboobs• Very strong horizontal winds over desert regions create sandstorms

called haboobs (from the Arabic word meaning “wind”). • Occur regularly in arid regions throughout the world.

Video: MicroburstVideo: Microburst

http://www.youtube.com/watch?http://www.youtube.com/watch?v=NT3YbuGHfC4 v=NT3YbuGHfC4

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

Virga in the skyVirga in the sky: defined as : defined as wisps or streaks of water or wisps or streaks of water or ice particles falling out of a ice particles falling out of a cloud but evaporating before cloud but evaporating before reaching the earth's surface reaching the earth's surface as precipitation. (NOAA 2001)as precipitation. (NOAA 2001)

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

Wet microburst: visual identificationWet microburst: visual identification

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

Causes of atmospheric turbulenceCauses of atmospheric turbulence

1.1. ThermalsThermals - Heat from the sun - Heat from the sun makes warm air masses rise and makes warm air masses rise and cold ones sink.cold ones sink.

2.2. Jet streamsJet streams - Fast, high-altitude - Fast, high-altitude air currents disturb the air nearby.air currents disturb the air nearby.

3.3. MountainsMountains - Air passes over - Air passes over mountains and causes wave and mountains and causes wave and turbulence on the other side.turbulence on the other side.

4.4. Wake turbulenceWake turbulence - Near the - Near the ground a passing plane or ground a passing plane or helicopter sets up small, chaotic air helicopter sets up small, chaotic air currents, orcurrents, orMicroburstsMicrobursts – dry, wet – dry, wet and hybrid.and hybrid.

SummarySummary

1.1. 2 types of mesoscale convective systems (mesoscale 2 types of mesoscale convective systems (mesoscale convective complex, squall line)convective complex, squall line)

2.2. Structure of squall lines: four componentsStructure of squall lines: four components

3.3. 3 types of downbursts (derechos, haboobs, 3 types of downbursts (derechos, haboobs, microbursts)microbursts)

4.4. Derechos. Definition (swath wider than 240 miles, wind Derechos. Definition (swath wider than 240 miles, wind speed>57 mph). 4 types (serial, progressive, hybrid, low speed>57 mph). 4 types (serial, progressive, hybrid, low dewpoint). Which state has the largest annual number of dewpoint). Which state has the largest annual number of derecho events? Most derechos happen in the three derecho events? Most derechos happen in the three months of May, June and Julymonths of May, June and July

5.5. Haboobs (dust storms). Haboobs (dust storms).

6.6. 3 types of microbursts (wet, dry, hybrid). 3 types of microbursts (wet, dry, hybrid).

7.7. 4 causes of atmospheric turbulence.4 causes of atmospheric turbulence.

Works citedWorks cited

http://www-frd.fsl.noaa.gov/mab/microburst/ http://www-frd.fsl.noaa.gov/mab/microburst/ http://www.cimms.ou.edu/~doswell/microbursts/http://www.cimms.ou.edu/~doswell/microbursts/

Handbook.html Handbook.html http://usatoday30.usatoday.com/weather/wcomform.htm http://usatoday30.usatoday.com/weather/wcomform.htm NOAA SPC Derecho websiteNOAA SPC Derecho website