Mesoscale convective systems
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Mesoscale convective systems. Review of last lecture. 3 stages of supercell tornado formation. Tornado outbreak (number>6) Tornado damage: Enhanced Fujita Scale (EF-0 65-85 mph, EF-5 >200 mph)

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Review of last lecture

  • 3 stages of supercell tornado formation.

  • Tornado outbreak (number>6)

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

  • 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)


Convective systems
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.


2 types of Mesoscale Convective Systems

1. MesoscaleConvective Complex

2. Squall line


Video mesoscale convective complex
Video: Mesoscale convective complex

https://www.youtube.com/watch?v=3y7mxqNXYpc


Mesoscale Convective Systems

I. Mesoscale Convective Complex (MCC)

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.



Global distribution of mccs
Global distribution of MCCs

From Doswell et al. 2000


Mesoscale convective systems ii squall line
Mesoscale Convective SystemsII. 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
Vertical structure of squall lines

Convective updrafts (controlled by lower troposphere temperature and moisture)

Mesoscale updrafts

Mesoscale downdrafts

Convective downdrafts

Zipser (1977), modified by Houze (1993)


Development of a squall line
Development of a squall line

Thunderstorm stage

Bow echo stage

Comma or line echo stage


Developement of a squall line horizontal view
Developement of a squall line:Horizontal view

1. Thunderstorm

2. Bow echo

3. Comma echo


Video the june 2012 derecho
Video: The June 2012 Derecho

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


Derechos
Derechos

  • 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 derecho
Clouds associated with a derecho

From NOAA SPC Derecho website


The line storm by john steuart curry 1897 1946
"The Line Storm," by John Steuart Curry, 1897-1946


Different types of derechos
Different types of derechos

Serial derechos

Progressive derechos

Hybrid derechos

Low dewpoint derechos


Serial derecho
Serial derecho

From NOAA SPC Derecho website

typically is associated with an extensive squall line or quasi-linear convective system with multiple embedded bow echoes.

usually occur in conjunction with well-defined, migratory low pressure system.

Overall convective system (and, therefore, derecho) motion is dominated by advection --- the downstream movement of individual thunderstorm cells by the environmental wind.


Progressive derecho
Progressive derecho

June 2012 derecho over the Midwest and Ohio Valley

From NOAA SPC Derecho website

typically is associated with a squall line of restricted length that may involve a single bow echo or multiple bows.

usually are associated with a stagnant large scale weather pattern.

Overall convective system (and, therefore, derecho) motion is dominated by thunderstorm propagation --- that is, the development of new storm cells downstream from existing ones.



Timing of derecho events
Timing of derecho events

From NOAA SPC Derecho website


Summary

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

  • Structure of MCCs

  • Structure of squall lines: four components

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


Works cited
Works cited

http://usatoday30.usatoday.com/weather/wcomform.htm


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