Mesoscale convective system

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A shelf cloud such as this one can be a sign that a squall is imminent
A shelf cloud such as this one can be a sign that a squall is imminent

A mesoscale convective system (MCS) is a complex of thunderstorms which becomes organized on a scale larger than the individual thunderstorms, and normally persists for several hours or more. Mesoscale convective systems may be round or linear in shape, and include systems such as tropical cyclones, squall lines, lake-effect snow events, and Mesoscale Convective Complexes (MCCs), among others, and form near weather fronts. They have been noted across North America and Europe, with a maximum in activity during the late afternoon and evening hours during the warm season (i.e. late spring and summer) on both continents. Mesoscale convective systems over the Plains of the United States bring the region about half of their annual warm season rainfall.

Contents

[edit] Definition

Mesoscale convective systems may be round or linear in shape, and include systems such as tropical cyclones, squall lines, and Mesoscale Convective Complexes (MCCs), among others. MCS often is used to describe a cluster of thunderstorms that does not satisfy the size, shape, or duration criteria of an MCC. They tend to form near weather fronts and move into areas of 1000-500 mb thickness diffluence. Their formation has been noted worldwide, from the Mei-Yu front in the far East to the deep tropics.[1]

[edit] Mesoscale Convective Complex

Main article: Mesoscale Convective Complex

A mesoscale convective complex (MCC) is a unique kind of mesoscale convective system which is defined by characteristics observed in infrared satellite imagery. Their area of cold cloud tops exceeds 100,000km² with temperature less than or equal to -32 °C; and an area of cloud top of 50,000km² with temperature less than or equal to -52 °C. Size definitions must be met for six hours or greater. Its maximum extent is defined as when cloud shield reaches maximum area. Its eccentricity (minor axis/major axis) is greater than or equal to 0.7 at maximum extent. They are long-lived, nocturnal in formation and commonly contain heavy rainfall, wind, hail, lightning and possibly tornadoes.[2]

[edit] Squall line

Main article: Squall line

A squall line is a line of severe thunderstorms that can form along or ahead of a cold front. In the early 20th century, the term was used as a synonym for cold front. It contains heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes and waterspouts. Severe weather along squall lines can be expected if it displays a line echo wave pattern (LEWP) or if the line is in the shape of a bow echo.

[edit] Great Plains of the United States

Typical evolution of thunderstorms (a) into a bow echo (b, c) and into a comma echo (d). Dashed line indicates axis of greatest potential for downbursts. Arrows indicate wind flow relative to the storm. Area C is most prone to supporting tornado development.
Typical evolution of thunderstorms (a) into a bow echo (b, c) and into a comma echo (d). Dashed line indicates axis of greatest potential for downbursts. Arrows indicate wind flow relative to the storm. Area C is most prone to supporting tornado development.

The convective season for the Plains ranges between May and September. Mesoscale convective systems develop over the region during this time frame, with a bulk of the activity occurring between 6 and 9 p.m. local time. Mesoscale convective systems bring 30 to 70 percent of the annual warm season rainfall to the Plains.[3] A subset of these systems known as mesoscale convective complexes lead to up to 10% of the annual rainfall across the Plains and Midwest.[4] Squall lines account for 30% of the large thunderstorm complexes which move through the region.[5]

[edit] Europe

While most form over the continent, some MCSs form during the second half of August and September over the western Mediterranean Sea. MCS triggering over Europe is strongly tied to mountain ranges. On average, a European MCS moves east-northeast, forming near 3 p.m. local solar time, lasts 5.5 hours, dissipating near 9 p.m. LST. Around 20% of the MCSs over Europe do not form during maximum heating. Their average maximum extent is around 9000 km[2].[6]

[edit] See also

[edit] References

  1. ^ UCAR. PHYSICS OF MESOSCALE WEATHER SYSTEMS. Retrieved on 2008-03-01.
  2. ^ Maddox, R.A., 1980: Mesoscale convective complexes. Bulletin of the American Meteorological Society, Vol.61, 1374-1387.
  3. ^ William R. Cotton, Susan van den Heever, and Israel Jirak. Conceptual Models of Mesoscale Convective Systems: Part 9. Retrieved on 2008-03-23.
  4. ^ Walker S. Ashley, Thomas L. Mote, P. Grady Dixon, Sharon L. Trotter, Emily J. Powell, Joshua D. Durkee, and Andrew J. Grundstein. Distribution of Mesoscale Convective Complex Rainfall in the United States. Retrieved on 2008-03-02.
  5. ^ Brian A. Klimowski and Mark R. Hjelmfelt. Climatology and Structure of High Wind-Producing Mesoscale Convective Systems Over the Northern High Plains. Retrieved on 2008-03-01.
  6. ^ MOREL C. and SENESI S. A climatology of mesoscale convective systems over Europe using satellite infrared imagery. II: Characteristics of European mesoscale convective systems. Retrieved on 2008-03-02.

[edit] External links