Lecture #15: 
Mesoscale Convective Systems and Squall Lines
Friday, 23 February 2001

Text Reading for Lecture #15
Severe Thunderstorms (385-395)

MESOSCALE SYSTEMS and SQUALL LINES

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- Highly organized, long-lived lines of convective
   storms
- The most intense region of convection tends to
  be rather narrow, and the line can extend for
  hundreds of miles

- Typically form along or ahead of cold fronts or
   drylines

 

- Gravity waves generated by a front
  can trigger squall lines tens of miles ahead of
  the front, as shown below.  Such lines are called
  pre-frontal squall lines (page 390 on text).

- Typically contain numerous individual cells, but
  can contain a few supercell storms

- Rarely produce tornadoes, except at the southern
   end, where mutual interference of cells is
   minimized. 
- Often exhibit a region of trailing stratiform
  precipitation that can extend backward, behind
  the main convective area, for 100+ miles
- Leading edge exhibits strong gradients in radar
  reflectivity (precipitation rate)

- Long-lived squall lines require sufficient CAPE as
  well as specific wind profiles

Isolated cloud in non-sheared environment (airmass storm)

Isolated cloud with gust front

Conditions for long-lived squall line