Interesting Weather Information

Sunday, March 17, 2013

Thunderstorm Primer - Part 2 - Linear Mesoscale Convective Systems - Squall Lines

During the first half of the 20th Century lines of thunderstorms were generally called squall lines and the thunderstorms in the line were classified as frontal thunderstorms.  In some instances the term squall line was synonymous with the term cold front. But in those years the modern concepts of cold and warm fronts were still not universally recognized.  The first organization in the U.S. to analyze fronts on weather maps was the United States Navy and that was not until the 1930s.

The situation as we understand it today  is much more complicated. So we now speak of single-cell thunderstorms (formerly called air mass thunderstorms), which can grow into mesoscale convective systems as storms grow and merge. Or the linear mesoscale convective system can develop in a relatively straight or gently curved line (squall line), a warped line (bow line or LEWP) or a round to elliptical cluster of storms (mesoscale convective complex).

Scales of Atmospheric Motion

This is a topic with too many details for this short post but I have created a webpage (primarily for my meteorology students at The College of Mount St. Joseph) covering scales of atmospheric motion and here is  the link:

More Information on the Sales of Atmospheric Motion - CLICK

What is important for understanding this discussion is the mesoscale is smaller than synoptic scale and larger than both storm scale and micro scale. Synoptic scale is the scale of a national weather map with fronts, highs and lows. Storm scale is the size of individual cumulonimbus and microscale can be the size of the dust devil on a baseball field or leaves whirling around the corner of a building on a blustery autumn day.

So when we talk about a mesoscale convective system we mean a middle sized system, made of multiple thunderstorms that share a source of lift (in addition to differential surface heating most of the time). The storms may or may not work cooperatively and may or may not compete for resources. Most of the time the storms compete for low-level moist air  which is high-octane fuel for convective systems. Often new storms form and old ones dissipate as the mesoscale convective system propagates, moves and evolves.

Liner Mesoscale Convective Systems - Squall Lines

The video shows a squall line developing, ahead of a cold front on June 14, 2012.

The individual cells share a source of lift, in this case the front and surface convergence ahead of the front along with the heating of the day. Warm moist air to the south and southeast is pulled into the thunderstorms, after all they are low pressure systems at the storm scale, and when the vapor condenses tremendous amounts of heat are released to the environment and drive the storm beyond what the initial energy sources would.

In summary, the sources of energy for this system are:
1. differential surface heating
2. surface convergence
3. frontal lift
4. condensation of water vapor in the updraft.

Below are two additional examples of squall lines or as we call them now linear mesoscale convective systems.

Linear MCS June 6, 2008
In this exc
Linear MCS just to the west of Cincinnati May 25, 2011.

Tornado Watch #370 2011  a PDS Watch (Particularlly Dangerous Situation)  along tje Linear MCS on May 25, 2011. Note the date on the image says 2011 0526/0134 UTC which is 8:34 PM CDT May 25.

Linear MCS Systems rarely spawn large rotating supercells leading to large tornadoes. Most of the time given sufficient instability when a tornado is spawned by a Linear MCS like this it is a leading edge vortex and usually rated as EF0 to EF2 (occasionally EF3) on the Enhanced Fujita Scale. This is discussed in a later post.

The Linear MCS of May 25, 2011 caused 5 small tornadoes in southern Indiana, two were rated as EF2 and three rated as EF1.

The situation gets more complicated when Linear MCSs interact with their environments in complex ways and give birth to bow line echoes or  line echo wave patterns (or LEWPs - pronounced loops). Both are squall lines, but squall lines with complications. The bow line is covered in the next post and the LEWP follows.

I end this post as I did the previous one. What's in a name? The term squall line evokes a more ominous  tactile experience  than the term Linear MCS. The term squall line can refer to any line of thunderstorms. A Linear MCS on the other hand is a category that includes squall lines, bow echoes and  line echo wave patterns (LEWPs).

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