The World of "Mesoscale" Convective Systems

This evening's radar composite was quite the sight.

Fig 1 -- NEXRAD composite base reflectivity from 0228Z, June 27, 2011.

There are three areas of storms moving across the northern plains and upper midwest.  The leading batch at this time was crossing the Iowa-Illinois border.  The next line was in western Iowa and southeastern Nebraska, and the furthest west group was in western Nebraska and South Dakota.  Quite the active night with line after line of storms marching through.

These are "special" types of storm systems that are called mesoscale convective complexes (or MCCs) by meteorologists.  Let me back up a minute and describe a bit more of the meteorological parlance going on here.

In meteorology we often refer to different types of weather phenomena in terms of their scale, be it spatial scale or temporal scale.  It helps to classify various weather processes and keep track of what is going on in different scopes.  Here's a quick list of some of the commonly-used scales (taken from a discussion in Stull 2000):

1. Global/Planetary Scale -- This describes phenomena that are on the order of tens of thousands of kilometers in length and usually days/months/years in time scale.  When we talk about the jet stream pattern across the planet or about the state of El Nino, these are things on the Global/Planetary scale.
2. Synoptic Scale -- This starts getting into the realm of things I usually discuss here.  The synoptic scale generally deals with phenomena on the order of 1,000-10,000 km across and is often what we focus on when looking on the scale of, say, the United States.  Features like troughs and ridges, surface lows and highs, and local jet streaks are generally considered to be on the synoptic scale.  Synoptic comes from the Greek meaning "with the eye", and originally this scale was used to describe the features on weather maps that we would easily describe by just eyeballing them--troughs, ridges, lows, highs and so on.  Sometimes I wonder with the amount of detail and layers upon layers we put on our weather maps now if "synoptic" is the best way of describing them.  But, the term persists...
3. Mesoscale -- This describes anything from the scale of 10-100 km scale ('meso' means 'middle'), and boy does it cover a lot.  Fronts are usually considered borderline between synoptic scale and mesoscale.  Think about it -- a front can be several hundred to a thousand kilometers long but only be a few dozen kilometers "across".  So, it sits right in the middle between the two scales.  On the larger side of the mesoscale, we have features like hurricanes and the types of complexes we saw in the radar image above.  But individual thunderstorms and supercell thunderstorms also fall into this scale.  Local phenomena like mountain winds or sea breezes are also considered to be in the mesoscale.
4. Microscale -- This is a somewhat looser term and generally just applies to anything less than a kilometer or so in size.  On the large end of this scale we have things like tornadoes--not quite large enough to be mesoscale phenomena (though because they come from thunderstorms that ARE large enough to be considered mesoscale, we often lump them in with the mesoscale). Turbulence is another phenomenon that is also considered to be in the microscale.

These are just terms that meteorologists use to keep track of what they're talking about.  For instance, the Storm Prediction Center (SPC) will often publish "Mesoscale Discussions" when they're talking about a region they're considering for a new watch.  The area they're talking about isn't the size of the entire country, so it's not in the synoptic scale.  It's definitely mesoscale-sized.  In the SPC's general Convective Outlooks, however, they often talk about the pattern of the jets aloft and the forecast positions of troughs and ridges in the coming days.  That's more of a synoptic-scale discussion.  Things at different scales all do interact with each other, but this just helps keep them apart.

Now let's get back to these storms.  Here is the infrared satellite image from about the same time as the radar image above.

Fig 2 -- GOES Infrared satellite image from 0245Z, June 27, 2011.

Notice that the area affected by these storms is on the scale of the size of a few states.  I still like keeping that image of the entire continental United States in my head when I think about the definition of synoptic scale.  Anything significantly smaller than that is into the mesoscale.  So, these storms are definitely in the mesoscale.  It's kind of difficult to see, but notice how that even though on radar the groups of storms formed elongated lines, the satellite image shows more circular blobs over the storms.  This shows a lot of high, thick clouds trailing the leading edge.  It also helps meteorologists to define these collections of storms in a special way. 

What do I mean by that?  It turns out that meteorologists have even more classifications for different types of phenomena.  Any time a collection of thunderstorms organizes itself on a scale larger than individual storms, it can be referred to as a mesoscale convective system (MCS).  We now know what mesoscale means, and convective just means we're referring to convective processes--meaning unstable air lifting (well, it's more complicated than that, but still...).  The term "mesoscale convective system" (or, often its acronym MCS) has become a very common term used to describe any collection of storms.  I guess it just sounds more technical to say "an MCS is moving in" as opposed to "a line of storms is moving in."  But the term MCS can describe any area of convection where multiple convective updrafts organize themselves on a scale where they work to reinforce each other.  So, individual supercell thunderstorms are NOT MCSs.

However, there are many different kinds of MCSs, including:

1. Squall Lines
2. Mesoscale Convective Complexes
3. Mesoscale Convective Vortices
4. Hurricanes
5. Lake-Effect Snow

So, I personally think it's a bit general to use the term "MCS" all the time--it can mean many things.  Particularly when you're trying to separate things like squall lines and mesoscale convective complexes.  In fact, even in the image above, one might argue that some of the groups of storms I was referring to as mesoscale convective complexes might simply be squall lines.  Specificity helps.  But, people still use the term MCS to describe just about any collection of storms.  And, technically they'd be correct.  But, not very specific.

So just what DOES separate a squall line from an MCC?  In my next few blog posts I'm going to go into some of these "technical" descriptions of these different mesoscale phenomena.  To tell you the truth, I've always been curious as to why MCCs are so special.  I was always told, "If the infrared satellite return from the storms makes a big circle, its an MCC.  If it makes a line, it's a squall line."  Why this odd distinction?  Stay tuned to my next few blog posts to find out.