Tuesday, June 28, 2011

What is a mesoscale convective complex?

As I mentioned in my last post, I want to go into some detail about the definition and structure of several classes of storms.  We use the term mesoscale convective system to describe any collection of storms (or other convective activity) that is organized on a scale larger than a single storm.  Today I want to focus on one particular type of mesoscale convective system--the mesoscale convective complex.

Much of this discussion is going to follow Robert Maddox's 1980 paper "Mesoscale Convective Complexes," which is the first paper to define and describe these types of storms.  You can read the full paper here (which is actually very understandable if you can follow the kinds of things I write in my blog).

In Maddox (1980), he describes a breakdown of different types of large-end mesoscale convective phenomena using this convenient chart which follows nicely from my last blog post:
Table -- Describing a classification of meso-alpha (large mesoscale) convective phenomena, broken down by geometric shape and location.  From Maddox (1980).
The main point from this chart is that these mesoscale convective complexes are "circular" in structure and they can occur in both the tropics and the midlatitudes.  Maddox proposed a definition that still stands today (at least it still matches the American Meteorological Society's definition of a mesoscale convective complex) though there have been proposed amendments over the years:
Table -- A definition of a mesoscale convective complex from Maddox (1980).
MCCs are mostly determined by the shape and size of their infrared satellite signature.  This has always struck me as somewhat arbitrary--what's so significant about the cloud returns being more than 100,000 sq. kilometers in area?  Why does the structure have to have a near-circular shape (eccentricity greater than 0.7)?  It turns out that these parameters themselves are indeed arbitrary--they exist as a way of describing MCCs in the context of the observations that we have and in as succinct of a way as possible. 

From Maddox's paper and several subsequent descriptions, there are several key points that describe MCCs uniquely and separate them from other features such as squall lines:

  1. MCCs do not require significant large-scale forcing to form.  Whereas squall lines often form along fronts and are associated with deep troughing and strong upper-air support,  MCCs can form in areas where there is relatively weak support for convection--only mild lifting motion and no significant support aloft.
  2. MCCs require a very warm and moist layer that extends from the surface relatively deeply up into the atmosphere.  Because MCCs typically form in areas with weak upper-level support (as mentioned above), they need a lot of latent heat energy to sustain themselves.  Basically, with structures like squall lines along a front, you have an ambient, synoptic-scale environment that works to force and maintain thunderstorms.  With MCCs, it's more like you have storms that work to change and modify the synoptic-scale environment.
  3. Because of the strong dependence on latent heat release to maintain the structure of a MCC, they tend to share many charactaristics in common with a warm-core system as opposed to a cold-core system. Typically we think of hurricanes as the strongest warm-core-type storms.  However, MCCs also exhibit this kind of structure.
Let's look at a more direct comparisons between MCCs and squall lines.  Maddox provides some examples from 1979 of both in an attempt to illustrate the difference in structure.
Figures taken from Maddox (1980).  The left side illustrates a MCC on July 12, 1979 and the right side illustrates a squall line on June 20, 1979.  The top panels show infrared satellite images, the middle panels show radar returns and the lower panels show the surface analysis.
The left-hand side of the above image shows an MCC whereas the right-hand side shows a squall line event.  In the satellite images (the top panels), the shape difference is clear--the MCC is very round in shape and covers a large, continuous area.  The squall line, in contrast, has a much more linear shape and lots of variation in the infrared satellite returns--it's not just one big blob.

In the middle panels showing the radar returns, we can see that in the squall line, the bulk of the radar return (and therefore most of the precipitation) is confined to the immediate area of the squall line (ignoring the other precipitation off to the northwest associated with the surface low).  In the MCC, however, there's a rather large area of radar returns underneath the big circular infrared satellite feature.  Furthrmore, there are embedded areas of deeper convection in the MCC--some of which do look "linear".  But, the satellite return and other radar return areas are very large and aren't just confined to that deep convective area.  This sets the MCC apart from the squall line.

Finally, in the lower panels, for the squall line case on the right we can see that the squall line is out ahead of a deep surface low to the west.  The only way you would get a deep surface low like that is to have significant upper-level support (i.e., a strong jet streak with its exit region above).  We can see a lot of pressure contours, and they're fairly close together, indicating strong pressure gradients.  The squall line itself has moved out ahead of the main cold front, but the fact that its structure so well mirrors the main cold front hints that the original storms probably formed along the cold front before strong winds aloft started moving them out ahead of the front.  This is a classic scenario that I've talked about in this blog multiple times.

However, in the MCC case, we notice a very different synoptic setup.  There's no strong surface low nearby at all.  The pressure contours are rather few and far between, indicating weak pressure gradients.  There is a cold front analyzed off to the northwest, but it doesn't appear to be very strong.  Furthermore, the shape of the convective area doesn't really match well with the shape of the cold front.  This implies that while the front may have forced initial storm development, it was not the primary mechanism for organization in these storms.  You can see that in the MCC surface analysis, they've drawn outflow boundaries on the southern and eastern edge of the complex, with a mesoscale area of high pressure underneath the middle of the complex.  This contrasts greatly with the squall line case, and gets back to the difference I mentioned earlier--

Squall lines--their forcing and maintenance--are a product of the surrounding synoptic environment whereas MCCs tend to force and change the surrounding synoptic environment.

In the surface analyses above, the MCC analysis is dominated by features (like the outflow boundaries and meso-high pressures) created by the MCC itself.  In the squall line case, the dominant features (the surface low, the strong fronts and pressure gradients) were NOT created by the squall line.  They worked to create the squall line.  This is a fundamental difference.

Let's explore some of the other features of MCCs by looking at some potential MCC candidates from the other day.  Remember this radar image from Sunday night?
NEXRAD composite base reflectivity from 0228Z, June 27, 2011.
Now--do we call these features squall lines or MCCs?  On radar the deep convection does look rather linear, but we saw even in Maddox's example MCC that we could see linear-like convective elements within an MCC.  There's also extended areas of precipitation to the north and the west of these convective lines in the radar image.  However, the technical definition revolves around the infrared satellite image, so I'll bring that back too.
GOES Infrared satellite image from 0245Z, June 27, 2011.
Immediately we can see that the shapes of these storm complexes on the satellite image is more rounded--there's no linear feature or "comma shape" like we'd expect if there was a deep surface low or a squall line. I haven't measured the area of the satellite returns explicitly, but knowing that the state of Iowa has an area of about 147,000 square kilometers seems to imply that these are large enough to qualify as MCCs (the exact definition(s) of area needed is in the first table above).  So just from these satellite images, I'd call these MCCs.

But lets look at some analyses of the surrounding environment.  Let's go back to 18Z (around the middle of the day) to when these storms were just forming.  Here was the radar image then:
NEXRAD composite radar reflectivity from 18Z, June 26, 2011.
Scattered storms were beginning to develop throughout the Dakotas and into northern Nebraska.  Other storms were beginning to fire in southeastern Nebraska.  Now, let's look at the 300mb chart for this time:
SPC mesoanalysis 300mb winds (barbs and blue colors), divergence (pink) and height (black) for 18Z, June 26, 2011.
Is there a deep trough moving in?  Not really.  Are we in the exit region of a strong jet streak?  Kind of--you can see the blue shadings highlighting the core of the jet winds off to the west.  There's some divergence aloft (shown by the pink contours) but this doesn't seem to be tied to the jet streak.  This divergence is right above the area where storms were developing on radar.  This makes sense--the rising motion in the convection implies that air above has to get out of the way, so we expect divergence aloft above the storms.  But the fact that this divergence is so localized to just the areas where the storms are seems to indicate that this is a storm-induced phenomenon--the divergence is not significantly coming from the overall synoptic pattern.  This agrees nicely with what I said about MCCs before--they modify the surrounding environment more than it forces them.

Continuing to the time when the MCCs had matured, here's the new 300mb map.
SPC mesoanalysis 300mb winds (barbs and blue colors), divergence (pink) and height (black) for 2Z, June 27, 2011.
We can see the divergence aloft (once again, the pink contours) associated with the MCCs in eastern and western Iowa--they're nowhere near any significant jet at 300mb.  There are some wiggles in the height contours, but no troughs or anything supporting these storm clusters.  Once again, the storms are modifying the environment. 

Also in this 300mb image, I find it interesting how the jet aloft has changed surrounding the storms that were developing in central South Dakota at this time.  Notice how the jet streak seems to curve up and around the top of the strong area of divergence associated with those storms in central South Dakota.  If you follow that curvature, you'd see that the jet streak is starting to become anticyclonically curved--it curves clockwise.  Interestingly, one of the features of a warm-core storm is that it promotes an area of anti-cyclonic flow (and "higher" pressure) aloft.  Is this anticyclonic curvature of the jet evidence of a warm-core structure in the storms in central South Dakota?  Remember, in my list of features of an MCC, I said that these storms often exhibited warm-core charactaristics.  So it could be...

Finally, some more interesting facts about MCCs:
  1. Most MCCs develop in the afternoon hours, then strengthen through the evening and perisist overnight until the next morning.  Thus they provide a lot of nocturnal rainfall.
  2. In fact, MCCs are thought to be one of the largest contributors to annual rainfall for much of the central US (several studies have looked at this, including Fritsch (1986), McAnelly and Cotton (1986, 1989) and Kane et al. (1987)).  Fritsch (1986) claims that most states in the plains and midwest receive 30-70% of their annual rainfall from MCC events.
  3. Because of their heavy rainfall contribution (remember--MCCs have precipitation over a much larger area than squall lines and usually occur when there is a very deep moist layer), MCCs are responsible for many flash flood events across the central US.
  4. Severe weather is definitely possible within a MCC.  Severe wind and hail are frequently found with the stronger convective elements.  If those elements can organize along one of the MCC's outflow boundaries, the wind threat can get even greater.  Tornadoes are also possible from stronger convective elements within the MCC.  Here's the SPC storm reports for that group of MCCs on Sunday night:
SPC storm reports for June 26, 2011.
And with that I'm going to wrap up my discussion of what makes a mesoscale convective complex.  It's a peculiar type of storm that is very frequent in the late spring/early summer months.  While superficially it can look like a squall line, it's a unique type of organized storm that:
  • Doesn't need strong forcing aloft
  • Actually works to change the synoptic pattern aloft instead of being driven by the synoptic pattern
  • Usually forms with a very deep layer of moisture in the low-levels
  • Has warm-core charactaristics
  • Brings a lot of rainfall, usually at night, to the central part of the US
  • Has a very recognizable circular-type structure on infrared satellite images.
Now you'll know what to look for the next time that the term "MCC" comes up.

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