Tuesday, July 5, 2011

The Mesoscale Convective Vortex (MCV)

For my last post in this series about different kinds of mesoscale convective systems (MCSs), I'm going to briefly talk about one of the rarer types of MCSs--the mesoscale convective vortex (MCV).  These sorts of storms usually form from parent mesoscale convective complexes (MCCs), so we have a full line of mesoscale convective classification:

MCS --> MCC --> MCV

In trying to trace the history of MCVs in the meteorological literature, it seems that the MCV was recognized very soon after Maddox's 1980 paper describing the features of MCCs. It was noted that in some cases, strong mesoscale low-pressure centers would develop in the wake of MCCs.  Johnston (1981) talks about mesoscale vorticity maxima induced by mesoscale convective compexes. Such maxima were theorized to have helped organize the storm that caused the 1977 Johnstown, Pennsylvania, floods by  Zhang and Fritsch (1987).  However, the first actual mention I could find of the term "mesoscale convective vortex" being used to describe this phenomenon came from a paper by Menard and Fritsch in 1989, where they describe a "mesoscale, convectively-generated vortex (MCV)" over Oklahoma and Arkansas.  So you can see that our understanding of this particular phenomenon is pretty young, having only been recognized over the past 20-30 years of reseach.

So what is a mesoscale convective vortex?  In my discussion of MCCs, I talked about how the large area of rising air in the circular MCC had lots of condensation going on.  That condensation of water vapor into rain releases a lot of latent heat, warming the core of the storm.  That warming causes the air to expand, reducing the pressure relative to the surrounding environment.  This, in effect, creates a "warm-core low" with a structure analogous to the structure of a hurricane.  Sometimes, after convection and rainfall associated with an MCC dissipates, this area of low-pressure can actually linger on.  The cyclonic flow associated with the low-pressure center can later help organize new areas of rising motion and convection, often with a charactaristic cyclonic "swirl".

Here's an example of a visible satellite image of this organization going on from the CIMSS satellite page:
GOES visible satellite image from 1615Z, Jul 8, 1997 showing a MCV over western Missouri.  From http://cimss.ssec.wisc.edu/goes/misc/970708.html.
Sometimes you'll see MCVs described as a "mesoscale vorticity center".  This is an apt description, and it means essentially the same thing--it just changes the acronym to MVC. You can see on that example image how the MCC has organized itself into a charactaristic swirl-shape.  The analogies between MCVs and tropical cyclones really start to make sense when you see this kind of organization.

Here's a radar image from Patrick Marsh's blog last year when an MCV was observed moving through the Houston area:
NEXRAD base reflectivity composite of an MCV near Houston at 2220Z on June 3, 2010.  From http://www.patricktmarsh.com/2010/06/day-154-mesoscale-convective-vortex-near-houston/.  Annotated by Patrick Marsh.
Patrick has annotated that radar image with streamlines showing the wind patterns and an L marking the center of this mesoscale vortex.

You can see in these two images why these features are considered to be "mesoscale".  Unlike the banded "swirls" or comma-shapes we see with strong synoptic-scale low-pressure centers, these bands do not mark the locations of frontal boundaries.  They are much more akin to hurricane rain bands than frontal bands.  Furthermore, the size of these vortexes is only at most a few hundred kilometers across--often less.  You can see above that these storms are about the size of a state or less.  This still puts them in the "mesoscale" category--smaller than the synoptic scale.

It turns out that, though mesoscale convective complexes are relatively common across the central US, only a very few actually give way to the eventual development of a mesoscale convective vortexBartels and Maddox (1991) did a survey over seven and a half years of visible satellite data and only observed 24 cases of MCVs, working out to around three or four per year.  So, they're a fun feature to see when you can actually find them.

That's just a quick look at MCVs, a rarer, but still important type of mesoscale convective system.  In my next blog, I think I'll finally move away from talking about MCSs and get back to talking about the weather that's currently going on.

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