WX BRIEF

Mesoscale convective systems

Recognizing and understanding these midsized but dangerous storm systems.

By Karsten Shein
Comm-Inst, Climate Scientist


A squall line leaves behind a wet tarmac at BLV (Scott AFB, Belleville IL) in Jun 2006. Midsized storm systems such as squall lines are known as MCSs and pose significant dangers to aviation.

On the afternoon of May 8, 2009, residents of southern Illinois experienced what many could only refer to as an “inland hurricane.” This odd weather system moved in with little warning and, at its height, was delivering a deluge of rain, high winds and tornados.

It even appeared to have a distinctive “eye” when viewed on local radar screens. At MDH (Carb­on­dale IL), the ASOS anemo­meter measured a sustained wind of 68 mph with gusts to 81 mph before failing.

A nearby rooftop anemometer registered a gust of 106 mph. There was significant damage to structures as roofs were peeled away and trees toppled. Despite the substantial similarities to a tropical cyclone, however, this storm was not a hurricane.

Hurricanes do not form over the middle latitudes, nor will they retain their eye for very long over land. But, like a tropical cyclone, this storm was a mesoscale convective system (MCS), and the accompanying wind storm is known as a derecho.

Mesoscale convective systems

Hurricanes and midlatitude cyc­lones are fairly close relatives. Both are the result of a low-pressure disturbance moving through the atmosphere. Convergence of air at the surface coupled with the divergence of air aloft pumps heat and moisture aloft, generating convective activity around the central low.

But while there is generally more heat energy available in the tropics from which a hurricane can draw, their midlatitude cousins are commonly far more powerful than even the strongest Category 5 hurricane.

Fortunately, due to their size—often covering half a continent—they tend to be far less intense than even a weak tropical storm, as the power a midlatitude cyclone contains is distributed over a large area.

But, at times, localized conditions converge to produce much smaller and more intense storm systems that can reap the same level of destruction as a compact hurricane. These storm systems are what are known as MCSs.

The term “mesoscale convective system” describes just about any organized convective storm system larger than an individual thunderstorm but not big enough to be classified as a synoptic system.

MCSs include tropical cyclones, squall lines and mesoscale convective complexes. At its most fundamental level, an MCS is simply a cluster of thunderstorms held to­gether by some central circulation pattern.

Like thunderstorms, these systems may develop where warm, humid air has been trapped beneath a stable low-level layer of air. Event­ually, the buoyancy of the surface air is able to overcome the ability of the stable air to restrain it and it rises convectively to higher altitudes. Individual thunderstorms can be touched off in several ways.

Idealized growth of a thunderstorm radar echo (a) into a bow echo (b,c) and finally into a comma echo with a bookend vortex (d). Arrows show the air flow. The dashed line is the location of the greatest downburst threat.

The surface energy can simply exceed the air’s ability to contain it, or a disturbance, such as a transitory low pressure, a front, or even an atmospheric wave propagating several hundred kilometers ahead of a fast moving cold front, can set convective cells in motion.

Initiation of the thunderstorms over a wider region generally requires the presence of a larger-scale disturbance such as a weak low pressure or an atmospheric wave. Within this region of favorable convective conditions, several dozen individual thunderstorm cells may exist simultaneously.

As one decays, its outflow can provide the lifting force to initiate the development of a new cell next door. In this way, the system of storms can sustain itself for many hours. In addition, the prevailing flow of the atmosphere will dictate the char­acter and movement of the MCS.

Strong winds aloft, all moving in rough­ly the same direction, will help to create an MCS that is fairly oblong in shape—the system itself will move slightly to the left of the prevailing currents. The outflow from each storm is directed more toward the left of the storm than to the right, presenting a preferential location for the outflow to act as the initiation mechanism for a subsequent storm.

Such behavior is normally found in squall lines and MCSs associated with fronts. If the winds aloft are moving from a significantly different direction than the winds at the middle levels and the surface, it’s likely that the MCS will develop more of a circular shape on the radar and satellite.

This situation often results in the development of a low pressure center within the MCS and a resultant spiraling of the storm cells around that central low. In rare instances, the low can become strong enough to draw in a return flow to the center, resulting in a rain-free “eye”—although, unlike a hurricane, the eye will likely not be cloud-free or visible on a satellite image.

A mesoscale convective complex (MCC) is a special kind of MCS. MCCs tend to form overnight as afternoon thunderstorms are subjected to the strong windshear out ahead of an upper-level trough in the jet stream, and organize into a roughly circular area of cumuliform cloud coverage.

The MCS itself must have at least 100,000 km2 of cloud top temperatures below –32°C, and 50,000 km2 below –52°C for at least 6 hours in order to be classified as an MCC. On satellite images, MCSs are relatively easy to identify.

As the individual storm cells hit the base of the tropopause, the updrafts can no longer continue to rise, and the tops of the clouds spread out to form the thunderstorm’s anvil-shaped top.

Because there are a number of cells in close proximity to one another, the anvils will merge together, and the MCS will appear on a satellite image as a solid area of relatively uniform cloud cover, interspersed with shadows from bulges in the cloud top where the strongest up­drafts have managed to push the clouds a bit further into the tropopause.

Different types of MCS will appear differently on satellite images. A squall line MCS will appear as a long, thin line, while a tropical cyclone will be more circular with cloud bands radiating from its rotational center.

An MCC will also appear roughly circular, but without any spiral banding as the inflow and rotation will generally be much weaker. Individual storm cells within an MCS can be strong, and may in­clude supercells, though regular airmass-style storms are more common within the system. Weak to moderate tornadoes have also been observed in conjunction with stronger mesoscale convective systems.

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