Meet the mesoscale convective complex
A color-enhanced infrared image of a mesoscale convective complex at 4a.m. Central time June 5, 1994. The bright red area indicates high, cold cirrus clouds over the top of the individual thunderstorms in the system. |
By May the large-scale storms that sweep across North America from fall to early spring have faded away, making the weather generally pleasant across the flat countryside from the Rockies to the Appalachians.
In the spring most weather concern turns to thunderstorms, which certainly can be fierce, but even the largest isn't likely to be more than 15 miles across. Compared with a winter storm that can spread snow across the Midwest and sweep the South with heavy rain at the same time, thunderstorms are tiny.
Most weather training for pilots focuses on either small-scale weather, such as thunderstorms and localized fog, or big weather systems, such as fronts that can stretch hundreds of miles.
Middle-sized weather systems also make life difficult for pilots, especially from May into August from the Rockies to the Appalachians. Meteorologists use the term mesoscale for middle-size weather systems, which are from a few miles wide to maybe 300 miles across.
Larger systems, such as extratropical storms and their fronts, are synoptic-scale events. The weather maps that include all of the United States, or maybe a large region such as the Southeast, are synoptic-scale maps. Smaller weather systems -- individual thunderstorms, for example -- are microscale, but you don't hear that term too often.
While most weather training doesn't include the word mesoscale, it usually includes at least a mention of one kind of mesoscale system: squall lines of thunderstorms. A squall line is composed of individual thunderstorms in a line that can stretch 300 miles or longer. The line stays organized for hours as the individual storms in it form, grow, and die. Squall-line thunderstorms can bring heavy rain, damaging wind, hail, and sometimes tornadoes.
If you are planning to fly across the Great Plains at night from May through July, maybe into August, you should be alert to the possibility of another kind of middle-size weather system known as a mesoscale convective complex (MCC). The mesoscale part of the name tells us that it's a phenomenon that can be up to a few hundred miles long or across. Convective refers to the up-and-down air motions that are needed to create any kind of thunderstorm. Meteorologists often use "convective" as a synonym for "thunderstorm." Complex is part of the name because an MCC is more than a bunch of thunderstorms that happen to form close to each other. Instead, the individual storms interact among each other and with the surrounding atmosphere in ways that create a long-lasting system.
While MCCs surely have been sweeping across the Plains since the end of the last Ice Age, meteorologists began to realize only in the 1970s that these complexes represent a unique kind of weather system. Infrared satellite images, which indicate the temperatures of cloud tops, were the first clue that MCCs are something more than a bunch of thunderstorms that happen to be near each other.
Of course, meteorologists and Great Plains residents knew that some nights brought thunderstorms with more lightning than most storms, and that on some of these nights especially rainy storms hit. Today, the factors that meteorologists use to say that an MCC has formed include an area of cloud-top temperatures lower than minus 25 degrees Fahrenheit that covers around 39,000 square miles, an area a little smaller than the state of Ohio. The area also has to be pretty much round, not a line of thunderstorms -- a squall line.
Since researchers first identified and began studying mesoscale convective complexes, they've learned that these systems, on the average, account for about 80 percent of the needed summer rain in the farming region between the Rockies and the Appalachians.
In addition to being organized and often unusually wet or windy, the thunderstorms in MCCs differ from most other thunderstorms in an important way: They reach their peak well after dark, maybe around midnight, not in the late afternoon as most other thunderstorms do.
A strong temperature contrast between temperatures at low levels, such as at the ground, and temperatures aloft create the very unstable atmosphere that thunderstorms need. Late in the afternoon on a sunny day, hot air from near the ground rises into the chilly upper atmosphere to create towering cumulonimbus clouds -- thunderstorms. These often continue producing lightning, rain, hail and winds after sunset, but they run out of steam as the ground cools off after dark.
Formation of a low-level jet stream, which brings in warm, humid air from over the Gulf of Mexico, is a key to a batch of ordinary bunch of afternoon thunderstorms becoming a mesoscale convective complex.
A low-level jet is a fast-moving stream of air maybe 1,000 to around 2,500 feet above the ground, which begins blowing when the ground south of the thunderstorms cools after sunset. As this happens, an inversion -- air aloft that's warmer than the air at ground level -- forms because the air aloft doesn't cool as fast as air next to the ground. The inversion shuts down the up-and-down air motions that make flights on sunny days bumpy at low altitudes. Meteorologists say that an inversion decouples the upper-altitude winds from the surface. Without slow-moving air rising from below and some of the fast-moving air aloft sinking, the air aloft is no longer "connected" to slow-moving air near the ground.
The warm, humid air that the low-level jet is feeding into the bottoms of the thunderstorms helps keep that level of the atmosphere relatively warm. Also, as the humid air rises into the thunderstorms, its humidity condenses, releasing heat. Meanwhile, the clouds at the top of the MCC are losing heat into the darkness above the Earth, cooling the cloud tops and helping maintain the strong temperature contrast between the system's top and bottom.
The final step in forming an MCC is the low-pressure center that forms about 18,000 feet above sea level. Heat being released as water vapor condenses into cloud drops warms the air at this level. As the air is warmed it becomes less dense than the air at the same altitude outside the system. Air pressure inside this area of warm, less-dense air is lower than the pressure of the air outside the system at the same altitude. In other words, it's a low-pressure center. As higher pressure air around the "low" begins moving toward the low-pressure area, the Earth's rotation creates a counter-clockwise swirl, just like the swirl around any Northern Hemisphere area of low-pressure.
In recent years meteorologists have started calling this area of low pressure a mesoscale vorticity center, and you might see this term if you read forecasters' discussions on a day when an MCC is around. (Vorticity refers to a spinning motion in a fluid, such as the atmosphere.)
In an MCC, when rain begins falling it drags down colder air from aloft, which spreads out as a gust front when it reaches the ground. Gust fronts push into warm, humid air near the ground, giving it the upward shove needed to trigger formation of a new thunderstorm and helping to keep an MCC going late into the night.
A full-blown MCC is a meteorological mix of layers of stable and unstable air and boundaries between cool and warm air that look like miniature cold and warm fronts. This gives MCCs areas of steady rain typical of areas ahead of warm fronts, as well as the heavier but on-and-off rain of thunderstorms. Frequent lightning often marks MCCs. They contain all of the hazards of thunderstorms, icing, hail, dangerous turbulence, and sometimes tornadoes. The worst of all of this goes on at night with nothing but flashes of lightning to illuminate the sky for any pilot who's flown into an MCC. Usually this system begins to fizzle some time before sunrise; the rain and lightning ease up and thunderstorms fade away.
But, even when this happens, winds continue circling around the area of low air pressure around 18,000 feet up. This upper-atmosphere low drifts generally toward the east with the general flow of the upper atmosphere. The next day, when conditions are ripe for thunderstorms, the upper-level low -- which is often about all that's left of the MCC -- can trigger new thunderstorms, bringing the MCC back to life for another night.
An MCC can bring several days of storms as it makes its way across the Plains. Fortunately, each night's storms will be to the east of those of the previous night. MCCs sometimes cross the Appalachians to bring overnight rain and wind to the East.
The thunderstorms and low visibility in an MCC aren't the only potential hazards to pilots. The low-level jets that feed MCCs can catch pilots by surprise as they climb into a clear night far to the south of the MCC. (See "The Weather Never Sleeps: Shear Energy," May 2004 AOPA Flight Training, for more on low-level jets.)
As a practical matter, you can avoid flying into the middle of an MCC by heeding the warnings and alerts about thunderstorms or possible thunderstorms you hear during a preflight weather briefing.
Knowing at least a little about MCCs will ensure that you don't become dangerously optimistic that thunderstorms which are blocking a planned flight early in the evening will soon go away. They could last almost all night.
If nothing else, understanding what's going on when a sky full of overnight thunderstorms is keeping you on the ground will keep you from feeling like a victim of the weather gods.
Jack Williams is coordinator of public outreach for the American Meteorological Society. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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