When convective cells organize, it can mean trouble for pilots.
By Karsten Shein
Comm-Inst Climate Scientist
Neither pilot had ever experienced conditions this bad. Entering the wall of clouds, they knew they faced several active storm cells, but there had seemed to be enough room to navigate in the clear of any of the darkest echoes. Besides, deviating around this line of storms would have taken close to 200 nm and made the sales team late for their meeting.
Rethinking their decision as they corrected course for what seemed the dozenth time to clear a magenta return on their radar scope, the air ahead of them suddenly lit up in red on the scope. Their pathway was blocked. Turning another 70 degrees, the pilts saw a narrow path on the scope to what they though should be clear air behind this squall line.
They would have to punch through the clouds ahead in the promise that their radar was showing only some light precipitation returns ahead. But heading through that last hot tower cumulus, the aircraft was slammed by a strong updraft – a sign the convective cell was growing to maturity. As quickly as it hit, the aircraft popped out of the clouds and into clear air.
With the squall line behind them, they headed back on course. On approach, they found they couldn’t keep the aircraft from vibrating. A post-landing walk around revealed that the updraft they’d flown through had overstressed the airframe and knocked a gear door out of alignment. As soon as they’d put the wheels down, a hinge on the door had fractured, causing the vibration.
The aircraft remained grounded for 3 days while the repairs were made, and the sales team returned home on a commercial flight. From our first flight at the controls, we are normally taught to keep clear of thunderstorms. Hundreds of aviators over the decades have perished as a result of poor decisions or inadvertent encounters that took them too close to an active thunderstorm.
Frequently, thunderstorms are easily seen and avoided. However, a greater problem emerges when the sky seems filled with active convection and pilots think that there is a reasonable way through to the other side. In many such scenarios, the atmospheric conditions are simply conducive to occasional to frequent storms punching through a weak temperature inversion above the surface, and developing into towering cells that emerge from an otherwise benign cumulus deck at 3000 or 4000 ft.
Such storms may occur in close proximity to one another, and flight between 2 active cells is never recommended, especially if there’s a separation shorter than 40 nm between the cells (20 nm minimum clearance per storm). Although a broad landscape of pop up storms may appear daunting, it is possible to safely navigate such a minefield as long as one can remain in clear air.
Most of the times, storms that occur under these conditions remain simple airmass storms that mature and decay in under an hour and tend not to drop hail, produce damaging turbulence, or become a supercell.
Other times, however, clusters of convective cells appear to take on properties that suggest they are being organized by broader atmospheric conditions. Squall lines and mesoscale convective complexes are 2 such phenomena, although their dynamic mechanisms are related.
To a lesser extent, any warm, cold, stationary, or occluded front can provide the lifting necessary to kick off convection, but one of the most frequent types of organized convection that pilots face are squall lines associated with cold fronts.
The wedge of colder, denser air that undercuts warmer, humid air has great propensity to fire up the convective engine all along the frontal boundary. However, most cold fronts don’t spawn an endless line of solid convection along their entire length.
Rather, the forced lifting that comes from the intrusion of the denser air at the surface helps displace the warmer, energy-rich air upward.
The thing that has kept that air conditionally stable is normally a lower level temperature inversion (or cap) that simultaneously allowed the surface layer air to accumulate energy and prevented that air from rising into the free atmosphere above.
The first storms to fire up along a front generally appear in places where solar radiation and a darker, moisture-rich surface has locally enhanced the heating.
Frequently, the cap will be weak enough for the heated air to push through without any additional help from a front. These are the conditions that lead to widespread areas of isolated to small clusters of storms. However, if the cap is strong enough, perhaps more than about 4° C, even the hottest surface air may not be able to punch through it before the sun sets.
But, add in the lifting created by an advancing cold front, and suddenly the heated air is forced up through the cap to the colder free atmosphere, where it can rise explosively. In a frontal situation, this may occur at dozens of places along the frontal boundary, and, owing to the vertical profile of the front, it also happens a few kilometers behind the frontal boundary at the surface.
The key ingredients in the development of these frontal storms are the difference in air density on either side of the front and the latent energy available in the warm surface air. The speed of the front doesn’t really have too much to do with the frequency and strength of the storms that form as the air rising past the cap moves much faster.
Organized squall lines
There is another – perhaps more important – mechanism at work to transform isolated storms into an organized squall line. This is the creation of a vertical circulation and small (mesoscale) highs along the front. Along the front, denser air from aloft sinks in replacement for the warmer air rising from the surface.
This is enhanced by thunderstorm outflow that may extend in any direction once it leaves the bases of the storms, but often, given the parallel flow of air ahead of the frontal boundary, will preferentially outflow ahead and to the left of the thunderstorm from which it separated.
The gust front from the storm creates a convergence area that makes possible lifting both along and ahead of the surface front.
In generating convergence along the front, the storm is initiating convection in the space adjacent to it. Newly formed updrafts quickly fill the free space between the storms with towering cumuli.
As the storms mature, they create a more and more solid wall of convection and precipitation. The mesohighs building behind the storms also can push them out ahead of the front by several km.
Outflow pushing ahead of the frontal squall line, coupled with periodic accelerations of the front itself create a disturbance wave that propagates far, often several hundred km, into the warm air ahead of the front.
This prefrontal wave tends to travel a few thousand feet off the ground and works to disrupt the capping inversion that is keeping warm sector convection from initiating.
Meanwhile, the cooler, denser surface outflow from the frontal squall line may be helping to provide lifting to the warmer surface air beneath the wave. The combination creates prefrontal squall lines that are frequently more severe, wider, and more solid than the squall line along the cold front itself.
Squall lines extend for hundreds of kilometers
The characteristics of squall lines, whether they be frontal or prefrontal, are that they tend to extend for several hundred km. In some cases, even thousands, stretching across entire continents.
They can be as narrow as a few km, but in general that is just the leading edge, or squall line front, containing the strongest storms and radar echoes. Normally, post-frontal weaker and more sporadic convection occurs up to 30 km behind the squall front.
Because of the strong mesohighs behind the front, coupled with the deep but highly-localized meso-lows of the squall line itself, storms within the squall line can frequently develop mesocyclones and become supercells. In addition, the continued advancement of the cold front ensures that, even if ordinary airmass storms mature and decay, new cells will fire up to continue the squall line as it moves across the landscape.
This development of extreme storms into mesocyclonic storm cells is enhanced because, while the surface flow of air ahead of the front is parallel to the front itself, winds above the front tend to flow perpendicular to it.
This dichotomy creates a windshear situation that twists updrafts around downdrafts to form a rotating cell, and which tilts the upper regions of the storm downstream of the storm bases near the front.
Furthermore, because the general flow of warm sector surface air in the lower levels moves parallel to the front, squall lines along or ahead of the front tend to move both forward with the front, and in line with the front toward the central low at one end of it. This results in areas getting hit with one storm after the next until the front eventually moves past.
These “training” storms (as in in-line, like a train) are most dangerous to an airport when the front is moving slowly, allowing several storms to transit across the runways in a matter less than an hour or so. Some squall lines develop a particularly strong mesohigh behind them due to the descent of denser air from aloft.
In turn, this downdraft pushes the middle of the squall line outward, into what is known as a bow echo due to its arced appearance on radar. Storms near the center of the echo tend to be strong to severe, often producing derechos. A derecho occurs when the outflow from an organized line of storms, usually a bow echo, produces an outflow beyond the ordinary individual gust fronts.
Instead, the derecho is an organized straight-line wind storm in which sustained winds reach at least 93 kph (58 mph) along a line at least 460 km (290 miles) in length. Derechos are most common to the United States, but also occur in other places, such as the Indian subcontinent.
Mesoscale convective complexes
Not all organized convection happens in the presence of a front. Mesoscale convective complexes (MCCs) organize from clusters of isolated airmass thunderstorms, primarily during warm months. The downdraft outflow from these storms creates a high pressure at the surface, along with a mesoscale cold front.
At the middle levels, the system is held together by a rotating mesoscale low pressure. In the upper levels, near the top of the troposphere, the diverging air produces a rotating high-pressure area.
Surface convergence and upper-level divergence of air basically create a giant circulation cell that sustains the individual storm cells within the MCC. MCCs are large. To be an MCC, the system’s cloud shield, as seen from satellite, must either be at least 100,000 sq km in area with a cloud top temperature below -32° C, or tops to 50,000 ft with temperatures below -52° C, for at least 6 hours.
Most MCCs form in the midafternoon and often are strongest by late afternoon, continuing their destructive paths well through the night. A well developed MCC can last over 100 hours and travel well over 1000 km in that time.
Flying through an MCC is a very bad idea
MCCs tend to be a mostly solid area of storms, with little way through. Given their extent, even if a path seemed available on radar, the shear size of the MCC ensures that your airborne radar can’t see all the way through it. You may get 10 or 15 km in before you realize there’s no way out and the storms extend at least 50 to 100 km in any direction. MCC’s are most commonly encountered in the southern plains of the US in early summer, migrating northward as the summer progresses.
They have also been recorded in southern Canada, India, Pakistan and Bangladesh during monsoon season, Australia, Africa, and South America. A larger version of an MCC is a tropical cyclone. Tropical cyclones form in a similar manner. Normally, an easterly wave in the tropical jet stream will support and organize a cluster of airmass thunderstorms over warm and humid ocean waters.
Unlike an MCC, however, tropical cyclones require little to no windshear aloft, and they form a distinct circulation rotating around a central eye.
Flying with organized convection
The good news is that organized convective activity, especially synoptic-based events such as frontal squall lines are usually forecast with reasonable accuracy several days ahead of time.
Even MCCs and tropical cyclones are not too difficult to forecast, although their tracks tend to be much more difficult to accurately predict more than a day or so in advance. Although it is possible to transect a squall line if there are wide enough breaks, and it is often tempting to do so if the line extends for hundreds of miles in either direction, pilots should be absolutely certain that they can make it all the way through without encountering an active cell.
The general rule of thumb for staying clear of individual thunderstorms applies even if they are organized. Remain at least 20 mi from any thunderstorm and at least 1 mi for every 1000 ft of storm height for any storm with tops above 20,000 ft MSL. So, ideally, pilots should not try to fly through a gap between 2 storms that is less than 40 mi wide.
Unfortunately, many pilots attempt to fly through far smaller gaps between squall line storms. Some encounter no trouble, while others find themselves in the middle of a fully mature storm cell that, seconds before, had simply been a towering cumulus with light radar returns.
In general, if you are flying fast and you have a clear air path (no clouds) through to the other side, there’s a pretty good chance you’ll be fine, even if your gap width is only 10 or 20 miles.
However, it is still a pretty big risk. The reason is that, even in the clear air between storms, a squall line or other organized convective system is loaded with strong wind shear and turbulence. That space between 2 storms may contain not only outflow from one or both storms, but wicked horizontal shear coming off of the front itself.
Strong to severe turbulence encounters are quite likely anywhere within 20 mi or so of an active squall line or MCC. The best course of action when faced with a large area of organized convective activity is to ascertain if there is a way around it that doesn’t put you, your passengers and your aircraft in danger. Depending on your position relative to the line, a quick check of ground-based radar, or even a call to Flightwatch, will give you an idea about where the edge of the convection is.
Because the storms tend to track toward the low that’s driving the front, heading around the trailing edge of the line is more likely to keep you clear of storms by the time your deviation gets you to the line. Of course, if a deviation will put you into the red zone for your reserves, you can estimate the system’s speed by looking at sequential radar loops and locate a suitable airport you can divert to to wait for the system to pass.
If you approach the system and are able to land within perhaps 20–40 mi of it, it is likely to mean only a few hours on the ground before you can continue your flight in the clean and clear air behind the front. As always, if you do encounter weather that’s not quite what you expected, be sure to file a pirep.
Karsten Shein is cofounder and science director at ExplorEiS. He was formerly an assistant professor at Shippensburg University and a climatologist with NOAA. Shein holds a commercial license with instrument rating.