WX BRIEF

Thunderstorms

From isolated cells to squall lines, thunderstorms remain a consistent threat to pilots flying in unstable airmasses.

By Karsten Shein
Comm-Inst, Climate Scientist


Anatomy of a supercell thunderstorm. These massive storms often top out above FL500 and have a characteristic anvil top. Severe flight conditions, including turbulence, lighting and hail can be present anywhere within 20 nm or more of the cloud itself.

In the soup at 6000 ft, the Cessna 310 was getting knocked around by some moderate turbulence. The high-time pilot had flown in the military and for a commercial airline before landing a job flying globe-hopping business jets for a fractional operator. It was a dream job and he normally enjoyed the relaxed airborne commute to and from his rural fly-in community to his employer's home airport about 80 miles away.

But on this occasion, bad weather made things a bit dicey, and on days when the weather was really bad, the pilot would usually drive into the city to make his flight time. Today however, heavy rain had resulted in a wreck on the main highway, snarling traffic for miles. Checking the weather on his computer, the radar image showed isolated storms across the region, and the satellite picture confirmed they were embedded in a broken cloud deck.

But the storms left ample room between them and his aircraft was equipped with a GPS nav system that provided weather radar imagery. Figuring he could pick his way around the worst of the storms, the pilot filed an IFR flight plan and launched into the murky sky.

What happened after he picked up his clearance is uncertain. ATC cleared him to 6000 ft direct to the airport's IAF and he noted several storms near to his route. The pilot's last transmission to ATC was to acknowledge the clearance and that he saw the storms on his nav screen.

About 30 miles south of his home, witnesses stuck in traffic on the highway reported seeing several lightning flashes in the clouds above them, and then seeing parts of the aircraft falling from the base of the clouds. The debris continued to fall over the next several minutes. Investigators later determined the pilot had likely flown into a rapidly developing storm cell that had not appeared on the aircraft's GPS screen, which only updated the radar image every 5 minutes.

Before losing contact, ATC tracking showed the Cessna 310 climbing quickly to 7300 ft and then falling to 5200 ft before disappearing from radar. The forces within the cell had caused structural failure and the ultimate disintegration of the aircraft.

Many of us routinely fly aircraft that are capable of cruising over all but the tallest cumulonimbus clouds, and which are equipped with the latest airborne radar and weather information systems. Yet every year, experienced professional pilots are killed when they fly their aircraft into or near thunderstorms.

Many of these incidents occur when pilots either apply their weather avoidance experience from more advanced aircraft to smaller aircraft that may not be as well equipped, or they choose to accept the risk of weather danger in favor of making good time to their destination. Unfortunately, when it comes to thunderstorms, the risks are very high and too many pilots continue to find themselves on the wrong side of the weather.

Convective cells

The 3 stages of the life cycle of a single-cell airmass thunderstorm are the cumulus, mature and dissipating stages. Transitions between each stage can occur in seconds, providing little warning to pilots approaching the area.

Thunderstorms are the product of a rapid repositioning of energy in the atmosphere. This energy becomes concentrated in the surface layer air by solar heating of the ground coupled with a temperature inversion aloft that prevents the immediate exchange of heated surface air with the cooler air of the free atmosphere aloft. The storm cell kicks off when there is either enough energy to break through the temperature inversion "cap," or something comes along to push the surface air aloft.

The most vicious storms tend to occur when the cap is moderately strong. Too weak of an inversion (less than around 2° C) will prevent energy buildup to storm supporting quantities, while too strong of an inversion (more than around 4° C) means that the surface layer may never accumulate enough energy to break through. Bear in mind though that some of the strongest storms build energy under a moderate to strong cap that then rapidly weakens later in the day.

Of course, a cap and energy near the surface are not alone enough to produce a thunderstorm. There also must be favorable conditions in the free atmosphere above. The troposphere is heated from below, and so becomes cooler with increasing distance from the surface. Rising air also cools as it ascends, but for a different reason – expansion. Rising air is subjected to lower and lower pressure as it rises, allowing it to expand.

Since pressure and temperature are directly related, the expanding air also cools down. The rate at which it cools depends on whether it is saturated or unsaturated. Unsaturated air cools rapidly, but the release of heat by condensing water vapor in saturated air slows the rate of cooling.

For heated air to rise convectively through the troposphere, it must cool at a rate less than that of the surrounding environment. Otherwise it will quickly equalize temperature and cease rising. The difference between the temperature of the atmosphere and the temperature of a rising parcel of heated air helps us determine its ability to form a continual updraft of heated air higher into the troposphere.

Integrating through all altitudes in which this is the case results in a value, called the Convective Available Potential Energy (CAPE), that gives an idea of the strength of the convection that may occur. CAPE values over 1500 Joules/kg are considered large enough to support sustained updrafts and strong convection. Values above 2500 are extreme and support rapid uplift and storms capable of producing hail and tornadoes. Cap and CAPE values are usually provided on atmospheric sounding charts.

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