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Kirk Mellish's Weather Commentary

Posted: 5:47 a.m. Thursday, June 4, 2009

Tropical Weather and crash of Air France flight 447 

By Kirk Mellish

Satellite imagery, balloon soundings, and global lightning networks reveal clues about the weather that may have affected the Air France Airbus A330 last Sunday night. The plane flew into thunderstorms, and the updrafts or turbulence associated with those storms, in addition to lightning and maybe icing may have played a role. This is not unprecedented. Look up Pulkovo aviation flight 612 in August of 2006 and Northwest flight 705 in February 1963.

The projected flight path of flight 447 took the aircraft near Sao Luis, Brazil, where it may have first encountered thunderstorms. Later in the flight, the plane seems to have flown into a large cluster thunderstorms northeast of Fernando De Noronha, off Brazil's northern coast, and along the Intertropical Convergence Zone (ITCZ), the belt of low pressure that surrounds the Earth at the equator.

Based on upper-air data near Fernando De Noronha, the updrafts associated with the thunderstorms could easily have reached 50-100 mph, rain cooled air from vertical mixing of dry  air could have lead to even stronger downdrafts. Such an updraft would lead to severe turbulence for any aircraft. In addition, the storms were towering up to at least 56,000 feet (some analysis suggests overshooting tops of over 65,000 feet--penetrating the Troposphere) and would have been producing lightning. The Air France plane would have encountered these stormy conditions, which could have resulted in either some structural failure or electrical failure as noted in the automated computer communications between the airplane and Air France headquarters.

Based on satellite images, the Air France flight had little chance of going around the storms because they stretched for over 400 miles and were along the flight path. The airplane was flying at cruising altitude of 35,000 feet. With the updrafts pushing the storms up to 56,000 feet or higher, the plane had to fly through the storms (without turning back) because it could not fly over or around them, they would have used on-board weather radar to navigate through, but this type radar would not show wind shear just rain. Commercial jetliners routinely go through or near thunderstorms at flight level but avoid them at take-off and landing.

Despite the presence of the storms Sunday night, the only lightning detected along the flight path was near Sao Luis, Brazil.  Excessive lightning strikes were not being detected with the storms northeast of Fernando De Noronha and along the ITCZ at the time. However, the ITCZ is known for highly electrified storm clouds, and the nearest detectors may simply have been too distant for detection.

Tropical thunderstorms and the lightning patterns generated by them are different from storms that typically occur over the United States. Studies have shown that the top region of a tropical thunderstorm is highly charged and more conducive to lightning, which indicates that an airplane flying near the top of a tropical thunderstorm could be more susceptible to a lightning strike. Tropical thunderstorms are also notorious for producing frequent cloud-to-cloud, as well as cloud-to-air, lightning. The thermodynamics of the region at the time suggest minimal threats from large hail or icing, although some ice could be encountered. Severe turbulence pockets of heavy rain and lightning are more greatly supported by the thermal profile estimates. Flight track data indicate the aircraft would have flown through stormy conditions for some 12 minutes and 75 miles before contact was lost.

The ITCZ is one of Earth's largest and most influential weather systems. It is a year-round phenomenon that actually helps to drive the world's "weather engine." The ITCZ surrounds the Earth's tropics over both land and water, although it tends to be best developed over water. It is, as the name implies, a belt of converging winds known as the trade winds.

Driven by sub-tropical high pressure in both hemispheres, the trade winds tend to converge and pile up steamy, water-laden air that is squeezed aloft into showers and towering tropical thunderstorms. Vast clusters of such tropical rain have the power to alter the paths of storms in the middle latitudes as they release immense energy into the atmosphere. These ITCZ thunderstorms also have the potential to spin up to form tropical storms and hurricanes.

The Northern Hemisphere has more landmass than the Southern Hemisphere, and because of this, it heats more in the Southern Hemisphere during its summer. The ITCZ reaches farthest from the equator during the Northern Hemisphere summer. The average latitude of the ITCZ is 5 to 10 degrees north of the equator.

On Sunday night, Air France Flight 447 intersected the ITCZ north of the equator over the open Atlantic Ocean. At the time, there were thunderstorms within its belt of converging winds, which is a common occurrence. It's worth noting that flying through the ITCZ is not something out of the ordinary, as countless passenger flights have navigated this area routinely for many years, and no doubt countless tropical oceanic flights have encountered similar thunderstorm complexes without such a tragic incident. So it may have been a cascade of factors, including the weather, that lead to the crash.
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