10 July 1997 -- Severe Turbulence Injures Passengers and Forces Jet to Make an Unscheduled Landing

GOES-8 IR image

20:45 UT GOES-8 IR image

GOES-8 IR image

23:45 UT GOES-8 IR image

An American Airlines passenger jet encountered severe turbulence while flying from Seattle WA to New York City on 10 July, injuring 22 of the passengers and forcing the Boeing 757 jet to make an unscheduled landing in Denver CO. According to the pilot report, the event occurred 35 miles southeast of Dickinson ND (airport identifier DIK), around 21:52 UT at an altitude of 37,000 feet.

Atmospheric turbulence at typical jetliner cruising altitudes can result from convective activity or from a variety of non-convective phenomena (jet stream circulations, deformations zones, orographic waves, gravity waves, Kelvin-Helmholtz instability). The GOES-8 infrared imagery above reveals that a cluster of thunderstorms was rapidly developing across southwestern North Dakota around the time of the incident. The image 1 hour prior to the event (left) shows disorganized convection building over far eastern Montana and the Black Hills region of Wyoming and South Dakota, while the image 2 hours after the event (right) shows a well-developed convective cluster with cloud top temperatures colder than -60 C over the Dickinson ND region (see magnified IR image). The rawinsonde report from Bismarck ND (just east of the convection) indicates that these -60 C cloud tops would have penetrated above the level of the aircraft (the temperature at the 37,000 foot level was about -52 C). A visible image at this time revealed cloud shadows from overshooting tops, indicative of vigorous thunderstorm updrafts.

Turbulence due to jet stream circulations was not likely a factor in this event -- wind fields from the ETA model indicate that the core of the strongest jet stream winds (45 ms-1 or about 90 kts) was over far western Montana into southern Alberta, and the flow over the Dickinson ND region was southwesterly at only 15-20 ms-1 (30-40 kts).

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