Session 11.4 Gravity wave turbulence generated by convection over the Bahamas on 17 January 1996

Thursday, 7 October 2004: 5:15 PM
Lee E. Branscome, Climatological Consulting Corporation, Palm Beach Gardens, FL; and M. N. Campbell

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On 17 January 1996 an Airbus 300 aircraft encountered severe turbulence at about 34,000 feet over the Bahamas, causing injuries, while en route from Miami to San Juan. Several years of litigation related to this incident recently ended. The weather conditions along the route of flight and surrounding area, prior to, during and shortly after the severe turbulence encounter were examined through satellite data and imagery, lightning data, upper air charts and soundings, and pilot reports.

The weather in the Bahamas was affected by a short-wave trough traveling eastward in the middle and upper troposphere (from about 19,000 to 35,000 feet altitude). General cloudiness, with embedded widely scattered showers and thunderstorms, was generated by upward motion on the eastern side of the trough over the central Bahamas. An International SIGMET issued for the area described thunderstorms, embedded in cirrus, as intensifying and moving north at 5 to 10 knots.

Various aircraft reported turbulence in and around the Bahamas on the day of the incident. Satellite images show the generation and propagation of gravity waves in cirrus as developing convective cells encountered an inversion layer at about 31,000 to 35,000 feet. A smaller aircraft encountered severe turbulence at 31,000 feet as the likely result of such waves emanating from a convective cell in the Central Bahamas. In addition to the turbulence induced by convection, evidence for Kelvin-Helmholtz instability was present in soundings and satellite imagery.

Of particular interest to the Airbus incident is a coherent disturbance, seen in both visible and infrared imagery, that was generated in an area of strong convection and traveled northwestward away from its source at about 42 knots for 2 hours, when it intersected the route of flight at the time of the turbulence encounter. The Airbus crew reported only light returns on airborne radar and no visual clues of local convective turbulence.

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