A case study of mid-level turbulence outside regions of active convection
Historically, the principal focus of research on clear-air turbulence of concern to aircraft has been on jet stream and mountain (orographic) induced turbulence. Relatively little research has focused on the turbulence hazard outside of, but in the vicinity of, convective storms, known as Convective Induced Turbulence (CIN). In this paper, we present our analysis requested by the National Transportation Safety Board (NTSB) of the meteorological conditions leading to severe turbulence and near loss of flight control of a commercial passenger jet and find that they fall into the CIN category. On 12 May 1997, at approximately 1929 UT, an American Airlines Airbus A300 en route from Boston, MA to Miami, FL encountered severe turbulence off the coast of West Palm Beach, FL. Near the time of the incident the crew had been directed to hold at 16,000 ft because of weather and traffic near Miami International. While approaching the holding position, the aircraft experienced severe turbulence and dropped over 3000 vertical feet in 30 seconds. A detailed postevent analysis by the NTSB failed to find any causal evidence for the turbulence and no single sensor, data set, or pilot report examined by the NTSB provided justification for the magnitude of the event. Our independent analysis of the incident was conducted primarily using recorded Miami WSR-88D base data. The analysis revealed a small-scale vertical shear zone may have emanated from a thunderstorm upstream of the Airbus. Animated cross-sectional images also suggested that a rotor may have propagated with the mean wind and intersected the flight path at the time the severe turbulence was reported. This paper will focus on meteorological conditions that led to the upset and provide evidence for several possible causes of the turbulence.