Characterizing the severe turbulence environments associated with commercial aviation accidents. Part 1: A 44-case study synoptic observational analyses

In this paper, we describe the results of 44 case study analyses of synoptic scale data sets that define the atmospheric structure prior to the development of accident-producing turbulence. First, the 44 case studies are categorized as a function of the location, altitude, time of year, time of day, and turbulence environment, i.e., in clear air, cloudiness, convection, near mountains, or in the proximity of deep convection. It is noteworthy that this later category was much more ubiquitous than was anticipated. Second, NCEP Reanalysis data sets as well as both visible and infrared satellite imagery are employed to diagnose predictor fields associated with the synoptic-scale environment preceding severe turbulence. These predictor fields are calculated based on jet stream configuration, kinematic, dynamical, and thermodynamic analyses of the synoptic-scale atmosphere. The results of these analyses indicate a prevalence of severe accident-producing turbulence within the entrance region of the polar or subtropical jet stream at the synoptic-scale. Typically, there is a region of flow curvature located just upstream within the jet entrance region, convection is present within 100 km of the accident, the vertical motion is upward typically within the curved entrance region, absolute vorticity is low, the vertical wind shear is increasing with time, and horizontal cold air advection is substantial. Not all of the 44-case studies conform to this entrance region paradigm. However, most do and the most consistent predictor of severe turbulence is upstream curvature in the synoptic-scale flow. Nearby convection is the second most ubiquitous predictor field. Upward vertical motion, low absolute vorticity, and horizontal cold air advection are all typical predictors in case studies occurring both within the entrance and exit regions of the polar or subtropical jet stream.