Observational and Numerical Simulation-Derived Factors That Characterize Turbulence Accident Environments

Dual approaches are employed to determine the synoptic, meso-alpha, and meso-beta scale dynamical factors that organize an environment conducive to severe turbulence that affects commercial aircraft. The first approach involves performing a 44-case-study turbulence categorization analysis. 2.5 degree 6-hourly data sets of NCEP Reanalyses including wind, temperature, relative humidity and height on mandatory pressure surfaces along with high frequency GOES infra-red and visible satellite imagery are employed to perform diagnostic analyses of 44 case studies where turbulent conditions lead to accidents on commercial aircraft. The analyses indicated that a synoptic and meso-alpha scale environment predisposed to severe turbulence is favored by proximity to the following phenomena: 1) the curved right entrance region of a jet streak, 2) convection, 3) low relative vorticity, 4) upward vertical motion, 5) horizontal cold advection, 6) strong vertical wind speed and directional shear, and 7) shallow layers of low Richardson number. In particular, accidents are favored near convection and low relative vorticity.

The second approach involves developing meso-beta scale numerical simulations of the precursor environment that organizes severe turbulence. The Mesoscale Atmospheric Simulation System (MASS) numerical weather prediction model is utilized to perform 12 km nested-grid simulations during two turbulence events that led to accidents: one in proximity to deep convection and a second which is devoid of deep convection. Both accidents occur in a region of low relative vorticity within the curved right entrance region of a jet streak. In an effort to refine the meso-beta scale precursor conditions that result in turbulence, both of these simulations are evaluated for the wind, temperature, and moisture conditions that are conducive to inertial instability, conditional symmetric instability, and shearing instability. These instability signals indicate the potential for internal gravity wave generation mechanisms as a precursor to extreme turbulence.