Probability of Convectively Induced Turbulence Derived from High-Resolution Ensemble Simulations: Implications for Turbulence Avoidance Guidelines

Turbulence generated by convective clouds is among the most important contributors to aircraft turbulence encounters. These encounters can occur within convective regions, within lower-reflectivity (stratiform or cirrus) clouds, or in clear-air outside of cloud (i.e., near-cloud turbulence). The two latter regions are arguably the most hazardous to aircraft because the turbulence may not be detectable by onboard radar. Recent case studies have identified some of the mechanisms contributing to near-cloud turbulence, which includes gravity wave breaking and instabilities within upper-level storm outflows. Among other things, these studies have demonstrated that the current guidelines for turbulence avoidance near thunderstorms are inadequate and are inconsistent with the underlying storm dynamics, with turbulence encounters well outside the recommended lateral separation distance. The challenge is to use recent improvements in our understanding of the governing processes and ongoing research to improve these guidelines.

This study focuses on characterizing the turbulence hazard around mesoscale convective systems. Very high-resolution simulations of idealized squall lines are conducted; these simulations are three-dimensional and use 125-m grid spacing in all directions, and therefore resolve part of spectrum of turbulence that affects commercial aircraft. A variety of storm configurations are examined, including upright systems and systems with notable tilt, which are used to contrast systems with significant differences in their upper-outflow structure. Post-processing of the model simulations is used to estimate turbulence intensities and a series of ensemble simulations with perturbed initial conditions are also conducted to determine the probability of exceeding certain turbulence thresholds. These results are used to define the most hazardous regions in and around the storms, and will be discussed in the context of current observational data and the existing turbulence guidelines.