(Invited talk) Recent advances in the detection and understanding of turbulence associated with thunderstorms

Unexpected encounters with turbulence pose a safety risk to airline passengers and crew, can occasionally damage aircraft, and indirectly increase the cost of air travel. Deep convective clouds are one of the most important sources of turbulence. Convective-induced turbulence (CIT) can occur both within the cloud visible boundaries and in the clear air surrounding clouds, and accounts for about half of the moderate-or-greater turbulence encounters recorded by commercial aircraft in cruise.

In this talk, recent advances in the detection of turbulence within cloud and in the understanding of the dynamics of near-cloud turbulence will be reviewed. These advances are also due in part to the availability of more accurate in situ measurements of turbulence now routinely provided by some commercial aircraft, which allow unambiguous identification of storm-related turbulence events.

Turbulence within cloud can now be detected using the experimental Nexrad Turbulence Detection Algorithm (NTDA) which provides 3D maps of in-cloud turbulence intensity levels over the contiguous U. S., and this mosaic is updated every 15 min.

Turbulence outside of cloud, but ultimately caused by the presence of convective cloud is also an important hazard. While this near-cloud turbulence (NCT) is probably weaker than turbulence within convective cores, it is arguably more dangerous because it is invisible and undetectable by standard on-board or ground-based radar. Theories surrounding the origins of NCT are at best incomplete, relying on empirical evidence and pilot experience to determine the most hazardous regions. However, high resolution simulation case studies of the storm environment have identified some important dynamical processes responsible for near-cloud turbulence, both above the cloud and laterally away from the cloud. These processes include atmospheric wave breaking, unstable upper-level thunderstorm outflows, shearing instabilities, and cirrus cloud bands.

These new understandings of near-cloud turbulence underscore the need for revising the current operational guidelines for turbulence avoidance associated with thunderstorms. For example we have found the vertical wind shear near cloud top to be an extremely important parameter in controlling the presence and location of turbulence near cloud, however this parameter is not mentioned in the current operational FAA avoidance guidelines.

This research is in response to requirements and funding by the Federal Aviation Administration (FAA). The views expressed are those of the authors and do not necessarily represent the official policy or position of the FAA.