Recent Advances in the Understanding of Near-Cloud Turbulence

Abstract

Anyone who has flown in a commercial aircraft is familiar with turbulence. 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. Cloud-induced turbulence can occur both within clouds and in the surrounding clear air. Turbulence associated with but outside of clouds is of particular concern because it is more difficult to discern using standard hazard identification technologies (e.g., satellite and radar) and thus is often the source of unexpected turbulence encounters. Although operational guidelines for avoiding near-cloud turbulence exist, they are in many ways inadequate because they were developed before the governing dynamical processes were understood. Recently, there have been significant advances in the understanding of the dynamics of near-cloud turbulence. Using examples, this article demonstrates how these advances have stemmed from improved turbulence observing and reporting systems, the establishment of archives of turbulence encounters, detailed case studies, and high-resolution numerical simulations. Some of the important phenomena that have recently been identified as contributing to near-cloud turbulence include atmospheric wave breaking, unstable upper-level thunderstorm outflows, shearing instabilities, and cirrus cloud bands. The consequences of these phenomena for developing new en route turbulence avoidance guidelines and forecasting methods are discussed, along with outstanding research questions.

Aviation Turbulence: Dynamics, Forecasting, and Response to Climate Change

“…The smaller the jet width, the larger the wind shear, and therefore, the closer to the cloud top the gravity wave breaking would be. Lane et al (2012) also showed turbulence can be found in cirrus bands (banding caused by thermal instabilities), and also ducted gravity waves that propagate far from the updraft, reducing the Richardson number and causing turbulence much farther away than the 20 km outlined in the guidelines. The new guidelines from FAA (2017a) no longer reference the acceptable height above the thunderstorm pilots can fly.…”

“…Lane et al (2012) explore our current understanding of near-cloud turbulence or Convectively Induced Turbulence (CIT). They explain that the FAA regulations at the time were not sufficient for avoiding severe turbulence.…”

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“…In order to derive the amplitude, a per unit mass turbulent kinetic energy (TKE) is computed in [28] as…”

Convective Instability Underneath Midlevel Clouds: Comparisons between Numerical Simulations and VHF Radar Observations

“…Near-cloud turbulence occurs outside of strong convective systems through gravity waves and their breaking, enhancement of vertical shear, radiative effects, and a combination of these mechanisms (Lane and Sharman 2008;Trier et al 2010;Lane et al 2012;Kim et al 2014). Inertia-gravity waves produced by highly unbalanced flows can be a source of turbulence through the local modification of the environmental Richardson number to be less than a critical value (Knox 1997;McCann 2001;Koch et al 2005).…”