Prediction and Warning of Transported Convective Turbulence in Long-Haul Aircraft Operations

The occurrence of unexpected severe turbulence, whose origin is believed to be convective systems well upstream from commercial jet aircraft, has been reported in a number of instances, resulting in injuries to flight crews or passengers. One incident involving a Hawaiian Airlines A330 jet near Vanuatu in the South Pacific in 2012 initiated a research effort to determine if such events can be anticipated, and the flight crews warned of impending turbulence during long duration flights, so that preventative measures can be taken. Only cirrus clouds were present in the Vanuatu case, but a convective storm had formed and dissipated upstream almost six hours earlier.

An analysis of the National Transportation Safety Board (NTSB) database of inflight turbulence accidents was conducted to estimate how frequently such events, referred to here as “transported turbulence” have been observed. Out of 180 weather-related accidents/incidents reported to the NTSB over a 20 year period (1995-2014), 92 (51%) were due to convection. Of those, approximately 5% were possibly due to transported turbulence encountered in clear air, anywhere from 1 to 5 hours following the initiation of strong convection upstream from the aircraft. These results are consistent with a larger study of inflight near-cloud turbulence (NCT) based on Eddy Dissipation Rate (EDR) data from commercial aircraft. Analysis of likely turbulence causes for the NTSB cases was determined from a combination of geostationary infrared (IR) satellite imagery, ground-based radar (when available), flight crew narratives, and the NTSB conclusions.

The development of a prototype Transported Turbulence Product (TTP) for use by air crews was based on the assumption that  trackable remnants of the cirrus anvil generated by strong convective systems could produce moderate to severe turbulence hours later, after being advected (transported) downstream, .  The process by which this re-invigorated turbulence occurs is uncertain, but there are a variety of mechanisms that can regenerate turbulence as the cirrus remnants are transported downstream.

Transported Turbulence source regions are identified when and where satellite IR temperatures indicate cloud tops reaching jet cruising altitudes with frequent in-cloud lightning observed by ground-based time-of-arrival systems. The potential turbulence regions are advected using the NOAA Air Resources Laboratory’s (ARL) Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model with Global Forecast System (GFS) model wind data. Segments of flight tracks intersecting these trajectories are identified and displayed in a format which allows immediate import into Common Operating Environment (COE) and Geographic Information Systems (GIS) now common in Electronic Flight Bag (EFB) equipment available on many commercial jets.

Several real-time examples were produced during 8-10 December 2015 for Hawaiian Airlines flights over the South Pacific enroute to and from Hawaii, Australia, and New Zealand. Several flights were alerted for potential turbulence using the TTP and real-time flight information on a Google Earth COE. Pilot reports (PIREPs) indicated the presence of turbulence in the predicted TTP areas in otherwise smooth conditions. The TTP has potential applications in both general and commercial aviation, scientific field operations, and even space operations. Future enhancements will assign probability of moderate or greater turbulence, determine false alarm rates, and automate TTP in collaboration with future commercial partners.