Revisiting Clear Air Turbulence and Synoptic Weather Systems

Clear air turbulence (CAT) forecast was once relying on the recognition of synoptic weather systems in upper-air weather charts. Various weather systems, including jet streams, upper-level fronts, synoptic troughs and ridges, and tropopause folds are linked to the occurrence of CAT through analyzing pilot reports. Furthermore, moist processes like the convection embedded in extratropical cyclones are suggested to have caused CAT in recent research. The weather system perspective not only helps the development of useful forecast tools but also the process understanding of CAT. However, while automated turbulence reporting using eddy dissipation rate (EDR) measured by commercial aircraft is introduced and provides extensive observational data, a systematic and detailed investigation of the relationships between different weather systems and CAT is not performed using these new observations.

A 45-month EDR measurement archive (2019-2022) is employed in this study as the observational CAT database and around 5000 events with moderate or greater (MoG) intensity are identified. The observed events are analyzed together with synoptic weather systems affecting the tropopause region, which are identified with feature-based methods in the ERA5 reanalysis. While the jet stream showed a high frequency of co-occurrence with CAT, Rossby wave breaking (RWB; using potential vorticity streamers as a proxy) and warm conveyor belt (WCB) ascent are also frequently concurrent with the CAT events. A categorization of the events with respect to these two weather systems shows that they account for around 70% of the observed events and the very different properties of the two categories are revealed, with high horizontal deformation associated with the RWB-concurrent events and higher frequency of negative potential vorticity being present in the vicinity of the WCB-concurrent events. Deformation and the diabatic processes related to WCBs are hence suggested to be potential triggering processes of these two types of CAT.