Clear-air turbulence derived from in situ aircraft observation – a weather feature-based typology using ERA5 reanalysis

Abstract. Clear-air turbulence (CAT) endangers aviation safety and early understanding of the phenomenon was obtained mainly by analysing the corresponding synoptic weather situation. In this study, the relationship between CAT and different synoptic weather features is revisited based on in situ eddy dissipation rate measurements by commercial aircraft and modern reanalysis data (ERA5 reanalysis). In the years from 2019 to 2022, 4880 moderate-or-greater turbulence events are identified in predominantly clear-air conditions in the Northern Hemisphere. Most of the events identified occur over the contiguous U.S. and along the major flight corridors in the North Atlantic and western North Pacific. They are associated frequently with potential vorticity (PV) streamers, which are used as a proxy to Rossby wave breaking (RWB), and/or warm conveyor belt (WCB) ascents at the event locations. Events which are concurrent with RWB in the absence of WCB ascents are classified as type I. They constitute around 40 % of the events and are found evenly across the contiguous U.S. Events which are concurrent with WCB ascents are classified as type II. They account for around 30 % of the events and are more concentrated over the eastern U.S. and the East China Sea. Analysing the environmental conditions associated with the events, higher values of horizontal deformation are found on average in the vicinity of type I events, and the high horizontal deformation associated with RWB is considered as the possible cause of this type of CAT. Type II events occur more frequently in the presence of negative PV, together with higher averaged cloud ice water content and wind speed. The presence of negative PV, which is most likely due to diabatic PV reduction in clouds, may indicate that inertial or symmetric instabilities or enhanced local wind shear due to the strengthened outflow from WCBs are possible causes of CAT for type II events. The suggested linkages are further supported by examining the ERA5 grid point data. When grid points with high horizontal deformation are examined, they are mostly found in RWB regions and show an enhanced chance of turbulence. This collocation of RWB, high horizontal deformation, and turbulence is particularly prominent over the Western U.S. Similarly, grid points with negative PV values also show a higher probability of turbulence and a noteworthy fraction is collocated with WCB ascents. The results thus (i) reveal the important roles of RWB and WCB ascents for CAT, (ii) provide a better explanation of the physical mechanisms triggering CAT in the presence of RWB and WCB ascents, and (iii) highlight the importance of in situ observations for deepening the understanding of CAT. Furthermore, the weather-feature perspective employed in this study may also provide insights to interpret the climatology of CAT or projected changes of CAT in the future.