Increased Light, Moderate, and Severe Clear-Air Turbulence in Response to Climate Change (Invited Presentation)

New observational evidence shows that the atmospheric jet stream in the North Atlantic region has become 15 per cent more sheared at aircraft cruising altitudes since 1979. This strengthening of the vertical wind shear is an expected consequence of anthropogenic climate change. Such a strengthening is expected to increase shear instabilities and thereby to increase the production of shear-driven clear-air turbulence. Correspondingly, climate modelling studies have indicated that the amount of moderate-or-greater clear-air turbulence on transatlantic flight routes in winter will increase significantly in future as the climate changes. However, the individual responses of light, moderate, and severe clear-air turbulence have not previously been studied, despite their importance for aircraft operations.

Here we use climate model simulations to analyse the transatlantic wintertime clear-air turbulence response to climate change in five aviation-relevant turbulence strength categories. We find that the probability distributions for an ensemble of 21 clear-air turbulence diagnostics generally gain probability in their right-hand tails when the atmospheric carbon dioxide concentration is doubled. By converting the diagnostics into equivalent eddy dissipation rates, we find that the ensemble-average airspace volume containing light clear-air turbulence increases by 59% (with an intra-ensemble range of 43–68%), light-to-moderate by 75% (39–96%), moderate by 94% (37–118%), moderate-to-severe by 127% (30–170%), and severe by 149% (36–188%). These results suggest that the prevalence of transatlantic wintertime clear-air turbulence will increase significantly in all aviation-relevant strength categories as the climate changes.

A further analysis shows that these results are not limited to transatlantic wintertime turbulence, but apply globally in all seasons, at multiple flight levels, and according to multiple climate models. We conclude that an increase in clear-air turbulence from climate change is a robust finding, being supported by new observational evidence of stronger shear in the jet stream since the start of the satellite era, and being consistent with the thermal wind response to climate change.