Evidence of gravity wave contribution to vertical shear and mixing in the lower stratosphere: a WISE case study

Abstract. Evidence is presented which illustrates the role of atmospheric gravity wave (GW) induced shear as a mechanism for the occurrence of clear air turbulence and exchange of air masses with different chemical composition in the lower stratosphere. This study investigates the characteristics of GWs and their impact on the distribution of trace species in the lowermost stratosphere during an extratropical cyclone over the North Atlantic using airborne in-situ observations, ERA5 reanalysis data as well as IFS and ICON forecast data. Tracer observations as well as model simulations reveal fine scale structures around the tropopause which are embedded in a region influenced by the inertia gravity waves, warm conveyor belt ascent and mesoscale modifications of the tropopause structure. The GWs propagate through highly sheared flow above the jet stream maximum, perturbing background wind shear and static stability, and thereby creating conditions conducive to turbulent mixing in the lowermost stratosphere. The observed significant correlation between GW-induced momentum flux and enhanced shear perturbations confirms the role of GWs in driving potential turbulence and facilitating trace gas exchange in the lower stratosphere. Further analysis of turbulence diagnostics suggests that GWs produce shear which leads to the occurrence of clear air turbulence.