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

Atmospheric gravity waves (GWs) play a crucial role in the dynamics of the middle atmosphere, transporting energy and momentum and substantially influencing the atmospheric energy budget. In the upper troposphere and lower stratosphere (UTLS), the composition is shaped by horizontal transport, vertical transport associated with convective systems and warm conveyor belts (WCBs), as well as turbulent mixing. GWs can drive cross-isentropic fluxes of trace gases through turbulence generation; however, their role in enhancing shear and turbulent mixing within the extratropical transition layer (ExTL) remains poorly understood.

This study investigates the characteristics and dynamics of GWs generated near an extratropical cyclone using observations from the WISE (Wave-driven ISentropic Exchange) campaign over the North Atlantic on 23 September 2017, supported by ERA-Interim and ERA5 reanalysis data. Additionally, convection-permitting simulations with the ICOsahedral Non-hydrostatic (ICON) model were conducted on a global and two higher resolution nested domains. The tracer observations reveal fine scale structures around the tropopause which are embedded in a region affected by the WCB ascent, inertia gravity waves, a mesoscale modifications in the tropopause structure.

These GWs propagate through highly sheared flows above the jet stream, perturbing background wind shear and static stability, creating conditions conducive to turbulent mixing in the lowermost stratosphere (LMS). The observed significant correlation between GW-induced absolute momentum flux and enhanced small-scale shear confirms their role in driving potential turbulence and facilitating trace gas exchange in the lower stratosphere. Bands of low Richardson number, indicative of potential turbulence, suggest regions prone to clear air turbulence (CAT).

Our findings underscore the critical role of GWs in enhancing vertical wind shear and facilitating turbulent mixing in the LMS, thereby contributing to the formation of the ExTL. These results highlight the necessity of accurately representing GWs in atmospheric models to improve predictions of clear-air turbulence and associated mixing in the UTLS.