Potential vorticity modification by turbulence in the upper troposphere and lower stratosphere – Part 1: Mechanistic understanding in an upper-level jet-front system

Abstract. Turbulence has been recognised as an important non-conservative process that modifies potential vorticity (PV) in the upper troposphere and lower stratosphere (UTLS) since the 1970s from research flight measurements. While recent studies based on numerical simulations affirmed the significance of turbulence in changing the PV distribution in the UTLS, the understanding of the underlying mechanism and properties of the PV modification by turbulence remains limited. Therefore, a comprehensive and detailed analysis is required to determine if turbulence may systematically modify PV, which would permit an important up-scale dynamical influence. In this study, we develop an analytical framework and perform a numerical simulation using the Integrated Forecasting System (IFS) model from the European Centre for Medium-Range Weather Forecasts to provide insight into turbulence-induced PV modification. Inspired by the coherent signal near an upper-level jet-front system in the simulation, we first employ an idealised quasi-two-dimensional framework to explore the characteristics of PV modification by turbulence. Both the turbulent heat and the momentum fluxes cause a transfer of PV away from the turbulent zone in most situations, leading to the formation of a tripole pattern of instantaneous PV tendencies. The relevance of the idealised framework to realistic atmospheric flows is validated in the IFS simulation, which features prominent PV tendency tripoles in the frontal zones associated with the upper-level jet stream, consistent with the theory. Material PV changes along air parcel trajectories are further examined by accumulating the instantaneous PV tendencies due to different non-conservative processes in the simulation along the flow. In the vicinity of the upper-level jet-front system, turbulence is shown to be the dominant contributor to accumulated PV changes, which are organised into tripolar bands along the jet axis. By analysing vertical profiles of the atmospheric conditions traced along individual trajectories, air parcels are found to travel consistently through the PV tendency tripoles induced by turbulence. This allows them to steadily accumulate PV changes, which ultimately form the observed tripolar bands. Our results thus (i) provide a mechanistic explanation for the emergence of the tripolar PV tendency pattern due to turbulence with the idealised framework, (ii) demonstrate the possibility of turbulence in systematically modifying PV in the UTLS, which may influence the flow evolution subsequently, and (iii) highlight the importance of the upper-level jet-front systems in providing the spatial and temporal coherence required for the organised behaviour of turbulence in modifying PV. This study therefore advances our understanding of PV modification by turbulence in the UTLS and provides the foundation for future studies, e.g., on the relevance of turbulence to forecast errors and flow dynamics in the UTLS.