Jet streaks from a PV gradient perspective: A Lagrangian analysis of diabatic-adiabatic interaction in km-scale simulations

The jet stream is a circumpolar global band of high wind speeds in the upper troposphere. Meridional meanders of the jet lead to high-impact weather events and may synchronize them over thousands of kilometres. As a Rossby waveguide, the jet influences paths of synoptic-scale eddies, which in turn alter jet dynamics. Coherent regions of enhanced wind speed, so-called jet streaks, typify the jet locally and anomalously strong jet streaks often coincide with extreme precipitation and wind events.

Despite their relevance, process understanding remains limited regarding the formation of jet streaks in interaction with lower-level weather systems. The same is true for the influence of jet streaks on Rossby wave evolution. One way to further the understanding of jet streak dynamics is to study the interaction between adiabatic and diabatic processes during jet streak evolution. However, the relative importance of those processes is difficult to disentangle. 

This study utilises a Lagrangian-based PV gradient perspective by applying it to a jet streak relative coordinate system to obtain composites throughout the lifecycles of multiple jet streak events. The theoretical foundation of this approach is the link between the horizontal isentropic PV gradient and wind speed. Local maxima of the normalised isentropic PV gradient are collocated with centers of jet streaks. As PV is conserved under adiabatic and frictionless flow, the PV gradient perspective allows for an investigation of diabatic-adiabatic interaction.

We analyse a convection-resolving 1.1 km COSMO simulation in the eastern North Atlantic in autumn 2016, using PV gradient analysis and online air parcel trajectories to separate diabatic and adiabatic contributions to jet streak development. Cloud processes play an important role in the establishment and maintenance of a strong event, while the dynamics of a weaker jet streak is dominated by effects of adiabatic deformation.

After demonstrating the approach in two case studies, we present results from a composite analysis of multiple jet streak events to achieve a more systematic understanding of diabatic-adiabatic interaction during their evolution.