Jet stream dynamics from a potential vorticity gradient perspective: The method and its application to a kilometre-scale simulation

The influence of adiabatic and diabatic processes on the midlatitude circulation is a formidable research question, especially considering their projected changes under global warming. This study presents the prospects, merits, and caveats of a potential vorticity (PV) gradient perspective as a means to disentangle the contributions of adiabatic and diabatic processes affecting the midlatitude circulation. Theoretical considerations reassess the link between the PV gradient and the jet stream. They reveal that the maximum isentropic PV gradient is consistently located on the stratospheric side of the jet, whereas the gradient of ln(PV)$$ \ln \left(\mathrm{PV}\right) $$ is shifted to the tropospheric side but, in general, is better aligned with the jet axis. The stratospheric shift of the PV gradient results from variations in stability across the tropopause, whereas the tropospheric shift of the ln(PV)$$ \ln \left(\mathrm{PV}\right) $$ gradient results from variations in vorticity. Regions of high PV gradient may serve as a proxy for the curvature of the wind field in the case of sufficiently small variations in stability. Otherwise, they depict variations in both wind and thermal stratification along tropopause-intersecting isentropic surfaces. Lagrangian "PV gradient thinking" is demonstrated in two case studies of jet streak evolution in a simulation with 1.1 km grid spacing performed with the graphics-processing-unit-enabled numerical weather prediction model Consortium for Small-Scale Modelling featuring on-line air parcel trajectories. Dry deformation drives the Lagrangian evolution of the PV gradient in the first case, whereas there is a pronounced influence of diabatic modification in the second case. The Lagrangian PV gradient perspective presented offers fresh insight into adiabatic and diabatic processes underlying the midlatitude circulation variability and change.