Disentangling diabatic and adiabatic drivers during the life cycle of a jet streak from a Lagrangian PV-gradient perspective

 

 The jet stream is a circumpolar global band of high wind speeds in the upper troposphere. Meridional jet meanders lead to high-impact weather events and synchronize them over thousands of kilometres. Locally, the jet stream can be typified by concentrations of extreme wind speeds in so-called jet streaks, whose formation mechanisms are largely unknown. 

The mean position and intensity of the jet is to first order determined by the meridional temperature gradient and its variation with altitude, following the thermal wind relationship. On synoptic timescales, however, jet streaks arise from a complex interaction between adiabatic and diabatic processes with their relative importance difficult to disentangle. To obtain a mechanistic understanding of the formation of jet streaks, however, a proper understanding of the relative role of adiabatic and diabatic processes is crucial. 

Here, a novel Lagrangian PV-gradient framework is introduced that allows to quantify the role of adiabatic and diabatic processes during the life cycle of jet streaks. First, the theoretical foundation of the widely used link between horizontal isentropic PV-gradients and the jet is revisited. It is shown that the zone of highest PV gradient must be located always on the stratospheric side of the jet. Further, the jet evolves hand in hand with the natural logarithm of the PV gradient for wave lengths exceeding approximately 50 km on Earth-like planets. 

Next, using a case study, the novel diagnostic is discussed in the context of Lagrangian PV-gradient tendencies computed fully online in a convection resolving 1-km COSMO simulation. Individual contributions are separated into adiabatic processes such as deformation, direct diabatic contributions such as condensation and indirect diabatic contributions from deformation acting along non-conserved PV. 

The presented case study analyses two jet streaks of different origin and intensity during the NAWDEX period. The relative roles of diabatic and adiabatic processes in the strengthening and weakening of the PV gradient varies widely between the two and between different phases of their life cycles. Cloud processes play an important role for the jet streak at time of its main intensification phase while during most of the life cycle, adiabatic deformation dominates the PV gradient evolution.