On the dissipation of negative potential vorticity in the upper troposphere

Potential vorticity (PV) in mid-latitudes of the Northern Hemisphere is predominantly positive. Nevertheless, recent studies have shown that coherent and elongated negative PV (NPV) features can be generated in the upper troposphere by diabatic heating in a vertically sheared environment. These NPV features may persist for a few hours and interact with the jet, affecting the large-scale flow evolution. However, in contrast to its formation, the dissipation of NPV features is not well-understood, and the involved processes have not been investigated yet.

In this study, we carry out case studies on the dissipation of NPV near jet streams using numerical simulations from the Integrated Forecasting System by the European Centre for Medium-Range Weather Forecasts (ECMWF). Temperature and momentum tendencies from each parametrisation scheme are output, allowing a quantification of PV tendencies due to individual processes along air parcel trajectories. By launching forward trajectories in coherent NPV features, the contribution to the increase in PV, i.e., to the dissipation of NPV, by different diabatic processes are traced and compared. Turbulence appears to stand out as the dominant process that dissipates NPV. Detailed analysis on selected trajectories further demonstrates that the PV increase is usually associated with the tripole pattern of PV tendencies created by turbulence, which can be understood with a two-dimensional framework of the upper-level jet-front system. A special case that is consistent with the framework, but with a reversed tripole pattern is also found in a region of NPV. The study therefore provides further insight and understanding of the process by which NPV is dissipated in the upper troposphere.