Evolution of Rossby wave trains in a quantitative potential vorticity – potential temperature framework

Rossby wave trains are a fundamental ingredient of the synoptic- to large-scale dynamics of the midlatitudes. It is well known that these wave trains are linked to surface cyclones, blocking, and convective systems and thus play an important role in producing midlatitude weather events. While the linear dynamics of Rossby wave trains are understood very well, open questions about the evolution of finite-amplitude, synoptic-scale Rossby wave trains in the real atmosphere still exist. Real-atmospheric wave trains exhibit a strong baroclinic coupling to low-level weather systems and are modified by diabatic processes. A primary goal of our work is to quantify the relative importance of the individual processes for the evolution of Rossby wave trains in a potential vorticity (PV) – potential temperature framework.

Here, Rossby wave trains are identified by the distribution of PV anomalies on an isentrope intersecting the midlatitude tropopause. The local tendencies of these PV anomalies characterize the evolution of the wave train. Individual contributions to these tendencies are quantified using piecewise PV inversion complemented by a Helmholtz partition of the flow and evaluation of diabatic heating terms. For the piecewise PV inversion, a time-averaged background state is defined and the resulting PV anomalies are separated in upper-level and low-level (below 600hPa) anomalies. The tendencies associated with the upper-level anomalies are interpreted as barotropic wave propagation and those associated with the low-level anomalies as baroclinic feedback. Select Rossby wave trains are diagnosed using this PV framework and a brief comparison to a more conventional wave activity flux diagnostic is presented. It is shown that, locally and temporally, baroclinic feedback and diabatic processes are of the same importance to the evolution of the upper-tropospheric Rossby wave train as barotropic wave propagation. Potential implications for the predictability of Rossby wave trains will be discussed briefly.