Modification of potential vorticity near the tropopause by nonconservative processes in the ECMWF model

Abstract

The upper-level potential vorticity (PV) structure plays a key role in the evolution of extratropical weather systems. PV is modified by nonconservative processes, such as cloud latent heating, radiative transfer, and turbulence. Using a Lagrangian method, material PV modification near the tropopause is attributed to specific parameterized processes in the global model of the European Centre for Medium-Range Weather Forecasts (ECMWF). In a case study, several flow features identified in a vertical section across an extratropical cyclone experienced strong PV modification. In particular clear-air turbulence at the jet stream is found to be a relevant process (i) for the PV structure of an upper-level front-jet system, corroborating previous observation-based findings of turbulent PV generation; (ii) for the purely turbulent decay of a tropopause fold, identified as an effective process of stratosphere-troposphere exchange; and (iii) in the ridge, where the Lagrangian accumulated turbulent PV modification exhibits a distinct vertical pattern, potentially impacting the strength of the tropopause inversion layer. In contrast, cloud processes affect the near-tropopause PV structure above a warm conveyor belt outflow in the ridge and above cold-sector convection. In agreement with previous studies, radiative PV production dominates in regions with an anomalously low tropopause, where both radiation and convection act to increase the vertical PV gradient across the tropopause. The particular strengths of the Lagrangian diagnostic are that it connects prominent tropopause structures with nonconservative PV modification along the flow and that it quantifies the relative importance of turbulence, radiation, and cloud processes for these modifications.