Characteristics of Potential Vorticity anomalies associated with mesoscale extremes in the extratropical troposphere

Abstract

Severe convective weather, which has a huge impact on society, is influenced by a large range of scales. However, extremes, by definition rare events, are especially hard to forecast. This study proposes to use Potential Vorticity (PV) as an analytic tool for diagnosing Deep Moist Convection (DMC) on the convective weather scale (≈ 10 km). On synoptic and planetary scales, PV is a useful conceptual variable. Given a balance condition, and suitable boundary conditions, one can invert a given PV distribution to obtain wind velocity, pressure and (potential) temperature distribution associated with the PV distribution. Ideally, one would like to invert the PV distribution on the convective weather scale, too. Inversion of PV associated with DMC is problematic, because of the unsteady convection and the unknown balance condition. This study hypothesises, however, that PV anomalies might still be seen as “quasi”-balanced. This implies that one would expect consistent PV during DMC, associated with coherent significant flow anomalies. This study tests the coherency of PV anomalies by compositing the PV distribution around convective cells in the nonhydrostatic Numerical Weather Prediction (NWP) model COSMO-DE.<br /> A case study of two severe weather events (5 and 22 June 2011) shows that the COSMO-DE model is capable of reproducing the theoretically described PV dipole around a convective updraft. During the 22 June event, bands of PV are generated. Possible explanations for these bands are advection due to large scale wind shear and the preferential generation of new cells downshear of old cells. Tracks of the evolution of PV dipoles show that for both cases there is a consistent development. In this study 9 severe weather cases are selected, with a large range of synoptic backgrounds. The consistent evolution is confirmed by composites of storm cells during all severe weather cases. Especially intense cells, characterised by e.g. severe precipitation rates or strong PV, have a more monopole morphology. Moreover, strong PV cells last relatively long. This indicates that these intense cells might be regarded as “supercells”: longlasting intense rotating updrafts. This also implies that PV might be an indicator for severe weather. Spectra of potential enstrophy show a consistent correlation with extreme precipitation and the spectral variance on the convective weather scale in COSMO-DE.