Towards a diagnostic framework unifying different perspectives on blocking dynamics: insight into a major blocking in the North Atlantic-European region

Abstract

Abstract. Atmospheric blocking describes a situation in which a stationary and persistent anticyclone blocks the eastward propagation of weather systems in the midlatitudes and can lead to extreme weather events. In the North Atlantic-European region blocking contributes to life cycles of weather regimes, which are recurrent, quasi-stationary, and persistent patterns of the large-scale circulation. Despite progress in blocking theory over the last decades, we are still lacking a comprehensive, process-based conceptual understanding of blocking dynamics. Here we combine three different perspectives on blocking, namely the commonly used Eulerian and Lagrangian perspectives, complemented by a novel quasi-Lagrangian perspective. Within the established framework of mid-latitude potential vorticity (PV) thinking the joint consideration of the three perspectives enables a comprehensive picture of the dynamics and quantifies the importance of dry and moist processes during a blocking life cycle. We apply the diagnostic framework to a European Blocking weather regime life cycle in March 2016, which was associated with a severe forecast bust in the Atlantic-European region. All three perspectives highlight the importance of moist processes during the onset and maintenance of the ‘blocked’ weather regime. The Eulerian perspective, which identifies the processes contributing to the onset and decay of the regime, indicates that dry quasi-barotropic wave dynamics and especially the eastward advection of PV anomalies (PVAs) into the North Atlantic-European sector are associated with the onset of the regime pattern. By tracking the negative upper-tropospheric PVA associated with the ‘block’, the quasi-Lagrangian view reveals, for the same period, abrupt amplification due to moist processes. This is in good agreement with the Lagrangian perspective indicating that a large fraction of air parcels that end up in the negative PVA experiences diabatic heating. Overall, the study shows that important contributions to the development take place outside of the region in which the blocked weather regime eventually establishes, and that a joint consideration of different perspectives is important in order not to miss processes, in particular moist-baroclinic dynamics, contributing to a blocking life cycle.

Similarity and variability of blocked weather-regime dynamics in the Atlantic-European region

“…The term DIV represents the impact of the divergent flow. Large values of this term are usually associated with latent heat release below (see detailed discussion in Teubler and Riemer (2021); explicitly verified in a case study by Hauser et al (2022b)) and can thus be interpreted as an indirect contribution by moist processes. The term EDDY describes the nonlinear redistribution of PV in terms of the convergence of the eddy flux of PV anomalies (−∇ • (v rot q ), where v rot is the non-divergent wind; hereafter eddy flux convergence for the sake of brevity).…”

“…A detailed discussion of this decomposition technique is given in (Teubler and Riemer, 2021). Similar as in Hauser et al (2022b), the tendency equation for q L can symbolically be written as…”

“…The prime denotes that the deviation from the climatological average of the individual tendencies is considered. Because we here deviate in the computation of that climatological average from Hauser et al (2022b), the derivation of Equation 4 is given in Appendix A, where we also explain why we do not consider noncoservative tendencies N explicitly in this study.…”

“…Davis et al, 1993;Pomroy and Thorpe, 2000;Chagnon et al, 2013;Riemer, 2016, 2021;Riboldi et al, 2019;Spreitzer et al, 2019), in addition to quasi-barotropic dynamics. This study adopts the PV perspective and employs the quantitative, piecewise PV tendency diagnostic developed in the context of mid-latitude Rossby wave packets (Teubler and Riemer, 2021) and extended to the dynamics of a blocked regime in a case study (Hauser et al, 2022b). In short, the decomposition of dynamical mechanisms in this framework can be interpreted in terms of linear (quasi-barotropic) Rossby wave dynamics, baroclinic interaction, divergent outflow associated with latent heat release below, direct diabatic PV modification, and nonlinear PV fluxes.…”

“…For example, projections do not account for processes that amplify PV anomalies outside of the regime pattern. This limitation may affect in particular the diagnostic of the impact of the moist processes, which tend to occur upstream of the regime pattern (Neal et al, 2022;Hauser et al, 2022b). A distinct advantage of projections is, however, that variability in terms of mere geographical location of different patterns is eliminated and thus the variability of dynamical processes is more directly intercomparable.…”