Contradictory results concerning the relative importance of dry and moist process in blocking dynamics: Is it the perspective that matters?

Atmospheric blocking describes extratropical circulation patterns, in which a persistent, quasi-stationary high-pressure system obstructs the eastward progression of weather systems. Despite progress in recent years, a full understanding of processes involved in blocking formation, maintenance and decay is still lacking. There are even contradictory theories, particularly concerning the importance of diabatic processes. Understanding how diabatic processes influence blocking is essential for improving weather forecasts and unraveling the complexities of climate dynamics.

This talk will provide an overview about research on blocking dynamics with a focus on the role of diabatic processes. The effects of radiation, surface heat fluxes and friction are addressed, and a particular focus is set on latent heat release in mid-latitude synoptic-scale weather systems, which is often linked to so-called ‘warm conveyor belts’. As studies often differ in conclusions about the relative importance of dry and moist processes, a novel quasi-Lagrangian perspective based on potential vorticity (PV) is introduced. This allows to bridge earlier Eulerian and Lagrangian perspectives and to reconcile initially contradictory results regarding blocking dynamics. The quasi-Lagrangian framework tracks anticyclonic anomalies linked to blocking and quantifies the relative importance physical processes in the amplitude evolution, using piecewise PV tendencies. Applied to four different blocked regime types in the North Atlantic-European region, it is demonstrated that the traditional perspectives (Eulerian vs. Lagrangian) are consistent on its own and reveal together with the quasi-Lagrangian perspective a holistic view on blocking dynamics, reconciling earlier thought contradictions. This underlines the necessity of a multifaced view to correctly capture the role of diabatic processes.

Finally, the representation of moist processes linked to blocking in numerical weather prediction and climate models is discussed. The found relationship between forecast busts associated with blocking and an underestimation of warm conveyor belt activity emphasizes the need to improve the representation of multi-scale interactions in models ranging from the cloud-microphysical scale to the large Rossby wave scale - particularly regarding global warming and the suspected stronger latent heating.