Lagrangian analysis of two flavours of Central European heatwaves: development under Omega blocking vs. initiation by subtropical ridges

In mid-latitude regions, the development of a heatwave is closely linked to a quasi-stationary anticyclonic flow anomaly. For many cases over Europe, these anomalies are associated with atmospheric blocking events, which in summer usually manifest themselves in form of a so-called Omega blocking. However, not all heatwaves necessitate atmospheric blocking. Indeed, some heatwaves are enabled by poleward extensions of the subtropical high pressure belt, forming an atmospheric ridge pattern. We hypothesize that both the origin of the involved air masses as well as the processes modulating the air mass along its path to Central Europe may differ fundamentally between heatwaves forming under an Omega blocking and those that are initiated by a subtropical ridge.

In this work, we therefore select the respective 20 most textbook-like cases of Omega and ridge-type Central European heatwaves in the period of 1950 to 2023. Based on high-resolution ERA5 data, we conduct a Lagrangian analysis into the properties of air masses and the relative importance of the three processes warming the involved air masses, namely advection, adiabatic warming by subsidence and diabatic warming through sensible heat fluxes. By computing a large number of backward trajectories using Lagranto and the subsequent application of a Lagrangian temperature decomposition algorithm, we quantify the relative importance of each of the three mentioned processes. This analysis is done separately for the onset day and the subsequent three days of the heatwave.

Omega- and ridge-type heatwaves feature some significant differences in both air mass origin and the relative importance of the processes leading to anomalously high near-surface temperatures, which tend to become more apparent in the more mature stage of the respective type of heatwave. Overall, ridge-type heatwaves tend to be characterized by a higher advective contribution to the overall temperature anomaly. This is directly related to the fact, that the involved air masses tend to originate from slightly more southern and climatologically warmer regions. Particularly two or three days after heatwave onset, anomalous subsidence and associated adiabatic heating contributes significantly more to warming in ridge-type than in omega-type heatwaves. In turn, omega-type heatwaves are characterized by a significantly stronger contribution of diabatic heating. This is mostly due to air masses spending more time in the planetary boundary layer and stronger short-wave radiation along the air masses' path.