Severe convective outbreaks and heatwaves – a continental-scale compound event

Recent summers in Europe were accompanied by significant convective storm outbreaks with widespread large hail, flash floods, and severe wind phenomena. Particularly severe outbreaks have occurred upstream of concurrent heatwaves. On a continental scale, this leads to considerable compound hazards from heatwaves and thunderstorm hazards. 

Utilizing reanalysis data, we investigate the connection between high temperatures and synoptic conditions leading to severe convective environments (SCE), which are conducive to severe convection. Our analysis reveals that SCE across Central and Western Europe are preceded by high temperatures and a slow-moving upper-level wave pattern. More strikingly, they reveal a strongly increased heatwave frequency downstream of SCE. 75% of SCE are associated with a heatwave, usually ~500km downstream. The remaining minority take place in much cooler, predominantly low-pressure situations, with less persistent SCE. Inversely, 80% heatwaves are also associated with upstream SCE. These heatwaves are significantly hotter by >1°C than those not associated with convection. 

This strong co-occurrence of severe convective outbreaks and heatwaves implies a dynamical link. The upper-level wave pattern may drive both the SCE through the advection of unstable airmasses and high wind shear in the prefrontal zone, as well as the heatwave by warm air advection, radiative heating, and a strong ridge. Further feedback between heatwaves and SCE is possible via diabatic heating processes and soil moisture feedback. This study highlights a previously underexplored continental-scale compound event relationship between severe convective environments and downstream heatwaves in Europe, suggesting a common synoptic driver and potential two-way interactions.