Understanding the vertical temperature structure of recent record-shattering heatwaves

Abstract. Extreme heatwaves are one of the most impactful natural hazards, posing risks to human health, infrastructure and ecosystems. Recent theoretical as well as observational studies suggested that the vertical temperature structure during heatwaves limits the magnitude of near-surface heat through convective instability. In this study, we thus examine in detail the vertical temperature structure during three recent record-shattering heatwaves, the Pacific Northwest (PNW) heatwave in 2021, the Western Russia (RU) heatwave in 2010, and the West European and UK (UK) heatwave in 2022 by decomposing temperature anomalies (T') in the entire tropospheric column above the surface into contributions from advection, adiabatic warming and cooling, and diabatic processes.

All three heatwaves exhibited bottom-heavy yet vertically deep positive T' extending throughout the troposphere. Importantly, though, the T' magnitude and the underlying physical processes vary greatly in the vertical within each heatwave, as well as across distinct heatwaves, reflecting the diverse synoptic storylines of these events. The PNW heatwave was strongly influenced by an upstream cyclone and an associated warm conveyor belt, which amplified an extreme quasi-stationary ridge and generated substantial mid- to upper-tropospheric positive T' through advection and diabatic heating. In some contrast, positive upper-tropospheric T' during the RU heatwave was caused by advection, while during the UK heatwave, it exhibited modest positive diabatic contributions from upstream latent heating only during the early phase of the respective ridge. Adiabatic warming notably contributed positively to lower tropospheric T' in all three heatwaves, but only in the lowermost 200–300 hPa. Near the surface, all three processes contributed positively to T' in the PNW and RU heatwave, while near-surface diabatic T' was negligible during the UK heatwave. Moreover, there is clear evidence for an amplification and downward propagation of adiabatic T' during the PNW and UK heatwaves, whereby the maximum near-surface T' coincided with the arrival of maximum adiabatic T' in the boundary layer. Additionally, the widespread "ageing" of near-surface T' over the course of these events now quantitatively supports the notion of heat domes, within which air recirculates and accumulates heat.

Our results for the first time document the four-dimensional functioning of anticyclone-heatwave couplets in terms of advection, adiabatic and diabatic cooling or warming, and shed light on the complex interplay between large-scale dynamics, moist convection and boundary layer processes that ultimately determines near-surface temperatures during heatwaves.