Three-dimensional analysis reveals diverse heat wave types in Europe

Heat waves are increasingly significant atmospheric hazards, causing substantial societal and environmental impacts. While near-surface temperature patterns are commonly studied, they offer limited insight into the complex 3D structure of heat waves. Recent major events, such as the 2021 Western North American, 2022 Eastern China, and 2022 Western European heat waves, underscore the need for a more comprehensive understanding of their physical drivers (advection, adiabatic and diabatic processes) and 3D profiles. Previous research using atmospheric profiling has revealed that heat waves can be characterized by excessive heat not only near the surface but also throughout the lower troposphere. Studies also suggest a link between the development of residual layers (storing heat from day to day), the diurnal atmospheric boundary layer, and soil desiccation. Soil moisture availability is a critical factor influencing daily temperature maxima due to latent/sensible heat flux partitioning. Heat waves can be amplified by local desiccated soils and advection of sensible heat from dry regions. The relationship between soil moisture and temperature is complex and non-linear, affected by orography and land cover. This research aims to improve understanding of heat waves' 3D structures by using the ERA5 reanalysis data across multiple European regions, providing a more holistic view considering vertical structure, temperature anomaly, duration, and spatial extent.

Existing research highlights the significant impacts of heatwaves and the need for a deeper understanding of their three-dimensional structure. Studies have shown the importance of vertical temperature profiles in understanding heatwave intensity and duration. The role of soil moisture in amplifying heatwaves through feedback loops with high temperatures has been established. Previous research has investigated specific heatwave events, analyzing vertical temperature profiles and the influence of factors such as soil desiccation, warm advection, and adiabatic warming. However, a systematic analysis of the diverse three-dimensional structures of heatwaves across different European regions was lacking, prompting this current study.

This study utilized temperature data from the ERA5 reanalysis, covering three European regions: the British Isles, France, and Middle Europe. These regions were selected from the PRUDENCE regions to minimize variability in spatial extent, altitude, and water bodies. Daily means of near-surface (2 m) air temperature, air temperature at twelve vertical levels (850–300 hPa), and volumetric soil water in the upper layer were analyzed from 1979 to 2022, focusing on the extended summer season (June–September). Three vertical layers were defined: near-surface (2 m), lower troposphere (850–600 hPa), and higher troposphere (550–300 hPa). Heat waves were identified based on temperature anomalies exceeding the 95th percentile for each layer, considering spatial and temporal criteria. Four heat wave types were classified: near-surface (HWG), lower-tropospheric (HWL), higher-tropospheric (HWH), and omnipresent (HWO). The extremity of each heatwave was calculated considering temperature anomalies, spatial extent, and duration across layers. Links between heatwave types and soil moisture (volumetric soil water in the upper layer from ERA5) were investigated. Statistical significance was determined using the Wilcoxon test. The ERA5 data was obtained from the Copernicus Climate Data Store.

The analysis revealed distinct characteristics for each heat wave type. HWG events, dominated by near-surface temperature anomalies, had the longest duration (up to 15–17 days), peaking in mid-summer and showing a strong link to low soil moisture preconditioning (below the 25th percentile, 14 days prior to onset). HWL events, concentrated in the lower troposphere, were shorter (5 days at most) and exhibited more even distribution within the summer season. HWH heat waves, primarily in the higher troposphere, were also short and showed less seasonal variation. HWO heat waves showed a relatively even distribution of anomalies across all layers. The most severe heat waves (2003, 2016, 2019, and 2022) occurred throughout the extended summer season and demonstrated diverse typing. The 2003 heatwave stood out as the most severe across all regions. Notably, soil moisture preconditioning was crucial only for HWG, pointing to different driving mechanisms for the various heat wave types. An analysis of the July 2022 UK heatwave (classified as HWL) showed significant lower-tropospheric warming, a shift in wind direction (westerly to southerly) at 500 hPa, and increased warm advection. No other heatwave type showed a significant relationship with soil moisture preconditioning, except for a less pronounced lower soil moisture content during HWL and HWO onset in Middle Europe.

The findings highlight the importance of considering 3D heat wave structures to understand their diverse driving mechanisms. The differences observed between heat wave types—particularly the link between HWG and soil moisture pre-conditioning—suggest that distinct physical processes are at play. While large-scale circulation patterns (blocking anticyclones and subtropical high-pressure ridges) are primary drivers, the relative importance of adiabatic warming, air subsidence, and near-surface advection varies regionally. The study's results align with previous research highlighting the role of surface fluxes in continental climates and the influence of adiabatic warming in Western Europe. The observed regional differences in heat wave types (e.g., higher proportion of HWG in Middle Europe) support these existing findings. The 3D approach enhances the ability to disentangle the complex interactions between atmospheric dynamics and land surface processes during heat waves.

This study provides the first systematic investigation of European heat waves' 3D structures, revealing diverse types with distinct characteristics. The identified differences in duration, timing, and soil moisture relationships for different types highlight the varied physical mechanisms involved. The findings emphasize the importance of a 3D perspective for improving heatwave projections and climate services. Future research could explore the use of higher-resolution datasets, integrate other variables (e.g., humidity, wind), and examine the influence of climate change on the frequency and intensity of these diverse heat wave types.

The study relies on ERA5 reanalysis data, which has inherent limitations, particularly in representing precipitation accurately. The precipitation-temperature relationship, especially in near-surface layers, may be affected by uncertainties in the ERA5 precipitation scheme. Future work should address this by incorporating higher-resolution data or independent precipitation datasets (such as E-OBS) for validation. The regional focus limits generalizability to other geographic areas with potentially different climate characteristics and driving mechanisms.