Towards a more comprehensive assessment of the intensity of European Heat Waves 1979-2019

Heat waves are among the most dangerous natural hazards worldwide. Central Europe has been affected by record-breaking heat waves in recent decades, especially in 2003, 2018 and 2019. Four frequently used indices are chosen in this study to diagnose heat waves in Europe based on both station data and ERA5 reanalysis: the Heat Wave Magnitude Index daily (HWMId), the Excess Heat Factor (EHF), the Wet Bulb Globe Temperature (WBGT) and the Universal Thermal Climate Index (UTCI). To improve the quantification of the events and comparability of the four indices, a normalisation is applied and the three metrics intensity, duration, spatial extent were combined by a cumulative intensity measure. The large-scale characteristics of the 1979 to 2019 European heat waves are analysed from a Lagrangian perspective, by daily tracking of contiguous heat wave areas. The events were ranked and visualized with bubble plots. The role of different meteorological input parameters like temperature, radiation, humidity and wind speed is explored to understand their contribution to the extremeness of heat waves and the variance in time series of the heat wave indices.

As expected, temperature explains the largest variance in all indices, but humidity is nearly as important in WBGT and wind speed plays a substantial role in UTCI. While the 2010 Russian heat wave is by far the most extreme event in duration and intensity in all indices, the 2018 heat wave was comparable in size for EHF, WBGT and UTCI. Interestingly, the well-known 2003 central European heat wave was only the fifth and tenth strongest in cumulative intensity in WBGT and UTCI, respectively. The June and July 2019 heat waves were very intense, but short-lived, thus not belonging to the top heat waves in Europe when duration and areal extent are taken into account. Overall, the proposed normalised indices and the multi-metric assessment of large-scale heat waves allow for a more robust description of their extremeness and will be helpful to assess heat waves worldwide and in CMIP6 climate projections.

Applying the normalization to the four indices and deriving the large-scale metrics of intensity, spatial extent and duration, as proposed in the present study, will facilitate trend studies using different sources of observations and models. As the combination of duration and intensity over large areas are responsible for the most severe health and economic impacts, interdisciplinary research (e.g. links to health effects) is recommended starting to better quantify the impacts of heat waves in a warming climate.