Near-past evolution of the magnitude and intensity of European heat waves

This work presents results from downscaling experiments using the Weather Research and Forecasting (WRF) in frame of the H2020-EXHAUSTION project for the period of 1980-2010 at 20km horizontal resolution over the European domain. Two simulations were carried out driven by ERA5 input by grid nudging (WRF_ERA5) and CESM2 output using 6 waves spectral nudging (WRF_CESM2), respectively. These near-past simulations have been rigorously evaluated with observations and reanalysis data including European Climate Assessment & Dataset (ECA&D), EOBS, and ERA5-land for the daily mean (TG), maximum (TX), and minimum (TN) surface temperatures over the whole Europe as well as five climate zones. The WRF simulations compared reasonably well with the observations. WRF_ERA5 showed a smaller root mean square error (RMSE) and higher correlations (r), while WRF_CESM2 performed better in terms of mean and normalized mean bias (MB and NMB). WRF_CESM2 is overall reliable to be used for future simulations.  In terms of the 30-year trend of TG, TX, and TN, WRF_CESM2 (0.6-0.66 °C/10yrs) showed faster increasing trends than WRF_ERA5 (0.29-0.35 °C/10yrs) and observations (0.27-0.41 °C/10yrs). Evaluations in different climate zones show smaller bias in north-western Europe and southern Europe. In terms of temporal evolution, eastern Europe showed the highest correlations. The worst model performance has been calculated for northern Europe. 

In addition, the Warm Spell Duration Index (WSDI) and the Heat Wave Magnitude Index daily (HWMId) have been calculated to represent the duration and magnitude of heat waves, respectively, for both simulations and observations. Strong and significant increasing trends are shown in eastern Europe and northern Europe for both WSDI and HWMId in all cases, with the fastest trends shown in EOBS (4 days/10yrs for WSDI, and 2/10yrs for HWMId), slowest trends in ECA&D (2 days/10yrs for WSDI, and 1/10yrs for HWMId), and trends in two WRF simulations are in between. No significant trends were found in southern Europe and north-western Europe in ECA&D, EOBS, and WRF_ERA5 simulation, while significant increasing trends were simulated in WRF_CESM2 in these two zones. The preliminary results suggested an increasing trend in the evolution of the future heat waves over Europe with implications on both direct impacts on human health, as well as indirect impacts through changes in exposure to pollutants such as ozone and particulate matter. Various future simulations are ongoing to address the impacts of climate change on the severity of heat waves under different levels of mitigation.