Revisiting heatwaves in a EURO-CORDEX RCM ensemble in comparison with a coupled regional climate system model with 3D subsurface hydrodynamics

​​Explicitly considering groundwater dynamics in regional climate models (RCMs) can significantly influence the simulation of states and fluxes at the land surface, leading to an altered land-atmosphere coupling. The modified flux partitioning is relevant for the simulation of heatwaves, and their future evolution under climate change. This study compares the representation of heatwaves at climate time scales in an ensemble of RCMs without, and one coupled RCM with explicit groundwater- and subsurface hydrodynamics. The Terrestrial Systems Modelling Platform (TSMP, https://www.terrsysmp.org) as a regional climate system model, couples the atmospheric model COSMO, the Community Land Model, and the hydrologic model ParFlow (https://www.parflow.org) through the OASIS3-MCT coupler. TSMP simulates a closed terrestrial water cycle from the groundwater to the top of the atmosphere with a 3D variably saturated subsurface flow representation and a free-surface overland flow boundary condition. TSMP is run in a EURO-CORDEX compliant setup at 12km resolution over the European EUR-11 domain. First, we compare heatwave area, frequency, duration, and intensity from 13 years of ERA-Interim driven evaluation runs. TSMP is analysed alongside a EURO-CORDEX RCM ensemble, gridded E-OBS observations, and the ERA5-Land reanalysis. Especially for heatwave intensities and the number of heatwave days, TSMP shows a clear tendency towards lower mean absolute deviations from the comparison data. A comparison to GLEAM-based evapotranspiration indicates low deviations in evapotranspiration anomalies. This is linked to an increased evaporative fraction that is affected by a redistribution of soil moisture and groundwater flow in a continuum approach and interactions with the land surface. In a further analysis, 30 years of selected EURO-CORDEX RCM ensemble members from historical simulations, driven by CMIP5 global climate models (GCM), and TSMP driven by the MPI-ESM-LR GCM are compared. Consistent with the evaluation run analysis and with large spatial and temporal heterogeneity, the soil moisture and groundwater treatment in TSMP attenuates hot events and heatwave extremes, in comparison to the RCM ensemble. The duration of heat events in TSMP decreases as the mean number of hot day events (duration > 3 days) and long hot events (duration > 6 days) decreases by a factor of 1.5-2.3; the mean number of short hot events (duration < 3 days) is higher in TSMP. Also, the frequency of heatwaves (heat events exceeding 6 consecutive days) with an amplitude (intensity) larger than 4K compared to the 90th temperature percentile, is decreased by a factor of 2 and more, while, the frequency of heatwaves with low amplitudes is increased. The results suggest that groundwater dynamics, due to their impact on the number of hot day events, and the frequency and intensity of heatwaves, has to be taken into account when analysing heatwave statistics from RCM ensembles.