Three-dimensional (3D) characteristics of heat waves in EURO-CORDEX climate models

Heat waves are regarded as one of the major hazards in relation to climate change but their climate change scenarios are laden with uncertainties due to the inability of RCMs to simulate their driving mechanisms properly. We aim to advance in understanding of processes governing heat waves in climate models by analysing them as three dimensional (3D) phenomena.

The ensemble of 9 EURO-CORDEX regional climate models (RCMs) with lateral boundary conditions provided by the ERA-Interim reanalysis is analysed over Middle Europe in the 1989–2008 period. We apply a novel approach to classify heat waves as the 3D phenomena and evaluate capabilities of the RCMs to reproduce their characteristics. Heat waves are defined based on positive mean daily temperature anomalies from the 95th percentile in near-surface, 850 hPa, and 500 hPa levels with temporal and spatial criteria imposed. Based on predominant locations of positive temperature anomalies we classify heat waves into four types: i) near-surface, ii) lower-tropospheric, iii) middle-tropospheric, and iv) omnipresent.

We find that all RCMs overestimate the number of heat wave days, especially of the near-surface and lower-tropospheric types. By contrast, they underestimate the occurrence of omnipresent heat waves which indicates their inability to reproduce properly processes leading to the vertically extensive heat waves.

Since large-scale atmospheric circulation is an important driver in heat waves development, we examine which circulation types are favourable and non-favourable to heat waves in the RCMs. We also analyse moisture characteristics of air masses linked to the days of heat waves. RCMs outputs are validated against the reference datasets (ERA5, EOBS) and by comparing their atmospheric circulation and humidity characteristics we explain some of the models’ biases in reproduction of the frequency and other characteristics of heat waves types.