Projected changes of heatwave occurrences and characteristics in Hungary using raw and bias-corrected EURO-CORDEX simulations

A clear sign of climate change is the increased frequency of certain weather and climate extremes, which trend can be observed in the South-Eastern European region, including the Carpathian Basin. Heatwaves are among those weather related phenomena that pose serious health risks, being not only dangerous to the human body but also to natural ecosystems and various sectors of the economy. These periods are expected to occur more frequently, with greater intensity and longer duration in the future, in addition, heatwave days may appear earlier in the year and become more frequent after the end of summer. 

In this study a heatwave detection method is introduced based on daily mean temperature thresholds optimalized for Hungary. Three main characteristics are counted: duration, intensity and the highest daily mean temperature during the heatwave originally performed on daily measurement data series of a given station. This method was implemented for fine scale regional climate model (RCM) simulations through using grid cell averages for the location of interest. For this research 5 RCMs of the EURO-CORDEX initiative were chosen (CCLM, HIRHAM, RACMO, RCA, REMO) available at a horizontal resolution of 0.11° following the RCP8.5 scenario. Beside the raw simulations, different bias-corrected versions of the above-mentioned RCMs were analysed, namely: the RCM projections available from the EURO-CORDEX initiative using MESAN as reference data, the FORESEE-HUN database and BC-HUCLIM which is a set of bias-corrected simulations created specifically for this research. For the latter, the internationally widely used percentile-based quantile mapping method was applied for bias correction using the quality controlled HuClim data as reference. The process was carried out on a monthly scale for each raw simulation separately.

The results are presented for two cities: Budapest, the capital of Hungary, and Szeged, the third populous city in Hungary,  located in the south-eastern part of the country. The average heatwave characteristics are calculated for the reference period 1976-2005, and the changes are expressed for the averages of two future time slices (2021-2050 and 2070-2099). In addition, the occurrence of heatwave days are analysed throughout the year according to the simulations from the different databases. Our results indicate that the average intensity of heatwaves may increase to a greater extent in the south-eastern area of Hungary, and the duration may increase by 3-9 days on average by the end of the 21st century. Under the RCP8.5 scenario, heatwave days are projected to occur between late April and early October at least once out of 30 years during the period 2070-2099.