Observed and projected changes in the diurnal temperature range at European cities with different base climate

Many regions across the globe have been witnessing changes in both mean climate and climatic extremes during the last decades. Cities in particular, where the concentration of urban population is high, have been in the spotlight of scientific research seeking to address climate-related issues. At the same time, climate change is expected to largely impacts cities until the end of 21^st century. However, changes in air temperature levels, as an essential element of the climate, are probably not uniform nor of the same rate across different areas of the globe. An important indicator of the global climate change is the diurnal temperature range (DTR), defined as the difference between the daily maximum and daily minimum air temperature, thus reflecting the temperature variation within a day.

This study analyses the distribution and long-term trends in DTR in seventeen European cities of different base climate. The response of DTR under exceptionally hot weather has also been investigated. The study uses observed and projected data of daily maximum and minimum air temperature over the periods 1961-2019 and 1971-2100, respectively. The projected data, over the studied cities for the closest land grid point to the stations’ location, were derived from the Regional Climate Model (RCM) RCA4 of the Swedish Meteorological and Hydrological Institute driven by the Max Planck Institute for Meteorology model MPI-ESM-LR, with the simulations carried out in the framework of the EURO-CORDEX modeling experiment. The projected data were bias adjusted applying the empirical quantile mapping technique. Future simulations were based on two climate scenarios, the Representative Concentration Pathways (RCPs) 4.5 and 8.5.

The distribution of the DTR frequency based on observations shows a similar pattern in cities that share the same background climate, while is clearly differentiated at diverse climate types. The mean DTR for normal summer days (maximum air temperature lower than 95 percentile) ranges between 8.0 and 10.5 ^oC for all cities, with the exception of Nice that shows lower mean DTR and Nicosia, Madrid and Bucharest that present higher. The change of the mean DTR between summer normal days and hot days (maximum air temperature higher than 95 percentile) is greater for cities in higher latitudes, while it is smaller for cities in lower latitudes. According to the projected data for the period from 1971 to 2100 under the RCP4.5 scenario, a statistically significant decreasing trend in mean DTR/yr is projected for the cities in the highest latitudes (Oslo, Stockholm and Helsinki), suggesting higher increasing rates in the minimum air temperature compared to the maximum air temperature. At the same time, the opposite result is expected in Madrid (statistically significant increasing trend), while no statistically significant trends in mean DTR are projected for the rest of the cities.