Compound daytime and nighttime high-temperature extremes in Poland

Since the maximum temperature increased in Poland significantly, more prolonged heat waves are expected in summer. Long-lasting heat waves create high thermal stress not only in the human body but also in the agriculture and energy systems. Far more dangerous is the case when very high temperature during the day is followed by high temperature during the night. This may pose a real challenge for the human thermoregulation system, which can not fully recover even at night after daytime heat stress. It is proven that hot day and night events are more harmful to the human body when they occur as compound events, but most of the studies considered warm daytime or nighttime events separately. The main issue of this study is to detect trends in the occurrence of warm spells during the day and night either separately or inseparable. Furthermore, these events are also investigated under different mechanisms that can favour their appearance. First of all, the circulation patterns under each event are analysed. Then, the humidity and radiation conditions are described for warm daytime or nighttime events and compound events. To distinguish the daytime and nighttime warm-spell, the maximum and minimum temperature is used. These events are selected based on the 90^th percentile threshold in the 15-day window. All datasets used in the study were acquired from the Institute of Meteorology and Water Management—National Research Institute (IMWM-NRI) and ERA5 Reanalysis for the period from 1966 to 2024.

This study is funded by the National Science Center (NCN) (grant number 2023/51/B/ST10/01926).