Daytime and Nighttime Heat Waves in Poland — Compound and Individual Events

The daytime (WD) and nighttime (WN) heat waves are defined as lasting at least three consecutive warm days and warm nights, respectively. These were calculated for late spring and summer (MJJA) from 1966 to 2024, when the 90th percentile, determined using a 15-day window, was exceeded. Temperature data was obtained from the Institute of Meteorology and Water Management National Research Institute (IMWM-NRI). The compound WD and WN were identified when these events overlapped for at least one day. During the study period, the number of WD and WN events increased. In the case of the former, the most significant increase started in the middle of the 90s, and in the latter, in the early 21st century. A notably higher trend is observed for the compound (of 1.2 events per decade) compared to individual events (0.7 events per decade for WN and 0.3 events per decade for WD). Until 2007, individual WD events dominated, especially from 1992 to 2006. Since then, compound events have become more prevalent, mainly due to a significant increase in the number of accompanying WN.

There are important differences in the background of compound and individual WD&WN event formation. To characterise these, we used the ERA5 reanalysis (geopotential height at 850 hPa, latent and sensible heat fluxes, specific humidity, mean surface direct short-wave radiation and mean top net long-wave radiation fluxes, total cloud cover, maximum and minimum temperature), for two terms, 03UTC and 15UTC, which correspond to WD and WN, respectively. In the afternoon, the compound WD&WN events occurred when the high-pressure system was located much closer to the northeastern border, associated with much higher direct short-wave and top net long-wave radiation flux and less cloud cover than during individual events. These conditions lead to considerably higher temperatures and lower latent heat flux anomalies. The specific humidity is above normal but still lower than in the case of individual WD events. At night, for individual events, the high-pressure center occupied eastern Poland and is associated with much lower cloud cover and higher top-of-atmosphere long-wave radiation flux. The minimum temperature and specific humidity increase, but not to the extent seen during compound heat waves. There are no notable differences between sensible and latent heat fluxes, both of which are slightly above average.

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