Attribution of fast changes in UTCI to synoptic systems

The UTCI index determines the impact of weather on human by combining four meteorological variables, that is, temperature, wind speed, humidity, and mean radiant temperature. In this contribution, the causes of fast changes in the UTCI index in the Czech Republic are assessed. We utilized the Aladin Reanalysis, which covers large area of Europe. The reanalysis has two advantages. Firstly, the 2.3 km resolution provides great detail on topography, and secondly, the UTCI is computed in the post-processing stage at hourly intervals. Hence, there is a possibility to examine the changes in the index in a subdaily time step. We calculated changes in the index in 12-hour intervals in the period 1991-2020 for separate seasons. The largest changes, reaching up to 30 °C, are detected in winter and spring, while smaller values being found particularly in summer. We find out that the passing of fronts accounts for a substantial portion of large changes in the UTCI index. For instance, cold fronts, responsible for the inversion disruption, may lead to a substantial increase in UTCI during the night in winter. Yet, in other cases, cold fronts accompanied by stronger winds may also lead to a substantial decrease in UTCI. Fast changes in UTCI in summer are always connected with fronts; we observe that increase (decrease) in UTCI is linked with warm (cold) fronts. Anticyclonic conditions take part in increase in UTCI on morning and before noon, particularly in cold months. In such situations, we observe that the growing MRT reflects in rise in UTCI. This work contributes to operational meteorology by describing the impact of subdaily weather changes on humans.