Explaining trends in UTCI in the Czechia between 1991 and 2020

The increase in thermal stress on human has been recently documented due to changing climatic conditions, mainly in the lowlands and the urban areas. The future evolution of the climate, as predicted by various scenarios, also proves that thermal stress on humans will further increase. Recently developed Aladin-Climate reanalysis for the broad area of Europe involves computation of the UTCI index (Universal Thermal Climate Index) at regular gridpoints. The advantage of the reanalysis is its resolution of 2.3 km and therefore the detailed orography mask, which is reflected in values of the UTCI, being substantially modulated by altitude. For separate seasons, the trends of UTCI are computed between 1991 and 2020. It has been shown that the most prominent increase is found in summer, mainly after 1990, probably due to rising temperatures. Insignificant increase of UTCI in autumn may be explained by increasing trends in cloudiness, affecting wind speed and the radiation balance on the surface. The UTCI trends are spatially inhomogeneous in separate seasons. Here, here we try to address factors causing discrepancies in the spatial distribution of the UTCI trends in individual seasons. For example, increasing thermal stress on human in northern and western parts of the country is detected in spring and autumn, whereas the increase in UTCI is concentrated in the lowlands in summer. The increasing temperature probably plays a major role, but changes in other factors associated with thermal stress, such as wind conditions and humidity, influence thermal stress as well. Those meteorological parameters are often tied to synoptic conditions, therefore, changes in surface pressure fields, front activity, etc. will be analyzed to uncover their influence on the spatial distribution of the UTCI trends.