Comparison of UTCI between the ERA5 reanalysis and the ALADIN model for Czechia

The UTCI (Universal Thermal Climate Index) combines selected meteorological elements (air temperature, wind, humidity, and radiation) into a single value (in °C) describing level of thermal stress on humans. Computation of UTCI is implemented in several datasets, comparison of two of which is presented here. The ALADIN is NWP (numerical weather prediction) model commonly used for forecasting; operational calculation of UTCI has started in 2019. In contrast, the ERA5 reanalysis is based on observed data spanning period from 1979 to 2021. Therefore, the aim here is a comparison of datasets different in their nature: meteorological model with reanalysis. Results can also be interpreted as a verification of UTCI in ALADIN. The analysis is conducted for years 2019 and 2020 due to the availability of data. After interpolation of both datasets on the same regular grid (resolution is 7.1 km) covering the Czech Republic, datasets are compared from both temporal and spatial perspectives. Subtraction of UTCI between ERA5 and ALADIN is made at every gridpoint in hourly step. Hourly differences are consequently aggregated in daily, monthly, and seasonally step. Considering seasonally and monthly differences, no substantial changes are evident; interesting results emerge when analyzing single hourly data (00-24): it is shown that the UTCI in ALADIN is overestimated in the morning (1-2 °C) whereas the opposite is found for the afternoon and early evening (1-3 °C) in all seasons.. Spatial differences are, however, mostly small at majority of gridpoints, although underestimation of UTCI in ALADIN is detected in higher altitudes (above 500/600 m), where differences at some gridpoints reach up to 6.5 °C. Differences are inherent to orography, owing to coarse grid resolution of ERA5 (30 km) compared to ALADIN (2.3 km). Flattened orography mask in ERA5 reflects in larger differences in higher altitudes.