Czech Hydrometeorological Institute

Study region: This study examines catchments in the Czech Republic that have generated flash floods in the past, with a focus on hydrological response and physiographic parameters. Study focus: The hydrological response of events with peak discharges exceeding the one-year return period during the summer half-year was evaluated using the flashiness index. Catchments were categorized into clusters I–III based on physiographic parameters, employing principal component analysis and k-medoids clustering. To evaluate Czech flash floods, a descriptive flashiness metric was computed for both the Czech and European flash-flood datasets, enabling cross-regional comparison. New hydrological insights for the region: The results revealed an increase in 1-h flashiness during the recent period from 2018 to 2023 compared to 2005–2010, observed across all three clusters. The highest flashiness values were recorded in a group of small, steep catchments characterized by high terrain roughness, maximum elevations, a dense river network, and compact shape. A comparison of flash floods in the Czech Republic with those in Europe and the Mediterranean indicated that Czech flash floods generally exhibit lower unit peak discharge and 1-h flashiness values, although they can occasionally reach extreme intensities significant within the European and Mediterranean contexts.

Rime ice is an effective winter ambient air pollution accumulator. Due to its higher ion content as compared to snow it is a non-negligible contributor to atmospheric deposition fluxes with potential environmental consequences, particularly in mountain regions. Here we explore spatio-temporal patterns of rime formation as a proxy for the propensity of individual sites to form rime ice. We present the recent time trends in rime ice occurrence and thickness measured by 23 professional meteorological stations in the Czech Republic in 2002–2023. In an exploratory data analysis, we found high year-to-year variability in rime occurrence and thickness at all sites. According to the annual mean number of hours with rime detected, the stations situated at the highest altitudes are significantly different (higher) from the rest of the sites. The highest rime hour and thickness records by far were observed at the LYSA station in the Beskydy (Beskid) Mts situated at the exposed mountaintop and highly elevated above the surrounding terrain. For advanced statistical modelling of rime thickness, we used two generalised additive models that account for long-term trends (potentially nonlinear), seasonal and daily variability. In an expanded model we further considered the effect of the North Atlantic Oscillation (NAO) index. All the parameters included in the models proved to be statistically significant, although the strength of their effect differed. Factors affecting the rime formation (meteorology and terrain) are strongly site-specific and identification of the significance of individual influencing factors remains a challenging task for our future research. Here, we explore a rare long-term rime record with detailed temporal resolution from multiple uniformly measured sites, which significantly enhances our understanding of rime formation. Additionally, the rime record is from a temperate zone, where rime forms only during a small part of the year.

The actual method of assessing dispersion conditions has ceased to meet current knowledge about their influence on the level of pollutant concentrations, and there is a need to find a simple and more accurate tool for assessing and predicting dispersion conditions, especially with a focus on suspended PM10 particles. Based on the requirements of the forecasting departments of the Czech Hydrometeorological Institute, the methodology for defining dispersion conditions was revised and subsequently updated in 2024. The new methodology is based on the calculation of the ventilation index that has been part of the ALADIN model since 2013. It is based on the intervals of PM10 concentration deciles and the corresponding ventilation index thresholds assigned to them, defining individual classes of dispersion conditions. The threshold values of individual classes of dispersion conditions correspond to the median values in the relevant decile classes (rounded to hundreds). Verification of the methodology on data not used in the classification design shows that the proposed classification is logical, functional, and applicable for routine operation. Compared to the previous classification, four classes of dispersion conditions are now proposed, namely poor, moderately poor, good and very good dispersion conditions. Dispersion conditions are essential for assessing the level of air pollution, therefore it is necessary to pay sufficient attention to the retrospective assessment of dispersion conditions from a long-term perspective. For this assessment, a comprehensive PTRP parameter was proposed within the individual classes of dispersion conditions, including a verbal assessment of deviations from the normal 1991–2023, using ventilation index data from the reanalysis of the ALADIN model.

This study assesses trends in the total ozone column (TOC) and the atmospheric factors influencing ozone variability at three Antarctic stations (Marambio, Troll/Trollhaugen, and Concordia) from 2007 to 2023. Ground-based TOC measurements were used, supplemented by satellite observations from the Ozone Monitoring Instrument on NASA's Aura satellite. TOC trends were derived using a multiple linear regression model provided by the Long-term Ozone Trends and Uncertainties in the Stratosphere (LOTUS) project. The selected LOTUS model was able to explain 94 %–97 % of the TOC variability at all three stations. The regression analysis showed that ozone variability at these stations is mainly driven by the lower stratospheric temperature, eddy heat flux, and the Quasi-Biennial Oscillation. A statistically significant increasing trend was found at the Marambio station (3.43 ± 3.22 DU per decade), while statistically insignificant trends were detected at the other two stations. Using MERRA-2 reanalyses, the LOTUS model was applied to each grid point in the 40–90° S region, which effectively illustrates the spatial distribution of the impacts of individual predictors. It was found that warmer conditions in the Antarctic stratosphere in September 2019 caused TOC to be up to 100 DU higher than normal, especially over East Antarctica. The results improve understanding of regional TOC trends and how the Antarctic ozone layer responds to changes in ozone-depleting substances.

Between January 23rd and 29th there were low stratus clouds above most of the Czech Republic. Light freezing drizzle was falling, even though the entire cloud layer was within a temperature range below 0 °C. The drizzle was hard to detect, all the forecasters often relied on observation from professional weather stations and eyewitness reports from the public on social media. However, locally there were major complications, especially in transport. This freezing precipitation was caused by the so called SWRP (Supercooled Warm-Rain Processes). A bit unusual precipitation development on such a large scale and its complicated detection were a true challenge for forecasters on duty, and showed the importance of cooperation between professionals, amateurs and public. Also observations from professional weather stations showed their irreplaceability.