Rime ice formation: Insight into time trends over the last two decades based on observations in a Central European country

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.