Rain-on-snow events in Romania: climatology and trend analysis using observation data (1961-2021)

Seasonal snowpack plays a significant role in the climate system. Its characteristics are monitored as climate change indicators considering the snow importance from ecological, hydrological, and economic perspectives. Observation data provides evidence of declining depth, spatial extent and duration of the snow cover, in connection to changes in seasonal air temperature and precipitation, especially in the lowlands and mid-elevation mountain areas. However, these changes underpin the occurrence of compound hazards resulting from the joint actions of rain and snow (rain-on-snow events - ROS). Such hazards entail a wide range of physical and socioeconomic impacts related to the complex processes occurring on and within the snowpack, which influence the occurrence of floods, landslides and snow avalanches. In this context, this study aims to provide a baseline understanding of ROS  climatology and how it changes in the present climate conditions. Our work investigates the spatial and temporal changes in the rain-on-snow (ROS) events over the 1961-2021 period, based on 114 weather stations in Romania. This snow hazard is considered in the analysis as it is responsible for important socioeconomic impacts in different sectors (e.g., tourism, forestry, transport). The ROS event detection and trend analysis were conducted at annual and seasonal scales. 

The results indicate that ROS events occur mainly in winter months in the lowlands (below 1,000 m) and in late fall, winter and spring in the mountains (above 1,000 m). Overall, in the Romanian Carpathians, the highest ROS frequency is in the December-April interval, but at the highest elevation, such events occur throughout the entire year. The years of maximum ROS frequency ( >60/year) were 1962, 1966, 1981, and 2021 and were recorded in the mountain areas. Observation data indicate a decreasing frequency of ROS events, especially in the lowlands, due to the significant changes in the solid/liquid precipitation ratio (more frequent liquid precipitation than solid) and the decline in snow cover duration in response to seasonal warming. In the mountains, the observed change signal in ROS frequency is mixed, showing both decreases and increases. The high elevation areas (above 1,800-2,000 m) register the highest number of ROS events yearly (over 35-40), where the snow avalanche release areas are mostly located. 

Acknowledgements

This research received funds from the project “Cross-sectoral framework for socio-economic resilience to climate change and extreme events in Europe (CROSSEU)” funded by the European Union Horizon Europe programme, under Grant agreement n°101081377.