Investigating the relationship between compound cold events and impacts on the Scottish rail sector.

Winter weather events can result in costly damages and severe disruption to affected regions. While compound events research has strongly focused on heat-related events, less focus has been placed on extreme cold hydrometeorological hazards. Cold events impact a range of sectors from energy and agriculture to transport and health. The rail sector is particularly sensitive to cold weather hazards resulting in service delays and cancellations. Snowfall can lead to blocked tracks, points failures and issues with electricity supply. Loss of traction, braking issues and frozen infrastructure can arise from ice formation. The impacts of these cold events can be amplified by the compounding effect of another meteorological variable, such as whether heavy precipitation is present or not, with subsequent impacts dependent on the nature of the cold event. For example, a cold-wet event could incur heavy snowfall, whereas a cold-dry event could result in extreme low temperatures and icy conditions. In this study, we analyse the occurrence of 10,000 rail incidents in Scotland over an extended winter period of October to March for 2006-2023 to investigate the relationship between impacts and compound cold events. We use an impact dataset from Network Rail to categorise high-impact days based on two classifications: (1) days with the highest number of aggregated incidents; (2) days with the highest number of accumulated customer minutes lost. Using daily gridded observations from HadUK-Grid at a 5 km resolution we then apply a localised percentile-based methodology to determine the occurrence of cold-dry and cold-wet events on these high-impact days. Initial results show that the majority of high-impact days consisted of incidents caused by severe snow and icing. Analysis reveal that these incidents occurred under hydrometeorological conditions that can be classified as cold-dry and/or cold-wet events. These findings highlight the importance of considering co-occurring hazards rather than single hazards. The results of this study provide a useful insight into compound cold events for rail sector early warning systems, with valuable information on cold weather event hazard characterisation and their associated impacts across varying timescales.