Analysis of the surface energy balance and its impacts on the extreme low temperatures in Spain after snowstorm Filomena

Storm Filomena, in January 2021, was one of the most extreme snowstorms of the last century in Spain, blanketing wide areas across central Spain in exceptional amounts of snow. Once the storm left the Iberian Peninsula, the snow layer persisted for many days due to stationary stable synoptic conditions, leading to extremely low surface temperatures, with several records being hit. The storm caused major disruption and economic impact.

Despite Filomena being an extreme weather event, not many studies have addressed the cold spell once the low pressure system left the Iberian peninsula. In this work, we analyse the physical mechanisms that explain such low temperatures, studying the influence of different scales (synoptic and microscale) by means of reanalysis data and data analysis of a micrometeorological weather station in a rural area in central Spain, where snow accumulation exceeded 40 cm. Results suggest that the stable synoptic-scale weather conditions led to enhanced influence of the microscale and strong surface-based temperature inversions. In particular, significant changes are observed in the surface energy balance (SEB) due to a reduction in the net radiation available at the surface as a result of a higher albedo and emissivity of the surface, with respect to the period before the arrival of Filomena. The effect of the snow layer is also clearly visible on the soil temperatures and ground heat flux, which remained essentially constant throughout the period of study as a result of its lower thermal conductivity. Thus, all the terms involved in the SEB will be thoroughly analysed to study the influence of the snow layer on them and on the surface temperatures reached.