Influence of the extraordinary Filomena snowstorm on the surface energy balance and its relationship with the subsequent very-low surface temperatures

Filomena snowstorm impacted a large part of the Iberian Peninsula during January 2021, especially affecting the central part of Spain, and covering the Madrid region with up to 50 cm of snow.

In this work, we analyse data from a micrometeorological station (GuMNet-Herrería) placed in a rural environment in the northwest of the Community of Madrid. The objective is to determine the influence of the microscale on the extremely low temperatures reached after Filomena's passage, as well as to better understand the physical processes associated with these extreme conditions.

Although the snowfall lasted only for 2 days, the snow was present on the surface for more than 10 days, already with stable synoptic conditions, accompanied by a weak turbulent transfer between the surface and the lower part of the atmospheric boundary layer. These days after, extremely low temperatures were recorded, reaching minima of up to -26.5 ºC in the central part of the Iberian Peninsula. The presence of snow on the surface modified significantly the components of the surface energy balance (SEB). This included a reduction in the available net radiation at the surface as result of increased albedo and emissivity of the surface, compared to standard conditions (previous period to Filomena), as well as in the turbulent fluxes, contributing to some records of minimum temperatures. The persistence of the snow cover also affected the soil temperatures, keeping the ground heat flux practically constant.