Characterization of snowfall events in the northern Iberian Peninsula and the synoptic classification of heavy episodes (1988–2018)

Abstract

Historic snowfall events in the northern Iberian Peninsula recorded between 1988 and 2018 are presented and analysed. This study makes use of data collected over a course of 31 years from 105 observation stations. These weather reports describe the temporal and spatial characteristics of five Spanish provinces facing the Cantabrian Sea. The average number of snow events observed per year (as recorded by all 105 stations) was 133, where a maximum of 421 snow events was recorded in 2010 and a minimum of 24 events were recorded in 2002. In addition, the monthly distribution of snow events per day had a maximum of 630 events, (February), with a mean monthly value of 170 snow events. Other features like the distribution of snow events depending on the altitude of each province studied and the corresponding spatial patterns are also shown. Furthermore, the circulation patterns responsible for heavy snowfall in the region were also examined. To carry out this study, we considered the daily patterns at 1,200 UTC of the geopotential height at 500 and 850 hPa pressure levels and sea-level pressure and temperature at 500 and 850 hPa, respectively. The synoptic situations were classified based on a principal component analysis coupled with a K-means clustering, and four groups associated with heavy snowfall events were subsequently identified. The analysis of the daily synoptic patterns showed that a trough was present over the Iberian Peninsula, or close by, and a low appeared over the Mediterranean Sea or Central Europe. The low-level flow was from the north (N) or northeast (NE) in 85% of the cases and the temperature at 850 hPa pressure level was lower than −3 C in 70% of the cases.

The unusually long cold spell and the snowstorm Filomena in Spain in January 2021

“…In winter, the Mediterranean receives its largest fraction of annual precipitation when it is frequently influenced by extratropical cyclones (Trigo et al, 1999;Wernli and Schwierz, 2006). In this region and season, cyclones and fronts are often involved in extreme precipitation events (Field and Wood, 2007;Pfahl and Wernli, 2012;Catto and Pfahl, 2013), however, they rarely lead to extreme snowfalls due to climatologically warm temperatures in the Mediterranean (Tayanç et al, 1998;Llasat et al, 2014;Gascón et al, 2015;de Pablo Dávila et al, 2021). In the region of Madrid, precipitation typically occurs as rain also in January, due to surface temperatures above the freezing level (DWD, 2021).…”

“…Cold air is typically quite dry and therefore advection and lifting of slightly warmer and moister air, ideally above the still very cold near-surface layer, is needed to produce large amounts of snow. Heavy snowfall events in the Iberian Peninsula are associated with an upper-level trough over southwestern Europe and a surface cyclone located either over the Mediterranean Sea or central Europe (Esteban et al, 2005;de Pablo Dávila et al, 2021). This large-scale pattern facilitates the advection of anomalously cold air from the north near the surface and of warm and humid air from the south towards the snowfall area above the cold layer (Llasat et al, 2014;Gascón et al, 2015).…”