New insights into the world's longest series of monthly snowfall (Parma, Northern Italy, 1777–2018)

Abstract

The emergence of decreasing trends in snowfall frequency and snow depth highlights the challenges arising from shifts in snow regimes. In particular, snow-dependent southern Europe regions may be negatively impacted by such changes. Snow regimes strongly influence the water availability in reservoirs and groundwater. This study presents the world's longest series of monthly snowfall (fresh snow depth, snow days per year, days with snow on the ground) for the Parma Observatory, northern Italy (44 48 0 N and 10 19 0 E) between 1777 and 2018. These datasets were collected over the centuries using early rain gauges and recently, SIAP tipping bucket models. Sources of inhomogeneity were identified and analysed through the study of metadata and of nonparametric tests on datasets. Homogenized time series were obtained after correcting the observed snowfall data for the instruments response and localization characteristics, for the observing practices with the help of the standard normal homogeneity tests. The Buishand and the Mann-Kendall tests were further applied to check the correction and detect possible monotonic trends. Over the study period, days per year of snow decreased after the change point detected in 1897, in association with a rise in surface air temperature (including a distinct urban warming trend). In this study high numbers of snowy days per year (hereafter referred as snowfall frequency) and snow depth values during the latest phase of the LIA at Parma are consistent with a cycle of minimum solar activity, suggesting that enhanced, solar-induced blocking activity dominated, with the arrival into the Mediterranean of large cold air masses developing over Siberia and northern Europe. Over the last century, snow appears to have resonance-like characteristics as similar trends were observed over the Northern Hemisphere, where the extent of the snow cover has been reported to markedly decline in the transition spring and autumn seasons.

Empirical modelling of snow cover duration patterns in complex terrains of Italy

“…The first 34 years are from the BNV, as derived from historical records, which are mostly mirroring extreme occurrences, i.e., years for which snow remained on the ground over many days. This is the case for the 16 years, which were characterised by snowfall events of remarkable impact for society and the agriculture (Diodato et al 2019(Diodato et al , 2020a(Diodato et al , c, 2021. Snow cover duration was not obtained for other years, for which evidence of snowfall occurrence is not reported in written documents prior to 1973.…”

“…), snow cover observations are available since 1938 (Diodato et al 2020a). Though occurring in the plain, snowfall at Parma reflects the important geographical and climatic complexity of this area, placed at the separation between the greatly snowy pre-Alps (to the north) and the Apennines (to the south), and the limited accumulations of snow to the east, where climate is less continental because of the proximity of the Adriatic Sea (Diodato et al 2021). This makes the occurrence of snow events not easily predictable because mountain ranges create different orographic effects by hindering the movement of air masses (Enzi et al 2014).…”

“…According to Stoppani (1876), between 1797 and 1806 the total number of days with snow in Milan had been 243 (i.e., 26 on average per year), but the situation had already substantially changed from 1857 to 1876 with a total of 166 snow days (i.e., eight snow days per year). Extensive evidence reveals in fact that Earth's snow cover and snow amount have been decreasing, especially in the Northern Hemisphere, where this declining trend is expected to affect climate variability by the albedo effect (Beniston 1997(Beniston , 2003Mote et al 2018;Brown et al 2017;Fontrodona Bach et al 2018;Pulliainen et al 2020;Diodato et al 2021). Although the importance of snowfall and snow cover on climate had already been highlighted in the nineteenth century (Woeikof 1885), it is only recently that the associated impacts on human livelihood, economy and ecosystems have been considered within the context of contemporary global warming (e.g., Xu et al 2009;Beniston 2012) or studied for historical times (e.g., Ljungqvist et al 2021).…”

“…The physician Salvatore De Renzi (1800-1872), in his report on the medical topography of the Kingdom of Naples in the nineteenth century (De Renzi 1829, p. 170), wrote "The snow falls in November above the inland mountains, and within the higher valley, and snow-cover lasts up to end of May." Today's emerging negative trends in snow depth and cover days suggest that snow-dependent regions in southern Europe could be affected by these changes because the number of snow days per year and the days with snow on the ground (DSG, day year −1 ) determine soil processes and water flow in rivers, streams, lakes and reservoirs (Diodato et al 2019(Diodato et al , 2020a(Diodato et al , c, 2021. The Italian territory (the focus of this study, excluding the islands) is roughly delimited to the north by the alpine range (around 46° N) and its southern-most point is roughly 38° N, divided into east and west by the Apennine mountains.…”

Multi-Decadal Variability in the Snow-Cover Reconstruction at Parma Observatory (Northern Italy, 1681–2018 CE)

“…Centrally located in the Po River Basin (PRB), the Parma OBS started weather observations on 1777 thanks to the local Jesuit community with the support of the University of Parma, but only in the 20th century regular and continuous daily snowfall measurements were recorded. Diodato et al (2020a)…”