Manara et al. (2025) MODIS (2001-2022) snow cover variability over the Italian territory

Identification

• Journal: Journal of Hydrology Regional Studies
• Year: 2025
• Date: 2025-10-30
• Authors: Veronica Manara, Cecilia Delia Almagioni, Guglielmina Diolaiuti, Maurizio Maugeri, Davide Fugazza
• DOI: 10.1016/j.ejrh.2025.102895

Research Groups

• Department of Environmental Science and Policy, University of Milan, Italy
• Department of Physics “Aldo Pontremoli”, University of Milano, Italy
• Institute of Atmospheric Sciences and Climate, CNR, Bologna, Italy

Short Summary

This study analyzes the spatial distribution and temporal evolution of snow cover variables (duration, onset, end) across Italy from 2000-2022 using MODIS data, complemented by ERA5-Land reanalysis for longer-term trends (since 1950s), revealing a strong elevation dependence and a significant overall negative trend in snow cover extent, particularly in the Alps.

Objective

• To analyze the spatial distribution and temporal evolution of snow cover variables (duration – SCD, onset – SOD, end - SED) over the entire Italian territory, including the southern Alps and Apennine chain, between 2000 and 2022 using MODIS data.
• To provide a climatology of snow cover duration and discuss its relationship with geographical parameters.
• To evaluate longer-term trends in snow cover extent since the 1950s using ERA5-Land reanalysis data.

Study Configuration

• Spatial Scale: The entire Italian territory, including the southern part of the Alps and the Apennine chain, divided into 5 subregions (Alps, Apennines West, Apennines East, Sicily, Sardinia). Data resolutions: 500 meters (MODIS), 30 meters (ALOS AW3D30 DEM), 0.1° x 0.1° (ERA5-Land).
• Temporal Scale: MODIS data from 2000 to 2022. ERA5-Land data from 1950 to 2022. Hydrological year defined from 1 October to 30 September.

Methodology and Data

• Models used: ECMWF-ERA5 Land reanalysis dataset.
• Data sources:
  • MODIS Terra Snow Cover Daily L3 Global 500 m SIN Grid, Version 61 (MOD10A1 V61)
  • MODIS Aqua Snow Cover Daily L3 Global 500 m SIN Grid, Version 61 (MYD10A1 V61)
  • ALOS Global Digital Surface Model “ALOS World 3D-30m (AW3D30)” Digital Elevation Model (30 m resolution)
  • ECMWF-ERA5 Land reanalysis dataset (0.1° x 0.1°)
• Processing:
  • Merging of MODIS Terra and Aqua products, reprojection to WGS84 coordinate system.
  • Classification of pixels based on Normalized Difference Snow Index (NDSI > 0.4 for snow, < 0 for snow-free).
  • Gap filling methodology combining six steps: combination of Terra/Aqua products, temporal combination of adjacent dates, snow transition elevation, spatial interpolation, combination of spatial interpolation with elevation of surrounding pixels, and temporal interpolation.
  • Calculation of Snow Cover Duration (SCD), Snow Onset Date (SOD), and Snow End Date (SED) for each hydrological year.
• Analysis:
  • Multilinear regression and linear regression to investigate dependence of snow cover metrics on geographical parameters (elevation, aspect, slope, latitude, longitude).
  • ANOVA type I SS analysis to determine variable importance.
  • Sen-Theil method for quantifying temporal trends.
  • Mann-Kendall non-parametric test for evaluating trend significance.

Main Results

• Spatial Distribution: Snow Cover Duration (SCD) shows a strong dependence on elevation, explaining 76% of the variance at the Italian scale. However, high variability at each elevation highlights the influence of other parameters (e.g., slope, aspect, longitude, latitude).
• Regional Differences in Spatial Variability: The variance explained by geographical parameters is highest in the Alps (>86%), followed by Apennines East (~70%), Apennines West (~50%), and lowest in Sicily and Sardinia (30-40%). Slope explains 24-30% of variance at the Italian scale, and latitude 14-19%.
• Interannual Variability (2001-2022): SCD exhibits very high interannual variability across Italy.
• Temporal Trends (2001-2022, MODIS):
  • A significant negative trend in SCD (between -5.1 and -0.6 days per decade, p < 0.1) is observed at elevations > 3500 meters above sea level (m a.s.l.) over Italy, primarily driven by the Alps.
  • Snow End Date (SED) shows a significant negative trend (between -5.7 and -0.5 days per decade, p < 0.1) for elevations > 3000 m a.s.l. in Italy and the Alps.
  • Snow Onset Date (SOD) shows a significant positive trend (0.2-0.3 days per decade, p < 0.05) in the elevation band between 4000 and 4500 m a.s.l. in the Alps.
  • In the Apennines, SCD shows significant negative trends (between -6.2 and -9.9 days per decade, p < 0.05) for elevations between 500 m and 1500 m a.s.l.
• Longer-term Trends (1950-2022, ERA5-Land):
  • A strong agreement (correlation = 0.91) is found between MODIS SCD and ERA5-Land Snow Cover Extent (SCE) temporal evolution.
  • The ERA5-Land SCE series reveals a significant decreasing trend of approximately -4% per decade over the Italian territory since the 1950s, mainly attributed to the Alpine region.

Contributions

• Provides an updated and comprehensive climatology and trend analysis of snow cover variables (SCD, SOD, SED) for the entire Italian territory, addressing a gap in literature for central and southern Italy.
• Quantifies the relative importance of various geographical parameters (elevation, slope, aspect, latitude, longitude) in explaining snow cover spatial variability across different Italian regions and elevation bands.
• Extends the temporal analysis of snow cover trends back to the 1950s by validating and integrating MODIS data with ERA5-Land reanalysis, offering a longer-term perspective on snow cover changes in Italy.
• Highlights the socioeconomic implications of observed snow cover changes for agriculture, human consumption, tourism, and industry in Italy.

Funding

• Research Funds from the Italian Ministry for University and Research (PRIN 2022-CN4RWK-CCHP-ALPS-Climate Change and HydroPower in the Alps, funded by the European Union (Programme Next Generation EU)).
• Ministero dell’Università e della Ricerca of Italy [grant FSE-REACT EU, DM 10/08/2021 n. 1062] (for Veronica Manara).

Citation

@article{Manara2025MODIS,
  author = {Manara, Veronica and Almagioni, Cecilia Delia and Diolaiuti, Guglielmina and Maugeri, Maurizio and Fugazza, Davide},
  title = {MODIS (2001-2022) snow cover variability over the Italian territory},
  journal = {Journal of Hydrology Regional Studies},
  year = {2025},
  doi = {10.1016/j.ejrh.2025.102895},
  url = {https://doi.org/10.1016/j.ejrh.2025.102895}
}