Berghald et al. (2026) Climatology and trends of observed daily and hourly extreme precipitation in the French Alps

Identification

• Journal: Hydrology and earth system sciences
• Year: 2026
• Date: 2026-10-21
• Authors: Sebastian Berghald, Juliette Blanchet, Antoine Blanc, David Penot
• DOI: 10.5194/hess-30-6421-2026

Research Groups

• University Grenoble Alpes, CNRS, INRAE, IRD, Grenoble INP, IGE, 38000 Grenoble, France (Sebastian Berghald, Juliette Blanchet)
• Office National des Forêts – Service RTM de l'Isère, 38000 Grenoble, France (Antoine Blanc)
• KU Leuven, Department of Earth and Environmental Sciences, 3001 Leuven, Belgium (Sebastian Berghald)
• EDF-DTG, 38000 Grenoble, France (David Penot)

Short Summary

This study provides an updated climatology and trend analysis of daily and hourly extreme precipitation in the French Alps using long-term in situ observations and extreme value theory. It reveals distinct spatial and temporal patterns, with daily extremes showing positive trends in the south and negative in the north, while hourly extremes exhibit more variable, localized positive trends in high mountain ranges.

Objective

• To provide an updated climatology and trend analysis of daily and hourly extreme precipitation across the French Alps using long time series of in situ observations.
• To statistically confirm or refute the perceived increase in sub-daily extreme precipitation events reported by local stakeholders.
• To gain a deeper understanding of the dynamics underlying these trends by examining monthly and seasonal levels and utilizing an existing weather pattern classification.

Study Configuration

• Spatial Scale: French Alps, covering nine French departments (Savoie, Haute-Savoie, Isère, Drôme, Hautes-Alpes, Alpes-de-Haute-Provence, Vaucluse, Var, Alpes-Maritimes) and parts of Ain and Bouches-du-Rhône. The region is divided into Northern and Southern Alps based on a climatological divide.
• Temporal Scale:
  • Observations: Long time series of daily and hourly precipitation.
    • Météo-France: 89 stations (daily), 87 stations (hourly) with at least 25 years of data between 1980 and 2023.
    • EDF: 67 stations (daily and hourly) with data from 1980s to 2022. 30 stations have combined data from 1955-2022 (weekly maxima for earlier period), others 1980-2022. Longest series: 71 years.
  • Analysis Periods:
    • Stationary analysis: 1980–2023 (Météo-France & EDF, 25-42 years).
    • Trend analysis: 1985–2022 (EDF stations, 34-71 years), with a breakpoint at 1985.
  • Temporal Resolution: Daily and hourly precipitation maxima, aggregated to monthly, seasonal (January–March, April–June, July–September, October–December), and annual scales.
  • Return Levels: Focus on 20-year return levels.

Methodology and Data

• Models used:
  • Extreme Value Theory (EVT) based on block maxima (monthly, seasonal, annual).
  • Generalized Extreme Value (GEV) distribution for stationary analysis (location (μ), scale (σ), shape (ξ) parameters).
  • Non-stationary GEV model: Location (μ) and scale (σ) parameters allowed to vary linearly with time (t) after a breakpoint year (1985), while shape (ξ) parameter is kept constant.
  • Model selection: Akaike Information Criterion (AIC) to choose between models with trends in μ, σ, or both.
  • Parameter estimation: Generalized maximum likelihood estimation (stationary), maximum likelihood estimation (non-stationary).
  • Neighbourhood approach: For non-stationary trend estimation, data from neighbouring stations (within 0.3° radius) are scaled by mean annual maxima and combined to increase robustness.
  • Significance assessment: Bootstrap approach (500 resamples) to determine if the 5%–95% confidence interval of 20-year return level changes includes zero.
• Data sources:
  • In situ precipitation observations:
    • Météo-France: Hourly (6-minute intervals aggregated to hourly) and daily data, quality-controlled, from 1990s (RADOME project) to December 2023.
    • Électricité de France (EDF): Hourly and daily precipitation from heated large-surface (2000 cm²) rain-gauges. Recent data (1980s-2022) and "old" digitized data (late 1950s-early 1980s, weekly maxima for 1, 2, 3, 6, 12, 24 hours).
  • Weather pattern classification: Daily classification into eight main classes (grouped into Atlantic, Mediterranean, Northeast Circulation, Anticyclonic) by Garavaglia et al. (2010), updated by EDF.
  • Quality control: EDF raw hourly data scaled daily to match quality-controlled daily sums; visual inspection for spurious maxima. Météo-France data used as provided.

Main Results

• Climatology of Daily Extremes:
  • 20-year return levels (RL) are lowest from February to August (< 70 mm/d) and highest between September and January.
  • Highest values (> 200 mm/d) are found in the Alpes-Maritimes department in autumn.
  • Northern Alps: Atlantic influence dominates precipitation maxima year-round.
  • Southern Alps: Mediterranean influence is dominant from April to June and September to November, and seasonally throughout the year.
• Climatology of Hourly Extremes:
  • 20-year RL are lowest in February (< 15 mm/h) and highest in late summer (August and September).
  • In summer (JAS), a distinct gradient shows higher intensities in the pre-Alps and lower in the high inner Alps.
  • Highest values (up to 75 mm/h) are found in September on the Mediterranean coast and in the Rhône Valley, and in July in western Alpes-de-Haute-Provence.
  • Annual hourly extremes are dominated by the autumn season (OND) in the Rhône valley and along the coast, and by the summer season (JAS) in the inner Alps.
• Trends in Daily Extremes (1985-2022):
  • Trends generally range between -10 and +10 mm/d per decade.
  • Northern Alps: Mostly significant positive trends in May, and significant negative trends in June and September.
  • Southern Alps: Significant negative trends in August and September, followed by significant positive trends in October to December.
  • Annual trends show a transition from significant positive trends in the south (up to +24 mm/d per decade or +50% in 2020 relative to 1990) to negative trends in the north (up to -8 mm/d per decade or -20% in 2020 relative to 1990).
  • In the south, daily trends are negative when Mediterranean influence is weak and positive when it is stronger, indicating the importance of weather patterns.
• Trends in Hourly Extremes (1985-2022):
  • Trends are highly variable spatially and seasonally, with statistical significance at fewer stations compared to daily extremes.
  • Northern Alps: Trends are almost always centered around zero.
  • Southern Alps: Overall positive trends are found from April to June and October to December, while they tend to be slightly negative in August and September.
  • November stands out with spatially coherent significant positive trends in the south (up to +3.5 mm/h per decade or +100% in 2020 relative to 1990).
  • In summer (JAS), daily extremes show overall negative trends, whereas hourly extremes are mostly close to zero or positive, particularly in the high mountain ranges in the northeastern parts of the study domain.
  • The clear north-south pattern observed in daily trends for autumn (OND) is absent in hourly data, which show weakly non-significant and weakly negative trends across the region.
  • A significant decrease in daily extremes in September across the study region is observed, potentially attributable to a shift in seasonality with a later onset of the rainier autumn season.

Contributions

• First study to analyze trends of sub-daily (hourly) extreme precipitation in the French Alps using long-term observations, addressing a critical knowledge gap.
• Provides an updated and comprehensive climatology of daily and hourly precipitation extremes, extending previous work with an additional 30 years of data.
• Highlights distinct spatial and temporal patterns of extreme precipitation and their trends, emphasizing the need for region-specific and timescale-specific approaches.
• Links observed trends to changes in large-scale atmospheric circulation patterns (e.g., strengthening Mediterranean influence in autumn) and local thermodynamic effects (e.g., albedo feedback from decreasing snow cover in high Alps).
• Confirms local stakeholders' perception of increasing short-duration, high-intensity precipitation events, particularly in regions and seasons already at high risk.

Funding

• Office National des Forêts (ONF)
• Agence Nationale de la Recherche – France 2030 (PEPR TRACCS programme, grant number ANR-22-EXTR-0005)

Citation

@article{Berghald2026Climatology,
  author = {Berghald, Sebastian and Blanchet, Juliette and Blanc, Antoine and Penot, David},
  title = {Climatology and trends of observed daily and hourly extreme precipitation in the French Alps},
  journal = {Hydrology and earth system sciences},
  year = {2026},
  doi = {10.5194/hess-30-6421-2026},
  url = {https://doi.org/10.5194/hess-30-6421-2026}
}