1. What are the expected changes in extreme snowfall in the French Alps under a high emission scenario?
Based on climate projection datasets, changes in extreme snowfall in the French Alps under a high emission scenario (RCP8.5) are assessed between an historical and a future period. The mean temperature for the historical period between specific months is used as a threshold to define regions where extreme snowfall are expected to increase or decrease. The study estimates projected changes in heavy (mean annual maxima) and extreme (100-year return level) snowfall. The evolution of the elevation threshold below or above which extreme snowfall is projected to decrease or increase, respectively, is also provided. This threshold is computed as a function of global warming level using a recent methodology based on non-stationary extreme value analysis and designed for climate projection ensembles. This study is the first to compute such a threshold for high return periods and as a function of global warming level in the French Alps.
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2. What are the annual maxima of daily snowfall observed for the 23 massifs of the French Alps from the S2M reanalysis, and how are they expressed?
The annual maxima of daily snowfall are provided for the 23 massifs of the French Alps by the S2M reanalysis (Durand et al., 2009a; Vernay et al., 2019 Vernay et al., , 2022) ) which combines large-scale reanalyses, meteorological forecasts and ground measurements. These reanalyses of daily snowfall data, expressed in kg m-2 , span the time period August 1958 to July 2019. Here, for any year t between 1959 and 2019, we consider the annual maxima for the period between the 1st of August for the year t-1 and the 31st of July for the year t.
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3. What statistical methodology is used in the study?
The study utilizes a recent statistical methodology that projects the evolution of extreme variables from climate projection ensembles. This methodology employs flexible non-stationary generalized extreme value (GEV) models, incorporating piecewise linear functions to model changes in the three GEV parameters. Additionally, adjustment coefficients are used to align the GEV distributions of GCM-RCM pairs with past observations. This approach allows for modeling changes in both the body and tail of the GEV distribution, providing a comprehensive understanding of climate extremes. However, a drawback of this methodology is the fixed knots in the linear pieces, which are determined by the selected number of linear pieces.
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4. What is the link between elevations and changes in intense snowfall?
The study shows a consistent link between elevations and changes in intense snowfall. Figure 4 and Figure 5 demonstrate that both heavy (mean annual maxima) and extreme (100-year return levels) snowfall show average changes in elevation. The three types of evolution (increase, increase followed by a decrease, decrease) correspond to different outcomes of the trade-off between projected temperature increase and precipitation phase, as well as the projected increase in extreme winter precipitation in the French Alps. For elevations around 3000 m, an increase in intense snowfall is followed by a decrease at +3 * C. Lower elevations are directly projected to decrease. The average relative changes in heavy and extreme snowfall are not substantial, ranging between -15% and +8% for the 100-year return levels, and between -26% and +3% for the mean annual maxima. The elevation threshold for extreme snowfall increase is projected to rise from 2600 m to 3000 m for 100-year return levels at +1.5 * C of global warming. This indicates that critical infrastructure design above 2600 m may need to be reevaluated due to projected changes in extreme snowfall.
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