Regional pattern of annual snow cover duration in the Greater Alpine Region (2000 – 2018)

This study analyses regional differences in annual snow cover duration as quantified by the annual number of days with snow cover (D_sc) and investigates differences in sensitivity of D_sc to climatic variability across the Greater Alpine Region over the 2000-2018 period. MODIS snow cover data were used to estimate D_sc based on the Regional Snowline Elevation (RSLE) method, a spatial filter technique for large-scale cloud cover reduction.

D_sc over the study period closely follows the relief, with a mean D_sc of ~10–60 days at elevations of 500 m that increase to about 100–150 days at 1500m. South of the main alpine ridge, D_sc is, at the same elevation, consistently lower than north of it with differences of ΔD_sc  ~25–50 days. Similarly, the eastern part of the study region experiences longer snow cover duration than the western part. This difference is particularly pronounced at elevations below 1500m where ΔD_sc ~25 days. Throughout the study period, a general upward shift of the RSLE was observed for most parts of the Greater Alpine Region. This upward shift, characterized by later onset of snow accumulation (∆D_start ~14–30 d) and earlier melt-out at the end of the snow season (∆D_end ~10–20 d), translates into reductions of the annual number of snow-covered days by up to ΔD_sc = -46 days over the study period. The data suggest that, in particular, low-elevation  (< 600m.a.s.l.) regions in the north-eastern part of the Greater Alpine Region, as well as elevations between 1400 and 2000 m in the north-western part of the study region experienced the most pronounced reductions of D_sc., whereas ΔD_sc remained very limited south of the main Alpine ridge. The spatially integrated MODIS-derived estimates of D_sc correspond well with D_sc estimates derived from longer-term point-scale observations at >500 ground station observations across the region. In the majority of regions, the temporal evolution of D_sc over the 2000-2018 study period also reflects the longer-term D_sc trends as estimated from these point-scale observations (1970-2014). This provides supporting evidence that the widespread decline of D_sc across the Greater Alpine Region as estimated based on MODIS data is largely not caused by isolated short-term climatic variability but coincides with multi-decadal fluctuations. A comparison of the sensitivities of D_sc to climatic variability indicates that neither mean winter temperatures T_w nor annual solid precipitation totals P_s, are consistent first order controls on D_sc across  elevations and regions. Rather, the data highlight the importance of the interaction between the two variables: depending on the respective sensitivities of D_sc to changes in either variable, T_w or P_s, respectively, the interplay between them can reinforce or largely off-set potential effects on D_sc in different regions in the Greater Alpine Region. The regional differences in ΔD_sc with a less pronounced decline south of the main Alpine ridge are largely a consequence of this interplay: while T_w evolved similarly North and South of the Alpine ridge, many southern regions, unlike the northern regions, experienced an increase in P_s that offsets the effects of positive temperature trends.