Regional pattern of annual snow cover duration in the Greater Alpine Region (2000 &#8211; 2018)
Authors: Hrachowitz Markus , Fugger Stefan , Schulz Karsten 
Published: March 2020

&lt;p&gt;This study analyses regional differences in annual snow cover duration as quantified by the annual number of days with snow cover (D&lt;sub&gt;sc&lt;/sub&gt;) and investigates differences in sensitivity of D&lt;sub&gt;sc&lt;/sub&gt; to climatic variability across the Greater Alpine Region over the 2000-2018 period. MODIS snow cover data were used to estimate D&lt;sub&gt;sc&lt;/sub&gt; based on the Regional Snowline Elevation (RSLE) method, a spatial filter technique for large-scale cloud cover reduction.&lt;/p&gt;&lt;p&gt;D&lt;sub&gt;sc&lt;/sub&gt; over the study period closely follows the relief, with a mean D&lt;sub&gt;sc&lt;/sub&gt; of ~10&amp;#8211;60 days at elevations of 500 m that increase to about 100&amp;#8211;150 days at 1500m. South of the main alpine ridge, D&lt;sub&gt;sc&lt;/sub&gt; is, at the same elevation, consistently lower than north of it with differences of &amp;#916;D&lt;sub&gt;sc&lt;/sub&gt;&amp;#160; ~25&amp;#8211;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 &amp;#916;D&lt;sub&gt;sc&lt;/sub&gt; ~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 (&amp;#8710;D&lt;sub&gt;start&lt;/sub&gt; ~14&amp;#8211;30 d) and earlier melt-out at the end of the snow season (&amp;#8710;D&lt;sub&gt;end&lt;/sub&gt; ~10&amp;#8211;20 d), translates into reductions of the annual number of snow-covered days by up to &amp;#916;D&lt;sub&gt;sc&lt;/sub&gt; = -46 days over the study period. The data suggest that, in particular, low-elevation&amp;#160; (&lt; 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&lt;sub&gt;sc&lt;/sub&gt;., whereas &amp;#916;D&lt;sub&gt;sc&lt;/sub&gt; remained very limited south of the main Alpine ridge. The spatially integrated MODIS-derived estimates of D&lt;sub&gt;sc&lt;/sub&gt; correspond well with D&lt;sub&gt;sc&lt;/sub&gt; estimates derived from longer-term point-scale observations at &gt;500 ground station observations across the region. In the majority of regions, the temporal evolution of D&lt;sub&gt;sc&lt;/sub&gt; over the 2000-2018 study period also reflects the longer-term D&lt;sub&gt;sc&lt;/sub&gt; trends as estimated from these point-scale observations (1970-2014). This provides supporting evidence that the widespread decline of D&lt;sub&gt;sc&lt;/sub&gt; 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&lt;sub&gt;sc&lt;/sub&gt; to climatic variability indicates that neither mean winter temperatures T&lt;sub&gt;w&lt;/sub&gt; nor annual solid precipitation totals P&lt;sub&gt;s&lt;/sub&gt;, are consistent first order controls on D&lt;sub&gt;sc&lt;/sub&gt; across&amp;#160; elevations and regions. Rather, the data highlight the importance of the interaction between the two variables: depending on the respective sensitivities of D&lt;sub&gt;sc&lt;/sub&gt; to changes in either variable, T&lt;sub&gt;w&lt;/sub&gt; or P&lt;sub&gt;s&lt;/sub&gt;, respectively, the interplay between them can reinforce or largely off-set potential effects on D&lt;sub&gt;sc&lt;/sub&gt; in different regions in the Greater Alpine Region. The regional differences in &amp;#916;D&lt;sub&gt;sc&lt;/sub&gt; with a less pronounced decline south of the main Alpine ridge are largely a consequence of this interplay: while T&lt;sub&gt;w&lt;/sub&gt; evolved similarly North and South of the Alpine ridge, many southern regions, unlike the northern regions, experienced an increase in P&lt;sub&gt;s&lt;/sub&gt; that offsets the effects of positive temperature trends.&lt;/p&gt;