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. Knowledge of current snow cover extent isessential for characterizing energy and moisture fluxes at the Earth'ssurface. The snow-covered area (SCA) is often estimated by using opticalsatellite information in combination with the normalized-difference snowindex (NDSI). The NDSI thereby uses a threshold for the definition if asatellite pixel is assumed to be snow covered or snow free. Thespatiotemporal representativeness of the standard threshold of 0.4 ishowever questionable at the local scale. Here, we use local snow cover mapsderived from ground-based photography to continuously calibrate the NDSIthreshold values (NDSI thr ) of Landsat satellite images at twoEuropean mountain sites of the period from 2010 to 2015. TheResearch Catchment Zugspitzplatt (RCZ, Germany) and Vernagtferner area(VF, Austria) are both located within a single Landsat scene. Nevertheless, thelong-term analysis of the NDSI thr demonstrated that theNDSI thr at these sites are not correlated ( r =  0.17) and differentthan the standard threshold of 0.4. For further comparison, a dynamic and locallyoptimized NDSI threshold was used as well as another locally optimizedliterature threshold value (0.7). It was shown that large uncertainties inthe prediction of the SCA of up to 24.1 % exist in satellite snow covermaps in cases where the standard threshold of 0.4 is used, but a newlydeveloped calibrated quadratic polynomial model which accounts forseasonal threshold dynamics can reduce this error. The model minimizes theSCA uncertainties at the calibration site VF by 50 % in the evaluationperiod and was also able to improve the results at RCZ in a significant way.Additionally, a scaling experiment shows that the positive effect of a locally adaptedthreshold diminishes using a pixel size of 500 m or larger,underlining the general applicability of the standard threshold at larger scales.

the cryosphere

On the need for a time- and location-dependent estimation of the NDSI threshold value for reducing existing uncertainties in snow cover maps at different scales