Recent Ice Trends in Swiss Mountain Lakes: 20-year Analysis of MODIS Imagery

Depleting lake ice can serve as an indicator for climate change, just like sea level rise or glacial retreat. Several Lake Ice Phenological (LIP) events serve as sentinels to understand the regional and global climate change. Hence, it is useful to monitor long-term lake freezing and thawing patterns. In this paper we report a case study for the Oberengadin region of Switzerland, where there are several small- and medium-sized mountain lakes. We observe the LIP events, such as freeze-up, break-up and ice cover duration, across two decades (2000-2020) from optical satellite images. We analyse time-series of MODIS imagery by estimating spatially resolved maps of lake ice for these Alpine lakes with supervised machine learning (and additionally cross-check with VIIRS data when available). To train the classifier we rely on reference data annotated manually based on webcam images. From the ice maps we derive long-term LIP trends. Since the webcam data is only available for two winters, we also validate our results against the operational MODIS and VIIRS snow products. We find a change in complete freeze duration of -0.76 and -0.89 days per annum for lakes Sils and Silvaplana, respectively. Furthermore, we observe plausible correlations of the LIP trends with climate data measured at nearby meteorological stations. We notice that mean winter air temperature has negative correlation with the freeze duration and break-up events, and positive correlation with the freeze-up events. Additionally, we observe strong negative correlation of sunshine during the winter months with the freeze duration and break-up events.