On the Use of COSMO-SkyMed X-Band SAR for Estimating Snow Water Equivalent in Alpine Areas: A Retrieval Approach Based on Machine Learning and Snow Models

This study aims at estimating the dry snow water equivalent (SWE) by using X-band synthetic aperture radar (SAR) data from the COSMO-SkyMed (CSK) satellite constellation. The time series of CSK acquisitions have been collected during the dry snow period in the Alto Adige test site, in the Italian Alps, during the winter seasons from 2013 to 2015 and from 2019 to 2021. The SAR data have been analyzed and compared with the in situ measurements to understand the X-band SAR sensitivity to SWE, which has been further assessed by dense media radiative transfer (DMRT) model simulations. The sensitivity analysis provided the basis for addressing the SWE retrieval from the CSK data, by exploiting two different machine learning (ML) techniques, namely, artificial neural networks (ANNs) and support vector regression (SVR). To ensure statistical independence of training and validation processes, the algorithms are trained and tested using SWE predictions of the fully distributed snow model AMUNDSEN as reference data and are subsequently validated on the experimental dataset. Due to its influence on the CSK estimates, the effect of forest canopy was accounted for in the analysis. Depending on the algorithm, the validation resulted in a correlation coefficient 0.78 <= R <= 0.91 and a root-mean-square error (RMSE) 55.5 mm <= RMSE <= 87.4 mm between estimated and in situ SWE. Further analysis and validation are needed; however, the obtained results seem suggesting the CSK constellation as an effective tool for the retrieval of the dry SWE in alpine areas.