TanDEM-X Dual-Polarization Time Series for the Estimation of Snow Accumulation on an Alpine Glacier

Abstract

Snow accumulation on glaciers represents the main positive contribution to mass balance and, therefore, its knowledge is of high interest. Glaciers store about 75% of all available fresh water on Earth, which most living beings rely on for their water supply. The ongoing global warming is significantly accelerating glacier melting and, therefore, can have severe consequences on the global ecosystem. So far, snow accumulation is mostly being measured by means of sparse weather stations which provide only point measurements. Snow maps for larger areas are obtained by spatially interpolating data from stations at different locations with the help of meteorological models. The introduction of space-borne radar remote sensing techniques has led to significant developments in the field of glaciology, due to the large spatial coverage with relatively short revisit time and high spatial resolution. Among them, radar remote sensing provides sensitivity to the surface as well as subsurface layers of glaciers and ice sheets due to the penetration capability of microwaves into dry snow and ice. In this sense, radar sensors and, in particular, Synthetic Aperture Radar (SAR) systems provide a tool to access information related to the structural properties of ice and snow layers. This study investigates the potential of dual-polarimetric (HH/VV) TanDEM-X time series to monitor snow accumulation (depth) over the Aletsch glacier, in the Swiss Alps. In particular, co-polarization phase differences are employed to detect snowfall events, as shown already in [1] for the case of snow-covered soil. Furthermore, the propagation model described in [2] is adopted to link phase differences to fresh snow properties, like structural anisotropy, density and thickness, and to invert fresh snow depth from TanDEM-X data. Based on the inversion results, a simple algorithm has been developed to estimate the temporal evolution of the total snow depth on the glacier surface during several accumulation seasons. The obtained results are validated with snow depth measurements from a weather station nearby the glacier and show that the proposed approach provides good performance despite a number of simplifying assumptions introduced to keep the model complexity low. However, a number of limitations remain, mainly related to the relatively low temporal sampling (11-day) offered by TanDEM-X and the need of some a-priori knowledge about fresh snow density and structural anisotropy, which require further investigations. [1] Leinss S., Parrella G. and Hajnsek I.: Snow height determination by polarimetric phase differences in X-band SAR data, JSTARS, vol. 7, no. 9, pp. 3794-3810, 2014. [2] Leinss S., Loewe H., Proksch M., Lemmetyinen J., Wiesmann A. and Hajnsek I.: Anisotropy of seasonal snow measured by polarimetric phase difference in radar time series, The Cryosphere, vol. 10, pp. 1771-1797, 2016.