Detection of snow change using polarimetric TerraSAR-X time-series (Svalbard, Norway)

Due to recent climate change conditions, i.e. increasing temperatures and changing precipitation patterns, arctic snow cover dynamics exhibit strong changes in terms of extent and duration. Arctic amplification processes and impacts are well documented expected to strengthen in coming decades. In this context, innovative observation methods are helpful for a better comprehension of the spatial variability of snow properties relevant for climate research and hydrological applications. Microwave remote sensing provides exceptional spatial and temporal performance in terms of all-weather application and target penetration. Time-series of Synthetic Active Radar images (SAR) are becoming more accessible at different frequencies and polarimetry has demonstrated a significant advantage for detecting changes in different media. Concerning arctic snow monitoring, SAR sensors can offer continuous time-series during the polar night and with cloud cover, providing a consequent advantage in regard of optical sensors. The aim of this study is dedicated to the spatial and temporal variability of snow in the Ny-Ålesund area on the BrØgger peninsula, Svalbard (N 78°55’ / E 11° 55’) using a high temporal TerraSAR-X Stripmap time series from November 2018 to June 2022, providing four consecutive winter datasets. The dual-cross polarized (HH/VV) SAR data were acquired from two different orbits (ascending and descending) with high incidence angles (36° to 39°) increasing snow volume backscattering and reducing topographic constraints. Additionally, a high spatial resolution Digital Elevation Model (NPI 5-m), consistent in-situ measurements of meteorological data, and snow profiles including lowlands and glaciers sites are available. Polarimetric processing is based on the Kennaugh matrix decomposition, co-polar phase coherence (CCOH) and co-polar phase difference (CPD). The Kennaugh matrix elements K0, K3, K4, and K7 are the total intensity, phase ratio, intensity ratio, and shift between HH and VV phase center, respectively. Their interpretation allows analyzing the structure of the snowpack linked to the near real time of in-situ measurements (snow profiles). The X-band signal is strongly influenced by the snow stratigraphy: internal ice layers reduce or block the penetration of the signal into the snow pack. The best R2 correlation performances between estimated and measured snow heights are ranging from 0.50 to 0.80 for dry snow conditions. Therefore, the use of the X-band for regular snow height estimations remains limited under these conditions. Conversely, this study shows the benefit of TerraSAR-X thanks to the Kennaugh matrix elements analysis. A focus is set (i) on the K0 element corresponding to the total power (Span), and (ii) on the Copolar Phase Difference (CPD, Leinss 2016) between VV and HH polarization: ɸ CPD = ɸ VV - ɸ HH. Our results indicate that the CPD values are related to the snow metamorphism: positive values correspond to dry snow (horizontal structures), negative values indicate recrystallization processes (vertical structures)

Combining in situ observations and remote sensing data to determine the spatial extent of rain-on-snow events on the Brøgger peninsula

Climate change is particularly impacting the Arctic, where the temperature increase is stronger than the global mean due to Arctic Amplification. Long-term observations at sites such as Ny-Alesund on the Brøgger’s peninsula in Svalbard allow understanding meteorological changes taking place in the Arctic. In the last decades, Ny Alesund was affected by a large increase of winter temperatures leading to occasional periods of positive temperatures lasting few days. As a result, the number of rainfall events also increased, contributing to an early degradation of the snowpack on the Brogger peninsula. Meteorological measurements such as at Ny Alesund allow to quantify the temporal variability of these “rain on snow” (ROS) events at specific points. The goal of this study is (i) to spatialize recent ROS events on the Brøgger peninsula during the period 2019-2022 using remote sensing radar data and (ii) to characterize the atmospheric origin of these events using anomalies of 500 hPa height or the identification of cyclonic systems. We use SAR satellite images from each event, mainly TSX and RCM images provided by German and Canadian Space Agencies. We processed the images with a thresholding method in order to find the spatial elevation limits between wet and dry snow after the events. PlanetScope optical images are used for snow extent validation. During the ROS episodes, the snow remains generally dry upstream of the glaciers, while at lower altitudes the snow on the peninsula is systematically wet. These ROS episodes are associated to cyclonic systems originated from the Northern Atlantic Ocean, and to a strong Z500 gradient from high pressure centered in Norway and low pressure centered in Greenland. These results are important to better characterize the origins and the spatial variability of ROS events and to evaluate the impact of these specific events on glaciers, permafrost, or ecology