Simulating snow properties and Ku-band backscatter across the forest-tundra ecotone

Abstract

Sophisticated snowpack models are required to provide accurate estimation of snowpack properties across the forest-tundra ecotone where in situ measurements are sparse. As snowpack properties strongly influence radar scattering signals, accurate simulation is crucial for the success of spaceborne radar missions to retrieve snow water equivalent (SWE). In this study, we evaluate the ability of default and Arctic versions of Crocus embedded within the Soil, Vegetation and Snow version 2 (SVS2-Crocus) land surface model to simulate snowpack properties (e.g. depth, density, SWE, specific surface area; SSA) across a 40 km transect of the Northwest Territories, Canada, using two winter seasons (2021–22 and 2022–23) of in situ measurements. An ensemble of simulated snowpack properties (120 members from default and Arctic SVS2-Crocus) was used in the Snow Microwave Radiative Transfer (SMRT) model to simulate Ku-band (13.5 GHz) backscatter. Simulated backscatter from multi-layer (∼ 20-layer) SVS2-Crocus snowpack simulations and simplified 3-layer “radar-equivalent snowpack” SVS2-Crocus simulations were compared to simulated backscatter from snow pit observations (with no snow layer simplification). Results highlight that Arctic SVS2-Crocus wind-induced compaction modifications were spatially transferable across the forest-tundra ecotone, reducing the RMSD of surface density by an average of 29 %. Basal vegetation modifications were less effective in simulating low-density basal snow layers at all sites (2022 and 2023; default RMSD: 67 kg m−3; Arctic RMSD: 69 kg m−3) but were necessary to simulate a physically representative Arctic density profile. SVS2-Crocus underestimated SSA leading to high errors in the simulation of snow backscatter (2022 and 2023; default RMSD 3.5 dB; Arctic RMSD: 4.8 dB). RMSD of backscatter was reduced by implementing a minimum SSA value (8.7 m2 kg−1; 2022 and 2023; default RMSD: 1.5 dB; Arctic RMSD: 1.5 dB). A radar-equivalent snowpack was effective in retaining the scattering behaviour of the multi-layer snowpack (RMSD < 1 dB) providing a means to estimate SWE with increased computational efficiency.