Assimilation of synthetic observations of radar backscatters at Ku-band improves SWE estimates

Abstract

In cold regions, snow serves as the primary water source for downstream rivers and lakes. Accurate gridded snow water equivalent (SWE) estimation is hindered by the sparse ground observation network and the low resolution of satellite passive microwave products. To address this, Environment and Climate Change Canada (ECCC), the Canadian Space Agency (CSA), and Natural Resources Canada (NRCan) are developing the Terrestrial Snow Mass Mission (TSMM), a dual Ku-band satellite mission designed to measure backscatter at 13.5 and 17.25 GHz across the Northern Hemisphere at a 500 m spatial resolution with a weekly temporal resolution. This study assesses the feasibility of assimilating Ku-band backscatter to enhance SWE estimates in a synthetic experiment. We used the Soil-Vegetation-Snow version 2 (SVS2) land surface model, which incorporates the snowpack model Crocus, coupled with the Snow Microwave Radiative Transfer model (SMRT). Synthetic observations of SWE and backscatter extracted at weekly intervals from synthetic truths (model simulations) were assimilated with a particle filter at point-scale. This was done at three sites representing three different Canadian climates (Arctic, humid continental, Alpine) over three winter seasons. Meteorological forcing derived from the high-resolution Canadian meteorological model was perturbed to generate ensembles of snow simulations for assimilation. Results indicate that assimilating synthetic observations of backscatter improved SWE estimates at the Arctic and humid continental sites, reducing the mean continuous ranked probability score (CRPS) by up to 32 % compared to the open-loop ensemble. This performance was comparable to assimilating the SWE synthetic observations with observation errors larger than 20 %. Assimilating synthetic observations of backscatter at the Alpine site only improved the SWE estimates by 5 % as backscatter signals seemed to lose sensitivity to SWE values greater than ∼300 kgm-2 in our experimental setup. Assimilating backscatter and SWE synthetic observations also improved the estimations of vertical profiles of snow density and specific surface area. These findings demonstrate the potential of direct assimilation of Ku-band backscatter to enhance both estimates of SWE and snowpack properties.