Assessment of snow depth retrievability from passive microwave observations over Arctic sea ice: a global sensitivity analysis

Abstract. The complexity of passive microwave (PM) retrieval of snow depth over Arctic sea ice stems from non-linear interactions between snow microstructure, wetness, and basal ice properties. These mechanisms remain insufficiently quantified, resulting in large uncertainties in PM-based snow products. We employ the snow microwave radiative transfer (SMRT) model together with a global sensitivity analysis, i.e. the Extended Fourier Amplitude Sensitivity Test, to decompose SMRT-simulated TB variance into contributions from individual parameters and their interactions. Averaging kernel analysis is then used to quantify snow depth retrievability across standard PM channels from 6 to 89 GHz under single- and multi-layer snowpack scenarios. 1) For single-layer dry snow, snow depth, density and grain radius are strongly coupled to each other, dominating the PM signals. When liquid water is present in the snow, the PM signals are primarily controlled by snow density and liquid water content. 2) In multi-layer dry snow, channels below 23 GHz are strongly influenced by the basal snow ice, while those above or equal to 23 GHz are dominated by depth hoar. At 6 GHz, retrievability is limited to dry snow with grain radius ≥ 0.5 mm and density ≤ 250 kg m-³, expanding toward finer grains and higher densities with increasing frequency. Regarding gradient ratio (GR), GR(18/6) provides limited retrievability for grain radius < 0.5 mm, whereas GR(36/18) remains effective for grain radius ＞0.2 mm. Notably, incorporating 89 GHz in GR improves the retrievability for new snow. Furthermore, sea ice type exerts a significant constraint on GR retrievability of snow depth and becomes increasingly pronounced under fine-grained snow conditions.