In situ monitoring of seasonally frozen ground using soil freezing characteristic curve in permittivity–temperature space

Abstract. Seasonally frozen ground (SFG) is a critical component of the cryosphere, yet its freezing dynamics are often oversimplified in large-scale monitoring frameworks – particularly in remote sensing (RS) and land surface modeling – through the use of binary 0 °C thresholds. This approach overlooks the physically significant “transitional” state where liquid water and ice coexist, leading to systematic errors in quantifying the timing and duration of the frozen season. To address this, we recast the Soil Freezing Characteristic Curve (SFCC) framework directly into permittivity–temperature space. By operating in dielectric space, we bypass the high uncertainty associated with soil-specific liquid water content calibrations and enable a robust categorization of soil into unfrozen, transitional, and frozen states. We fitted this model to in-situ measurements from eight monitoring networks (87 sites) across Canadian boreal forest, prairie, and tundra ecozones. Using Bayesian hierarchical partial pooling, we derived stabilized estimates of the freezing onset (Tf) and transition sharpness (b). Network-level Tf ranged from 0.15–0.44 °C, while b varied from 0.92–3.47 °C−1, reflecting distinct freezing regimes. We found that the transitional state is a dominant seasonal feature at these sites, challenging binary 0 °C assumptions used in RS evaluation. In high-moisture sites characterized by thick organic insulation (e.g., within the observed eastern boreal forest networks), this state persisted for over 100 d – effectively the entire winter – despite persistent subzero air temperatures. In contrast, sites in the western boreal and prairie networks, which generally lack thick surface organic layers and have lower soil moisture, exhibited shorter but still significant transitional periods (30 and 60 d, respectively). Even in the extreme cold of the tundra network sites, the transitional phase persisted for over 40 d. These results confirm that surface insulation and soil moisture, rather than air temperature alone, govern the SFG regime at the observed locations, providing a reproducible, physically-based reference framework for the next generation of freeze–thaw products.