Precipitation phase-partitioning schemes introduce substantial uncertainty into simulations of the permafrost thermal regime over the Tibetan Plateau

Abstract. Precipitation phase-partitioning schemes determine how precipitation forcing is divided into snowfall and rainfall at each time step in land surface modelling. However, their influence on simulated permafrost thermal regimes remains poorly quantified. In this study, we use the one-dimensional transient permafrost model CryoGridLite to assess the uncertainty in permafrost simulations induced by different schemes over the Tibetan Plateau during 1951–2024. By keeping the meteorological forcing dataset, model configurations, and input parameters identical, we implemented eight commonly used schemes, including binary threshold schemes, temperature-ramp schemes, piecewise schemes, and wet-bulb/humidity-dependent schemes, allowing the effect of precipitation phase assignment to be isolated. Our simulations indicate that precipitation phase-partitioning schemes produce substantial differences in snowfall and rainfall forcing, which propagate into simulated ground thermal regimes. Across the ensemble-defined permafrost area, 83.4 % of the area shows an inter-scheme range in mean annual ground temperature at 10m depth exceeding 0.5 °C, and 48.9 % exceeds 1.0 °C. Uncertainty in simulated thaw depth is more spatially localized than that in mean annual ground temperature, but 47.2 % and 18.3 % of the ensemble-defined permafrost area still show inter-scheme thaw depth ranges exceeding 0.25 and 0.5 m, respectively. These thermal differences further affect threshold-based permafrost diagnosis. Differences among schemes lead to measurable changes in diagnosed permafrost area, with the largest diagnostic uncertainty occurring in marginal permafrost regions where simulated ground temperatures are close to 0 °C. Although precipitation phase assignment is commonly treated as a preprocessing step in meteorological forcing preparation, our controlled experiments show that the choice of scheme can generate pronounced differences in simulated ground thermal states and permafrost area diagnostics. Phase partitioning should therefore be considered a previously underexamined forcing-related source of uncertainty in high-elevation permafrost thermal simulations.