Resolving freeze-thaw surface energy exchange in soils through phase-dependent thermal measurements.

Surface energy exchange regulates permafrost stability and cold region climate feedback, yet key thermal parameters governing freeze-thaw transitions remain poorly constrained. In particular, soil emissivity and conductive heat transfer are often treated as static or one-dimensional properties, despite strong phase-dependent changes during freezing and thawing. Here we experimentally resolve the coupled evolution of surface emissivity, thermal conductivity, volumetric heat capacity, and conductive heat flux in sand-, silt-, and clay-dominated soils between -15 °C and 20 °C under varying saturation, organic content, and salinity. Frozen soils exhibit systematically lower emissivity than thawed soils, with distinct nonlinear transitions across phase change. Thermal conductivity increases substantially in the frozen state due to ice-enhanced grain contacts, while latent heat effects generate pronounced peaks in volumetric heat capacity near 0 °C. By explicitly quantifying both vertical and lateral conductive components, we demonstrate that neglecting lateral heat flux can underestimate total conductive energy transfer by up to 31.65%. These results provide physically constrained, phase-resolved parameterizations for land-surface and permafrost models and reduce uncertainty in infrared surface temperature retrievals across thermodynamically sensitive temperature ranges. Our findings establish a mechanistic framework linking phase change, radiative properties, and multidimensional heat transfer in cold region soils.