A unified hydro-thermal framework for improved skin conductivity and skin temperature in the ECLand model

Accurate numerical weather prediction (NWP) and climate modelling depend critically on high-fidelity simulation of land surface processes and their interactions with the atmosphere. These interactions are governed by land surface state variables (LSSVs) such as soil moisture, soil temperature, and land surface (skin) temperature, which regulate the energy, water, and carbon fluxes at the land-atmosphere interface. LSSVs strongly influence near-surface atmospheric state variables, including air temperature and relative humidity, which are key to reliable NWP and climate forecasts. To enhance the representation of soil and vegetation processes in land surface models (LSMs), focussing on ECLand in first instance, we are developing a unified hydro-thermal framework for improved coupling of soil moisture and heat transport, and related land-atmosphere coupling. It integrates soil hydraulic and thermal properties, which are typically modelled independently, to improve the simulation of energy and water fluxes. For ECLand, we introduced two key modifications. First, the van Genuchten (1980) soil water retention curve (SWRC) was replaced with a formulation which explicitly accounts for adsorbed and capillary water content (e.g., Lu, 2016). This modification allows for a more physically realistic representation of soil hydraulic properties, particularly under dry conditions. Secondly, the thermal conductivity function of Peters-Lidard et al. (1998), currently used in ECLand, was replaced with an equation which directly links thermal conductivity to the SWRC parameters (Lu & McCartney, 2024), ensuring consistent coupling between soil hydraulic and thermal properties. This new set of equations is being developed to improve the representation of the below-ground part of the skin conductivity, a key parameter for predicting skin temperature, which is critical for accurate energy balance predictions at the land surface, including skin heat flux. While ECLand currently uses a lumped approach, whereby the skin conductivity controls heat flow through topsoil and vegetation combined, the JULES model explicitly separates the contributions of soil and vegetation. We plan to adopt equations from the JULES model for the above-ground part of skin conductivity and integrate them into the updated ECLand model, with the aim to enhance the physical representation of surface heat flux dynamics. The updated model will be tested at multiple sites, including Cabauw, to evaluate its performance. We aim to demonstrate significant improvements in the simulation of soil moisture, soil temperature, and energy fluxes, showcasing the potential of this new framework. However, broader validation across a range of climatic and soil conditions will be required to ensure robustness and scalability. Future work will focus on developing a global soil hydro-thermal parameter set tailored to the new equations, enabling global application of the framework in the IFS. Once thoroughly tested and calibrated, this advancement is expected to improve the predictability of both land surface and atmospheric state variables, ultimately enhancing the reliability of ECMWF’s seasonal to sub-seasonal forecasts.