STEMMUS-UEB v1.0.0: Integrated Modelling ofSnowpack and Soil Mass and Energy Transfer with ThreeLevels of Soil Physical Process Complexities

Abstract. Snowpack, as the indispensable component in cold regions, has a profound effect on the hydrology and surface energy conditions through its modification of the surface albedo, roughness, and insulating property. Although the modelling of the snowpack, soil water dynamics, and the coupling of the snowpack and underlying soil layer has been widely reported, the analysis of coupled liquid-vapor-air flow mechanisms considering the snowpack effect was not yet investigated in detail. In this study, we incorporated the snowpack effect (Utah Energy Balance model, UEB) into a common modeling framework (Simultaneous Transfer of Energy, Mass, and Momentum in Unsaturated Soils with Freeze-Thaw, STEMMUS-FT), with various complexities of mass and energy transfer physics (from the basic coupled, to advanced coupled water and heat transfer, and further to the explicit consideration of airflow, termed BCD, ACD, and ACD-air, respectively). We then utilized the in situ observations and numerical experiments to investigate the effect of snowpack on soil moisture and heat transfer with the above-mentioned model complexities. Results indicated that the abrupt increase of surface albedo after precipitation events can be only reproduced by models considering snowpack. The BCD model tended to overestimate the land surface latent heat flux. Such overestimations were largely reduced by ACD and ACD-air models. Compared with the simulation considering snowpack, there is less surface latent heat flux from no-snow simulations due to the neglect of snow sublimation. With coupled models, the enhanced latent heat flux after precipitation events can be sourced from the surface ice sublimation, snow sublimation, and increased surface soil moisture, while the simple BCD model cannot provide the realistic partition of surface latent heat flux. The ACD model, with its physical consideration of vapor flow, thermal effect on water flow, and snowpack, can identify relative contributions of different components (e.g., thermal or isothermal liquid and vapor flow) to the total mass transfer fluxes. With the ACD-air model, the relative contribution of each component (mainly the isothermal liquid and vapor flows) to the mass transfer was significantly altered during the soil thawing period.