A Flexible Snow Model (FSM 2.1.0) including a forest canopy

Abstract. Multiple options for representing physical processes in forest canopies are added to a model with multiple options for representing physical processes in snow on the ground. The canopy processes represented are shortwave and longwave radiative transfer, turbulent transfers of heat and moisture, and interception, sublimation, unloading and melt of snow in the canopy. There are options for Beer's Law or two-stream approximation canopy radiative transfer, linear or non-linear canopy snow interception efficiency, and time/melt-dependent or temperature/wind-dependent canopy snow unloading. Canopy mass and energy balance equations can be solved with one or two model layers. Model behaviour on stand scales is compared with observations of above and below canopy shortwave and longwave radiation, below canopy wind speed, snow mass on the ground and subjective estimates of canopy snow load. Large-scale simulations of snow cover extent, snow mass and albedo for the Northern Hemisphere are compared with observations and land-only simulations by state-of-the-art Earth System Models. Without accounting for uncertainty in forest structure metrics and parameter values, the ranges of multi-physics ensemble simulations are not as wide as seen in intercomparisons of existing models.