A Flexible Snow Model (FSM 2.1.1) including a forest canopy

Abstract. Multiple options for representing physical processes in forest canopies are added to FSM, which is 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- and melt-dependent or temperature- and 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. FSM2 provides a platform for rapid investigation of sensitivity to model structure and parameter values or ensemble-based data assimilation for snow in open and forested environments.