Climate forcing time step requirements for firn modeling

The firn layer regulates how an ice sheet responds to climate change, by modifying how changes in surface temperature, snow accumulation and ablation affects the ice sheet mass balance.  The firn layer can be simulated with a firn densification model. However, a variety of climate forcing time steps have been used to force such firn models in literature, ranging from 3-hourly to daily, monthly, or even annually. Reasons for selecting certain climate forcing time steps are most often practical: the data are not available at smaller time steps, the amount of forcing data becomes too large, or computational resources are insufficient. Evaluation of the impact of the selected forcing time step is often absent or based on different lines of reasoning.In this study, we force the firn model IMAU-FDM with different surface climate forcing time steps for the Antarctic Peninsula and southern Greenland Ice Sheet. We show that the modelled firn layer contains more pore space for larger forcing time steps. Locations with limited firn pore space due to seasonal melt are most sensitive.The largest differences in modelled firn pore space arise when there is no diurnal cycle in the climate forcing input data. This allows for coexisting snowmelt and sub-zero surface temperatures, leading to immediate shallow refreezing of meltwater. We also found that parameterizations for e.g. fresh snow density can become unsuitable when applied outside the physical conditions or climate forcing time step on which they are based. We argue that (1) firn models require input with at least a sub-daily forcing time step, (2) use of parameterizations should be critically assessed and only used consistently with the way they were originally developed, and (3) the forcing time step should be considered when interpreting firn model output.