Impact of climate forcing time step on the modelled ice-sheet firn layer

Abstract. The firn layer regulates how an ice-sheet responds to climate change by modifying how changes in surface temperature, snow accumulation and ablation affect the ice-sheet mass balance. Firn properties are often simulated with a firn densification model. Firn models are forced with surface mass balance components, surface energy balance components and/or meteorological variables. In literature, a variety of climate forcing time steps have been used to force such firn models, ranging from 3-hours to 1-day, 1-month, or even annual. To investigate the impact of these different time steps, we force the firn densification model IMAU-FDM with different climate forcing time steps at the surface 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, and that locations with limited firn pore space due to seasonal melt are most sensitive. The climate forcing time step impacts the creation of pore space by snowfall and the depletion of pore space by snowmelt and firn densification. The key in causing the differences in firn pore space is the presence or absence of a diurnal cycle in the input data. A climate forcing time step greater than a day allows for a non-physical coexistence of snowmelt and sub-zero surface temperatures, leading to immediate shallow refreezing of meltwater. Subsequent melting removes refrozen ice rather than porous firn, reducing the amount of firn air that is lost through melting. Therefore, the decoupled temperature and snowmelt in the upper layers give more firn air with a climate forcing time step larger than a day. We also found that firn density parameterizations can become unsuitable when applied outside the physical conditions or climate forcing time step on which they are based. These parameterizations lead to unrealistic firn densification and accumulation behavior in the model. We argue that (1) firn models require a timestep small enough to capture at least the diurnal cycle, (2) use of parameterizations should be critically assessed and used consistently with the way they were originally developed, and (3) the forcing time step should be considered when interpreting firn model output.