Impact of climate forcing time step in an ice-sheet firn model

The firn layer regulates how an ice sheet responds to atmospheric 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. Prior studies have used a variety of time steps in the climate forcings of such firn models, ranging from 3 h to 1 d, 1-month, or even annual. The climate forcing time step impacts the creation of pore space by snow accumulation and the depletion of pore space by snowmelt and firn densification. To investigate this effect, we force the firn densification model IMAU-FDM with surface mass balance components and meteorological variables at different time steps for the Antarctic Peninsula and southern Greenland Ice Sheet. We show that the final modelled firn layer contains more pore space for larger forcing time steps, and that the magnitude of this effect depends on the climate regime. Locations with limited firn pore space due to seasonal melt, and regions with emerging firn aquifers, are most sensitive. 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 equal to or 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 higher density firn rather than porous firn, reducing the amount of firn air that is lost through melting. Therefore, for locations experiencing surface melt, the decoupled temperature and snowmelt in the upper layers results in more firn air with a climate forcing time step equal to or greater than a day. We also found that model parameterizations can become unsuitable when applied outside the physical conditions or climate forcing time step on which they are based, leading to unrealistic firn densification behavior in the model. We argue that (1) firn models forced with surface mass balance terms and meteorological variables require a timestep small enough to capture at least the diurnal cycle, (2) parameterizations should be used in a way that is consistent with the development data.