A model study of the response of dry and wet firn to climate change

We study the response of firn to a stepwise surface temperature change, using a firn model that includes meltwater hydrology and is driven by an idealized surface climate. We find that adjustment of dry firn (i.e. without surface melt) to surface warming takes longer than a subsequent cooling to the original, colder climate, mainly because firn compacts faster at higher firn temperatures. In contrast, wet firn adjusts faster to a surface warming than to a cooling. Increased meltwater percolation enhances the downward transport of latent heat, whereas there is no such mechanism that can enhance the downward transport of a cooling signal. Thus, wastage of firn after surface warming is faster than its regeneration if the warming were reversed. Furthermore, the response of wet firn to temperature change exhibits a complex relation between accumulation rate and the steady-state deep-firn temperature. For high accumulation rates, the deep-firn temperature is higher because latent heat release upon refreezing is isolated by winter snow. As a result, the response of wet firn to a temperature change varies strongly with accumulation rate. In general, the magnitude and the rate of density change is larger in wet firn than in dry firn.