A mechanism for ice layer formation in glacial firn

There is ample evidence for ice layers and lenses within glacial firn. The standard model19 for ice layer formation localizes the refreezing by perching of meltwater on pre-existing20 discontinuities. Here we argue that even extreme melting events provide insufficient flux21 for this mechanism. Using a thermomechanical model we demonstrate a different mech22 anism of ice layer formation. After a melting event when the drying front catches up with23 the wetting front and arrests melt percolation, conductive heat loss freezes the remain24 ing melt in place to form an ice layer. This model reproduces the depth of a new ice layer25 at the Dye-2 site in Greenland. It provides a deeper insight into the interpretation of firn26 stratigraphy and past climate variability. It also improves the simulation of firn densi27 fication processes, a key source of uncertainty in assessing and attributing ice sheet mass28 balance based on satellite altimetry and gravimetry data.