Firnification and the effects of wind-packing on Antarctic snow

Abstract

Using firn-core data from ten widely spread Antarctic sites, the dependence of firnification on temperature, wind and accumulation rate has been examined with two empirical models. One model relates the square of the porosity to the logarithm of the overburden pressure, and yields good fit to data through the first stage of firnification up to around 0.70-0.75 Mg m-3, beyond which it severely overestimates density. All three meteorological factors enter into this model, with higher temperatures and stronger winds increasing firnification rates, whilst higher accumulation rates have the opposite effect at any given depth. A temperature increase of 10°C has the equivalent effect to a wind-speed increase of 5 m s-1, or an accumulation rate decrease of 0.10 m a-1 w.e. A second model equates the logarithm of the porosity to overburden pressure and gives a much better fit to field data at higher densities where values asymptote to the bubble-free density of pure ice. This model generally yields a poor match to field data in the upper layers, with surface densities generally overestimated. Annual mean wind speed appears to be the least important of the local variables in this case, consistent with the success of the model at greater depth in matching data profiles.