Abstract

As part of the cal/val experiments for the CryoSat radar altimeter, density profiles in the upper 10-14 m of snow have been measured along a 500 km traverse across the Greenland Ice Sheet, using a neutron scattering technique. Repeat measurements, over periods ranging from a few days to 5 years allow strain rates and densification rates to be determined as a function of depth. As expected, the strain rate decreases as the ratio of pore space to ice content decreases. Very large strain rates are observed in the surface layer of snow over summer periods. However, for multi-year snow, once the effect of porosity has been removed, the remaining mean response is constant with depth, that is the effect of increasing overburden pressure is counteracted by increasing strength of the material. The mean strain rate for multi-year snow at a given site is related to the mean annual accumulation rate and mean annual temperature by an expression consistent with the Herron and Langway equation for first-stage densification. However, there are fluctuations in strainrate associated with the annual layering which indicate that fine and coarse-grained snow have differing strengths. Futhermore, the temperature-dependent process equations proposed by previous workers are not consistent with the data. An alternative approach is suggested.