Temporal variability in snow accumulation and density at Summit Camp, Greenland ice sheet

A 3-year record of weekly snow water equivalent (SWE) accumulation at Summit Camp, central Greenland ice sheet, obtained by direct sampling, is presented. While the overall SWE accumulation of 24.2 cm w.e. a−1 matches long-term ice core estimates, variability increases at shorter timescales. Half of the annual SWE accumulation occurs during a few large events, with the average accumulation rate decreasing 35% between the first and second halves of the record coinciding with exceptional anticyclonic conditions in the spring and summer of 2019. No seasonality in accumulation is detected. Rather, local accumulation rates appear to be significantly impacted by wind redistribution that obscures temporal patterns in snowfall. Surface snow density is consistent, on average, with previously measured values but does not correlate with near surface temperature or wind speed. Two surface mass balance reanalysis models significantly underestimate accumulation rates at Summit Camp. This is concerning because such models are often used to estimate ice-sheet mass loss.

Methods
A shallow, rectangular pit is excavated, and a piece of plywood is placed over the floor of the pit. The pit is then allowed to fill with snow and settle over a period of 2 weeks. A plastic tube is used to remove a core sample of the snow from the surface to the plywood, which serves as a depth reference for each subsequent sample. The sample is taken from a different location on the board each time, as measured from flagged poles at the corner of the plywood, to provide an undisturbed sample. The snow water equivalent (SWE) of the sample is obtained from both its mass and water volume, providing redundancy for quality control. To obtain the SWE thickness of the sample from its mass, the sample is brought indoors in its sampling tube and weighed. The weight of the empty sampling tube is subtracted, and this weight is divided by the cross-sectional area of the tube. To obtain the SWE thickness from the sample volume, the sample is allowed to melt and the liquid volume is divided by the cross-sectional area of the core. The snow depth at each sampling site is also recorded. The SWE estimate divided by this depth gives an estimate of sample density.

When no undisturbed locations remain on the board, or the snow becomes too deep to sample, sampling moves to a new, adjacent snow board site. During the change to a new site, samples are taken at the same time at both the old and new sites. These are termed tie points. Subtracting the SWE value of the tie point at the new site from that of the old site, and adding this difference to later measurements, gives the cumulate change in SWE across site transitions.

The snow board sampling sites were in a designated area of undisturbed snow, upwind of other field instrumentation and buildings to minimize their influence on accumulation.